INVESTIGATION OF A SUSTAINABLE ALTERNATIVE -

WOOD BIO-PLASTIC COMPOSITES

By

MENG-HSIN TSAI

A dissertation submitted in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

WASHINGTON STATE UNIVERSITY

Materials Science and Engineering Program

December 2011

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To the Faculty of Washington State University

The members of the Committee appointed to examine the dissertation of MENG-HSIN TSAI find it satisfactory and recommend that it be accepted.

____________________________________

Michael P. Wolcott, Ph.D., Chair

____________________________________

Jinwen Zhang, Ph.D

____________________________________

Karl Englund, Ph.D

____________________________________

Long Jiang, Ph.D

____________________________________

Vikram Yadama, Ph.D

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ACNOWLEDGMENT  

I would like to appreciate the faculties and staffs of Department of Civil

Engineering, Materials Science and Engineering Program, and Composite Materials

and Engineering Center (CMEC) for your assistance and support throughout the years.

Special thanks to my advisor Dr. Michael P. Wolcott and my committee Dr. Karl

Englund, Dr. Vikram Yadama, Dr. Jinwen Zhang, and Dr. Long Jiang for their

valuable guidance and support on my research. A special thanks to Bob Duncan, Scott

Lewis and Dr. Brent Olson for their assistance and dedication. I would not have been

able to complete this research without their help. Additionally, I would like to thank

the financial support from Department of Energy. My thanks also go to my colleagues

in the CMEC for their help and emotional support. Finally, I also appreciate my wife,

Lee-Wen Chen, my son, Eric Tsai, and all my family for their love and support.

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INVESTIGATION OF A SUSTAINABLE ALTERNATIVE –

WOOD BIO-PLASTIC COMPSOITES

Abstract

by Meng-Hsin Tsai, PhD. Washington State University

December 2011

Chair: Michael P. Wolcott

Bio-polymers have become a focus for developing wood bio-plastic composites

since they can be sustainably renewed from agricultural resources. One of the

bio-polymers, poly-3-hydroxybutyrate (PHB), has been widely studied for its

performance enhancement by blending with wood flour and coupling agents. One

riddle of the influence from coupling agents on PHB/wood flour (WF) composites

was discussed in this study. A lab-scale composting evaluation test was developed

based on ASTM standards in order to understand the compostability of PHB/WF

composites. During the compostability evaluation, multiple phases of biodegradation

such as lag, biodegradation, and slow down phases were observed. Theoretically,

microcracks occurred during the lag phase, which is a combination of physical

(microcracks) and chemical (decomposition) degradation. The following phase of

biodegradation occurs when the organic matter of PHB/WF composites rapidly

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converts to carbon dioxide, known as mineralization behavior in its biological

definition.

In addition, this carbon conversion process is involved in a carbon cycling

ecosystem. Mineralization results indicated that PHB was consumed faster than wood

flour. Wood flour was then believed to challenge the compostability of the PHB/WF

composite. It was also observed that hypha could not reach the PHB located in the

core of the composite. This phase is the slow down phase. Without coupling agents,

an in accordance with ASTM standards and the results found this study, the PHB/WF

composite is certified as a compostable polymeric material. Furthermore, purification

of producing neat PHB has been reported as consuming more energy than producing

polyolefin. By eliminating the purification step of PHB production, energy

consumption could be limited and seen to reduce. In this study, the PHB/wood

flour/cell debris (PWC) composites were successfully developed and produced

through injection molding and extrusion processes. The mechanical performance and

moisture diffusion behaviors of the PWC composites were comparable with a

commercial wood plastic composite. Therefore, a sustainable alternative - wood

bioplastic composite was developed by maintaining biodegradability and its carbon

footprint was effectively controlled, both of which are sustainable requirements.

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TABLE OF CONTENTS

Page

ACKNOWLEDGEMENT ….…………………………………………………….. iii

ABSTRACT …....……………………………...……………………………………. iv

LIST OF FIGURES ...……………………………………………………………... xiii

LIST OF TABLES ………………………………………………………………… xvi

CHAPTER 1 - PROJECT INTRODUCTION

1.1 Introduction ..……………………………………………………………. 1

1.2 Background ..……………………………………………………………. 3

1.2.1 Poly-hydroxyalkanoates (PHAs) ..………………………………… 3

1.2.2 Definition of “Biodegradable plastic” and “Compostable plastic” .. 5

1.2.3 Composting mechanism and carbon cycling ecosystem ...………... 6

1.2.4 Mechanics of enzymatic degradation .…………………………… 10

1.3 Researches and Developments (Biocomposites) ...……………………… 15

1.4 Objectives …....………………………………………………………….. 18

1.5 References ………………………………………………………………. 21

CHAPTER 2 - COMPOSTABILITY AND MINERALIZATION BEHAVIORS OF

A POLY-3-HYDROXYBUTYRATE/WOOD FLOUR COMPOSITE

2.1 Introduction ……………………………………………………………... 27

2.2 Materials and Methods ..………………………………………………… 31

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2.2.1 Materials ..………………………………………………………. 31

2.2.2 Testing specimen preparation …………………………………... 32

2.2.3 Composting apparatus and procedure …………………………... 34

2.2.4 Thermal analysis ………………………………………………... 39

2.2.5 Fickian diffusion coefficients …………………………………... 40

2.2.6 Enzymatic kinetic ………………………………………………. 41

2.2.7 Morphology …………………………………………………….. 43

2.3 Results and Discussion …………………………………………………. 43

2.3.1 Thermal degradation ……………………………………………. 43

2.3.2 CO2 evolution and mineralization .……………………………… 44

2.3.3 Thermal analysis ………………………………………………... 50

2.3.4 Fickian diffusion coefficient ……………………………………. 52

2.3.5 Enzymatic kinetic ……………………………………………….. 53

2.3.6 Morphology ……………………………………………………... 54

2.4 Conclusion ………………………………………………………………. 56

2.5 Acknowledgements ……………………………………………………... 58

2.6 References ………………………………………………………………. 59

CHAPTER 3 - A NOVEL INVESTIGATION OF HYGROTHERMAL EFFECTS

ON PROPERTIES DEGRADATION AND PROPAGATION OF PHB/WF

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COMPOSITES BY USING THE CONTINUOUS DAMAGE MECHANISM

3.1 Introduction ……………………………………………………………... 65

3.2 Materials and Methods ………………………………………………….. 68

3.2.1 Materials ………………………………………………………... 68

3.2.2 Methods ………………………………………………………… 69

3.2.2.1 Composite extrusion preparation ……………………….. 69

3.2.2.1.1 Melt compounding …………………………… 69

3.2.2.1.2 Sheet extrusion ……………………………….. 70

3.2.2.2 Testing specimen preparation and hygro- / hydro- thermal

conditioning ...…………………………………………... 71

3.2.2.3 Mechanical properties and density evaluation ……......... 72

3.3 Results and Discussion ………………………………………………….. 73

3.3.1 Moisture absorption …………………………………………….. 73

3.3.2 Moisture effects on specific tensile properties …………………. 74

3.3.3 Hygrothermal effects (Moisture and mechanical degradation) … 79

3.3.4 Microcracking evaluation by the Continuous Damage

Mechanism …...…………………………………………………. 83

3.3.5 Proposed microcracking mechanism under hygrothermal

conditions …………………………………………………….… 86

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3.4 Conclusion ………………………………………………………………. 87

3.5 References ……….……………………………………………………… 89

CHAPTER 4 - INVESTIGATION OF EFFECTS OF COUPLING AGNETS ON

COMPOSTABILITY AND MINERALIZATION BEHAVIORS OF

POLY-3-HYDROXYBUTYRATE/WOOD FLOUR COMPOSITES

4.1 Introduction ……………………………………………………………... 94

4.2 Materials and Methods ………………………………………………….. 98

4.2.1 Materials ………………………………………………………... 98

4.2.2 Testing specimen preparation …………………………………... 99

4.2.3 Composting apparatus and procedure …………………………. 101

4.2.4 Fickian diffusion coefficient …………………………………... 106

4.2.5 Enzymatic kinetic ……………………………………………... 106

4.2.6 Morphology …………………………………………………… 108

4.3 Results and Discussion ………………………………………………... 109

4.3.1 CO2 evolution and mineralization results ……………………... 109

4.3.2 Fickian diffusion coefficient …………………………………... 114

4.3.3 Enzymatic kinetic ……………………………………………... 116

4.3.4 Morphology …………………………………………………… 118

4.3.5 Enzymatic surface erosion mechanism ……………………….. 121

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4.4 Conclusion …………………………………………………………….. 123

4.5 Acknowledgments …………………………………………………….. 125

4.6 References …………………………………………………………….. 126

CHAPTER 5 - DEVELOPING ENERGY EFFICIENT AND COST EFFECTIVE

POLY-3-HYDROXYBUTYRATE /WOOD FLOUR COMPOSITES: EXTRUSION

5.1 Introduction ……………………………………………………………. 132

5.2 Materials and Methods ………………………………………………… 136

5.2.1 Materials ………………………………………………………. 136

5.2.2 Methods ……………………………………………………….. 137

5.2.2.1 Particle size distribution analysis of ground cell debris .. 137

5.2.2.2 Torque rheometry ……………………………………… 137

5.2.2.3 Preparation of composites ……………………………... 138

5.2.2.4 Preparation of testing specimens ………………………. 141

5.2.2.5 Morphology ……………………………………………. 141

5.2.2.6 Mechanical properties and sample density …………….. 142

5.2.2.7 Moisture resistance …………………………………….. 142

5.3 Results and Discussion …………...……………………………… 143

5.3.1 Torque rheometry ………………………………………... 143

5.3.2 Morphology ……………………………………………… 148

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5.3.3 Density and mechanical properties ………………………. 150

