Biodegradable Polymer Blends and Their Biocomposites

i

Biodegradable Polymer Blends and Their Biocomposites: Compatibilization

and Performance Evaluation

By

Rajendran Muthuraj

A Thesis

Presented to

The University of Guelph

In partial fulfilment of requirements

for the degree of

Doctor of Philosophy

in

Engineering

Guelph, Ontario, Canada

©Rajendran Muthuraj, November, 2015

ii

ABSTRACT

BIODEGRADABLE POLYMER BLENDS AND THEIR BIOCOMPOSITES:

COMPATIBILIZATION AND PERFORMANCE EVALUATION

Non-biodegradable polymers, polymer blends and composites are known to persist in the

environment over a long time. The use of certain biodegradable polymers is limited as they often fail

to match some of the non-biodegradable counterpart perfromances. Blends of biodegradable

polymers and composites with complementary attributes can provide materials that strike a

balance between cost and performance. This research was focused on the fabrication and

performance evaluation of biodegradable polymer blends and composites, as potential

alternatives to non-biodegradable polymeric materials. Industrially viable melt processing

techniques like extrusion and injection molding were adopted to fabricate and evaluate the

structure-property-relationship of biodegradable polymer blends and composites. In this research

work, two different types of commercially available biodegradable polyesters, namely

poly(butylene adipate-co-terephthalate) (PBAT) and poly(butylene succinate) (PBS) were used

to fabricate binary blends and composites. Melt blending these two polymers yielded synergistic

properties, which are not present in the respective polymers. The optimal PBS/PBAT blend was

selected based on its overall performance and it was used as the standard biocomposite matrix.

Miscanthus is a purpose grown energy crop and has not been explored much for polymer

Advisor: Dr. Amar K. Mohanty

Co-advisor: Dr. Manjusri Misra

Rajendran Muthuraj

University of Guelph, 2015

iii

composite applications. This fiber was used as reinforcing agents in the PBS, PBAT and blend of

PBS/PBAT matrix to compare the effects of matrix properties upon the performance of the

resulting composites.

One important aspect of this study was reactive compatibilization to improve the

interfacial adhesion between the miscanthus fibers and polymer matrix. Maleic anhydride grafted

polyesters were synthesized as a compatibilizer, which was used to improve the compatibility

with the miscanthus fibers and polymer matrices. The improved fiber-to-polymer matrix

adhesion exhibited in better mechanical performances of the resulting composites compared to

that of uncompatibilized counterparts. The influence of major processing parameters such as

processing temperature, screw speed, fiber length, and holding pressure on the mechanical

performance were statistically analyzed by factorial design of experiment. The impact strength of

the PBS/PBAT/miscanthus fiber composites was significantly dependent on the fiber length. The

durability of the biodegradable polymer (PBS, PBAT and PBS/PBAT blend) was investigated

after being exposed to elevated temperature (50oC) and humidity (90%) for 30 days. It was found

that the mechanical properties of the samples were heavily affected under the selected

environmental conditions and exposure time.

An optimum biocomposite formulation was successfully extruded and injection molded

for continuous prototype manufacturing in pilot-scale production facilities.

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Dedicated to my family

v

Acknowledgements

It would not have been possible to accomplish this PhD thesis without my advisors, Dr.

Amar Kumar Mohanty and Dr. Manjusri Misra. I would like to thank them for their consistent

support, encouragement and guidance during the course of this project. I am also grateful to my

advisory committee members, Dr. Animesh Dutta and Dr. Loong-Tak Lim for their fruitful

comments towards my research work and support for my research work. Dr. Fantahun Defersha

is gratefully acknowledged for his support in statistical analysis.

I would like to express my sincere thanks to my parents Mr. P. Muthuraj, Mrs. M.

Karuppathal, and my brother Mr. M. Venkatachalam and my sisters Miss. M. Bhaghyalakshmi

and Mrs. M. Manoranjitham who provided never-ending support and kindness throughout my

life. Also, I am very much thankful to the Bioproducts Discovery and Development Center

(BDDC) colleagues and my friends who have directly and indirectly helped me throughout this

project.

I would like to express my gratitude to funding agencies that provided financial support

to carry out this research: Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA),,

Ontario Ministry of Economic Development and Innovation (MEDI), Ontario Research Fund -

Research Excellence Round 4 program (ORF-RE04), Natural Sciences and Engineering

Research Council (NSERC) − Discovery Grant, National Centre of Excellence (NCE) AUTO21

Network and our private sector partner - New Energy Farms for providing miscanthus fiber

samples.

vi

List of publications

Patent

Mohanty. A.K., Misra. M., Zarrinbakhsh. N., Muthuraj. R., Wang. T., U-Rodriguez. A.,

Vivekanandhan. S., Biodegradable polymer-based composites with tailored properties

and method of making those, US provisional application, Application number-62128736,

filed on March-2015.

Peer-reviewed journal publications

Muthuraj. R., Misra. M., Mohanty. A.K., 2014, Biodegradable Poly(butylene succinate)

and Poly(butylene adipate-co-terephthalate) Blends: Reactive Extrusion and Performance

Evaluation, Journal of Polymers and the Environment, 22, 336–349.

Muthuraj. R., Misra. M., Mohanty. A.K., 2015, Hydrolytic degradation of biodegradable

polyesters under simulated environmental conditions. Journal of Applied Polymer

Science, 132, 42189-42201.

Muthuraj. R., Misra. M., Mohanty. A.K., Injection Molded Sustainable Biocomposites

From Poly(butylene succinate) Bioplastic and Perennial Grass, ACS sustainable

chemistry & engineering, 2015, 3, 2767−2776.

Muthuraj. R., Misra. M., Defersha.F., Mohanty. A.K., Influence of Processing Parameters

on the Impact Strength of miscanthus composites: A Statistical Approach, Composites

Part A: Applied Science and Manufacturing, 2015,

DOI:10.1016/j.compositesa.2015.09.003

Book chapter

Muthuraj. R., Misra. M., Mohanty. A.K., 2015, Chapter 5: Studies on mechanical,

thermal, and morphological characteristics of biocomposites from biodegradable polymer

blends and natural fibers. In: Misra. M, Pandey. J. K, Mohanty. A.K., (Eds.)

Biocomposites: Design and Mechanical Performance, Woodhead Publishing Limited,

Cambridge, UK, pp. 93-140.

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Peer-reviewed conference publications

Muthuraj. R., Misra. M., Mohanty. A.K., 2015, “Durability Studies of Biodegradable

Polymers under Accelerated Weathering Conditions”, Society of Plastic Engineering

(ANTEC), March 23-25, Orlando, Florida, USA.

Muthuraj. R., Misra. M., Mohanty. A.K., 2015, “Binary Blends of poly(butylene adipate-

co-terephthalate) and poly(butylene succinate): A new matrix for biocomposites

applications”, AIP Conf. Proc. 1664, 150009-1.

Muthuraj. R., Misra. M., Mohanty. A.K., 2013, “Plasticization of Co-products from

Bioethanol Industries: Potential Uses in Biocomposites”, The 19th

International

Conference on Composite Materials (ICCM), pp 7764-7771.

xxi

Table of Contents Acknowledgements ........................................................................................................... v

List of publications .......................................................................................................... vi

List of Tables ............................................................................................................... xxix

List of Figures ............................................................................................................. xxxii

List of abbreviations and defined terms ......................................................................... xlii

Chapter 1: Introduction ..................................................................................................... 1

Abstract ............................................................................................................................. 1

1.1 Research problems ...................................................................................................... 1

1.2 Objectives and Hypotheses ......................................................................................... 2

1.3 Thesis organization ..................................................................................................... 4

Chapter 2: Studies on Mechanical, Thermal and Morphological Characteristics of

Biocomposites from Biodegradable Polymer Blends and Natural Fibers* ...................... 9

2.1 Introduction ................................................................................................................. 9

2.2. Biodegradable and compostable polymeric materials ............................................. 10

2.3. Renewable resource based biodegradable polymers: Some examples .................... 11

2.3.1 Poly(lactic acid), PLA ............................................................................................ 12

2.3.2 Microbial polyesters-Polyhydroxyalkanoates (PHAs) .......................................... 14

2.4 Fossil fuel based biodegradable polymers: Some examples ..................................... 17

2.4.1 Poly(butylene succinate), PBS ............................................................................... 17

2.4.2 Poly(butylene adipate-co-terephthalate), PBAT .................................................... 19

2.4.3 Poly(caprolactone), PCL ........................................................................................ 21

2.5. Recyclability of biodegradable polymers ................................................................ 21

2.6. Durability of biodegradable polymers ..................................................................... 23

2.7. Polymer blends: Some examples ............................................................................. 24

2.7.1 Miscible biodegradable polymer blends ................................................................ 26

2.7.2 Immiscible biodegradable polymer blends ............................................................ 28

2.7.3 Compatibilization of polymer blends..................................................................... 32

2.7.4 Non-reactive compatibilization of biodegradable polymer blends ................ 33

2.7.5 Reactive compatibilization of biodegradable polymer blends: Few specific

examples ......................................................................................................................... 35

xxii

2.7.5.1 Reactive compatibilization of PLA/PBAT blends: ............................................. 35

2.7.5.2 Reactive compatibilization of PLA/PBS blends: ................................................ 37

2.7.5.3 Reactive compatibilization of PLA/PHB and PHBV/PBS blends:..................... 38

2.7.5.4 Reactive compatibilization of PLA/PCL blends: ................................................ 40

2.8 Natural fibers ............................................................................................................ 41

2.8.1 Classification of natural fibers ............................................................................... 41

2.8.2 Natural fibers: nature and behavior........................................................................ 42

2.8.3 Advantages and challenges in using natural fibers ................................................ 42

2.9. Biocomposites .......................................................................................................... 43

2.9.1 Advantageous of natural fiber composites ............................................................. 44

2.9.2 Attributes of natural fiber composites.................................................................... 45

2.10. Biocomposites based on biodegradable blends as matrix material: Some

specific examples ............................................................................................................ 47

2.10.1 Biocomposites based on PHBV blends................................................................ 48

2.10.2 Biocomposites based on PLA blends ................................................................... 54

2.11. Natural fiber composites market and their applications ........................................ 61

2.12 Conclusions ............................................................................................................. 62

References ....................................................................................................................... 64

Chapter 3: Fully Biodegradable Poly (butylene succinate) and Poly (butylene

adipate-co-terephthalate) Blends: Reactive Extrusion and Performance Evaluation* ... 88

Abstract ........................................................................................................................... 88

3.1 Introduction ............................................................................................................... 89

3.2 Experimental section ................................................................................................. 91

3.2.1 Materials ................................................................................................................ 91

3.2.3 Fourier transform infrared spectroscopy (FTIR) ................................................... 92

3.2.4 Mechanical properties ............................................................................................ 92

3.2.6 Differential scanning calorimetry (DSC) ............................................................... 93

3.2.7 Dynamic mechanical analysis (DMA) ................................................................... 93

3.2.8 Heat deflection temperature (HDT) ....................................................................... 94

3.2.9 Thermogravimetric analysis (TGA) ....................................................................... 94

3.2.10 Rheological studies .............................................................................................. 94

xxiii

3.2.11 Polarizing optical microscopy (POM) ................................................................. 94

3.2.12 Scanning electron microscopy (SEM) ................................................................. 95

3.3 Results and Discussion ............................................................................................. 95

3.3.1 Fourier transform infrared spectroscopy (FTIR) ................................................... 95

3.3.2 Mechanical properties ............................................................................................ 97

3.3.3 Melt flow index .................................................................................................... 100

3.3.4 Differential scanning calorimetry ........................................................................ 101

3.3.5 Dynamic mechanical analysis .............................................................................. 105

3.3.6 Heat deflection temperature ................................................................................. 107

3.3.7 Thermogravimetric analysis................................................................................. 108

3.3.8 Rheological properties ......................................................................................... 110

3.3.9 Polarizing optical microscopy.............................................................................. 115

3.3.10 Scanning electron microscopy ........................................................................... 117

3.4 Conclusions ............................................................................................................. 119

References ..................................................................................................................... 119

Chapter 4: Preparation and Characterization of Maleic Anhydride Grafted

Biodegradable Polyesters .............................................................................................. 124

Abstract ......................................................................................................................... 124

4.1 Introduction ............................................................................................................. 125

4.2 Materials and Methods ............................................................................................ 128

4.2.1 Synthesis of MAH grafted PBS, PBAT and their blend ...................................... 128

4.2.2 Grafting mechanism ............................................................................................. 130

4.2.3 Purification of MAH grafted samples .................................................................. 133

4.2.4 Determination of grafting percentage .................................................................. 133

4.2.5 Gel percentage measurement ............................................................................... 134

4.2.6 Fourier transform infrared (FTIR) spectroscopy ................................................. 135

4.2.7 Thermogravimetric analysis (TGA) ..................................................................... 135

4.2.8 Differential scanning calorimetry (DSC) ............................................................. 135

4.3 Results and Discussion ........................................................................................... 135

4.3.1 Infrared spectroscopy ........................................................................................... 135

4.3.2 MAH grafting percentage calculation .................................................................. 139

xxiv

4.3.3 Gel content measurement ..................................................................................... 142



4.4 Conclusions ............................................................................................................. 147

References ..................................................................................................................... 148

Chapter 5: Enhanced Mechanical Performances of Fully Biodegradable Miscanthus

Fibers Reinforced Poly (butylene succinate) Composites* .......................................... 156

Abstract ......................................................................................................................... 156

5.1 Introduction ............................................................................................................. 157

5.2 Materials and Methods ............................................................................................ 159

5.2.1 Materials .............................................................................................................. 159

5.2.2 Thermal property ................................................................................................. 160

5.2.3 Biocomposite preparation .................................................................................... 161

5.2.4 Mechanical testing ............................................................................................... 161

5.2.5 Statistical analysis ................................................................................................ 162


5.2.7 Melt flow index (MFI) ......................................................................................... 162


5.2.9 Scanning electron microscopy (SEM) ................................................................. 163

5.3 Results and discussion ............................................................................................ 164


5.3.2 Efficiency of compatibilizer ................................................................................ 165

5.3.3 Mechanical properties versus fiber loading ......................................................... 167


5.3.5 Adhesion factor calculation ................................................................................. 179


5.3.7 Melt flow analysis ................................................................................................ 182


5.3.9 Morphological analysis ........................................................................................ 184

5.4 Conclusions ............................................................................................................. 188

References ..................................................................................................................... 189

xxv

Chapter 6: Mechanical Performances of Biocomposites Made From Miscanthus

Fibers and Poly(butylene adipate-co-terephthalate) Matrix ......................................... 196

Abstract ......................................................................................................................... 196

6.1 Introduction ............................................................................................................. 197

6.2 Materials ................................................................................................................. 198

6.3 Biocomposite fabrication method ........................................................................... 199

6.4 Characterization methods........................................................................................ 199


6.5.1 Mechanical properties .......................................................................................... 200

6.5.2 Melt flow index and Heat deflection temperature ............................................... 203

6.5.3 Scanning electron microscopy ............................................................................. 204

6.6 Conclusions ............................................................................................................. 205

References ..................................................................................................................... 206

Chapter 7: Biocomposites Consisting of Miscanthus Fibres in a Biodegradable

Binary Blend Matrix: Preparation and Performance Evaluation* ................................ 208

Abstract ......................................................................................................................... 208

7.1 Introduction ............................................................................................................. 209

7.2.1 Materials .............................................................................................................. 212

7.2.2 Processing of polymer blend and their composites.............................................. 213


7.2.4 Density ................................................................................................................. 213

7.2.5 Dynamic mechanical analysis (DMA) ................................................................. 214


7.2.8 Thermogravimetric analysis (TGA) ..................................................................... 215


7.2.10 Rheological property .......................................................................................... 215



7.3.2 Theoretical approximation of Young’s modulus of the PBS/PBAT

biocomposites ............................................................................................................... 223

7.3.3 Dynamic mechanical properties ........................................................................... 226

7.3.4 Density ................................................................................................................. 229

xxvi




7.3.8 Measurements of fiber diameter, length, and aspect ratio ................................... 235

7.3.9 Morphology of composites .................................................................................. 237

7.3.10 Rheological property .......................................................................................... 239

7.4 Conclusions ............................................................................................................. 241

References ..................................................................................................................... 242

Chapter 8: Influence of Processing Parameters on the Impact Strength of

Biocomposites: A Statistical Approach* ...................................................................... 250

Abstract ......................................................................................................................... 250

8.1 Introduction ............................................................................................................. 251

8.2 Full factorial design methodology .......................................................................... 254

8.3 Materials ................................................................................................................. 255

8.4 Experimental procedure .......................................................................................... 256

8.4.1 Samples preparation ............................................................................................. 256

8.5 Characterization methods........................................................................................ 257

8.5.1 Fiber dimension measurement ............................................................................. 257

8.5.2 Mechanical testing and scanning electron microscopy (SEM) ............................ 258



8.6.2 Analysis of variance (ANOVA) for impact strength ........................................... 260

8.6.3 Effect of processing parameters on the impact strength ...................................... 262

8.6.4 Fiber length distribution ....................................................................................... 267

8.6.4 Scanning electron microscopy ............................................................................. 270

8.6.5 Mathematical model development ....................................................................... 271

8.6.6 Diagnostic verification of the developed model .................................................. 272

8.7 Conclusions ............................................................................................................. 274

References ..................................................................................................................... 275

Chapter 9: Hydrolytic Degradation of Biodegradable Polyesters under Simulated

Environmental Conditions* .......................................................................................... 280

xxvii

Abstract ......................................................................................................................... 280

9.1 Introduction ............................................................................................................. 281

9.2 Materials and Methodology .................................................................................... 284

9.2.1 Materials .............................................................................................................. 284

9.2.2 Sample preparation and conditioning .................................................................. 284

9.2.3 Moisture absorption ............................................................................................. 285

9.2.4 Fourier transform infrared spectroscopy (FTIR) ................................................. 285



9.2.7 Dynamic mechanical analysis (DMA) ................................................................. 287


9.2.9 Polarizing optical microscopy (POM) ................................................................. 287

9.2.10 Morphological analysis ...................................................................................... 287


9.3.1 Moisture absorption ............................................................................................. 288

9.3.2 Hydrolytic degradation mechanism of PBS and PBAT ....................................... 289

9.3.3 Changes in mechanical properties ....................................................................... 292




9.3.7 Polarizing optical microscopy.............................................................................. 308


9.4. Conclusions ............................................................................................................ 313

References ..................................................................................................................... 314

Chapter 10: Conclusions, Contributions, and Recommendations for Future Work ..... 318

Abstract ......................................................................................................................... 318

10.1 Overview ............................................................................................................... 318

10.2 Conclusions ........................................................................................................... 318

10.3 Significant contributions ....................................................................................... 321

10.4 Recommendations for future works ...................................................................... 323

xxviii

Appendix I: Binary Blends of Poly(Butylene Succinate) and Poly(Butylene Adipate-

co-Terephthalate): A New Matrix for Biocomposites Applications* ........................... 325

Abstract ......................................................................................................................... 325

A-I.1.Introduction ......................................................................................................... 325

A-I.2. Materials and Methods ....................................................................................... 326

A-I.3. Results and Discussion ....................................................................................... 328

A-I.4. Conclusions ........................................................................................................ 332

References ..................................................................................................................... 333

Appendix II: Durability Studies of Biodegradable Polymers under Accelerated

Weathering Conditions* ............................................................................................... 334

Abstract ......................................................................................................................... 334

A-II.1. Introduction ....................................................................................................... 334

A-II.2. Materials and Methods ...................................................................................... 336

A-II.3. Results and Discussion...................................................................................... 337

A-II.4. Conclusions ....................................................................................................... 342

xxix

List of Tables

Table 2.1. Classification and molecular structure of some biodegradable polymers ....... 11

Table 2.2. Properties of some biodegradable polymers in comparison to non-

biodegradable polymers ..................................................................................................... 16

Table 2.3. Ecoflex® based masterbatches for different applications [66,67] ..................... 20

Table 2.4. List of companies engaged in the production of some biodegradable polymer

blends (the table was modified after referene [9,94,95]). .................................................. 25

Table 2.5. Properties of some natural and synthetic fibers ................................................ 46

Table 2.6. Recently developed biodegradable polymer blend matrix based biocomposites56

Table 3.1. Melt flow index (MFI) of the neat polymers and their blends ........................ 100

Table 3.2. Solubility parameter values for polymers ....................................................... 104

Table 3.3. Heat deflection temperatures of the neat polymers and their blends .............. 108

Table 4.1. Properties of the neat PBS, PBAT and PBS/PBAT blend .............................. 128

Table 4.2. Proposed formulation for producing maleation of PBS, PBAT, and PBS/PBAT

blend ................................................................................................................................. 130

Table 4.3. Maleic anhydride grafting percentage of the PBS, PBAT and PBS/PBAT blend

....................................................................................................................................... 140

Table 4.4. Detailed DSC results of the maleated PBS and PBS/PBAT blend (MAH

grafted samples were prepared with 1 phr DCP and 5 phr MAH) ................................... 147

Table 5.1. Mechanical properties of PBS composites with two different MAH grafting

levels of compatibilizer .................................................................................................... 167

Table 5.2. A comparison of mechanical properties of injection molded PBS/natural fiber

composites (Note: the reported percentage differences were calculated based on the neat

PBS matrix properties) ..................................................................................................... 176

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Table 5.3. Heat deflection temperature (HDT) and melt flow index (MFI) of neat PBS

and its composites ............................................................................................................ 181

Table 5.4. Summary of differential scanning calorimetry traces of neat PBS and its

composites........................................................................................................................ 182

Table 6.1. Melt flow index (MFI) and Heat deflection temperature (HDT) measurement204

Table 7.1. Notched Izod impact strength of PBS/PBAT blend and its compatibilized and

uncompatibilized composites ........................................................................................... 223

Table 7.2. Heat deflection temperature (HDT) and density of PBS/PBAT blend and its

compatibilized and uncompatibilized composites ........................................................... 229

Table 7.3. Thermogravimetric data of miscanthus, PBS/PBAT blend and their composites

....................................................................................................................................... 232

Table 7.4. Detailed differential scanning calorimetry analysis of the PBS/PBAT blend

and their composites ........................................................................................................ 235

Table 7.5. Average fiber length (L), average fiber diameter (D), and aspect ratio (L/D) of

the miscanthus fiber before and after compounding ........................................................ 236

Table 8.1. Selected processing parameters and their respected levels ............................. 255

Table 8.2. Physical and mechanical properties of the PBS/PBAT blend and miscanthus

fibers ................................................................................................................................ 256

Table 8.3. The 16 investigated experimental conditions ................................................. 257

Table 8.4. A complete summary of all the experiments and the related mechanical

properties of PBS/PBAT/miscanthus composites ............................................................ 260

Table 8.5. Analysis of Variance (ANOVA) for notched Izod impact strength ............... 261

Table 8.6. Average fiber length (L), average fiber diameter (D), and aspect ratio (L/D) of

the miscanthus fiber before and after compounding ........................................................ 269

Table 9.1. Notched Izod impact strength of the samples before and after conditioned at

50oC with 90% relative humidity ..................................................................................... 300

xxxi

Table 9.2. DSC results for PBS, PBAT and their blend before and after 30 days

conditioned at 50oC with 90% relative humidity ............................................................. 303

Table 9.3. Relative molecular weight (M1/M2) of the PBS, PBAT and PBS/PBAT blend

before and after 6 days conditioned at 50oC with 90% relative humidity ....................... 308

Table A-II.1. General properties of PBS, PBAT, PBS/PBAT (60/40 wt%) and PP.

(a obtained from material data sheet,

b and PBS/PBAT (60/40 wt%) data were measured

in the lab) ……………………………………………………………………………....337

Table A-II.2. Notched Izod impact strength of PBS, PBAT, PBS/PBAT (60/40 wt%)

and PP before and after 18 days conditioning at 50oC with 90%

RH………………………………………………………………………………………340

xxxii

List of Figures

Figure 2.1. (a) Schematic representation of the evolution of morphology in a binary

immiscible blend, (b) matrix/dispersed morphology, and (c) co-continuous

morphology (adapted with kind permission from Ravati and Favis, Polymer, 2010,

51: 4547-4561, Copyright©

2015, Elesevier, Licence number 3591550612924 [113]) .. 30

Figure 2.2. Adding a compatibilizing agent, such as a diblock copolymer, to a polymer

blend can improve its stability, but is more likely to result in a dispersed morphology

rather than a co-continuous morphology. a) A two-dimensional slice of a compatibilized

blend with dispersed phase morphology, represented by minority dark blue phase and a

majority turquoise phase. b) A molecular schematic showing how the diblock copolymers

are segregated at the interface between the two phases (adapted with kind permission

from Ryan, Nature Materials, 2002, 1: 8-10. Copyright©

2015, nature publishing group,

License number 3557770952943 [142]). ........................................................................... 34

Figure 3.1. Evaluation of the normalized FTIR spectra of the carbonyl region (1800-1600

cm-1

) of PBS, PBAT and their blends ................................................................................ 96

Figure 3.2. Tensile stress-strain curves of PBS, PBAT, and their blends.......................... 98

Figure 3.3. Tensile strength and elongation at break of PBS, PBAT, and their blends: (A)

PBS; (B) PBS/PBAT(70/30 wt%); (C) PBS/PBAT (60/40 wt%); (D) PBS/PBAT (50/50

wt%); (E) PBAT. .............................................................................................................. 99

Figure 3.4. Second heating DSC thermograms of PBS, PBAT, and their blends after

cooling at 5 °C/min .......................................................................................................... 102

Figure 3.5. Theoretical and experimental values of Tg for PBS/ PBAT blends ............... 105

Figure 3.6. Storage moduli of PBS, PBAT, and their blends .......................................... 106

Figure 3.7. Tan curves of PBS, PBAT, and their blends .............................................. 107

Figure 3.8. TGA curves of PBS, PBAT, and their blends ............................................... 109

xxxiii

Figure 3.9. DTG curves of PBS, PBAT, and their blends ............................................... 109

Figure 3.10. Complex viscosity of PBS, PBAT, and their blends with different weight

fractions of PBAT at 140 oC ............................................................................................ 110

Figure 3.11. Loss modulus versus frequency for PBS, PBAT, and their blends with

different weight fractions of PBAT at 140 oC ................................................................. 112

Figure 3.12. Storage modulus versus frequency for PBS, PBAT, and their blends with

different weight fractions of PBAT at 140 oC ................................................................. 113

Figure 3.13. Cole–Cole plot of the PBS/PBAT blends at 140 oC .................................... 114

Figure 3.14. (a) Photograph of the film annealed at 80 oC: (i) PBS; (ii) PBS/PBAT

(70/30 wt%); (iii) PBS/PBAT (60/40 wt%) and (iv) PBS/PBAT (50/50 wt%). Figure (b)

Photograph of the film annealed at 90 oC: (i) PBS; (ii) PBS/PBAT (70/30 wt%); (iii)

PBS/PBAT (60/40 wt%) and (iv) PBS/PBAT (50/50 wt%) ............................................ 116

Figure 3.15. (a) SEM images of PBS and PBAT blends (left hand side) (i) PBS/PBAT

(70/30 wt%); (ii) PBS/PBAT (60/40 wt%) and (iii) PBS/PBAT (50/50 wt%) . (b) SEM

images of PBS and PBAT blends surface after et al.,hing with THF (right hand side): (i)

PBS/PBAT (70/30 wt%); (ii) PBS/PBAT (60/40 wt%) and (iii) PBS/PBAT (50/50 wt%)

....................................................................................................................................... 118

Figure 4.1. Proposed reaction mechanism of maleic anhydride grafted PBS (MAH-g-

PBS) ................................................................................................................................. 131

Figure 4.2. Proposed reaction mechanism of maleic anhydride grafted PBAT (MAH-g-

PBAT) .............................................................................................................................. 132

Figure 4.3. FTIR spectra of MAH, neat PBS and MAH-g-PBS with 1 phr DCP and 5 phr

MAH ................................................................................................................................ 136

Figure 4.4. FTIR spectra of the MAH, neat PBAT and MAH-g-PBAT with 1 phr DCP

and 5 phr MAH ................................................................................................................ 137

xxxiv

Figure 4.5. FTIR spectra of MAH, neat PBS/PBAT blend and MAH-g-PBS/PBAT blend

with 1 phr DCP and 5 phr MAH ...................................................................................... 139

Figure 4.6. Gel content of maleic anhydride grafted PBS, PBAT, and PBS/PBAT samples

with 5 phr MAH and different concentration of DCP ..................................................... 143

Figure 4.7. TGA thermograms of neat and maleated PBS, PBAT and PBS/PBAT blend

(the maleated samples were obtained with 1 phr DCP and 5 phr MAH) ........................ 144

Figure 4.8. DSC second heating curves of neat PBS, PBS/PBAT (60/40 wt%) and their

maleated samples with 1 phr DCP and 5 phr MAH......................................................... 145

Figure 4.9. DSC first cooling curves of neat PBS, PBS/PBAT (60/40 wt%) and their

maleated samples with 1phr DCP and 5 phr MAH.......................................................... 146

Figure 5.1.SEM micrograph of as received miscanhtus fibers……………………........160

Figure 5.2. Thermogravimetric analysis of miscanthus fiber under different environment

....................................................................................................................................... 164

Figure 5.3. Tensile properties of PBS and PBS/miscanthus composites: A) neat PBS, B)

PBS/miscanthus (70/30 wt%), C) PBS/miscanthus/compatibilizer (65/30/5 wt%), D)

PBS/miscanthus (60/40 wt%), E) PBS/miscanthus/compatibilizer (55/40/5 wt%), F)

PBS/miscanthus (50/50 wt%), and G) PBS/miscanthus/compatibilizer (45/50/5wt%)... 169

Figure 5.4. Reduced tensile strength of uncompatibilized and compatibilized

PBS/miscanthus composites plotted against volume fraction of fibers according to

equation 5.4. ..................................................................................................................... 171

Figure 5.5. Flexural properties of PBS and PBS/miscanthus composites: A) neat PBS, B)



PBS/miscanthus (50/50 wt%), and G) PBS/miscanthus/compatibilizer (45/50/5 wt%).. 172

xxxv

Figure 5.6. Nothced Izod impact strength of PBS and PBS/miscanthus composites: A)

neat PBS, B) PBS/miscanthus (70/30 wt%), C) PBS/miscanthus/compatibilizer (65/30/5

wt%), D) PBS/miscanthus (60/40 wt%), E) PBS/miscanthus/compatibilizer (55/40/5

wt%), F) PBS/miscanthus (50/50 wt%), and G) PBS/miscanthus/compatibilizer (45/50/5

wt%) ................................................................................................................................. 175

Figure 5.7. Dynamic mechanical analysis of PBS and PBS/miscanthus composites: A)

neat PBS, B) PBS/miscanthus (70/30 wt%), C) PBS/miscanthus/compatibilizer (65/30/5

wt%), D) PBS/miscanthus (60/40 wt%), E) PBS/miscanthus/compatibilizer (55/40/5

wt%), F) PBS/miscanthus (50/50 wt%), and G) PBS/miscanthus/compatibilizer (45/50/5

wt%) ................................................................................................................................. 178

Figure 5.8. Tan δ curves of PBS and its composites: A) neat PBS, B) PBS/miscanthus

(70/30 wt%), C) PBS/miscanthus/compatibilizer (65/30/5 wt%), D) PBS/miscanthus

(60/40 wt%), E) PBS/miscanthus/compatibilizer (55/40/5 wt%), F) PBS/miscanthus

(50/50 wt%), and G) PBS/miscanthus/compatibilizer (45/50/5 wt%) ............................. 179

Figure 5.9. Adhesion factor of PBS/miscanthus composites: A) PBS/miscanthus (70/30

wt%), B) PBS/miscanthus/compatibilizer (65/30/5 wt%), C) PBS/miscanthus (60/40

wt%), D) PBS/miscanthus/compatibilizer (55/40/5 wt%), E) PBS/miscanthus (50/50

wt%), and F) PBS/miscanthus/compatibilizer (45/50/5 wt%) ......................................... 180

Figure 5.10. DSC second heating thermograms of PBS and its composites: A) neat PBS,

B) PBS/miscanthus (70/30 wt%), C) PBS/miscanthus/compatibilizer (65/30/5 wt%), D)



Figure 5.11. DSC first cooling thermograms of PBS and its composites: A) neat PBS, B)




Figure 5.12. SEM micrograph of tensile fractured surface of uncompatibilized PBS

composites with low (150x) and high (500x) magnification; (a) and (b) are

PBS/miscanthus (70/30 wt%) composites; (c) and (d) are PBS/miscanthus (60/40 wt%)

composites; (e) and (f) are PBS/miscanthus (50/50 wt%) composites ............................ 186

xxxvi

Figure 5.13. SEM micrograph of tensile fractured surface of compatibilized PBS

composites with low (150x) and high (500x) magnification; (a) and (b) are

PBS/miscanthus (70/30 wt%) composites; (c) and (d) are PBS/miscanthus (60/40 wt%)

composites; (e) and (f) are PBS/miscanthus (50/50 wt%) composites ........................... 187

Figure 6.1. Tensile strength and tensile modulus of PBAT and its composites; (A) neat

PBAT, (B) PBAT/miscanthus fibers (70/30 wt%), (C) PBAT/miscanthus fibers/MAH-g-

PBAT (65/30/5 wt%), (D) PBAT/miscanthus fibers (60/40 wt%), and (E)

PBAT/miscanthus fibers/MAH-g-PBAT (65/30/5 wt%) ................................................ 201

Figure 6.2. Flexural properties of PBAT and its compatibilized and uncompatibilized

composites: (A) neat PBAT, (B) PBAT/miscanthus fibers (70/30 wt%), (C)

PBAT/miscanthus fibers/MAH-g-PBAT (65/30/5 wt%), (D) PBAT/miscanthus fibers

(60/40 wt%), and (E) PBAT/miscanthus fibers/MAH-g-PBAT (65/30/5 wt%).............. 202

Figure 6.3. Notched Izod impact strength of PBAT and its compatibilized and

uncompatibilized composites: (A) neat PBAT, (B) PBAT/miscanthus fibers (70/30 wt%),

(C) PBAT/miscanthus fibers/MAH-g-PBAT (65/30/5 wt%), (D) PBAT/miscanthus fibers

(60/40 wt%), and (E) PBAT/miscanthus fibers/MAH-g-PBAT (65/30/5 wt%).............. 203

Figure 6.4. SEM micrographs of uncompatibilized PBAT/misanthus fibers (60/40 wt%)

composites (A) and compatibilized PBAT/misanthus fibers/MAH-g-PBAT (55/40/5

wt%) composites (B)........................................................................................................ 205

Figure 7.1. Tensile properties of PBS/PBAT blend and their composites: (A) PBS/PBAT,

(B) PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers +

MAH-g-PBS/PBAT (65/30/5 wt%), (D) PBS/PBAT + miscanthus fibers (60/40 wt%),

(E) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (55/40/5 wt%), (F)

PBS/PBAT + miscanthus fibers (50/50 wt%), and (G) PBS/PBAT + miscanthus fibers +

MAH-g-PBS/PBAT (45/50/5 wt%). ................................................................................ 217

Figure 7.2. Stress-strain curves of PBS/PBAT blend and their composites: (A)

PBS/PBAT, (B) PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT +

miscanthus fibers (60/40 wt%), (D) PBS/PBAT + miscanthus fibers (50/50 wt%), (E)

PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (65/30/5 wt%), (F) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (55/40/5 wt%), and (G) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (45/50/5 wt%). ............................................... 219

xxxvii

Figure 7.3. Flexural properties of PBS/PBAT blend and their composites: (A)

PBS/PBAT, (B) PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (65/30/5 wt%), (D) PBS/PBAT + miscanthus

fibers (60/40 wt%), (E) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (55/40/5

wt%), (F) PBS/PBAT + miscanthus fibers (50/50 wt%), and (G) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (45/50/5 wt%). ............................................... 221

Figure 7.4. Expected reaction between the miscanthus and the compatibilizer .............. 221

Figure 7.5. Variation of experimental and theoretical values of Young’s modulus as a

function of fiber loading .................................................................................................. 226

Figure 7.6. Storage moduli of PBS/PBAT blend and their composites: (A) PBS/PBAT,

(B) PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers

(60/40 wt%), (D) PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (65/30/5 wt%), (F) PBS/PBAT+ miscanthus

fibers + MAH-g-PBS/PBAT (55/40/5 wt%), and (G) PBS/PBAT + miscanthus fibers +

MAH-g-PBS/PBAT (45/50/5 wt%). ................................................................................ 227

Figure 7.7. Tan δ of PBS/PBAT blend and their composites: (A) PBS/PBAT, (B)

PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers (60/40

wt%), (D) PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT + miscanthus

fibers + MAH-g-PBS/PBAT (65/30/5 wt%), (F) PBS/PBAT+ miscanthus fibers + MAH-

g-PBS/PBAT (55/40/5 wt%), and (G) PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (45/50/5 wt%). .............................................................................................. 228

Figure 7.8. Thermogravimetric traces for miscanthus, PBS/PBAT blend and its

composites........................................................................................................................ 231

Figure 7.9. DSC second heating thermograms: (A) PBS/PBAT, (B) PBS/PBAT +

miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers (60/40 wt%), (D)

PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT + miscanthus fibers +

MAH-g-PBS/PBAT (65/30/5 wt%), (F) PBS/PBAT+ miscanthus fibers + MAH-g-

PBS/PBAT (55/40/5 wt%), and (G) PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (45/50/5 wt%). .............................................................................................. 233

xxxviii

Figure 7.10. DSC first cooling thermograms: (A) PBS/PBAT, (B) PBS/PBAT +


PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT + miscanthus fibers +

MAH-g-PBS/PBAT (65/30/5 wt%), (F) PBS/PBAT+ miscanthus fibers + MAH-g-

PBS/PBAT (55/40/5 wt%), and (G) PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (45/50/5 wt%). .............................................................................................. 234

Figure 7.11. Fiber length distribution before and after compounding: (A) as received

miscanthus fibers distribution, (B) fibers distribution in 30 wt% composites, (C) fibers

distribution in 40 wt% composites, and (D) fibers distribution in 50 wt% composites. . 237

Figure 7.12. SEM micrographs of uncompatibilized PBS/PBAT blend composites with

different fiber loads: (A) PBS/PBAT + miscanthus fibers (70/30 wt%), (B) PBS/PBAT +

miscanthus fibers (60/40 wt%), and (C) PBS/PBAT + miscanthus fibers (50/50 wt%) . 238

Figure 7.13. SEM micrographs of compatibilized PBS/PBAT blend composites with

different amount of fiber loads: (A) PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (65/30/5 wt%), (B) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT

(55/40/5 wt%), and (C) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (45/50/5

wt%). ................................................................................................................................ 239

Figure 7.14. Complex viscosity of PBS/PBAT blend and its composites: (A) PBS/PBAT,

(B) PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers

(60/40 wt%), (D) PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (65/30/5 wt%), (F) PBS/PBAT + miscanthus

fibers + MAH-g-PBS/PBAT (55/40/5 wt%), and (G) PBS/PBAT + miscanthus fibers +

MAH-g-PBS/PBAT (45/50/5 wt%). ................................................................................ 240

Figure 8.1. Main effect plot for the impact strength ........................................................ 263

Figure 8.2. Plot of interaction effects for the impact strength of biocomposites ............ 264

Figure 8.3. Half Normal probability plot of the standardized effects for impact strength of

the PBS/PBAT/miscanthus composites ........................................................................... 265

xxxix

Figure 8.4. Pareto chart of the standardized effects for the impact strength of the

PBS/PBAT/miscanthus biocomposites ............................................................................ 266

Figure 8.5. Tensile stress-strain curves of PBS/PBAT/miscanthus composites with

changing fiber length 4 mm (A) and 2 mm (B) ............................................................... 267

Figure 8.6. Histograms of miscanthus fiber length distribution before and after

compounding in a twin screw extruder ............................................................................ 268

Figure 8.7. Represents the SEM micrographs of the PBS/PBAT/miscanthus composites;

(a) PBS/PBAT composites with 2 mm miscanthus (b) PBS/PBAT composites with 4 mm

miscanthus........................................................................................................................ 270

Figure 8.8. Normal probability plot of the residuals for impact strength ........................ 273

Figure 8.9. Residual plots versus fitted values for impact strength ................................. 273

Figure 8.10. Variation of the residuals with observed order values of the impact strength

of the PBS/PBAT/miscanthus composites. ...................................................................... 274

Figure 9.1. Moisture absorption curves as a function of conditioning time .................... 289

Figure 9.2. Hydrolysis reaction of PBS ........................................................................... 290

Figure 9.3. Hydrolysis reaction of PBAT ........................................................................ 290

Figure 9.4. FTIR spectra of PBS, PBAT and PBS/PBAT before and after 30 days exposed

to 50oC with 90% relative humidity................................................................................. 292

Figure 9.5. Tensile strength of PP, PBS, PBAT and PBS/PBAT as a function of exposure

time at 50oC with 90% relative humidity ......................................................................... 294

Figure 9.6. Flexural strength of PP, PBS, PBAT and PBS/PBAT as a function of

exposure time at 50oC with 90% relative humidity ......................................................... 295

xl

Figure 9.7. Testing failure mode of PBS, PBAT, PBS/PBAT and PP after 30 days

exposed to 50oC with 90% relative humidity .................................................................. 296

Figure 9.8. Percentage elongation of PP, PBS, PBAT and PBS/PBAT as a function of


Figure 9.9. Tensile modulus of PP, PBS, PBAT and PBS/PBAT as a function of exposure

time at 50oC with 90% relative humidity ......................................................................... 298

Figure 9.10. Flexural modulus of PP, PBS, PBAT and PBS/PBAT as a function of


Figure 9.11. DSC heating cycles for PBS, PBAT and PBS/PBAT before and after

exposed to 50oC with 90% relative humidity for 30 days ............................................... 301

Figure 9.12. DSC cooling curves for PBS, PBAT and PBS/PBAT before and after

exposed to 50oC with 90% relative humidity for 30 days ............................................... 302

Figure 9.13. Storage modulus of PBS, PBAT and PBS/PBAT before and after exposed to

50oC with 90% relative humidity for 30 days .................................................................. 304

Figure 9.14. Loss factor peak (tan δ) of PBS, PBAT and PBS/PBAT before and after 30

days exposed to 50oC with a relative humidity of 90% ................................................... 305

Figure 9.15. Shear viscosity curves for PBS, PBAT and PBS/PBAT before and after 6

days exposed to 50oC with a relative humidity of 90% ................................................... 308

Figure 9.16. Polarized optical micrographs of PBS, PBAT and PBS/PBAT before and

after 30 days conditioned at 50oC and 90% relative humidity......................................... 310

Figure 9.17. SEM micrographs of PBS, PBAT and PBS/PBAT before and after 30 days

conditioned at 50oC and 90% relative humidity .............................................................. 312

xli

Figure 10.1. Prototypes were made from biodegradable polymers/miscanthus fiber by

extrusion and injection molding method.......................................................................... 323

Figure A-I. 1. Tensile properties of (A) PBAT, (B) PBS, (C) PBAT/PBS (60/40 wt%) and

(D) PBAT/PBS (70/30 wt%) blend ................................................................................. 329

Figure A-I. 2. HDT and MFI values of (A) PBAT, (B) PBS, (C) PBAT/PBS (60/40 wt%)

and (D) PBAT/PBS (70/30 wt%) blend ........................................................................... 330

Figure A-I. 3. SEM image of the cryofractured PBAT/PBS (60/40 wt%) blend ........... 331

Figure A-I. 4. POM image of the (i) PBS and (ii) PBAT/PBS (60/40 wt%) blend ........ 332

Figure A-II. 1. Tensile strength of PBS, PBAT, PBS/PBAT (60/40 wt%) and PP before

and after 18 days exposed to 50oC with 90% RH. ........................................................... 338

Figure A-II. 2. Flexural strength of PBS, PBAT, PBS/PBAT (60/40 wt%) and PP before

and after 18 days exposed to 50oC with 90% RH. ........................................................... 339

Figure A-II. 3. SEM micrographs of PBS, PBAT, and PBS/PBAT (60/40 wt%) before (A,

B and C) and after (D, E and F) 18 days exposed to 50oC with 90% RH ...................... 341

xlii

List of abbreviations and defined terms

ABS Acrylonitrile butadiene styrene

ANOVA Analysis of Variance

APTMS (3-aminopropyl) trimethoxysilane

ASTM American society for testing and materials

ATR Attenuated total reflectance

BDO 1,4-butanediol

DCP Dicumyl peroxide

DDGS Distiller’s dried grains with solubles

DMA Dynamic mechanical analysis

DSC Differential scanning calorimetry

DTG Derivative thermogram

EBA-GMA Ethyl-butyl acrylate and glycidyl methacrylate copolymer

EGMA Ethylene-glycidyl methacrylate

EMAA-Zn Ethylene methacrylic acid zinc ionomer

EMA-GMA Ethylene methyl acrylate-glycidyl methacrylate terpolymer

ENR Epoxidized natural rubber

EPA Environmental protection agency

EPDM Ethylene-propylene-diene terpolymer

FDA Food and drug administration

FRCs Fiber reinforced plastic composites

FTIR Fourier Transform Infrared Spectroscopy

GMA Glycidyl methacrylate

HB Hydroxybutyrate

HDPE High-density polyethylene

xliii

HDT Heat deflection temperature

HSD Honestly significant difference

HV Hydroxyvalerate

ISO International organization for standardization

KOH Potassium hydroxide

LTI Lysine triisocynate

LVE Linear viscoelastic

MA-g-PP Maleic anhydride-grafted-polypropylene

MAH Maleic anhydride

MAH-g-PBS/PBAT Maleic anhydride-grafted-poly(butylene

succinate)/poly(butylene adipate-co-terephthalate)

MFI Melt flow index

NaOH Sodium hydroxide

PBAT Poly(butylene adipate-co-terephthalate)

PBS Poly(butylene succinate)

PBSA Poly(butylene succinate-co-adipate)

PBT Poly(butylene terephthalate)

PC Polycarbonate

PCL Polycaprolactone

PDI Poly dispersity index

PDLA Poly(D-lactic acid)

PE Polyethylene

PEBA Poly(ether-b-amide) copolymer

PET Poly(ethylene terephthalate)

PHAs Polyhydroxyalkanoates

xliv

PHB Poly(3-hydroxybutyrate)

PHB-g-PBS Poly(hydroxybutyrate)-grafted-poly(butylene succinate)

PHB-g-PLA Poly(hydroxybutyrate)-grafted-poly(lactic acid)

PHBV Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

PHBV-g-PBS Poly(3-hydroxybutyrate-co-3-valerate)-grafted-poly(butylene

succinate)

PLA Poly(lactic acid)

PLLA Poly(L-lactic acid)

PMDI Polymeric methylene diphenylene diisocyanate

PMMA Poly(methyl methacrylate)

PP Polypropylene

PPC Poly(propylene carbonate)

PS Polystyrene

PVA Poly(vinyl alcohol)

PVAc Poly(vinyl acetate)

PVDF Poly(vinylidene fluoride)

RH Relative humidity

ROP Ring opening polymerization

RWF Recycled wood fiber

SAN Styrene acrylonitrile

SEM Scanning electron microscopy

TFC Twice functionalized nanoclay

TGA Thermogravimetric analysis

T-GMA Random terpolymer of ethylene, acrylic ester and glycidyl methacrylate

THF Tetrahydrofuran

TMPTA Trimethylolpropane triacrylate

xlv

TPP Triphenyl phosphite

Nomenclature

% Percentage

Crystallization enthalpy

Melting enthalpy

Theoretical melting enthalpy of 100% crystalline polymers

Young’s modulus of the fibers

Young’s modulus of the matrix

Volume fraction of fiber

Volume fractions of matrix

∆Gm Gibbs free energy

∆Hm Heat of mixing

∆Scm Mixing of combinatorial entropy

∆Sem Mixing of excess entropy

DF Degree of freedom

E’ Storage modulus

EL Longitudinal modulus

ET Transverse modulus

F F-value (statistics)

G'' Loss modulus at melt state

G' Storage modulus at melt state

g/10min Grams per 10 minutes

g/cm3

Gram per cubic centimeter

GPa Giga Pascal

xlvi

h Hours

J/g Joule per gram

kN kilonewton

kV kilovolts

l/d length/diameter of the reinforcement

M1/M2 Relative molecular weight

mA milli-ampere

Mc Young’s modulus of the composites

mol% mole percentage

MPa Mega pascal

MS Mean square

Mw Weight average molecular weight

NA Not applicable

P P-value (in ANOVA table)

Pa.s Pascal second

phr Parts per hundred

R Universal gas constant

R2 R-squared

R2

adj Adjusted R-squared

SS Sum of square

T Absolute temperature

Tan δ Loss modulus to storage modulus ratio

Tc Crystallization temperature

Tg Glass transition temperature

xlvii

Tm Melting temperature

Tmax Maximum degradation temperature

Tonset Onset degradation temperature

wt% Weight percentage

Greek symbols

Volume fractions of the fiber

Volume fractions of the matrix

Densities of the composites

Densities of the fiber

Densities of the matrix

T

B Load-bearing capacity of the dispersed component

ƞo Zero shear viscosity

x Stress transfer constant

α Alpha level (statistics)

δ Solubility parameters

η* Complex viscosity

λ Ratio of length measured before (L0) and after (L) tensile test

σ Tensile strength of composites

χ Crystallinity

χ Flory-Huggins interaction parameter

Group molar attraction constant of the polymer

Reinforcement geometry

1

Chapter 1: Introduction

Abstract: This chapter briefly describes the research problems associated to this research work.

In order to address these research problems, this chapter provides a hypothesis followed by

objectives of the present work. The main objectives to be accomplished in this research work are

presented. This chapter also discusses the thesis organization and a short summary of each

following chapters.

1.1 Research problems

After use, petroleum-based non-biodegradable polymers persist in the soil for hundreds

of years, creating environmental concerns. Carbons from these polymers are not renewable and

their supply is quickly depleting. In addition, non-biodegradable polymers are not satisfactory

materials for short-lifespan applications such as packaging, catering, textile, agricultural,

household uses, and surgery, as incineration of many non-biodegradable polymeric material

wastes can produce non eco-friendly emissions. Recycling these products may require high-

energy consumption. Therefore, biodegradable/compostable polymers are an alternative for these

non-biodegradable/non-compostable polymers. The insufficient performances of the

biodegradable polymers are limiting their certain applications. This can be overcome by melt

blending two or more dissimilar biodegradable polymers while preserving biodegradability of

the parent components. Furthermore, the cost of biodegradable polymer is quite expensive

compared to that of commonly used polymers. Biodegradable polymers and their blends are

often used to produce biocomposites to modify overall properties. The use of biodegradable

polymers and their blends as a matrix for producing biocomposites will result in products with

enhanced performances and lower cost. The relatively hydrophobic nature of biodegradable

polymer matrix is less compatible with hydrophilic natural fibers due to the polarity difference

2

between them. The incompatibility between the polymers and fibers leads to weak stress transfer

from one phase to another phase. This weak stress transfer causes inferior mechanical properties

of the resulting composites. This problem can be effectively addressed by compatibilization

strategy.

1.2 Objectives and Hypotheses

The optimal blend can be selected based on its overall performance and it can be used as

the standard biocomposite matrix. Increase in the cost of biocomposites could be mitigated by

adding environmentally friendly low cost fibers/reinforcement such as miscanthus fibers [1]. The

selected fiber crop does not currently have good value added applications in composites sectors

[2]. Finding value-added applications for this material would improve the economics for the

respective producers. Compatibilization chemistry has a predominant role in the performances of

yielded biocomposites. Maleic anhydride (MAH) is a well-known functional monomer for

grafting onto the polymer backbone because of its chemical reactivity, lower efficiency of self

polymerization, reactive compatibilizer, and low toxicity. The MAH grafted copolymer can

significantly improve interfacial adhesion and thereby result in good mechanical properties of the

composites. Carlson et al., [3] observed the enhanced interfacial adhesion/interaction between

the polylactide (PLA)/starch composites with the addition of MAH grafted PLA compatibilizer.

Some other researchers have reported improved interfacial adhesion in the PLA composites with

the addition of MAH grafted PLA as a reactive compatibilizer [4, 5]. Similarly, poly(butylene

adipate-co-terephthalate) (PBAT) and poly(butylene succinate) (PBS) based composites were

compatibilized by the use of MAH grafted PBAT [6] and MAH grafted PBS [7], respectively. To

our knowledge, there was no literature available on the compatibilized PBS/miscanthus

3

composites, PBAT/miscanthus composites and PBS/PBAT/miscanthus composites with MAH

grafted reactive compatibilizer.

The purpose of this study was to use reactive compatibilization strategy to increase the

compatibility between the PBS/miscanthus fiber, PBAT/miscanthus fiber and

PBS/PBAT/miscanthus fibers. In this study, MAH grafted biopolymers were synthesized as a

compatibilizer, which was anticipated to have a high compatibility with the hydroxyl

functionality of the reinforcements (e.g., miscanthus). Hence, a MAH grafted biopolymer was

used for the target compatibilization of this work. Improved fibers-polymer matrix adhesion was

hypothesized to result in better mechanical properties of the resulting biocomposites. Moreover,

fibers-matrix adhesion was evaluated through theoretical methods, microscopical analysis, and

the performances of the resulting biocomposites.

The objectives of this project were as follows:

Objective 1 was to generate optimum property for biodegradable binary polymer blends

of poly(butylene adipate-co-terephthalate), PBAT and poly(butylene succinate), PBS, which can

be used as a matrix for biocomposite fabrication. In addition, the durability of PBS, PBAT and

their blend was investigated after being exposed to elevated humidity (90%) and temperature

(50oC).

Objective 2 was to functionalize PBS, PBAT and their blend by the melt free radical

grafting of maleic anhydride (MAH) onto its backbone. These MAH functionalized polymers

were use as a compatibilizing agent in subsequent steps.

The first part of Objective 3 was to investigate the compatabilization effect of MAH

grafted PBS on performances of the PBS/miscanthus composites. Second part of this objective

4

was to understand and investigate how the addition of miscanthus fibers and MAH gafted PBAT

to PBAT influences the melt processes as well as their mechanical properties.

Objective 4 was to fabricate and to evaluate performance of miscanthus fiber reinforced

PBS/PBAT blend matrix based biocomposites. The effect of compatibilizer (MAH-grafted-

PBS/PBAT blend) on the resulting biocomposites was investigated by means of mechanical,

thermo-mechanical, morphological, and rheological properties. A statistical analysis was used to

map the relations between the mechanical properties of the PBS/PBAT/miscanthus

biocomposites and the processing parameters.