5.3.4 Moisture behavior ………………………………………... 154

5.4 Conclusions ……………………………………………………... 160

5.5 Acknowledgements ……………………………………………... 161

5.6 References ………………………………………………………. 162

CHAPTER 6 - DEVELOPING ENERGY EFFICIENT AND COST EFFECTIVE

PHB/WF COMPOSITES: INJECTION MOLDING

6.1 Introduction ……………………………………………………………. 166

6.2 Materials and Methods ………………………………………………… 170

6.2.1 Materials ……………………………………………………….. 170

6.2.2 Methods ………………………………………………………... 171

6.2.2.1 Preparation of composites ……………………………... 171

6.2.2.2 Injection molded testing specimens …………………… 173

6.2.2.3 Thermal degradation analysis of raw materials ……….. 174

6.2.2.4 Mechanical properties and sample densities …………... 174

6.2.2.5 Morphology ……………………………………………. 175

6.2.2.6 Moisture behavior and diffusion ………………………. 176

6.3 Results and Discussion ………………………………………………... 177

6.3.1 Thermal degradation …………………………………………... 177

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6.3.2 Material density and mechanical properties …………………... 178

6.3.3 Morphology …………………………………………………… 182

6.3.4 Moisture diffusion behavior …………………………………... 186

6.4 Conclusion …………………………………………………………….. 192

6.5 Acknowledgement …………………………………………………….. 193

6.6 References ……………………………………………………………... 194

CHAPTER 7 – CONCLUSION

APPENDIX A – MALEATION OF POLY-3-HYDROXYBUTYRATE BY

REACTIVE EXTRUSION

A.1 Introduction …………………………………………………………… 201

A.2 Materials and Methods ………………………………………………... 202

A.2.1 Materials ………………………………………………………. 202

A.2.2 Methods ……………………………………………………...... 202

A.2.2.1 Reaction Extrusion of MA-grafting on PHB …………. 202

A.2.2.2 Evaluation of MA-grafting Ratio …………………….. 203

A.3 Results and Discussion ………………………………………………... 205

A.4 Conclusion …………………………………………………………….. 206

A.5 References ……………………………………………………………... 207

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LIST OF FIGURES Page

Figure 1.1 Biological enzyme, substrate, enzyme- substrate complex, and products concentration change with time …………………………………………………….. 13 Figure 1.2 Plot of Michaelis-Menten equation with determination of Michaelis constant, Km ………………………………………………………………………… 14 Figure 1.3 Lineweaver-Burk Plot of reciprocal Michaelis- Menten equation for determining Km/Vmax, and 1/Vmax …………………………………………………... 15 Figure 2.1 Schematic of the lab-scale material compostability evaluation set-up system ………………………………………………………………………………. 38 Figure 2.2 Weight loss and derivative thermogravimetry of PHB, WF, and PHB/WF composite …………………………………………………………………………... 44 Figure 2.3 The accumulative CO2 evolving production of CP, PHB, PHB/WF composite, and control compost material ………………………………………….. 48 Figure 2.4 The accumulative mineralization of CP, PHB, and PHB/WF composite. 49 Figure 2.5 Lineweaver-Burk plot of rate of mineralization and substrate content … 54 Figure 2.6 Comparison of initial samples (CP, PHB, and PHB/WF composite) and after 90days composted sample debris ……………………………………………... 55 Figure 2.7 SEM Morphology of different sample (CP, PHB, PHB/WF composite) surfaces on day 1 (a) and after 90 days (b) ……………………………………........ 56 Figure 3.1 Relative water absorption behaviors of three different PHB/WF composites …………………………………………………………………………. .74 Figure 3.2 Plot of relation of moisture content and specific tensile strength of three different PHB/WF composites ……………………………………………………... 76 Figure 3.3 Plot of relation of relative moisture content and specific tensile modulus of three different PHB/WF composites ……………………………………………….. 76 Figure 3.4 Plot of relation of relative moisture content and strain at break of three different PHB/WF composites ………………………………………………………77 Figure 3.5 Plot of relation of relative moisture content and strength adjusting factor of three different PHB/WF composites with regression curves ………………………..78 Figure 3.6 Plot of relation of relative moisture content and modulus adjusting factor of three different PHB/WF composites with regression curves ………………...…….. 79 Figure 3.7 Plot of relation of specific tensile strength against conditioning time .…. 82 Figure 3.8 Plot of relation of specific tensile modulus against conditioning time .… 82 Figure 3.9 Plot of relation of relative moisture absorption against hygro-thermal conditioning time …………………………………………………………………… 83 Figure 3.10 Plot of relation of theoretical calculated damage parameter (ω) in percentage against conditioning time ………………………………………………. 86

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Figure 4.1 Schematic of lab-scale material compostability evaluation set-up system ……………………………………………………………………………... 105 Figure 4.2 Plot of accumulative CO2 evolving production against square root of time in hours of polymeric composites and materials ………………………………….. 114 Figure 4.3 Plot of accumulative mineralization against square root of time in hours of polymeric composites and materials ……………………………………………… 115 Figure 4.4 Lineweaver-Burk plot of rate of mineralization and substrate content ... 118 Figure 4.5 Morphology of PHB (A) and eroded and/or biodegradated surface of PHB (B – F) …………………………………………………………………………….. 120 Figure 4.6 Morphology of a PHB/WF composite (A) and eroded and/or biodegradated surfaces of PHB/WF composites (B – F) …………………………………………. 121 Figure 4.7 Morphology of crystal region of PHB (A) and evidence of hypha (B and C) on the surface of PHB/WF composite …………………………………………….. 122 Figure 5.1 Torque rheometry behavior at different temperatures on the control formulation (PW) …………………………………………………………………. 146 Figure 5.2 Torque rheometry behavior of the six formulations of PWCCs at 165

oC ………………………………………………………………………………….. 147 Figure 5.3 Torque rheometry behavior of six different formulations of PWCCs with processing temperature of 160 and 165 oC …………………………………………147 Figure 5.4 SEM micrographs of cell debris (left) and pine wood flour (right) …… 150 Figure 5.5 Interfacial bonding of wood flour (a) and cell debris (b) with PHB matrix on microtomed surface of PWCC ………………………………………………… 150 Figure 5.6 Plot of the relation of cell debris content and specific MOR of the PWCCs ……………………………………………………………………………. 153 Figure 5.7 Plot of the relation of cell debris content and specific MOE of the PWCCs …………………………………………………………………………..... 153 Figure 5.8 Plot of the relation of cell debris content and specific impact strength of the PWCCs …………………………………………………………………………..... 154 Figure 5.9 Plot of moisture absorption versus square root of time of seven composites ………………………………………………………………………… 156 Figure 5.10 Plot of thickness change versus square root of time of seven composites ………………………………………………………………………… 156 Figure 5.11 Plot of true moisture diffusion constant versus cell debris content ….. 160 Figure 6.1 Functional group reactions between isocyanate chemicals and wood/moisture …………………………………………………………………….. 169 Figure 6.2 Ground cell debris particle size distributions ………………………….. 171 Figure 6.3 Plot of the thermogravimetric curves of pine wood flour, cell debris, and neat PHB ………………………………………………………………………….. 178

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Figure 6.4 Plot of the relation of tensile properties and cell debris contents ……... 180 Figure 6.5 Plot of the relation of flexural properties and cell debris contents ……. 181 Figure 6.6 Plot of the relation of impact strength and cell debris contents ………. .181 Figure 6.7 SEM micrographs of cell debris (a) and pine wood flour (b) …………. 184 Figure 6.8 Fracture surfaces of neat PHB (a), PHB/wood flour composite (b), and PHB/wood flour/cell debris composite (c). Magnification: X80 …………………. 184 Figure 6.9 Fracture surfaces of neat PHB (a), PHB/wood flour composite (b), and PHB/wood flour/cell debris composite (c). Magnification: X500 ………………... 185 Figure 6.10 Sectioned surface of PHB/wood flour/cell debris composites ………. 185 Figure 6.11 Wood lumens filled with polymer …………………………………… 185 Figure 6.12 Plot of moisture absorption against square root of time in seconds …. 187 Figure 6.13 Plot of thickness swelling against square root of time in seconds …… 188 Figure 6.14 Plot of true moisture diffusion constant versus cell debris content ….. 192

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LIST OF TABLES Page

Table 2.1 The proportion of compost feedstocks …………………………………... 31 Table 2.2 The chemical characteristics of the materials ………………………….... 38 Table 2.3 The rates of CO2 evolution and mineralization of each sample in different phases of biodegradation ………………………………………………………….... 46 Table 2.4 The average moisture content and pH value of control, CP, PHB, and PHB/WF composite ……………………………………………………………….... 49 Table 2.5 Thermal properties of PHB and PHB/WF in different sampling times ..... 51 Table 2.6 Enzyme kinetic constants of different materials ………………………… 54 Table 4.1 Proportion of compost feedstocks ……………………………………… 100 Table 4.2 Chemical characteristics of materials …………………………………... 106 Table 4.3 Rates of CO2 evolution and mineralization of each sample in different phases of biodegradation ………………………………………………………….. 111 Table 4.4 Mineralization diffusion coefficient of different polymeric material and composites ………………………………………………………………………… 117 Table 4.5 Enzyme kinetic constants of different materials ……………………….. 118 Table 5.1 Particle size distribution of cell debris after hammer milling ………….. 138 Table 5.2 Formulations of PHB/WF control and PHB/WF/Cell debris mixtures for torque rheometry study ……………………………………………………………. 139 Table 5.3 Formulations of HDPE/WF, PHB/WF control, and PHB/WF/cell debris composites ………………………………………………………………………… 141 Table 5.4 Temperature profiles used for extrusion of the seven formulations ……. 141 Table 5.5 Densities of HDPE/wood flour, PHB/WF control and PHB/wood flour/cell debris composites …………………………………………………………………. 151 Table 5.6 Actual output date of extrusion processing of the different composites .. 151 Table 5.7 Comparison of moisture diffusion parameters of extruded PHB composites and commercial HDPE composite ………………………………………………... 159 Table 6.1 Formulations of two controls of PHB/WF (PW) and PHB/cell debris (PC) and PHB/WF/cell debris (PWC) composites ……………………………………... 173 Table 6.2 Densities and mechanical properties with standard deviation of four formulations ………………………………………………………………………. 180 Table 6.3 Apparent (DA) and true (D) diffusion constants for the six formulations ………………………………………………………………………. 190

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DEDICATION

This dissertation is dedicated to my wife Lee-Wen Chen, my son Eric (Eitzer) Tsai and all my family members who have provided their emotional supports throughout

these years.

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Chapter 1 Project Introduction

1.1 Introduction

Development of fossil fuel derived polymers has facilitated the convenience of

our life style since the early 20th century. Durability is the biggest advantage of these

polymers. Polyolefin is one broad example. Using fillers to reinforce polymers and

form polymeric composites has been seen as a solution to expand the performance

envelope, such as improving stiffness, tensile strength, impact, dimensional stability,

and also to reduce cost. The two main categories of these fillers are: 1) inorganic

fillers, such as talc, mica, calcium carbonate, glass fiber, etc.; and 2) organic/natural

fibers, such as wood flour/fiber, cellulose, rice hull, wheat straw, etc. Nowadays,

“green” aspects and lightweight have become another focus for maintaining

sustainable requirements, e.g. less carbon dioxide emission during transportation and

reduction of the carbon footprint of products. Natural fibers such as wood flour/fiber

demonstrate most of these sustainable advantages. Wood polymer composites (WPCs)

have been successfully developed in recent decades. WPCs play a major role in the

replacement of polymeric materials in building and construction, such as decking,

siding, fencing, etc. [1][2][3]. Now, the application is moving into the field of

injection molding, such as for automotive parts, containers, and consumer goods, due

  2  

to its low cost and lightweight (0.8 g/cm3 of average wood density vs. 2.8 g/cm3 of

talc). Unfortunately, the advantage of durability of polymers is now seen as

problematic in some product applications, e.g. consumer goods, because these

polymers may require hundreds of years to degrade after the products are disposed.

According to the US Environmental Protection Agency (EPA), one statistical

report [4] indicates that in 2009 only approximately 9% of non-durable plastic goods

were recovered in the US. To be specific, about 17.3 million tons of plastic waste was

land-filled in 2009. This statistical report indicates that recycling is still a challenge in

the US. Fortunately, biologically derived polymers (bio-polymers) have been

discovered and developed by engineers and scientists. These bio-polymers can be

derived not only from natural resources, but can also be biodegraded under enzymatic

conditions [5][6][7][8]. The most commonly studied examples of bio-based polymers

are poly-lactic acid (PLA) and poly-hydroxyalkanoates (PHAs). Utilization of these

bio-polymers is now seen as a solution for achieving the reduction of petroleum

dependence. The reason behind this idea is that bio-polymers, for example PHAs, can

be sustainably synthesized and fermented from natural carbohydrate resources via

biological processes [9]. Furthermore, the utilization of a sustainable material (wood

flour) as a reinforcement in bio-polymers to form wood bio-polymer composite is

  3  

now creating more attention as a solution to achieve sustainability while maintaining

performance requirements.

1.2 Background

1.2.1 Poly-hydroxyalkanoates (PHAs)

One family of microbial polyesters, PHAs, is biologically synthesized by

bacteria with carbohydrate feedstock and reserved as an energy source for bacteria.

PHAs have widely different properties, from stiff to rubbery (thermoplastic to

elastomer), which are associated with different species of bacteria and feed sources

[9][10][11][12]. Because of that, PHAs have been viewed as an alternative to lessen

dependence on crude oil. Many previous studies [9][10][11][12] have indicated that

the first PHA was discovered in 1925 and known as polyhydroxybutyrate (PHB)

French scientist Maurice Lemoigne found that PHB was an energy reserve made up of

many different bacteria with carbohydrate substrate feeding. PHB is a highly

crystalline substance with a melting point of about 175oC [10]. PHB possesses a

similar processability to many commodity polymers with no issue in using

conventional polymer processing techniques and equipment [13]. Moreover, PHB is

biodegradable and compostable [14][15], which means that it can be mineralized by

  4  

biologically converting organic carbon to CO2 as a metabolism of microorganisms

[16][17].

However, the cost of PHAs is much higher than that of polyolefin.

Conventionally, PHAs is converted from starch through biological synthesis; 1 kg of

PHAs requires about 3.3 kg of starch for conversion [18]. Additionally, further

procedures of sterilization and purification are required to obtain purified PHAs

polymer. These processes not only consume food resources such as starch, but also

require more energy input [18][19]. After this cradle-to-grave analysis [18], it appears

that producing 1kg of PHA from corn would require 65% more fossil fuel to generate

the same quantity of petroleum-based polystyrene.

The addition of natural organic fillers to reinforce the PHAs can maintain

biodegradability and expand the performance envelope with cost reductions. Starch,

soy protein, wood flour (WF), and other natural fibers are not frequently used. The

performance of natural fibers has often been studied in engineering biocomposites

through integration with PHB due to the low cost and high strength/stiffness from

natural fibers [20][21][22]. However, the discrepancy of surface tensions of

hydrophobic bio-polymer and hydrophilic organic fillers causes incompatibility in the

blend system. In morphology, it has been observed that the interfacial gap between

the fiber and the matrix causes stress concentration while applying a load [22][23][24],

  5  

which then induces low performance of the biocomposites. The reason for this result

is that applied loads cannot be transferred properly between fiber and matrix. Hence,

it is desirable to modify the surface adhesion between two materials by adding

chemicals such as a compatibilizer, which is also known as a coupling agent or

interfacial modifier. The performance of the biocomposites can therefore be regarded

as being improved. Research results have indicated that enhanced mechanical

properties and reduction of moisture absorption were observed by adding

compatibilizers to wood-plastics composites (WPCs) [23][24][25][26].

1.2.2 Definition of “Biodegradable” and “Compostable”

As mentioned earlier, PHB has been proven and classified as a biodegradable

and compostable bio-polymer [14][15]. However, these two terminologies are

commonly misstated or addressed. According to ASTM D 6400, the definitions of

biodegradable and compostable are: “Biodegradable plastic (polymer) is defined as a

polymer in which the degradation results from the action of naturally occurring

microorganisms such as bacteria, fungi, and algae”; “Compostable plastic (polymer)

is defined as a polymer that undergoes degradation by biological processes during

composting to yield CO2, water, inorganic compounds, and biomass at a rate

consistent with other known compostable materials and leaves no visible,

  6  

distinguishable or toxic residue.” Notably, the testing period for this research will be

conducted for a period of 180 days. For plastic products consisting of a single

polymer, 60% of the organic carbon of the polymer must be converted to CO2 within

the 180 day testing period. The compostability of biopolymers or biocomposites will

be carried out by a laboratory scale evaluation in accordance with ASTM D5338.

1.2.3 Composting Mechanism and Carbon Cycling Ecosystem

In municipal waste management (MSW), one method of reducing municipal

waste is to decompose all compostable products, e.g. kitchen wastes, by a composting

technique. During the composting process, the organic carbon of these compostable

products can be converted to carbon dioxide (CO2) by aerobic enzymatic

bioactivation/bioreactions [16][17]. By composting the compostable plastic products,

a reduction of the annual landfill capacity can therefore be expected. In the

mechanisms of composting, organic matter is decomposed by the metabolism of

aerobic microorganisms under a condition of high humidity and high temperature.

Under such conditions, a hygrothermal condition will first influence the substrate

materials, e.g. dimensional swelling (escalation of microcracking occurs [27]) and

deterioration of properties. In this stage, moisture will penetrate into the substrates

potentially by diffusion and capillary movements. Following hydrolysis, in which

  7  

only the polymer chain is broken down to small chain length, could possibly occur

before biodegradation [6]. Biological degradation next occurs and produces

by-products such as CO2, water, heat energy, and composted soil. The composting

degradation mechanism can be schematically seen in Scheme 1.1). In accordance with

the eco-system of carbon cycling, plants can further photosynthesize the released

carbon from organic matter, and the carbon can also be sequestered as feedstock. For

example, vascular plants can store carbon and produce food sources, e.g.

polysaccharides or carbohydrates, as well as structural components, such as starch and

cellulose. These two components can further be harvested and utilized to produce

PHAs by utilizing biological synthesizing processes. Therefore, a carbon cycling

ecosystem can be expected to develop (See Scheme 1.2).