1.3 Thesis organization

This thesis is separated into several chapters. Chapter 1 explains outline of the thesis,

including hypothesis, and objectives of this project.

Chapter 2 reviews the literature on mechanical, thermal and morphological

characteristics of biodegradable polymer blends and their natural fiber reinforced composites

(biocomposites) made from biodegradable polymer blends and natural fibers. Structure-property-

performance of biodegradable polymer blends and their biocomposites fabricated using melt

processing methods is the prime focus of this chapter. In addition to the economical advantages,

natural fibers also offer several other benefits and challenges in fabrication of biocomposite

materials, which are all reviewed in detail in different sections of this chapter. Strategies like

reactive compatibilization have been widely studied to overcome the compatibility issue between

the matrix and fiber phases. This chapter has also attempted to correlate the enhanced

fiber/matrix adhesion with the resulting biocomposite performances.

Chapter 3 discusses the preparation and performance evaluation of biodegradable PBS

and PBAT binary blends. The mechanical properties of the resulting blends were correlated with

5

compatibility of the blended components and infrared spectroscopy. The processability and

phase morphology of the prepared PBS/PBAT blends were investigated by rheological properties

and microscopic analysis, respectively. Furthermore, the performance of the PBS/PBAT blends

including thermal and thermo-mechanical properties were investigated and reported.

Chapter 4 describes experiments carried out in accomplishment of an objective 3. The

melt state graft copolymerization of biodegradable polymers with maleic anhydride (MAH). The

MAH grafting efficiency on the PBS, PBAT and PBS/PBAT blend was investigated while

varying concentrations of the dicumyl peroxide (DCP) free radical initiator. The percentage of

maleic anhydride grafting on the polyesters backbone was quantified based on acid–base titration

method. In addition, the graft copolymerization was confirmed by FTIR spectroscopic analysis.

Differential scanning calorimetry (DSC) and thermogravimertic analysis (TGA) were used to

study the effect of MAH grafting on the polyesters backbone.

Chapter 5 discusses the effects of compatibilizer concentration on the mechanical

performances of the PBS matrix based biocomposites. In order to achieve maximum

performance of the PBS/miscantus composites, the composites were prepared as a function of

compatibilizer concentrations and fiber loadings. Two different compatibilizers, i.e., higher and

lower degree of maleic anhydride grafted PBS (MAH-g-PBS), were used to explore the influence

of compatibilizer on mechanical performance of resulting composites. It was observed that the

composites with 5 wt% compatibilizer exhibited optimum mechanical performances. The load-

bearing capacity/reinforcing effect of the miscanthus fiber composite was analyzed with the help

of theoretical methods. In addition, the degree of interaction between the components was

demonstrated from the height of tan δ peak values as well as SEM analysis. The melt

6

processability of the PBS composites with fiber loading up to 50 wt% was assessed by melt flow

index (MFI) measurement.

Chapter 6 provides manufacture and characterization of biocomposites made from

miscanthus fibers and PBAT matrix. In this chapter, PBAT biocomposites were produced at

various loadings of miscanthus fibers with and without maleic anhydride grafted PBAT (MAH-

g-PBAT) compatibilizer. The effect of fiber loading on melt flow and mechanical properties was

also studied.

In Chapter 7, preparation and performance evaluation of biocomposites consisting of

miscanthus fibers in a biodegradable binary blend matrix were conducted. The mechanical

performance of the composites was investigated with different weight percentage of fiber

loadings. The elastic modulus of the composites was evaluated by parallel, series, Hrisch and

Halpin-Tsai theoretical models and the values were compared with the experimental values.

Maleic anhydride functionalized PBS/PBAT (MAH-g-PBS/PBAT) blend used as a

compatibilizer improved interfacial bonding between the phases in the resulting composites.

Thermal, thermo-mechanical, rheological and physical properties were studied for resulting

compatibilized and uncompatibilized composites. SEM analysis was employed to demonstrate

the interfacial bonding between matrix and fibers in the compatibilized composites.

In Chapter 8, effects of processing parameter on the impact performance of

biodegradable polymer blend matrix based biocomposites were studied. A biocomposite

consisting of miscanthus fibers and a biodegradable PBS/PBAT blend matrix was produced by

extrusion and injection molding method. A full factorial experimental design was used to predict

the statistically significant variables on the impact strength of the PBS/PBAT biocomposites.

The main and interaction effects of the variables were studied using analysis of variance

7

(ANOVA) at 95% confidence level. The accuracy of the developed model was examined by

residuals plots and coefficients. Among the selected independent processing parameters, the

most and least significant processing parameters on the impact strength were fiber length and

holding pressure, respectively.

Chapter 9 investigated the durability of PBS, PBAT and their blend was assessed by

exposure to elevated temperature and humidity. The influence of moisture and temperature on

the mechanical performances was examined as a function of exposure time. The mechanical

properties of the PBS, PBAT and PBS/PBAT blend were heavily affected after being exposed to

90% humidity and 50oC. The change in crystallinity of the exposed samples was correlated with

observed modulus. At last, Chapter 10 provides an overall conclusion and recommendations for

future work in this research.

References

[1] http://www.newenergyfarms.com/news.php. (Accessed on Jaunary 2015)

[2] R. M. Johnson, N. Tucker, S. Barnes, Impact performance of Miscanthus/Novamont Mater-

Bi® biocomposites, Polymer Testing, 2003, 22 (2): 209-215.

[3] D. Carlson, L. Nie, R. Narayan, P. Dubois, Maleation of polylactide (PLA) by reactive

extrusion, Journal of Applied Polymer Science, 1999, 72 (4): 477-485.

[4] L. Petersson, K. Oksman, A. P. Mathew, Using maleic anhydride grafted poly(lactic acid) as

a compatibilizer in poly(lactic acid)/layered-silicate nanocomposites, Journal of Applied Polymer

Science, 2006, 102 (2): 1852-1862.

[5] D. Plackett, Maleated Polylactide as an Interfacial Compatibilizer in Biocomposites, Journal

of Polymers and the Environment, 2004, 12 (3): 131-138.

http://www.newenergyfarms.com/news.php

8

[6] H.-S. Kim, B.-H. Lee, S. Lee, H.-J. Kim, J. Dorgan, Enhanced interfacial adhesion,

mechanical, and thermal properties of natural flour-filled biodegradable polymer bio-composites,

Journal of Thermal Analysis and Calorimetry, 2011, 104 (1): 331-338.

[7] Y. Nabar, J. M. Raquez, P. Dubois, R. Narayan, Production of Starch Foams by Twin-Screw

Extrusion: Effect of Maleated Poly(butylene adipate-co-terephthalate) as a Compatibilizer,

Biomacromolecules, 2005, 6 (2): 807-817.

9

Chapter 2: Studies on Mechanical, Thermal and Morphological Characteristics of

Biocomposites from Biodegradable Polymer Blends and Natural Fibers*

*A version of this chapter has been published in:

R. Muthuraj, M. Misra, A. K. Mohanty, Chapter: 5 Studies on mechanical, thermal, and

morphological characteristics of biocomposites from biodegradable polymer blends and natural

fibers. In: M. Misra, J.K. Pandey, A. Mohanty (Eds.) Biocomposites: Design and Mechanical

Performance, Woodhead Publishing, 2015, pp. 93-140.

2.1 Introduction

Bioplastics can contribute significantly to sustainable development in terms of the

environment and ecological systems. Over the past two decades, the utilization of biobased

and/or biodegradable polymeric materials has gained significant research attention from both

academia and industry. Bioplastics are a group of plastics with wide range of properties and

applications. According to European Bioplastics, a bioplastic can be biobased, biodegradable or

a combination of both biobased and biodegradable. In 2010, the worldwide annual production of

plastics was reported to be 265 Mt [1], reached at 299 Mt in 2013 [2]. Meanwhile, bioplastics

production was estimated to be 1.6 Mt in 2013 [3]. European bioplastics market report has

predicted the annual bioplastic production to be ~6.7 Mt in 2018 [3], which is a 22% hike

compared to bioplastics production in 2012. Currently, bioplastics contribute 10-15% of the

total plastic market share and by 2020 the market share is expected to be around 25-30%, mainly

driven by the gradual increase in the number of bioplastics processing companies [4]. Based on

overwhelming chemical and material demand, the US Department of energy aims to reach 10%

renewable resource based chemical production by 2020 and 50% by 2050 [5]. Main reasons for

biopolymers being on prime focus are reduction of CO2 emissions, reduced waste problems, and

establishment of sustainable alternatives to conventional materials.

10

Petroleum based non-biodegradable polymers can remain intact in landfills for decades,

causing grave environmental concerns. Such non-biodegradable polymers are not suited for

applications requiring short lifespan. Incineration of many non-biodegradable plastics waste after

their end use can produce harmful emissions. The US Environmental Protection Agency (EPA)

has reported that most of the synthetic plastic waste enters landfills, oceans, and lakes [6]. This

problem can be overcome by using biodegradable polymers that can offer effective solution to

this scenario as they are degradable in the presence of naturally occurring microorganisms.

Biodegradable polymers, whether obtained from renewable resources or petroleum resources, are

potential substitutes for some non-biodegradable polymers.

2.2. Biodegradable and compostable polymeric materials

All biodegradable plastics fall under the bioplastic category and for it to be

biodegradable, it should possess the ability to break down into smaller molecules through the

action of naturally occurring microbes [7]. In order to assess the biodegradability of polymeric

substances, standard experimental procedures are established by national and international

organizations. These standards are used to investigate the biodegradability of polymeric

materials in specific environmental conditions. The ASTM D6400 standard defines compostable

polymeric materials as follows: “a plastic that undergoes biological degradation during

composting to yield CO2, H2O, inorganic compounds and biomass at a rate consistent with other

known compostable materials and leaves no visually distinguishable or toxic residues” [7]. This

(ASTM D6400) test method is similar to that given in the ISO 17088. Detailed biodegradability

and compostability mechanisms of polymeric materials have been reviewed by Muniyasamy et

al., [8]. In 1980s, synthetic biodegradable polymers were first introduced and were considered

potential substitutes for non-biodegradable polymers [9]. Currently, there are many

11

biodegradable polymers produced from petrochemical resources and renewable resources [10].

Biodegradability of the polymeric materials generally depends on the molecular formula

and does not depend on the monomer origin [11,12]. Therefore, biodegradable polymers

can be derived from non-renewable resources and/or renewable resources. We can sort

these biodegradable polymers based on the source of the feedstock used to make them

(Table 2.1).

Table 2. 1. Classification and molecular structure of some biodegradable polymers

2.3. Renewable resource based biodegradable polymers: Some examples

Renewable resource based biodegradable polymers have their origin either from plant or

biological resources. On the other hand, non-renewable resources based biodegradable polymers

are generally made from fossil fuel based monomers. Few examples of renewable resource based

biodegradable polymers are discussed below.

12

2.3.1 Poly(lactic acid), PLA

PLA was first synthesized by DuPont scientist Wallace Hume Carothers in 1931 as a

typical linear aliphatic thermoplastic polyester, derived from agricultural feedstock like corn,

sugar cane, and sugar beet [13,14]. PLA is synthesized from lactic acid (2-hydroxypropanoic

acid) monomers through application of heat under vacuum conditions. PLA can be produced by

various methods, like direct condensation polymerization, ring-opening polymerization, and

azeotropic dehydration [15]. Low molecular weight PLA is not acceptable for some of the

applications due to poor mechanical performance. High molecular weight PLA preparation by

direct dehydration condensation is not feasible due to the equilibrium not favoring a high

molecular weight polymer. Therefore, Cargill Dow LLC developed a new technology (ring-

opening polymerization) to produce high molecular weight PLA through an economic route

[16,17]. This process involves three separate steps [15,16]. First step involves a

polycondensation reaction, which converts lactic acid into PLA oligomers (1000-5000 Daltons).

In the second step, PLA oligomers are catalytically converted into a mixture of lactide

stereoisomers. After vacuum distillation of lactide stereoisomers, ring-opening polymerization

(ROP) is typically preferred to produce high molecular weight PLA due to its high degree of

controllability. One of the biggest advantages of the ROP method is the elimination of water

formation step, allowing to attain high molecular weight structures to be attained [18]. This was

the first attempt to synthesize renewable resources based economically and commercially viable

biodegradable polymer (PLA) by melt stage rather than in solution [19]. Stereochemistry of PLA

plays a vital role in the synthesis of stereoisomers such as L (+), D (-), and meso (L, D) lactic

acid by using an appropriate microorganism. Four different types of PLA can be produced using

different kinds of lactide monomers: L-PLA (PLLA), D-PLA (PDLA), mixtures of L and D, and

meso-PLA. The L-PLA and D-PLA are optically active, as opposed to mixtures of L and D, and

13

meso-PLA, which are optically inactive. These steriochemical structures determine the

crystallinity of PLA. For instance, PLA with >93% L-lactic acid is semicrystalline in nature

whereas PLA containing 50-93% L-lactic acid becomes amorphous in nature [15]. Due to

amorphous nature, PDLA does not show crystallization or melting enthalpies under differential

scanning calorimetry tests, while PLLA have melt crystallization, cold crystallization, and

melting enthalpies due to the formation and melting of crystals. Other parameters affecting PLA

properties include polymerization conditions, thermal history, purity, and molecular weight.

In terms of tensile strength, tensile modulus (Table 2.2) [20] and gas (O2 and CO2) barrier

properties [21], PLA is more superior than polypropylene, however, toughness and impact

strength are lower than commodity polymers [16]. PLA can be processed in conventional

processing equipments such as extrusion, injection, stret al.,h blow molding, extrusion blow film,

thermoforming, foaming, melt spinning, cast film and sheet [16]. Therefore, PLA has potential

applications ranging from food packaging, compostable bags, household items, and textiles to

medical supplies. Due to these attractive properties of PLA, the global production of PLA is

expected to reach $5.2 billion by 2020 with growth rate of 19.5% during 2013 to 2020 [22].

Unfortunately, PLA has some inherent drawbacks including low heat deflection temperature

[23], poor melt strength in comparison with polyolefins [24], slow crystallization rate [19], and

too brittle. Moreover, PLA is not able to biodegrade at room temperature i.e., it is not home

compostable [23]. These properties prevent PLA from being applied in varied applications.

Several attempts have been made to address these issues using plasticization, rubber toughening,

blending with other polymers, and reinforcement with natural fibers [25-27]. Recently, the

toughens modification of PLA has been reviewed exhaustively by Kfoury et al., [26]

14

2.3.2 Microbial polyesters-Polyhydroxyalkanoates (PHAs)

Microbial polyester synthesis began in the year 1901 and detailed investigations were

conducted in 1925 [13]. In the late 1920s, Maurice Lemoigne first discovered

polyhydroxyalkanoates (PHAs) using the Bacillus megaterium bacteria [28]. Two different types

of bacterial polyesters along with their chemical structure are shown in Table 2.1. PHAs are a

family of linear thermoplastic polyesters, which can degrade by the microorganisms [29]. PHAs

can be found as homopolymers or copolymers. The performance of PHAs is dependent on their

chemical composition. After the 1970s, researcher’s curiosity and an impending energy crisis

initiated the commercialization of poly(3-hydroxybutyrate), PHB. PHB is the first microbial

polyester synthesized through fermentation of sugar and starch with the help of a microorganism.

PHB is a saturated linear homopolymer, which behaves like traditional thermoplastics. It can be

processed in traditional processing equipments such as extrusion, injection and compression

molding. PHB has properties that are very similar to those of a commodity polymer like

polypropylene (PP) [30,31]. However, poor thermal stability [29,32], embrittlement due to high

crystallinity [13,32] and narrow processing window [33] of PHB hinder its wide range of

applications. The brittleness/impact strength of PHB can be modified by copolymerization of

hydroxybutyrate (HB) and hydroxyvalerate (HV) [13] or blending with tough polymers [31]. For

instance, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is more ductile and flexible

compared to PHB [32]. In 1990, poly(3-hydroxybutyrate-co-3-valerate) (PHBV), was introduced

into the market by Imperial Chemical Industries Inc. (ICI) [13]. Currently, the major PHAs

producers are Tianan (EnmatTM

), Kaneka (NodaxTM

), PHB Industrial (Biocycle®

), Mitsubishi

Gas Chemicals (BiogreenTM

), Biomer® biopolyester, and Metabolix (Mirel

®). PHAs have

desirable properties like higher melting point (Table 2.2) and higher crystallinity, comparable to

commodity polymer (PP), with added advantageous like good biodegradability, compostability

15

and biocompatibility. PHAs have potential applications in medical fields, like wound

management, vascular system, drug delivery, orthopedic, and urology [34]. However, the

thermal degradation temperature range of PHBV is 180-200 oC, which is very close to their

melting temperature. In addition, the main drawback of PHB and low valerate content PHBV is

their poor impact strength and low elongation at break caused by low nucleation density. All the

above factors combined with high cost hinder the application of PHBV in a wide range of fields

[35]. Increasing the valerate unit up to 34 mol% can help in decreasing the brittleness of PHBV

[30]. However, increase in the HV content can decrease the melting temperature, tensile strength,

modulus of elasticity and glass transition temperature [30]. Most of the commercially produced

PHBV contains HV content below 15 mol% [32]. The melting temperature (Tm) of such PHBV

is still high (e.g. HV = 11 mol%, Tm = 157 °C) and thermal degradation may occur during melt

processing [30]. In addition, it possesses very low ductility, very similar to PHB. The synthesis

of PHBV with higher valerate content requires a lot of energy consumption for production,

sterilization, and purification. Therefore, intensive research needs to be directed towards

reducing the production cost of PHBV, and in bringing the cost to be on par with traditional

plastics like PP [36]. Extensive research has been conducted on polymers from PHA family to

overcome the above discussed drawbacks [30,32,33,37]. Different approaches adopted to address

the challenges of PHAs are discussed in the following blend and composites sections. In addition

to PHBV based polymer blends, natural fibers are potential candidate to reduce the cost of

PHBV based materials while enhancing some of their mechanical properties [27].

16

Table 2. 2. Properties of some biodegradable polymers in comparison to non-biodegradable polymers

Polymers

Density

(g/cm3)

Melting point

(oC)

Tensile strength

(MPa)

Tensile modulus

(MPa)

Elongation at

break (%)

References

PLA 1.21-1.25 150-162 21-60 350-3500 2.5-6 [36, 38]

PLLA 1.24-1.30 170-200 15.5-150 2700-4140 3-10 [36, 38]

PDLA 1.25-1.27 Amorphous 27.6-50 1000-3450 2-10 [38]

PHB 1.18-1.26 168-182 40 3500-4000 5-8 [36, 38]

PHBV 1.23-1.25 144-172 20-25 500-1500 17.5-25 [36, 39]

PBS 1.26-1.32 96-114 19-36 324-647 200-807 [40]

PCL 1.11-1.15 58-65 21-42 210-440 300-1000 [38, 41]

PBAT 1.25 110-115 36 80 820 [39, 42]

HDPE 0.92 110 10 177 700 [40-41]

PP 0.9-1.16 161-170 30-40 1100-1600 20-400 [36]

17

2.4 Fossil fuel based biodegradable polymers: Some examples

Biodegradability of a polymer is basically based on chemical structure unlike from the

source from which it is obtained. Thus, biodegradable polymers besides renewable based sources

(as discussed above) can also be made from fossil fuel sources. Few examples of such polymer

are reviewed below.

2.4.1 Poly(butylene succinate), PBS

PBS is a promising aliphatic biodegradable thermoplastic polyester (chemical structure is

shown in Table 2.1). It is synthesized by condensation polymerization of 1,4-butanediol (BDO)

and succinic acid in the presence of a catalyst [40]. Low molecular weight PBS is weak and

brittle in nature. Therefore, Showa Highpolymer Co. Ltd in Japan, produced relatively high

molecular weight PBS under the trade name BionolleTM

[40]. High molecular weight PBS was

achieved through melt condensation polymerization followed by coupling reaction with a chain

extender (e.g. hexamethylene diisocynate). The condensation polymerization can be performed

using either direct melt polymerization or solution polymerization. Direct melt polycondensation

is the most promising method for PBS synthesis as it can produce the high molecular weight PBS

suitable for food contact applications [43]. Currently some of the companies producing PBS

include Xinfu pharmaceutical (China), Hexing (China), Ire Chemical (Korea). Nippon Shokubai

(Japan), Mitsubishi Chemical (Japan), Showa Highpolymer (Japan), and Jinfa Tech (China)

[44,45].

Interestingly, PBS can also be produced from either biobased, fossil fuel based monomers

or a combination of both bio and fossil fuel based monomers. There are several companies

around the world producing pilot scale biobased succinic acid, including BioAmber and Myriant.

Mitsubishi Chemical has developed a new technology to produce partially biobased PBS having

18

54% biobased content derived from biobased succinic acid [46]. It can be used in applications

requiring food contact and applications besides non-food contact uses while meeting global

biodegradability standards. Traditionally, 1,4-BDO is produced from fossil fuel resources.

Alternatively, the biobased 1,4-BDO is produced through a fermentation process using glucose

monomers. Therefore, there is a possibility to produce PBS from 100% renewable resource

based monomers [47]. PBS is a biodegradable polyester with balanced thermo-mechanical

properties compared to other polymers such as polypropylene, polyethylene, PHB and PLA [48].

Mechanical and thermal properties of PBS depends mainly on crystallinity, crystal structure and

molecular weight [49]. Good thermal stability, processing properties, semi crystalline nature and

an ideal melting point range, makes PBS a promising biodegradable polymer [50-52]. Notably,

the strength of PBS is comparable to those of low-density polyethylene (LDPE) and

polypropylene (PP), as shown in Table 2.2. PBS based products are widely used for many

applications, such as packaging film, agriculture mulch film, sheets, laminates, bags, hygiene

products, non-woven, split yarn, monofilament and multifilament [53]. However, the stiffness,

water vapor barrier, melt viscosity and impact strength of PBS are often not sufficient for many

applications. These drawbacks can be effectively addressed by blending with other polymers

[54], fillers [55,56] and additives [51]. The stiffness of the PBS has been improved by the

addition of natural fibers [55] and blending with brittle polymers like PHB [57]. The melt

viscosity and gas barrier properties of the PBS were improved with the help of reactive agents

[51] and nanofillers [56], respectively.

Poly(butylene succinate-co-adipate), PBSA, is a random copolyester, made from 1,4-

butanediol, succinic acid, and adipic acid by polycondensation reaction. It has excellent impact

strength, elongation and processability. Co-monomer content dictates the physical properties of

19

the copolymer of PBS like PBSA. For instance, the adipate content must be lower than 15 mol%

in order to maintain the melting point of PBSA above 100 oC [58]. In addition, adipate co-

monomer content has a greater influence on the biodegradability and mechanical properties of

PBSA [40]. PBSA has higher impact strength and elongation as compared to PBS, whereas the

tensile strength and melting point of the PBSA are lower than that of PBS.

2.4.2 Poly(butylene adipate-co-terephthalate), PBAT

Polymers that are solely aromatic polyesters such as poly(ethylene terephthalate) (PET)

and poly(butylene terephthalate) (PBT) are resistant to microbial attack [59]. However,

copolymerization of aliphatic monomers with aromatic monomers results in a biodegradable

polymer, like PBAT. PBAT, a biodegradable aliphatic-aromatic copolyester, is derived from

fossil fuel based adipic acid, terephthalic acid, and butane diol by a polycondensation reaction

[60]. The chemical structure of PBAT is shown in Table 2.1. Due to its mixed chemical

structure, PBAT can be enzymatically hydrolyzed in the presence of microorganisms followed

by mineralization like aliphatic polyesters [11]. The concentration of aromatic functionality plays

a vital role in the performance of the PBAT. PBAT shows higher thermal and mechanical

properties with a terephthalic acid concentration above 35 mol% but good biodegradability was

achieved with an aromatic moiety concentration lower than 55 mol% [33]. Therefore, PBAT

with a range of about 35 to 55 mol% of terephthalic acid can offer an optimal performance [61].

In 1998, PBAT with 22.2 mol% of aromatic moiety was commercialized under the trade

name, Ecoflex®, by BASF [11]. BASF studied the biodegradability of PBAT under composting

conditions. More than 90% of PBAT was observed to be metabolized within three months [62].

Witt and coworkers [11] studied the biodegradability of Ecoflex® under composting conditions

according to DIN-V-54900 standard and concluded that there is no environmental risk after

20

composting Ecoflex®. Mechanical properties of PBAT are comparable to polyethylene (Table

2.2) [63], but with water vapor permeability of PBAT (240 gm-2

d-1

) being higher than LDPE (3

gm-2

d-1

) [62]. In order to overcome this shortcoming, BASF is producing a wide range of

biodegradable master-batched Ecoflex® (mixed with wax, talc, carbon black, chalk or silica et

al.,) to fulfill the requirements set for different applications. Details of the masterbatches are

shown in Table 2.3 [64]. Among them, wax based masterbatch film has potential to reduce the

water vapor permeability up to 75% compared to virgin PBAT [62]. PBAT possesses excellent

toughness, biodegradability and processability. Therefore, PBAT is widely used for compostable

organic waste bags, agricultural mulch films, as well as lamination/coatings for starch-based

products [65]. Moreover, it can be used to tailor the properties of starch, PLA, and PHAs,

thereby opening up new applications for PBAT based materials. Currently, there are few

companies producing PBAT, including BASF (Germany), Ire Chemical (Korea), Shandong

Fuwin New Material (China), and Xinfu Pharmaceutical (China).

Table 2. 3. Ecoflex® based masterbatches for different applications [66,67]

Masterbatches Filler/additive

types

Filler

content

Proposed applications

Ecoflex® Batch

AB1

Chalk 60% Packaging films, compost bags and

agricultural films

Ecoflex® Batch

SL 05

Erucamide ESA

(lubricant)

5% Packaging films, compost bags and

agricultural films

Ecoflex® Batch C

Black

Carbon black 35% Packaging films, compost bags and

agricultural films

Ecoflex® Batch C

White

Titanium

dioxide (TiO2)

60% Packaging films, compost bags and

agricultural films

Ecoflex® Batch

SL 2

Lubricant (Wax) 5% Packaging films, compost bags and

agricultural films

21

2.4.3 Poly(caprolactone), PCL

PCL is a linear aliphatic biodegradable polyester derived from caprolactone by ring-

opening polymerization. PCL can also be produced by anionic, cationic, coordination, radical

and enzymatic catalyzed polymerization [68,69]. PCL is a typical semi-crystalline synthetic

thermoplastic with crystallinity ~50%, melting temperature of 58-65 oC (depending on the

crystallinity), and glass transition temperature of -60 oC [38]. PCL has been most widely used in

drug delivery, tissue engineering, and dentistry as it is approved by the US Food and Drug

Administration (FDA). PCL is commercially available under different trade names: Capa™

(Perstorp, UK), PlaccelTM

(Daicel Chemical Indus, Japan) and Tone® (Dow, USA). A well-

known PCL/starch biodegradable blend is commercially available in the market under the brand

name Mater-Bi®

Z grades, and is produced by Novamount, Italy [70]. This blend is mainly used

for biodegradable and/or compostable films and sheets. Inherent toughness and biocompatibility

of PCL are desired attributes that makes it an ideal choice of blending partner. In this view, PCL

has been blended with PLA, PHAs and PBS for different biomedical applications [71-75].

2.5. Recyclability of biodegradable polymers

Recyclability of plastics is a process that extends their service lives before discarding.

Plastic waste can be recycled in two different ways: mechanical and chemical recycling.

Mechanical recycling involves a polymer being re-melted and re-processed into desired products

through different processing techniques [76]. Chemical recycling is a process (depolymerization)

that converts polymers into their monomers, which are then used as a feedstock for further

polymerization process [9]. Among these recycling methods, mechanical recycling is most

favored method for bioplastics recycling. Many studies have been investigated on the mechanical

recyclability of biodegradable polymers, like PLA [77-80], PCL [81], PHBV [80, 82],

PLA/PHBV blend [80] and PBS [83]. Multiple extrusion and injection molding of the PLA

22

(3002D NatureWorks®, USA) has been studied by Żenkiewicz [77]. After re-extruding PLA for

10 times, the tensile strength, tensile strain and impact strength were reduced by 5.2, 2.4, and

20.2%, respectively. On the other hand, melt flow rate, oxygen and water vapor permeability

rates were significantly increased. After the 10-time extrusion cycle of PLA, the oxygen and

water vapor permeability rates were increased by 39 and 18%, respectively. Thermal properties,

such as thermal stability, crystallization, and melting point were slightly reduced while there was

no change in glass transition temperature. Another study investigated the recyclability of PLLA

(L9000, Biomer, Germany) up to seven-injection molding [78]. The authors found that the

tensile modulus was not heavily affected, whereas rheological properties, molecular weight,

hardness, tensile strength and strain at break were reduced considerably. A recent study also

proved that eight times mechanical recycling of PLLA (L9000, Biomer, Germany) leads to

considerable reduction in its mechanical properties as well as thermal stability [79]. Meanwhile,

the melt flow index (MFI) of the eight times recycled PLLA showed 15 times higher value than

unprocessed PLLA. The reduced mechanical properties were attributed to thermo-mechanical

degradation of PLLA during melt re-processing. Consequently, the molecular weight, thermal

stability and viscosity of the PLLA decreased. Such reduction in properties can be overcome by

using thermal stabilizers such as quinine and tropolone [78]. However, the study concluded that

PLLA (L9000, Biomer) can be recycled up to five times without a significant change in the

tensile properties and flexural modulus [79]. Zaverl and coworkers [82] investigated thermal and

mechanical properties of PHBV being recycled for seven times. Mechanical properties such as

impact, tensile, and flexural properties were not affected after recycling up to four times;

however, there was slight decrease after a fourth cycle. According to this study [82], there was

no change observed in the molecular weight of PHBV after two cycles. A considerable

23

molecular weight reduction was observed after the third (8.7%), fourth (13.5%) and fifth cycles

(16.6%). Even after fifth round of recycling, melting temperature and thermal stability of the

PHBV were not affected significantly. The chemical structure of the PHBV was not changed

during melt reprocessing, which was confirmed by Fourier transform infrared (FTIR)

spectroscopy. On the contrary, PHBV is more susceptible to thermo-mechanical degradation

compared to PLA [80]. Therefore, blending of PHBV with PLA can minimize the degradation of

PHBV during recycling. Recyclability of PCL has been investigated by Moraczewski [81]. After

eight cycles, there were no changes observed in tensile strength and elongation at break.

However, MFI, thermal stability, and Charpy impact strength of PCL were dependent on the

number of cycles. This study suggests that the recycled PCL can be blended with virgin PCL.

According to Kanemura et al., [83], PBS (Bionolle® 1020) can be recycled without affecting its

bending strength and modulus when reprocessed up to three times at 140°C using a compression

molding method. In addition, the molecular weight of the PBS was not heavily affected after

three times of reprocessing at 140oC. Therefore, this study concluded that PBS is a suitable

candidate for material recycling.

2.6. Durability of biodegradable polymers

Generally, durability is very important for polymers as it increases their suitability for a

wide range of applications. The durability of polymeric materials can be investigated by

exposing to simulated environmental variables such as temperature, and humidity. Only a few

studies have examined the durability of biodegradable polymeric materials [84-88]. These

literature sources report that biodegradable polymers are very sensitive to elevated temperature

and humidity. Recently, Harries and Lee [86,87] assessed the durability of PLA at elevated

temperature and humidity with respect to time. Their experimental findings suggest that after 28

24

days of exposure at 70 oC temperature with 90% relative humidity, PLA materials experienced

severe loss in mechanical properties. Therefore, commercial PLA cannot fulfill long-term

durability requirements set by the automotive industry. This drawback can be addressed either by

reducing the hydrolysis reaction through addition of scavengers/sacrificial compounds or by

blending/alloying with other durable polymers [86]. The degradation of PHBV has been

examined in distilled water and marine environments at different temperatures [89,90]. The

mechanical property performance of the PHBV gradually deteriorated with increasing immersion

time. This effect is attributed to the hydrolytic degradation of ester bonds in the PHBV

backbone. Usually, the durability of the polyesters depends on the chemical structure. Among

PLA, PBS, PBAT, and PBSA, PLA and PBAT are more durable compared to PBS and PBSA

[84]. However, the durability of PBS and its composites has been noticed to improve with the

addition of trimethylolpropane triacrylate (TMPTA) as an anti-hydrolysis agent [84]. The

durability of PBS, PBAT and their blend has been investigated by exposure to 50 oC and 90%

relative humidity for a duration of up to 30 days [91]. This study concluded that the

biodegradable polymers (PBS, PBAT and PBS/PBAT) could readily undergo hydrolytic

degradation in the presence of elevated temperature and humidity. The durability of

biodegradable polymers need to be improved under high humidity and temperature for

expanding their applications.

2.7. Polymer blends: Some examples

Neat polymers for commercial applications usually cannot fulfill all product

requirements. One of the alternatives for modifying the properties of polymers are to blend with

other polymers (one or more), which have complementary properties. Blending two or more

structurally different polymers might improve or tailor some of the properties and its cost

25

performance, making a blend that is able to meet specific end-use requirements [26].

Furthermore, polymer blending can be performed in standard processing equipment such as

twin-screw extruders. Generally, polymer blends possess unique physico-mechanical properties,

which are usually not present in their individual components. Polymer blends have a wide range

of applications in fields such as automotive, household items, electrical and electronics. Some of

the commercially available biodegradable polymer blends are listed in Table 2.4. A blend of

polylactic acid/lignin/fatty acid/wax shows mechanical properties similar to acrylonitrile

butadiene styrene (ABS). It has strong thermal insulation properties as well as excellent chemical

resistance against polar substances [92]. The properties of STARCLA™

(20 µm film) are

comparable to polyethylene [93]. BASF is producing biodegradable PBAT/PLA (55/45 wt%)

blend under the trade name of Ecovio®. The mechanical performance of Ecovio

® is similar to the

properties of high-density polyethylene [62].

Table 2. 4. List of companies engaged in the production of some biodegradable polymer blends

(the table was modified after referene [9, 94,95]).

Blends Applications Trade name Manufacturer

PLA/PBAT Blown films Ecovio®

BASF, Germany

PLA/copolymer Injection molding:

housewares, tackle

Cereplast

sustainable®

Cereplast Inc., USA

PCL based blend Films and injection molded

items

Mater-Bi®

Novamount, Italy

PLA/co-polyester

blend

Blown film and co-

extrusion

Bio-Flex® FKuR, Germany

PLA based blend Extrusion and blown film StarclaTM

Showa Denko

Europe

Biodegradable co-

polyester blend

Blown film and injection

molded parts

TerraloyTM

Teknor Apex

PLA/PBS or PBSA Food service ware IngeoTM

AW

240D

NatureWorks, USA

PBAT/PHA Fiber and non-woven BioTufTM

970 Heritage Plastics Inc.

USA

26

As discussed previously, high cost and inferior mechanical properties relative to several

commodity plastics are the downside of most of the biodegradable polymers and therefore wide

scale adaptability of these polymers have been significantly hampered. In order to effectively

overcome these drawbacks, melt blending is a fast, economical and a convenient approach

compared to developing a novel polymeric material through synthetic polymerization technique

[96]. For example, slow crystallization behavior and poor impact resistance of PLA hinders its

wide range of industrial applications. Researchers have found that crystallization and cold

crystallization were enhanced by incorporating PBS into PLA systems [97]. Yokohar and

Yamaguchi [98] have conducted studies on the structure-properties correlation of PLA/PBS

blend. They found that the cold-crystallization temperature of PLA was reduced in the blend

compared to neat PLA. This result indicated that PBS acted as a nucleating agent in the PLA

matrix. Blends of biodegradable polymers are being used for short-term applications, such as

absorbable medical implantations, compostable food packaging containers, and agricultural

mulch films. Kim and coworkers [85, 88] have reported the durability of talc filled biodegradable

PBS/PBAT (55/45 wt%) blends under marine environments. Better elastic properties were

observed in biodegradable materials compared to that of the recycled PE.

2.7.1 Miscible biodegradable polymer blends

At thermodynamic equilibrium, mixtures of polymers that exist in a single phase at the

molecular scale are referred to as miscible (soluble) blends [99]. Miscibility of polymers in the

blend system can be detected through glass transition temperature (Tg) and phase morphology.

Miscible polymer blends show single glass transition temperature. In most cases, the miscibility

of two polymers depends on the molecular weight, chemical structure of the polymer, and

crystallinity [100]. In the polymer blend system, some of the properties are determined by

27

miscibility, interaction and phase morphology. For example, the mechanical properties of the

miscible blends follow the rule of mixture [101]. A blend is considered partially miscible if there

exists phase separation but each polymer rich phase contains a sufficient amount of the other

polymer to alter the properties of that phase (e.g., the glass transition temperature) [101].

Normally, polymers are miscible with each other up to a certain limit. The Flory-Huggins theory

is most frequently used to calculate mutual solubility of polymer blends using an interaction

parameter. The Flory-Huggins interaction parameter (χ) can be obtained by the following

equation 1.1 [102]:

=

( ) (1.1)

The term Vr is reference volume, T is absolute temperature, δ1 and δ2 are the solubility

parameters of the components and R is the gas constant. The solubility parameters can be

calculated using group contribution methods [103]. If the interaction parameter (χ) value is less

than 0.5, it can be considered as miscible blend [104]. The Flory-Huggins theory always yields

negative interaction parameter values for miscible blends. For instance, a PHB/

poly(epichlorohydrin-co-ethylene oxide), PEEO blend has an interaction parameter value of -

0.089 [105] while a PLA/PBS blend has an interaction parameter value of -0.15 [106]. Another

study showed limited miscibility of PHB with 20% PBS blend [107]. This miscibility was

evidenced by shift in glass transition temperature and depression of the PHB melting

temperature. Bhatia et al., [108] found that a PLA/PBS blend system has partial miscibility when

the PBS composition was lower than 20 wt%. This result has been confirmed by rheological

properties. However, tensile strength and modulus of the PLA/PBS blends are lower compared to

neat PLA. The brittleness of the PLA is reduced with the addition of PBS, thus making it a

28

suitable material for packaging applications [108]. Park and Im [106] studied blends of two

typical biopolyesters (PLA and PBS) by melt blending through an extruder. In this polymer

blend system, a single glass transition temperature was observed, which was attributed to

enhanced miscibility, between the polymer phases. Spherulites morphology of the blends was

observed through optical polarizing microscopy and they concluded that crystallization induced

phase separation occurred when the PBS volume increased by more than 40 wt% in the blend

system. Moreover, PLA/poly(ethylene glycol) [109-110], PLA/poly(ethylene oxide) [104, 111],

and PHB/poly(ethylene oxide) [112] blends were able to form miscible blend system.

2.7.2 Immiscible biodegradable polymer blends

If mixtures of two polymers exhibit separate phase morphology (i.e., matrix-droplet and

co-continuous morphology), they are referred to as immiscible blends [113], as shown in Figure

2.1a. In the binary blends, the matrix-droplet phase morphology (Figure. 2.1b) changes to co-

continuous morphology (Figure. 2.1c) with an increase in the volume fraction of the dispersed

phase. Immiscible polymer blends exhibits different glass transition temperatures characteristic

of their pure blend components and core-shell morphology. The core-shell morphology of the

immiscible blends depends on the viscosity of the individual blend components, processing

aspects, composition, and compatibility between the components. Inherently some

biodegradable polymers (PBS, PBSA, PBAT, certain PHAs and PCL) have good toughness and

elongation. Therefore, they are considered as a promising candidate for modifying brittle

biodegradable polymers while preserving the biodegradability. However, a large amount of

literature shows that most of the polymer blends are immiscible due to poor dispersion of

the inclusion phase in the continuous phase, weak interfacial adhesion, and instability

between the components [26]. For instance, biodegradable polymer blends of PHBV/PCL

29

[114], PHBV/PBAT [115], PHBV/PBS [116], PHBV/PLA [117], PLA/PBS [98], PLA/PBAT

[118,119], PLLA/PCL [120], PLA/PHB [121], PCL/PBS [122], and PBS/PBAT [54] are all

immiscible blends. In order to improve the interaction between the pure polymers in the

blend system, compatibilization has proven to be an effective strategy for the immiscible

polymer blend systems.

Jiang et al., [119] studied the toughening of PLA through melt blending with

PBAT. Dynamic mechanical analysis (DMA) results showed two independent glass

transition temperature (Tg) peaks corresponding to their individual components, suggesting

that the PLA/PBAT blends were immiscible. Mechanical properties of the PLA blends

showed the greatest improvement in elongation and toughness with incorporation of

PBAT from 5 to 20 wt%. These improvements were correlated with de-bonding-induced

by shear yield and weak interfacial adhesion between the components in the blends. Wu et

al., [123] studied the PBS/PLA blend morphology, rheological properties, and interfacial

tension between the polymers. They observed that PBS/PLA blends were

thermodynamically not miscible and possessed co-continuous phase morphology in the

50/50 wt% blend. DSC analysis reveled that PBS has no significant effect on melting point

and crystallization behavior of PLA, but on the other hand, PLA had little effect on

melting point and cold crystallization of PBS.

30

Figure 2.1. (a) Schematic representation of the evolution of morphology in a binary

immiscible blend, (b) matrix/dispersed morphology, and (c) co-continuous morphology

(adapted with kind permission from Ravati and Favis, Polymer, 2010, 51: 4547-4561,

Copyright©

2015, Elesevier, Licence number 3591550612924 [113]).

Furthermore, Zhou et al., [124] studied PBS/PLA blends for biomedical

applications. They also made similar observations i.e., PBS/PLA blends were immiscible

and their mechanical properties followed the rule of mixtures. The hydrolysis behavior of

PBS, PLA and their blends was investigated under simulated body fluid for 16 months at

37 oC. The PBS/PLA blends lost their tensile properties earlier than the neat polymers due

to faster hydrolysis reaction facilitated by the interface present between the blend

components. Throughout the hydrolysis time, the molecular weight reduction of the blends

was faster than that of parent polymers. A blend of PHBV/PCL has been studied by

Jenkins et al., [125] and they suggested that the blend is not miscible.Melt-blended immiscible

PHBV/PLA blend was studied by Nanda et al., [117]. The mechanical performances of the

blends were dependent on processing parameters and composition of the constituents. The

31

elongation at break observed in PHBV/PLA blend [117] increased slightly compared to neat

PLA and PHBV. Similar trends were observed in compostable PLA/PHBV blend by Ma et al.,

[126]. It was demonstrated that matrix yielding and interfacial debonding were responsible for

the increased toughness in PLA/PHBV blends. According to Qiu et al., [116], a PHBV/PBS

blend is not miscible due to the independent Tg of two phases.

Some of the immiscible blends are considered compatible blends when they show fine

phase morphology and satisfactory performance in their mechanical properties [96]. These types

of blends show their characteristic Tg but most often the Tg values are shifted towards other

components in the blends indicating improved compatibility. The performance of the compatible,

immiscible polymer blends depends on many factors including their interaction, component

properties, composition, and compatibility between the polymers phases. Among all the

parameters, interaction is a factor of greater importance because it determines the thickness of

the interface between the components. John et al., [122] have investigated the compatibility and

miscibility of PBS, PBAT, and PCL blends. From DSC analysis, the blends of PCL/PBAT and

PCL/PBS are seen to have two melting points, which correspond to their individual melting

points. This result clearly indicates that these blends are not completely miscible. However, from

DMA analysis, a single Tg was observed for the PBS/PBAT blend. This is because the Tg

values of both the neat PBS and PBAT are very close to each other. Moreover, these blends

showed an interesting behavior when tested for tensile strength. The tensile strength of

PBS/PBAT (70/30 wt%) and PCL/PBS (30/70 wt%) blends was higher than that of neat PBS,

PBAT and PCL. This improvement was attributed to the interaction between the blended

components as well as a change in crystallinity. Recently, Muthuraj et al., [54] have reported a

similar type of synergistic effect in the PBS/PBAT blends.

32

2.7.3 Compatibilization of polymer blends

Compatibilization is an interfacial phenomenon in the heterogeneous polymer blends.

This interfacial activity can be modified by reactive and non-reactive processing strategies.

Compatibilization is a process by which the blend properties are enhanced while increasing

adhesion between the phases, reducing the interfacial tension, and stabilizing morphology [127].

To achieve these goals, there are several strategies to the method of compatibilization. The

degree of compatibility between the components in the blends can be enhanced through the

addition of compatibilizers [60-62]. The resulting mechanical properties of the immiscible,

compatible blend will have a balance of their parent polymer properties or show a synergistic

improvement [99]. Many commercially available polymer blends on the market are mostly

compatibilized using a compatibilizer [1]. Polymers with polar groups can interact between the

components in the blend through non-bonded interactions such as dipole-dipole interactions and

hydrogen bonding [1]. Therefore, biopolymers have more mutual miscibility than polyolefins.

Compatibilization of polymer blends was reviewed in detail by Koning et al., [96], and Imre and

Pukanszky [1]. According to these sources, the process of adding a premade compatibilizer

(block-copolymers) that has a strong affinity towards the blend components is referred to as

physical compatibilization. Introducing a compatibilizer that can promote chemical reaction

and/or specific interaction between the blend components is referred to as reactive or in-situ

compatibilization. Varieties of reactive compatibilizers have been identified in the earlier

publications [128-136]. Next section of this chapter discusses these compatibilization methods.

Generally, premade graft or block copolymers are mostly used to modify the

heterogeneous polymer blends interface by a non-reactive compatibilization method. In order to

make effective compatibilization of the blends by block-copolymers, the copolymer must have a

33

maximum solubility with components in the blend. This leads to strong interfacial adhesion

while lowering the interfacial tension between the components. The molecular weight and

concentration of compatibilizer (block copolymer) should be slightly higher than that of critical

entanglement and critical micelle concentration, respectively [137]. In addition, compatibilized

blends exhibit dispersed particle size at a sub-micron level which prevents coalescence during

subsequent processing [96]. Non-reactive biopolymer blends with block-copolymers have been

widely studied in the literature [138-140]. However, non-reactive compatibilization is less

efficient than reactive compatibilization [96]. Reactive extrusion generates reactive sites, which

interact with the other polymer component and localize themselves at interface between the

components in the blends. Such localization at an interface enhances the mechanical properties

and decreases the domain size of the inclusion phase. Reactive extrusion of the polymer blends

can be performed by one step (in-situ) or two-step extrusion. All of the components are

introduced simultaneously in the one-step reactive extrusion [141]. In the two-step reactive

extrusion, the first step involves functionalization of polymer with reactive agents followed by

the second step in which the functionalized polymers are blended with other components through

extrusion [142].

2.7.4 Non-reactive compatibilization of biodegradable polymer blends

Historically, immiscible polymer blends can be compatibilized through the addition of a

graft or block copolymers [127]. The block-copolymer acts as a compatibilizer between the

components in the immiscible polymer blend systems. As a consequence, the block copolymer

can reduce the interfacial tension between the components [142], as shown in Figure 2.2.

34

Figure 2.2. Adding a compatibilizing agent, such as a diblock copolymer, to a polymer blend

can improve its stability, but is more likely to result in a dispersed morphology rather than a co-

continuous morphology. a) A two-dimensional slice of a compatibilized blend with dispersed

phase morphology, represented by minority dark blue phase and a majority turquoise phase. b) A

molecular schematic showing how the diblock copolymers are segregated at the interface

between the two phases (adapted with kind permission from Ryan, Nature Materials, 2002, 1: 8-

10. Copyright©

2015, nature publishing group, License number 3557770952943 [142]).

There are many commodity polymer blends compatibilized with a block-copolymer. For

example, styrene acrylonitrile, SAN, and styrene butadiene styrene, SBS blends, have been

compatibilized by the addition of a diblock-copolymer [143]. These compatibilized blends

exhibit good mechanical properties compared to uncompatibilized blends. A similar type of

compatibilization effect has been reported for polystyrene/low and high density PE blends via

addition of butadiene-styrene block-copolymers [144]. In this blend system, the compatibility

between the components has been found to be dependent on processing parameters. Recently,

similar attempts have been made for bioplastic blends [120, 138, 140, 145-149]. LDPE-PLA

block copolymers have been used as a compatibilizer in the PLA/LDPE blends in order to

improve the properties of resulting blends [149]. These blends showed considerable increase in

deformability with tensile strength lower than that of neat PLA. Another study also showed a

remarkable increase in ductility with the addition of a block-copolymer, PHB-block-poly(methyl

methacrylate), PMMA in the PHB/PMMA blend [148]. PLLA/PCL [120] and PBS/PCL [150]

35

blends were compatibilized with triblock copolymer, (polyethylene oxide–polypropylene oxide–

polyethylene oxide). These two studies concluded that the blends with compatibilizer achieved

better compatibility and toughness compared to those of the uncompatibilized blends. The

enhanced compatibility between the components was further confirmed by a Tg shift and

decrease in PCL domain size in the matrix.

2.7.5 Reactive compatibilization of biodegradable polymer blends: Few specific examples

Poor compatibility between the phases is a major drawback in multi phase immiscible

polymer blends. Immiscible polymer blends can be compatibilized by introducing a

compatibilizer. Extensive work has been conducted on improving the compatibility between the

phases in heterogeneous biodegradable polymer blend systems.

2.7.5.1 Reactive compatibilization of PLA/PBAT blends: Recently, significant research has

been directed towards producing compatible PLA/PBAT blend through reactive processing. For

instance, Sirisinha and Somboon [128] investigated the reactive PLA/PBAT blends by adding 0

to 0.5 wt% of dicumyl peroxide (DCP) for blown-film processing. This study was completed

using fixed PLA/PBAT ratio (70/30 wt%) and varied the DCP loading from 0 to 0.5 wt%.

Rheological, melt flow index, and melt strength properties were measured to investigate the

synergistic effect of DCP addition into the blend system. The addition of DCP into a PLA/PBAT

blend was found to increase the elasticity and viscosity. In addition, good melt strength of the

blend allowed for easy processing when blowing the plastic into a film.

According to the literature [129], solubility parameter values of PLA and PBAT are 10.1

and 22.95 (cal/cm3)1/2

, respectively. The difference in solubility parameter leads to the formation

of immiscible blends as well as a reduction in their mechanical performance. These drawbacks

are addressed by using glycidyl methacrylate (GMA) as a reactive compatibilizer in melt blended

36

PLA/PBAT. Kumar and coworkers [129] studied the effect of mechanical, thermal as well as

morphological properties of PLA/PBAT blends with GMA. They incorporated GMA (3 and 5

wt%) into the optimum PLA/PBAT blend system in order to improve compatibility between the

components. They found that the impact strength of the PLA/PBAT/GMA (70/25/5) blends

increased (72%) as compared to virgin PLA. The increment in properties was attributed to the

formation of ethylene acrylic ester at the interface with the addition of GMA. Morphological

examinations confirmed that compatibility between the PLA and PBAT phases was in fact

improved in the presence of GMA. Thermo-mechanical analysis (DMA) further corroborated the

improvement noticed in compatibility between the PLA and PBAT and heterogeneous phase

morphology of the PLA/PBAT blends Furthermore, with the addition of GMA within

PLA/PBAT blends, reduction in crystallinity and inward shift in melting point were observed by

DSC analysis, supporting the enhanced compatibility between the two polymers.

Zhang et al., [130] investigated reactive blending of PLA/PBAT with a reactive

processing agent, a random terpolymer of ethylene, acrylic ester and glycidyl methacrylate

(abbreviated as T-GMA in their work). They prepared reactive blends of PLA/PBAT with T-

GMA at a varying concentration of 1 to 10 wt% by a melt blending technique. They found that

the tensile toughness and impact strength of the blends were improved with 1 wt% of T-GMA

incorporation, but no changes occurred with increasing T-GMA concentrations. The toughness

and impact strength improvement were attributed to increased miscibility between PLA and

PBAT phase, which was confirmed by SEM analysis. From the DSC analysis, they found two

glass transition temperatures from the blends, indicating that the blends were not homogenous

after adding T-GMA. During melt blending, the PLA/PBAT blend can be compatibilized by a

transesterification product with the help of terbutyl titanate (TBT) [131]. The mechanical and

37

thermo-mechanical properties of this compatibilized blend were improved significantly

compared to the non-compatibilized PLA/PBAT blend. Best elongation at break (298%), impact

strength (9 kJ/m2), and tensile strength (45 MPa) of PLA/PBAT blend were obtained with 0.5%

TBT concentration. The compatibility between PLA and PBAT has been improved through

transesterification reaction products, which was demonstrated by SEM micrographs.

2.7.5.2 Reactive compatibilization of PLA/PBS blends: PLA/PBS blends have been

compatibilized using lysine triisocynate (LTI) as a reactive processing agent [135]. Significantly

higher level of improvement was observed in the impact strength (2-4 fold increase compared to

PLA), and elongation at break with a decrease in melt flow rate. This improvement could

possibly be due to the grafting or crosslinking of LTI with PBS or PLA in the blends. However,

this reactive compatibilization was detrimental to tensile strength of PLA/PBS blends. A similar

trend was observed in the PLLA/PBS blend with a dicumyl peroxide (DCP) reactive agent [51].

These blends achieved stronger interfacial adhesion and fine dispersed phase morphology owing

to the increased compatibility between the blended components. Consequently, the PLLA/PBS

blend with 0.1 parts per hundred (phr) of DCP showed 12-fold increment in the Izod impact

strength compared to virgin PLLA. This improvement was attributed to debonding through shear

yielding. However, both flexural strength and flexural modulus continuously decreased with

increasing DCP concentration. Yet, DCP is not an appropriate choice when trying to improve

elongation at break of PLLA/PBS blends. Unlike PLLA/PBS blends, PLA/PBAT blends with

0.1-0.2 phr DCP showed significant improvements in elongation at break as well as increased

impact toughness [141]. Chen et al., [151] investigated the compatibility between the PLA and

PBS blends with twice functionalized nanoclay (TFC). SEM revealed that the PBS domain size

gradually decreased when 2 wt% TFC was incorporated into PLA/PBS blends, due to increased

38

compatibility between the polymer phase in the blend system. Elongation at break was increased

with the addition of TFC into PLA/PBS blends. Flory-Huggins theory suggested that TFC has a

high compatibility with the PLLA phase when compared to the PBS phase. In addition, there was

no improvement in the elongation at break when nanoclay without functionalization was

incorporated, which indicates that no compatibility existed between the PLA/PBS blends. Ojijo

et al.,[152] have improved the toughness of PLA/PBSA blends by in-situ compatibilization with

triphenyl phosphite (TPP). Comparing the impact strength and elongation at break for neat PLA

and PLA/PBSA blend containing 10% PBSA and 2% TPP, the impact strength and elongation at

break of compatibilized PLA/PBSA blend showed 166% and 516% higher value compared to

neat PLA. These improvements were attributed to shear yielding of the matrix, which was

initiated by the debonding between the blended components. In addition, there was no significant

change in the tensile strength and thermal stability of the compatibilized blends. The authors also

suggested that the barrier properties, thermal properties and mechanical properties of this

toughened PLA/PBSA blend system could be improved by the addition of clay into such a

system.

2.7.5.3 Reactive compatibilization of PLA/PHB and PHBV/PBS blends: Reactive

compatibilized PLA/20 wt% PHB (MirelTM

M4300) copolymers showed 1.3 J tensile toughness

and 200% elongation at break [121]. These values are significantly higher than neat PLA and

non-reactive blends of PLA/20 wt% PHB. It is important to note that this amorphous PHB is

thermodynamically immiscible with PLA. However, reactively compatibilized PLA/PHB blend

shown that the notched Izod impact strength of immiscible PLA/20 wt% PHB copolymer blend

is 20 times greater than that of virgin PLA (from 0.4 ft.lb/inch to ~8.2 ft.lb/inch) [121]. The same

39

blend displayed a tensile modulus of ~2 GPa and tensile strength of ~50 MPa. Therefore, the

stiffness-toughens performance of this blend is superior compared to PBT and polypropylene.

PBS is another biodegradable thermoplastic polyester with tensile strength comparable to

PP and stiffness similar to LDPE with fair processability using processing equipments of

conventional polyolefin thermoplastics [153]. PBS has a glass transition temperature of around -

30oC, which gives it a high toughness compared to the polymer at room temperature. Also, the

high elongation of PBS (more than 200%), makes it an ideal candidate to blend with a brittle

biopolymer [116]. Therefore, PBS seems to be one of the biodegradable polymers that can be

effective in improving the toughness of PHBV. Ma et al., [57] investigated the toughening

mechanism of PHB/PBS blend and PHBV/PBS blend by using an in-situ compatibilizer DCP

through melt blending. They found that the PBS domain size was considerably reduced by

incorporating DCP into those two blends. The domain size reduction was directly related to the

interfacial adhesion between the components in the blend systems. During melt blending,

addition of DCP into the blends could generate a grafting reaction between the components

(PHBV, PHB, and PBS) in the blends such as PHBV-g-PBS and PHB-g-PBS. The PHBV-g-PBS

and PHB-g-PBS acted as a compatibilizer in the blend system which in turn improved the

interfacial adhesion between the polymer phases. Consequently, the mechanical properties of the

PHB/PBS and PHBV/PBS blends were considerably increased. For example, after incorporation

of DCP (0.5 wt%), the elongation at break of PHBV/PBS (80/20) blend was increased by 39-

fold, in comparison to the corresponding uncompatibilized blend. In addition, a significant

improvement was observed in un-notched Izod impact toughness of the PHB/PBS blends with

0.5 wt% DCP. Similarly, the poor compatibility between the PHB and PLA has been improved

by using DCP as reactive agent [154]. The strength and impact toughness of the compatibilized

40

blends were improved in comparison to uncompatibilized PHB/PLA blends. The improved

compatibility was attributed to the formation of crosslink network and/or copolymer (PHB-g-

PLA) at the interfaces. Consequently, the the inclusion phase (PLA) domain size was observed to

have reduced significantly.

2.7.5.4 Reactive compatibilization of PLA/PCL blends: Shin and Han [132] showed melt

blending of PLA/PCL resulted in immiscible blend without compatibilizer and that the

compatibility was improved by adding GMA into the blend. The improved compatibility has a

direct relationship with mechanical properties and fine morphology. In addition, they have

observed that GMA acted as a reactive monomer between the PLA/PCL blend interfaces. Liu et

al., [133] have studied super toughened ternary blends with PLA/ethyl-butyl acrylate and

glycidyl methacrylate copolymer (EBA-GMA)/ethylene methacrylic acid zinc ionomer (EMAA-

Zn). This reactive blend exhibited remarkable improvements in notched Izod impact strength

(from 38 to 770 J/m) and elongation at break (from 5 to 229%). These improvements were

attributed to the compatibilization that occurred between the components in the resulting blends.

In another study, reactive blending of PLA and ethylene-glycidyl methacrylate (EGMA) showed

a dramatic improvement in impact strength (70 kJ/m2) and elongation at break (>200%)

compared to virgin PLA [134]. Zhang et al., [155] have studied a super-toughened multiphase

PLA blends with series of renewable poly(ether-b-amide) (PEBA) copolymer and ethylene

methyl acrylate-glycidyl methacrylate (EMA-GMA) terpolymer. This reactive multiphase blend

(PLA/EMA-GMA/PEBA (70/20/10 wt%) system showed a massive improvement in the impact

toughness (~500 J/m) and elongation at break (~75%). The authors concluded that the

improvements are attributed to the combination of strong interfacial adhesion between the phases

and massive shear yielding phenomena. Both the miscibility and impact energy of the PLA/PCL

41

blends were dramatically improved with the addition of lysine triisocynate (LTI) [136]. This was

due to reduction in size of the PCL phase in the PLA matrix; this behavior reduced the localized

stress concentration triggered by interfacial failure. A blend of PLA/PCL (85/15) was melt

processed with and without LTI (1 wt%) as a reactive additive [136]. The average fracture

energy of PLA/PCL/LTI was measured to be five times higher than that of PLA/PCL blend.

Similar to other studies, addition of LTI was found to increase the miscibility of PLA with PCL.

The improved compatibility between the PLA and PCL contributed to the reduction of PCL

domain size, which facilitated increased energy absorption during the fracture process. Wang et

al., [156] have improved the toughness of PLA/PCL blends by in-situ compatibilization with

triphenyl phosphite (TPP). The percentage elongation of the reactively compatibilized

PLA/PCL/TPP (80/20/2 or 20/80/2 by weight) blends improved considerably when compared to

neat PLA and corresponding uncompatibilized blend. This result suggested that a synergism

existed for certain compositions of PLA/PCL blends. Moreover, the compatibilized blends

showed higher susceptibility to enzymatic degradation when compared to neat PLA, PCL and

PLA/PCL blend without reactive agent.

2.8 Natural fibers

2.8.1 Classification of natural fibers

Natural fibers are are classified based on their origin as it can be obtained from either

plant or animal sources. The major component of animal fibers (e.g., wool, feather, angora fiber,

and silk fiber) is protein, whereas the major component of plant fibers is cellulose [157]. Plant

fibers (cellulosic/lignocellulosic fibers) can be classified based on their origin: seed/fruit fibers

(e.g., cotton, coir, et al.,), leaf fibers (e.g., sisal, pineapple), bast fibers (e.g., jute, keneaf, flax et

al.,), agriculture residues (e.g., corn straw, wheat straw, corn stover, et al.,) and grass/reed fibers

42

(switch grass, miscanthus, bamboo, et al.,). Plant fibers (excluding cotton) are mainly composed

of water-soluble organic components, lignin, waxes, cellulose, hemicellulose, and pectin. Based

on the type of the biomass, the chemical compositions of fibers can vary widely [158].

2.8.2 Natural fibers: nature and behavior

The performance of the plant fiber depends on its internal structure, architecture,

chemical composition, crystallinity and micro-fibril orientation angle [159] (Table 2.5). Among

the natural fiber constituents, cellulose has higher crystallinity. In contrast, lignin is amorphous

in nature with a highly complex aromatic structure. Because of its stiffness, lignin acts as a

protective barrier for cellulose in the plant. Hemicellulose is a highly branched polymer with a

degree of polymerization 10-1000 times lower than cellulose [160]. Pectin is usually found in

primary cell wall of most plant fibers. Pectin is more hydrophilic component in plant fiber due to

presence of carboxylic acid group. Generally, native cellulose has strength > 2GPa and a

stiffness of 138 GPa [161]. The stiffness of natural fibers depends on the microfibril angle.

Therefore, plant fibers with high cellulose content (~60-80%) and low microfibril angles have a

strong reinforcing effect in composites. Both pectin and hemicelluloses play important roles in

the water absorption, swelling, elasticity, wet strength and fiber bundle integration of the fibers

[41].

2.8.3 Advantages and challenges in using natural fibers

Unlike synthetic fibers, natural fibers are lightweight and inexpensive. They possess high

toughness, reduced tool wear, good insulation properties, reduced human health hazard,

enhanced energy recovery, recyclability, and biodegradability [153]. The specific modulus and

strength of the natural fibers are comparable to glass fibers [46, 160, 162], as shown in Table 2.5.

Furthermore, the natural abundance, renewability, CO2 neutrality and lower density of natural

43

fibers have caused an increasing demand for these fibers in material applications. However, there

are some major drawbacks with respect to plant fibers, including complex supply chain,

geographical availability, moisture absorption, and inconsistency. The inconsistency of the plant

fibers is related to many factors, such as harvesting time, species type, soil quality, fertilization,

moisture content of the fibers, location of the plant growth, as well as climatic conditions during

plant growth [157]. Moisture absorption of the natural fiber can vary greatly depending on the

fiber types. This moisture absorption creates performance problems for products made out of

such natural fiber composites. Due to low thermal stability, natural fibers are recommended for

process/use below 200oC [36, 157]. These issues can be effectively addressed by appropriate

material selection/screening techniques [162, 163].

2.9. Biocomposites

The term composites originated from the Latin word, compositus. In general, composites

are materials made from two or more distinct constituent having significantly different physical

and/or chemical properties. At least two components are required in order to prepare composites

material: matrix or continuous phase and reinforcement or discontinuous phase. The continuous

phase acts as a binder while the discontinuous phase acts as reinforcement. Often both

continuous and discontinuous phases are easily distinguishable in the resulting composite

materials. Generally, the mechanical performances of the reinforcements are higher than those of

the matrix phases. Composite materials properties can be tailored to suit specific application

requirements. Composites are generally classified based on their matrix sources like metal,

ceramic and polymer (thermoplastic and thermoset). Based on their reinforcements, the

composites can be further categorized as fibre (short or long) composites and particulate

(powder, flacks and spherical) composites, fabric based composites and long fiber reinforced

44

thermoplastic composites. Most common fibers, such as aramid, carbon and glass are used as a

reinforcement in conventional polymer matrices, such as PP, epoxy resins, unsaturated polyesters

and polyurethane [164], while E-glass fibers dominate in the fiber reinforced plastic composites

(FRCs) market [161]. Biocomposites offer a combination of high mechanical properties,

lightweight, moldability and design flexibility. Thermoplastic polymer matrices usually

dominate the class of natural fiber composites.

FRCs have diversified applications in automotive interior parts, packaging, construction,

furniture, consumer goods, electrical, musical instruments, wind turbine rotor blade and decking

industries [161]. A combination of flax and carbon fiber reinforced composites are used in

sporting equipment (snowboards, tennis racket and bicycle frames) because of its superior

vibration damping properties and mechanical performance [157]. In 2002, fiber reinforced

composites share of the global market was 771 million kg and this has continued to grow

extensively [41]. The amount increased to 8.7 million tons in 2011 [161]. In 2002, fiber

composites had a maximum share of 31% in automotive applications followed by construction

applications (26%) [10]. In Europe, this trend increased significantly because of mounting

environmental concerns. Increasing interest in biodegradable and/or biologically derived

materials has fueled vivid research in biocomposites [46]. Either a composite produced from the

combination of non-biodegradable polymer with a natural fiber/filler or a bioplastics with

synthetic fibers/fillers can be called as biocomposites.

2.9.1 Advantageous of natural fiber composites

Natural fiber composites (NFCs) have numerous benefits compared to synthetic fibers

such as abundant availability, lightweight, eco-friendly, nonabrasive, inexpensive, good fatigue

and specific strength [165]. Biocomposites made from plant based fiber and renewable resource

45

based polymers can be termed eco-composites or all green composites [46]. Any biocomposites

that are compostable, recyclable, environmentally friendly and commercially adequate can help

in waste management [162]. Biocomposites are in huge demand for automotive uses, as they

reduce both manufacturing costs and fuel consumption while meeting the consumer pull for

greener products [36]. Because of global warming, environmental concerns, and an increased

interest in greener products, biocomposites based on bioplastics have been developed.

2.9.2 Attributes of natural fiber composites

Properties of NFCs are strongly influenced by many factors, including stress transfer

between the fiber-matrix, fiber orientation, fiber-matrix interaction, fiber volume, fiber length

and aspect ratio. There are some drawbacks associated with natural fiber composites, like

incompatibility with some polymer system, poor wetting, moisture sensitive, non-destructive

behavior, and lower thermal stability that could affect the composites performance [36, 164].

Natural fibers are mainly hydrophilic in nature making them incompatible with the

hydrophobic polymer matrix and their addition can lead to a reduction in the mechanical

performance of the resulting products [153]. The polarity difference between the

thermoplastics matrix and the natural fibers may result in poor fiber-matrix adhesion in the

resulting composites. A lack of interaction/compatibility between reinforcement and matirx

reduces the overall performance of the resulting materials [41]. The compatibility between

the reinforcement and matrix can be improved by chemical surface modification (sodium

hydroxide, permanganate, peroxide, maleic anhydride, organosilanes, and isocynates) and

physical surface modification (cold plasma treatment, corona treatment) of the fibers [41].

46

Table 2.5. Properties of some natural and synthetic fibers

Fibers Density

(g/cm3)

Specific tensile

strength

(MPa)

Specific tensile

modulus

(GPa)

Elongation

at break

(%)

Cellulose

content

(wt%)

Cellulose

Crystallinity

(%)

Microfibril

angle

(Degree)

Moisture

(wt%)

Price

(US$/kg) References

Flax 1.5 535-1000 18.4-53 1.2-3.2 64-71 50-90 5-10 8-12 2.1-4.2 [157, 159, 161]

Hemp 1.47 372-608 47.3 2-4 70-74 50-90 2-6 6.2-12 1.0-2.1 [153, 157, 159,

161]

Jute 1.3-1.5 269-548 6.85-20.6 1.5-1.8 61-72 50-80 8 12.5-13.7 0.35-1.5 [153, 157, 159,

161]

Kenaf 1.5-1.6 641 36.55 1.6 31-39 - - 6.2-12 0.26-0.52 [157, 159]

Ramie 1.5-1.6 147-625 29.3-85 2-3.8 68.6-76.2 - 7.5 12-17 1.5-2.5 [153, 157, 159]

Sisal 1.45 366-441 6.5-15.2 3-7 66-78 50-70 10-25 11 0.6-0.7 [157, 159, 161]

Bamboo 1.1 454 32.6 1.4 26-60 40-60 8-11 9.16 0.45-0.5 [157]

Coir 1.2 146 3.3-5 30 32-43 27-33 30-49 11.36 0.25-0.5 [159, 161]

Cotton 1.6 179-373 3.44-7.9 7-8 82.7-91 - - 7.85-8.5 2.1-4.2 [159, 161]

Abaca 1.5 267 8 3-10 56-63 20-25 15 0.345 [36, 159]

Pineapple 1.4-1.6 118-446 4-27 1.6 70-82 44-60 14 11.8 0.4-0.55 [153, 161, 165]

Banana 1.35 444 13.2 3.36 44-64 45-55 10-12 10.71 - [159, 161]

Miscanthus 1.41 - 6.7 - 38 - - - - [166, 167]

Switchgrass 1.40 37.7 6.4 - 32 - - - - [166, 168]

Softwood 1.5 667 26.6 4.4 - - - - - [159]

E-glass 2.55 1333 28.63 3.4 - - - - 2 [36, 159, 165]

47

Another strategy is to improve the interfacial adhesion through the addition of a

“compatibilizing/coupling agent” into the composites during processing [36]. Generally, the

“compatibilizer/coupling” agents are used to modify the interface between the matrix and

reinforcement in the composite structures. The interfacial modification may be in the form of

a physical and/or chemical interaction between the components in the composites. Enhanced

compatibility between the phases of the composites helps in improving fiber-matrix adhesion,

resulting in improved performance of the resulting composites. Natural fiber composites with

compatibilizers are reviewed in detail in subsequent sections.

2.10. Biocomposites based on biodegradable blends as matrix material: Some specific

examples

There have been many reviews on natural fiber composites [157, 159, 163, 169], [161],

[41, 170, 171], [10, 36, 46, 172]. Most of these publications reviewed single polymer matrix

based biocomposites. Reviews of biocomposites fabricated using biodegradable blends as matrix

material are very limited. Recently, biocomposite fabrication using blends of polymer matrices is

part of the growing trend because blend matrices could provide an optimum stiffness-toughness

balance, or tailored properties for resulting biocomposites. Currently, few biodegradable pre-

blends are commercially available in the market, for instance PHBV/PBAT blend, available

under the name of ENMATTM

from Tianan Biologic Materials Company. Ltd., China [173].

BASF commercialized a biodegradable polymer blend under the trade name Ecovia®. It is a

blend of PLA/PBAT (45/55 wt%) with 45 wt% biobased content [174]. In addition, FKuR is

producing PLA-Copolyester blends under the trade name Bio-Flex. All of these blends are

successful matrix material for the incorporation of natural fibers. Zhang et al., [175] fabricated a

biodegradable and renewable ternary blends from PHBV, PLA and PBS. The ternary blends

(PLA/PHBV/PBS) showed a unique stiffness-toughness balanced mechanical properties

48

compared to their neat components. The authors concluded that the ternary blend

(PLA/PHBV/PBS) system is very promising for the biocomposites application. Similarly,

Muthuraj et al., [54] have also developed a binary blend of PBS and PBAT with good thermal,

mechanical and thermo-mechanical properties to be used as a matrix material for composite

applications. These research efforts are indicative of the fact that the multiphase polymer blend

provides a simple and an effective way to develop a new polymer matrix system for natural fiber

composites.

2.10.1 Biocomposites based on PHBV blends

As stated previously PHBV is a brittle and biodegradable polymer. By blending PHBV

with tough, biodegradable polymers, their mechanical and thermal properties can be tailored

without sacrificing the biodegradability [117]. Therefore, many researchers have extensively

studied the toughened PHBV blend biocomposites (Table 2.6). For example, Javadi et al., [173]

investigated the preparation and performance of solid and microcellular pre-blend of

PHBV/PBAT and its recycled wood fiber (RWF) based composites with and without silane

treatments. There were no significant changes in the SEM morphology of the

PHBV/PBAT/silane-treated-RWF composites and PHBV/PBAT/untreated-RWF composites,

suggesting that silane treatment was not worthy for improving thermal properties and interfacial

bonding between the fiber-matrix. However, addition of 10% RWF into PHBV/PBAT blends

increased the crystallinity, storage modulus, specific tensile strength, tensile modulus and

decreased strain at break and toughness compared to the neat PHBV/PBAT blend. Overall, the

authors concluded that silane-treated-RWF reinforced PHBV/PBAT composites did not induce

any significant changes in morphology, mechanical and thermal properties of

PHBV/PBAT/RWF untreated-RWF composites.

49

Nagarajan et al., [168] studied the performance of biocomposites fabricated from a pre-

blend of PHBV/PBAT (45/55%, ENMATTM

) as matrix and switchgrass fiber (20-40 wt%) as

reinforcement. An obvious increase was observed in the modulus of the composites because of

the reinforcing effect of the switchgrass fibers. However, the resulting composites showed poor

interfacial adhesion between the fiber-matrix. Therefore, the interfacial adhesion was improved

by adding poly (diphenylmethane diisocynate), (PMDI) as a compatibilizer (0.5, 0.75 and 1 phr).

SEM analysis indicated the occurrence of a better stress transformation between the fibers and

matrix through improved interfacial adhesion, which lead to an increase in the mechanical

performance of the resulting composites. The critical concentration of compatibilizer was found

to be 0.75 phr. This critical concentration yielded 80, 40 and 56% improvements in notched Izod

impact strength, flexural strength and tensile strength, respectively. Furthermore, the composites

with and without compatibilizer showed a considerable improvements in storage modulus and

heat resistance properties compared to the matrix. The melt flow of both compatibilized and

uncompatibilized composites sharply reduced in comparison to the PHBV/PBAT blend matrix.

For example, with addition of 30 wt% switchgrass fibers into PHBV/PBAT matrix, melt flow

index (MFI) reduced to 7 g/10min. It could be due to the restriction of polymer chain mobility in

the presence of stiff fibers. The MFI of the composites with 30 wt% switchgrass fibers and 0.75

phr compatibilizer is around 5 g/10 min. Authors suggest that this biocomposites can be injection

molded with the help of some processing additives.

Biocomposites was prepared from a PHBV/PBAT(45/55%) blend and distiller’s dried

grains with solubles (DDGS) using melt processing technique by Zarrinbakhsh et al., [176]. This

study explored the influence of compatibilizer (0.5 and 1% PMDI) and lubricating agent (3 and

6% corn oil) on the performance of the resulting biocomposites. Use of PMDI as a

50

compatibilizer was observed to increase mechanical properties significantly when compared to

uncompatibilized composites. The composites with only 0.5% PMDI showed a maximum

improvement in tensile strength (39%) and modulus (24%). There was no significant change of

these mechanical properties while increasing PMDI content from 0.5 to 1%. Similar trends were

observed in flexural properties. These improvements may be due to the good interfacial adhesion

that occurred through cross-linking reactions between the components in the composites. This

reaction was confirmed by MFI reduction, as well as by the increased melt viscosity of the

composites. However, the addition of only corn oil reduced both the stiffness and strength of the

composites. Interestingly, combination of both corn oil and PMDI displayed a synergistic effect

in impact strength and elongation at break. Authors attributed this improvement to the lubrication

effect of corn oil, which enhanced the chemical reaction between the components. Water

absorption rate of all the biocomposites was higher than PHBV/PBAT blend matrix. However,

the initial water absorption rate of the compatibilized composites was lower than that of the

uncompatibilized composites. Reduction of free hydroxyl groups in the DDGS particles after

chemically reacting with the matrix was believed to be the reason. The interfacial interaction

between matrix and DDGS was justified by scanning electron microscopy analysis. In few other

studies, the biodegradability of DDGS/PBAT [177], DDGS/PLA [178], and DDGS/PHA [179]

composites has been investigated under different environmental conditions. These DDGS based

biocomposites exhibited superior biodegradation rate compared to their biodegradable polymer

matrix.

In a different study, Nagarajan et al., [158] studied the influence of adding 30 wt% of

various natural fibers (miscanthus, switchgrass, wheat straw, corn stalk, and soy stalk) into

PHBV/PBAT (45/55 wt%) blend by extrusion and injection molding technique. They have

51

evaluated the water uptake behavior, mechanical, thermal and thermo-mechanical properties.

Among biocomposites with different types of fibers, miscanthus based composites exhibited

higher mechanical properties, heat deflection temperature, higher thermal stability, and

comparatively low water absorption. The observed property differences between the composites

were due to difference in fiber composition, fiber length distribution, and the interaction between

the fibers-matrix. Moreover, SEM micrographs showed debonding and fiber pullout from the

matrix, which was attributed to the inefficient stress transfer occurring between the components.

Similar to this study, various types of agricultural residues reinforced PLA composites have also

been reviewed [180]. The biodegradability rate of these nature fiber reinforced composites can

be expected to be much faster in comparison to their parent components [181].

The influences of man-made cellulose, jute and abaca fiber on the mechanical properties

of PHBV/PBAT (70/27.6% and 2.6% processing additive) blend were also investigated by

Bledzki and Jaszkiewicz [182]. The composites samples were produced in extrusion and

injection molding process. The fillers had positive effects on tensile properties and notched

Charpy impact strength. Among the abaca, jute fibers and man-made cellulose, abaca fiber has a

high tensile strength (980 MPa) and a high modulus (27-32 GPa) [182]. However, abaca fiber

based composites underperformed compared to man-made cellulose and jute fiber composites.

The composites with man-made cellulose showed a massive improvement (> 500%) in notched

Charpy impact strength and a 50% improvement in tensile strength over the PHBV/PBAT

matrix. SEM micrograph of PHBV/PBAT composites showed a poor adhesion between the

fiber-matrix. This was attributed to the partial degradation of PHBV/PBAT blend during melt

processing and post-molding shrinkage of the PHBV/PBAT blend matrix. Similar to a previous

study, a massive increase in the tensile modulus was observed for the PHBV/PBAT/(70/27.6%

52

and 2.6% processing additive) blend with jute fiber composites [183]. The observed results were

attributed to chemical composition, aspect ratio and the geometry of the fibers. SEM analysis

revealed that the compatibility between the fiber and the matrix was poor due to the shrinkage

nature of PHBV in the matrix. Even still, PHBV/PBAT composite properties are comparable

with PP biocomposites.

The effects of sodium hydroxide (NaOH) treated and pectinase retted kenaf fibers on

PHBV/PBAT composite properties were studied using compression molding [184]. This study

investigated the dynamic mechanical analysis and crystallization behavior of composites with

two different fiber lengths (5 and 10 mm). The dynamic mechanical properties of the composites

showed that the storage modulus of the composites increased with the addition of fiber compared

to the PHBV/PBAT matrix. The storage modulus of pectinase retted fiber reinforced composites

was higher compared to corresponding composites with NaOH retted fiber. The composites with

5 mm retted fibers showed a higher storage modulus than the 10 mm fiber and their hybrid (1:1)

composites. In the presence of both pectinase and NaOH retted fibers, the glass transition

temperature of PBAT increased and the damping factor of PHBV phase reduced. The fiber

retting had influence on crystallization, melting and spherulite morphology of the PHBV/PBAT

matrix, which was confirmed by polarizing optical microscopy (POM) analysis. POM was used

to study the nucleation density and spherulite growth of PHBV/PBAT blends and their

biocomposites. The nucleation density of the PHBV/PBAT blend increased with the addition of

fibers and it lead to a change in the crystal morphology of the resulting composites.

Consequently, the melting point of the composites was reduced as compared to the PHBV/PBAT

matrix with increase in the melting enthalpy. This suggested that the pectinase retted kenaf fibers

have more surface activity and thus increased the crystallinitythan the unmodified kenaf fiber.

53

The complex viscosity of the composites was higher than the matrix because of the restricted

polymer chain mobility in the presence of rigid fibers. NaOH retted fiber composites showed less

nucleation density and larger spherulites as compared to pectinase retted fiber composites. This

was attributed to the enhanced roughness of the NaOH retted kenaf, which tend to form tiny

spherulites.

Zarrinbakhsh et al., [185] investigated green composites made from distiller’s dried

grains with solubles (DDGS) and an optimized blend of PHBV/PBS (70/30 wt%) matrix.

Mechanical, thermal, morphological, and thermo-mechanical properties of the injection molded

composites were studied. Interestingly, after water washing of DDGS, the thermal stability of

DDGS was improved because of water solube moleculaes were washed away. The improved

thermal stability allowed for higher processing temperatures without any thermal degradation of

DDGS up to 180oC. The water-washed DDGS composites showed an improved tensile and

flexural strength compared to non-washed DDGS composites. When poly(diphenylmethane

diisocynate) (PMDI) was introduced as a compatibilizer into water-washed DDGS composite,

there were significant improvements in flexural strength and tensile strength. The flexural and

tensile modulus of the DDGS composites has an increasing trend in the following order:

PHBV/PBS blend matrix<non-washed DDGS composites<water-washed DDGS

composites<water-washed/compatibilized DDGS composites. The improved tensile and flexural

properties were attributed to the enhanced interfacial adhesion between the fillers and matrix,

which was confirmed through SEM. The impact strength of water-washed DDGS composites

and water-washed/compatibilized DDGS composites was lower compared to non-washed DDGS

composites. It was hypothesized that the poor interfacial bonding between the phases needs more

54

energy to break the fillers because the crack propagation occurred around the fillers rather than

braking fillers.

Recently, Zhang et al., [186] studied preparation and performance evaluation of

sustainable green composites from ternary PHBV/PBAT/epoxidized natural rubber (50/35/15

wt%) blend matrix and miscanthus fibers. The selected ternary blend system has non-break

behavior under notched Izod impact test. However, the addition of 10 wt% miscanthus fiber into

this ternary blend matrix, the impact strength of the resulting composites was observed around

273 J/m. A similar trend was observed in the elongation at break and tensile strength. This

reduction was mainly due to weak interfacial interaction between the fiber-matrix as well as poor

load transfer capability between the fiber-matrix. Interestingly, this drawback was overcome by

adding 0.3 wt% DCP as a reactive agent in the composites. In the presence of DCP, the polymers

formed graft copolymers and partially crosslink network between the polymers, which improved

the compatibility between the blend components. The authors also investigated the effect of 0.3

wt% DCP while increasing fiber content up to 20 wt% in the resulting composites. The impact

energy of the composite with 20 wt% fiber and 0.3 wt% DCP still remained at 240.5 J/m. The

flexural properties and storage modulus were found to increase with increasing fiber content up

to 20 wt%, which is a fairly common observation in natural fiber reinforced polymer composites.

2.10.2 Biocomposites based on PLA blends

PBAT, PBS, and PCL are the best candidates for increased toughness of PLA in their

respective blends. Blending PLA with tough polymers (PBAT, PBS, and PCL) resulted in a

higher toughness but lower stiffness in the blends in comparison to virgin PLA. The low stiffness

can be addressed by adding fibers/filler to PLA blends. Therefore, many researchers have

extensively studied toughened PLA based biocomposites, as shown in Table 2.6. Commercially

55

available pre-blends of PLA/PBAT, 45/55 wt%, (BASF-Ecovio®) with chemically (alkali

treatment) treated curaua fiber reinforced biocomposites were studied by Harnnecker et al.,

[174]. There was a slight (4oC) increase in Tg of the composites compare to the PLA/PBAT

blend. This fact was attributed to the intermolecular (hydrogen bonds) interaction between the

fibers and polymer matrix. Moreover, DSC thermograms showed an endothermic peak during

heating cycle for all the composites whereas PLA/PBAT blend did not show any endothermic

peak during heating cycle. This was believed to be due to the nucleation effect of curaua fiber in

the composites. Performance of these composite was studied with and without the addition of

compatibilizer (2 wt%) i.e., maleic anhydride-grafted-polypropylene (MA-g-PP). Tensile, impact

and flexural strength of the compatibilized composites increased with increased fiber content

from 5 to 20 wt%. For example, the compatibilized composites with 20 wt% curaua fibers

showed 56% increase in flexural strength and 75% increase in the tensile strength.

56

Table 2.6. Recently developed biodegradable polymer blend matrix based biocomposites

Blend matrix Fiber/filler Compatibilizer/Coupling agent Manufacturing

process

References

Type Content Name Chemical structure

PHBV/PBAT Recycled wood fiber 10% Gamma-

methacryloxypropylt

rimethoxysilane

CH2=C(CH3)

CO2CH2CH2CH2Si

(OCH3)3

Injection

molding

[173]

PHBV/PBAT Switchgrass 20-40% Poly

diphenylmethane

diisocyanate (PMDI)

Injection

molding

[168]

PHBV/PBAT Distiller’s dried grains

with solubles (DDGS)

20% Poly

diphenylmethane

diisocyanate (PMDI)

Injection

molding

[176]

PHBV/PBAT Miscanthus, switchgrass,

wheat straw, corn stalk,

and soy stalk

30% NA NA Injection

molding

[158]

PHBV/PBAT Jute, abaca and man-

made cellulose

30% NA NA Injection

molding

[182]

PHBV/PBAT Cellulose, Jute, abaca

and flax

30% NA NA Injection

molding

[183]

PHBV/PBAT Kenaf 5 and

10%

NA NA Compression

molding

[184]

PHBV/PBS Distiller’s dried grains

with solubles (DDGS)

30% Poly

diphenylmethane

diisocyanate (PMDI)

Injection

molding

[185]

PHBV/PLA Soy hull, switchgrass,

and miscanthus

30% NA NA Injection

molding

[166]

PHBV/PLA Soy fiber 30 and

50%

2,5-Bis(tert-

butylperoxy)-2,5-

dimethylhexane

Injection

molding

[187]

57

PLA/PBAT Kenaf 10-50% (3-aminopropyl)

trimethoxysilane

(APTMS)

Compression

molding

[188]

PLA/PBAT Curaua 5-20% Maleic anhydride

grafted

polypropylene (MA-

g-PP)

Compression

molding

[174]

PLA/PBAT Softwood flour 30-50% NA NA Injection

molding

[189]

PLA/PBAT Wood flour 20-30% NA NA Compression

molding

[190]

PLA/PBAT Ramie 10% NA NA Compression

molding

[191]

PLA/PCL Alkali treated palm fiber 10-25% Dicumyl peroxide

(DCP)

Injection

molding

[192]

PLA/PCL Silane treated Jute and

Untreated Jute

50% Trimethoxy (methyl)

silane

CH3Si (OCH3)3 Compression

molding

[193]

PLLA/PBS Flax 25.5% NA NA Injection

molding

[194]

58

The increased mechanical properties were attributed to the improvement of interfacial

adhesion between the fibers and matrix, which was confirmed by cross-sectional fracture

analysis. In addition, water absorption of the compatibilized composite was almost 100% lower

compared to the uncompatibilized composite. Sis et al., [188] examined the impact, flexural, and

tensile properties of the PLA/PBAT/kenaf fiber composites with and without coupling agent ((3-

aminopropyl) trimethoxysilane), APTMS, by melt blending. The tensile, flexural and impact

strength of PLA/PBAT/kenaf composites decreased with increasing fiber loading. This was

because of inadequate fiber-matrix interaction and poor dispersion of fiber in the matrix. The

composites were modified with different amount (from 1 to 5 wt%) of APTMS coupling agent.

The optimum properties were obtained for PLA/PBAT/kenaf composites with the addition of 2

wt% coupling agent. This composite showed 22, 42 and 63% improvement in Izod impact

strength, tensile strength, and flexural strength, respectively. In addition, the tensile and flexural

moduli of the composites with coupling agent were considerably higher than composites without

coupling agent. The storage modulus of the composites with and without coupling agent exhibits

similar trends, like the tensile and flexural modulus. This behavior was ascribed to good

adhesion between the phases.

Unlike PLA/PBAT/kenaf and curaua fiber composites, PLA/PBAT/wood flour

composites exhibited an increase in both tensile and flexural properties compare to the neat

PLA/PBAT (45/55 wt%) blend (Ecovio®

) [189]. With increasing fiber load from 30 to 50 wt%, a

48.2% reduction was observed in the Charpy impact strength of the resulting composites

compared to PLA/PBAT blend matrix. The loss in impact strength was attributed to low fiber

aspect ratio of wood flour [189]. The water resistance property of the composites was reduced

because of hygroscopic nature of the wood flour. However, the composites with an addition of

59

50 wt% wood flour showed significant improvements in tensile strength (27.2%), modulus

(174.3%), and flexural strength (20.6%) as compared to neat PLA/PBAT blend. These

improvements were related to good fiber-matrix adhesion and better distribution of fibers in the

matrix. This trend is in contradiction to the finding of Georgiopoulos et al., [190]. Another study

observed similar results for PLA/PBAT(70/30 wt%) biocomposites [195]. More recently, a

comparison study between PLA/30% ramie composites and PLA/PBAT(95/5, 90/10 and

85/15%) with 30 wt% ramie composites has been investigated by Yu and Li [191]. In general,

strong interfacial bonding between fiber and the matrix decreases the water absorption of the

natural fiber composites. The Vicat softening temperature, flexural and tensile strength of the

PLA/PBAT/ramie (85/15%) composites were lower than PLA based composites, which was due

to the soft elastomeric nature of the PBAT phase. Yet, the notched Izod impact strength of the

PLA/PBAT/ramie composites was superior compared to PLA/ramie composites. This suggests

that the PBAT phase induced stress concentration under impact load. Consequently, the PBAT

phase absorbed more energy during failure of the PLA/PBAT composites. In order to maintain

energy dissipation source in the PLA/PBAT composite system, concentration of PBAT was

considered an important factor. Furthermore, thermal stability of the PLA/PBAT based

composites was studied by thermogravimetric analysis. It was found that the thermal stability of

PLA/PBAT based ramie composites was higher than PLA based composites because the PBAT

phase enhanced the charring process for the composites [191].

Ibrahim et al., [192] studied the performance of uncompatibilized and compatibilized

PLA/PCL (10/90 wt%)/alkali treated palm fiber composites with 10 to 25 wt% fiber loading.

With the addition of 0.01 phr DCP, the flexural modulus, impact strength, tensile strength, and

flexural strength of the composites were superior when compared to uncompatibilized

60

composites. These improvements were attributed to strong fiber and matrix adhesion and good

fiber distribution in the matrix. Fiber and matrix adhesion was confirmed by damping behavior

as well as by SEM analysis. DCP increased the viscosity and crosslinking while reducing the

brittleness of the composites. Consequently, the tensile modulus of compatibilized composites

was lower than uncompatibilized composites in the entire fiber composition. A similar trend was

noticed in storage modulus of the compatibilized composites. The storage modulus of the

composites increased up to 15 wt% fiber loading and then decreased with increasing fiber up to

25 wt%. The increased storage modulus was due to the reinforcing effect of the fibers. The

decreased storage modulus with increasing fiber loading was a result of free volumes that were

created due to crosslinking. Furthermore, the thermal stability of the composites was higher than

that of the matrix. Overall, the composites with 15 wt% fiber load showed balanced mechanical,

thermal and thermo-mechanical properties. Recently, PLLA/PBS blends with 25.5% flax fiber

composites showed well balanced mechanical properties without any compatibilizer [194].

The consequence of incorporating 30 wt% miscanthus, switchgrass, and soy hull their

hybrids as a reinforcement in the PHBV/PLA (70/30 wt%) blend matrix has been investigated by

Nanda et al., [166]. The performance of the green composites was examined by means of their

thermo-mechanical properties. Due to lack of interfacial adhesion between the components, the

tensile strength of all the composites was reduced significantly when compared to PHBV/PLA

matrix. Yet, the tensile and flexural moduli of all the composites were higher compared to

PHBV/PLA blend. There was no significant difference observed in the notched Izod impact

strength of all the composites compared to the PHBV/PLA matrix. Lower aspect ratio, and

smaller cellulose and lignin content of soy hull in comparison to miscanthus and switchgrass

resulted in soy hull composites displaying inferior mechanical properties. All of the composites

61

showed superior heat resistance properties compared to their matrix. Soy hull based composites

showed lower thermal stability compared to miscanthus and switchgrass based composites. This

could probably be due to the less thermally stable compounds, such as pectin and proteins,

present in the soy hull [187]. The melting temperature and melting enthalpy of the PLA and

PHBV were not affected in the presence of all the fibers. Soy hull based composites showed

more water absorption compared to other composites. This was due to the smaller amount of

lignin present in the soy hull [187]. This trend was changed in compatibilized PHBV/PLA/soy

fiber composites.

2.11. Natural fiber composites market and their applications

Natural fiber reinforced polymeric composites were produced early in 1908 [46]. Since

then, natural fiber composites (NFCs) have become attractive in both automotive and building

industries because of its low cost, low CO2 emissions and because it is lightweight. The specific

mechanical properties of NFCs are superior to synthetic fiber (glass, carbon and kevlar)

composites [160]. Currently, PP and polyurethane based biocomposites are being used in

automotive interior and exterior parts [10, 36]. In 2010, 430.7 million pounds of natural fiber

composites were produced and its worth was 289.3 million US$. This is expected to reach 531.3

million US$ by 2016, which means an 11% growth rate over the next 5 years [196]. Automotive

companies have set a goal and are working towards the possible replacement of some non-

biodegradable polymer products by using biocomposites and bioplastics. An average vehicle

contains 113 kg of plastic parts and approximately 50% of automotive interior parts are made up

of plastics [197]. Annually 10-11 million vehicles are discarded, and 25% of their parts cannot

be recycled or reused because they are made from foams, rubber, and synthetic fibers [197].

62

Biocomposites from PLA/kenaf and PBS/bamboo are used for making automotive spare

tire cover and tailgate trim, respectively [36, 198]. There exists the possibility of producing

durable car parts entirely from biodegradable materials. More specifically, a vehicle (Agri-Car)

with 90% biodegradable materials has been jointly designed by the University of Akron and

Ohio State University [46]. In 2009, the University of Warwick developed a prototype World

First Formula 3 racing car by using green composites [46]. In addition, the mechanical and heat

resistance properties of PLA/kenaf biocomposites were almost equal to those of conventional

polycarbonate/glass fiber composites. PLA based biocomposites are used to produce prototype

kayaks, wind turbine rotors, laptop computers and cellular phones [46, 198-200]. According to

this resource [159], the NFC market is expected to grow by up to 20% and more than 50% in

automotive and building applications, respectively. The global NFC market is expected to grow

to 531.3 million US$ by 2016 [196]. This forecast is significantly higher than the 2010 world

annual production (US$ 289.3 million) of the NFCs market. The European biocomposites market

growth rate is predicted at 21% per year. In 2008, biocomposite production totalled 129,000 tons

in Europe and this growth is expected to increase to 427,000 tons in 2014. Parallel to this, hemp

and flax fibers are major players in the natural fiber composites market share in the Europe

[163]. In 2000, the North American natural fiber market was worth 155 million US$ and it is

expected to reach US$ 1.38 billion by 2025 [157].

2.12 Conclusions

Recently, the utilization of biodegradable polymers has been accelerated due to the

environmental concerns associated with disposal of non-biodegradable plastics in landfills.

Higher cost, thermal instability, low melt strength and brittleness of biodegradable polymers

currently limit their extended application. Great efforts have been taken to overcome these

63

drawbacks by blending technique as well as incorporation of natural fibers. However, most of the

polymer blends are thermodynamically immiscible due to a difference in solubility. Generally,

the immiscible polymer blends lead to a weak interface between the blend partners. The poor

interfacial adhesion between the blend components can be modified using various additive,

compatibilizer and compatibilization strategies. The compatibilization effect and its influence on

the thermal, thermo-mechanical, morphological and mechanical properties of the blends have

been reviewed in this chapter. With the increasing cost of fossil fuels, more research attention is

being focused on developing renewable resource-based materials for various industrial

applications. Natural fiber reinforced biodegradable polymers provide a sustainable alternative to

non-biodegradable polymeric composites. It was found that biocomposites could be fabricated

from a stiffness-toughness balanced blend matrix with various natural fibers. The difference in

strength and modulus values between the natural fibers is attributed to the difference in

individual fiber compositions. Recently, multiple binary blend matrix based biocomposites have

been studied and they all exhibit superior performance compared to single polymer based

biocomposites. Due to uncompatibility between the matrix and fiber, several approaches have

been explored to modify both matrix and fiber to enhance the compatibility between them. The

mechanical properties of all the compatibilized composites are significantly higher in

comparison to uncompatibilized one. However, there are only a limited number of applications

for biodegradable polymers and their biocomposites. It is believed that significant research into

existing processing technology will help to widen the application potential of biodegradable

polymer-based blends and their biocomposites.

64

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[200] http://www.mastersoflinen.com/eng/technique/19-secteurs-d-application, (Last accessed in

January, 2015).

http://indiacompositesshow.com/bio-composites-in-automotive-applications/

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Chapter 3: Fully Biodegradable Poly (butylene succinate) and Poly (butylene adipate-co-

terephthalate) Blends: Reactive Extrusion and Performance Evaluation*

*A version of this chapter has been published in:

R. Muthuraj, M. Misra, A. K. Mohanty, Biodegradable poly (butylene succinate) and poly

(butylene adipate-co-terephthalate) blends: Reactive extrusion and performance evaluation,

Journal of Polymers and the Environment, 2014, 22:336–349 (adapted with kind permission

from Springer, Jul 09, 2015, License number 3664990312761).

Abstract

Two biodegradable polyesters, poly(butylene succinate) (PBS), and poly(butylene

adipate-co-terephthalate) (PBAT) were melt-compounded in a twin screw extruder to fabricate a

novel PBS/PBAT blends. The compatibility of the blends was attributed to the transesterification

reaction that was confirmed by Fourier transform infrared spectroscopy. The Gibbs free energy

equation was applied to demonstrate the miscibility of resulting blends. Dynamic mechanical

analysis of the blends exhibits an intermediate tan δ peak compared to the individual components

which suggests that the blend achieved compatibility. One of the key findings is that the tensile

strength of the optimized blends is higher compared to blended partner. Rheological properties

revealed a strong shear-thinning tendency of the blend by the addition of PBAT into PBS. The

phase morphologies of the blends were examined through scanning electron microscopy, which

revealed that phase separation occurred in the blends. The spherulite growth in the blends was

highly influenced by the crystallization temperature and composition. In addition, the presence of

a dispersed amorphous phase was found to be a hindrance to the spherulite growth, which was

confirmed by polarizing optical microscopy. Furthermore, the increased crystallization ability of

PBAT in the blend systems gives the blend a balanced thermal resistance property.

89

3.1 Introduction

The development of biodegradable material as a potential substitute for non-biodegradable

material is an emerging field of research and development. In recent years, different types of

biodegradable polymers have received an immense amount of attention for developing various

new materials and to reduce environmental concerns [1]. Some biodegradable polymers are

commercially available in the market, such as poly (propylene carbonate) (PPC), poly (butylene

adipate-co-terephthalate) (PBAT), polyhydroxyalkanoates (PHAs), polycaprolactone (PCL),

poly (lactic acid) (PLA), poly (butylene succinate) (PBS), and thermoplastic starch [2-5].

Biodegradable polymers are not currently widely used due to some limitations such as their cost,

mechanical properties, and thermal stability.

Researchers have been trying to address these issues by utilizing blending techniques to

obtain biodegradable blends with tailored properties. Compared to other methods, melt blending

is a cost effective and less time-consuming process for the development of new material with

balanced properties [6,7]. During melt blending, dipole interactions, hydrogen bonding, or a

combination of these occur naturally, and such interactions can enhance the overall performance

of the resulting products [8,9]. The modification can also be made during processing to further

improve the strength of the material. Well established literature is available for modification

studies of polymer blends using techniques such as in-situ compatibilization [10], graft

copolymerization [11], copolymerization [12], and transreactions [8, 13]. Transreactions include

an alcoholysis, acidolysis, and ester interchange reaction. These three reactions are generally

referred to as transesterification. The transesterfication reaction is an exchange mechanism which

can help to form a new type of ester linkage between the components in the blends [14]. The

resulting transesterification products very often play important roles in the miscibility,

compatibility, crystallinity, and mechanical properties of the blends. During the past several

90

years of research, many studies have been done on the transesterification of polyester blends

such as PBS/PCL [6], poly(ethylene terephthalate)/poly(ether imide) [15], poly(triethylene

terephthalte)/polycarbonate [16], PHB/PLA [17], and polycarbonate/poly(trimethylene

terephthalate) [18].

Among the biodegradable polyesters, both PBS and PBAT have been widely studied

because of their commercial availability and inherent biodegradability [1, 19]. PBS is an

aliphatic polyester which is synthesized from the polycondensation reaction of petroleum based

aliphatic dicarboxylic acid (succinic acid) and 1,4-butane diol [20-22]. The biodegradability of

PBS is similar to cellulose and bacterial polyesters like poly(hydroxybutyrate-co-valerate)

(PHBV) [23]. Only limited biodegradable thermoplastics are produced from renewable

resources. Recently, the PBS is produced from biobased monomer i.e., bio-succinic acid [24].

Therefore, exploring PBS as a matrix system for polymer blends and composites could diversify

the renewable resource based material applications. PBS is a good candidate for making

biodegradable products as well as having some unique physical properties such as

semicrystalline nature, thermal stability, good processing properties, good gas barrier properties,

and a lower melting point [25-28]. The main drawback of the PBS is low impact strength, which

is limiting its applications.

PBAT is commercially synthesized from petroleum based adipic acid, 1,4-butane diol,

and terephthalic acid, which is a good biodegradable polymer in the presence of naturally

occurring microorganisms [29-31]. Furthermore, it has excellent toughness and is mostly used

for film extrusion and coatings [32]. PBAT is a promising material to improve the toughness of

polymer blends which contain brittle polymers like poly(lactic acid) [33], polycarbonate [34],

and poly(hydroxybutyrate-co-valerate) [35]. As noted above, PBS and PBAT are the most

91

promising candidates for future biodegradable materials in various potential applications. Many

studies have reported the blending of either PBS or PBAT with other biodegradable polymers.

For instance, PBS has been incorporated with many polymers such as poly(triethylene succinate)

[21], poly(ethylene oxide) [36], poly(propylene carbonate) [37], poly(butylene terephthalate)

[28], copolyesters [30, 38], polyhydroxybutyrate [27], and polycaprolactone (PCL) [6]. Although

many studies have reported stiffness-toughness balanced biodegradable binary blends, to the best

of our knowledge no literature is available for PBS/PBAT binary blends. As two typical

thermoplastic biodegradable polyesters, the blend of PBS and PBAT are of great interest due to

their unique properties, which can extend their applications in diversified areas. Hence, the

present work focused on the fabrication of a novel high performance PBS/PBAT blend. The

binary blend was fabricated by an extrusion-injection molding technique. The resulting binary

blend was characterized by different analytical techniques.

3.2 Experimental section

3.2.1 Materials

PBS pellets (Bionolle 1020) with a weight average molecular weight (Mw) of 1.4 ×105

g/mol and PDI of 1.82, manufactured by Showa Highpolymer, Japan, and were used. The

commercially available PBAT (Biocosafe 2003F) was purchased from Xinfu Pharmaceutical,

China. The molecular structures of the neat PBS and PBAT are shown in Scheme 3.1.

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Scheme 3.1. Molecular structure of PBS and PBAT

3.2.2 Blend preparation

Prior to blending, PBS and PBAT pellets were vacuum dried at 80 oC for 12 h. Samples

with different compositions of PBS/PBAT were prepared in a DSM Xplore® micro-compounder.

The micro-extruder was equipped with a co-rotating twin screw and had a barrel volume of 15

cm3. A twin-screw aspect ratio of 18 and length of 150 mm was used in the melt blending

process. The process temperature, cycle time, and screw speed were kept constant at 140 oC, 2

min, and 100 rpm, respectively for different compositions of the PBS/PBAT blend. The molten

polymer was collected and injected into the mold at 30 oC using a 12 cm

3 micro-injection molder

(DSM Xplore®) kept at 140

oC. The molded test specimens were used for further

characterization.

3.2.3 Fourier transform infrared spectroscopy (FTIR)

FTIR spectra of PBS, PBAT and PBS/PBAT blends were obtained by using a Thermo

Scientific NicoletTM

6700 at ambient temperature. All the scans were performed from 400 to

4000 cm-1

with a resolution of 4 cm-1

. For each sample of the spectrum, 32 accumulated scans

were produced and the absorbance was recorded as a function of wavenumbers.

3.2.4 Mechanical properties

Tensile properties of the neat PBS, PBAT and PBS/PBAT blends were obtained by

Instron 3382, using a constant crosshead speed of 50 mm/min at room temperature. The tensile

properties were measured according to ASTM D638 test method using dumbbell shaped

93

samples. Data was collected by Blue hill software. All the reported values are an average of at

least five samples for each formulation.

3.2.5 Melt flow index (MFI)

MFI measurement of the neat polymers and their blends was carried out according to the

ASTM D1238 standard using a Qualitest (model 2000A) Melt Flow Indexer at 190 °C with a

standard weight of 2.16 kg. The presented results are an average of five replicates of each

formulation.

3.2.6 Differential scanning calorimetry (DSC)

DSC analysis was carried out in a thermal analysis instrument (TA Q-200), and the

analysis was performed under the nitrogen atmosphere. The sample was first scanned from 25 to

150 oC with a rate of heating 10

oC min

-1 and subsequently cooled down from 150 to -50

oC at a

cooling rate of 5 oC min

-1. A second heating scan of the samples was performed from -50 to 150

oC at a heating rate of 10

oC min

-1. The first heating cycle was used for the removal of thermal

history and the reported results are from the second heating cycle. The data were analyzed using

TA Instrument software.

3.2.7 Dynamic mechanical analysis (DMA)

The storage modulus and tan delta of PBS, PBAT and PBS/PBAT blends were measured

by a DMA Q800 from TA Instruments. The analysis was performed between -50 to 100 oC at a

heating rate of 3 oC/min. The experiment was carried out in a dual cantilever clamp with 1 Hz

frequency and 15 µm oscillating amplitude.

94

3.2.8 Heat deflection temperature (HDT)

HDT measurement was performed based on the ASTM D648 standard at a constant load

0.455 MPa in the same DMA Q800 thermal instrument. The analysis was performed at a heating

rate of 2 oC/min from ambient temperature to 100

oC in a three point bending mode.

3.2.9 Thermogravimetric analysis (TGA)

Thermal stability of the PBS, PBAT and PBS/PBAT blends was measured using a TA

Q500 Instrument. The experiment was carried out under the nitrogen environment from 25 to

600 oC with a rate of heating 20

oC min

-1. The maximum rate of degradation was observed from

the derivative thermogram (DTG).

3.2.10 Rheological studies

A strain-controlled rheometer (Anton Paar Modular Compact Rheometer MCR– 302)

was used to observe the rheological properties of neat polymers and their blends. Injection

molded samples were placed between the parallel-plates (diameter of the parallel plate is 25

mm), and the experiment was performed at 140 oC. The plates subsequently compressed the

samples, and the distance between the parallel plates was adjusted to 1 mm. Dynamic properties

were determined by a dynamic frequency sweep test. During the test, the range of frequency and

strain used was 500 to 0.01 rad/s and 3 %, respectively. These limits were fixed based on the

polymer torque sensitivity and their thermal stability.

3.2.11 Polarizing optical microscopy (POM)

Polarizing optical microscopy was performed on a Nikon, Universal Design Microscope.

The microscope was equipped with a Linkam LTS 420 hot stage, which is used to control the

temperature. A DS-2Mv (with DS-U2) color video camera with the capture NIS element imaging

software was used for POM observations. Samples were sandwiched between two glass slides.

95

Subsequently, samples were annealed at the crystallization temperature. The spherulities growth

was observed at two different crystallization temperatures of 80 and 90 oC.

3.2.12 Scanning electron microscopy (SEM)

Morphology of the PBS/PBAT blends was captured by an Inspect S50-FEI Company. The

cryofractured samples were used to observe the phase morphology of the blends. A selective

dissolution of polyester in tetrahydrofuran (THF) was used to distinguish the polymer phases. All

the samples were dried and sputtered with gold prior to imaging in order to make them

conductive.