  8  

Scheme 1.1 The mechanism of composting degradation

Scheme 1.2 The carbon cycling ecosystem

Complex carbohydrates (Starch/Cellulose)

Packaging products Bio-composites

PHB

Fermentation

Polymerization

PLA

CO2, H2O, Compost soil, Energy

Biodegradation Photooxidation

Recycling

Agricultural feedstocks

Photosynthesis

Thermal p

roces

sing

Composting

Food sources

Solar energy

O

O

C

=CH3

CHn

nO

OC

=CH3CH CH2

Extra

ctio

n Waste, Debris, etc.

Complex carbohydrates (Starch/Cellulose)

Packaging products Bio-composites

PHB

Fermentation

Polymerization

PLA

CO2, H2O, Compost soil, Energy

Biodegradation Photooxidation

Recycling

Agricultural feedstocks

Photosynthesis

Thermal p

roces

sing

Composting

Food sources

Solar energy

O

O

C

=CH3

CHn

O

O

C

=CH3

CHn

nO

OC

=CH3CH CH2 n

OOC

=CH3CH CH2O

OC

=CH3CH CH2

Extra

ctio

n Waste, Debris, etc.

  9  

Theoretically, the bio-degradation reaction involves three phases: lag phase,

biodegradation phase, and plateau phase [14][28]. The lag phase is a period of

adjustment to microorganisms after the introduction of a new substrate into the culture

mixture. In the first step of this phase, material consequently uptakes moisture and

results in dimensional swelling. This change happens because moisture penetrates into

the material and fills the spaces between polymer chains of the material. Over time,

the polymeric material experiences creep damage because of its nature of

viscoelasticity properties due to internal stress generated by the swollen dimension

after moisture absorption. Previous studies indicated that internal microcracks

occurred after polymeric materials absorbed moisture [29][30]. This result is

recognized as hygrothermal condition effects, and further caused nonlinear Fickian

moisture diffusion due to escalated moisture of microcracks [27][31]. Possibly,

polymer chains, especially polyester, could break down to scissions known as

hydrolysis, but may not exhibit molecular weight loss [6].

In the second step, enzymes or depolymerases from microorganisms begin to

attach or bind with the substrate materials and begin to operate the

biodegradation/catalyzing reaction while moisture and oxygen are provided

uninterruptedly. The decomposed materials further supply nutrients to the

microorganisms [16][17]. Hydrolysis and biodegradation might simultaneously occur

  10  

during this period, but it is believed that in this case there is more biodegrading

decomposition than hydrolysis. In the last step of lag phase, by-products such as CO2

are generated by enzymatic catalysis bio-reaction, which is related to the metabolism

of microorganisms.

The following example of the biodegradation phase can be observed during a

rapidly CO2 generation rate. In this period, the exponentially grown enzymes react

with the substrate material. After most of the substrate material is consumed, the

plateau phase occurs with stable or close-to-no bio-reaction. Hence, the

biodegradation ends. The simplified mechanism of compost is that organic matter

converts to carbon dioxide (mineralization) by enzyme reactions from

microorganisms. As the organic matter can be released back to the atmosphere via

composting techniques, the capacity of waste landfill is then to be seen as a reduction,

if all compostable materials are indeed composted. In sustainable views assessing the

environment impact of products, the results of the accumulation of CO2 evolution and

mineralization could estimate the required time period to convert polymeric products

back to nature, which is also been considered as life-cycle assessment [19].

  11  

1.2.4 Mechanics of Enzymatic Degradation

As mentioned earlier, the rate of enzyme bioreaction can be understood by

reflecting on the results of CO2 evolution during composting evaluation. The higher

the rate of CO2 evolves, the more rapidly enzyme bioreaction occurs. Enzyme kinetics

has often been studied to understand enzyme-substrates bioreactions. Enzyme kinetics

is also broadly applied in the areas of biology, bio-engineering, food science,

agriculture, environmental/soil science, etc. Also, enzyme kinetics can be recognized

as a rate/velocity of enzyme bio-reaction [32]. As bioreactions of composting

processes are mainly operated and dependent on enzymes, enzyme kinetics can be

theoretically applied to evaluate the enzyme kinetic during composting. The concepts

of enzyme kinetics were empirically and theoretically developed on composting

[33][34]. Further, enzyme kinetic results will specifically utilize exploration and

numerical quantification of the behavior of the biodegradation process during

composting. In enzyme mechanisms and kinetics, under single-substrate mechanisms,

Brown concluded the first biological and mathematical fundamental concept of

enzyme kinetics, which was presented as follows as a stoichiometric scheme model

[32][35][36] in 1902:

E+ S  ±!!  ES  

!!  E+ P (1)

  12  

Where E is free enzymes, S is substrate, ES is complex of activated enzyme

attached on substrate surface or substrate binds with enzyme, + k1 is the reaction rate

constant of enzyme adsorb to the S, - k1 is reaction rate constant of ES desorbing E, P

is by/end-products irreversibly produced from ES, and k2 is the reaction rate constant

of enzyme producing P.

Later, as summarized by Briggs and Haldane [37], in 1913, Michaelis and

Menten concluded the concept to a theoretical model. The concept equations have

been proved successfully in empirical cases of enzyme actions in 1924 by Kuhn.

Therefore, by the law of mass action with two crucial assumptions that are 1) under a

quasi-steady state, concentration of ES changes much slower than those of P and S,

and 2) total enzyme does not change over time, the Michaelis-Menten equations

expressed by following:

d[ES]/dt = k1 [E][S] – (-k1+ k2)[ES] (2)

In the assumption of the quasi-equilibrium/steady state (see Fig. 1.1 [36]), which

is d[ES]/dt = 0, after an initial burst phase on ES:

k1[E][S] = (-k1+k2)[ES] (3)

  13  

Figure 1.1. Biological enzyme, substrate, enzyme- substrate complex, and products concentration change with time

As it is understood that the concentration of free enzyme [E] is equal to the

concentration of total enzyme [ET] minus [ES], the equation (3) turns to:

k1([ET] - [ES])[S] = (-k1+k2)[ES] (4)

To solve equation (4) to get [ES]

[ES] = [ET] [S] /{[S] + (-k1+k2)/ k1} (5)

where (-k1+k2)/ k1 = Km is the Michaelis constant, then it turns to :

ES =   [!!][!]! !!!

ES = !! !! !!!

(6)

The production rate, d[P]/dt, is equivalent with consumption rate of [ES] and

expressed as v, then :

  14  

v = d[P]/dt = k2[ES] (7)

Since a theoretical maximum reaction velocity, Vmax, can be observed when [ES]

equal [ET]. When all concentration of enzymes are saturated with substrate, this can

be expressed as:

Vmax = k2 [ET] (8)

Further, substituting (5) into (6) with concept of (7):

v =   !!"#[!]! !  !!

(9)

The equation (9) is called the Michaelis-Menten equation and is used to predict

the velocity of enzyme reaction as a function of substrate concentration. Also, the

equation was been empirically proved by Monod [38]. In this equation, the Vmax and

Km are the kinetic parameters to the enzyme reaction. Figure 1.2 is the plot of the

Michaelis-Menten equation.

Figure 1.2. Plot of Michaelis-Menten equation with determination of Michaelis constant, Km

  15  

In order to extrapolate and determine the kinetic parameters, the

Michaelis-Menten equation is reciprocated as follows:

!!=   !!

!!"#∙   ![!]+   !

!!"# (10)

Further the Lineweaver-Burk plot (Fig. 1.3) is used for determining the slope,

Km/Vmax, and intercept, 1/Vmax, of a plot of equation which incorporates 1/v and 1/[S].

The lower the value of Km, the higher the affinity of enzyme that occurs for the

substrate catalyst (k1) and/or less efficient of product production rates (k2). The higher

the value of Vmax, the faster the rate of reaction occurs during substrate catalyst

[33][34][39].

Figure 1.3. Lineweaver-Burk Plot of reciprocal Michaelis- Menten equation for determining Km/Vmax, and 1/Vmax

  16  

1.2 Research and Development (biocompositses)

As WPC has been developed for decades, researchers have found that the key

enhancement of mechanical and physical (moisture) properties is to modify the

interface between wood and plastic. Thus, the compatibilizer effects of WPCs have

then been widely studied. The conclusion of most of research studies has been that

maleic anhydride (MA) grafted on polyethylene and polypropylene (MA-PE / MA-PP)

are the common potential compatibilizers for WPCs [23][24][40][41]. In these studies,

approximately 20 to 30 percent improvement was observed on mechanical properties.

Water absorption also received considerable attention, resulting in about

50-percentage reduction [24]. With the performance requirements of wood

biopolymer composites, such as PHB/wood flour (WF) composites, adding

compatibilizers is now seen as one promising solution to solve the disparity of

hydrophobic-hydrophilic (PHB/wood flour) [22]. Better filler dispersion and/or

chemical interaction between filler and matrix are expected to occur. With improved

interfacial adhesion between wood and PHB, applied loads could be properly

transferred between filler and matrix without stress concentration, which is believed

to cause failure. Moreover, with the improved interfacial bonding (less gap), the rate

of moisture diffusion will decrease. Anderson [22] concluded that MA grafted on

PHB (PHB-g-MA) is a suitable compatibilizer for enhancing PHB/WF composites. In

  17  

his thesis, about 10 to 30 percent difference in strength was observed with different

processes because of side group of PHB-g-MA, succinic moieties, can possibly

provide hydrogen and/or chemical bonding with the hydroxyl group (-OH) of wood

fiber [23][40][41][42]. Moreover, the isocyanate group (-NCO) of isocyanate

adhesives is expected to bond chemically on the hydroxyl-rich functional group of

wood and partially with a polymer matrix [24][43][44]. A polymeric diphenylmethane

diisocyanate (pMDI) can be used as a compatibilzer for the WPCs. Anderson [22]

indicated that approximately 20~50 percent variation of mechanical properties and

water uptakes were found in the pMDI added PHB/WF composites. Also, the pMDI

had a more significant improvement than the other compatibilizers. This could be due

to the fact that the functional groups of isocyanates highly reacts between wood and

PHB. In conclusion, the PHB-g-MA and pMDI can provide an interfacial modifying

option and create physical and/or chemical bonding between wood and PHB.

Unfortunately, there is a lack of focus on compatibilizer effects of the

compostability of biocomposites. Adding compatibilizers provides an enhancement of

interfacial bonding between matrix and fillers, but the compostability of

biocomposites could also be influenced. It can be concluded that the possible reasons

are: 1) Reduction rate of moisture diffusion was observed and it retards bioreaction

from enzymes with substrates. This is because the enzymes are generated from living

  18  

microorganisms, which require water and oxygen to subsist; and 2) Less surface area

and internal microcracks were observed in the geometrically highly-packed (improved

interfacial bonding) of the biocomposite. For these reasons, internal defects such as

gaps and microcracks can provide capillary motion and trapping moisture, and will

not be seen to benefit enzyme molecular diffusion on the compatibilizer added

PHB/WF composites.

1.3 Objectives

As many of the above motivations described, this study will extend and contribute

to the fundamental and relevant knowledge of mechanisms of composting on

PHB/WF composites by crossing science from biological and agriculture science to

engineering bio-composites. For instance, initially (lag phase), escalation of

microcracking in the composites should be observed due to hygrothermal

conditioning effects. The concept of continuous damage mechanics (CDM) will be

applied to assist numerical evaluation of the growth of microcracking. The details of

the CDM will be further discussed in the Chapter 3. In the biodegradation phase,

enzymes highly active and mineralize the substrates (biocomposites). Recognizing the

theoretical enzyme reaction and kinetics can be conducted by applying the

Michealis-Menton concept with the results of substrate concentration changes and

  19  

mineralization rate. In addition, the scenario of microcracks explains the behavior of

non-linear moisture (non-Fickian) diffusion. Additionally, with sustainability

requirements, eliminating one energy consumption step of PHB purification can be

seen as one solution to achieve this goal, by blending wood flour and PHB-laden cell

debris to produce WPC. This process is essential to assist in the reduction of energy

consumption from production of PHB while maintaining its performance. Therefore,

this innovative study will be expected to provide knowledge and understanding of a

sustainable alternative PHB/WF composite.

The main objectives to be accomplished in this research study are :

a. To investigate the compostability and mineralization behavior of PHB/WF composites under a lab-scale composting evaluation

b. To understand hygrothermal effects of compatibilizers added PHB/WF composites by using continuous damage mechanisms to determine microcracking accumulation

c. To investigate the compatibilizers’ effects on compostability and enzymatic degradation mechanism of PHB/WF composites

d. To investigate processability and performance of extrusion grade of PHB/WF/Cell debris composites

e. To investigate the performance of injection molding grade PHB/WF/Cell debris composites

  20  

The expected outcomes and contributions of this innovative research are 1)

facilitating fundamental knowledge based of the hygrothermal and compatibilizer

effects on compostability of biocomposites; 2) providing estimation of carbon

footprint and life-cycle assessment of a compostable biocomposites through the

results of mineralization; and 3) developing using unpurified PHB for extrusion and

injection molding graded of PHB/WF composites.

In future studies, the assistances from this study will serve as an example of

evaluating and certifying compostable biocomposites and understanding

compatibilizer effects on biocomposites. Moreover, the innovated method of crossing

the science of enzymatic degradation kinetics with diffusion assumption can help to

recognize mineralization behavior and biodegradation mechanism of compostable

biocomposites. Finally, it can further provide information to support estimation of

carbon footprint (emission) and product life-cycle assessment by noteworthy

exploration of mineralization during composting.

  21  

1.4 References

[1] C. Clemons. 2002. Wood-Plastic Composites in the United States The Interfacing of Two Industries. Forest Products Journal. 52(6): 10 - 18

[2] P. M. Smith and M. P. Wolcott. 2006. Opportunities for Wood/Natural Fiber-Plastic Composites in Residential and Industrial Applications. Forest Products Journal. 56(3):4-11

[3] 「Wood-Plastic Composite Lumber vs. Wood Decking | Dovetail Partners INC.」 [Online]. Available: http://www.dovetailinc.org/reportsview/2010/responsible-materials/pdr-jim-bowyerp/wood-plastic-composite-lumber-vs-wood-decking. [Accessed: 22-Feb-2011].

[4] 「Municipal Solid Waste in the United States: Facts and Figures | Municipal Solid Waste | Wastes | US EPA」. [Online]. Available: http://www.epa.gov/epawaste/nonhaz/municipal/msw99.htm. [Accessed: 06-July-2011].

[5] R. Chandra and R. Rustgi. 1998. Biodegradable polymers. Progress in Polymer Science. 23(7): 1273-1335

[6] M. Vert, J. Feijen, A. Albertsson, G. Scott, and E. Chiellini. 1992. Biodegradable polymers and plastics. Cambridge [England]: Royal Society of Chemistry

[7] M. Flieger, M. Kantorová, A. Prell, T. Řezanka, and J. Votruba. 2003. Biodegradable plastics from renewable sources. Folia Microbiologica. 48(1):27-44

[8] Y. Doi and A Steinbüchel. 2001. Biopolymers - Polyesters III Applications and Commercial Products. 4(10). Weinheim  ;Chichester: Wiley-VCH

[9] C. S. K. Reddy, R. Ghai, Rashmi, and V. C. Kalia. 2003. Polyhydroxyalkanoates: an overview. Bioresource Technology. 87(2): 137-146

[10] P. A. Holmes. 1985. Applications of PHB - a microbially produced biodegradable thermoplastic. Physics in Technology. 16(1):32-36

[11] S. Y. Lee. 1996. Bacterial polyhydroxyalkanoates. Biotechnology and Bioengineering. 49(1): 1-14

[12] R. A. J. Verlinden, D. J. Hill, M. A. Kenward, C. D. Williams, and I. Radecka, 2007. Bacterial synthesis of biodegradable polyhydroxyalkanoates. Journal of Applied Microbiology. 102(6): 1437-1449

[13] J. Zhang, S. McCarthy , and R. Whitehouse. 2004. Reverse temperature injection molding of BiopolTM and effect on its properties. Journal of Applied Polymer Science. 94(2): 483-491

  22  

[14] U. Pagga, D. B. Beimborn, J. Boelens, and B. De Wilde. 1995. Determination of the aerobic biodegradability of polymeric material in a laboratory controlled composting test. Chemosphere. 31(11-12):4475-4487

[15] D. S. Rosa, R. P. Filho, Q. S. H. Chui, M. R. Calil, and C. G. F. Guedes. 2003. The biodegradation of poly-β-(hydroxybutyrate), poly-β-(hydroxybutyrate-co-β-valerate) and poly(ε-caprolactone) in compost derived from municipal solid waste. European Polymer Journal. 39(2):233-237

[16] N. Trautmann. 1998. Composting in the classroom  : scientific inquiry for high school students. Dubuque, Iowa: Kendall/Hunt Pub

[17] The BioCycle Guide to the Art & Science of Composting. Emmaus, Pa: JG Press, 1991.

[18] T. U. Gerngross. 1999. Can biotechnology move us toward a sustainable society?. Nat. Biotech. 17(6):541-544

[19] R. Narayan. 2004. Drivers And Rationale For Use Of BiobasedMaterials Based On Life Cycle Assessment (LCA). Global Plastics Environmental Conference, Atlanta, GA

[20] L. Jiang, J. Huang, J. Qian, F. Chen, J. Zhang, M. Wolcott, and Y. Zhu. 2008. Study of Poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV)/Bamboo Pulp Fiber Composites: Effects of Nucleation Agent and Compatibilizer. Journal of Polymers and the Environment. 16(2): 83-93

[21] A. Gregorova, R. Wimmer, M. Hrabalova, M. Koller, T. Ters, and N. Mundigler. 2009. Effect of surface modification of beech wood flour on mechanical and thermal properties of poly (3-hydroxybutyrate)/wood flour composites. Holzforschung. 63(5): 565-570

[22] Scott P. Anderson. 2007. Wood Fiber Reinforced Bacterial Biocomposites: Effects of Interfacial Modifiers and Processing on Mechanical and Physical Properties. Master thesis, Washington State University

[23] D. Harper and M. Wolcott. 2004. Interaction between coupling agent and lubricants in wood-polypropylene composites. Composites Part A: Applied Science and Manufacturing. 35(3): 385-394

[24] M. J. A. Chowdhury, and M. P. Wolcott. 2007. Compatibilizer selection to improve mechanical and moisture properties of extruded wood-HDPE composites. Forest Products Journal. 57(9): 46-53

[25] K. Oksman Niska, M. Sain, Institute of Materials, Minerals, and Mining. 2008. Wood-polymer composites. Cambridge [England]: CRC Press  ;Woodhead Pub.