3.3 Results and Discussion


The FTIR analysis was performed to identify the physical and chemical interaction

between the neat polymers during melt blending. The FTIR spectra of PBS, PBAT and

PBS/PBAT blends are shown in Figure 3.1. The carbonyl group frequency of neat polymers and

their blends was detected at 1712 cm-1

and 1716 cm-1

, respectively. The peak of the carbonyl

group was shifted towards higher wavenumbers for the PBS/PBAT blends compared to each of

the neat polymers, which clearly indicates that a strong chemical interaction occurred during the

melt process at 140 oC. Kwei [39] has reported that the shift (from 1722 to about 1705 cm

-1) of

the carbonyl group peak in the blends occurred as a result of chemical interactions between the

parent polymers. John et al., [6] also observed a similar type of transesterification reaction in the

PBS/PCL and PCL/EASTAR blends. These results can be explained by the formation of

copolyester of PBS and PBAT by an ester-ester interchange reaction. The resulting copolyester

which is compatible with the homopolymer of the unreacted PBS and PBAT may act as a

compatibilizer in the blend system. In the present study, no external transesterification catalyst

96

was added into the blends. Even though a transesterification reaction was observed during melt

blending, the amount of reaction gradually decreased with increasing PBAT wt%. This resultant

ester exchange reaction was due to the residual catalysts existing in the homopolymer synthesis,

which was supported by Wang et al., [40-42]. The transesterification product can further enhance

the mechanical performance of the resultant blends. Scheme 3.2 shows the expected chemical

structure of the transesterification product in the PBS/PBAT blends.

Figure 3.1. Evaluation of the normalized FTIR spectra of the carbonyl region (1800-1600 cm-1

)

of PBS, PBAT and their blends

97

Scheme 3.2. Expected transesterification product of PBS/PBAT blend


Figure 3.2 shows the stress-strain curves of PBS, PBAT, and their blends. Neat PBS

showed a higher tensile yield strength but lower elongation compared to neat PBAT. For PBS,

no apparent strain hardening was observed during the tensile test. On the other hand, PBAT

showed excellent elongation and obvious strain hardening regions in the stress-strain curves,

while its’ tensile and yield strength was poor. As for the blends, all the samples presented three

clear regions such as elastic, plastic deformation, and strain hardening. The first region showed

linear stretching with recoverable deformation, followed by the second region which revealed

plastic deformation which is a non recoverable deformation of the samples. The second region

indicated cold drawing behavior after the neck forming occurred in the samples. The third region

showed strain hardening, the tensile stress gradually increasing until the samples broke.

Crystalline slippage was also observed. Interestingly, after the strain increased to 150%, each

composition of the blend showed clear evidence of cold drawing which affected the polymer

chain alignment and resulted in strain hardening. Generally, amorphous and semi crystalline

98

polymer chain entanglement can lead to strain hardening. Strain hardening behavior is of great

importance in polymer processing such as film blowing, thermoforming, and blow molding due

to its good resistance against stretching of polymer segments. Also, strain hardening behavior

can make the process easier and lead to higher quality products [43]. Figure 3.3 shows tensile

strength and percentage elongation data. A significant property improvement was observed in the

blends.

The tensile strength of the PBS/PBAT (70/30 wt%) blend increased by 30% and 148%

over the neat PBS and PBAT, respectively. The percent elongation of PBS/PBAT (70/30 wt%)

blend was 150% higher than neat PBS. Tensile strength improvement is directly related to the

intermolecular forces, crystallinity, miscibility, compatibility, and molecular orientation of the

polymers in the blend [6]. In the present study, tensile strength improvement was directly related

to the amount of PBS present in the blend.

Figure 3.2. Tensile stress-strain curves of PBS, PBAT, and their blends

99

Tensile strength gradually reduced with increasing PBAT concentration in the blend

system, which reveals that the blend transesterification ability was reduced. Furthermore, the

increased PBAT content in the blend system may cause phase separation because of decreased

compatibility in the blend which can lead to the reduction of tensile strength. These results were

also observed from morphological analysis of the blends. John et al., [6] observed that a similar

phase separation occurs when increasing one component in a PBS/EASTAR and PCL/EASTAR

blend leading to reduced tensile strength in the blends. Tensile properties of the PBS/PBAT

blends were sharply increased compare to their parent polymers. Apparently, the PBS/PBAT

blends mechanical properties are comparable with literature polyethylene mechanical properties

[44]. Therefore, we believe that the PBS/PBAT blends can be potential substitute for no-

biodegradable polymers in the packaging applications.

Figure 3.3. Tensile strength and elongation at break of PBS, PBAT, and their blends: (A) PBS;

(B) PBS/PBAT(70/30 wt%); (C) PBS/PBAT (60/40 wt%); (D) PBS/PBAT (50/50 wt%); (E)

PBAT.

100

3.3.3 Melt flow index

MFI measurement is a common technique for studying the flow behavior of the

polymers [45]. Table 3.1 shows the melt flow index values of neat polymers and their blends.

The MFI values of extruded PBAT and PBS were 9 g/10min and 25 g/10min, respectively. After

blending both polymers, MFI of all the blends increased compared to the neat polymers. The

reduction of molecular weight, and changing thermal properties during the melt blending may be

responsible for this [3]. PBS/PBAT (70/30 wt%) blend had the highest melt flow rate compared

to neat polymers and other PBS/PBAT blends. The observed MFI improvement of the blends is

probably attributed to the residual catalyst in the PBS system which is accelerating the PBAT

degradation during MFI measurement. Possible degradation can be occurring by chain scission

of the polymers, depolymerization, oxidative degradation, and transesterification reactions. In

addition, reactive end groups, residual catalyst, unreacted starting monomers in the polymers,

and other impurities can accelerate the thermal degradation of the polymers. A similar

observation has been observed in the literature [46]. Increased MFI of the blends indicate better

flow behavior compared to neat polymers. Therefore, this blend system is suitable to use as a

new matrix system for polymer composites.

Table 3.1. Melt flow index (MFI) of the neat polymers and their blends

Samples MFI (g/10min)

Neat PBS

25.3±2.4

PBS/PBAT (70/30 wt%)

41.5±3.2


33.3±2.8


33.6±1.6

Neat PBAT 9.4±1.8

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3.3.4 Differential scanning calorimetry

The thermal behaviors of the neat PBS, PBAT, and their blends were measured through

non-isothermal DSC analysis. Non-isothermal DSC results of PBS, PBAT, and PBS/PBAT

blends are shown in Figure 3.4. The PBS and PBAT showed melting points at 114 and 116oC,

respectively. Interestingly, the second heating cycle showed a double melting peak for neat PBS

and their blends due to the melt re-crystallization of the polymers. The second heating cycle DSC

result is given in Figure 3.4, the imperfect crystals melt at lower temperatures but the more

structurally perfect crystals melt at higher temperature [3]. Another possible reason may be that

the molecular weight distribution could also affect the melt of the polymers [47]. The

PBS/PBAT blend showed a similar melting behavior to that of PBS. The enthalpy of fusion for

neat PBS and PBAT was found to be 32 and 10 J/g, respectively. The blends showed a high

enthalpy of fusion compared to the neat polymers. The PBAT phase may act as a nucleating

agent for the PBS phase, which will improve the crystallization of PBS in the blend. Another

reason is a change in the regular structure of the interchange reaction product, which may lead to

the production of thicker lamellar crystals that melt with a higher enthalpy of fusion [6]. In all

the blends, single glass transition temperature (Tg) was observed because the Tg values of both

the neat polymers are very close to each other. Therefore, the values may be overlapping. The Tg

value of the blends shifted to lower temperatures compared with that of the neat polymer. A

similar observation was found through DMA analysis. This variation in Tg could be the cause of

an interchange reaction which occurs during the melt blending process and it is also evidence for

compatibility of the polymer in the blends. John et al., [6] have observed similar synergistic

effects in PBS/PCL blends. Miscibility of the binary polymer blends can be predicted from

Gordon- Taylor (G -T) equation (3.1) [48-49]

102

Figure 3.4. Second heating DSC thermograms of PBS, PBAT, and their blends after cooling at 5

°C/min

Tg =

(3.1)

where W1 and Tg1 are the weight fraction and glass transition temperature of PBAT, respectively.

The W2 and Tg2 are the weight fraction and glass transition temperature of PBS, respectively, and

the parameter k is the fitting constant. The Tg values are observed from the DSC analysis. If

k=1, the Gordon and Taylor theory represents a good interaction between two blended

components. If the k value is lower or higher than 1, it indicates poor interaction between the

components in the blend. Figure 3.5 shows the Tg value obtained from the Gordon-Taylor

equation. From the DSC, the Tg values observed for all the blends were -35 oC, close to the G -T

curve. This indicates that the theoretical and experiment Tg values are close to each other. In our

present study, the k value was found to be 0.98; this semi-quantitatively measured value can

further support the interaction, which occurred between the polymers in the blends. The k value

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of the diglycidyl ether of bisphenol-A/poly(ethylene terephthalate) blend was 0.10; the small

value of k suggests that only weak interactions exist between the components in the blend [8,

50]. Richard et al., [50] observed a k value of 0.18 for the PLA/PHBV blend indicating poor

miscibility, which was confirmed through SEM analysis. These results are supportive of our

present work: that the components have a good compatibility in the blend system.

Miscibility and compatibility of the blends can be explained by Gibbs free energy.

Thermodynamically compatible PBS/PBAT blends were analyzed according to the Gibbs free

energy equation (3.2) [51]:

∆Gm = ∆Hm-T (∆Scm+∆S

em) (3.2)

where ∆Gm is Gibbs free energy, T is absolute temperature, ∆Hm is heat of mixing, ∆Scm is

mixing of combinatorial entropy and ∆Sem is the mixing of excess entropy. The molar volume of

the components inversely depends on the combinatorial entropy. Hence, molecular weight of the

polymers is directly related to combinatorial entropy. If the polymers have higher molecular

weight, the ∆Scm becomes zero. Therefore, the system is spontaneous; it could lead to a ∆Gm

which is less than zero while ∆Hm is less than zero. In practical fields, this is rarely possible and

so can be ignored. The ∆Hm is calculated using expression (3.3) [51]:

∆Hm = ( )2 (3.3)

where and are the solubility parameter values of PBS and PBAT; and are the

volume fraction of PBS and PBAT. The solubility parameter ( ) value of the PBS and PBAT

was calculated according to the expression (3.4) [51]:

= (3.4)

104

Where M, G and are the monomer molecular weight, group molar attraction constant of the

polymer, and density of the polymer, respectively. The group molar attraction constant was

calculated by Mark [52]. The Gibbs free energy and solubility parameter values for PBS and

PBAT were calculated by equation 3.3 and 3.4, and the values are given in Table 3.2. Gibbs free

energy values for PBS/PBAT blends are low and very close to each other, indicating that some

extent of compatibility was achieved in the blend system. Previous studies have reported similar

observations for some of the biopolymer blends such as PLA/PCL, PLA/PHBV, and PHBV/PCL

blends, and they too have reported slight miscibility was achieved in their melt blend process [7,

51, 53].

Table 3.2. Solubility parameter values for polymers

Samples

Group Molar attraction

constant G

(J1/2

cm3/2

mol-1

)

Solubility Parameter

(J1/2

cm3/2

)

Gibbs free energy

∆Gm

(J g-1

m-3

)

PBS 2990 20.93 -

PBAT 6154 22.28 -

PBS/PBAT (70/30 wt%) - - 0.382

PBS/PBAT (60/40 wt%) - - 0.437

PBS/PBAT (50/50 wt%) - - 0.455

105

Figure 3.5. Theoretical and experimental values of Tg for PBS/ PBAT blends

3.3.5 Dynamic mechanical analysis

Figure 3.6 shows the storage modulus of neat PBS, PBAT, and their blends. The storage

modulus value of the PBS and PBAT at room temperature was found to be 0.6 and 0.1 GPa,

respectively. PBS had a higher storage modulus compared to PBAT at all temperatures, and their

blends had values in between the PBS and PBAT. A similar trend was observed in the tensile and

flexural modulus values. Reduction in modulus while increasing temperature is attributed to

increasing polymer chain mobility. Generally, above the alpha transition temperature, molecular

motion increases and polymer segments move from glassy to a rubbery state, which is

accompanied by an increase in the molecular relaxation in the polymers [54].

106

Figure 3.6. Storage moduli of PBS, PBAT, and their blends

Figure 3.7 depicts the Tan curves of PBS, PBAT, and their blends. It shows the primary

and secondary transition peaks in neat PBAT at -20 and 62 oC, respectively. The primary

transition peak corresponds to the poly(butylene adipate) segment mobility, and the secondary

transition peak corresponds to the terephthalate unit mobility [55]. The Tg value of the PBS,

PBAT and PBS/PBAT blends was calculated from the maximum height of the Tan peak.

Generally, an incompatible blend shows two transition peaks which correspond to the Tg of

individual components in the system [56]. For a highly compatible and partially compatible

blend, a single transition peak can be seen lying between the transition temperature of individual

components with an increased broadness in the transition peak [9, 57]. In our present study, all

the blends were observed to have a single transition peak. The Tg values of the PBS/PBAT

blends were shifted towards lower temperatures compared to the neat PBS, which is a dilution

effect with the addition of PBAT into PBS. Another possible reason is that for partially or

completely compatible blends, the Tg shifts towards lower or higher temperatures as a function

107

of composition [56]. Moreover, the small variation in the Tg value shows further evidence of an

interchange reaction occurring between the neat homopolymers. The Tg shift was observed by

the influence of a transesterification reaction when polycarbonate was incorporated into the

poly(trimethylene terephthalate) [56].

Figure 3.7. Tan curves of PBS, PBAT, and their blends

3.3.6 Heat deflection temperature

HDT represents the maximum working temperature of materials and is defined as the

temperature at which a material will be deformed by 250 µm under a constant load of 0.455 MPa

[58]. The HDT value of the neat polymers and their blends is shown in Table 3.3. The HDT

value of the neat PBS and PBAT is 88 and 46 oC, respectively. In general, the HDT of

amorphous polymers is low, and close to their glass transition temperature. In the crystalline

polymers, the HDT is close to its melting point [7, 59]. In the present study, a balanced HDT

value of PBS/PBAT blends was observed due to the PBAT having a lower crystallinity and

thermal resistance compared to PBS.

108

Table 3.3. Heat deflection temperatures of the neat polymers and their blends

Samples HDT (oC)

Neat PBS

88.06±0.4


73.66±1.2


70.27±2.1


65.88±2.3

Neat PBAT 46.12±1.5

3.3.7 Thermogravimetric analysis

Figure 3.8 shows the thermal stability of PBS, PBAT, and their blends as a function of

temperature. PBS undergoes cyclic degradation mechanism and some of the predominant

byproducts are anhydrides, olefins, carbon dioxide, and esters [60]. PBAT degradation takes

place by the breakdown of the ester groups and chain scission of C-O and C-C bonds on the

polymer backbone. The onset degradation temperature (Tonset) of PBAT was 377oC and PBS

was 372oC. This suggests that PBAT has slightly more thermal stability compared to PBS. TGA

results reveal that PBS and PBAT present a relatively good thermal stability up to 300 oC.

These data are similar to published literature results [55, 61]. The maximum degradation

temperature (Tmax) was observed at 405, 413, 408, 410, and 413oC for extruded PBS, PBAT,

PBS/PBAT (70:30 wt%), PBS/PBAT (60:40 wt%), and PBS/PBAT (50:50 wt%), respectively.

The Tonset and Tmax of the blends were quite similar to those estimated for PBS and PBAT

homopolymer. All the blends showed single step degradation because the neat polymer

degradation temperatures were close to each other, which was made clear through a derivative

thermogram (Figure 3.9). According to the melt flow rate results, all the blends showed a

molecular weight reduction, but the thermal stability of the blends was gradually increased

compared to neat PBS. This may be improved compatibility between the polymer phases. While

109

increasing the PBAT content, thermal stability increased because PBAT is more thermally

stable compared to PBS. The results indicate that a compatible blend was achieved and supports

the DSC results.

Figure 3.8. TGA curves of PBS, PBAT, and their blends

Figure 3.9. DTG curves of PBS, PBAT, and their blends

110

3.3.8 Rheological properties

Rheological properties were investigated to identify the interaction between polymer

phases in the blends. Figure 3.10 shows the complex viscosity (η*) of the neat polymers and

their blends as a function of frequency at 140oC. Apparently, a higher complex viscosity was

observed in the lower frequency range compared to its higher frequency region. This rheological

behavior of the blends indicates that the blend is a pseudo plastic liquid. Furthermore, the neat

PBS and PBAT exhibited almost Newtonian behavior at below 1 rad/s frequency, and a strong

shear thinning behavior was observed beyond 1 rad/s frequency range.

Figure 3.10. Complex viscosity of PBS, PBAT, and their blends with different weight fractions

of PBAT at 140oC

The PBS/PBAT blends had a higher complex viscosity compared to the neat polymers at

lower oscillation frequencies. At higher frequencies, the complex viscosity of the blends was

between that of the neat polymers. This increased viscosity may have occurred because

transesterification can form pseudo structures [62] that can withstand shear forces. In addition,

111

this is due to phase morphology of the blends and compatibility between the phases. The higher

compatibility between the two phases leads to good dispersion of the discrete phase in the blend

system. As we can see in the SEM image, when PBAT content increases from 30 to 50 wt% in

the blend, the discrete phase (PBAT) morphology is changed form spherical droplet to co-

continuous morphology. This indicates that, the PBS/PBAT 70/30 wt% blend was more

compatible than PBS/PBAT 60/40 and 50/50 wt% blends. Consequently, the PBS/PBAT 70/30

wt% blend had higher melt viscosity than PBS/PBAT 60/40 and 50/50 wt% blend. A similar

behavior of the PLA/PBS and PLA/PBAT blends was reported in literature [63, 64]. In addition,

the transesterification reaction also plays a predominant role in viscosity improvement of the

blends. At lower frequency, the transesterification product acts as a solid-like particle in all the

PBS/PBAT blends and leads to a higher viscosity compared to the parent polymers. However,

the FTIR results showed transesterification product gradually decreased with increasing PBAT

content from 30 to 40 and 50 wt%. The higher content of PBAT reduces the transesterification

reaction in the blend system and thus reduces the viscosity compared to PBS/PBAT 70/30 wt%

blend. Li et a.l [64] have reported similar behavior for PLA/PBAT blends at lower frequencies

and also that the interaction between the polymers can increase the melt viscosity.

Figure 3.11 and 3.12 show the dynamic loss modulus and the storage modulus of PBS,

PBAT, and PBS/PBAT blends. Generally, dynamic loss modulus and storage modulus represent

the amount of energy dissipated in the viscous portion and the ability of a material to store

energy during deformation, respectively. Figure 3.11 shows that the storage modulus (G') of each

sample increased with increase in frequency. It was also observed that with the blending of

PBAT into PBS, there were no changes in the storage modulus (G') at higher frequencies. All the

blends showed a higher storage modulus at lower frequencies compared to the neat polymers.

112

The 70:30 wt% of PBS/PBAT blend had a higher loss modulus (G'') than other blends (Figure

3.12).

Figure 3.11. Loss modulus versus frequency for PBS, PBAT, and their blends with different

weight fractions of PBAT at 140oC

When the PBAT phase was finely dispersed in the blend, the fine dispersal could be

reason of interaction existing between the two phases. Stronger interactions were observed in the

PBS/PBAT (70:30 wt%) blend, which were exhibited as a higher loss modulus. In addition, the

increased storage modulus is attributed to the PBAT molecular chain entanglement with PBS

molecular chain mediated by transesterification product acting as a compatibilizer. The higher

entanglement density of the blends would store more recoverable energy. Generally, the

entanglement density of the blend is depends on the existing compatibility between the two

phases. The PBS/PBAT 70/30 wt% blend had more compatibility than PBS/PBAT 60/40 and

50/50 wt% blends as shown in SEM. The higher entanglement density of the PBS/PBAT 70/30

wt% blend leads to higher storage modulus. However, the storage modulus gradually decreased

113

at lower frequency while increasing PBAT content in the blends. A similar trend was observed in

the complex viscosity. This is consistent with higher trasesterification product present in 70/30

wt% blend. The reduced storage modulus of the blends is due to the morphology changes [64]

and entanglement density due to lower lower transesterification product present in the blend.

Figure 3.12. Storage modulus versus frequency for PBS, PBAT, and their blends with different

weight fractions of PBAT at 140oC

The Cole–Cole plot was used to explain the phase structure of PBS/PBAT blends and the

plot was performed between the real and imaginary viscosity components of the blends. If the

blend gives a single arc curve, it can suggest phase homogeneity at the melt stage [65].

Furthermore, if any deviations from a single arc are observed, they are evidence for

inhomogeneous morphology and phase separation occurring in the blends due to a second

relaxation mechanism occurring in the samples. The Cole-Cole plot for PBS/PBAT blend at

140oC is depicted in Figure 3.13.

114

Figure 3. 13. Cole–Cole plot of the PBS/PBAT blends at 140oC

A second circular arc was observed on the right-hand side of the curve, and it is clear

evidence for a second relaxation mechanism happening for all PBS/PBAT blends. Nevertheless,

when the PBAT reaches 40 and 50 wt% in the blends, the blends showed a tail on the right hand

side of the plot. This is probably due to the phase inversion occurred in the blends. This result

shows that co-existing phase morphology was formed in the entire blend system and that the

formed morphology may be the droplet-matrix or co-continuous phase morphology.

Consequently, the Cole–Cole plot shows an inhomogeneous morphology formed in PBS/PBAT

blend systems. There have been reports of PLA/PBAT blend systems with similar observations

of phase behavior when the PBAT concentration is more than 30 wt% in the matrix [64]. The

Cole-Cole plot clearly shows that the PBS/PBAT 70/30 wt% blend is more heterogeneous

compared to the 50/50 wt% blend.

115

3.3.9 Polarizing optical microscopy

The spherulite morphology of the blends was investigated by optical microscopy. The

dark and light regions represent the amorphous and crystalline phases, respectively. Figure 3.14

(a) shows that PBS/PBAT blends were annealed at a crystallization temperature of 80oC for 30

min. When the PBAT concentration was 50 wt% in the blend, an increased number of PBS

spherulites were observed with decreasing spherulite size. This decrease in size suggests that the

PBS chain mobility was disrupted and PBAT acted as a nucleating site to promote the formation

of crystalline nuclei. A PBS/polyvinylidene fluoride (PVDF) blend exhibited a similar

phenomenon when PVDF was the predominant species in the blend compared to the PBS [66].

The PBS chain mobility was slowed in the presence of highly viscous PBAT, thereby decreasing

spherulite growth. With increasing PBAT content from 30 to 50 wt% in the blends, the texture of

PBS spherulites became coarse. Figure 3.14 (b) shows the PBS/PBAT blends after being

annealed at a crystallization temperature of 90oC for 30 min. The increasing PBAT composition

in the blends caused a reduced number of spherulites because PBAT has less crystallinity and the

PBS chain mobility increases at the crystallization temperature of 90oC compared to 80

oC. With

increasing PBAT content in the blends, the spherulites became rougher. For 30 wt% PBAT

content in the blend, the PBS spherulites were uniformly distributed with uniform dimensions

after being annealed for a given time. Consequently, our present results conclude that the

spherulite size controls the mechanical properties and morphology of the blends. For example,

the large spherulite size leads to the fracture along the spherulite boundaries and only

occasionally through the spherulites. Therefore, small spherulite size can yield better tensile

stress as well as strain [67].

116

Figure 3.14. (a) Photograph of the film annealed at 80oC: (i) PBS; (ii) PBS/PBAT (70/30 wt%);

(iii) PBS/PBAT (60/40 wt%) and (iv) PBS/PBAT (50/50 wt%). Figure (b) Photograph of the

film annealed at 90oC: (i) PBS; (ii) PBS/PBAT (70/30 wt%); (iii) PBS/PBAT (60/40 wt%) and

(iv) PBS/PBAT (50/50 wt%)

117

3.3.10 Scanning electron microscopy

Phase morphology of the blends depends on the second components, processing

parameters, molecular weight of the virgin polymers, and compatibility between the polymers.

If the blending components have a similar melt viscosity, the resulting morphology will be

very fine and both polymers will be uniformly distributed throughout the blend whether it is

the minor or major phase. The same is true if the blend consists of similar melt viscosity

components. If the minor phase has a lower or higher viscosity compared to the major phase, it

leads to the spherical domains of finely or coarsely dispersed morphology in the matrix. As

shown in Figure 3.15, phase morphology in the blend was identified by the solvent et al.,hing

method. Being a good solvent for PBAT while unable to dissolve PBS, THF was used as the et

al.,hing solvent. The observed morphology of PBAT phase selectively removed from the

blends without disturbing the PBS matrix is shown in Figure 3.15 (b). The PBAT phase was

completely extracted under these conditions. The corresponding unextracted samples are

shown in Figure 3.15 (a). This indicates that the holes are the extracted PBAT phase by THF.

The surface morphology of the blend reveals that spherical PBAT particles were uniformly

distributed throughout the matrix. Finer dispersions were observed in lower PBAT composition

in the blend. When the PBAT composition was increased in the blend, the domain shape and

size gradually changed due to the coalescence phenomenon. This may also act to decrease the

tensile strength of the blends.

118

Figure 3.15. (a) SEM images of PBS and PBAT blends (left hand side) (i) PBS/PBAT (70/30

wt%); (ii) PBS/PBAT (60/40 wt%) and (iii) PBS/PBAT (50/50 wt%) . (b) SEM images of PBS

and PBAT blends surface after et al.,hing with THF (right hand side): (i) PBS/PBAT (70/30

wt%); (ii) PBS/PBAT (60/40 wt%) and (iii) PBS/PBAT (50/50 wt%)

119

3.4 Conclusions

We have succeeded in fabricating a high performance and biodegradable PBS/PBAT

blend through the melt blending technique. There is a significant improvement in tensile strength

and elongation by the incorporation of PBAT into PBS, indicating that a good level of

compatibility is achieved between the polymers. The observed compatibility is caused by the

formation of copolyester due to transesterification between the neat polymers, which was

confirmed by FTIR analysis. DSC and DMA analysis suggested that the blends show

compatibility between the PBS and PBAT. The rheological properties of blends such as the

complex viscosity, loss modulus, and storage modulus were increased with the addition of PBAT

into PBS. As the PBAT composition was increased, phase morphology changes occurred in the

blends, leading to decreased values of complex viscosity, loss modulus, and storage modulus.

The phase morphology of the PBS/PBAT blends shows a two phase structure in which PBAT is

the minor phase. Furthermore, polarizing optical microscopy analysis revealed that the PBAT

has disturbed the spherulite growth of the matrix. The prepared biodegradable PBS/PBAT blend

is a potential substitute for non-biodegradable packaging films, blow molding bottles and

flexible tubes.

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124

Chapter 4: Preparation and Characterization of Maleic Anhydride Grafted Biodegradable

Polyesters

Abstract

Maleic anhydride (MAH) grafting was quantified onto poly(butylene adipate-co-

terephthalate) (PBAT), poly(butylene succinate) (PBS), and their blend in the presence of free

radical initiator (dicumyl peroxide, DCP) by reactive melt processing in an internal batch mixer.

Fourier transform infrared (FTIR) spectroscopy confirmed the successful grafting of MAH on

the polyesters backbone. The dependence of grafting yield on initiator and MAH concentration

was calculated by back-titration. Compared to the MAH grafted PBS, the MAH grafted PBAT

and MAH grafted PBS/PBAT blend had slightly lower grafting yield over the entire range of

DCP concentrations. This difference in grafting yield could be due to the structural difference

and proton abstraction capability of the polymer backbone. Then, MAH grafting efficiency of

batch and continuous process was compared. The result shows that the batch processed samples

had slightly higher grafting yield than continuous processed samples. The slightly higher grafting

yield as observed in the internal batch process may be attributed to higher residence time and air

contact of the reaction medium. The crystallization and melting temperatures of the MAH

grafted samples were significantly decreased compared to their counterparts. This is possibly due

to the MAH group preventing the lamella growth and nucleation of MAH grafted samples, thus

leading to the formation of imperfect crystal structure compared to the virgin polymer. The onset

temperatures for thermal degradation of the MAH grafted samples were reduced compared to

neat polymers while their maximum and final decomposition temperature remained unchanged.

125

4.1 Introduction

Recently, biodegradable polymeric materials have gained great interest because of certain

environmental concerns in using non-biodegradable polymers. In order to reduce the problems

regarding the non-biodegradable polymer disposal in the environment, some of the

biodegradable polymers have been commercialized. However, many biodegradable polymers are

expensive compared to the commonly used polymers. Therefore, the use of biodegradable

polymers as a matrix for producing biocomposites will result in products with higher costs.

Increase in the cost of biocomposites could be mitigated by adding low-cost natural fillers/fibers

to make them cost competitive with traditional non-biodegradable composites.

Nevertheless, there are some technological challenges in successfully developing a

natural fiber/bioplastic composite. The hydrophilic character of the natural fibers/fillers hinders

its compatibility with the hydrophobic polymer matrix due to its lack of interfacial adhesion. The

incompatibility between the reinforcement and polymer leads to weak stress transfer from one

phase to another phase. This weak stress transfer causes inferior mechanical properties of the

resulting composites. Furthermore, very often polymer blends and composites are

thermodynamically immiscible. In most cases, immiscibility of the components of the

composites and blends is due to solubility parameter difference. Often such blends and

composites are not able to target specific applications. The immiscibility and poor compatibility

of the composites and blends can be overcome through a compatibilizer. A compatibilizer helps

to improve the better stress transfer between the components through interfacial adhesion,

uniform dispersed phase and reduced surface tension between the two phases [1].

Generally, the compatibilization of blends/composites can be performed in two ways i.e.,

non-reactive compatibilization and reactive compatibilization. Usually block copolymers are

126

used as a non-reactive compatibilizer because one constitutive block is miscible with one blend

component while the second block is miscible with the other component. This type of

compatibilizers is not economically viable for industrial applications and it is not used

extensively as a compatibilizer [2]. A reactive compatibilizer is a good choice for producing

compatibilized composites/blends. The reactive compatibilization involves forming the block or

graft copolymer in-situ during blend preparation via interfacial reaction of added functionalized

polymeric components. The maleic anhydride (MAH) grafted polymer is a well-known reactive

compatibilizer. For example, maleated polypropylene is extensively studied as a compatibilizer

in polypropylene based blends [3] as well as composites [4].

The polymer grafting reaction has a predominant role in preparing reactive compatibilizer

because it can introduce new reactive functional groups on the polymer backbone. The grafting

can be performed in different ways such as melt state, solution state, solid state, irradiation,

suspension in aqueous/organic solvents [5, 6] and ball milling [7]. Among them, melt state

reactive extrusion is a very common and economical way to create the grafted polymers. A

variety of functional monomers has been used as grafting agent on the polymer backbones [8-

10]. MAH is a very common functional monomer for grafting because of its unique free radical

reactive double bonds, reactive anhydride groups, and poor homopolymerization tendency during

free radical graft polymerization [11]. The poor homopolymerization of MAH enhances the

grafting efficiency of the resulting grafted polymers. In addition, MAH functional groups have

high reactivity with specific functional groups such as carboxyl, amine, and hydroxyl [12]. The

MAH grafting onto molten polypropylene (PP) [13], polyethylene (PE) [14], and ethylene-

propylene-diene (EPDM) terpolymer [15] has been investigated in the literature. In addition, the

MAH grafted polyolefins are commercially available in the market and act as a good

127

compatibilizer for PP and PE based blends/composites [16, 17]. Thus, MAH reactive functional

group on the polymer backbone significantly reduces the dispersed domain size and surface

tension, and increases the interfacial adhesion in the composites/blends.

Recently, this research has expanded to the biodegradable/biocompostable polymers like

polylactide (PLA) [18], poly(hydroxybutyrate-co-valerate) (PHBV) [19], poly(butylene adipate-

co-terephthalate) (PBAT) [20], poly(butylene succinate) (PBS) [21], and polycaprolactone (PCL)

[22]. MAH grafted copolymers have been show to be a good compatibility enhancer in the

biodegradable polymer based blends [23] and composites [24]. Research findings confirmed that

a small quantity of maleated compatibilizer increased the compatibility between two

thermodynamically immiscible materials. Few researchers have done detailed research on maleic

anhydride grafting onto PLA backbone by melt process [25, 26]. The use of MAH grafted PLA

as a compatibilizer has been studied while investigating the performance of composites materials

[18, 27, 28]. This literature has clearly shown that the compatibility was improved between the

matrices, and reinforcement through MA grafted PLA meanwhile improved their mechanical

performances. Similar observations were found in some other biodegradable polymer composites

with MAH grafted compatibilizers [29-31]. Many researchers have performed maleation of

biodegradable and non-biodegradable polymers but none of the literature was reported maleation

of PBS/PBAT blend.

This study synthesized and characterized MAH grafted biodegradable polymers. The

MAH grafted PBS, PBAT and PBS/PBAT blend were synthesized by varying initiator

concentration from 0.5 to 1 phr while keeping MAH concentration constant at 5 phr. The

grafting percentage was determined by back-titration method. In addition, the maleated samples

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were characterized by thermal analysis, and FTIR spectroscopy. In addition, melt viscosity and

elasticity of the MAH grafted samples was studied by rheological analysis.

4.2 Materials and Methods

Commercially available PBAT (Biocosafe 2003) and PBS (Biocosafe 1903) pellets were

procured from Xinfu Pharmaceutical Co., Ltd, China. PBS, PBAT and their blend (60/40 wt%)

were processed by micro compounder (DSM Xplore®, The Netherland) using 140

oC processing

temperature. General properties of the virgin PBS, PBAT and blend of PBS/PBAT (60/40 wt%)

were measured and are reported in Table 4.1. Organic free radical initiator, dicumyl peroxide

(DCP-99% purity) was purchased from Acros Organics with half-life time about 9.2 s at 150°C

and maleic anhydride (MAH) was obtained from Sigma-Aldrich, USA. All the materilas were

used as received.

Table 4. 1. Properties of the neat PBS, PBAT and PBS/PBAT blend

Polymers Densitya

(g/cm3)

Melting

Pointb (

oC)

Glass Transition

Temperatureb (

oC)

Melt Flow Indexc

(g/10 min)

PBAT 1.26 115 -34 9.4±1.8

PBS 1.26 114 -31 25.3±2.4

PBS/PBAT (60/40 wt%) 1.26 114 -35 33.3±2.8

a: Measured by Archimedes method, b: Measured by DSC with heating rate 10 oC/min, c: 190

oC

with 2.16 kg

4.2.1 Synthesis of MAH grafted PBS, PBAT and their blend

Before processing the polymer pellets were dried at 80oC for 12 h. Based on our previous

research results, PBS/PBAT (60/40 wt%) blend was chosen as an optimum blend for further

study [33]. Hereafter, PBS/PBAT (60/40 wt%) blend will be referred to as PBS/PBAT blend.

Maleation of PBS (MAH-g-PBS), PBAT (MAH-g-PBAT) and PBS/PBAT blend (MAH-g-

PBS/PBAT) were prepared by varying the initiator concentration. The MAH-g-PBS, MAH-g-

129

PBAT, and MAH-g-PBS/PBAT samples were prepared in a laboratory-scale internal batch mixer

(Torque Rheometer, Haake PolyLab QC, Thermo scientific). The formulations are shown in

Table 4.2. The batch mixer barrel was divided into three heating zones. The temperatures of

these three heating zones were kept constant at 160oC with a screw speed of 60 rpm and reaction

time of 6 min for synthesizing all the samples. Reaction time of 6 min was divided into three

steps. In the first step, polymers were melted at 160oC for 2 min and in the second step a free

radical initiator was introduced into the molten polymers to make reactive sites for 2 min.

Finally, MAH was added into the reaction medium and reaction was carried out for another 2

min to obtain desired amount of grafting content onto polymers. It’s known that

homopolymerization of MAH is difficult with molten polymers because of the 1,2-disubstituted

double bond present in the MAH. In order to diminish the homopolymerization of the MAH in

the reaction medium, the reaction was carried out above the ceiling temperature (150oC). During

the reaction, the reaction mixture torque was monitored over time showing a significant

increment. The increased torque was likely due to grafting and gel formation occurred after

addition of MAH and DCP into polymers. The MAH grafted samples were taken out and ground

into small pieces for further analysis.

In addition, the MAH grafting of PBS/PBAT was performed in a continuous process

using a Leistritz extruder Micro 27, USA. The extruder had three kneading disc block regions

and self-wiping co-rotating twin screws. The extruder was consisting of 11 heating zones with

electric heating and a water cooling system. Appropriate amount of monomer, initiator and

polymers were manually premixed and fed into the hopper with a feed rate of 5 kg/h. In order to

compare batch process with continuous process, the continuous process was performed with

similar batch processing parameters such as processing temperature and screw speed. In

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continuous process, 11 heating zones were set at a constant temperature of 160oC and screw

speed of 60 rpm. The MAH grafted molten materials were pumped through the extruder die,

quenched and pelletized.

Table 4.2. Proposed formulation for producing maleation of PBS, PBAT, and PBS/PBAT blend

Samples DCP (phr) MAH (phr)

MAH-g-PBS 0.5 5

MAH-g-PBS 0.7 5

MAH-g-PBS 1.0 5

MAH-g-PBAT 0.5 5

MAH-g-PBAT 0.7 5

MAH-g-PBAT 1.0 5

MAH-g-PBS/PBAT blend 0.5 5



4.2.2 Grafting mechanism

Melt state free radical grafting mechanism is a widely accepted method in academia and

industry. In the presence of peroxide free radical initiator, the possible MAH grafting reaction of

PBS and PBAT is shown in Figure 4.1 and 4.2 [20, 34]. Basically, the free radical mechanism

occurs in three steps of initiation, propagation and termination. The initiation step is well

established, which is the homolytic rupture of the initiator and produces free radicals at higher

temperature (peroxide decomposition temperature). Then the second step, propagation, is to form

macroradicals through abstraction of hydrogen atoms from the polymer α-carbon atom.

131

Figure 4.1. Proposed reaction mechanism of MAH grafted PBS (MAH-g-PBS) [34]

132

Figure 4.2. Proposed reaction mechanism of MAH grafted PBAT (MAH-g-PBAT) [20]

133

Once the radicals are formed, they can directly react with MAH onto the polymer

backbone. Finally, grafting process is terminated by recombination of radicals. This termination

reaction leads to the formation of succinic type anhydride structure on the polymer chains that

further undergo β-scission reaction on the grafted copolymers (Figure 4.1 and 4.2). There are

other possibility to form chain scission reaction either through backbiting or thermo hydrolysis

reactions [26]. Thermodynamically, when the maleation reaction takes place above ceiling

temperature, the homopolymerization of MAH (poly-MAH) grafting will not occur on the

polymer backbone [35]. In addition, if the grafting reaction takes place above ceiling temperature

the formed homopolymers (poly-MAH) decompose, unlike other functional monomers [36].

4.2.3 Purification of MAH grafted samples

The purification of the MAH grafted samples was performed according to a modified

procedure [20]. The unreacted MAH was removed by vacuum drying at 80oC for 24 h. Vacuum

dried MAH grafted sample was dissolved in chloroform at room temperature overnight. After

dissolution of MAH grafted sample, they were selectively precipitated in methanol and filtered.

The filtered sample was repeatedly washed with excess methanol to remove residual MAH and

DCP, followed by drying at 80oC under vacuum for 24 h. These dried samples were used for

further analysis.

4.2.4 Determination of grafting percentage

The MAH grafting percentage was determined by back-titration, which was modified

from Nabar et al., [20]. The purified MAH grafted sample was dissolved in 100 ml of

chloroform at ambient temperature for 2 h. Subsequently, the MAH groups were hydrolyzed into

carboxylic acid by adding a few drops of 1 N hydrochloric acid. Immediately, the solutions were

titrated against 0.025 N alcoholic KOH in the presence of few drops of phenolphthalein

134

indicator. Under this condition, maleated samples were completely soluble in chloroform and did

not precipitate during titration against alcoholic KOH. The grafting percentage was calculated as

follows:

Grafting Percentage (%) =

x98.06x100 (4.1)

where is the weight (g) of the maleated sample, is the volume of the KOH in liters,

is the normality of KOH dissolved in methanol, and MAH molecular weight is 98.06

(g/mol).

4.2.5 Gel percentage measurement

Soxhlet extraction was used to measure the gel percentage of the samples. Both grafted

and ungrafted PBS, PBAT and their blend are completely soluble in chloroform but cross-linked

PBS, PBAT, and blend of PBS/PBAT were found to be insoluble in chloroform. Based on the

PBS, PBAT and their blend solubility, the extraction was perfromed by refluxing chloroform for

24 h. After 24 h extraction, the obtained insoluble fractions on the filter paper were dried at 60oC

for 12 h to remove the residual chloroform. The dried filter papers were weighed, and gel (cross-

linked) content was calculated as follow:

Gel percentage =

X100 (4.2)

where Wi and Wf are the initial weight of sample in the filter paper and the final weight of

sample left in the filter paper after extraction, respectively. The gel content measurement was

repeated at least three times.

135

4.2.6 Fourier transform infrared (FTIR) spectroscopy

Structural differences between the samples were evaluated by FTIR spectroscopy

(Nicolet 6700-Thermo scientific) at room temperature with 36 consecutive scans. The analysis

was performed by measuring transmittance versus wavenumbers in the range of 4000-400 cm-1

at a resolution of 4 cm−1

.


Thermal stability of the ungrafted and MAH grafted PBS, PBAT and their blend was

measured using a TA Q500 Instruments. The experiment was performed in the N2 atmosphere at

a flow rate of 60 mL min-1

with a heating rate of 20 oC/min.


The melting temperature (Tm), crystallization temperature (Tc) and crystallization

enthalpy (∆Hc) of MAH grafted and ungrafted samples were analysised by TA Q200 DSC

Instruments. Accurately weighed samples were encapsulated into the aluminum pan and placed

into the DSC machine. The samples were heated in the presence of N2 atmosphere with a flow

rate of 50 mL min-1

. The reported melting temperature and crystallization temperature were

collected from the second heating and first cooling scans, respectively.


4.3.1 Infrared spectroscopy

The PBS, PBAT, PBS/PBAT blend and their respective MAH grafted samples were

analyzed trough FTIR spectroscopy (Figures 4.3-4.5). The FTIR spectra of MAH, PBS and

maleated PBS samples are shown in Figure 4.3. The broad and medium intensity peak at 955 cm-

1 is attributed to –C–OH bending of carboxylic groups in PBS. The stretching vibration of ester

carbonyl (>C=O) group was observed at 1718 cm-1

. Most of the saturated hydrocarbons contain

136

methyl groups. These methyl groups show a symmetric stretching band at 2962 cm-1

and an

asymmetric stretching band at 2872 cm-1

. In PBS, methyl and methylene C-H stretching bands

occur at 2945 and 2854 cm-1

, respectively. Two new small peaks (1859 and 1788 cm-1

) were

formed in MAH grafted PBS, which corresponds to the saturated cyclic anhydride carbonyl ring

(succinic anhydride group) [37, 38]. These characteristic peaks confirm that the MAH moieties

were successfully grafted on to the PBS backbone. The characteristic functional group of alkyl

ether is C-O-C. A strong symmetric C-O-C stretching peak can be seen at 1151 cm-1

in PBS,

which confirms that the PBS contains an alkyl ether group. In Figure 4.3, observed peaks at 955,

806, and 654 cm-1

were attributed to C-O stretching, CH2 in OC(CH2)2CO in-plane bending, and

–COO bending bands of the PBS and MAH grafted PBS. These peaks are clear evidence to

differentiate PBS from other polymers.

Figure 4.3. FTIR spectra of MAH, neat PBS and MAH-g-PBS with 1 phr DCP and 5 phr MAH

FTIR spectra of MAH, PBAT and maleated PBAT samples are presented in Figure 4.4.

The neat PBAT had peaks at 2951, 2864, 1716, 1463, 1402, 1259, 1157, 1111, 729 and 875 cm-1

.

137

The maleated PBAT sample shows additional peaks at 1786 and 1855 cm-1

compared to neat

PBAT. These additional peaks correspond to symmetric and an asymmetric stretching for the

carbonyl functionality of MAH [20]. The observed peaks at 2951 and 2864 cm-1

were assigned to

-CH3 and -CH2- stretching vibrations. The band at the 1111–1259 cm–1

results from the –C–O–

C– group in the PBAT ether linkage. The peak at 1716 cm–1

was attributed to the C=O stretching

of ester groups in PBAT. Peaks at 875 and 729 cm-1

were due to out-plane bending vibration of

=C–H in benzene ring. Generally, out-plane bending vibration of =C–H group in benzene ring

should occur at 830 cm-1

. In PBAT, conjugated C=O groups were influenced and the benzene

ring out-plane bending of =C–H groups appeared at 729 cm-1

. This is a characteristic peak of

PBAT in the FTIR spectra. Furthermore, PBAT has characteristic peaks at 729, 1111 and 1157

cm-1

and they can be used as identification of PBAT structure [39].

Figure 4.4. FTIR spectra of the MAH, neat PBAT and MAH-g-PBAT with 1 phr DCP and 5 phr

MAH

138

FTIR spectra of MAH, PBS/PBAT blend and MAH grafted PBS/PBAT blend were

scanned separately and are shown in Figure 4.5. Peaks at 2956 and 2860 cm-1

were assigned to -

CH2- and -CH3 stretching vibration in the PBS/PBAT blend. More specifically, the peak at 1716

cm-1

was due to the C=O stretching vibration in the ester groups. The bands at 1259 and 1157

cm-1

attributed to the stretching vibration of ether groups in the PBAT and PBS, respectively.

The out plane and in plane bending vibration of terephthalic acid (benzene =C-H) unit in the

PBAT was observed peaks at 731 and 1036 cm-1

. The succinic acid -COO bending mode peak

appeared at 648 cm-1

and the succinic acid -CH2- group peak was observed at 806 cm-1

.

However, MAH grafted PBS/PBAT blend exhibited two distinguish peaks at 1857 and 1782 cm-

1, which were assigned to symmetric and asymmetric stretching of C=O groups in the grafted

MAH functionality [5]. This literature suggests that the FTIR absorption peak near 1850 cm-1

(symmetric stretching) and a peak near 1780 cm-1

(asymmetric stretching) are due to vibration

of anhydride. As a result, our FTIR spectra confirmed that the MAH groups are grafted on the

PBS/PBAT polyester backbone.

The characteristic peaks of MAH-g-PBS/PBAT (1857 cm-1

) were between that of MAH-

g-PBS (1859 cm-1

) and MAH-g-PBAT (1855 cm-1

). This suggests that the MAH was

successfully grafted on both the PBS and PBAT backbone in the PBS/PBAT blend. A similar

trend was observed by Li et al., [40] in the MAH grafted polyolefin (PE/PP) blend. In addition,

elimination of unreacted MAH in the grafted samples can be confirmed by disappearance of the

MAH C=C characteristic peak at 684 cm-1

in the FTIR spectra [13]. The FTIR peak at 1590 cm-1

belongs to the MAH C=C stretching. This cyclic C=C peak was not found in the MAH grafted

PBS, PBAT or PBS/PBAT blend, which suggests that the unreacted MAH was not present in the

MAH grafted samples.

139

Figure 4.5. FTIR spectra of MAH, neat PBS/PBAT blend and MAH-g- PBS/PBAT blend with 1

phr DCP and 5 phr MAH

4.3.2 MAH grafting percentage calculation

The MAH grafting percentage can be controlled by many parameters including reaction

temperature, rotor speed, resistance time, and monomer to initiator ratio [5, 22]. In this literature,

initiator concentration was changed upto 1% while MAH concentration, screw speed, and

processing temperature were kept constant. The main reason for keeping MAH content constant

was based on literature suggesting a higher influence on the grafting efficiency of the

concentration of initiator (DCP) than concentration of MAH [5]. Mani et al., [5] studied the

MAH grafting of PBS, poly(butylene succinate adipate) (PBSA), PLA, and Eastar co-polyester

with various initiator and MAH concentrations at different temperatures. They found that there is

more possibility to form cross-linking when the initiator concentration higher than 1%.

Therefore, in the current study, initiator concentration was kept below 1 phr for all the

140

experiments. Moreover, the MAH grafting reaction was conducted at only one temperature (160

oC) because previous researchers have reported that the temperature does not have a significant

influence on the MAH grafting efficiency of polyesters backbone [20, 26].

The MAH grafting was performed in a twin-screw extruder as well as through an internal

batch mixer. The MAH grafting percentage of the internal batch processed samples is shown in

Table 4.3. The grafting yields of the MAH-g-PBS, MAH-g-PBAT and MAH-g-PBS/PBAT

blend were gradually increased with increasing DCP concentration from 0.5 to 1 phr. However,

the MAH-g-PBS in the presence of 0.7 and 1 phr DCP initiator had a grafting yield of 2.45 and

2.56%, respectively. This observed small difference in the grafting yield may be due to the faster

termination reaction of free radicals at higher free radical concentrations in the reaction medium

[40].

Table 4.3. MAH grafting percentage of the PBS, PBAT and PBS/PBAT blend

Samples DCP (phr) MAH (phr) Grafting Percentage (%)

MAH-g-PBS 0.5 5 1.6

MAH-g-PBS 0.7 5 2.45

MAH-g-PBS 1.0 5 2.56

MAH-g-PBAT 0.5 5 1.34

MAH-g-PBAT 0.7 5 1.56

MAH-g-PBAT 1.0 5 1.90

MAH-g-PBS/PBAT blend 0.5 5 1.16



141

The MAH grafting onto the PBAT was performed in the presence of 0.5 phr DCP and 5

phr MAH, and the grafting yield observed was 1.34%. In a previous study, Chen and Zhang [29]

performed the MAH grafting onto PBAT with initiator and MAH concentrations of 0.5 and 5

phr, respectively. They also found that the MAH grafting yield was 1.34%.This result suggested

that the grafting yield variation in the PBS and PBAT found in the current study is reasonable.

When comparing grafting efficiency of PBS and PBAT with 1 phr DCP and 5 phr MAH, PBS

showed higher MAH grafting yield than PBAT (Table 4.3). This observed difference in MAH

grafting yield could be due to the structural difference and proton abstraction capability of the

polymer backbone. The same trend was observed in MAH grafted PBS/PBAT blends. In PBAT,

the observed lower MAH grafting yield compared to PBS may be attributed to a lower number of

free radical formations on its backbone than the PBS backbone. This lower number of radicals

leads to lower grafting efficiency [5]. In addition, the grafting yield difference may be due to the

viscosity difference between the polymers. The comparatively lower viscosity of PBS compared

to PBAT might allow the MAH to better disperse in the reaction medium and it can enhance

their grafting yield. This phenomenon may be limited in the high viscosity PBAT. An optimal

concentration of initiator and monomer are required to achieve a better grafting efficiency. The

present study shows that the optimal concentration of initiator and grafting monomer were 1 and

5 phr, respectively.

There is literature available for comparison of MAH-g-PBS and MAH-g-PBAT in an

internal mixer and continuous mixer [20, 21, 29, 30, 34, 41]. There is no literature available for

comparison of MAH-g-PBS/PBAT blend in both internal mixer and continuous mixer.

Therefore, in the present study investigated that MAH-g-PBS/PBAT blend in batch as well as in

continoues process. In both these processes, 1 phr DCP and 5 phr MAH were used to graft MAH

142

onto PBS/PBAT blend. The batch process and continuous process grafting yields were 2.05 and

1.65%, respectively. The grafting efficiency was significantly lower in continuous processed

MAH-g-PBS/PBAT blend compared to in batch process. This significantly lower grafting yield

in continuous process may be due to lower residence time compared to batch process. Generally,

the total residence time in a twin-screw extruder is lower when compared to batch mixture [42].

The slightly higher grafting yield as observed in internal batch process may be attributed to

higher residence time and air contact of the reaction medium [37]. Similarly, the internal batch

process had higher grafting efficiency than intermeshing co-rotating twin-screw extrusion

process [43]. Moreover, the grafting yields for MAH-g-PBS, MAH-g-PBAT and MAH-g-

PBS/PBAT blends were higher than that the commercially available MAH-g-PP (grafting yield

between 0.5–1.2%) [4]. Therefore, we believe that MAH grafted PBAT, PBS, and their blend

samples might act as an effective compatibilizer in their composites.

4.3.3 Gel content measurement

Generally, in the presence of DCP initiator, PBAT and PBS can form complex reactions

such as copolymers of PBAT and PBS (e.g. PBAT-g-PBS) as well as cross-linked/branched

PBAT and PBS [44]. The gel formation (cross-linking) was investigated by a solvent extraction

method and the results are shown in Figure 4.6. The gel content of the MAH grafted PBAT, PBS

and their blend steadily increased with increasing DCP content. This is reasonable because

increasing DCP content increases the free radical formation. Consequently, the formed excess

radicals can easily react with each other and leads to a cross-linking reaction. It was observed

that the PBS had more gel content than PBS/PBAT blend and PBAT in three different

concentrations of DCP content with the same MAH content (5 phr). The gel formation may be

reduced by increasing MAH concentration in the reaction medium. Increasing MAH content in

143

the reaction medium can react with excess free radicals on the polymer backbone. This may

increase the grafting percentage while reducing the gel formation. Ma et al., [44] examined

cross-linking behavior of PBS, poly(hydroxybutyrate-co-valerate) (PHBV), and PBS/PHBV

blend using DCP and they found that PBS forms more cross-linking than PHBV and PBS/PHBV

blend. A similar type of cross-linking reaction was observed in MAH grafting on PLA in the

presence of DCP [25, 45]. They found that the cross-linking of the PLA increases with

increasing DCP content.

Figure 4.6. Gel content of MAH grafted PBS, PBAT, and PBS/PBAT samples with 5 phr MAH

and different concentration of DCP

A study [46] reported that the gel formation could be controlled by adding styrene as a

co-monomer in the free radical initiated MAH grafting reaction. This co-monomer not only

reduces the gel formation in the reaction but also accelerates the grafting yield and grafting

reaction rate in the melt state. Nevertheless, using styrene as a co-monomer is not recommended

for certain applications. Therefore, it needs to be replaced by some other co-monomers [47]. A

144

few researchers have tried to prevent the gel formation during MAH grafting of polyolefin by

using electron donor additives [48]. Although, they found a gel-free MAH grafted polyolefin

product, the grafting yield was reduced.