[26] A. Klesov. 2007. Wood-plastic composites. Hoboken N.J.: Wiley-Interscience

  23  

[27] J. Zhou and J. P. Lucas. 1995. The effects of a water environment on anomalous absorption behavior in graphite/epoxy composites. Composites Science and Technology. 53(1): 57-64

[28] R. M. Maier, I. L. Pepper, and C. P. Gerba. 2009. Environmental microbiology, Second Edition. Academic Press

[29] E. G. Wolff. 1980. Microcracking in Graphite-Epoxy Composites. The Airspace Corporation. Report: SD-TR-80435.

[30] Y. J. Weitsman. 1991. Moisture in Composites: Sorption and Damage, in Composite Materials Series 4 - Fatigue of composite materials, K. L. Reifsnider, Eddit. Amsterdam  ;New York: Elsevier. pp.385-429.

[31] L.-W. Cai and Y. Weitsman. 1994. Non-Fickian Moisture Diffusion in Polymeric Composites. Journal of Composite Materials. 28(2):130 -154

[32] R. T. Haug. 1993. The practical handbook of compost engineering. CRC Press [33] P. Agamuthu, L. C. Choong, S. Hasan, and V. V. Praven. 2000. Kinetic

Evaluation of Composting of Agricultural Wastes. Environmental Technology. 21(2): 185-192

[34] D. S. Whang and G. F. Meenaghan. 1980. Kinetic Model of Composting process. Compost Science and Utilization. 21(3):44 - 46

[35] A. J. Brown. 1902. Enzyme Action. Journal of the Chemical Society . 81:373-388 [36] H. Bisswanger. 2002. Enzyme kinetics  : principles and methods. Weinheim

(Federal Republic of Germany): WILEY-VCH [37] G. E. Briggs and J. B. S. Haldane. 1925. A Note on the Kinetics of Enzyme

Action. Biochemical Journal. 19(2):338-339 [38] J. Monod. 1949. The Growth of Bacterial Cultures. Annual Review of

Microbiology. 3(1): 371-394 [39] H. D. Belitz, W. Grosch, and P. Schieberle. 2009. Food Chemistry. Fourth

Edditon. Springer [40] H. Dalväg, C. Klason, and H.-E. Strömvall. 1985. The Efficiency of Cellulosic

Fillers in Common Thermoplastics. Part II. Filling with Processing Aids and Coupling Agents. International Journal of Polymeric Materials. 11(1): 9-38

[41] R. Gauthier, C. Joly, A. C. Coupas, H. Gauthier, and M. Escoubes1998. Interfaces in polyolefin/cellulosic fiber composites: Chemical coupling, morphology, correlation with adhesion and aging in moisture. Polymer Composites. 19(3):287-300

[42] A. K. Bledzki, S. Reihmane, and J. Gassan. 1996. Properties and modification methods for vegetable fibers for natural fiber composites. Journal of Applied Polymer Science. 59(8):1329-1336

  24  

[43] E. Petinakis, L. Yu, G. Edward, K. Dean, H. Liu, and A. Scully. 2009. Effect of Matrix–Particle Interfacial Adhesion on the Mechanical Properties of Poly(lactic acid)/Wood-Flour Micro-Composites. Journal of Polymers and the Environment. 17(2):83-94

[44] A. Karmarkar, S. S. Chauhan, J. M. Modak, and M. Chanda. 2007. Mechanical properties of wood-fiber reinforced polypropylene composites: Effect of a novel compatibilizer with isocyanate functional group. Composites Part A: Applied Science and Manufacturing. 38(2):227-233

  25  

Chapter 2 Compostability and Mineralization Behaviors of a

Poly-3-hydroxybutyrate/Wood Flour Composite

Abstract

243 million tons of municipal solid waste was generated with a limited recovery

rate in 2009. One solution for this issue is to compost compostable materials, which is

the conversion of organic carbon to carbon dioxide (CO2) by metabolism of

organisms. Assessing the compostability and biodegradability of any compostable

materials can be achieved in accordance with ASTM standard D6400 and D5338.

Poly-3-hydroxybutyrate (PHB) is a biodegradable plastic produced by fermenting

natural carbon sources. With physical and mechanical properties similar to

polypropylene, PHB is being targeted to replace petroleum plastics in many

applications. Our hypothesis is that natural fibers such as wood flour (WF) can be

used as a reinforcement to expand the performance envelope while maintaining the

biodegradable properties of the polymer. A composting test was established in

accordance with ASTM D 5338 to evaluate the compostability of PHB and the

PHB/WF composite. The result showed that the PHB/WF composite reached over

60% mineralization after 117 days, which means that it can be classified as a

compostable material in accordance with ASTM D6400. The PHB/WF composite

  26  

resulted in a lower rate of CO2 and mineralization (34.2mg/g/hr0.5 and 17.5%/hr0.5)

than both the neat PHB and the positive control (cellulose paper). It is believed that

this result is because the complex structure and chemicals of wood influenced the rate

of mineralization in the composite. The composite formulation may be used to tailor

degradation rates. Notably, after being composted, the crystallinity of PHB resulted in

a 17% improvement. Additionally, the concept of diffusion is that small molecules,

such as moisture and enzymes, diffuse into a substrate over time. For this reason,

enzyme diffusion behavior can be recognized and numerically presented, while CO2 is

simultaneously generated as the end-product of bioreaction. After application of the

Fickian diffusion equation on the biodegradation phases, the mineralization Fickian

diffusion coefficient of PHB was about twice higher than that of the PHB/WF

composite (7.72 vs. 3.69 x 10-5 mm2/hr0.5), which could imply that the enzymes

showed a slower diffusion in the PHB/WF composite than in PHB. Additionally, after

applying the Michaelis-Menten equation to determine enzyme kinetics, the results

indicated that enzymes had a higher bioreaction rate with PHB than with the PHB/WF

composite.

Key words: Poly-3-hydroxybutyrate, Compostability, Mineralization, Degradation,

Fickian Diffusion

  27  

2.1 Introduction

Durability, a benefit of most plastics, is now viewed as problematic with some

products because it may require more than 100 years to degrade after the product is

disposed of. According to the US Environmental Protection Agency (EPA), 243

million tons of municipal solid waste was generated in 2009 and only ca. 2 million

tons out of 30 million tons of disposed plastic were recovered in the US during 2009

[1]. These plastic wastes contained ca.19 million tons of non-durable plastic goods

including disposable products, containers, and packaging plastics, and only ca. 9% of

those materials were recycled. Interpretation of these statistics indicates that about

17.3 million tons of plastic wastes were land-filled in 2009[1]. This statistical result

indicates that recycling is still a challenge in the US. Another solution in waste

management is to utilize biodegradable biopolymers that are now emerging as a

biodegradable option to reduce petroleum dependent and natural organic fillers, such

as natural fibers and starch; such reinforcement may provide additional benefits to

expand the performance envelope while maintaining biodegradability. Typically,

organic materials (wood, paper, etc.) and food wastes are used as carbon sources for

the composting process. With respect to the carbon cycling ecosystem, this process

converts organic carbon to carbon dioxide (CO2), while the remaining organic

material can be used as a soil amendments that provides nutrient retention and reduces

  28  

environmental impact by using chemical nutrients and also reduces the cost of

landscaping. Several studies [2][3][4] have indicated that organic matter is carried out

from the compost mixture and then used as an energy source for microorganisms

during composting. Moreover, because biopolymers are mainly comprised of carbon,

hydrogen and oxygen, they may also be converted to soil amendments in this process.

To evaluate the compostability or biodegradability of an organic material under

aerobic conditions, a test was developed to understand biodegradability and

specification of compostable polymeric materials in accordance with ASTM D 5338

and D 6400. This test can estimate and monitor the rate of CO2 evolution and

mineralization on the compostable materials for further analysis.

Currently, biopolymers are commercially produced by fermentation of sugars and

other natural carbon sources. Two such bio-polymers, polylactic acid (PLA) and

poly-3-hydroxybutyrate (PHB), one of most common used biopolymer in a family of

Polyhydroxyalkanoates (PHAs), have been studied for their biodegradability.

Commercial PLA bottles [5] have been evaluated for compostability and

biodegradation using both full and laboratory scaled tests in accordance with ASTM

D 5338. Moreover, PHAs and PHAs/natural filler composites have been studied, and

it has been concluded that these materials can be biodegraded under different

environments, such as compost [6][7][8][9], seawater [10] , enzymatic condition

  29  

[11][12][13][14][15][16], and sewage sludge [17]. However, biodegradability varies

based on the type of PHAs. After a certain period of biodegradation, PHAs test

specimens become undetectable, causing difficulty in collecting tested samples for

further evaluation, such as morphology and thermal properties. Also, after a

significant weight loss of the PHAs natural fillers-reinforced composites, the

mechanical properties become undetectable because the samples disintegrate during

recovery or dimensional reduction made it unable to carry the load [6]. In this case,

the dimensional reduction behavior of specimens can be used as a mechanical safety

factor.