Figure 4.7 shows the thermogravimetric result of neat and maleated PBS, PBAT and their

blends with respect to the temperature. Thermal stability of PBAT was higher than PBS and

PBS/PBAT blend. The onset degradation temperatures of the samples were detected from the

deviation of the baseline thermogravimetric curve by tangent plots. MAH grafted PBS, PBAT,

and blend of PBS/PBAT showed lower onset thermal degradation compared to neat PBS, PBAT

and PBS/PBAT. This onset of thermal degradation reduction may be attributed to the polymer

chain scission occurs during maleation reaction. Similar thermal stability reduction was found in

maleated PLA with peroxide initiators [25, 26]. However, maximum and final degradation

temperatures were not heavily affected in the current study.

Figure 4.7. TGA thermograms of neat and maleated PBS, PBAT and PBS/PBAT blend (the

maleated samples were obtained with 1 phr DCP and 5 phr MAH)

145


In polymer processing, non-isothermal DSC analysis is more practical interest than

isothermal DSC analysis. The heating and cooling non-isothermal DSC thermograms of the neat

and maleated samples are shown in Figures 4.8 and 4.9. Table 4.4 shows the Tc, ∆Hc, and Tm of

neat and maleated PBS and PBS/PBAT blend, and the reported values are obtained from second

heating and first cooling cycles. Both PBS and PBS/PBAT blends had almost the same melting

points because both PBS and PBAT melting points are very close to each other. Therefore, the

melting peaks may be overlapping. John et al., [49] also observed a similar phenomenon in

PBS/aliphatic–aromatic copolyester blend. The major melting points of virgin PBS, PBAT and

blend of PBS/PBAT are 114, 115 and 114oC, respectively.

Figure 4.8. DSC second heating curves of neat PBS, PBS/PBAT (60/40 wt%) and their maleated

samples with 1 phr DCP and 5 phr MAH.

146

Figure 4.9. DSC first cooling curves of neat PBS, PBS/PBAT (60/40 wt%) and their maleated

samples with 1phr DCP and 5 phr MAH

Interestingly, the PBS and blend of PBS/PBAT showed two distinct endothermic peaks at

108 (Tm1) and 114oC (Tm2), which may be attributed to two different lamellar thicknesses

presented in the PBS and PBS/PBAT blend. Usually double melting peaks are observed for

semicrystalline polymers and they can be explained by a melt re-crystallization mechanism [50].

These two melting points had a significantly lower temperature shift after MAH grafting on both

PBS and PBS/PBAT blend. This is possibly due to the fact that the MAH group may prevent the

lamella growth and nucleation of MAH grafted samples, thus leading to imperfect crystal

structure compared to their parent polymers [51]. However, all the MAH grafted samples had a

sharp melting point and a weak shoulder melting point. The weak shoulder melting peak and

sharp melting peak are attributed to lamella with more imperfect crystals and lamella with

perfect crystals, respectively. In addition, after MAH grafting, the PBS and blend of PBS/PBAT

showed one additional exothermic peak prior to melting points. This additional exothermic peak

147

resulted from recrystallization of PBS during heating [21]. The crystallization temperatures of

PBS/PBAT and PBS blend were considerably affected with the addition of MAH group on the

polyester backbone. An obvious crystallization temperature reduction was observed in PBS and

PBS/PBAT blend from 93 to 63oC and from 94 to 55

oC, respectively. The significant reduction

in crystallization temperature was attributed to the decrease in polymer chain regularity which

hinders crystal growth, leading to lower crystallization temperatures [25]. The crystallization

enthalpies (Table 4.4) of fusion for MAH-g-PBS and MAH-g-PBS/PBAT blend were found to

be 65 and 35 J/g, respectively. Both MAH-g-PBS and MAH-g-PBS/PBAT blend crystallization

enthalpies were lower than those of ungrafted PBS and PBS/PBAT blend. An apparent decrease

in crystallization enthalpy of MAH grafted samples was due to the reduction of polymer chain

regularity. A similar result for solid state MAH grafting onto PP was reported [52].

Table 4.4. Detailed DSC results of the maleated PBS and PBS/PBAT blend (MAH grafted

samples were prepared with 1 phr DCP and 5 phr MAH)

Samples Tm1 (oC) Tm2 (

oC) Tc (

oC) ∆Hc(J/g)

PBS 108 114 93 67

MAH-g-PBS 98 109 63 65

PBS/PBAT blend 108 114 94 39

MAH-g-PBS/PBAT blend 98 110 55 35

4.4 Conclusions

The radical grafting reaction of MAH onto the PBAT, PBS, and blend of PBS/PBAT was

investigated succefully. The characteristic peaks of MAH in the MAH grafted PBAT, PBS and

blend of PBS/PBAT were observed by FTIR spectroscopy. The characteristic peaks of MAH-g-

PBS/PBAT blend were located between that of MAH-g-PBS and MAH-g-PBAT. This revealed

148

that MAH was successfully grafted on both PBS and PBAT backbone in the MAH-g-PBS/PBAT

blend. The maximum MAH grafting yield was reached at MAH (5 phr) and DCP (1phr)

concentration in PBS, PBAT and PBS/PBAT blend. However, a higher grafting yield on the PBS

backbone was observed (2.56%) compared to MAH grafted PBAT (1.90%) and PBS/PBAT

blend (2.05%). The MAH grafting yield was compared in the batch and continuous process. The

batch processed sample had a slightly higher yield than the continuous processed sample. The

slightly higher grafting yield as observed in internal batch process may be attributed to higher

residence time. It was also observed that both grafting reaction and cross-linking can occur

during MAH grafting in the presence of DCP initiator. The crystallization temperature was

significantly reduced after MAH grafting on the polymer backbone. This crystallization

temperature reduction is attributed to chain branching that occurred on the polymer backbones.

Thermal stability of all the maleated samples (MAH-g-PBS, MAH-g-PBAT and MAH-g-

PBS/PBAT blend) were found to be reduced compared to their counterparts. This reduction can

be correlated with molecular chain scission that occurred in the maleated samples. These MAH

grafted polymers are expected to be a good adhesion promoter for natural fiber reinforced

composites.

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Chapter 5: Enhanced Mechanical Performances of Fully Biodegradable Miscanthus Fibers

Reinforced Poly (butylene succinate) Composites*

*A part of this chapter has been published in: R. Muthuraj, M. Misra, A. K. Mohanty, Injection

Molded Sustainable Biocomposites From Poly(butylene succinate) Bioplastic and Perennial

Grass, ACS Sustainable Chem. Eng., 2015, 3, 2767−2776. (adapted with kind permission from

American Chemical Society, Nov 01, 2015).

Abstract

Miscanthus fiber reinforced poly (butylene succinate) (PBS) composites were fabricated

at various concentrations of fiber loadings. Two different compatibilizers, i.e., higher (2.5%) and

lower (1.6%) degree of maleic anhydride grafted PBS (MAH-g-PBS), were used to investigate

the influence of compatibilizer on mechanical performance of resulting composites. The

composites compatibilized with a higher degree of MAH-g-PBS showed superior mechanical

properties compared to neat PBS, uncompatibilized composites, and compatibilized composites

with a lower degree of MAH-g-PBS compatibilizer. The improved mechanical properties were

attributed to the enhanced interfacial adhesion that occurred between the fibers and the matrix.

The optimum coupling efficiency of the compatibilizers was determined by examining 3, 5 and

10 wt% compatibilizer while fiber content was kept constant at 30 wt%. It was found that the

PBS composites with 5 wt% MAH-g-PBS exhibit optimal performance. Consequently, the PBS

composites were prepared with fiber loading up to 50 wt% while the higher degree (2.5%) of

MAH-g-PBS compatibilizer was kept constant at 5 wt%. All the compatibilized composites had

greater thermo-mechanical and mechanical properties as compared to their corresponding

uncompatiblized composites as well as the neat PBS matrix. These improvements were attributed

to the enhanced interfacial bonding between the components with the addition of compatabilizer.

This phenomenon was confirmed by scanning electron microscopy (SEM) and theoretical

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adhesion parameter values. Over all, this study provides an option for preparing a sustainable

biocomposite with superior mechanical and thermo-mechanical properties.

5.1 Introduction

Increasing environmental pollution, global warming and waste accumulation issues are

impetus for developing sustainable and environmentally friendly biodegradable materials to

replace non-biodegradable materials. One category of biodegradable materials is green

composites, which can be produced from biodegradable polymer matrices with natural fibers as

reinforcement. These green composite materials have been finding increased favor across

packaging, horticultural, automotive and biomedical applications [1-4]. There are several

biodegradable polymers commercially available in the market that are being used for green

composite fabrication. Among them, poly(butylene succinate), PBS, is one of the promising

candidates for green composite fabrication because it has good melt processability, relatively

higher heat deflection temperature than other biodegradable polymers, good thermo-mechanical

properties and biodegradability under composting environments [5]. PBS is typically produced

from petroleum based monomers but also can be produced from renewable-resource-based

succinic acid with bio-based content of ~54% [4]. These attractive properties may be responsible

for the increase in the use of PBS based green composites for various applications [3]. The cost

of PBS is more expensive than conventional non-biodegradable polymers [6]. It is well known

that this shortcoming can be overcome by blending with inexpensive natural fibers/fillers while

maintaining or enhancing the matrix performance. Therefore, a variety of natural fibers are used

to fabricate composites with PBS matrix [4].

In addition to being inexpensive, natural fibers are renewable, sustainable, biodegradable,

abundant and have good specific properties and low density compared to synthetic fibers such as

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glass fiber [7]. Among the natural fibers, miscanthus fiber is mainly used for green energy

production, soil preservation and composite application [8]. It is an attractive fiber for composite

fabrication because of suitable fiber properties [7, 8], higher yield per hectare, lower production

cost [8] and thus realistic price [9]. Similar to many other natural fibers, miscanthus fiber is

thermally stable up to 200oC without any major thermal decomposition [7, 10]. It can be

processed with low melting (< 200oC) temperature polymers in conventional processing

equipment. According to Bourmaud and Pimbert [11], the miscanthus fibers have modulus of

9.49 GPa which is between hemp (12.14 GPa) and sisal (8.52 GPa) fibers. Kirwan et al., [9]

reported that miscanthus fibers have mechanical properties similar to commodity thermoplastics.

As a result, it can be expected that miscanthus fibers could offer good reinforcing effect in the

resulting composites. Due to these inherent properties, recently, miscanthus fiber is widely used

as reinforcement in biodegradable polymer matrix such as polylactide (PLA) [7], Mater-Bi® [12],

poly(vinyl alcohol) (PVA) [9], PLA/poly(hydroxybutyrate-co-valerate) (PHBV) blend [8] and

pre-blend of poly(butylene adipate-co-terephthalate) (PBAT)/PHBV [13] matrices. From these

studies, the properties of PLA/miscanthus fiber composites are comparable to some other fiber

reinforced composites. Interestingly, the impact load of Mater-Bi® is increased up to 30% with

the addition of miscanthus fibers. Furthermore, the flexural modulus of PVA/miscanthus fiber

composites is significantly higher when compared to PVA matrix. A similar occurrence has been

observed in the PHBV/PLA/miscanthus and PHBV/PBAT/miscanthus composites.

In general, compatibility between the natural fibers and biopolymer matrix is not

adequate because of their polarity differences. Because of incompatibility between the matrix-

fibers, the resulting composites have yielded an inferior mechanical performance than their neat

counterparts. For instance, the flexural strength and tensile strength of PHBV/PLA/miscanthus

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composites are considerably reduced when compared to PHBV/PLA blend matrix [8]. Another

study by Nagarajan et al., [13] showed a drastic reduction in impact strength of PHBV/PBAT

pre-blend after incorporation of 30 wt% miscanthus fiber. Therefore, it is well documented that

the compatibility/interfacial bonding between the fiber-matrix can be enhanced by two ways.

The first method is surface modification of fibers through chemical, physical and biological

process [14]. In second method, a reactive compatibilizer/coupling agent can be introduced into

the composites system in order to improve the matrix-fiber interfacial adhesion [15]. In the

natural fiber/filler composites, most commonly used method is reactive compatibilizer i.e.,

maleic anhydride (MAH) grafted functional polymers. It has tendency to form a chemical bond

with surface free hydroxyl groups of the natural fibers in the resulting composites [15, 16].

Until now several types of natural fiber/filler reinforced PBS composites have been

investigated in the literature [5,17-20]. There is no data available for miscanthus fibers

reinforced PBS composites with improved performance. In order to overcome insufficient impact

strength and stiffness of PBS, the present study was aimed to develop PBS/miscanthus fiber

biocomposite with and without a reactive compatibilizer i.e., MAH grafted PBS (MAH-g-PBS).

Besides, the effect of compatibilizer concentrations, grafting level of MAH and fiber loading up

to 50 wt% upon the performance of resultant biocomposites were also evaluated by means of

mechanical and thermo-mechanical properties.

5.2 Materials and Methods

5.2.1 Materials

Injection molding grade poly(butylene succinate) (PBS, Biocosafe 1903F) pellets were

purchased from Xinfu Pharmaceutical Co., Ltd, China. The PBS pellets had melt flow index (at

190 oC with 2.16 kg load) of 22.23±1.78 g/10min, density of 1.26 g/cm

3 and melting point of

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115oC. Miscanthus fibers with average length of 4.65±2.5 mm were kindly supplied by New

Energy Farms, Ontario, Canada. The chemical composition of miscanthus fiber is reported in our

group earlier publications [8, 13]. The surface morphology of as received miscanthus fibers is

shown in Figure 5.1. From the SEM analysis, it can be seen that the miscanthus fibers are in the

form of bundules. Two different grafting level of maleic anhydride grafted PBS (MAH-g-PBS)

compatibilizer (C1 and C2) is synthesized according to procedure reported in our previous

Chapter (Chapter 4). The grafting level of compatibilizer C1 and C2 is 2.5 and 1.6%,

respectively.

Figure 5.1. SEM micrograph of as received miscanhtus fibers

5.2.2 Thermal property

Thermogravimetric analysis was performed in a TGA Q500 (TA Instruments, USA) at a

heating rate of 20 ºC/min. In order to compare the thermal degradation of the samples under

different atmosphere, the experiments were performed both inert (nitrogen) and air atmosphere

with a flow rate of 50 mL/min. The experiment results were analyzed in TA instruments

software.

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5.2.3 Biocomposite preparation

Both PBS and miscanthus fibers were dried for 12 h at 80 oC. After drying, the remaining

moisture content of both polymers and fibers was measured in an electrical moisture analyzer

(Denver Instrument) and the percentage of moisture content was 0.1% for PBS and 2.5% for

miscanthus fibers. The biocomposites fabrication was perfromed in an extrusion followed by

injection molding devices (DSM Xplore® 15 cc microcompounder, Netherlands). The extruder

consists of three collectively control heating zones, twin screws with an length and aspect ratio

of 150 mm and 18, respectively. Compounding was achieved by using following processing

parameters: the screw speed was set at 100 rpm; the processing temperature was set at 140oC;

and the processing/dwell time of the material inside the barrel was 2 minutes. All the test

samples were molded with mould temperature of 30oC, injection pressure of 10 bar and injection

time 8 s. The composites were prepared with three different concentrations (3, 5 and 10 wt%) of

compatibilizer and two different amount of functionality (C1 and C2) compatibilizer to study the

influence of compatibilizer concentration and functionality upon the performance of the

PBS/miscanthus composites. Furthermore, the PBS/miscanthus composites were prepared with

fiber loading up to 50 wt% in the presence and absence of optimum compatibilizer (C1)

concentration (5 wt%).

5.2.4 Mechanical testing

Before performing the mechanical test, test samples were conditioned at room

temperature for at least 40 h. The tensile, flexural and impact test were performed according to

ASTM D 638, ASTM D790 and ASTM D256, respectively. Tensile and flexural properties of

the test specimens were measured in an Instron-3382 at room temperature. The tensile properties

of the prepared samples were measured at a strain rate of 50 mm/min for neat PBS and 5mm/min

for all the composite samples. The flexural properties were measured with a cross-head speed of

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14 mm/min and a support span length of 52 mm in a three point bending mode. Notched Izod

impact strength measurement was carried out in a TMI digital impact testing machine with 5 ft-

lb pendulum.

5.2.5 Statistical analysis

Statistical significant differences of mechanical properties were identified by Minitab®

17

software at 95% confidence level (P< 0.05). In order to determine significant difference among

mean values of mechanical properties, one-way analysis of variance (ANOVA) with Tukey’s

honestly significant difference (HSD) tests was conducted with a sample size of 5 for each

group.


The temperature dependent storage modulus (E’) of PBS and its composites is examined

in a dynamic mechanical analyzer (DMA, Q800). The experiments were performed from -50 to

100 oC with a heating ramp of 3

oC/min as per ASTM D4065 standard. The storage modulus

measurements were investigated in a dual cantilever mode using strain of 1Hz frequency and

15µm oscillating amplitude. Heat deflection temperature (HDT) analyses were also performed

using a same DMA machine according to ASTM D648. The HDT measurements were carried

out from 25 to 115 oC with a heating rate of 2

oC/min in a three point bending mode. According

to ASTM D648, the reported HDT values are obtained from the temperature at which sample

deflection occurred at 250 µm with a constant load of 0.455 MPa.

5.2.7 Melt flow index (MFI)

MFI values of the samples were determined according to ASTM D1238 standard. All the

MFI measurements were performed in a melt flow indexer (Qualitest model 2000A) at 190oC

163

with a constant load of 2.16 kg. Five measurements were made for each batch and their average

values are reported here with standard deviations.


DSC anlysis was employed to study the thermal properties of PBS and its composites.

The experiments were carried out in the presence of nitrogen atmosphere with a flow rate of 50

mL/min. Exactly weighed samples were heated from -50 to 160oC with a heating rate of 10

oC/min subsequently the samples were cooling until -50

oC with a rate of 5

oC/min. After end of

cooling cycle, a second heating cycle was performed from -50 to 160oC with a heating ramp of

10 oC/min. A first heating cycle was used to erase the thermal history of the samples. A second

heating and first cooling cycles are considered for the analysis. The percentage of crystallinity

was calculated as follows [21]:

χ =

X 100%

χ =

X 100% (5.1)

The parameter ∆Hc is crystallization enthalpy, respectively and ∆Hom is theoretical melting

enthalpy of one-hundred percentage crystalline PBS taken to be 110.3 J/g [21]. The term wf is

weight fraction of the fibers in the composite samples.

5.2.9 Scanning electron microscopy (SEM)

To examine the fracture, surface morphology of the fractured sample was analyzed using

an Inspect S50-FEI Company SEM at an accelerating voltage of 20 kV. Before observing sample

morphology, all the samples were gold coated with a final thickness of 20 nm with 20 mA in

order to make them electrically conductive.

164

5.3 Results and discussion


Thermal stability of the miscanthus fiber under air and nitrogen atmosphere is shown in

Figure 5.2.

Figure 5.2. Thermogravimetric analysis of miscanthus fiber under different environment

The miscanthus fiber showed only a 2% weight loss by 105oC in both nitrogen and air

atmosphere; this 2% weight loss is attributed to the residual moisture. There was no significant

further weight loss observed by 200oC under air or nitrogen atmospheric conditions, suggesting

that the miscanthus fiber can be compounded with polymers up to 200oC without severe thermal

degradation. Furthermore, the thermal stability of the miscanthus fiber was found to be similar to

other natural fibers [12]. Beyond 200oC, the degradation of miscanthus fiber was more

pronounced under air atmosphere compared to nitrogen atmosphere, perhaps due to the effect of

oxygen on degradation. Unlike under the nitrogen atmosphere, the weight of the char residue of

the miscanthus fiber under oxygen atmosphere was almost negligible at 790oC, this lack of char

165

residue may be due to the oxidation of char residue in air [22]. There were two and three major

derivative weight loss peaks observed for the miscanthus fiber under N2 and air atmosphere,

respectively. The oxidation of char residue under air atmosphere can be confirmed by the

observed extra derivative peak around 460oC (Figure 5.2).

5.3.2 Efficiency of compatibilizer

The efficiency of the two different MAH grafting level compatibilizers (C1 and C2) was

accessed by means of PBS composites mechanical properties. For this series of experiments, the

mechanical properties of the PBS/miscanthus composites are presented in Table 5.1 with (3, 5

and 10 wt%) and without MAH-g-PBS compatibilizers. All the mechanical properties were

increased except for tensile strength with addition of 30 wt% miscanthus fiber into the PBS

matrix. The observed flexural strength and modulus improvement of the composites can be

attributed to the reinforcing capability of miscanthus fibers [9]. The impact strength of short-

fiber composites can be influenced by several mechanisms, namely matrix shear yielding, fiber

pullout, fiber-matrix debonding, and fracture of fiber and matrix [23-25]. Among the impact

fracture mechanisms, the fiber pullout and strong interfacial adhesion are the most effective

mechanisms to improve the fracture toughness of the composites [24,26]. The impact strength of

PBS was increased upon addition of 30 wt% miscanthus fibers. This increased strength could be

due to the energy dissipation that occurres when matrix shear yielding and fiber pull-out

terminate the unwanted growth of crazes [25].

The tensile and flexural strength values of the composites higher with addition of both

compatibilizers (C1 and C2) as compared the strength of uncompatibilized composites. This

increased strength suggests that the MAH grafting on the PBS backbone was successful and it

acts as a compatibilizing agent between the matrix-fibers [27]. Similar tensile strength

166

improvement was observed in PLA/wood and PLA/wheat straw composites in the presence of

MAH grafted PLA compatibilizer [27, 28]. It can be seen (Table 5.1) that the composites with

C1 compatibilizer showed significantly higher flexural and tensile strength than did the

composites with C2 compatibilizer. This strength improvement is likely due to the difference in

molecular weight between C1 and C2 as well as number of MAH groups of the compatibilizer

[29]. An appropriate molecular weight and number of MAH groups of the compatibilizer could

provide better entanglement and stronger interfacial bonding between the phases [29, 30]. The

formation of chemical interaction between the fiber and MAH grafted compatibilizer has been

found to enhance the interfacial adhesion [31]. Since all the composites had the same amount of

fiber content (30 wt%), there was no considerable change observed in flexural and tensile

modulus of the composites with and without compatibilizer. The elongation at break of the PBS

was substantially reduced after incorporation of miscanthus fibers; this reduction has good

agreement with observed modulus of the PBS composites. Both compatibilized and

uncompatiblized PBS composites showed an insignificant difference in impact strength. This

similarities in the impact strength could be due to the same amount of energy being absorpted

during impact fracture, no matter whether the composites were compatibilized (fiber break along

with matrix) or not (fibers pull-out from matrix) [25].

Overall, it can be concluded that the composites with C1 compatibilizer (5 wt%) showed

an optimum mechanical performance in comparison to uncompatibilized and compatibilized

composites with C2 compatibilizer. Consequently, the PBS composites were prepared with C1

compatibilizer (5 wt%) increasing as a function of miscanthus fiber loading.The performances of

the prepared composites are discussed in the next section of this research work.

167

Table 5.1. Mechanical properties of PBS composites with two different MAH grafting levels of

compatibilizer

Samples

Tensile

strength

(MPa)

Elongation

at break

(%)

Tensile

Modulus

(MPa)

Flexural

Strength

(MPa)

Flexural

Modulus

(MPa)

Notched

Izod impact

strength

(J/m)

Neat PBS 39.4±0.7

(B, C)

246±10.5

(A)

642±12.7

(B)

33.18±1.0

(E)

759±29

(D)

28.40±0.91

(C)

PBS 70 wt%+30 wt%

Miscanthus

33.2±1.7

(D)

3.76±0.6

(B)

2210±84.9

(A)

55.04±1.3

(D)

2379±54

(A, B, C)

46.42±1.4

(A, B)

Biocomposites prepared with C1 compatibilizer

PBS 67 wt%+3 wt%

MAH-g-PBS +30 wt%

Miscanthus

41.3±1.5

(B, C)

3.67±0.2

(B)

2180±121

(A)

73.59±1.8

(A, B)

2277±104

(C)

47.48±4.8

(B)

PBS 65 wt%+5 wt%

MAH-g-PBS +30 wt%

Miscanthus

44.4±2.0

(A)

3.66±0.3

(B)

2300±91

(A)

75.32±2.4

(A)

2346±98

(B, C)

53.23±3.5

(A)

PBS 60 wt%+10 wt%

MAH-g-PBS +30 wt%

Miscanthus

45.3±1.6

(A)

4.02±0.3

(B)

2210±47

(A)

75.51±2.0

(A)

2347±211

(B, C)

52.38±1.2

(A)

Biocomposites prepared with C2 compatibilizer

PBS 67 wt%+3 wt%

MAH-g-PBS +30 wt%

Miscanthus

37.9±1.1

(C)

3.41±0.2

(B)

2240±39

(A)

65.28±2.0

(C)

2556±60

(A)

45.65±1.0

(B)

PBS 65 wt%+5 wt%

MAH-g-PBS +30 wt%

Miscanthus

38.4±0.6

(C)

3.61±0.2

(B)

2220±69

(A)

63.49±1.6

(C)

2503±56

(A, B)

46.22±0.98

(B)

PBS 60 wt%+10 wt%

MAH-g-PBS +30 wt%

Miscanthus

41.3±0.6

(B)

3.61±0.2

(B)

2200±15

(A)

70.79±1.4

(B)

2565±83

(A)

47.01±1.70

(B)

Means that do not share a letter in parentheses are significantly different at p<0.05 level.

5.3.3 Mechanical properties versus fiber loading

The tensile properties of the natural fiber reinforced composites are dependent on both

the matrix and fiber characters. The tensile strength of natural fiber composite is mainly

influenced by three factors such as interfacial interaction between the components fiber

orientation, and stress concentration [23]. On the other hand, wettability of fibers by the matrix,

168

volume fraction of fibers, and aspect ratio of fibers determine tensile modulus of the natural fiber

composite [23]. Figure 5.3 shows the variation of tensile strength and tensile modulus of PBS

composites with different fiber loadings with and without compatibilizer. The neat PBS had

tensile strength of 39 MPa and tensile modulus of 0.66 GPa. Due to lack of interfacial interaction

between the phases and incompatibility between the miscanthus fiber and the PBS, the tensile

strength decreased significantly after inclusion of miscanthus fibers into the PBS. Particularly,

the addition of the 50 wt% miscanthus fiber into the PBS matrix yielded a 22% reduction in

tensile strength. As expected, the tensile modulus of the composites can be influenced by the

fiber content in the composite matrix. Consequently, the tensile modulus of the PBS composites

was steeply increased with increasing miscanthus fiber content up to 50 wt%. For instance, the

PBS composite with 50 wt% miscanthus fiber showed a maximum tensile modulus value of 3.88

GPa, which is 475% higher than that of neat PBS. This improvement was due to the reinforcing

effect of miscanthus fiber in the PBS matrix. Among the tensile modulus and tensile strength of

the composites, tensile strength was influenced by the addition of compatibilizer. The tensile

strength of all the compatibilized composites was significantly higher than that of the

uncompatibilized composites as well as of PBS matrix. The observed improvement in the tensile

strength of the compatibilized composites is possibly due to an enhanced interfacial adhesion

between the components. In the presence of compatibilizer, the tensile strength improvement of

the PBS/miscanthus composite leveled off beyond 30 wt% fiber loading. Furthermore, the tensile

modulus of the compatibilized composites did not show any substantial difference compared to

that of the corresponding uncompatibilized composites. This observation is consistent with

PLA/wood composites [27].

169

Figure 5.3. Tensile properties of PBS and PBS/miscanthus composites: A) neat PBS, B)



PBS/miscanthus (50/50 wt%), and G) PBS/miscanthus/compatibilizer (45/50/5 wt%)

The load-bearing capacity/reinforcing effect of the particulate or short-fiber-filled

composites can be expressed quantitatively with the help of a simple model developed earlier

(equation 5.2) [32-36]. This equation can be used to describe the composition-dependent tensile

properties of the biocomposites [27, 33].

σ =

exp (Bφ) (5.2)

The term is tensile strength of neat matrix; σ is tensile strength of composites; λ is ratio

(L/L0) of length measured before (L0) and after (L) the tensile test; n is the strain hardening

parameter; B is the load-bearing capacity of the dispersed component, which depends on

interfacial adhesion/interaction; φ is the volume fraction of fibers in the composites. From

equation (5.2), ‘n’ can be neglected when strain hardening tendency of the matrix is low [32].

Then, equation (5.2) can be written as

170

σ =

exp (Bφ) (5.3)

Equation 5.4 is obtained from simplified linear form of equation 5.3 [32, 36]

ln σred = ln σ

= ln + Bφ (5.4)

If we plot the natural logarithm of reduced tensile strength as a function of fiber content,

this reduced tensile strength must give a straight line with a slope of B, which corresponds to

load-bearing capacity [34,36]. In Figure 5.4, the tensile strength of both compatibilized and

uncompatibilized composites is plotted against fiber content in equation 5.4. It was observed that

a linear correlation occurred for compatibilized and uncompatibilized composites with dissimilar

slope (B) value. The B value was proportional to the load-bearing capacity/reinforcing effect of

the resulting composites. The compatibilized composites showed superior load-bearing capacity

with B value of 4.85 while uncompatibilized composites yielded least reinforcing effect with B

value of 3.65. The enhanced load-bearing capacity of the compatibilized composites was

consistent with observed tensile modulus and strength of the PBS/miscanthus composite. The B

value (4.85) of the compatibilized PBS/composites found in this study was much higher than

compatibilized PLA/wood composites [27, 32, 35], suggesting that the load-bearing capacity of

compatibilized PBS/miscanthus composites is greater than compatibilized PLA/wood

composites.

171

Figure 5.4. Reduced tensile strength of uncompatibilized and compatibilized PBS/miscanthus

composites plotted against volume fraction of fibers according to equation 5.4.

Flexural properties of PBS and its compatibilized and uncompatibilized composites are

given in Figure 5.5. Unlike tensile strength, the flexural strength of all the uncompatibilized

composites is remarkably higher as compared to neat PBS. This increase can be attributed to the

enhanced stiffness of the PBS matrix after incorporation of the miscanthus fibers. However,

there is a slight reduction in flexural strength of uncompatibilized composites with increasing

fiber loadings from 30 to 50 wt%. For instance, the flexural strength of uncompatibilized

composites with 50 wt% miscanthus fiber is 12% lower than that of uncompatibilized

composites with 30 wt% fiber. This reduction is attributed to the agglomeration of fibers and

thus leads to uneven dispersion of the fibers in the matrix. In contrast, flexural modulus of the

composites gradually increased with increasing fiber content up to 50 wt%. This trend has good

agreement with tensile modulus observation in this current study.

172

Figure 5.5. Flexural properties of PBS and PBS/miscanthus composites: A) neat PBS, B)




Regarding flexural modulus, there was insignificant difference observed between the

compatibilized and uncompatibilized composites. In contrary, the compatibilized composites

exhibit superior flexural strength compared to uncompatibilized composites. Similar to tensile

strength of compatibilized PBS composites; the flexural strength of compatibilized PBS

composites levels off when fiber loading is increased from 30 to 50 wt%. Compared to neat PBS,

the flexural strength and flexural modulus of compatibilized PBS composites with 50 wt%

miscanthus fibers were found to increase 139 and 515%, respectively. The observed

improvement in the compatibilized composites could be due to the strong reinforcing effect of

miscanthus fibers and reduced fiber agglomeration in the matrix.

The notched Izod impact strength of PBS and its composites with and without

compatibilizer is shown in Figure 5.6. The impact strength of neat PBS is around 28 J/m, which

173

can be comparable to homopolypropylene [37]. Unlike impact strength of compression molded

PBS/30 wt% kenaf composites [38] and injection molded PLA/30 wt% wheat straw composites

[28], there is a trend for impact strength of injection molded PBS composite to increase with

miscanthus fibers loading up to 40 wt%. In contrast, the PBS composites with 50 wt%

miscanthus fibers did not show any significant impact strength improvement when compared to

neat PBS. This could be due to the uneven fiber dispersion in the matrix when composites

contain more fibers i.e., 50 wt%. The impact strength improvement was more pronounced in the

compatibilized composites in comparison to their corresponding uncompatibilized composites.

When impact strength of compatibilized and uncompatibilized PBS/miscanthus composites was

compared, a maximum improvement (47%) was noticed in compatibilized composites with 50

wt% fiber loading. Avella et al., [39, 40] have observed a similar trend in compatibilized

PHBV/kenaf composites and compatibilized PLA/kenaf fiber composites. According to these

studies, the observed improvement was due to the better adhesion between the components, less

fiber pullout under impact load, and enhanced uniform fiber dispersion in the matrix. However,

the impact strength of compatibilized composites gradually declined with increasing fiber

loading from 30 to 50 wt%, which follows a similar trend to uncompatibilized composites. This

could be due to the fiber-fiber contact that was increased with increasing fiber loadings. When

the fiber content is increased in the polymer matrix system, the fibers tend to form an

agglomeration in the matrix due to strong hydrophilic fiber to fiber interaction. The

agglomeration creates strong stress concentration regions that require less energy to elongate the

crack propagation under selected impact testing condition. Therefore, the composites with fiber

agglomeration lead to less impact strength [41].

174

In order to compare the effectiveness of this work, mechanical properties of injection

molded PBS biocomposites as described in the literature are presented in Table 5.2. It can be

seen that bamboo, ramie and flax fiber reinforced PBS composites tensile strength is greater than

that of PBS/miscanthus composites. In contrast, miscanthus fiber reinforced PBS composites

enhanced tensile modulus by 473% which is significantly higher than other natural fiber

(bamboo, flax, hemp, wood and waste silk) reinforced PBS composites [5, 42, 43]. This tensile

modulus increase indicates that the reinforcing effect of miscanthus fibers is obvious. Even

though chopped short miscanthus fibers were used in the current study, the flexural properties of

the PBS/miscanthus composite are quite comparable with continuous basalt fiber roving/PBS

composites [42]. In addition, the PBS/miscanthus fiber composites exhibit superior flexural

properties compared to PBS composites with ramie, kenaf and waste silk fibers. The notched

Izod impact strength of PBS/miscanthus fibers composite was about 47% higher in comparison

to neat PBS. When kenaf (30 wt% ) short fibers were incorporated into PBS, the impact strength

of neat PBS was reduced by ~79% (from 40 to 11 J/m2) [38]. In the present study, the impact

strength was improved about 47% in comparison to neat PBS. From the above, it clearly

suggests that the improved mechanical performances of the PBS/miscanthus composites can be

achieved by the simple method.

175

Figure 5.6. Nothced Izod impact strength of PBS and PBS/miscanthus composites: A) neat

PBS, B) PBS/miscanthus (70/30 wt%), C) PBS/miscanthus/compatibilizer (65/30/5 wt%), D)



176

Table 5.2. A comparison of mechanical properties of injection molded PBS/natural fiber composites (Note: the reported percentage

differences were calculated based on the neat PBS matrix properties)

Fiber

type

Optimum

fiber

(wt%)

Type of compatibilizer used Change in tensile

properties (%)

Change in flexural

properties (%)

Change

in Izod

impact

strength

(%)

References

Strength Modulus Strength Modulus

Bamboo 15 poly(ethylene glycol)

methacrylate

~76 ~52 NA NA NA [51]

Ramie 30 Alkali treatment ~55 ~431 ~86 ~267 NA [5]

Kenaf 30 Maleic anhydride grafted PBS NA NA ~23 ~399 ~(-79) [38]

Flax 30 Maleic anhydride grafted PBS ~28 ~358 NA NA NA [43]

Wood 30 Maleic anhydride grafted PBS ~5 ~329 NA NA NA [43]

Hemp 30 Maleic anhydride grafted PBS ~13 ~353 NA NA NA [43]

Bamboo 30 Maleic anhydride grafted PBS ~(-16) NA NA NA NA [52]

Wood 30 Maleic anhydride grafted PBS ~(-40) NA NA NA NA [52]

Waste silk 40 NA ~17 ~140 ~26 ~54 NA [53]

Miscanthus 50 Maleic anhydride grafted PBS ~22 ~473 ~139 ~515 ~47 Present

study

177


DMA was used to investigate the viscoelastic properties of PBS and its composites with

respect to temperatures. The interfacial adhesion between the matrix and the fibers can be

estimated by loss factor (tan δ) and storage modulus (E’) properties. For example, the E’ values

of the natural fiber composites are sensitive to the dispersion of fibers in the matrix as well as

interfacial bonding between the phases. Figure 5.7 illustrates the storage modulus of PBS and its

composites over the wide temperature range. It can be remarked that the E’ value of composites

monotonically increased with the addition of miscanthus fibers up to 50 wt%. This can be

attributed to the reinforcing effect of the miscanthus fibers in the PBS matrix. In addition, the E’

of compatibilized composites is slightly higher with respect to corresponding uncompatibilized

composites. For instance, the storage modulus of compatibilized composites with 50 wt%

miscanthus fibers showed 6.8 GPa at -60oC, which is higher than corresponding

uncompatibilized composites (6.52 GPa) as well as neat PBS (2.98 GPa) at the glassy region (-

60oC). The same trend has been observed across the entire investigated range of temperature.

This suggests that compatibilized composites had uniform fiber dispersion and the high degree of

interaction between the phases. However, all the samples showed a drastic reduction of E’ at -

16oC, which is attributed to the glass transition temperature (tan δ) of PBS. Moreover, the E’ of

all the samples were gradually decreased with increasing temperature up to 100oC. This decrease

in E’ is an expected consequence of the molecular motion/relaxation while increasing

temperatures.

178

Figure 5.7. Dynamic mechanical analysis of PBS and PBS/miscanthus composites: A) neat

PBS, B) PBS/miscanthus (70/30 wt%), C) PBS/miscanthus/compatibilizer (65/30/5 wt%), D)



Figure 5.8 shows the tan δ curves of PBS and its composites with different weight

percentage fiber loadings. The glass transition temperature (position of tan δ peak maximum) of

the PBS is not affected considerably as content of miscanthus fiber increases up to 50 wt%. In

contrast, the height of the loss factor (tan δ) of the PBS was reduced by incorporation of

miscanthus fibers. This reduction corresponds to the stiffness improvement of the PBS in the

presence of fibers and is evidenced by increase of the E’. The height of the tan δ value was

reduced in the compatibilized composites compared to their corresponding uncompatibilized

counterparts. These reductions were due to the improved interaction between the phases with

addition of compatibilizer. This can be further confirmed by evaluating the interface adhesion

factor (A) between the fiber and matrix.

179

Figure 5.8. Tan δ curves of PBS and its composites: A) neat PBS, B) PBS/miscanthus (70/30

wt%), C) PBS/miscanthus/compatibilizer (65/30/5 wt%), D) PBS/miscanthus (60/40 wt%), E)

PBS/miscanthus/compatibilizer (55/40/5 wt%), F) PBS/miscanthus (50/50 wt%), and G)

PBS/miscanthus/compatibilizer (45/50/5 wt%)

5.3.5 Adhesion factor calculation

From the height of tan δ peak values, Kubar et al., [44] described a methodology to

evaluate degree of interaction between the composite components. When there is strong

interfacial interaction/adhesion between the phases and reduction of macromolecular mobility

around the reinforcement surface, the value of adhesion factor (A) decreases. Thus, lower values

of adhesion factor (A) are evidence of a high degree of interactions between the fibers and the

matrix. According to this methodology, the adhesion/interaction between the fiber and the matrix

can be estimated by adhesion factor (A), which was calculated in the following equation [44]:

A =

– 1 (5.5)

where tan δM (T) and tan δC (T) represent the relative damping values of the neat matrix and the

composite at a given temperature, respectively, and Vf is the volume fraction of fiber.

180

Figure 5.9. Adhesion factor of PBS/miscanthus composites: A) PBS/miscanthus (70/30 wt%),

B) PBS/miscanthus/compatibilizer (65/30/5 wt%), C) PBS/miscanthus (60/40 wt%), D)

PBS/miscanthus/compatibilizer (55/40/5 wt%), E) PBS/miscanthus (50/50 wt%), and F)

PBS/miscanthus/compatibilizer (45/50/5 wt%)

Figure 5.9 shows the adhesion factor values of the composites with respect to

temperatures. Unlike composites with higher fiber content (40 and 50 wt%), the composites with

lower fiber content (30 wt%) showed lower adhesion factor values. Better interfacial interaction

between components in the composites with 30 wt% fibers in contrast to composites with 50

wt% fibers content indicates the lower adhesion factor value. Moreover, the adhesion factor

value of all the compatibilized composites was found to be lower in comparison to that of

corresponding uncompatibilized composites. The lower adhesion factor value may be result of

further enhancement of the interfacial adhesion between the fibers and the matrix in the presence

of compatibilizer.


HDT value is used to determine physical deformation of a polymeric material at elevated

temperatures with a set of testing conditions. The HDT values of neat PBS and its compatibilized

181

and uncompatibilized composites are presented in Table 5.3. The HDT value of neat PBS was

around 90oC, and PBS shows significant increase in HDT with increasing miscanthus fiber

loading from 30 to 50 wt%. For instance, the PBS with 40 wt% miscanthus fiber composites

showed a 29% improvement in comparison to neat PBS. The observed HDT improvements of

PBS composites were attributed to the enhanced stiffness/reinforcement effect of resulting

composites, as reports in literature [8]. The enhanced stiffness of all the PBS composites has

good agreement with the observed tensile and flexural modulus. Similarly, the HDT value of

PBS/basalt fiber (85/15 wt%) composites [42] and PBS/switchgrass (50/50 wt%) composites

[19] was increased by 40 and 36%, respectively, in comparison to neat PBS. The HDT values of

compatibilized and uncompatibilized composites were not significantly different in the present

study. This can be ascribed to the HDT values of all the composites being very close to their

melting temperature i.e., ~114 oC. Therefore, it can be concluded that an optimum HDT value of

PBS composite is 115oC, which was able to be achieved in the PBS composites with 40 wt%

miscanthus fiber loading.

Table 5.3. Heat deflection temperature (HDT) and melt flow index (MFI) of neat PBS and its

composites

Samples

HDT (oC) MFI (g/10 min)

Neat PBS 89.59±4.15 22.23 ± 1.78

PBS/miscanthus fibers (70/30 wt%) 108.63±3.97 7.78 ± 1.11

PBS/miscanthus fibers/MAH-g-PBS (65/30/5 wt%) 112.25±0.31 9.99 ± 0.55





182

5.3.7 Melt flow analysis

The melt processability of the PBS and its composites was assessed by MFI analysis.

Table 5.3 represents the effect of fiber content and compatibilizer on the MFI value of PBS

composites. It was found that the MFI value of neat PBS is around 22 g/10 min. Unlike neat

PBS, the MFI value of PBS composites was drastically decreased with increasing fiber content

from 30 to 50 wt%. For example, the lowest MFI value (~1 g/10min) was observed for the

uncompatibilized PBS with 50 wt% miscanthus fiber composites. In general, the ability of

polymer melt flow is hampered in the presence of rigid fibers and fillers. Recently, this type of

MFI reduction was observed in PLA/50 wt% jute fiber composites and PLA/50 wt% hemp fiber

composites by Gunning et al., [45]. However, it is worth noting that all the compatibilized

composites showed a marginal improvement in MFI value when compared to that of

corresponding uncompatibilized composites. This increased MFI value of the compatibilized

composites is attributed to the good fiber dispersion in the matrix, which can enhance the

flowability of composite materials [45]. Over all, the observed MFI value of the PBS composites

is appropriate for injection molding applications [46].

Table 5.4. Summary of differential scanning calorimetry traces of neat PBS and its composites

Samples Tc

(oC)

∆Hc

(J/g)

Tm

(oC)

∆Hm

(J/g)

Crystallinity

(%)

Neat PBS 92.97 69.57 113.39 67.60 61.28

PBS/miscanthus fibers (70/30 wt%) 92.48 48.81 112.95 47.86 61.98

PBS/miscanthus fibers/MAH-g-PBS

(65/30/5 wt%)

92.63 52.06 113.03 51.00 66.03



(55/40/5 wt%)

92.33 42.53 114.34 42.47 64.17



(45/50/5 wt%)

92.30 32.74 113.61 32.93 59.70

183


The effect of the incorporated miscanthus fibers on the melting temperature (Tm), melting

enthalpy (∆Hm), crystalline temperature (Tc), crystalline enthalpy (∆Hc) and percentage

crystallinity of PBS was analysed by DSC. For PBS and its composites, the DSC second heating

and first cooling cycle results are presented in Figures 5.10 and 5.11, respectively. Table 5.4

shows the melting enthalpy, crystallization enthalpy and percentage crystallinity of PBS before

and after incorporation of miscanthus fibers. The melting temperature and crystallization

temperature of neat PBS are found to be 113 and 93oC, respectively. The melting temperature of

PBS and its composites is very similar, which suggests that the PBS melting temperature is not

affected in the presence of miscanthus fibers.

Figure 5.10. DSC second heating thermograms of PBS and its composites: A) neat PBS, B)




A similar trend was observed in crystallization temperature and percentage of

crystallinity of PBS after addition of miscanthus fibers. During the DSC heating cycle, a broad

bimodal melting peak for neat PBS was observed and can be attributed to the

184

meltrecrystallization phenomena. This bimodal melting peak of PBS became more distinct after

addition of miscanthus fibers. PBS crystallinity percentage was not heavily affected after

incorporation of miscanthus fibers; consequently, it can be expected that the biodegradability of

the PBS composites will be similar to that of neat PBS [47].

Figure 5.11. DSC first cooling thermograms of PBS and its composites: A) neat PBS, B)




5.3.9 Morphological analysis

SEM was employed to investigate the interfacial bonding between the component in the

composites and the degree of fiber dispersion in the matrix. The tensile fractured surface

morphology of uncompatibililized and compatibilized PBS composites with different fiber

loadings is presented in Figures 5.11 and 5.12. SEM micrographs of uncompatibilized

composites (Figure 5.12) reveal interfacial gaps between the fiber and the matrix, poor fiber

wetting, fibers pullout traces, and poor dispersion of fibers in the matrix. Such phenomena can

clearly be observed in higher (500x) magnification SEM images (Figure 5.12. b, d and f). A

185

similar observation has been reported in PLA/kenaf composites [40], PHBV/kenaf composites

[39], PHBV/PLA/miscanthus composites [8], PBS/bamboo composites [48], PBS/kenaf fiber

composites [38] and PP/bio-flour composites [29]. In addition, increased fiber

bundles/aggregates are evident with increasing fiber content up to 50 wt%. This is possibly due

to an increase fiber-fiber interaction with increasing fiber contents in the matrix, which could

hinder the interaction between the phases in the composites [49]. This contributes to a reduction

in the tensile strength of resulting PBS composites with increasing fiber content from 30 to 50

wt%. It is well documented that most of the natural fibers are not compatible with a hydrophobic

polymer matrix, this lack of compatibility is responsible for fiber debonding from the matrix

during tensile fracture [29, 50]. Figure 5.12 shows SEM micrographs of compatibilized PBS

composites at lower (150x) and higher (500x) magnification. All the compatibilized composites

clearly showed paucity of fiber pullout traces from the matrix, good fiber dispersion in the

matrix, and fibers wholly embedded into the PBS matrix due to strong adhesion at the interfacial

regions compared to that of uncompatibilized composites. These observations are more

pronounced in higher (500x) magnification SEM micrographs (Figure 5.12. b, d and f). In the

compatibilized composites, it can be seen (Figure 5.13) that many fibers are coated with PBS

matrix without the interfacial gap formation. This result was attributed to the enhanced

fiber/matrix adhesion with the help of a reactive compatibilizer [29]. Consequently, all the

compatibilized composites enhanced mechanical performances as compared to their

corresponding uncompatibilized composites, which is also consistent with the observed lower

adhesion factor A values of compatibilized composites. Similar findings have been recently

reported for the compatibilized PBS/kenaf fiber composites [38].

186

Figure 5.12. SEM micrographs of tensile fractured surface of uncompatibilized PBS composites

with low (150x) and high (500x) magnification; (a) and (b) are PBS/miscanthus (70/30 wt%)

composites; (c) and (d) are PBS/miscanthus (60/40 wt%) composites; (e) and (f) are

PBS/miscanthus (50/50 wt%) composites.

187

Figure 5.13. SEM micrographs of tensile fractured surface of compatibilized PBS composites

with low (150x) and high (500x) magnification; (a) and (b) are PBS/miscanthus (70/30 wt%)

composites; (c) and (d) are PBS/miscanthus (60/40 wt%) composites; (e) and (f) are

PBS/miscanthus (50/50 wt%) composites.

188

5.4 Conclusions

Biocomposites from PBS and miscanthus fibers were successfully prepared by extrusion

and injection molding methods with different fiber loadings. The performances of the resulting

composites were evaluated based on mechanical, thermo-mechanical, and morphological

properties. The strong reinforcing effect of micanthus fibers led to an increase in the

stiffness/modulus of resulting PBS composites. The tensile strength of uncompatibilized

PBS/miscanthus composites was much lower compared to that of neat PBS. Unlike tensile

strength, the flexural and impact strengths were significantly enhanced after incorporation of

miscanthus fibers into the PBS matrix. The enhanced flexural strength was attributed to the

reinforcing effect of miscathus fibers. The fiber pullout mechanism is likely responsible for the

observed impact strength improvement. Addition of 5 wt% MAH-g-PBS into PBS composites

resulted a significant improvement in tensile and flexural strength compared to the corresponding

uncompatibilized composites and neat matrix. For example, the PBS composites with 50 wt%

miscanthus and 5 wt% MAH-g-PBS resulted in 22, 139 and 47% improvements in tensile,

flexural and impact strength compared to neat PBS. These improvements were attributed to the

enhanced interfacial interaction between the components, as confirmed by adhesion parameter

values and by surface morphological analysis. Furthermore, the enhanced thermo-mechanical

properties were consistent with tensile and flexural modulus. Although, the MFI value of the

PBS was considerably reduced after incorporation of miscanthus fibers, the observed MFI value

of the PBS composites is still appropriate for the injection molding process. Because the PBS

crystallinity was not affected by incorporation of miscanthus fibers, it can be expected that the

biodegradability of the PBS composites will be similar to that of neat PBS.

189

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Chapter 6: Mechanical Performances of Biocomposites Made From Miscanthus Fibers and

Poly(butylene adipate-co-terephthalate) Matrix

Abstract

Miscanthus fibers reinforced biodegradable poly(butylene adipate-co-terephthalate)

(PBAT) based biocomposite was successfully produced by traditional melt processing methods.

The material properties of the produced PBAT/miscanthus composites were evaluated by means

of mechanical, thermal and morphological properties. Compared to neat PBAT, the flexural

properties, and tensile modulus were increased after the incorporation of miscanthus fibers into

the PBAT matrix. These improvements were attributed to the strong reinforcing effect of

miscanthus fibers. Relatively high hydrophilic nature of the miscanthus fibers and the relatively

high hydrophobic character of PBAT matrix lead to weak interfacial interaction between the

components in the resulting PBAT/miscanthus composites. This weak interface was evidenced in

the impact and tensile strength of the uncompatibilized PBAT composite. The interfacial

bonding between the miscanthus fibers and PBAT was modified with 5 wt% of maleic anhydride

grafted PBAT (MAH-g-PBAT) as a reactive compatibilizer. In the compatibilized PBAT

composites, the improved interfacial interaction between the PBAT and the miscanthus fiber was

corroborated with mechanical, thermal and morphological properties. The compatibilized PBAT

composite with 40 wt% miscanthus fibers exhibited an average heat deflection temperature of

81oC, notched Izod impact strength of 184 J/m, tensile strength of 19.4 MPa, and flexural

strength of 22 MPa. From the scanning electron microscopy analysis, a better interfacial

interaction between the components can be observed in the compaitibilized PBAT composites,

which contribute to enhanced mechanical properties.

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6.1 Introduction

Natural fiber composites (biocomposites) are an attractive material for many applications

because natural fibers are renewable, sustainable, low cost, lightweight, environmental friendly

nature and easy processing. Recently, biodegradable polymer matrix based biocomposites are

widely studied because of some environmental impacts such as waste disposal of non-

biodegradable plastic based materials. Among the biodegradable polymers, poly(butylene

adipate-co-terephthalate) (PBAT) is an aliphatic-aromatic copolyester, which is typically

synthesized from fossil fuel based monomers. PBAT has properties similar to non-biodegradable

polymers like polyethylene. However, the high cost, low heat resistance property, inferior

stiffness and strength are major shortcomings of the PBAT to extend its applications. These

shortcomings can be overcome by the addition of natural fibers/fillers into PBAT matrix. In

order to reduce the cost of PBAT, Muniyasamy et al., [1] and Torres et al., [2] prepared

biocomposite from PBAT and biofuel industry co-products. Due to the polarity difference

between the natural fibers/fillers and polymer matrices, most of the biocomposites are not

compatible between the phases. As a result, chemically modified (sulphuric acid hydrolysis,

silanization and acetylation) curua fibers reinforced PBAT biocomposites were investigated by

Marques et al., [3]. The authors suggest that the chemically modified curua fibers were enhanced

the performance of the resulting biocomposites owing to improve the fiber-matrix

interaction/adhesion. Similarly, PBAT/peanut husk composites were prepared with and without

maleic anhydride grafted PBAT (MAH-g-PBAT) compatibilizer [4]. The compatibilized

PBAT/peanut husk composites exhibited superior mechanical properties than that of

uncompatibilized composites and neat PBAT. Furthermore, the degradability of PBAT,

compatibilized PBAT/peanut husk composites and uncompatibilized PBAT/peanut husk

composites were investigated under biological environments [4]. It has been found that the

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compatibilized PBAT/peanut husk composites showed slightly lower degradation rate than

corresponding uncompatibilized composites. Interestingly, the rate of PBAT biodegradation was

enhanced after incorporation of natural fillers into PBAT [4]. This observation had good

agreement with another study investigated by Muniyasamy et al., [1].

Perennial grasses are good candidate for polymer matrix based composite applications

because of its good fiber properties [5] and reducing “food Versus fuel” competition. Very

limited literature is available for perennial grass (switchgrass, miscanthus, indiangrass,

napiergrass, Arundo donax L, big bluestem, little bluestem, et al.,,) reinforced polymer

composites. Miscanthus (elephant grass) is a typical lignocellulosic C4 grass, which grows in

North America, Europe and Asia. Miscanthus is mostly used for biofuel production and animal

bedding uses. Furthermore, miscanthus fiber based composites are developed for packaging

application under the name of SunaturaTM

[6]. Only few researchers have been investigated the

performance of miscanthus fiber reinforced biocomposites. Consequently, the present study was

aimed to produce a biocomposite from PBAT and miscanthus fibers. In addition, this study was

also employed to improve the compatibility between the PBAT matrix and the miscanthus fibers

by a reactive compatibilization strategy.

6.2 Materials

Commercially available Oyster white PBAT pellets (Biocosafe 2003) were purchased

from Xinfu Pharmaceutical Co., Ltd, China. According to the manufacturer, the density and

melting temperature of PBAT are 1.26 g/cm3 and 110-115

oC, respectively. New Energy Farms,

Ontario, Canada, kindly provided miscanthus fibers with an average length of 4.65±2.5 mm. It

was used as received without further purification and/or modification. In this work, a maleic

anhydride grafted PBAT (MAH-g-PBAT) was used as a compatibilizer. A detailed MAH-g-

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PBAT (MAH grafting level 1.9%) was explained in Chapter 4. Melt flow index (MFI) value of

the MAH-g-PBAT was measured at 190oC with 2.16 kg load according of ASTM D1238. The

MFI value of the MAH-g-PBAT was 44.46±9.96 g/10 min.

6.3 Biocomposite fabrication method

Both PBAT granules and miscanthus fibers were dried at 80oC for 10 h before

composites fabrication. The composite preparation was performed in a lab-scale extrusion and

injection molding technique. The extrusion process was carried out in a co-rotating twin-screw

extruder (DSM Xplore®, The Netherlands) with a barrel volume of 15cc. In order to produce

desired test specimens, the molten extrudate was immediately transferred into 12cc injection

molding device (DSM Xplore®, The Netherland). The extrusion processing was conducted at

140oC for 2 min while screw speed kept constant at 100 rpm and injection mould temperature of

30oC. All the samples including neat PBAT, compatibilized and uncompatibilized composites

were prepared using above said processing conditions. In the present study, the compatibilizer

(MAH-g-PBAT) concentration was fixed at 5 wt% for all the compatibilized composites

preparation. The chosen compatiblizer concentration (5 wt%) was based on optimal

performances of the composites in our previous work (Chapter 5).

6.4 Characterization methods

Tensile, flexural and impact properties of the PBAT and its composites were measured

according to the procedure reported in Chapter 5 (Section 5.2.4). The heat deflection temperature

(HDT), melt flow index (MFI) and fracture surface morphology of the samples were analyzed

based on the procedure reported in Chapter 5 (Section 5.2.6, 5.2.7 and 5.2.9).

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The tensile properties of the neat PBAT and its composites are presented in Figure 6.1.

Neat PBAT had an average tensile strength of 20 MPa. There was a significant reduction in

tensile strength of PBAT after incorporation of miscanthus fibers; this reduction, suggesting that

the compatibility between the PBAT and miscanthus was not sufficient [4]. Similar trend has

been noticed in the PBAT/peanut husk composites [4] and PBAT/sisal fiber composites [7]. In

the presence of compatibilizer (MAH-g-PBAT), the PBAT/miscanthus fiber composites showed

a substantial increase in tensile strength as compared to uncompatibilized PBAT/misccanthus

composites. The observed increased in tensile strength for the compatibilized composite was

attributed to the enhanced compatibility between the components. This observation agreed with

other research works reported elsewhere for PBAT composites [4, 7]. The tensile modulus of the

PBAT was increased with the addition of miscanthus fibers, which can be attributed to the strong

reinforcing capability of miscanthus fibers in the PBAT matrix. The modulus of compatibilized

and uncompatibilized composites was not different. A maximum tensile modulus (~700 MPa)

was observed in the PBAT composite with 40 wt% fiber loading. The observed tensile strength

and modulus values of PBAT/miscnthus fiber composites are comparable with low-density

polyethylene (LDPE) [8].

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Figure 6.1. Tensile strength and tensile modulus of PBAT and its composites; (A) neat PBAT,

(B) PBAT/miscanthus fibers (70/30 wt%), (C) PBAT/miscanthus fibers/MAH-g-PBAT (65/30/5

wt%), (D) PBAT/miscanthus fibers (60/40 wt%), and (E) PBAT/miscanthus fibers/MAH-g-

PBAT (65/30/5 wt%)

The flexural properties of the PBAT and its composites are depicted in Figure 6.2. Both

flexural modulus and flexural strength of PBAT were increased with increasing miscanthus fiber

content up to 40 wt%. Zhang et al., [9] also observed such a type of flexural properties

improvement in the miscanthus fiber reinforced toughened green composites. In another study by

Kirwan et al., [10] found that the flexural properties of the composites with miscanthus were

superior compared to their matrix of the composite. They concluded that the enhanced flexural

properties were attributed to the strong reinforcing effect of miscanthus fibers in the matrix. In

the present study, the flexural strength of the compatibilized composites was higher compared to

their uncompatibilized counterparts and neat PBAT. The enhanced flexural strength of the

compatibilized composite has same trend like tensile strength of compatibilized composites.

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Figure 6.2. Flexural properties of PBAT and its compatibilized and uncompatibilized

composites: (A) neat PBAT, (B) PBAT/miscanthus fibers (70/30 wt%), (C) PBAT/miscanthus

fibers/MAH-g-PBAT (65/30/5 wt%), (D) PBAT/miscanthus fibers (60/40 wt%), and (E)

PBAT/miscanthus fibers/MAH-g-PBAT (65/30/5 wt%).

Notched Izod impact test result of the neat PBAT and its composites are demonstrated in

Figure 6.3. Neat PBAT showed non-break impact strength while PBAT composites showed

hinge break impact strength under selected test conditions. There was a significant reduction in

impact strength after incorporation of 30 wt% miscanthus fibers into the PBAT matrix. This

impact strength reduction was more pronounced in the PBAT composite with the addition of 40

wt% miscanthus fiber. A similar trend was observed in the composites produced from

miscanthus fiber and toughen polymer matrix [9]. In this study, the compatibilized composites

yielded better impact strength compared to their corresponding uncompatibilized composites. In

particular, the impact strength of PBAT/miscanthus/MAH-g-PBAT (55/40/5 wt%) composites

showed 76% improvement in comparison to uncompatibilized PBAT/miscanthus (60/40 wt%)

composites. The observed effect of compatibilization upon the impact strength of resulting

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PBAT/miscanthus composites is consistent with compatibilized miscanthus fiber reinforced

toughen biocomposites impact strength [9].

Figure 6.3. Notched Izod impact strength of PBAT and its compatibilized and uncompatibilized

composites: (A) neat PBAT, (B) PBAT/miscanthus fibers (70/30 wt%), (C) PBAT/miscanthus

fibers/MAH-g-PBAT (65/30/5 wt%), (D) PBAT/miscanthus fibers (60/40 wt%), and (E)

PBAT/miscanthus fibers/MAH-g-PBAT (65/30/5 wt%)

6.5.2 Melt flow index and Heat deflection temperature

Table 6.1 summarises the melt flow index (MFI) and heat deflection temperatures (HDT)

of the neat PBAT, as well as uncompatibilized and compatibilized composites. MFI is one of the

important properties for composite materials because it can determine the possible processing

methods for different application. The MFI of the PBAT composites was lower than that of neat

PBAT. The observed MFI value reduction of the PBAT composites was attributed to the

restriction of polymer chain mobility in the presence of fibers. Due to the high MFI value of the

MAH-g-PBAT compatibilizer (44.46±9.96 g/10 min), the compatibilized PBAT/miscanthus

composites showed slightly high MFI value compared to their corresponding uncompatibilized

composites. The observed MFI values of the both compatibilized and uncompatibilized PBAT

composites are not high enough for mass production of household injection moulding articles

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[12]. The low MFI value of PBAT composites could be modified by the addition of flow

additives. Table 6.1 depicts the HDT values of neat PBAT and its composites with different

concentration of miscanthus fibers. It can be seen that the HDT value of PBAT was around 46oC.

The HDT value of PBAT composites was considerably higher compared to that of neat PBAT.

This increased HDT value of PBAT composites was attributed to the increased stiffness of the

resulting composites. The HDT value of the composite was more pronounced in the presence of

compatibilizer. This may be due to the enhanced compatibility between components as discussed

earlier. Such HDT improvement has been observed in the compatibilized perennial grass

reinforced biocomposites [9, 11].

Table 6.1. Melt flow index (MFI) and heat deflection temperature (HDT) measurement

Samples MFI (g/10 min) @

190oC with 2.16 kg

HDT (oC)

Neat PBAT 9.4±1.8 46.12 ± 1.5

PBAT/miscanthus (70/30 wt%) 1.28±0.13 67.56±2.05

PBAT/miscanthus/MAH-g-PBAT

(65/30/5 wt%)

1.47±0.10 70.92±2.75

PBAT/miscanthus (60/40 wt%) 1.19±0.09 75.74±1.11

PBAT/miscanthus/MAH-g-PBAT

(55/40/5 wt%)

1.59±0.12 80.72±1.97


Figure 6.4 shows the SEM micrographs of tensile fractured compatibilized and

uncompatibilized PBAT composites. In the uncompatibilized composites (Figure 6.4 A), there is

a clear evidence for lack of adhesion between the fibers and matrix phase and fiber pullout.

Furthermore, the fibers were not completely coated with PBAT matrix in the resulting

uncompatibilized composite. This suggests that the affinity between the matrix and fibers is very

205

poor. The observed insufficient interaction between the components is responsible for the

inferior mechanical performance of uncompatibilized PBAT composites. On the other hand, the

compatibilized PBAT/miscanthus fiber composites morphology (Figure 6.4 B) showed better

interfacial adhesion and the miscanthus fibers were well embedded with PBAT matrix. Unlike

uncompatibilized composites, most of the fibers were covered by PBAT matrix in the composites

with compatibilizer, indicating that the compatibilizer (MAH-g-PBAT) played a crucial role to

enhance the interfacial bonding between the phases. Consequently, the compatibilized

composites can lead to stronger stress transfer between the phase compared the PBAT

composites without compatibilizer. The enhanced compatibility between the PBAT and

miscanthus fibers was consistant with its mecahcnial properties.

Figure 6.4. SEM micrographs of uncompatibilized PBAT/misanthus fibers (60/40 wt%)

composites (A) and compatibilized PBAT/misanthus fibers/MAH-g-PBAT (55/40/5 wt%)

composites (B)

6.6 Conclusions

The effect of miscanthus fibers on the performance of PBAT/miscanthus fiber composite

was examined. Due to strong reinforcing effect of miscanthus fibers in the PBAT matrix, the

stiffness of the resulting PBAT/miscanthus composites is higher than neat PBAT. At the same

206

time, the tensile strength and impact strength had negative effect on the addition of miscanthus

fibers into PBAT matrix. This detrimental effect of the uncompatibilized composites tensile

strength and impact strength was considerably improved by compatibilization strategy. The

compatibilized PBAT/miscanthus composites exhibited superior performances more than that of

uncompatibilized composites. The enhanced compatibility was confirmed by microscopic

analysis. Overall, the compatibilized PBAT composite with 40 wt% miscanthus fibers showed a

maxima improvement in mechanical properties. As a result, the prepared sustainable

PBAT/miscanthus composite is possible alternative for non-biodegradable composite materials.

References

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[2] S. Torres, R. Navia, R. Campbell Murdy, P. Cooke, M. Misra, A. K. Mohanty, Green

Composites from Residual Microalgae Biomass and Poly(butylene adipate-co-

terephthalate): Processing and Plasticization, ACS Sustainable Chemistry and

Engineering, 2015, 3 (4): 614-624.

[3] M. V. Marques, J. Lunz, V. Aguiar, I. Grafova, M. Kemell, F. Visentin, A. Sartori, A.

Grafov, Thermal and Mechanical Properties of Sustainable Composites Reinforced with

Natural Fibers, Journal of Polymers and the Environment, 2014, 1-10.

[4] C.-S. Wu, Utilization of peanut husks as a filler in aliphatic–aromatic polyesters:

Preparation, characterization, and biodegradability, Polymer Degradation and Stability,

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[5] W. Liu, A. K. Mohanty, P. Askeland, L. T. Drzal, M. Misra, Influence of fiber surface

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[7] C.-S. Wu, Process, Characterization and Biodegradability of Aliphatic Aromatic

Polyester/Sisal Fiber Composites, Journal of Polymers and the Environment, 2011, 19

(3): 706-713.

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Preparation and properties of compatibilized LDPE/organo-modified montmorillonite

nanocomposites, European Polymer Journal, 2005, 41 (5): 1115-1122.


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Chapter 7: Biocomposites Consisting of Miscanthus Fibres in a Biodegradable Binary

Blend Matrix: Preparation and Performance Evaluation*

*A part of this chapter has been filed US provisional patent application:

A. K. Mohanty, M. Misra, N. Zarrinbakhsh, R. Muthuraj, T. Wang, A. U-Rodriguez, and

S.Vivekanandhan, Biodegradable polymer-based composites with tailored properties and method of

making those, US provisional patent application, Application number-62128736, 2015.

Abstract

Biocomposites were fabricated from miscanthus fibers and a blend composed of

poly(butylene succinate) (PBS)/poly(butylene adipate-co-terephthalate) (PBAT) matrix by

extrusion and injection molding process. The performance of the composites was investigated

with different weight percentages of fiber loading. Due to the strong reinforcing ability of the

miscanthus fibers, the elastic modulus increased dramatically to 2.1 GPa when fiber loading

increased to 50 wt%. The elastic modulus of the composites was evaluated by parallel, series,

Hrisch and Halpin-Tsai theoretical models and the values were compared to experimental values.

Addition of miscanthus fibers into PBS/PBAT blend; it sharply reduced both tensile and impact

strength. These reductions were attributed to the incompatibility between the miscanthus fibers

and PBS/PBAT blend resin. As a result, maleic anhydride functionalized PBS/PBAT blend was

prepared and used as a compatibilizer to improve interfacial interaction between the different

phases in the resulting composites. With the addition of 5 wt% compatibilizer to the composites

a significant improvement in the mechanical properties as compared to corresponding

uncompatibilized counterparts was found. SEM analysis demonstrated strong interface between

fiber and matrix in the compatibilized composites. The aspect ratio of the fibers was drastically

reduced after compounding because of fibers underwent attrition during processing. The shear

thinning behavior was found to increase with increasing fiber content. It could be due to the

209

reduced entanglement of the polymer chains in the composites. The density of the resulting

biocomposites was lower than glass fibers. Over all, the addition of miscanthus fibers into a

PBS/PBAT blend matrix to form composites can offer a significant benefit in terms of economic

competitiveness and functional performance.

7.1 Introduction

Due to global environmental concerns, recent academic research and literature in the field

of polymer has been focused on research into biobased and/or biodegradable polymeric

materials. The US Environmental Protection Agency has reported that most non-durable plastic

waste goes to landfills because recycling of these plastics is still a challenge. Because of this,

material scientists are interested as this can be reduced through the use of biodegradable

polymeric materials. Biodegradable polymers are easily degraded in the presence of naturally

occurring microbes to CO2 and H2O under aerobic conditions or CH4 and H2O under anaerobic

conditions [1]. Commercially available aliphatic and aliphatic-aromatic biodegradable polyesters

such as poly(butylene adipate-co-terephthalate) (PBAT) and poly(butylene succinate) (PBS) are

promising for use in natural fiber composites. Both PBS and PBAT are produced from fossil fuel

based monomers. PBS can be produced from renewable resource based succinic acid with ~54%

biobased content [2], however, these polymers are currently have poor cost-performance as

compared to traditional polymers. This poor cost performance is due to limited production of

these polymers [3].

Interest into this kind of polymers has increased recently because of the reduced carbon

footprint and greenhouse gas emissions associated with their use. Despite this interest and the

potential environmental benefits, these polymers cannot yet be utilized on their own for some

commercial applications because they do not satisfy many product requirements. These

210

shortcomings can be overcome by reinforcing some inexpensive natural fibers into polymer

matrices [4]. As such, in 2010 natural fiber composites production amounted to 430.7 million

pounds in the United States with an associated worth amounting to 289.3 million USD [5]. As a

commodity this market is estimated to reach 531.3 million US$ by 2016, with a short-term

growth rate of 11% in the next 5 years. Effective use of renewable resource derived fibers

provides environmental benefits with respect to ultimate disposability. Natural fibers have many

economic advantages compared to synthetic fibers in addition to being biodegradable,

renewable, and lightweight. Reduce reliance on petroleum oil, reduced tool wear, good specific

strength and minimized hazardous materials emission (noxious gases or solid residue) during

combustion are also beneficial [6, 7].

Miscanthus is a typical lignocellulosic biomass and is a promising fast growing non-food

crop. If this biomass could be successfully reinforced into a polymer matrix, it could increase

revenue for miscanthus growers. Miscanthus fibers are advantageous because it is low cost, high

yield, has low input conditions and a low maturation time, has potential for soil remediation, can

help balance carbon dioxide in the environment, and is able to sequester carbon underground.

Due to these benefits, the best strategy is to combine biodegradable polymers with miscanthus

fibers in order to create cheap sustainable biocomposites with good reinforcement properties.

Bourmaud and Pimbert [7] measured the modulus and hardness value of miscanthus fibers using

the nanoindentation method. They found average modulus and hardness values are 9.49 and 0.34

GPa, respectively. These properties are quite comparable with other agro fibers such as hemp

and sisal [7].

Green composite materials can be derived from natural/bio fibers and biodegradable

polymers and have been developed for many industrial applications [8]. The performance of the

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resulting composite mainly depends on orientation in the matrix, volume fraction of fibers, fiber

distribution, fiber-matrix interaction and ability of stress transfer between the components [8].

Natural fibers are inherently hydrophilic in nature [9] and are as such incompatible the

hydrophobic polymer matrix. This leads to inferior mechanical performance of the resulting

biocomposites. Additionally, during processing, natural fibers have a tendency to disperse poorly

in the matrices due to agglomeration [7]. Therefore, various strategies have been developed in

order to overcome these drawbacks in biocomposites [10]. Some of these strategies include using

a compatibilizing agent/interfacial modifier to improve interfacial bonding between the phases in

the composites [11-15]. Keener et al., [16] used a commercially available and economically

produced maleic anhydride (MAH) grafted polymer as a compatibilizing agent for fabricating

biocomposites. This study has proven that the relatively polar nature of maleic anhydride groups

may have a tendency to form covalent bonds, secondary bonds and mechanical interlocking in

the biocomposites. Another study reported by Tserki et al., [11] showed the effect of cotton

fibers on the mechanical properties of poly(butylene succinate-co-butylene adipate) (PBSA) with

and without MAH grafted PBSA (MAH-g-PBSA) as a compatibilizing agent. This study showed

significant improvement in all mechanical properties as compared to uncompatibilized

composites. A similar trend has been reported in the poly(3-hydroxy-co-3-hydroxyvalerate)

(PHBV)/kenaf fiber biocomposites [13]. Kim et al., [12] used MAH grafted polymers as a

compatibilizer for bamboo and wood flour filled PBS and poly(lactic acid), PLA biocomposites.

In these composites, MAH grafted PBS (MAH-g-PBS) and MAH grafted PLA (MAH-g-PLA)

showed improvements in mechanical and thermal properties when compared to other maleated

compatibilizers. However, only a few researchers have investigated miscanthus fiber reinforced

biodegradable polymer composites. Johnson et al., [17] have studied Mater-Bi®/miscanthus fiber

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composites and found that the impact performances were significantly increased as compared to

neat polymer. Nanda et al., [18] examined a composite made from agro fibers (miscanthus,

switchgrass and soy hull) and a binary blend matrix of PLA/PHBV. The authors concluded that

the miscanthus fiber based biocomposites have superior mechanical and thermo-mechanical

properties compared to many other agro fibers (switchgrass and soy hull) based PLA/PHBV

blend matrix composites. However, un-to-date no literature has been reported on fabrication of

composites from miscanthus fiber with a PBS/PBAT blend matrix. The present study aims to

explore the fabrication and performance evaluation of miscanthus fiber reinforced PBS/PBAT

based biocomposites. The effect of compatibilizer (MAH-grafted-PBS/PBAT blend) on the

resulting biocomposites was investigated by means of mechanical, thermo-mechanical,

morphological, and rheological properties.

7.2 Materials and Methodology

7.2.1 Materials

Miscanthus fibers were kindly supplied by New Energy Farms, Ontario, Canada. The

miscanthus fibers were used as received without any further purification and/or modification.

The predetermined fiber lengths and diameters were 4.65±2.5 and 0.074±0.024 mm,

respectively. Commercially available PBAT (Biocosafe 2003F) and PBS (Biocosafe 1903)

granules were procured from Xinfu Pharmaceutical Co., Ltd (China). Maleic anhydride grafted

PBS/PBAT blend (MAH-g-PBS/PBAT) was prepared in Haake PolyLab at 160oC with 5 phr of

MAH and 1 phr of dicumyl peroxide (DCP) according to the procedure reported in Chapter 4 and

it was used as a compatibilizer in this chapter. The grafting percentage of the compatibilizer was

determined by acid-base titration and the grafting percentage was 2.05%. The melt flow index

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(MFI) of the compatibilizer was 81±24 g/10 min (MFI measured @ 190 oC, 2.16 kg according to

ASTM D1238).

7.2.2 Processing of polymer blend and their composites

Before melt processing, both polymers and fibers were dried at 80oC for at least 10 h in

an oven. According to our previous research findings, PBS/PBAT (60/40 wt%) blend was taken

as a matrix for composites fabrication in the present study [20]. Here after PBS/PBAT (60/40

wt%) blend will be referred as PBS/PBAT blend. This blend matrix based composites were

prepared by incorporating 30, 40 and 50 wt% of miscanthus fibers. For comparison, these

composites were also prepared with the addition of 5 wt% MAH-g-PBS/PBAT. All the

compounding and injection molding were carried out in a DSM Xplore®, The Netherlands. The

capacity of DSM extruder and injection molder was 15 and 12 cm3, respectively. Compounding

of the formulation was done with co-rotating twin-screw extruder with a length of 150 mm and a

L/D of 18. The desired composite test specimens were produced at 140oC with residence time of

2 min, mould temperature at 30oC, and screw speed of 100 rpm. Injection molding was done

with injection pressure of 10 bar for 8 s, and holding and packing pressures of 10 bar for 10 s

each.


The tensile, flexural and impact properties of the PBS/PBAT blend and its composites

were measured according to the procedure reported in Chapter 5 (Section 5.2.4).

7.2.4 Density

An electronic densimeter (Alfa Mirage, model MD-300S) was used to measure the

density of the PBS/PBAT blend and their composites. The density measurement was performed

214

by Archimedes principle. The density of the miscanthus fiber was calculated by rule of mixture,

i.e.,

= + (7.1)

where , and , are the densities and volume fractions of the fiber and matrix,

respectively.


Dynamic mechanical properties were measured according to ASTM D4065 standard in a

DMA Q800, TA instruments Inc, USA. The storage modulus and tan delta results were obtained

as a function of temperature with a temperature ramp of 3 oC/min. The tests were carried out in a

dual cantilever clam with a 15 µm oscillating amplitude and 1 Hz vibrating frequency. Heat

deflection temperatures (HDT) of the samples were measured using same DMA machine as per

ASTM D648 standard. The experiments were carried out in a three point bending clamp at a

heating ramp of 2 oC/min from room temperature to desire temperature. The reported HDT

values of the samples are an average of two measurements.

7.2.6 Fiber length measurements

In order to measure the fiber length and diameter after processing, the composite samples

were dissolved in chloroform and then fibers were isolated by filtering. The isolated fibers were

rinsed thoroughly with the same solvent and dried at 70oC for 24 h. The processed and

unprocessed fibers were photographed through a digital camera and the fiber dimensions were

measured by Image J software (at least 85 individual fibers were measured). The measured fibers

length and diameter were inserted in Minitab®17 statistical software to get fiber distribution

histogram.

215


DSC analyses were performed under controlled nitrogen environment (50 mL/min) by

using TA instruments DSC Q200. The data were recorded for heat flow with respect to

temperatures using standard aluminum pan. The heating ramp rate was kept at 10 °C/min while

cooling ramp rate was kept at 5 °C/min for conventional DSC. The experiment was carried out

from -50 to 180oC, followed by cooling to -50

oC. Then they were reheated from -50 to 180

oC.

The first heating scans were used to remove thermal history of the sample. The results were

collected from the second heating and first cooling scans.


The thermal degradation behavior of the samples was evaluated using TA instruments

TGA Q500. The experiments were performed in a controlled nitrogen flow rate of 60 mL/min

with a heating rate of 20 oC/min.


In order to enhance the electrical conductivity of the specimen surface, the samples were

gold coated prior to observe the composites samples morphology. Morphology of the impact

fractured specimens was observed by Inspect S50-FEI Company scanning electron microscopy.

7.2.10 Rheological property

The rheological property was measured at 140oC using Anton Parr Rheometer MCR 301.

The experiments were carried out in a parallel-plate with 25 mm diameter and 1 mm gap

between the plates. The shear viscosity of the samples was measured from 629 to 0.1 rad/s by

frequency sweep test. Disc shape injection molded samples were used to study the rheological

behaviour of PBS/PBAT blend and their composites.

216



The tensile properties of PBS/PBAT blend and their composites are presented in Figure

7.1. Mechanical properties of the natural fiber reinforced composites are significantly dependent

upon many factors including fiber type, aspect ratio of fibers, orientation of fibers, fiber-matrix

adhesion, fiber dispersion in the matrix, and chemical composition of the fibers [21]. Apparently,

all the composites showed higher tensile modulus value than the PBS/PBAT blend matrix. The

tensile modulus of the composites gradually increased with increasing fiber loading from 30 to

50 wt%. Generally the addition of stiff fibers into polymer matrices restricts polymer chain

mobility and thus increases the stiffness of the resulting composites [22]. However, a reduction

in tensile strength was observed with an increase in fiber content up to 50 wt%. Incorporation of

hygroscopic natural fibers into the hydrophobic polymer matrix has a negative effect on tensile

strength of the composite. A number of researchers have reported a similar effect with natural

fiber composites [18, 22]. This reduction is associated with uncompatibility between the fiber-

matrix as well as lack of sufficient fiber dispersion in the matrix [23]. These drawbacks can be

overcome by modifying composite matrices or by reinforcement. In general, commercially

available MAH grafted polyolefins are more widely used in composite industries as a

compatibilizer [14]. Many studies have documented that the compatibilizers could connect the

fibers and the matrix through chemical bonds like covalent bonds or hydrogen bonds [15].

Therefore, MAH grafted compatibilizers have great potential to surpass the performance of

composites through strong interfacial adhesion between the different phases.

In this study, MAH grafted PBS/PBAT blend was used as a compatibilizer in order to

improve compatibility between the PBS/PBAT blend matrix and the miscanthus fibers. One of

217

the main advantages of modifying PBS/PBAT matrix as a compatibilizer for PBS/PBAT blend

based composites is that the chemical structures are identical and, consequently, compatible. All

the compatibilized composites showed a distinct enhancement in tensile properties in comparison

to their corresponding uncompatibilized composites. These improvements were likely due to the

enhanced interfacial bonding between the fiber-matrix. For instance, incorporation of

compatibilizer (5 wt%) into 50 wt% miscanthus fiber composite showed 69% tensile strength

and 7% tensile modulus improvement as compared to their corresponding uncompatibilized

composites. Similar improvements in the compatibilized biocomposites have been shown in the

literature [23,24]. The improved tensile strength of the compatibilized composites is likely due to

the physical entanglements, which can occur between the compatibilizer and matrix. Due to the

entanglement of polymer chains, the applied stress on the matrix can be effectively transformed

to the fibers [25].

Figure 7.1. Tensile properties of PBS/PBAT blend and their composites: (A) PBS/PBAT, (B)

PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (65/30/5 wt%), (D) PBS/PBAT + miscanthus fibers (60/40 wt%), (E) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (55/40/5 wt%), (F) PBS/PBAT + miscanthus fibers

(50/50 wt%) and (G) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (45/50/5 wt%).

218

Figure 7.2 shows the typical stress-strain curves of the PBS/PBAT blend and their

composites. It was noted that the stress-strain curve of the PBS/PBAT blend clearly shows three

types of behavior i.e., elastic deformation, plastic deformation and strain hardening. However,

the stress-strain curves of all the composites samples shows elastic and plastic deformation after

maximum stress has reached. There was no strain hardening observed in the composites samples.

These observations suggest that composites are less ductile than the PBS/PBAT blend. This is

not surprising because the short fibers reinforced composite does not necessarily increase the

ductility of the composites. Sahoo et al., [26] found a similar trend in PBS based biocomposites.

This ductility reduction is also evidence for the improved stiffness of the composites when

compared to PBS/PBAT blend. A drastic reduction was observed in elongation at break after

incorporation of miscanthus fibers into the PBS/PBAT blend matrix. This is common

observation for almost all the fiber reinforced composites [11]. All the compatibilized

composites exhibited slightly higher elongation at break compared to their corresponding

uncompatibilized composites. This implies that the interfacial adhesion was improved between

the phases [27]. Our findings agreed well with the previous works by Wang et al., [27] and

Zhang et al., [28]. However, the percentage elongation values of their compatibilized composites

were much lower than the PBS/PBAT blend.

219

Figure 7.2. Representative stress-strain curves of PBS/PBAT blend and their composites: (A)

PBS/PBAT, (B) PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus

fibers (60/40 wt%), (D) PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (65/30/5 wt%), (F) PBS/PBAT + miscanthus fibers +

MAH-g-PBS/PBAT (55/40/5 wt%), and (G) PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (45/50/5 wt%).

Figure 7.3 illustrates the influence the flexural properties of PBS/PBAT blend matrix

based composites with and without compatibilizer. The flexural modulus and strength of the

PBS/PBAT blend matrix were 380 and 17 MPa, respectively. Similarly, to the tensile modulus,

the addition of miscanthus fibers into PBS/PBAT blends lead to a remarkable improvement in

the flexural modulus of the composites. Similar occurrence has been reported in flexural

modulus with increasing filler content in different thermoplastic matrix systems [29, 30].

Moreover, the addition of miscanthus fibers into poly(vinyl acetate) (PVAc)/poly(vinyl alcohol)

(PVA) blend resulted in superior flexural properties than PVAc/PVA blend [31]. Normally the

polymer chain mobility is hindered by presence of fibers, consequently the composites become

stiffer and have a higher modulus as compared to their matrix [15]. The compatibilized

220

composites showed slight improvement in flexural modulus compared with their corresponding

uncompatibilized composites. This indicates that the compatibilizer may reduce the fiber

agglomeration in the resulting biocomposites. Additionally, the flexural strength of the

uncompatiblized composites reduced after incorporation of 50 wt% fibers. This could be

associated with weak fiber-matrix interaction, insufficient wetting between the fiber-matrix, and

agglomeration (fiber-fiber interaction) of fibers in the matrix [29]. Interestingly, the flexural

strength of the uncompatibilized composites is still superior in comparison to PBS/PBAT blend

matrix. This phenomenon once again proves that the miscanthus fibers have good reinforcing

capabilities in the PBS/PBAT blend matrix. Overall, flexural properties of the compatibilized

composites were superior to their uncompatibilized counterparts. Similar trends have been

reported elsewhere [15]. The compatibilized composite with 50 wt% miscanthus fiber exhibited

the highest flexural strength (192%) and modulus (520%) compared with PBS/PBAT matrix.

The enhanced fiber-matrix adhesion can be hypothesized as follows: the grafted maleic

anhydride groups of MAH-g-PBSPBAT are able to interact with surface hydroxyl groups of the

miscanthus fibers, while the PBS/PBAT segments are miscible with the bulk PBS/PBAT blend

matrix phase through cocrystallization [32]. The expected reaction mechanism of MAH-g-

PBSPBAT with miscanthus fibers is shown in Figure 7.4.

221

Figure 7.3. Flexural properties of PBS/PBAT blend and their composites: (A) PBS/PBAT, (B)

PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (65/30/5 wt%), (D) PBS/PBAT + miscanthus fibers (60/40 wt%), (E) PBS/PBAT +

miscanthus fibers + MAH-g-PBS/PBAT (55/40/5 wt%), (F) PBS/PBAT + miscanthus fibers

(50/50 wt%) and (G) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (45/50/5 wt%).

Figure 7.4. Expected reaction between the miscanthus fiber and the compatibilizer

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The results in Table 7.1 represent the Izod impact strength of uncompatibilized and

compatibilized composites as a function of miscanthus fibers. The PBS/PBAT blend showed

non-break impact strength. Contarory, with the addition of miscanthus fibers into PBS/PBAT

blend matrix resulted in a significant reduction in impact strength. This is attributed to the failure

mode of PBS/PBAT blend changed from ductile to brittle failure in the presence of fibers. As

explained before, the lignocellulosic fibers are restricting the polymer chain mobility in the

resulting composites, and thus reduce capability to absorb energy during impact fracture [33]. In

addition, the weak interface between the components is also important factor for deteriorated

impact strength of the composites. This can also be confirmed by area reduction under the tensile

stress-strain curves. However, a considerable amount of impact strength was increased after

incorporation of compatibilizer into the composites. The compatibilized composites with 30, 40

and 50 wt% fibers showed 59, 62, and 36% improvement in impact strength when compared to

their corresponding uncompatibilized composites, respectively. These improvements could be

due to enhanced interfacial bonding between the matrix-fiber. Good interfacial adhesion helps to

promote stress transfer between the phase through covalent and/or hydrogen bonds [34].

Furthermore, there is a possibility of mechanical interlocking in the compatibilized composites,

which may occur between the compatibilizer-fiber and/or between the compatibilizer-matrix. It

can be noted that the compatibilized composite with 50 wt% fiber showed lower impact strength

compared to compatibilized composites with fiber content of 30 and 40 wt%. This could be due

to the high fiber content lead to agglomeration and thus weaken the resulting composites [18].

Overall, the observed mechanical properties of the compatibilized PBS/PBAT matrix based

composites suggest that the compatibility between the fiber-matrix has been greatly improved.

223

Table 7.1. Notched Izod impact strength of PBS/PBAT blend and its compatibilized and

uncompatibilized composites

Samples

Notched Izod Impact strength

(J/m)

PBS/PBAT Non-break

PBS/PBAT + miscanthus fibers (70/30 wt%) 64.50 ± 3.59



PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT

(65/30/5 wt%)

102.47 ± 4.36


(55/40/5 wt%)

88.15 ± 2.59


(45/50/5 wt%)

63.42 ± 3.12

7.3.2 Theoretical approximation of Young’s modulus of the PBS/PBAT biocomposites

There are several mathematical models that have been proposed to predict composite

properties [35]. Some models such as parallel, series, Hirsch’s, and Halpin-Tsai models are very

often used to determine the randomly oriented rigid short fiber composite behavior.

In parallel and series models, Young’s modulus can be calculated by using following equations,

Parallel model

Mc = + (7.2)

Series model

Mc =

(7.3)

In both models, Mc, Mm and Mf represent the Young’s modulus of the composites,

matrix, and fibers while Vf and Vm represent the volume fractions of fiber and matrix,

respectively.

224

Hirsch’s model

This model is combination of both parallel and series models. In a fiber composite with

random fiber orientation, the elastic modulus can be predicted by Hirsch’s model. The model is

shown below,

Mc = x ( + ) + (1-x)

(7.4)

where χ is a value (0 to 1), which describes the stress transfer between the fiber and matrix.

Halpin-Tsai and Tsai Pagano model

Theoretical moduli of the aligned discontinuous fiber composites can be determined

according to the Halpin-Tsai (H-T) equation. According to the H-T model, the longitudinal (EL)

and transverse modulus (ET) can be calculated using the following equations (7.5 and 7.6):

EL= Em

(7.5)

ET= Em

(7.6)

where

=

(7.7)

=

(7.8)

where is the measure of reinforcement geometry and it can be defined as:

= 2( (7.9)

225

Aspect ratio of the reinforcement is l/d. In a fiber composite with random (Erandom) fiber

orientation, the elastic modulus can be determined using the Tsai Pagano model and it is written

as:

Erandom = (7.10)

Figure 7.5 compares the theoretical and experimental Young’s modulus of the composites

with different volume fraction of fibers. Both the theoretical and experimental Young’s moduli

of the composites were increased with increasing fiber content from 30 to 50 wt%. However, the

predicated modulus by parallel and series models are significantly deviated from experimental

modulus values. These deviations could be due to the model operating under the assumption that

there is no interaction between the fiber-matrix in the composites. According to this literature

[36], there is possibility for interaction between the composite components. It was observed that

the experimental moduli of the composites had good agreement with theoretically calculated

moduli by the Hirsch model. In the Hirsch model, the parameter x determines the stress transfer

between the fiber and the matrix. In order to determine a linear-fit value with experimental

values, the x values varied from 0 to 1 in the equation 7.4. A best fit was found between

theoretical and experimental values when the x value 0.33 in equation 7.4. The obtained x value

is slightly higher than the carbon fiber reinforced PLA and PHBV composites [37,38]. This

suggests that the PBS/PBAT/miscanthus composites have a more effective stress transfer than

carbon fiber reinforced PLA and PHBV composites. It was found that the experimental and

predicted modulus values of the composites are in good agreement with 30 wt% fiber loading.

When increasing fiber content from 30 to 50 wt% the modulus values are slightly deviating from

the experimental values. This deviation probably attributed to the agglomeration of fibers in the

matrix and thereby the applied stress is not able to transfer uniformly between aggregated fibers

226

to dispersed fibers [35]. It is clear that the predicated moduli using the H-T model are slightly

deviated from the experimental modulus values. This behavior could be attributed to the modulus

difference in the nodes, internodes, stem and leaves of the miscanthus fiber [7]. A similar trend

has been observed in the switchgrass reinforced PBAT/PHBV blend matrix composites [39].

Figure 7.5. Variation of experimental and theoretical values of Young’s modulus as a function

of fiber loading

7.3.3 Dynamic mechanical properties

As shown in Figure 7.6, the storage modulus for PBS/PBAT blend and their composites

varies with respect to temperature. The storage modulus (E’) values of PBS/PBAT blend and

their composites were higher below the glass transition temperature (Tg) which is -19 oC.

However, the E’ values of all the samples observed an abrupt decrease at the Tg of PBS/PBAT

blend. This is due to the fact that the polymer chain mobility increases above the Tg of

PBS/PBAT blend. It was found that the storage modulus values of compatibilized and

uncompatibilized composites were significantly higher as compared to the PBS/PBAT blends.

227

Therefore, it was concluded that the miscanthus fibers had a strong influence on the storage

modulus improvement of the resulting composites. The improved storage modulus was in

accordance with the flexural and tensile moduli values of the PBS/PBAT blend composites.

Nanda et al., [18] reported that the stiffness of the miscanthus based PHBV/PLA composites was

higher than PHBV/PLA matrix due to reinforcing effect of miscanthus fibers. The E’ values of

the compatibilized composites were slightly higher as compared to the corresponding

uncompatibilized composites. A similar observation has been reported in the compatibilized PLA

and PBS composites [12].

Figure 7.6. Storage moduli of PBS/PBAT blend and their composites: (A) PBS/PBAT, (B)

PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers (60/40 wt%),

(D) PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (65/30/5 wt%), (F) PBS/PBAT+ miscanthus fibers + MAH-g-PBS/PBAT (55/40/5

wt%), and (G) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (45/50/5 wt%).

The molecular transitions and energy dissipation of polymeric materials can be

determined by damping factor peaks (tan δ). Figure 7.7 shows the tan δ peaks of PBS/PBAT

blend and their composites. There is no significant shift observed in tan δ peak after the addition

228

of miscanthus fiber into PBS/PBAT blend. This observation is consistent with elsewhere [40].

Incorporation of fibers into PBS/PBAT blends reduces the tan δ peak intensity, which is

attributed with the restriction of the polymer chains mobility in the presence of fibers. In the neat

polymer systems, the polymer chain segments are free from restraints, thus have high intensity

tan δ peaks when compared to the composites. In the present study, compatibilized composites

had a slightly higher damping effect than uncompatibilized composites. This indicates that the

fiber-matrix adhesion was improved with the addition of compatibilizer. The molecular motion

at the interfacial regions contributes to the damping of the materials [31]. As seen in Figure 7.7,

the broadness of the tan δ peak is increased with increasing fiber from 30 to 50 wt%. This is due

to the increased heterogeneity in the composite system, which was supported by SEM analysis.

Figure 7.7. Tan δ of PBS/PBAT blend and their composites: (A) PBS/PBAT, (B) PBS/PBAT +


PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT + miscanthus fibers + MAH-g-



229

7.3.4 Density

It is well known that the density of natural fibers is lower than glass fibers. The low

density of natural fibers enables a reduction in weight of the composite material for many

applications including automotive industries. Density of the PBS/PBAT blend and their

composites is shown in Table 7.2. The densities of the miscanthus fibers reinforced composites

are above 1.3 g/cm3, which is comparatively higher than PBS/PBAT blend. Moreover, the

composites density was gradually increased with increasing fiber content from 30 to 50 wt%.

This is mainly due to the higher density of the miscanthus fiber (1.412 g/cm3) when compared to

neat PBS/PBAT blend (1.252 g/cm3). The calculated density of miscanthus fiber is 1.412 g/cm

3,

which can be compared to some other natural fibers that are reported in the literature [41, 42].

There is no significant difference observed in the compatibilized and corresponding

uncompatibilized composites densities. The observed densities of the both compatibilized and

uncompatibilized PBS/PBAT blend composites are lower than glass fibers [43].

Table 7.2. Heat deflection temperature (HDT) and density of PBS/PBAT blend and its

compatibilized and uncompatibilized composites

Samples HDT (oC) Density (g/cm

3)

PBS/PBAT 74.34 ± 0.63 1.252 ± 0.002

PBS/PBAT + miscanthus fibers (70/30 wt%) 98.98 ± 0.79 1.300 ± 0.001



PBS/PBAT + miscanthus fibers + MAH-g-

PBS/PBAT (65/30/5 wt%)

99.76 ± 2.98 1.306 ± 0.001



100.95 ± 4.95 1.322 ± 0.003



105.68 ± 3.62 1.338 ± 0.002

230


According to ASTM D648, the temperature at which the test specimen deforms by 250

µm under a bending stress of 0.455 MPa is called heat deflection temperature (HDT) of

polymeric materials. HDT plays a vital role in selecting polymeric materials for specific

applications because it represents the maximum working limit temperature of materials. Table

7.2 shows the HDT value of the PBS/PBAT blend and its composites. All the composites have

higher HDT than PBS/PBAT blend. The HDT of the 30, 40 and 50 wt% fiber reinforced

composites was around 100, 100 and 105oC, respectively. According to Nanda et al., [18],

miscanthus fiber reinforced PHBV/PLA composites have higher HDT values than switchgrass

based PHBV/PLA composites. It has been suggested that this is due to the higher stiffness and

reinforcing effect of miscanthus fibers. A marginal improvement was observed while increasing

fiber content from 30 to 50 wt%. This could be because the HDT value of the composites is very

close to the melting point (115 oC) of PBS/PBAT blend. The HDT values of the compatibilized

composites were not significantly different as compared to their corresponding uncompatibilized

composites. From these observations, we can conclude that the PBS/PBAT composite with 30

wt% miscanthus fiber reached an optimum HDT value.


Figure 7.8 shows the thermal stability of miscanthus fibers, PBS/PBAT blend and their

composites as a function of temperature. Table 7.3 summarizes the temperature at 5 (T5), 25

(T25), and 50 (T50) percent weight loss of all the samples. It can be seen that the miscanthus

fibers had a lower onset thermal stability (256oC) as compared to PBS/PBAT blend and its

composites. This could be attributed to the less thermally stable components such as pectin,

proteins and residual moisture. Despite this temperature is being much higher than the processing

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temperature (140oC) of PBS/PBAT blend based composites. As a result, there is no possibility

for thermal degradation of the miscanthus fibers in the prepared composites. A two-step thermal

degradation was observed in all the composites and miscanthus fibers whereas PBS/PBAT had

only one-step degradation. The two-step degradation of miscanthus is due to the degradation of

cellulose (294oC) and lignin (353

oC).

Figure 7.8. Thermogravimetric traces for miscanthus, PBS/PBAT blend and its composites

The thermal decomposition mechanism of the PBS/PBAT blend has been explained in

our previous publication [20]. The first and second step degradation of the composites

corresponds to the miscanthus and PBS/PBAT blend matrix, respectively. Apparently, the

thermal stability of the composites monotonically decreased with increasing fiber loading up to

50 wt%. All the compatibilized composites showed slightly lower thermal stability compared to

the corresponding uncompatibilized composites. This behavior may be due to the less thermal

stable compatibilizer counterbalancing the PBS/PBAT blend content in the composites. Residue

232

remains even after heating to 500 oC under the nitrogenous atmosphere due to the carbonaceous

residue and inorganic matter in the materials [44]. In miscanthus, about 20% char residue was

obtained at 600 oC. Consequently, the char residues of the composites were increased with

increasing fiber loading from 30 to 50 wt%.

Table 7.3. Thermogravimetric data of miscanthus, PBS/PBAT blend and their composites

Sample T5 (oC) T25 (

oC) T50 (

oC) Char residue

at 600 oC (%)

Miscanthus 256 318 354 20.60

PBS/PBAT 365 395 409 1.36

MAHgPBS/PBAT 339 388 405 0.87

PBS/PBAT + miscanthus fiber (70/30 wt%) 297 368 396 6.56





281 357 386 6.60



274 350 383 8.66



271 345 379 10.42


Thermal properties of composites were investigated by non-isothermal DSC analysis.

Figures 7.9 and 7.10 show the melting temperatures (Tm) and crystallization temperatures (Tc) of

the PBS/PBAT blend and their composites with and without compatibilizer. The detailed second

heating and first cooling cycles are summarized in Table 7.4. All the composites and blend

matrix showed bimodal endoderm peaks, which correspond to two different types of crystal

lamella formed during cooling. In the presence of miscanthus fibers, the melting point (114 °C)

233

of PBS/PBAT blend is not affected significantly, as shown in Figure 7.9. However, the melting

enthalpy (∆Hm) of the composites decreased considerably (Table 7.4). This can be attributed to

the reduced volume fraction of the polymer in the resulting composites. The ∆Hm values of both

compatibilized and uncompatibilized composites were significantly reduced in comparison to

blend of PBS/PBAT. This result suggests that the composites require less energy to melt in

comparison to the blend of PBS/PBAT. The melting enthalpy of the composites is directly

related to the amount of polymer present in the composites. As such, less energy is required to

melt a smaller amount of polymer in the composites than in the neat polymers. A double melting

peak of PBS/PBAT blend has clearly separated in the composites. This can be attributed to the

enhanced heterogeneous crystal formation with the addition of miscanthus fiber into PBS/PBAT

blend. The crystallization temperature of PBS/PBAT blend was observed at 94.85oC (Figure

7.10).

Figure 7.9. DSC second heating thermograms: (A) PBS/PBAT, (B) PBS/PBAT + miscanthus

fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers (60/40 wt%), (D) PBS/PBAT +

miscanthus fibers (50/50 wt%), (E) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT

(65/30/5 wt%), (F) PBS/PBAT+ miscanthus fibers + MAH-g-PBS/PBAT (55/40/5 wt%), and

(G) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (45/50/5 wt%).

234

The crystallization rate is slightly decreased after the addition of miscanthus fiber into

PBS/PBAT blend matrix. This is attributed to the miscanthus fibers restricting the polymer chain

mobility and diffusion to the surface of the nuclei [45]. This result is in accordance with the

PHBV/bamboo fiber composites [45], PHBV/pineapple leaf fiber composites [46] and

PHBV/kneaf fiber composites [13]. Lee and Wang [47], however, reported that the PBS

crystallization rate can be enhanced with the addition of natural fibers. Their finding contradicts

results from the present study. This discrepancy could be due to the differences in the nature of

the fibers [46]. Both compatibilized and uncompatibilized composites had a lower ∆Hc value

than PBS/PBAT blend (Table 7.4). This can be attributed to the amount of PBS/PBAT blend

present in the composites.

Figure 7.10. DSC first cooling thermograms: (A) PBS/PBAT, (B) PBS/PBAT + miscanthus

fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers (60/40 wt%), (D) PBS/PBAT +

miscanthus fibers (50/50 wt%), (E) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT

(65/30/5 wt%), (F) PBS/PBAT+ miscanthus fibers + MAH-g-PBS/PBAT (55/40/5 wt%), and

(G) PBS/PBAT + miscanthus fibers + MAH-g-PBS/PBAT (45/50/5 wt%).

235

Table 7.4. Detailed differential scanning calorimetry analysis of the PBS/PBAT blend and their

composites

7.3.8 Measurements of fiber diameter, length, and aspect ratio

The average length and the diameter of miscanthus fibers before and after processing is

summarized in Table 7.5. Before processing, the average length of miscanthus fibers was

4.65±2.5 mm with aspect ratio (L/D) of 63. After processing, however, the fiber length, diameter

and aspect ratio were significantly reduced. The measured fiber lengths were within the range of

0.72-1.07 mm. The reduced fiber length is attributed to the strong shear force, which developed

during the compounding process. During the course of processing, the fibers can undergo

defibrillation. This defibrillation leads to reduce the diameter of the fibers in the resulting

composites.

Figure 7.11 shows the fiber length distribution before and after compounding. Before

compounding the fiber lengths were observed in the wide range of 1.5 to 15.5 mm. However,

after compounding the fiber length distribution became narrower (range of 0.1 to 2.5 mm).

Samples

Tc (oC)

∆Hc (J/g)

Tm (oC)

∆Hm

(J/g)

PBS/PBAT 94.85 36.11 113.94 40.37

PBS/PBAT + miscanthus fibers (70/30 wt%) 92.03 26.22 114.51 27.72





90.81 27.09 114.09 27.42



90.56 26.30 113.95 21.26



91.34 26.03 113.39 22.99

236

Before extrusion, the diameter of the fibers was 0.74±0.024 mm. After being subjected to an

extrusion process a drastic reduction was observed in the fibers diameter. This is because of the

fiber breakage during extrusion in a twin screw extruder [15]. Moreover, the individualization of

the fiber bundles during high mechanical shear produced in the compounding chamber is perhaps

responsible for this observation. Similar trends were observed in the sisal fiber reinforced PBS

composites [48] as well as kenaf fiber reinforced starch grafted PP composites [49].

Table 7. 5. Average fiber length (L), average fiber diameter (D), and aspect ratio (L/D) of the

miscanthus fiber before and after compounding

Samples Number

of fibers

Average

length (L)

(mm)

Average

diameter

(D)

(mm)

Aspect

ratio

(L/D)

Miscanthus fibers before

compounding

85 4.65±2.5 0.74±0.024 6.3

PBS/PBAT+miscanthus fibers

(70/30 wt%)

85 1.07±0.35 0.28±0.011 3.8


(60/40 wt%)

85 1.07±0.34 0.30±0.014 3.6


(50/50 wt%)

85 0.73±0.25 0.24±0.007 3.0

237

Figure 7. 11. Fiber length distribution before and after compounding: (A) as received

miscanthus fibers distribution, (B) fibers distribution in 30 wt% composites, (C) fibers

distribution in 40 wt% composites, and (D) fibers distribution in 50 wt% composites.

7.3.9 Morphology of composites

The performance of the composite materials mainly depends on the interfacial interaction

between the phases and dispersion of all components in a given matrix system. Generally,

efficiently reinforced natural fibers are withstanding from the fiber pullout during fracture

phenomenon. This type of fracture can be observed only in composites with good interfacial

bonding between the phases. A composite with dissimilar polarity constituents resulted in weak

interfacial adhesion between the constituents. The fracture surface SEM micrographs of

uncompatibilized and compatibilized composites with different weight percentage of fiber load

are shown in Figures 7.12 and 7.13. As can be clearly seen, all the uncompatibilized composites

had worse fiber dispersion, fiber debonding, fiber pullout, and fiber aggregation. This gives

238

evidence of poor interfacial bonding between the fiber-matrix and thus reduced performance of

the composites [14].

Figure 7.12. SEM micrographs of uncompatibilized PBS/PBAT blend composites with different

fiber loads: (A) PBS/PBAT+miscanthus fibers (70/30 wt%), (B) PBS/PBAT+miscanthus fibers

(60/40 wt%) and (C) PBS/PBAT+miscanthus fibers (50/50 wt%)

By contrast, the morphology of the compatibilized composites (Figure 7.13) showed less

fiber pullout and better interfacial bonding between fibers-matrix compared to the

uncompatibilized composites. Similar types of interactions have been reported for PBS and

PBAT based composites with MAH grafted compatibilizer [12, 24]. The improved fiber-matrix

adhesion is consistent with enhanced mechanical properties of the compatibilized composites.

Overall, morphological analysis concludes that the fiber-matrix adhesion has improved with the

help of compatibilizer.

239

Figure 7.13. SEM micrographs of compatibilized PBS/PBAT blend composites with different

amount of fiber loads: (A) PBS/PBAT+miscanthus fibers+MAH-g-PBS/PBAT (65/30/5 wt%),

(B) PBS/PBAT+miscanthus fibers+MAH-g-PBS/PBAT (55/40/5 wt%), and (C)

PBS/PBAT+miscanthus fibers+MAH-g-PBS/PBAT (45/50/5 wt%).

7.3.10 Rheological property

Rheological properties can offer a detailed structural-property relationship of polymer

composites. Therefore, the influences of fiber content and compatibilizer on the complex

viscosity of the composites were investigated (Figure 7.14). The PBS/PBAT blend showed

Newtonian flow behavior at lower frequency whereas a slight shear thinning behavior was

observed at higher frequency. This behavior was commonly found in polymer melts because

polymer chain entanglement density drastically reduced with increasing frequency. Additionally,

the average end-to-end distance of the polymer chains increased at higher frequency range.

Complex viscosity of the composites is higher at lower frequency as compared to the PBS/PBAT

blend. Theoretically, the addition of fillers/fibers into the thermoplastic polymer matrix will lead

240

to an increased viscosity of the melt. This is possibly due to the rigidity of the fibers which

restricts the polymer chain mobility in the melt state, thus causing viscosity improvement in the

composites [50].

Figure 7.14. Complex viscosity of PBS/PBAT blend and its composites: (A) PBS/PBAT, (B)

PBS/PBAT + miscanthus fibers (70/30 wt%), (C) PBS/PBAT + miscanthus fibers (60/40 wt%),

(D) PBS/PBAT + miscanthus fibers (50/50 wt%), (E) PBS/PBAT + miscanthus fibers + MAH-g-



In addition, an increase in viscosity can be observed with an increase in the fiber loading.