After introducing a biodegradable material into a composting environment, the

moisture first penetrates into the material, and then the microorganisms react.

Furthermore, during the composting, moisture plays a limiting factor in the operation

and degradation of organic matter in microorganisms. The rate of moisture

penetration (moisture diffusion behaviors), which has been studied on natural

filler-reinforced composites [18][19][20], takes on the role of operating

biodegradation. If mineralization vs. time can fit the Fickian diffusion equation, it can

indicate the limiting steps and also explain the mineralization diffusion behavior of

materials. Therefore, by understanding the rates of CO2 evolution and mineralization

diffusion behavior of the products, the carbon cycle period and amounts need to be

  30  

estimated. In addition, in biological study most bioreactions have three different

phases: lag, bioreaction (biodegradation), and decreased/plateau phase [8][21]. During

the lag phase, the enzyme starts reacting with the substrate followed by a rapid

reaction which is seen as the bioreaction phase. To fundamentally understand the

enzymatic reactions during composting, many studies [22][23][24] have indicated that

by using enzyme kinetic equations, i.e. the Michaelis-Menten equation, the reaction

can be numerically compared.

The goal of this study was to demonstrate that PHB/Wood Flour (WF) composite

can be classified as a compostable material by testing under laboratory scale

conditions in accordance with ASTM D5338 and that output data can be further

applied based on Fickian diffusion equation to understand mineralization behavior.

The specific objectives were to: 1) identify the compostability of PHB/WF composite

by using a lab-scale apparatus; 2) evaluate the mineralization, properties, and

morphology change of PHB/WF composite during composting; 3) apply the Fickian

diffusion equation and Michealis-Menten equation to understand mineralization

diffusion behavior and enzyme reaction kinetics.

 

 

 

  31  

2.2. Materials and methods

2.2.1 Materials

Purified PHB powder was provided by Ningbo Tianan Biologic Material Co.,

Ltd., Ningbo, China. 60-mesh wood flour (Pinus) was provided by American Wood

Fibers, Schofield, WI. Chromatograph paper (CP; Fisherbrand® CHR200) made from

cotton cellulose with a 3.3mm thickness was provided from Fisher Scientific, Inc.

The compost inoculums were about four-month-matured and provided by the

Washington State University compost facility. The compost was sieved through a

sieve screen with openings of 10 mm diameter. Proportions of the compost feedstock

[25] are shown in Table 2.1. Moisture content of the compost was adjusted to ca.

50%.

Table 2.1 The proportion of compost feedstocks

Manure Ash Compost Food Waste Yard Waste Dirt Wood Chips*

78% 10% 10% 1% 0.5% 0.5% 5%*

* added during the wet season to provide porosity

 

 

 

  32  

2.2.2 Testing sample preparation

In accordance with ASTM D 5338, analytical-grade cellulose was used as the

positive control for comparison with other plastic materials. In order to reduce error of

surface area, the control and different testing materials were made into a flake shape

with particle diameter of ca. 10mm and thickness of ca. 0.5mm).

The wood flour was dried in a rotary steam tube drier and its moisture content was

ca. 3.0%. PHB and dried wood flour were placed into a plastic bag with a ratio of

50/50 dried wt% and manually mixed by vigorous shaking and tumbling for ca. 5

minutes. The PHB/WF mixture was then compounded by a co-rotating twin screw

extruder (Leistriz ZSE-18) with a screw diameter of 18 mm and length/diameter (L/D)

ratio of 40 equipped with a volumetric feeder. To improve melt strength and reduce

thermal degradation, a declining temperature profile of the extruder was applied

during the extrusion process. The extrusion temperature profile was set at

170/175/170/165/164 /163/162/160oC, from the feeding throat to the die adapter. The

screw speed was maintained at 125 rpm. Under this speed, the residence time of the

materials in the barrel was estimated to be about 1.5 minutes. The extrudate was air

cooled and pelletized.

To maintain a highly polymer penetration into the wood lumens, the composite

pellets were injection molded by an injection molding machine (Sumitomo SE 50D).

  33  

Injection temperature profile was controlled at 175/180/175/170oC from the feeding

zone to the nozzle. Mold temperature was 60oC. The filling pressure was set at 1700

kgf/cm2 and the packing pressures were 1250 kgf/cm2 and 1360 kgf/cm2 for the first

and second stage, respectively. The injection molding cycling time was about 75

seconds. The molded specimens were then hot-pressed into 0.5 mm thickness

composite film with a multi-step press method until reaching 15MPa for 15 minutes at

170oC. The composite film was followed by air cooling and chopped to a flake shape

(a particle diameter of ca. 12 mm). Tensile properties and density of the injection

molded specimen were evaluated in accordance with ASTM D 638. The composite

had a density of 1.31, tensile strength of 23.3 ± 0.94 MPa, tensile modulus of 9.7 ±

0.28 GPa, and 0.4% strain at break.

PHB flake was prepared by hot-pressing PHB powder at 170oC and chopped to a

flake shape (a particle diameter of ca. 15 mm and thickness of ca. 0.7 mm).

Chromatograph paper was also chopped into flake shape with a particle diameter of ca.

12 mm.

  34  

2.2.3 Composting apparatus and procedure

In this study, the procedures outlined in ASTM D 5338 were used to assess

biodegradation of plastics in a laboratory scale-compost experiment, and the

apparatus set-up was also built in accordance with ASTM D 5338 (see Fig. 2.1). The

system consisted of determining the degree and rate of mineralization of plastic

materials from organic carbon to evolved carbon dioxide. A series of bioreactors

(each in triplicate) containing blank (compost only), cellulose flake (positive control +

compost), PHB flake (polymer reference + compost), and PHB/WF composite flake

(sample + compost) were tested via this system. The ratio of inoculums mixture was

6:1 (compost:flake samples) in dry mass. The biodegradation test was carried out

under a laboratory-controlled condition: CO2-free and humidified inlet air had a

constant flow of ca. 90 ml/min., outlet air after reactors was trapped by 12 CO2

trappers, and 12 bioreactors were placed in a 58 ± 2oC a water bath throughout the

entire experimenting period.

The CO2-free and humidified inlet air was generated by compressing air through a

sodium hydroxide (NaOH) solution to remove CO2 and then passed through deionized

(DI) water to humidify the inlet air. To maintain a consistent condition, the

moisturized and CO2-free air was divided and adjusted by flow meters, and further

input to the bottom of each bioreactor. The bioreactor was built using a glass jar of

  35  

internal volume 2 L with a screen placed at the bottom of jar to avoid channeling and

provide equally distributed inlet air in the compost. Each mixture of inoculums was

mixed and placed into jars with about 20 mm space from the lid for outlet air

distributed to CO2 trappers. All mixtures of inoculums were stirred weekly to avoid

channeling and to provide proper aeration and contact between sample and compost.

A set of condensed water collectors was placed before each CO2 trapper. The 200 ml

CO2 trappers contained 2.5 M NaOH solution to collect the amount of CO2 produced

by each bioreactor. The following CO2 trapping reaction was discussed as in ASTM D

5338:

NaOH + CO2 → NaHCO3 --------------------------------------------------- (1)

then

NaHCO3 + NaOH → Na2CO3 + H2O ----------------------------------------- (2)

5 ml aliquots were removed from each trapper and then titrated with 0.5M

hydrochloric acid (HCl) solution 3 times a week, and the CO2 traps were changed

weekly. During the titration, CO2 was chemically reacted and removed as follows:

NaCO3 + HCl → NaHCO3 + NaCl ------------------------------------ (3)

then

NaHCO3 + HCl → NaCl + H2O + CO2 ------------------------------------ (4)

  36  

Initially, 5 drops of phenolphthalein indicator (pink color) were added to the 5 ml

aliquots and further titrated until the end-point, as indicated when the color turned to

colorless (as in titration formula 3). In addition, 5 drops of methyl orange indicator

were then added, and then titrated until the color turned from orange to red (as in

titration formula 4). The amounts of HCl consumed during the second titration

(formula 4) were used to calculate and obtain the evolving CO2 in accordance with the

following equation [4]:

CO2 (g) = VHCl (ml) x CHCl (molar) x 44 /1000 ---------------------------- (5)

where VHCl is the volume of HCl consumed in the second titration; CHCl is the

concentration of HCl solution; and 44 is the molar mass of CO2.

Further, the percentage of mineralization can be calculated in accordance with

ASTM D 5338:

Mineralization (%) = [(CO2)sample –(CO2)blank]/ ThCO2 (eq. 2.1)

where (CO2)sample is the cumulative amount of CO2 obtained from the titration

result of each sample mixture inoculums with the time; (CO2)blank is the cumulative

amount of CO2 obtained from the titration result of each blank inoculums (compost

only) with the time; and ThCO2 is the theoretical quantity of each sample CO2

evolution, which can be calculated based on the following equation:

  37  

ThCO2 = Msample x TOCsample x 44 /12 (eq. 2.2)

where Msample is the total dried mass of the sample material which was introduced

to the compost inoculums; TOCsample is the proportion of total organic carbon (TOC)

in the dried sample material; 44 is molar mass of CO2; and 12 is the atomic mass of

carbon.