This may be due to agglomeration of fibers and the increased fiber-fiber interaction in the

composites. A similar effect is commonly observed in all the melt state composites. A steady

viscosity reduction was observed in all the composite samples with increasing frequency. The

complex viscosity of the compatibilized composites was slightly lower compared to the

uncompatibilized counterparts in the tested frequency range. This could be due to the lower

molecular weight of the compatibilizer, following the mixture rule [51]. The MAH grafted

241

PBS/PBAT compatibilizer showed much higher MFI value (81 g/10min at 190oC with 2.16kg)

than the neat PBS/PBAT blend MFI value (33 g/10min at 190oC with 2.16kg). The MFI

improvement of the maleic anhydride PBS/PPBAT compatibilizer is attributed to the molecular

weight reduction in the presence of free radical initiator. Maleated compatibilizer viscosity

should, thereby, be considered when analyzing compatibilized composites melt viscosities. The

same trend has been already reported in the compatibilized PP blends and their composites [52].

7.4 Conclusions

A biocomposite was prepared from PBS/PBAT blend matrix and miscanthus fibers by a

melt process. The moduli of the prepared composites were increased remarkably with the

incorporation of miscanthus fibers into blend of PBS/PBAT matrix. These improvements suggest

that the miscanthus fiber acts as reinforcement in the PBS/PBAT matrix system. The tensile and

impact strength of the composites deteriorated with the addition of miscanthus fibers to

PBS/PBAT blend matrix. This was due to the lack of interfacial bonding between the phases. In

order to improve this issue in the resulting composites, a reactive compatibilizer (MAH-g-

PBS/PBAT) was introduced into the composites. It was found that the mechanical properties of

the compatibilized composites were noticeably increased. This is mainly because of the

improved interfacial bonding between the components, which were demonstrated by SEM. The

DSC analysis revealed that the crystallization temperatures of the composites were lower

compared to the PBS/PBAT blend matrix. This can be attributed to the miscanthus fiber

restricting the mobility and diffusion of polymer chains to the surface of the nuclei. The length

and diameter of the fibers was reduced considerably after compounding. This is due to the

fibrillation, which occurred during compounding. It was also found that the density of all the

composites was slightly higher when compared to the PBS/PBAT blend. From this study, it can

242

be concluded that the prepared biodegradable polymer based composites are a possible candidate

to replace non-biodegradable composites in applications where biodegradability is essential but

extreme thermal and humidity exposure is not required.

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Chapter 8: Influence of Processing Parameters on the Impact Strength of Biocomposites: A

Statistical Approach*

*A version of this chapter has been published in: R. Muthuraj, M. Misra, A. K. Mohanty,

Influence of Processing Parameters on the Impact Strength of Biocomposites: A Statistical

Approach, Composites Part A: Applied Science and Manufacturing, 2015,

DOI:10.1016/j.compositesa.2015.09.003.

Abstract

A biocomposite consisting of miscanthus fibers and a biodegradable polymer blend

(poly(butylene succinate) (PBS) and poly(butylene adipate-co-terephthalate) (PBAT)) matrix

was produced. The flexural strength of the PBS/PBAT/miscanhtus composites was increased

considerably (~127%) when compared to the neat PBS/PBAT blend. This increase was attributed

to the strong reinforcing effect of miscanthus fiber. The tensile strength of the

PBS/PBAT/miscanthus composite was inferior to the PBS/PBAT blend matrix. The impact

performance of the composites was optimized by manipulating the processing parameters such as

processing temperature, fiber length, the holding pressure, and screw speed. A full factorial

experimental design was used to predict the statistically significant variables on the impact

strength of the PBS/PBAT biocomposites. Furthermore, a regression model was developed to

study the impact strength of the composites. The main effects and interaction effects of the

variables were studied using analysis of variance (ANOVA) and factorial plots at 95%

confidence level. The accuracy of the developed model was examined by using residuals plots

and coefficients. Among the selected independent processing parameters, fiber length has a most

significant effect on the impact strength of the PBS/PBAT/miscanthus composites. The least

significant effect on the impact strength was attributed to the holding pressures. A poor

interfacial interaction between the miscanthus fibers and the PBS/PBAT blend was observed by

means of impact fractured surface morphological analysis. From scanning electron microscopy

251

(SEM) analysis, it can be observed that the composites prepared with 2.07 mm miscanthus fibers

had more fiber pullouts than 4.65 mm counterparts. This phenomenon could be responsible for

the observed high impact strength of the composites with 2.07 mm fibers compared to

composites prepared with 4.65 mm fibers.

8.1 Introduction

The increasing environmental pollution throughout the world has placed great emphasis

on eco-friendly sustainable material development. Consequently, more attention has been

focused on a sustainable material development by using bio-based and/or biodegradable

materials instead of petroleum based non-biodegradable materials. Governments in many

countries are supporting the usage of green products, and a reduction of dependence on

petroleum because of the associated environmental benefits [1]. Currently there are many

bioplastics (biodegradable and/or bio-based polymers) available in the market. Among them,

poly(butylene succinate) PBS, and poly(butylene adipate-co-terephthalate) PBAT are promising

biodegradable polyesters. The impact toughness/strength of the PBS is insufficient for a wide

range of applications [2]. Blending PBS with PBAT can enhance the impact and tensile

toughness of the PBS [3]. However, these polymers still cannot be used for wide range of

applications on their own because they cannot fulfill some of the product requirements [4]. These

issues can be overcome by blending, reinforcing, and forming composites with inexpensive

natural fibers in the polymer matrices [1].

Natural fiber (Kenaf, flax, hemp and jute) reinforced composites have been used for

many applications including those in the automotive, electronic, horticultural, packaging,

consumer goods and construction sectors [5]. Miscanthus is an alternative fiber for viable

biocomposite applications. Miscanthus is a typical lignocellulosic perennial grass and is a

252

promising non-food crop, which grows rapidly compared to some other crops. There are many

advantages of utilizing miscanthus as reinforcement in composites such as high yield [6], low

moisture content at harvest [6], low input conditions [7], soil remediation potential [7], good

fiber properties (tensile strength, hardness, and modulus) [8], and thermal stability up to 200oC

[9]. Currently, miscanthus fibers have only limited applications though these could be diversified

by developing viable biocomposites. The key strategy is the combination of bioplastics with

miscanthus fibers, which could create an eco-friendly sustainable biocomposite. Recently, the

performance of miscanthus fibers reinforced in a biodegradable polymer matrix has been

investigated by few researchers [6-8, 10-13]. In order to compare the effect of miscanthus fibers

on the resulting composites, Nagarajan et al., [12] investigated the performance of five different

lignocellulosic fibers (miscanthus, switchgrass, wheat straw, soy stalk and corn stalk) reinforced

poly(hydroxybutyrate-co-valerate) PHBV/PBAT (45/55 wt%) composites. This study revealed

that the miscanhtus fiber reinforced PHBV/PBAT composites exhibited superior properties

compared to other fiber reinforced PHBV/PBAT composites. Similar observations were made in

the miscanthus fibers reinforced PHBV/polylactide (PLA) (60/40 wt%) composites [7]. A recent

study by Zhang et al., [11] reported that the toughened multiphase green composite can be

obtained from miscanthus fiber reinforced PHBV/PBAT/epoxidized natural rubber (ENR)

matrix.

In a multi-phase material, processing parameters and variables play a vital role in the

performance of the resulting material. Recently, many researchers conducted experimental

studies to investigate the performance of heterogeneous composite materials. Johnson et al., [13]

used a two-level factorial design to investigate the influence of processing parameters such as

temperature, screw speed, filler content, and size on the impact performance of Mater-

253

Bi®/miscanthus composites. In another study [6] a Box-Cox transformation method was used to

examine the influence of processing parameters on the performance of the Mater-Bi®/miscanthus

fiber composites. From these studies, it was noted that processing temperature has more

influence on the performance of the composites. The significant influences of processing

parameters (processing temperature, screw speed, humidity, filler content, and the aspect ratio of

filler) on the elastic modulus, heat deflection temperature and impact strength of the Mater-

Bi®/wood flour composites has been studied by Morreale et al., [14]. The selected processing

variables are based on specific industrial target applications including automotive indoor

furnishing and panels. The filler aspect ratio had more influence on the impact strength while

filler content exhibited more influence on the heat deflection temperature as well as the elastic

modulus. Another study by Kirwan et al., [5] studied the influence of processing parameters on

the flexural properties of poly(vinyl alcohol), PVA/poly(vinyl acetate), PVAc/miscanthus

composites. The authors found that the processing temperature to be the most influencing factor

on the flexural properties followed by the washing of fibers. Most studies in current literature

investigate the composites (short fiber reinforced biodegradable polymer blend matrix based

composites) characteristics with fixed processing parameters. The aim of this work was to

fabricate biocomposites using a bioplastic blend of PBS/PBAT (60/40 wt%) as the matrix and

miscanthus fibres as the reinforcement with several independent processing variables (processing

temperature, screw speed, holding pressure, and fiber length). The influence of the processing

variables on the performance of the biocomposites was investigated by using a statistical

approach, i.e., full factorial design and analysis of variance (ANOVA).

254

8.2 Full factorial design methodology

More than one factor can affect an experimental result. In general, a factorial experiment

studies the simultaneous effects of two or more factors on experimental results [15]. In the

literature, it was suggested that there are many independently controllable processing

parameters/factors (processing temperature, mixing speed, pressure, reinforcement amount, and

size) which influence the performance of the resulting biocomposite [6, 13, 14]. As such, a

detailed investigation was conducted to fix the upper and lower limits of the independent

processing parameters for PBS/PBAT/miscanthus composites fabrication. The miscanthus fibers

can be melted and compounded with polymers at up to 200 oC without exhibiting any major

thermal decomposition [6]. As a result the miscanthus fiber reinforced composites preparation

should be performed below said temperature. Shear force occurs during the extrusion process

and can damage the fiber geometry. The mixing speed or screw speed of the composites can play

a vital role in maintaining sufficient fiber geometry (aspect ratio). For instance, high screw speed

may cause more fiber breakage while low screw speed can lead to less homogeneity of the

components in the composites. In this study miscanthus is used as a short fiber which should

have a wide range of aspect ratio distributions. This work aims to study the effect of two

different fiber lengths (2.07 and 4.65 mm) on the mechanical performance of the composites.

One of the hypotheses that were tested in this work is that high holding pressure will lead to a

composite with higher performance. High holding pressure may also help to reduce the shrinkage

of the resulting composites.

The processing temperature, fiber length, holding pressure and screw speed were selected

to be the variables in the factorial design. Two levels were assigned for each of these parameters

for the composite fabrication as shown in Table 8.1.

255

Table 8.1. Selected processing parameters and their respected levels

S.No Factor Notation Lower level Higher level

1 Temperature (oC) A 140 180

2 Holding pressure (bar) B 6 10

3 Screw speed (rpm) C 100 150

4 Fiber length (mm) D 2 4

These parameter levels were selected after a series of screening experiments had been

conducted. In order to fully understand the interaction between the parameters, a full factorial

design was selected in a 2k experimental design. In this study, randomization was carried out to

increase the precision of the experimental results by reducing the sampling variability. It was

assumed that the experimental results between the two levels are linear. The experimental design

was performed in statistical software Minitab®17 and, the same software was used to analyze the

results by means of statistical plots (main effect plot, interaction effect plot, normal probability

plot, residual plots, and Pareto plot) at a 95% confidence level.

8.3 Materials

Poly(butylene succinate), PBS and poly(butylene adipate-co-terephthalate), PBAT were

obtained from Xinfu pharmaceuticals Co. Ltd, China. Both PBS and PBAT are semi-crystalline

grade produced from fossil fuel based monomers. The PBS and PBAT have onset thermal

degradation temperatures of 372 and 377oC, respectively [16]. Two different lengths (2.08±0.95

and 4.65±2.45 mm) of miscanthus fiber were kindly supplied by New Energy Farms, Ontario,

Canada. Hereafter, these two fiber lengths (2.08±0.95 and 4.65±2.45 mm) will be referred as 2

and 4 mm. All the materials were used without any further purification.

256

Table 8.2. Physical and mechanical properties of the PBS/PBAT blend and miscanthus fibers

Properties Values

Melt flow index of PBS/PBAT (60/40 wt%) 33±3 g/10min

(190oC with 2.16 kg)

Notched Izod impact strength of PBS/PBAT blend Non-break [3]

Onset thermal degradation of miscanthus fiber* ~260oC

Density of the miscanthus fibers* 1.41 g/cm3

Modulus of the miscanthus fibers 9.49 GPa [8]

*Thermal stability and density of miscanthus fibers were measured in our previous study

(Chapter 7)

8.4 Experimental procedure

8.4.1 Samples preparation

Based on our previous study [16], we have selected a PBS/PBAT (60/40 wt%) blend as

an optimum composition for composites fabrication. Hereafter, the PBS/PBAT (60/40 wt%)

blend will be referred as PBS/PBAT blend. Table 8.2 shows some of the general properties of

miscanthus fiber and PBS/PBAT blend. A 30 wt% miscanthus fiber is selected to fabricate

biocomposites with a blend of biodegradable polymer (PBS/PBAT) matrix. The choice of using

miscanthus fiber in this present work was because of its good fiber properties and the strong

potential supply. Prior to melt compounding, both polymers and miscanthus fibers were dried at

80°C at least 12 h. Appropriate amounts of dried polymers and fibers were manually pre-mixed

at the solid state and the composites fabrication was performed by changing four processing

variables, as shown in Table 8.3. The PBS/PBAT/miscanthus fiber composites were prepared in

a lab-scale extrusion and injection molding process. The lab-scale co-rotating twin-screw

extruder (DSM explore, Netherlands) and injection molding machine (DSM explore,

Netherlands) had volume of 15 and 12 cm

3, respectively. The composite samples were molded

257

with a mold temperature of 30oC and the residence time of the materials inside the extrusion

barrel was 2 min.

Table 8.3. The 16 investigated experimental conditions

Experiment Temperature (oC) Screw speed (rpm) Holding pressure (bar) Fiber length (mm)

1 140 100 10 2

2 140 150 10 4

3 180 150 6 2

4 180 100 6 2

5 180 100 6 4

6 180 150 10 2

7 180 100 10 2

8 140 150 10 2

9 140 150 6 4

10 140 150 6 2

11 140 100 6 4

12 180 100 10 4

13 140 100 10 4

14 180 150 6 4

15 180 150 10 4

16 140 100 6 2

8.5 Characterization methods

8.5.1 Fiber dimension measurement

In order to measure the fiber length after processing, the composite samples were

dissolved in chloroform and then fibers were isolated by filtering. The isolated fibers were rinsed

thoroughly with the same solvent and dried at 70oC for 24 h. The processed and unprocessed

fibers were photographed through a digital camera (Nikon AF-S DX) and the fiber dimensions

were measured by Image J software (at least 85 individual fibers were measured). The measured

fibers length was inserted in Minitab®17 statistical software to get fiber length distribution

histogram.

258

8.5.2 Mechanical testing and scanning electron microscopy (SEM)

All the prepared test specimens were conditioned at room temperature for at least 40 h

before evaluating mechanical performances. Universal Testing Machine (Instron, Model-3382)

was use to measure the flexural and tensile properties of the test samples in accordance with

ASTM D790 and ASTM D638, respectively. The flexural testing was performed with a cross-

head speed of 14 mm/min. The tensile properties of neat PBS/PBAT blend and all the

composites were measured with a cross-head movement of 50 and 5 mm/min, respectively.

Notched Izod impact testing was performed according to ASTM D256 in a TMI monitor impact

testing machine using a 5 ft.lb pendulum. The reported tensile and flexural properties are

averages of five samples for each formulation. Minimum six test samples were tested for each

formulation and the average values are reported for impact strength. The morphologies of the

fracture surface were observed by using a SEM. Prior to observation of the sample morphology;

the samples were sputter-coated with a thin layer of gold. The analysis has been performed in

Inspect S50-FEI SEM.



Table 8.4 represents the mechanical properties of biocomposites as well as the factors

that are used for each experiment. Due to the reinforcing effect of miscanthus fibers, the flexural

strengths of all the composites were higher than that of the neat PBS/PBAT blend (denoted as

control). A similar trend has been observed in the PHBV/PBAT/miscanthus composites and

rubber toughened PHBV/PBAT/miscanthus fiber composites [11, 12]. Kirwan et al., [5] have

found to improve flexural properties of miscanthus fiber reinforced PVA/PVAc blend. Due to

strong reinforcing capability of miscanthus fibers, both tensile and flexural modulus of the

composites increased (data not shown) compared to neat PBS/PBAT blend. On the other hand,

259

all the composites showed inferior tensile strength as compared to control sample (matrix). The

observed reduction is attributed to the incompatibility between the miscanthus fiber and the

PBS/PBAT blend matrix. Such a reduction is very often observed in natural fiber reinforced

thermoplastic composites [7]. The miscanthus fibers have higher wax and silica content

compared to wood fibers which may be responsible for the incompatibility between the

PBS/PBAT matrix and the miscanthus fibers [13]. The tensile and flexural strengths of the

PBS/PBAT/miscanthus composites are not significantly affected with varying processing

parameters. Similarly, tensile and flexural modulus of the PBS/PBAT/miscanthus composites

was not affected significantly with varying processing parameters (data not shown). The Izod

impact strength of the control sample (matrix) showed non-break behavior under tested impact

conditions. Contrastingly, all composite samples showed hinge break behavior under the selected

test conditions. This phenomenon was attributed to the incorporation of stiff fibers into a ductile

polymer matrix. Taking this into consideration, the observed impact strength of the

PBS/PBAT/miscanthus composites was still superior to carbon fiber reinforced composites such

as PLA/carbon fiber (70/30 wt%) [17], PHBV/carbon fiber (70/30 wt%) [18], poly(trimethylene

terephthalate), PTT/carbon fiber composites (70/30 wt%) [19]. It can be noted that the

processing parameters have more influenced on the impact strength of PBS/PBAT/miscanthus

composites.

260

Table 8.4. A complete summary of all the experiments and the related mechanical properties of

PBS/PBAT/miscanthus composites

Experiment Temperature

(oC)

Screw

speed

(rpm)

Holding

pressure

(bar)

Fiber

length

(mm)

Tensile

strength

(MPa)

Flexural

strength

(MPa)

Impact

strength

(J/m)

1 140 100 10 2 21.9±0.26 37.48±0.23 82.34±4.55

2 140 150 10 4 18.8±0.34 34.62±0.30 66.45±3.50

3 180 150 6 2 19.9±0.30 38.61±0.66 76.48±7.85

4 180 100 6 2 19.4±0.34 37.57±0.62 77.04±3.16

5 180 100 6 4 21.6±0.54 39.08±0.43 62.94±2.70

6 180 150 10 2 20.7±1.13 37.86±0.92 67.17±3.08

7 180 100 10 2 19.8±0.40 36.96±2.08 70.90±2.85

8 140 150 10 2 21.1±0.41 37.62±0.27 79.29±6.10

9 140 150 6 4 19.0±0.43 34.62±0.70 67.00±3.13

10 140 150 6 2 19.6±1.05 34.72±0.83 75.39±3.50

11 140 100 6 4 19.3±0.49 33.93±0.35 70.93±3.00

12 180 100 10 4 19.8±0.81 33.99±0.50 67.89±5.00

13 140 100 10 4 19.2±0.55 34.51±0.68 68.19±4.51

14 180 150 6 4 21.6±0.18 38.74±0.92 62.57±4.37

15 180 150 10 4 19.7±0.76 34.39±0.16 62.76±3.79

16 140 100 6 2 19.7±0.67 34.89±1.24 77.55±2.95

Control 140 100 6 0 32.9±1.24 17.12±0.27 non-break

8.6.2 Analysis of variance (ANOVA) for impact strength

ANOVA is a statistical model, which can be used to investigate the significant main and

interaction effects of factors with respect to response. The model had 15 degrees of freedom with

four factors and two levels. To estimate the individual and interaction factors upon the impact

strength, sum of square (SS), mean square (MS), F-test statistics and p-values are presented in

the ANOVA Table 8.5.

261

Table 8.5. Analysis of Variance (ANOVA) for notched Izod impact strength

Source DF Sum of

squares (SS)

Mean squares

(MS)

F P

Temperature 1 96.97 96.97 6.63 0.050

Screw speed 1 26.70 26.70 1.82 0.235

Holding pressure 1 1.51 1.51 0.10 0.761

Fiber length 1 374.71 374.71 25.61 0.004

Temperature*Screw speed 1 0.07 0.07 0.01 0.946

Temperature*Holding pressure 1 15.43 15.43 1.05 0.352

Temperature*Fiber length 1 2.70 2.70 0.18 0.686

Screw speed*Holding pressure 1 2.75 2.75 0.19 0.683

Screw speed*Fiber length 1 0.17 0.17 0.01 0.917

Holding pressure*Fiber length 1 4.63 4.63 0.32 0.598

Error 5 73.16 14.63

Total 15 598.81

S = 3.82523 R-Sq = 87.78% R-Sq(adj) = 63.35%

In this study we have used an alpha level (α) = 0.05 for each F test to analyze the factorial

design experiment. Usually, the higher value of F-ratio suggests more influence of that factor on

the experiment response. According to an F-test, F=25.61 has a p-value of 0.004. Since the p-

value is less than 0.05, we then have sufficient evidence to conclude that the mean impact

strength of the biocomposites was significantly influenced by fiber length. At a 95% confidence

interval (P<0.05), it should be noted that the screw speed and holding pressure do not show

significant effects on the impact strength of the composites. The interaction effects did not

significantly influence the impact strength of the composites. The square correlation coefficient

(R2) was used to judge the adequacy of the developed model fit. The R

2 value can be interpreted

262

as the percentage reduction in the total variation in the experiment obtained by using the

developed model. The typical R2 value is 0 ⩽ R

2 ⩽ 1. The value of R

2 (87.78%) and R

2adj

(63.35%) is substantial and hence the developed model fits the experimental results very well.

8.6.3 Effect of processing parameters on the impact strength

Among the mechanical properties, impact strength was more affected by the processing

factors than other mechanical properties. The impact strength of the short fiber composites

mainly influenced by many factors including matrix intrinsic properties, optimum fiber-matrix

interaction, fiber concentration, fiber geometry, fiber-matrix stress transfer efficiency, fiber

orientation, and fiber dispersion and distribution [20]. At the same time, the fiber bridging, fiber

pull-outs, crack propagation and matrix deformation mechanisms contribute a vital role in the

impact rupture of short fiber reinforced composites [21]. Many of these mechanisms contribute

simultaneously during impact tests, which make it complicated to determine the impact strength

of the composites. Therefore, it is important to investigate the statistically significant factors

upon the impact performance. Morreale et al., [14] have studied the impact performance of the

composites with varying processing parameters. In addition, John et al., [6, 13] have performed a

systematic study of impact performance of Mater-Bi®/miscanthus composites by using factorial

design.

In the present study, the effect of processing parameters/factors on the notched Izod

impact strength was statistically analyzed. More specifically, the statistical analysis was mainly

focused on determining which factors and interactions parameters had more influence on the

Izod impact energy of the biocomposites. Generally, the plot which provides a response with

respect to the changes in the levels of the factors is called the main effect plot. Figure 8.1 shows

the influence of the investigated factors on the impact strength of the resulting biocomposites.

263

Figure 8.1. Main effect plot for the impact strength

Factors with steeper slopes have larger effects and thus a greater influence on the results.

From the main effect plots, it can be observed that the temperature and fiber length factor levels

have more significant effect, which is evidenced with a strong line slope. On the other hand,

holding pressure has almost no effect on the response when varying its levels. The composites

prepared with low temperature processing (140oC), low screw speed (100 rpm), and small fiber

length (2 mm) have higher impact strength compared to those produced with high processing

temperature (180oC), high screw speed (150 rpm) and high fiber length (4 mm). The holding

pressure did not have a great effect on the impact strength upon changing levels such as 6 and 10

bar.

Figure 8.2 represents the statistically significant binary interaction between the selected

variables. The joint effects of two factors such as fiber length/holding pressure, fiber

length/screw speed, holding pressure/screw speed, fiber length/temperature, holding

pressure/temperature, and screw speed/temperature were investigated. If there was no interaction

between the selected variables, the lines on the display should have been approximately parallel.

264

When the response of two factors was not parallel, this indicates a possible interaction between

the selected factors.

Figure 8.2. Plot of interaction effects for the impact strength of biocomposites

Among the selected variable combinations, it can be noted that the most significant

interaction variables are holding pressure/temperature, fiber length/temperature, screw

speed/holding pressure, and fiber length/holding pressure. There is no significant interaction

between the temperature/screw speed and screw speed/holding pressure on the impact strength of

the resulting biocomposites. It can be concluded that the selected variable combinations

(temperature/screw speed and screw speed/fiber length) behave separately, which are not

dependent on each other. Out of the selected four variables the fiber length had the most

significant effect on impact strength while screw speed had the least significant effect.

265

Figure 8.3 shows the significant factors influencing the impact strength of the

PBS/PBAT/miscanthus composites with a confidence level (α) of 0.05. According to the half

normal probability plot the points which are farther away from the fitted line represent the

significant effect on the impact strength. The points which appear close to the straight line

indicate insignificant effects on the impact strength. In the Figure 8.3, it can be seen that all the

significant factors (temperature and fiber length) are represented as square symbols while those

not significant factors are presented as circle symbols.

Figure 8.3. Half Normal probability plot of the standardized effects for impact strength of the

PBS/PBAT/miscanthus composites

The individual and interaction factors for the impact strength of the biocomposites can be

investigated using a horizontal Pareto chart and the results are shown in Figure 8.4. A Pareto

chart is a bar chart that orders the bars from largest to smallest along with a vertical line. This

chart is often used to analyze the statistical significant difference of the individual and interaction

effects on the response. The vertical line in the Pareto chart indicates the significant factors on

266

the response. For example, the bars extended to the right hand side of the vertical line are

significant.

Figure 8.4. Pareto chart of the standardized effects for the impact strength of the

PBS/PBAT/miscanthus biocomposites

In the present study, a Student’s t-test was performed in a Pareto chart with 15 degrees of

freedom at a 95% confidence interval. The t-value (vertical line in the chart) was found to be

2.57 which determine the significant factors and/or interactions on the impact strength of the

composites. The fiber length (D) had significant effect upon the impact strength of the

composites because the standardized effect value is higher than vertical line standardized effect

value 2.57 (t-value). The processing temperature exhibited significant effect on the impact

performance of the Mater-Bi®/miscanthus composites [6, 13]. Contrary to our present result

particle size did not significantly influenced the impact performance of the

biopolymer/miscanthus composites [6, 13]. This could be due to the morphological difference

between the materials due to changing the processing variables.

267

Generally, the tensile toughness of the composites can be calculated from area under the

stress-strain curve. Figure 8.5 shows the stress-strain curves of PBS/PBAT composites with 2

mm miscanthus fibers (B) and 4 mm miscanthus fibers (A). These two composites are prepared

with same processing conditions whilst changing fiber lengths. It can be noticed that the

composites prepared with 2 mm fibers showed better tensile toughness compared to composites

prepared with 4 mm fibers. This result has good agreement with observed impact strength of the

composites with 2 mm fibers.

Figure 8.5. Tensile stress-strain curves of PBS/PBAT/miscanthus composites with changing

fiber length 4 mm (A) and 2 mm (B)

8.6.4 Fiber length distribution

Before and after processing, the length distribution of miscanthus fibers is shown in

Figure 8.6. After processing, the fiber length distribution is broader compared to before

processing. At the same time, the fiber length is drastically reduced. It can be observed that the

length of 4.65 and 2.07 mm miscanthus fibers is not significantly different after processing. For

instance, after processing, the miscanthus fibers with average length reduced from 4.65±2.5 to

1.07±0.34 mm and from 2.07±0.94 to 0.80±0.39 mm. After processing, the length distribution of

268

4.65 mm fibers is varied from 0.45 to 1.9 mm while 2.07 mm fibers varied from 0.2 to 1.9 mm.

This is because of the fiber breakage during extrusion in a twin-screw extruder [22].

Figure 8.6. Histograms of miscanthus fiber length distribution before and after compounding in

a twin screw extruder

Moreover, the individualization of the fiber bundles during high mechanical shear

produced in the compounding chamber is perhaps responsible for this observation. Similar trends

were observed in the sisal fiber reinforced PBS composites [23] as well as kenaf fiber reinforced

starch grafted PP composites [24]. The fiber length has a strong influence on mechanical

performances [25]. It can be noted from Figure 8.6, most of the fibers distributed with >0.9 mm

length in the composites fabricated with 4.65 mm fibers. On the other hand, the composites

15129630

25

20

15

10

5

0

Mean 4.649

StDev 2.453

N 85

Fiber Length (mm)

Nu

mb

er

of

Fib

ers

Long fibers before processing

1.81.51.20.90.60.30.0

18

16

14

12

10

8

6

4

2

0

Mean 1.073

StDev 0.3466

N 85

Fiber length (mm)

Nu

mber o

f fi

bers

Long fibers after processing

15129630

30

25

20

15

10

5

0

Mean 2.075

StDev 0.9477

N 85

Fiber length (mm)

Nu

mb

er

of

fib

ers

Short fibers before processing

1.81.51.20.90.60.3-0.0

12

10

8

6

4

2

0

Mean 0.8014

StDev 0.3950

N 85

Fiber length (mm)

Nu

mb

er

of

fib

ers

Short fibers after processing

269

processed with 2.07mm fiber composites showed most of the fibers distributed <0.9 mm length

in the resulting composites. Based on the fiber distribution after processing, more number of

fiber ends can be observed in the composites prepared with 2.07 mm miscanthus fibers compared

to 4.65 mm miscanthus fibers. Consequently, more number of fiber pullouts can be expected

from the composites prepared with 2.07 mm miscanthus fibers compared to 4.65 mm miscanthus

fibers counterpart. The occurrence of more fiber pullout may be responsible for the observed

high impact strength in the composites prepared with 2.07 mm miscanthus fibers.

Table 8.6. Average fiber length (L), average fiber diameter (D), and aspect ratio (L/D) of the

miscanthus fiber before and after compounding

In general, the composites with a higher aspect ratio fiber should provide superior impact

strength than the composites with lower aspect ratio fiber. For a given composites system, the

recovered fiber length and aspect ratio were examined (Table 8.6) which revealed that the longer

fibers had a higher aspect ratio (3.8) than short fibers (3.2) after processing. The composite with

lower aspect ratio showed higher impact strength while the composite with higher aspect ratio

had lower impact strength. This could be due to the difference in the fiber orientation during

sample preparation [5, 26, 27]. The high aspect ratio fibers can align across the samples and thus

fail to effectively transfer stress between the fiber and matrix. During impact fracture the crack

Samples Number

of fibers

Average length

(L) (mm)

Average diameter

(D) (mm)

Aspect

ratio(L/D)

Long fibers before

compounding

85 4.65±2.5 0.74±0.024 6.3

Short fibers before

compounding

85 2.07±0.94 0.29±0.13 7.13

Long fibers after compounding 85 1.07±0.34 0.28±0.11 3.8

Short fibers after

compounding

85 0.80±0.39 0.25±0.09 3.2

270

initiation and propagation are mainly influenced by matrix behavior and morphology of the

sample, respectively [28]. This phenomenon could play a vital role on the impact strength of

PBS/PBAT/miscanthus composites when changing the fiber lengths.


The impact strength of the short fiber reinforced composites is influenced by many

parameters including fiber pull-out and degree of adhesion [29]. In order to study the impact

fracture mechanism of PBS/PBAT/miscanthus composites, the surface morphology of the impact

fractured samples was investigated by SEM analysis. In the short fiber composites the fibers with

subcritical aspect ratio lead to fiber pullout during fracture [30]. Figure 8.7 (a) and (b) represent

the SEM morphology of the 2 mm miscanthus fiber reinforced PBS/PBAT composites and 4 mm

miscanthus fiber reinforced PBS/PBAT composites, respectively.

Figure 8.7. Represents the SEM micrographs of the PBS/PBAT/miscanthus composites; (a)

PBS/PBAT composites with 2 mm miscanthus (b) PBS/PBAT composites with 4 mm

miscanthus

The SEM micrographs of both composites indicate that the fiber pullout mechanism and

poor interfacial bonded regions played eminent role during impact fracture of the composites.

There was no clear morphological difference witnessed in the composites with 2 and 4 mm

271

fibers. However, the observed impact strength difference between the 2 and 4 mm fiber

composites may be due to the combined effects of pullout, energy dissipation mechanism and

fiber orientation [30, 31]. Further work could be performed to find out which mechanism is

responsible to determine the impact strength of miscanthus fibers reinforced PBS/PBAT

composites.

8.6.5 Mathematical model development

The predicated response of the composites is “Y” and it can be represented by equation

(8.1) as a function of independent factors:

Y=f(A,B,C,D) (8 .1)

The polynomial equation was used to explain the main and interaction effect of all the

independent variables [15]. The polynomial equation can be expressed as follows,

Y = X0 + X1 (A) + X2 (B) + X3 (C) + X4 (D) + X5 (AB) + X6 (AC) + X7 (AD) + X8

(BC) + X9 (BD) +X10 (CD) + X11 (ABC) +X12 (ABD) +X13 (ACD) + X14 (BCD) +

X15 (ABCD) (8.2)

The term X0 represents average response (impact strength) value, X1, X2,….X15 is the

regression coefficient of main and interaction effects, A is processing temperature, B is screw

speed, C is holding pressure and D is fiber length. In the equation (8.2), three and four factor

interactions are not considered due to their insignificance [32]. The equation (8.2) can thereby be

modified as;

Y = X0 + X1 (A) + X2 (B) + X3 (C) + X4 (D) + X5 (AB) + X6 (AC) + X7 (AD) + X8 (BC) +

X9 (BD) + X10 (CD) (8.3)

The regression coefficients were calculated using MINITAB®17 statistical software for impact

strength. Substituting significant factor coefficients into Equation 8.3, it can be rewritten as

follows:

272

Y (impact strength) = 70.931 – 2.462 (temperature) – 4.839 (fiber length) (8.4)

When compared to longer and shorter miscanthus fiber reinforced PBS/PBAT composites, the

composites with a shorter fiber showed higher impact strength than the composites with a longer

fiber. The observed impact strength difference could be mainly due to the difference in fiber

distribution in the matrix. Generally, the longer fibers can lead to agglomeration and thus

favoring for crack initiation and poor stress transfer during impact test.

8.6.6 Diagnostic verification of the developed model

The assumption underlying the analysis of variance for each experimental design is

similar to those required for a regression analysis. Assumptions for a completely randomized

design are that the data for the treatment have normal probability distribution with equal

variances. The assumptions can be checked with the residual plots. The normal probability

plot/normal plot for the notched Izod impact strength of the biocomposites is shown in Figure

8.8. To meet the normality assumption points should fall close to straight line on the normal plot.

The normal plot of the impact strength data is dispersed along a straight line, which indicates that

the assumption of normal distribution is valid. The plot of residuals versus fit can be used to

ensure the linear model adequacy.

273

Figure 8.8. Normal probability plot of the residuals for impact strength

Figure 8.9 shows the plot of the residual versus fit for impact strength of the

PBS/PBAT/miscanthus composites. Figure 8.9 shows the variation of impact strength from –1.5

to 1.5 J/m in between fitted and observed values. From the residual versus fit plot, the random

scatter of residuals around the horizontal line can be seen which indicates that the model is

adequate for impact strength data.

Figure 8.9. Residual plots versus fitted values for impact strength

274

The typical residual plot in Figure 8.10 represents the residual versus observation order of

the impact strength of PBS/PBAT/miscanthus composites. There is no distinct pattern observed

in the residuals plot. Both positive and negative residuals are evenly distributed along the

observation order in Figure 8.10. This observation suggests that the impact strength of

PBS/PBAT/miscanthus composites is distributed normally.

Figure 8.10. Variation of the residuals with observed order values of the impact strength of the

PBS/PBAT/miscanthus composites.

8.7 Conclusions

In conclusion the miscanthus fibers can be used as a reinforcing agent for tough

biodegradable polymers. The stiffness and flexural strength of the PBS/PBAT (60/40 wt%)

blends is improved with addition of miscanthus fibers. This is a common observation in natural

fiber reinforced composites. The impact strength of the PBS/PBAT blend was considerably

reduced after incorporation of miscanthus fiber into PBS/PBAT blend matrix. This could be due

to the phase separation of the components in the multiphase material. However, the composites

with 2 mm fiber showed superior impact resistance than 4 mm fiber reinforced composites. This

impact strength variation could be due to the difference in fiber pullout mechanism during

impact test. The influence of independent processing variables on the impact strength of

275

PBS/PBAT/miscanthus composites has been investigated by 24

full factorial design of

experiment. Using student’s t-test and F-test, the statistically significant main and interaction

variables were analyzed at a 95% confidence level. According to main effect plot, Pareto plot,

and half-normal plot, the fiber length plays an important role on the impact strength of the

composites as does processing temperature. From the normality plot, it was observed that the

data are normally distributed along the straight line with R2 value of 87.78%. Further work could

be performed by maximizing more number of variables as well as levels.

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280

Chapter 9: Hydrolytic Degradation of Biodegradable Polyesters under

Simulated Environmental Conditions*

*A version of this chapter has been published in: R. Muthuraj, M. Misra, A. K. Mohanty,

Hydrolytic degradation of biodegradable polyesters under simulated environmental conditions,

Journal of Applied Polymer Science, 2015, 132, 42189. (adapted with kind permission from John

Wiley and Sons, Jul 09, 2015, License number 3664990532946).

Abstract

In this study, the durability of poly(butylene succinate) (PBS), poly(butylene adipate-co-

terephthalate) (PBAT) and PBS/PBAT blend was assessed by exposure to 50 oC and 90%

relative humidity for a duration of up to 30 days. Due to the easy hydrolysis of esters, the

mechanical properties of PBS and PBAT are significantly affected with increasing conditioning

time. The PBS, PBAT and PBS/PBAT showed an increase in modulus as well as a decrease in

tensile strength and elongation at break with increased exposure time. Furthermore, the impact

strength of PBAT remains unaffected up to 30 days of exposure. However, it was clearly

observed that the fracture mode of PBS/PBAT changed from ductile to brittle after being

exposed to high heat and humid conditions. This may be attributed to the hydrolysis products of

PBS accelerating the degradation of PBAT in the PBS/PBAT blend. The differential scanning

calorimetry results suggested that the crystallinity of the samples increased after being exposed

to elevated temperature and humidity. This phenomenon was attributed to the induced

crystallization from low molecular weight polymer chains that occurred during hydrolysis.

Therefore, low molecular weight polymer chains are often favored to the crystallinity

enhancement. The increase in crystallinity eventually increased the modulus of the conditioned

samples. The enhanced crystallinity was further confirmed by polarizing optical microscopy

281

(POM) analysis. Moreover, the hydrolysis of the polyesters was evaluated by scanning electron

microscopy (SEM), rheology, and Fourier transform infrared (FTIR) spectroscopy analysis.

9.1 Introduction

During the past decade, biodegradable polymers and their blends have gained great

attention in wide range of applications due to their low environmental footprint. Among the

biodegradable polymers, poly(butylene succinate) (PBS) is a promising aliphatic polyester, made

from fossil fuel based 1,4-butanediol and succinic acid precursors, which can also be derived

from biobased succinic acid. PBS has many desirable properties including good toughness and

melt processability. The mechanical properties of the PBS fall between polyolefins with a wide

processing window [1, 2]. In addition, the mechanical and thermal properties of the PBS depend

on the degree of crystallinity and the spherulite morphology [3]. Degradability of the PBS has

been widely studied under different environmental conditions [4-8].These studies claimed that

the PBS is susceptible to hydrolysis in the presence of moisture/water. The main route of

hydrolytic degradation occurs through cleavage of ester linkages and leads to lower molecular

weight compounds.

Solely aromatic polyesters are resistant to biological degradation. Therefore, an attempt

has been made to introduce aliphatic moieties into aromatic polyesters in order to enhance the

hydrolytic degradation [9]. For example, poly(butylene adipate-co-terephthalate) (PBAT) is a

commercialized biodegradable aliphatic-aromatic copolyester [10]. The PBAT exhibits good

thermal and mechanical properties with a terephthalic acid concentration above 35 mol% [11].

At the same time, PBAT possesses good biodegradability with an aromatic moiety concentration

below 55 mol%. The properties of PBAT can be compared to that of low-density polyethylene

with regards to its tensile properties. Nowadays, PBS and PBAT are widely used for many

282

applications because of their inherent properties in addition to biodegradability. The only

shortcomings of PBS are its insufficient impact strength and gas barrier properties for certain

applications. This could be overcome by physical blending with a highly flexible PBAT while

maintaining biodegradability.

The application of the polymeric materials depends on their durability and performance

under different environments. The durability of the polymers and composites is strongly related

to the degradation mechanism. The degradation mechanism is a key factor for the lifetime

prediction of polymeric materials [12, 13]. If the polymeric materials maintain their required

mechanical performance at least 60 weeks at elevated temperature (50oC) and humidity (90%) it

may be used for 10 year durable applications [14]. The biodegradable polymers are sensitive to

hydrolysis under high temperature and humidity and thus limit their durability as well as long-

term performance under these conditions. In order to incorporate more widespread

semicrystalline biodegradable polymers in durable applications, including automotives and

electronics, the the performance of the polymers must be maintained throughout their lifetime. It

is well known that the amorphous regions are more susceptible to degradation than crystalline

regions in a semicrystalline polymer. This can be explained by the rate of moisture penetration

being higher in the amorphous regions than in the crystalline regions [15]. These drawbacks

could be overcome by blending or alloying polymers while tailoring the material’s overall

performance and cost [14]. With this regard, we have extensively studied the PBS/PBAT blend

in our previous research [16]. However, it is very important to understand the durability

behaviors of the PBS, PBAT and PBS/PBAT blend in order to diversify as well as in predicting

their applications. Such understanding will help to find out new areas in improving the required

durability of these polymeric systems in different applications.

283

Only limited research works have been reported on the long-term durability behaviours of

biodegradable polymers under simulated environmental conditions [13,14,17-19]. For instance,

the long-term durability of polylactide (PLA) samples has been studied by few researchers [13,

14, 20]. These studies showed that the mechanical performance of the PLA was significantly

affected after exposure to elevated temperature and moisture levels. Therefore, PLA is still an

underperforming biopolymer for long-term durable applications such as automotive parts. In

addition, Harris and Lee [13] have studied the hydrolytic degradation of PLA and a

PLA/polycarbonate (PC) blend exposed to elevated temperature and humidity for 28 days. They

have noticed a reduction in the mechanical performance of PLA and PLA/PC blend with

increasing conditioning time. Therefore, the author concludes that the PLA accelerated the

degradation of PC in the PLA/PC blend under these conditions. However, PLA/PC blend

exhibits superior flexural strength than neat PLA during the entire conditioning period. Another

study by Kim and Kim [17] showed that polypropylene (PP) has a more hydrolytic resistant

behaviour than biodegradable polymers (PBS, PBAT and PLA) because of its inherent non-

biodegradability character.

To the best of our knowledge, there have not been many studies available in literature on

the durability of PBS, PBAT and their blends at elevated temperature and humidity. Considering

the above, in the present study, our attention was to investigating the durability of PBS, PBAT

and PBS/PBAT blend at an elevated temperature and humidity level. In this sense, the present

study was aimed to investigate the effect of mechanical and physico-mechanical properties of

PBS, PBAT and PBS/PBAT blend at 50oC with 90% relative humidity for duration of up to 30

days. The samples were evaluated before conditioning and after 6, 12, 24 and 30 days of

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continuous conditioning. The hydrolytic degradation of the polyesters was examined by using

various analytical techniques.

9.2 Materials and Methodology

9.2.1 Materials

For this study, commercially available PBAT pellets were supplied by Xinfu

Pharmaceutical Co., Ltd, China, under the trade name of Biocosafe 2003F with a melting point

of 117oC. PBS pellets were supplied by the same company under the trade name of Biocosafe

1903F with a melting point of 115oC. PP-1350N homopolymer was procured from Pinnacle

Polymers (Garyville, LA). According to manufacturer information, the density and melt flow

index of the PP-1350N are 0.9 g/cm3 and 55 g/10min, respectively. Neat PBS and PBAT were

dried in an oven for six h at 80oC to remove the moisture prior to melt processing.

9.2.2 Sample preparation and conditioning

Neat PP, PBS, PBAT and blend of PBS/PBAT (60/40 wt%) were extruded in a Leistritz

extruder with a screw speed of 100 rpm. The extruder was equipped with co-rotating twin-screws

with a screw diameter of 27 mm and a L/D ratio of 48. Prior to the injection molding, the

extrudates were pelletized and dried in an oven at 80oC for 12 h. The dried extruded pellets were

injection molded in an ARBURG allrounder 370C (Model No: 370 S 700-290/70, Germany)

injection molding machine to obtain desired test specimens. The injection-moulding machine had

a maximum injection pressure of 2000 bar and a screw diameter of 35 mm. The extrusion and

injection molding process was carried out with a processing temperature of 140oC for PBS,

PBAT and PBS/PBAT and 180oC for PP.

In the literature, the durability of polymers, polymer blends and their composites was

studied at different accelerated environmental conditions, [21-23] in vehicle and in-field

285

conditions [13,14]. Furthermore, long-term durability of the polymeric material has been studied

in the presence of Xenon light, UV light, metal halide and carbon arc lamps by many researchers

[24, 25]. However, in order to model the PBS, PBAT and PBS/PBAT blend for automotive

interior applications; all the moulded samples were conditioned under simulated temperature

(50oC) and relative humidity (90%) [18]. These conditions were simulated using an

environmental chamber, Envirotronics Endurance C340. The samples were tested initially before

and after 6, 12, 24, and 30 days continuous conditioning at 50oC and 90% relative humidity

(RH). Except moisture absorption analysis, all other characterizations were performed after

drying the test samples at 80oC for 24 h in order to avoid plasticization effect of excess moisture

in the specimens.

9.2.3 Moisture absorption

Before performing moisture absorption test, all the samples were dried at 80 oC till a

constant weight is reached. The moisture absorption of the samples was calculated by taking out

the samples at required time interval for the set environmental exposure conditions (50 oC and

90% RH). The percentage of moisture uptake was calculated by using the equation:

Moisture uptake (%) =

x 100 (9.1)

where Wa and Wb are weight of the samples after and before moisture exposure. The reported

moisture absorption values are an average of three samples.


FTIR analysis was performed in a Thermo Scientific NicoletTM

6700 at room temperature

with a Smart Orbit attachment. FTIR spectrum was recorded in the range of 4000-400cm-1

with a

resolution of 4 cm-1

and averaged over 36 readings.

286


Tensile and flexural tests were performed in an Instron Universal Testing Machine

(Model 3382) according to ASTM D638 and D790, respectively. The crosshead movement

speeds of 14 mm/min for flexural test and 50 mm/min for tensile test were used as recommended

by the respective standards. The tensile tests were performed until the conditioned samples broke

at the grip region as a consequence of embrittlement. Therefore, the experiment was conducted

only up to 30 days. Notched Izod impact strength was assessed with an impact test machine from

TMI 43-02, USA, complying with ASTM D256.The results are reported as an average of five

samples for each formulation.


The DSC analysis was performed in a TA-Q200 instrument with a heating and cooling

rate of 10 and 5 oC/min, respectively. The samples were heated under a nitrogen flow rate of 50

mL/min. The melting enthalpy was calculated by measuring area under the curves using TA

analysis software. The first heating cycle was considered in order to measure sample crystallinity

before and after conditioning. The percentage crystallinity of the PBS and PBAT was calculated

by using the following formula:

% Crystallinity (χc) =

x 100% (9.2)

where ∆Hm100 is the theoretical enthalpy of melting for 100% crystalline PBS (110.3 J/g) [8] and

PBAT (114 J/g) [26]. ∆Hm is the measured enthalpy of melting. The PBS cystallinity in the

PBS/PBAT blend was calculated as follows:

χc =

x 100% (9.3)

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where wf is the weight fraction of the PBAT in the PBS/PBAT blend.


DMA analysis was performed using TA Instrument (DMA Q800), USA. The

experiments were conducted from -60 to 100oC. The selected temperature range was based on

the glass transition temperature and melting temperature of the samples. The scans were

performed at a constant rate of heating (3 oCmin

-1) with oscillating amplitude of 15 µm and a

frequency of 1 Hz in a dual cantilever clamp mode.


Rheological properties were obtained in an Anton Paar Rheometer MCR302. The

experiments were carried out in parallel plates with a gap of 1 mm and a diameter of 25 mm. In

order to avoid degradation of the samples during the experiments, all the samples were vacuum

dried at 80 oC for 4 h before performing the experiments. The shear viscosity values of the

samples both before and after conditioning were measured at 140oC from 300 to 0.01 rad/s.

9.2.9 Polarizing optical microscopy (POM)

Spherulite morphology of the samples was observed by using optical polarizing

microscope (Nikon Eclipse LV100) equipped with a Linkam LTS 420 hot stage. Thin films of

the samples were made by heating the sample between two transparent microscope glass slides.

All the samples were heated to 150⁰C for 60 s followed by the samples being quickly transferred

to 90⁰C in the microscope hot stage. Subsequently, the samples were kept at close to

crystallization temperature (90oC) and the spherulite growth was recorded using a Nikon camera.


The specimens were prepared by sputtering gold particles in order to avoid electrical

charging during examination. A scanning electron microscope, Inspect S 50, FEI Netherlands,

288

was utilized to examine the fracture surface morphology of the specimens. The surface

morphology of the specimens was examined at an accelerating voltage of 20 kV.


9.3.1 Moisture absorption

Moisture absorption of all the samples was investigated as a function of exposure time.

Figure 9.1 shows the moisture absorption curves in percent of the PP, PBS, PBAT and

PBS/PBAT blend up to 34 days. Generally, more or less; all the polymers tend to absorb

moisture in a humid atmosphere. Usually, polymers with strong polar functionality such as

carbonyl (>C=O) groups and amine groups are able to absorb moisture by hydrogen bonds [27].

Therefore, it is expected that the polyesters can absorb more moisture than the relatively non-

polar polymers such as PP. It can be seen that the PP absorbed a very small amount

(0.011±0.004%) of moisture and the moisture absorption curve has reached a typical Fickian

behavior. It has been reported that the PP is resistant to moisture absorption even at elevated

temperatures [28]. On the contrary, moisture absorption of PBS, PBAT and their blend was

monotonically increased with increasing exposure time up to 34 days. After 34 days exposure,

the PBS showed a maximum moisture absorption (1.11±0.002%) followed by PBS/PBAT

(1.05±0.01%) and PBAT (0.99±0.003%). The observed moisture absorption difference between

the PBS and PBAT may be due to polarity differences between the polymers [29]. Due to the

moisture absorption, it can be expected that the PBS and PBAT can undergo hydrolytic

degradation at elevated temperature and humidity. Normally, higher moisture absorption of

polyesters causes undesirable losses in mechanical performances [13, 30].

289

Figure 9.1. Moisture absorption curves as a function of conditioning time

9.3.2 Hydrolytic degradation mechanism of PBS and PBAT

It is known that the ester linkages of PBS and PBAT are more sensitive to elevated

temperature and moisture [17, 19]. Therefore, in the presence of moisture, the PBS and PBAT

primarily can undergo hydrolytic degradation through cleavage of ester linkages on the polymer

backbone. In addition, the hydrolysis reaction may occur in the form of depolymerization

process and random chain scission mechanism [30]. The possible hydrolytic degradation of PBS

and PBAT under elevated humidity and temperature is depicted in Figure 9.2 and 9.3,

respectively.

290

Figure 9.2. Hydrolysis reaction of PBS

The chain scission is frequently terminated by carboxylic acid end groups [13,30,31] and

hydroxyl end groups [32]. A similar type of hydrolytic degradation mechanism was proposed

for PLA [13], PBS [8, 33] and poly(ethylene terephthalate) (PET) [30]. When PBS is exposed to

high temperature and humidity environment, the surrounding moisture can interact with ester

functionality of PBS and thus can create the low molecular weight PBS through hydrolytic

degradation mechanism [33].

Figure 9.3. Hydrolysis reaction of PBAT

291

The hydrolytic degradation of PBS, PBAT and their blend was further confirmed by

FTIR analysis. Figure 9.4 shows FTIR spectrum of PBS, PBAT and PBS/PBAT before and after

30 days of being exposed to elevated humidity and temperature. In PBS, the band at 917 cm-1

was due to the –C-O-C- groups in the ester linkage of PBS [8]. The band at 1325 cm-1

resulted

from the asymmetric stretching of the -CH2- group in the PBS backbone. The peak at 1045 cm-1

was attributed to the -O-C-C- stretching vibration and the peak in the range 1151 cm-1

. The band

at 1712 cm-1

resulted from the C=O stretching vibration of the ester group in PBS [17]. After 30

days hydrolysis of PBS, a remarkable decrease of –C-O-C- and C=O absorption intensity was

observed. These reductions in absorption intensity were due to lowering of the molecular weight

and deterioration of the chemical structure by hydrolysis after being exposed to moisture and

heat [8, 18, 34]. The characteristic peaks of the PBAT can be described as follows: a sharp peak

at 1710 cm-1

represents the C=O functionality of the ester linkage; the band at around 1267 cm-1

assigned to the C-O group in the ester linkage; the peak at 727 cm-1

resulted from four or more

adjacent -CH2- groups in the PBAT backbone. The peaks in the range of 700-900 cm-1

were

attributed to benzene substitutes [35]. After 30 days of exposure to moisture and heat, there is no

significant change observed in the FTIR spectra of PBAT. This is possibly due to the partial

aromatic structure of PBAT. On the contrary, the FTIR spectra of PBS/PBAT showed a

remarkable decrease in the characteristic peak intensity. This phenomenon may be attributed to

the hydrolysis product of PBS accelerating the degradation of PBAT in the PBS/PBAT blend

[31].

292

Figure 9.4. FTIR spectra of PBS, PBAT and PBS/PBAT before and after 30 days exposed to

50oC with 90% relative humidity

9.3.3 Changes in mechanical properties

Mechanical properties are the main indicators in order to evaluate the durability of the

polymeric materials. The influence of moisture and heat on the mechanical properties was

measured by tensile and flexural properties as well as impact strength. The mechanical properties

of neat PBS, PBAT, PBS/PBAT blend and PP are provided in our previous study [19]. Figure 9.5

shows the tensile strength of PBS, PBAT, PBS/PBAT and PP before and after exposure at 50 °C

with 90% RH up to 30 days. In general, the mechanical properties of semicrystalline polymers

are dependent on their molecular weight, crystal size and percentage of crystallinity [36]. The

tensile strength of PBS and PP showed slight enhancement after 6 days of exposure to 50oC and

90% RH. This can be attributed to the post crystallization of the samples after being exposed to

elevated humidity and temperature. A similar result has been found for PLA [13],

293

poly(hydroxybutyrate-co-valerate) (PHBV) [37], and homo polypropylene [38] specimens when

exposed to different environmental conditions. However, after 6 days of exposure; PBAT as well

as PBS/PBAT blend showed a slight reduction in tensile strength. This could be due to the

plasticization effect of hydrolytically degraded amorphous region in the PBAT. It can be

observed that the tensile strength of PBS, PBAT and PBS/PBAT blend decreased significantly

with increasing hydrolysis time. For example, after 12 days exposure time, the tensile strength of

PBS, PBAT, and PBS/PBAT blend was reduced by 40, 39 and 11%, respectively. The reduced

tensile strength may be attributed to the combined effect of hydrolytic degradation and molecular

weight reduction after being exposed to the raised humidity and temperature [33]. Generally, the

hydrolytic degradation of the biodegradable polymers is higher in the amorphous regions than

crystalline regions under high humidity [39]. A similar type of observation has been made in

PBS, PBAT, PBS/PBAT and PP after being exposure to 18 days of elevated humidity and heat

[19]. However, after 30 days of conditioning, the tensile strength of the PBS and PBS/PBAT

blend exhibited extreme degradation in contrast to PBAT. This is possibly due to the accelerated

degradation of PBS with the increased time at elevated temperature and humidity [15]. Our

finding had good agreement with the recent study by Kim and Kim [17]. Usually, the hydrolytic

degradation and biodegradability of the polymers mainly depend on the easily hydrolysable ester

functionality in the polymer backbone. In the present study, PP did not show any significant

reduction in the tensile strength up to 30 days conditioning, which is due to the non-polar as well

as its hydrophobicity type of characteristic.

294

Figure 9.5. Tensile strength of PP, PBS, PBAT and PBS/PBAT as a function of exposure time at

50 oC with 90% relative humidity

Figure 9.6 demonstrates the flexural strength of the PBS, PBAT, PBS/PBAT and PP after

and before exposure to elevated temperature and humidity. After 6 days of conditioning, the

PBAT did not show any significant improvement in the flexural strength, which may be due to

PBAT possessing a high entanglement density. Interestingly, the flexural strength of PBS,

PBS/PBAT blend and PP were increased 13, 15 and 15% respectively with increasing exposure

time up to 6 days. After 18 days of continuous conditioning at 50oC with 90% RH, the flexural

strength of PBS, PBS/PBAT and PP samples was found to increase slightly [19]. The increased

flexural strength is probably due to the increased crystallinity of the samples after being exposed

to elevated temperature [40].

295

Figure 9.6. Flexural strength of PP, PBS, PBAT and PBS/PBAT as a function of exposure time

at 50 oC with 90% relative humidity

However, it is important to note that the PBS and PBS/PBAT blend samples became

more brittle after 30 days conditioning and leading to premature failure during flexural test, as

shown in Figure 9.7. Harris and Lee [13] found that the PLA and PLA/PC blend underwent

severe flexural strength reduction because of the hydrolytic degradation under the exposed

elevated temperature (70oC) and humidity (90% RH). On the other hand, they have noticed that

the PC/ABS blend did not show any significant changes in the flexural strength up to 30 days

conditioning because of the resistance to the hydrolysis.

296

Figure 9.7. Testing failure mode of PBS, PBAT, PBS/PBAT and PP after 30 days exposed to


Figure 9.8 represents the elongation at break of the samples with respect to the exposure

time. Except PBAT, all the samples showed drastic reduction in the elongation at break from

early exposure time. The PP, PBS and PBS/PBAT blend showed a drastic decrease in the percent

elongation after 6 days conditioning. Therefore, it is clear that the toughness is more sensitive

than the strength after being exposed to raised humidity and temperature. A similar trend has

been reported in the literature for PP [41], high-density polyethylene (HDPE) [42], and PHBV

[37]. After conditioning, PBS showed lower elongation than PP during the entire exposed time.

This implies that the PBS is more moisture sensitive than the PBAT and the PP. Toughness of

the polymer is mainly dependent on the tie molecules and entanglement of the polymer chains

[42-44]. When, the entanglement density decreased in the polymers it led to a reduction in the

toughness of the resultant materials. Apparently, PBAT is more ductile and less crystalline than

PBS due to the higher degree of chain entanglements. Therefore, PBAT maintains its elongation

297

up to 12 days conditioning even after extensive chain scission occurred. Interestingly, the

PBS/PBAT blend has higher elongation than PBS and PP up to 12 days due to the PBAT chain

entanglements. After 12 days of conditioning, the PBS/PBAT blend experienced severe loss in

elongation because of heavy chain scission of PBS leading to hydrolysis of the PBAT phase in

the blend system [31].

Figure 9.8. Percentage elongation of PP, PBS, PBAT and PBS/PBAT as a function of exposure

time at 50oC with 90% relative humidity

Figure 9.9 and 9.10 shows the tensile and flexural modulus of the PP, PBS, PBAT and

PBS/PBAT as a function of conditioning time. Both tensile and flexural modulus of the PP, PBS,

and PBS/PBAT gradually increased with increasing conditioning time, whereas PBAT remains

constant throughout the entire conditioning period. More specifically, the tensile and flexural

modulus of PP and PBS were improved by around 200 MPa after 30 days conditioning period.

This could be related to the increased crystallinity and subsequently increase in modulus [45,

46].

298

Figure 9.9. Tensile modulus of PP, PBS, PBAT and PBS/PBAT as a function of exposure time

at 50oC with 90% relative humidity

A number of researchers have observed a similar tendency in the modulus after exposure

to different weathering conditions [12, 37, 42]. These studies were concluded that the modulus

improvement of the conditioned samples is associated with structural relaxation, increase in

crystallinity, crystal perfection and increase of lamella thickness. In addition, the brittleness of

the PBS and PBS/PBAT blend sample was also improved with increasing conditioning time up

to 30 days, accounting for the reduction in impact toughness.

299

Figure 9.10. Flexural modulus of PP, PBS, PBAT and PBS/PBAT as a function of exposure

time at 50oC with 90% relative humidity

Among the mechanical properties, impact energy is more sensitive to the environmental

exposure. Table 9.1 shows the impact strength of PBS, PBAT, PBS/PBAT and PP after and

before conditioning at 50oC and 90% RH. Before conditioning, the PBAT showed non-break

impact strength of 211 J/m while PBS and PP showed complete break with impact strength of 25

and 30 J/m, respectively. The impact energy of the PBS and PP decreased for the first 6 days of

conditioning. This reduction is probably due to the inadequate degree of entanglement between

amorphous and crystalline phase after exposed to 50oC and 90% RH [45]. The notched Izod

impact strength of PBS, PBAT, PBS/PBAT blend, and PP samples after conditioning for 18 days

is explained elsewhere [19]. With increased exposure time (from 6 to 30 days), the impact

energies of both PBS and PP were not significantly affected. In contrast, the impact energy of the

PBAT remains unchanged up to 30 days of conditioning at elevated temperature and humidity.

This may be due to the PBAT having sufficient molecular weight to form a significant degree of

300

entanglement up to 30 days of hydrolysis environments [14, 31]. Furthermore, the impact

strength of PBS/PBAT changed from a ductile to brittle fracture with increasing conditioning

time, as shown in Figure 9.7. This could be due to the accelerated degradation of PBS with the

increased exposure time. Moreover, except PBAT, all the samples exhibit brittle failure with

increasing conditioning time. This observation corroborated with the modulus improvements.

Table 9.1.Notched Izod impact strength (J/m) of the samples before and after conditioned at


Samples Before

conditioning

After 6 days

conditioning

After 12 days

conditioning

After 24 days

conditioning

After 30 days

conditioning

PBS

24.80

±

6.55

12.54

±

6.20

13.19

±

1.91

12.90

±

1.003

12.41

±

1.07

PBAT

Non-break

(210.55

±

10.37)

Non-break

(211.62

±

46.54)

Non-break

(209.09

±

27.01)

Non-break

(203.97

±

25.22)

Non-break

(193.16

±

35.47)

PBS/PBAT Non-break

(226.77

±

43.95)

56.22

±

4.24

52.24

±

14.24

11.36

±

1.92

13.04

±

2.69

PP

30.40

±

7.08

21.77

±

0.79

24.47

±

2.33

19.74

±

0.79

20.97

±

0.90


After exposing the polymers to raised humidity and temperature, it is expected that the

spherulitic growth rates, lamellar thickness, and crystal interphase be modified due to the free

energy changes in the crystals formation. DSC traces of the samples before and after

conditioning are shown in Figure 9.11 and 9.12. The thermal properties of PBS, PBAT and

PBS/PBAT blends are summarized in Table 9.2. Melting enthalpy (∆Hm) of the sample was

301

calculated by measuring the area under the melting peak while crystallization enthalpy (∆Hc) was

the measured area under the crystallization peak.

Figure 9.11. DSC heating cycles for PBS, PBAT and PBS/PBAT before and after exposed to 50 oC with 90% relative humidity for 30 days

Before conditioning, all the samples showed a single melting temperature (Tm). In the

heating cycles (Figure 9.11), a small exothermic peak was also observed for PBS and PBS/PBAT

prior to melting peak. This resulted from the melt-recrystallization of PBS while heating [47].

However, after 30 days exposure, PBS and PBS/PBAT samples displayed a bimodal melting

peak, as shown in Figure 9.11. The hydrolytic degradation of the polymers leads to wide range of

molecular weight distribution. The shorter polymer chains are having tendency to form less thick

crystal lamella than the high molecular weight polymer chains. Due to the difference in the

lamella thickness, the hydrolytic degraded samples were melting at two different temperatures.

These observed double endothermic peaks are attributed to the different crystal lamella thickness

formation [8]. In addition, the Tm of PBAT shifted to low temperature after 30 days conditioning.

302

Either the change in amorphous-crystal surface energy or a decreased in the lamellar thickness

was responsible for the Tm decrease of a polyester after exposure to elevated temperature and

humidity [30]. For both PBS and PBS/PBAT, no change was observed in the melting

temperature (~115oC) after 30 days of exposure time.

Figure 9.12. DSC cooling curves for PBS, PBAT and PBS/PBAT before and after exposed to

50oC with 90% relative humidity for 30 days

In a semi-crystalline polymer, initially the amorphous regions are more susceptible for

hydrolysis [6]. From the DSC analysis, it was clearly observed that the ∆Hm and ∆Hc of the PBS,

PBAT and PBS/PBAT increased after 30 days of conditioning, indicating that degradation

mainly occurred in the amorphous regions. In addition, this phenomenon may be due to induced

crystallization from low molecular weight polymer chains that occurs during conditioning [13].

Therefore, low molecular weight polymer chains are often favored to the crystallinity

enhancement. Our findings have good agreement with previous studies [14]. According to these

studies, the chain scission leads to reduced entanglement density and tie molecules of the semi

303

crystalline polymers. The small molecular chains have potential to rearrange into the crystalline

region, which is called chemi-crystallization. This behavior has been observed in most of the

semicrystalline aliphatic biodegradable polymers including PBS [46]. The increased crystallinity

(χc) further accounts for the enhanced modulus as well as stiffness. The crystallization

temperature (Tc) of the PBS and PBS/PBAT significantly reduced after 30 days of conditioning.

This is attributed to the low molecular weight polymer chains leading to slow crystallization. A

similar crystallization behavior for PBS has been reported after exposure to raised humidity and

temperature [8]. Contrary, the crystallization temperature of PBAT was shifted to higher

temperature. This is probably due to the nucleation effect which is caused by oligomers [48].

Table 9.2. DSC results for PBS, PBAT and their blend before and after 30 days conditioned at


Samples Tm (oC) ∆Hm (J/g) Tc (

oC) χc (%) Tg

* (

oC)

PBS before 115.2 68.26 91.98 61.88 -16.72

PBS after 114.8 84.41 77.15 76.52 -14.24

PBAT before 117.11 9.29 81.11 8.14 -20.27

PBAT after 114.30 16.51 96.50 14.48 -25.00

PBS/PBAT before 114.97 41.04 93.91 62.01 -19.04

PBS/PBAT after 114.39 43.44 71.32 65.63 -20.71

*Tg obtained from tan δ peaks


Figure 9.13 shows the temperature dependence dynamic modulus of PBS, PBAT, and

their blend. It can be seen that the PBS had higher storage modulus than PBAT and PBS/PBAT.

Similar occurrence has been observed in the tensile and flexural modulus. However, the storage

304

modulus of all the samples gradually decreased with increasing temperature. This is attributed to

the enhanced polymer chain mobility with increasing temperatures [16]. As reported by Van der

wal et al., [49] above the glass transition temperature, storage modulus is dependent to the

degree of crystallinity. Below glass transition temperature, the modulus of crystalline as well as

amorphous phase is almost identical. Interestingly, after 30 days of conditioning, the storage

modulus of PBS, PBAT and their blend samples was found to increase slightly. This is because

the samples become stiffer, as evidenced by the increase in crystallinity after conditioning at

elevated temperature and humidity.

Figure 9.13. Storage modulus of PBS, PBAT and PBS/PBAT before and after exposure to 50oC

with 90% relative humidity for 30 days

Figure 9.14 shows the tan δ (loss factor) curves with respect to temperatures. In fact, the

peak temperature of the tan δ represents the glass transition temperature (Tg). The Tg values of

PBS, PBAT and PBS/PBAT blends are summarised in Table 9.2. The position of each tan δ peak

305

is affected slightly after conditioning at 50oC with 90% RH. In general, Tg value of the

amorphous phase in semicrystalline polymers depends on the degree of crystallinity [46].

Initially, the PBS and PBAT had Tg values of -17 and -20oC, respectively. In PBS/PBAT blend, a

single Tg (-19oC) was observed. This is due to the fact that Tg values of both neat PBS and PBAT

were very close to each other and thus Tg may be overlapping in the PBS/PBAT blend [16].

After 30 days conditioning, Tg of the PBS increased from -16.72 to -14.24oC. This slight change

can be attributed to the enhanced crystallinity, as corroborated by DSC result.

Figure 9.14. Loss factor peak (tan δ) of PBS, PBAT and PBS/PBAT before and after 30 days

exposed to 50oC with a relative humidity of 90%

As reported in Table 9.2, the crystallinity of the PBS increased from 58.62 to 73.43%

after 30 days exposed to 50oC with 90% RH. Similar observations have been reported by Harris

and Lee for PLA [14]. However, after 30 days conditioning, the Tg values of the PBAT and

PBS/PBAT reduced marginally with slightly increased in crystallinity. This can be related to the

plasticization effect by the diffused moisture, which induces an increase in the amorphous chain

306

mobility [30]. A similar type of negative Tg shift was observed in the PLA films [20] and

poly(ethylene terephthalate) (PET) composites [30] after exposure to elevated temperature and

humidity.


Figure 9.15 represents the shear viscosity of the samples before and after 6 days of

conditioning at the elevated temperature and humidity. It was observed that all the samples

showed Newtonian and non-Newtonian flow behavior at lower and higher frequencies,

respectively. The 6 days conditioned samples exhibit a slight decrease in the shear viscosity

compared to the before conditioned samples. As expected, this behavior should be due to the

molecular weight reduction by random chain scission after being exposed to elevated

temperature and humidity. The molecular weight changes can be correlated with shear viscosity

of the sample at low shear rate. According to the literature [50], the weight average molecular

weight (M) is directly proportional to the viscosity of the polymer melt at a zero shear rate.

However, molecular weight distribution is independent to zero shear viscosity (ƞo). Generally,

the ƞo is obtained from extrapolation of the shear viscosity at lower shear rate (Newtonian

region), which considered as weight average molecular weight [51]. This relationship can be

explained as follows[50]:

ƞo = KM3.4

(9.4)

where K and M are the material constant and molecular weight respectively. In the present study,

relative molecular weight (M1/M2) of the samples before and after conditioning can be calculated

by using following equation:

log (

) = 3.4log (

) (9.5)

307

where ƞ1 and ƞ2 are the Zero shear viscosity of the samples before and after conditioning.

The molecular weight reduction is permanent damage caused by hydrolysis of the ester

functionalities on the polyesters backbone. Phua and coworkers [8] have studied the molecular

weight of hydrolytically degraded PBS samples. The authors found that the molecular weight

reduction was higher with an increasing conditioning period. Table 9.3 reports the zero shear

viscosity, viscosity ratio and relative molecular weight (calculated from equation 9.5) of the

samples before and after 6 days of conditioning. After 6 days conditioning, a significant

reduction in molecular weight and viscosity were observed for all the samples. As mentioned

before, PBS and PBAT are susceptible to the moisture. Therefore, it can be expected that the

moisture can easily hydrolyze the PBS and PBAT at 90% RH and it leads to a decrease in the

molecular weight as well as viscosity [13]. The molecular weight of the PBS/PBAT blend was

1.70 times lower after being subjected to hydrolytic degradation. This is relatively high

compared to PBS and PBAT. This may be due to the hydrolysis product of PBS or PBAT

accelerating the molecular weight reduction of the PBS/PBAT blend. After 6 days of exposure to

heat (50oC) and humidity (90%), the molecular weight reduction occurred in the following order

PBS/PBAT>PBAT>PBS, as shown in Table 9.3. This result agrees with the observed

mechanical properties of the conditioned samples as studied.

308

Figure 9.15. Shear viscosity curves for PBS, PBAT and PBS/PBAT before and after 6 days

exposed to 50oC with a relative humidity of 90%

Table 9. 3. Relative molecular weight (M1/M2) of the PBS, PBAT and PBS/PBAT blend before

and after 6 days conditioned at 50oC with 90% relative humidity

Samples Zero shear viscosity (Pa.s) Viscosity ratio

(ƞ1/ƞ2)

Relative molecular weight

(M1/M2) Before (ƞ1) After (ƞ2)

PBS 621.51 198.54 3.13 1.40

PBAT 2004.7 411.9 4.86 1.59

PBS/PBAT 1897.1 318.74 5.95 1.70

9.3.7 Polarizing optical microscopy

Figure 9.16 shows the spherulite morphology of PBS, PBAT and PBS/PBAT before and

after 30 days of exposure to raised humidity and temperature. The spherulite morphology of the

samples was analyzed at close to crystallization temperature (90oC). Before being exposed to

hydrolysis conditions, it was difficult to notice clear spherulite morphology at 90oC for all the

samples. However, after 30 days the conditioned samples exhibited an obvious spherulite

309

structure. It is generally agreed that the amorphous region is more susceptible for hydrolysis than

crystalline regions in semicrystalline polymers. Therefore, these findings have good agreement

with the improved percentage of crystallinity, which was observed by DSC. This type of

phenomenon is commonly found in the polymers when exposed to a degradation environment

[52, 53]. Interestingly, the amount of spherulite formation was higher in the samples with a

lower percentage of crystallinity. For instance, the PBS and PBS/PBAT blend showed less

number of spherulites than PBAT after 30 days exposed to hydrolysis. This is attributed to the

nucleation density difference in the samples. A similar trend has been observed in the degraded

polypropylene sample [53]. In addition, the PBS had a less amount of nucleation sites than

PBAT because of the severe molecular weight reduction by hydrolytic degradation. This

phenomenon was consistent with observed crystallization temperature by the DSC analysis.

Furthermore, the spherulite morphology of the 30 days exposed PBS and PBS/PBAT exhibits

clear ring-banded spherulites, which can be attributed to their reduced molecular weight. This

finding has good agreement with a previous study [52]. According to Kfoury et al., [54] the

percentage of crystallinity, size of crystallites and spherulite morphology have great influence on

the impact strength. The stress concentration ability of crystallites has been increased with an

enhanced percentage of crystallinity. Consequently, this could lead to a reduction in the impact

strength. In the present study, observed impact strength had good agreement with the spherulite

morphology and crystallinity.

310

Figure 9.16. Polarized optical micrographs of PBS, PBAT and PBS/PBAT before and after 30

days conditioned at 50oC and 90% relative humidity

311


To investigate the hydrolysis caused by moisture and temperature, SEM analysis was

carried out before and after 30 days conditioned samples. SEM micrographs of the PBS, PBAT

and their blend are depicted in Figure 9.17. Before exposure to hydrolysis environment, a smooth

surface morphology was observed for all the samples. On the other hand, after 30 days of

hydrolysis test, the PBS, PBAT and PBS/PBAT blend showed deep holes, cavities as well as

eroded regions. This observation indicates that the biodegradable polyesters (PBS, PBAT and

PBS/PBAT blend) can readily undergo severe degradation after being exposed to elevated

humidity and temperature. A similar type of physical damage in the hydrolytically degraded PBS

and PLA samples has been observed by Kanemura et al., [33] and Deroiné et al., [55] These

studies suggest that the formed irregular surface morphology is ascribed to the dissolution of

the oligomers during hydrolysis process [55]. It can be seen that the SEM image (Figure 9.17)

of the PBS showed significant erosion pits and large eroded regions compared to PBAT and

PBS/PBAT. This is corresponding to the higher rate of hydrolytic degradation of PBS after

conditioning for 30 days under the simulated environment [8]. In addition, an irregular fractured

surface was observed in the 30 days conditioned PBS sample. This is possibly due to the

increased crystallinity after being exposed to the hydrolysis environment.

312

Figure 9.17. SEM micrographs of PBS, PBAT and PBS/PBAT before and after 30 days

conditioned at 50oC and 90% relative humidity

313

9.4. Conclusions

The hydrolytic degradation of PP, PBS, PBAT and PBS/PBAT samples was examined

after exposure to elevated temperature and humidity. Because of chain scission through the

hydrolysis mechanism, the elongation at break and tensile strength of the PBS, PBAT and

PBS/PBAT were significantly affected after conditioning. However, the flexural and tensile

modulus of the PP, PBS and PBS/PBAT were slightly improved after exposure to heat and

humidity. This could be due to the improved crystallinity by molecular weight reduction

during the exposure time. The increased crystallinity was consistent with observed spherulite

morphology. The zero shear viscosity of the 6 days exposed samples was lower compared to

corresponding unexposed samples. This suggests that the molecular weight of the exposed

sample is reduced because of hydrolytic degradation. Interestingly, it was found that the

impact strength of the PBAT was not affected significantly over the entire exposure time,

whereas for PP, PBS and PBS/PBAT impact strength decreased up to 6 days of conditioning.

Over the hydrolysis time, the samples had rough surfaces and corrosive holes in the SEM

micrographs. This result agrees with the considerable reduction in the mechanical properties of

the samples after being exposed to elevated temperature and humidity. Our findings allow us to

conclude that the hydrolytic degradation of biodegradable polyesters needs to be reduced under

high humidity and temperature for diversifying their applications.

314

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318

Chapter 10: Conclusions, Contributions, and Recommendations for Future Work

Abstract

This chapter reflects on the conclusions of the research study carried out in previous

chapters, and contribution to the knowledge made during this investigation. Furthermore, this

chapter presents recommendations/suggestions for future directions.

10.1 Overview

The objective of this project was to create a sustainable biocomposite with stiffness-

toughness balanced composites from biodegradable polymer matrix and miscanthus short fibers

by melt processing. Chapters 3 and 4 described the experiments performed on the preparation

and characterization of biodegradable polymer blends and compatibilizer for composite

applications. The effects of the compatibilizer synthesized in Chapter 4 were investigated in

Chapters 5, 6 and 7. Chapters 5 and 6 presented the performances of the single polymer matrix

based biocomposites with and without compatibilizer. In chapter 7, the composites were

produced with a binary blend matrix that showed optimum stiffness-toughness balanced

properties described in Chapter 3. In Chapter 8, a statistical approach was adopted to identify the

processing parameters that had the most significant effect on the performance of the

uncompatibilized composite prepared in Chapter 7. Finally, the durability of the selected

biodegradable polymer blends and their parent polymers were investigated in Chapter 9.

Individual conclusions were presented for the research described in each chapter. The following

section summarizes the interrelationship between results and objectives for this thesis.

10.2 Conclusions

Blending of polymers is an effective and economical way to obtain new materials with

desired properties. In order to design a biodegradable polymer blend with balanced mechanical

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properties for biocomposite matrix application, commercially available poly(butylene succinate)

(PBS) and poly(butylene adipate-co-terephthalate) (PBAT) were melt blended through extrusion

and injection molding. Due to compatibility between the PBS and PBAT, the tensile toughness

and tensile strength of the PBS were remarkably enhanced after the inclusion of PBAT. The

observed compatibility results from the formation of copolyester due to the transesterification

reaction between the parent polymers, which was observed using an infrared spectroscopy.

Furthermore, the enhanced compatibility between the blended components was corroborated

with DSC and DMA analysis. The PBS/PBAT blend mechanical properties appeared to be

comparable with polyethylene mechanical properties. The rheological properties indicate that the

PBS/PBAT blend has good processability, which can allow higher fiber loading for the

composite fabrication. The surface morphological analysis of the PBS/PBAT blends provided

evidence that the PBS and PBAT are not miscible at a molecular level. The prepared

biodegradable PBS/PBAT (60/40 wt%) blend can be considered as a potential candidate for

biocomposite applications.

As reported in the literature, the compatibility between the hydrophilic natural fibers and

hydrophobic polymer matrix is very poor. Therefore, this research aimed to synthesize

compatibilizer as the next step of this thesis work. Solvent free and economically viable maleic

anhydride grafted PBS, PBAT and PBS/PBAT blend were prepared in the presence of dicumyl

peroxide (DCP) as a free radical initiator. The FTIR analysis was used to confirm the structural

changes in MAH grafting samples. The MAH grafting yield was calculated by titration method.

Among the MAH grafted PBS, PBAT and PBS/PBAT blend samples, a higher MAH grafting

yield was observed on the PBS backbone. In addition, the MAH grafting efficiency was

compared in the batch and continuous process. The batch processed sample had a slightly higher

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yield than the continuous processed sample. During a MAH grafting reaction, both grafting and

cross-linking phenomena can occur in the presence of initiator. Thermogravimetic analysis

revealed that thermal stability of all the MAH grafted samples were found to be slightly reduced

compared to their counterparts.

Miscanthus fibers could be used as a reinforcement to produce PBS based biocomposites

while reducing cost and retaining PBS mechanical properties. Therefore, biocomposites were

produced from PBS and miscanthus fibers with and without compatibilizer by melt processing. A

strong reinforcing effect of micanthus fibers led to an increase in the tensile and flexural

modulus of the resulting PBS biocomposites. It was found that the tensile strength of

uncompatibilized PBS/miscanthus composites was reduced compared to neat PBS. At the same

time, the melt flow of the PBS composites was much lower compared to neat PBS. Indeed, the

observed MFI value of the PBS biocomposites is appropriate for some injection molding

processes. The compatibilizing efficiency of the MAH grafted PBS was investigated in the PBS

biocomposites by means of mechanical performances. It was observed that the compatibilized

PBS biocomposites showed superior impact strength, tensile strength, and flexural compared to

their uncompatibilized one and neat PBS. However, the shortcoming of the PBS/miscnathus

biocomposites was insufficient impact strength/toughness, which could limit its range of

applications. The impact strength of the polymeric materials is one of the most important

properties, which relates to the service life of the products. As a result, a higher impact strength

biocomposite was produced from PBAT and miscanthus fibers while lacking stiffness, flexural

strength and tensile strength of the resulting PBAT/miscanthus fiber composites.

Balanced stiffness, toughness and thermal properties of the biocomposites were produced

from PBS/PBAT blend matrix and miscanthus fibers. The tensile and impact strength of the

321

PBS/PBAT blend were decreased with the addition of miscanthus fibers. In order to overcome

this issue in the resulting formulations, a reactive compatibilizer (MAH-g-PBS/PBAT) was

introduced into the composite system. It was found that the mechanical properties of the

compatibilized composites were noticeably increased as compared to the uncompatibilized one.

The density of all the composites was found to be lower than synthetic fibers like glass fibers.

The comptibilized PBS/PBAT/miscanthus fiber composites showed balanced performance

compared to their individual compatibilized PBS/miscanthus composites and PBAT/miscanthus

composites. From this study, it can be concluded that the prepared biodegradable polymer blend

matrix based composites are a possible candidate to replace non-biodegradable composites in

applications where biodegradability is essential after use but extreme thermal and humidity

exposure are avoided.

The influence of independent processing parameters upon the impact strength of

PBS/PBAT/miscanthus composites was investigated by full factorial design of experiment. The

statistically significant variables were analyzed at a 95% confidence level. It was found that the

fiber length plays an important role to predict the impact strength of PBS/PBAT/miscanthus

composites. The durability of PBS, PBAT and blend of PBS/PBAT was examined after being

exposed to 50oC and 90% humidity with comparison to polypropylene. Due to hydrolytic

degradation of polyesters, the performances of the PBS, PBAT and their blend are not stable

like PP under selected environmental conditions. The hydrolytic degradation of the polyesters

was confirmed by FTIR, DSC, rheological, and morphological analysis.

10.3 Significant contributions

Most of the commercially available biodegradable polymers are not satisfying their

application requirements because of their cost and insufficient mechanical performances. In this

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research work, the developed biodegradable PBS/PBAT blends have good mechanical, thermal

and rheological properties, which are not commercially available in the market. This

biodegradable polymer blend can be used for different industrial applications including

composite fabrication, blow molding, films and packaging. The cost of the biodegradable

polymers and their blends can also be reduced by composite fabrication with miscanthus fibers.

Utilizing these biodegradable polymers as a matrix and miscanthus fibers as a reinforcement for

biocomposite fabrication can reduce green house gas emissions while meeting consumer’s short

term application requirement.

In the present study, the observed mechanical performances of the blends and composites

were superior compared to the composites made with PP and miscanthus fiber composites. The

developed biodegradable green composites are possible substitutes for a class of 100%

petroleum-based non-biodegradable plastics and composites, which are currently used in

different applications. The prepared fully biodegradable material is a promising candidate for

environmental policies and public awareness, which can increase environmentally friendly

material usage. Providing value addition for these environmentally friendly materials could

increase the revenues for miscanthus growers. Dissemination of the new knowledge discoveries

of the present study could result in the implementation of a wide range of bio-based materials.

The final optimized biocomposite formulations were extruded and injection molded to produce

prototypes in an industrial trial to further the commercialization prospects of the new technology,

as shown in Figure 10.1. The produced biocomposites could be optimized to meet the

requirements for use in automotive applications, construction panels, and consumer products.

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Figure 10.1. Prototypes were made from biodegradable polymers/miscanthus fiber by extrusion

and injection molding method.

10.4 Recommendations for future works

Various electron-rich co-monomers (e.g., styrene) could be used to investigate the

MAH grafting yield while reducing side reactions of the electron deficient monomers.

The developed biocomposites should be compatibilized by modification of

miscanthus fibers with sizing agents and using commercially available

compatibilizers to compare to MAH grafted compatibilizer.

The influence of fiber length on the performance of the resulting biocomposites

should be further explored by varying fiber length.

Further work could be performed by maximizing the number of processing

variables as well as levels to predict the most significant processing parameters on

the resulting biocmposites.

During processing, the odor of the composite fabrication process should be

eliminated to make industrial processing smooth and effective.

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If these developed materials are required for long term applications, the durability

of the biodegradable polymers and their composites needs to be improved by anti-

hydrolysis agents.

Future studies can be conducted on these developed materials to evaluate and

certify them as a compostable blend and biocomposite.

In order to compare the cost of the prepared composites with commodity plastics,

economic analysis of the prepared composites should be studied.

Life cycle analysis is necessary to prove that the developed biocomposites are an

environmentally superior alternative to synthetic fiber reinforced composites.

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Appendix I: Binary Blends of Poly(Butylene Succinate) and Poly(Butylene Adipate-co-

Terephthalate): A New Matrix for Biocomposites Applications*

*A version of this appendix has been published in: R. Muthuraj, M. Misra, and A. K. Mohanty,

Binary blends of poly (butylene adipate-co-terephthalate) and poly (butylene succinate): A new

matrix for biocomposites applications. PROCEEDINGS OF PPS-30: The 30th

International

Conference of the Polymer Processing Society–Conference Papers. Vol. 1664. AIP Publishing,

2015. (adapted with kind permission from AIP Publishing LLC, Jul 09, 2015, License number

3664990443173).

Abstract

In this study, biodegradable poly(butylene adipate-co-terephthalate) (PBAT) and

poly(butylene succinate) (PBS) binary blends were melt compounded. The mechanical, thermal

and morphological properties of the PBAT/PBS blends were investigated. The melt compounded

binary PBAT/PBS blends showed balanced mechanical properties (especially in tensile strength

and elongation) compared to neat components. The obtained melt flow index (MFI) value of the

blends is much higher than PBAT. This may be attributed to PBS phase residual catalyst because

it pronouncing thermal degradation of the polymers at higher temperature. The toughness of the

PBAT is not significantly affected with addition of 40 wt% PBS in the PBAT/PBS blend. This

could be the reason of good compatibility achieved between the PBAT and PBS phase in the

blends. The phase morphology and spherulite morphology were also correlated with

compatibility between the PBAT and PBS in the blends.

A-I.1.Introduction

In recent years, a new trend has arisen where polymer blends are being selected over

individual biopolymers as matrices for composite applications. This is because the process of

polymer blending is one of the most promising techniques to create materials with specific

desired properties by combining the different advantageous qualities of two or more neat

polymers. The resulting mechanical properties are typically a compromise between the parent

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polymers. In cases of very successful compatibility, performance can be an overall improvement;

however, blending typically results in a general decrease in mechanical performance. The most

frequently used solution to overcome this problem is to compatibilize the blends. Block

copolymers or grafted polymers are widely used as compatibilizers leading to finer phase

morphologies and better interfacial properties [1]. However, the development of stiffness and

toughness balanced polymer blends for biocomposite matrix applications is in high demand. PBS

and PBAT are promising biodegradable polymers, with many inherent advantages. They present

good biodegradability, excellent toughness, good thermal stability, and commercial availability

in market. In addition, PBS and PBAT have been extensively studied in blends with other brittle

biodegradable polymers. PBS is obtained from the petroleum-based succinic acid and 1,4-

butanediol monomers. Interestingly, new methods have recently been developed such that these

two monomers can also be synthesized from renewable resources [2]. Therefore, PBS has the

newfound potential to be a biopolymer that is both biodegradable and bio-based. This new

development may further diversify the PBS utilization in different fields of applications.

Blending PBAT and PBS is of great interest because it returns desirable, unique properties while

retaining the biodegradable nature of the base materials [3]. Although many studies have

reported on balancing the stiffness-toughness in biodegradable binary blends. We report here the

binary blends of PBAT and PBS which represents good examples of super tough blends from the

biodegradable aliphatic-aromatic (PBAT) and aliphatic (PBS) polyesters. The resulting binary

blends exhibit balanced mechanical and thermal properties.

A-I.2. Materials and Methods

Injection grade PBS, trade name Bionolle 1020, was procured from Showa Highpolymer

Co. Ltd, Japan, with a molecular weight (Mw) of 1.4 ×105 g/mol and PDI of 1.82. The PBAT

327

(Biocosafe 2003F) was purchased from Xinfu Pharmaceutical Co., Ltd, China. The chemical

structures of the neat PBS and PBAT are shown in Scheme A-I.1.

Scheme A-I.1. Chemical structures of PBS and PBAT

Prior to melt processing, the polymers were dried in conventional oven for 6 h at 80oC to

remove moisture and prevent degradation. Both PBAT and PBS were mixed and processed using

a lab-scale twin-screw extruder, and injected into moulds (DSM Xplore® 15 cc

microcompounder). For all samples prepared, the operating temperature was 140 oC, the screw

speed was 100 rpm, and the processing or dwell time of the materials inside the barrel was 2 min.

Tensile properties of the PBAT/PBS blends were measured according to ASTM D638 in a

Universal Testing Machine (Instron-3382) at a strain rate of 50 mm/min at room temperature;

results are reported as average values of five replicates for each experiment. Heat deflection

temperature (HDT) analyses were performed using a Dynamic Mechanical Analysis Q800 from

TA Instruments, according to ASTM D648. Melt flow index (MFI) was measured according to

ASTM D1238 at 190oC temperature using a 2.16 kg load. Cryofractured sample morphology was

examined using a Inspect S 50-FEI Netherlands scanning electron microscope (SEM) at an

accelerating voltage of 20 kV. Before observing sample morphology, the samples were gold

328

coated with a final thickness of 20 nm with 20 mA. The spherulite morphology of the

PBAT/PBS blend was observed using a Nikon polarizing optical microscope (POM) with hot

stage; these micrographs were taken to observe crystallization. Samples were sandwiched

between two microscope glass slides and heated to 150oC for 5 min before quickly transferring

the slide to the 150oC microscope hotplate. Subsequently, samples were annealed at the

crystallization temperature with a heating rate of 10 oC/min. The spherulite growth was recorded

at crystallization temperatures of 85oC using a Nikon camera.

A-I.3. Results and Discussion

The tensile properties of the PBAT, PBS and PBAT/PBS blends are shown in the Figure

A-I.1. The tensile strength of the PBAT/PBS blends was higher than that of the neat PBAT.

Specifically, the tensile strength of the PBAT/PBS (60/40 wt%) blend increased by 84%

compared to neat PBAT. At the same time, the percentage elongation of the PBAT/PBS (60/40

wt%) is similar to the PBAT. The tensile strength improvement is directly related to the

intermolecular forces, compatibility, and molecular orientation of the polymers in the blend [4].

In our previous study (Chapter 3) [3], it was concluding that the PBS/PBAT transesterification

product acts as a compatibilizer in the PBS/PBAT blends and leads to the improvement of their

tensile properties. In that specific work, the transesterification reaction was confirmed via

normalized FTIR spectra. Particularly in that research, the carbonyl peak of the neat PBS, PBAT

and PBS/PBAT blend was focused to observe its variation between the materials. The neat PBS

and PBAT carbonyl group frequency was observed at 1712 cm-1

. The carbonyl peak of the

PBS/PBAT blend shows a slight shift towards higher wavenumbers (1716 cm-1

). This shift could

be the reason a transesterification product formed during melt processing in the presence of

existing residual catalyst in the neat PBS and PBAT. In general, the resulting transesterification

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product is more compatible with the homopolymers of the unreacted PBS and PBAT in the

PBS/PBAT blends. A similar observation is reported in the literature for PBS/PCL blends [4].

The mechanical properties of PBAT/PBS blends are comparable with commercially available

polyethylene (PE) reported previously on literature [5]. As such, we believe this indicates that

PBAT/PBS blends could be a potential substitute for non-biodegradable polymers in packaging

applications.

Figure A-I.1. Tensile properties of (A) PBAT, (B) PBS, (C) PBAT/PBS (60/40 wt%) and (D)

PBAT/PBS (70/30 wt%) blend

MFI values of the PBAT, PBS and PBAT/PBS blends are presented in Figure A-I.2. The

MFI of the PBAT/PBS blends (both 30 and 40 wt% PBS) were higher than neat PBAT, with

tests resulting in blend values similar to neat PBS. This suggested that the viscosity of the blends

is highly influenced by the incorporation of PBS into PBAT matrix. Alternatively, the low

observed viscosities of the polymer blends may be due to thermal degradation resulting from

exposure to high test temperatures i.e., 190 oC. Figure A-I.2 also presents the HDT of the PBAT,

330

PBS and PBAT/PBS blends. The HDT value of the PBAT (46 oC) is lower than PBS (90

oC).

Both PBAT/PBS (60/40 wt% and 70/30 wt%) blends displayed HDT values intermediate of the

neat PBAT and PBS.

Figure A-I.2. HDT and MFI values of (A) PBAT, (B) PBS, (C) PBAT/PBS (60/40 wt%) and

(D) PBAT/PBS (70/30 wt%) blend

To confirm the previous discussion regarding phase morphology, cryofractured

PBAT/PBS blend samples were observed by SEM (Figure A-I.3). The surface morphology of the

blend reveals that spherical PBS particles were uniformly distributed throughout the matrix. This

uniform dispersion of the PBS phase is attributed to the enhancement of compatibility in the

blends [6].

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Figure A-I.3. SEM image of the cryofractured PBAT/PBS (60/40 wt%) blend

The spherulite morphology of PBS and the PBAT/PBS (60/40 wt%) blend are shown in

Figure A-I.4. Generally, PBAT has poor capability to form perfect spherulite morphology when

compared to PBS. Therefore, the PBAT spherulite morphology is not shown in this work. The

crystallization temperatures of PBAT, PBS, and the PBAT/PBS blend are 53, 83 and 85 oC,

respectively. The PBS and PBAT/PBS blend spherulite morphologies were observed at 85oC for

30 min. The selected temperature was based on the crystallization temperature of PBS and

PBAT/PBS blends. High levels of spherulite growth were observed in both PBS and the

PBAT/PBS blend. However, the higher number of nucleations in the PBAT/PBS blend interferes

with crystal growth and lead to distorted lamellae. The fine dispersion of PBS phase in the PBAT

matrix reduces the lamellae thickness of the blend compared to neat PBS lamellae.

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Figure A-I.4. POM image of the (i) PBS and (ii) PBAT/PBS (60/40 wt%) blend

A-I.4. Conclusions

The PBAT/PBS blends were produced on a lab-scale injection-molding machine. After

blending PBAT and PBS, the blends showed balanced stiffness-toughness properties compared

to neat samples. These balanced properties were attributed to the excellent compatibility between

the PBS and PBAT in the blends. The compatibility was caused by a transesterification product

formed during melt blending which acts as a compatibilizer in the blend. SEM image of the

PBAT/PBS blend confirms that there is a good compatibility achieved in the blends evidenced

by a uniform dispersion of the PBS phase in the PBAT matrix. Optical polarizing microscopy

results imply that the PBAT/PBS blend spherulite morphology was affected by the addition of

PBS into the PBAT matrix; this may be due to the poor crystalline nature of PBAT. The MFI

value of the PBAT/PBS blend is quite high compared to PBAT. This improved viscosity will

help to facilitate the production of high fiber-content composites. The PBAT/PBS blend-based

composites fabrication and performance evaluation are under investigation.

333

References

1. B. Imre, B. Pukánszky, Compatibilization in bio-based and biodegradable polymer

blends. Eur.Polym.J, 49, 1215-1233 (2013).

2. http://www.myriant.com/applications/pbs.cfm (accessed on Febraury 06, 2014).

3. R. Muthuraj, M. Misra, A.K. Mohanty, Biodegradable poly (butylene succinate) and poly

(butylene adipate-co-terephthalate) blends: Reactive extrusion and performance

evaluation. J.Polym.Environ, 22, 336–349 (2014).

4. J. John, R. Mani, M. Bhattacharya, Evaluation of compatibility and properties of

biodegradable polyester blends. J.Poly.Sci.Part A: Poly.Chem, 40, 2003-2014 (2002).

5. K. Joseph, S. Thomas, and C. Pavithran, Effect of chemical treatment on the tensile

properties of short sisal fibre-reinforced polyethylene composites. Polymer, 37, 5139-

5149 (1996).

6. J.M. Willis, B.D. Favis, Processing-morphology relationships of compatibilized

polyolefin/polyamide blends. Part I: The effect of an lonomer compatibilizer on blend

morphology. Polym.Eng.Sci, 28, 1416-1426 (1988).

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Appendix II: Durability Studies of Biodegradable Polymers under Accelerated Weathering

Conditions*

*A version of this appendix has been published in: R. Muthuraj, M. Misra, and A. K. Mohanty,

Durability Studies of Biodegradable Polymers under Accelerated Weathering Conditions,

Society of Plastic Engineering (SPE, ANTEC), 2015, Orlando, Florida.

Abstract

Poly(butylene adipate-co-terephthalate), (PBAT) and poly(butylene succinate), (PBS) are

promising biodegradable polyesters whose blends have gained great attention in wide range of

applications. However, there are some drawbacks to the use of these biodegradable polymer

blends in durable applications. The main disadvantage of these materials is hydrolytic

degradation at elevated temperature and humidity. In this study, we have assessed the durability

of PBAT, PBS and PBS/PBAT blends at 50oC with 90% relative humidity (RH) for duration of

up to 18 days. The mechanical properties of these polyesters were evaluated before and after 18

days of conditioning at 50oC with 90% RH. The mechanical properties of the polyesters were

affected with increasing conditioning time. This can be attributed to the susceptibility of ester

bonds to hydrolytic degradation at elevated temperature and humidity. The hydrolytic

degradation was further confirmed by scanning electron microscopy.

A-II.1. Introduction

PBAT, a biodegradable aliphatic-aromatic copolyester, is derived from adipic acid,

terephthalic acid, and butane diol by polycondensation reaction [1]. Solely aromatic polyesters

are insensitive to the microbial attack [2]. However, this insensitivity can be modified through

copolymerization of aliphatic monomers with aromatic monomers, which resulted a

biodegradable polymer i.e., PBAT. The mechanical properties of PBAT are comparable with low

density polyethylene (LDPE) [3]. PBAT is widely used for compostable organic waste bags,

agricultural mulch films as well as lamination/coatings for starch-based products [4]. However,

335

there are some aspects that limit the use this biodegradable polymer in large-scale applications.

PBAT possesses excellent toughness, biodegradability and processability, allowing it to be used

to tailor the properties of some biopolymers, thereby opening up new applications for PBAT

based materials.

PBS is an aliphatic polyester which is traditionally synthesized from fossil fuel based 1,

4- butanediol (BDO) and succinic acid by polycondensation reaction [1]. Recently, renewable

resource based succinic acid and BDO can be produced by fermentation process. These biobased

monomers allow for a biobased PBS production. PBS has potential as a commercial product as it

shows wide variety of commercial applications because of its good processability,

thermomechanical properties, relatively high heat deflection temperature and biodegradability.

Also, PBS has properties close to commercial commodity polymers such as polyethylene (PE)

and polypropylene (PP) [5]. However, insufficient impact strength of the PBS is limits its

extensive applications.

Polymer blending is an effective approach to creating a material with some desired

properties by combining the different advantageous of two or more polymers. The resulting

mechanical properties are typically a compromise between those of the parent polymers.

Therefore, blending of PBS and PBAT is of great interest because it returns desirable properties

while retaining the biodegradable nature of the parent polymers in the resulting blends. The

resulting blend exhibits good mechanical, thermal and rheological properties [1].

Generally, durability is very important for the polymers in order to increase their

suitability for a wider range of applications. However, only few studies have been examined the

durability of biodegradable polymer based materials [6-8]. These literature sources report that

336

the biodegradable polymers are very sensitive to the elevated temperature and humidity.

Additionally, Kim and coworkers [9,10] have reported the durability of talc filled biodegradable

PBS/PBAT blends under marine environment. The authors suggest that the biodegradable

material showed better elastic properties than that of the commercial non-biodegradable material.

To the best of our knowledge, the durability study of PBS/PBAT blend has not yet been reported

in the literature. We report here the durability of PBS/PBAT blend under simulated temperature

(50oC) and relative humidity (90%). The present study was aimed to investigate the durability of

PBS, PBAT, PBS/PBAT and PP under simulated environmental conditions. The durability was

analyzed by means of mechanical properties such as tensile, flexural and impact strength.

A-II.2. Materials and Methods

Commercially available PBS (Biocosafe 1903F) and PBAT (Biocosafe 2003F) pellets were

procured from Xinfu Pharmaceutical Co., Ltd, China. Polypropylene (PP-1350N) was obtained

from Pinnacle Polymers (Garyville, LA). The general properties of the PBS, PBAT, PBS/PBAT

(60/40 wt%) and PP are shown in Table A-II.1. All the polymers were dried in oven for at least 8

h at 80oC to remove the moisture prior to melt processing. Prior to injection molding, all the

samples were extruded in a Leistritz twin-screw extruder with a screw speed of 100 rpm. The

extruder had an L/D ratio of 48 and a screw diameter of 27 mm. The extrudates were pelletized

and dried at 80oC for 12 h prior to the injection molding. The dried extruded pellets were then

injection molded in an ARBURG injection molding machine to obtain desired test specimens.

The injection molding machine had a maximum injection pressure of 2000 bar and a screw

diameter of 35 mm. The extrusion and injection molding process were carried out with a

processing temperature of 140oC for PBS, PBAT, PBS/PBAT (60/40 wt%) and 180

oC for PP.

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Table A-II.1. General properties of PBS, PBAT, PBS/PBAT (60/40 wt%) and PP. (a obtained

from material data sheet, b and PBS/PBAT (60/40 wt%) data were measured in the lab)

Properties PBS PBAT PBS/PBAT

(60/40 wt%)

PP

Melt flow index (g/10min)a

≤30 ≤20 33 55

Melting point (oC)

a 110-120 110-120 110-120 165

Density (g/cm3)a 1.26 1.26 1.26 0.9

Moisture absorption after 14

days at 50oC with 90% RH

b

1.0 ± 0.004 0.83 ± 0.004 0.896 ± 0.002 0.008 ± 0.005

In order to evaluate the durability, the injection molded samples were exposed to 50 oC with

90% RH in an environmental chamber (Endurance C340, Envirotronics, Inc). The tensile and

flexural properties of the samples before and after conditioning were measured according to

ASTM standards in a Universal testing machine (Instron-3382) with a 50 kN load cell at room

temperature. The crosshead speed for tensile and flexural test was 50 mm/min and 14 mm/min,

respectively. Notched Izod impact strength was measured as per ASTM D256 in a TMI impact

testing machine. The results are reported an average values of five replicates for each set of

samples. Cryofractured sample morphology was examined using scanning electron microscopy

(Inspect S50-FEI Company) at an accelerating voltage of 20 kV. Prior to observing sample

morphology, the samples were gold coated with a final thickness of 20 nm with 20 mA.

A-II.3. Results and Discussion

Durability of PBS, PBAT, PBS/PBAT (60/40 wt%) and PP samples were examined in

terms of mechanical properties. The results obtained from the samples before and after

conditioning are depicted in Figures A-II.1 and A-II.2 and Table A-II.2. Figure A-II.1 shows the

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tensile strength of the samples before and after 18 days conditioning. The tensile strength of

PBS, PBAT and PBS/PBAT significantly reduced after 18 days exposed to 50oC with 90% RH.

Generally, biodegradable polymers undergo hydrolytic degradation in the presence of moisture

and heat [6]. After 14 days exposed to 50oC with 90% RH, biodegradable polymers showed

greater moisture absorption than PP (Table A-II.1). This is attributed to the relatively high

polarity of the biodegradable polymers studied here. The higher moisture absorption of PBS,

PBAT and their blend leads to a decrease in molecular weight and tensile strength via hydrolysis

of the ester bonds. Due to the non-polar nature of PP, this plastic samples maintains its tensile

strength after 18 days exposed to 50oC with 90% RH. Our findings have good agreement with

literature [8].

Figure A-II.1. Tensile strength of PBS, PBAT, PBS/PBAT (60/40 wt%) and PP before and after

18 days exposed to 50oC with 90% RH.

Flexural strength of the PBS, PBAT, PBS/PBAT and PP samples before and after 18 days

conditioning is shown in Figure A-II.2. The flexural strength of PBS and PP is higher than

PBAT and PBS/PBAT blend. Except PBAT, all the samples showed a slight increase in the

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flexural strength after 18 days conditioning. This increase can be ascribed to post-crystallization

phenomena which occurred during conditioning [11].

Figure A-II.2. Flexural strength of PBS, PBAT, PBS/PBAT (60/40 wt%) and PP before and

after 18 days exposed to 50oC with 90% RH.

The notched Izod impact strength of the samples is shown in Table A-II.2. After 18 days

conditioning, there is no significant change observed in the impact strength of PBAT. This could

be due to the PBAT maintaining its sufficient molecular weight in order to form a significant

degree of entanglement [12]. In the present study, impact strength of PBS was improved by

adding PBAT. It can be seen that the impact strength of PBS, PBS/PBAT decreased considerably

after 18 days exposed. In addition, it was clearly observed that the fracture mode of PBS/PBAT

changed from ductile to brittle after exposure to heat and humidity. This may be attributed to the

hydrolysis products of PBS accelerated the degradation of PBAT in the PBS/PBAT blend

system. As mentioned before, PP is resistant to hydrolytic degradation due to its non-polar

nature. Therefore, as expected, mechanical properties are not affected significantly after 18 days

exposure to the hydrolysis environment.

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Table A-II.2. Notched Izod impact strength of PBS, PBAT, PBS/PBAT (60/40 wt%) and PP

before and after 18 days conditioning at 50oC with 90% RH.

Samples PBS PBAT PBS/PBAT (60/40 wt%) PP

Before conditioning

(J/m)

24.80 ± 6.55 Non-break

Non-break 30.4 ± 7.08

After 18 days

conditioning (J/m)

13.53 ± 1.21 Non-break

17.46 ± 7.22 22.3 ± 5.45

Figure A-II.3 shows the SEM micrographs of the samples before and after 18 days of

conditioning at 50oC with 90% RH. The PBS, PBAT and PBS/PBAT exhibits relatively very

smooth and clear surface morphology before exposed to elevated temperature and humidity. It is

clear from the SEM images that after 18 days of conditioning, the samples had gained rough

surface, deep holes as well as cavities. This result agrees with the considerable reduction

observed in the mechanical properties of the samples after 18 days exposure [8]. Furthermore,

ongoing degradation can be seen (Figure A-II.3) in the form of slightly eroded regions. This

shows that the PBS, PBAT and PBS/PBAT underwent degradation with increasing conditioning

time. The formed holes and cavities are attributed to the dissolution of the oligomers during

hydrolysis mechanism. All these observations suggest that the PBS, PBAT and PBS/PBAT can

readily undergo hydrolytic degradation in the presence of high humidity and temperature.

341

Figure A-II.3. SEM micrographs of PBS, PBAT, and PBS/PBAT (60/40 wt%) before (A, B and

C) and after (D, E and F) 18 days exposed to 50oC with 90% RH.

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A-II.4. Conclusions

We have studied the durability of biodegradable PBS, PBAT, PBS/PBAT and non-

biodegradable PP at elevated temperature and humidity. The temperature and humidity had an

influence on the mechanical properties of the conditioned biodegradable polymers. After 18 days

exposed to 50oC with 90% RH, the samples morphology changed from smooth to an eroded

surface. These morphological changes suggest that the samples can undergo degradation within

18 days of conditioning as studied. Furthermore, our ongoing research will focus on the

extensive analysis of PBS, PBAT and PBS/PBAT blend at various stages of hydrolytic

degradation. In order to expand the potential of PBS and PBAT for durable applications, it is

essential to reduce the hydrolytic degradation behavior while maintaining compostability at the

end of life.

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Biodegradable Polymer Blends and Their Biocomposites

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