Because all of the sample materials used in this study were assumed to be

compostable material, the following test was based on an evaluation method on TOC

of compost and/or municipal solid waste (MSW). In compost materials, TOC can be

estimated via a formula of reduction from total organic matter (TOM) on a prediction

equation of MSWs (waste paper based) and city refuse compost experimental results

[26]:

TOC = 1.703 + 0.520 * TOM (eq. 2.3)

According to APHA 2540 D and E, TOM content determines ignition loss of oven

dried samples by using a muffle furnace at 550oC for 30 minutes, and then being

weighed after cooling in a desiccator. The chemical characteristics of each sample

material are shown in Table 2.2.

After the composting test ended, which was the day the PHB/WF composite

reached 60% of mineralization in this study, the mixture inoculums were taken out of

  38  

the bioreactors and moisture content and pH value of the inoculums were tested. A pH

meter was used to measure the pH values. Each sample was diluted in a 5:1 w/w ratio

of DI water to each dried mixture inoculums. The pH value of each sample was

recorded after each sample was placed in a 25± 1oC water bath for 30 minutes.

Table 2.2 The chemical characteristics of the materials  

Compost CP PHB PHB/WF

Composite WF

TOM (%) 81.1 99.4 99.7 99.4 99.2

Ash (%) 18.9 0.6 0.3 0.6 0.8

TOC (%) 43.9 53.4 53.5 53.4 53.3

ThCO2 402.2 81.2 81.5 81.2 81.1

Figure 2.1 Schematic of the lab-scale material compostability evaluation set-up system

 

  39  

2.2.4 Thermal analysis

To evaluate thermal degradation, thermal stability and weight loss, the sample

materials were first oven dried at 103 ± 2oC until weights were stabilized. About 5 to

10 mg of each dried sample was placed in an aluminum crucible and then placed in

thermo-gravimetric analysis (TA-TGA) with a heating rate of 20oC/min from room

temperature (RT) to 550 oC and air flow rate of 100 ml/min. of inlet inert gas, nitrogen

(N2), which prevents specimen oxidation during the heating process.

Thermal properties were determined and evaluated at day 1, 90, and 117

(incubation time) by using a TA Instrument DSC 2920 calorimeter. Testing samples

were removed from each bioreactor and as much compost or remaining material as

possible was washed out on the surface of the samples with DI water. After being

oven dried (ca. 80oC) and placed in a desiccator until cooled to RT, about 5-10 mg of

flake samples were placed into an aluminum crucible, and further, the sample pan was

put in the differential scanning calorimeter (DSC) at a heating rate of 10oC/min from

30 to 200oC in a N2 atmosphere with a air flow rate of 60 ml/min. The first scan was

used to evaluate the melting temperature (Tm) which was present in the endothermic

transition after material biodegraded, and then integrated to obtain the entropy of the

samples to compare the biodegradation effects on the crystallinity of the sample

materials. Crystallinity was calculated by the following equation:

  40  

Crystallinity (%) = (Δ Hmelt x 100) / (Δ Href x msample) (eq. 2.3)

Where ΔHmelt is the measured energy on melting sample; Δ Href is the reference

energy required to melt the sample (PHB :146J/g [27]); and msample is the sample mass

placed in the aluminum crucible.

2.2.5 Fickian Diffusion Coefficients

During the composting test, the moisture was first penetrated into sample

materials followed by biodegradation reaction. Moisture penetration behavior can be

discussed and studied by using the Fickian diffusion equation. Considering geometry

effects, a geometric edge correction factor was also calculated to obtain the true

diffusion constant (D). Furthermore, moisture diffusion behavior is an important role

of operating biodegradation. If the evolved CO2 vs. time can fit the Fickian diffusion

equation with a geometric edge correction, it will indicate the limiting steps and also

explain the mineralization diffusion behavior of the materials. The Fickian diffusion

equation [18] [19] is shown below:

!! =  π !!!!"#

! !!!! !

!     (eq. 2.4)       and         ! =   !!

!!! !!! !

!     (eq. 2.5)  

where DA is the apparent diffusion constant; Msat is the maximum mineralization

percentage at the end of the test period; dMt/d√t is the slope of mineralization over

  41  

square root of time relation; and h, L, and W are the height, length, and width of

sample.

2.2.6 Enzymatic Kinetic

The enzyme kinetic is known as an accurate and reliable way to determine

enzymatic reaction. It can also provide numerical elementary mechanisms of the

reactions. The concept of the Michaelis-Menten kinetic [22][23][28] is widely used in

compost and agriculture science. Since mineralization is driven by bioreaction (faster

bioreaction occurs at a higher rate of mineralization), the results of mineralization

may be used to obtain the kinetic constants from the Michaelis-Menten equation

[22][23][28]. This will further facilitate understanding of enzyme bioreaction

behavior on PHB/WF composites, and possibly provide knowledge concerning the

composting behavior on the composite for future research studies. The equation can

be presented as:

v =   !!"#[!]! !  !!

(eq. 2.6) or !!=   !!

!!"#∙   ![!]+   !

!!"# (eq. 2.7)

where Km is Michealis constant, v is production rate (rate of mineralization, %/day),

Vmax is theoretical maximum reaction velocity, and [S] is the substrate (ratio of

converted carbon/ash, g/g).

  42  

Further, the Lineweaver-Burk plotting method can be used to determine the slope,

Km/Vmax, and intercept, 1/Vmax of a plot of equation which incorporates 1/v and 1/[S].

The lower the value of Km, the higher the affinity of enzyme occurs for the substrate

catalyst and/or less efficient of products production rate (CO2 in this case). The higher

the value of Vmax, the faster the rate of reaction occurs during substrate catalyst

[22][23][28].

2.2.7 Morphology

The surface morphology of sample materials (initial and 90 days) was observed by

using a scanning electron microscope (FEI SEM, FEI 200F). The tested samples,

removed from mixture inoculums at day 90, were washed with DI water to remove

the compost debris and then dried at 80oC. The control samples were also oven dried

to remove moisture, which could influence the quality of coating. All the prepared

samples were mounted on aluminum stubs and gold-coated by a sputter coater.

  43  

2.3. Results and discussion

2.3.1 Thermal degradation

Studies have shown that the degree of crystallinity on PHB can be increased by

reinforcement with natural fibers [29][30]. It is believed that when polymer chains are

highly packed (i.e., they have a high degree of crystallinity) in the same free volume,

the polymer chains require more input energy to mobilize. Thermal degradation

continues to occur with further heating. TGA results (Fig. 2.2) indicated that wood

flour enhanced the thermal stability of PHB/WF composite compared to neat PHB,

wood flour, and the PHB/WF composite. The weight loss of the PHB/WF composite

started decreasing at ca. 10oC higher than neat PHB. In addition, the peaks of thermal

degradation (Td) for the PHB and PHB/WF composite had 10oC difference (273oC vs.

283oC) from the derivative thermogravimetry (DTG) curves.

  44  

Figure 2.2 Weight loss and derivative thermogravimetry of PHB, WF, and PHB/WF composite

2.3.2 CO2 evolution and mineralization results

In composting chemistry, organic matter chemical breakdown is mainly reacted

by enzymes that are produced from microorganisms. In this system, complex organic

matters of mixture inoculums are depolymerized by the enzymes, and then the

microorganisms absorb the breakdowns as energy sources into their cells and are

further metabolized [3][4]. Enzymes, in the case of aerobic microorganisms, can

convert organic matter to CO2 and remaining water, energy, and residuals. This

  45  

conversion is known as a mineralization process. Therefore, the rate of enzyme

reaction and rate of mineralization have a positive relation.

In this study, as observed for the three different materials (CP, PHB, and

PHB/WF composite), a lag phase of biodegradation occurred during the first week of

compost experimentation. In this lag phase, because the testing materials had just

been introduced into the mixture inoculums, the microorganisms did not fully attach

with the test samples, and/or the low moisture content (MC) of the samples did not

benefit the bioreaction from the enzymes. This resulted in a lower rate of CO2 gas

evolution than that in the biodegradation phase (Table 2.3). After the MC of the

samples equilibrated, the high bioreaction occurred within the biodegradation phases,

which followed after the lag phases. However, under high humidity and temperature

conditions, hydrolytic cleavage on the polyester [13] [31], PHB (in this case), can

occur. The PHB polymer chain may have fractured and reduced to shorter polymer

chains, but no CO2 or weight loss occurred [32] [33]. Moreover, the polymer chains of

hydrolyzed cleavage possibly provided more contact surface which could have

advanced the bioreactions. The biodegradation phase was followed after the lag phase

with a high rate of CO2 evolution appeared until day 47. An alleviated biodegradation

phase occurred beginning on day 49. Due to the similar amount of total theoretical

CO2 evolution of the different materials, the mineralization of each material had the

  46  

same trend as the results in the CO2 evolutions. Table 2.4 presents the rates of CO2

evolution and mineralization of each material in different phases of biodegradation.

The curves of accumulative CO2 evolution and mineralization are shown in Fig. 2.3

and Fig. 2.4. After being normalized, the blank compost had a constantly CO2

evolving rate of 0.33 mg/g/hr0.5, indicating that the compost inoculums had a constant

bioreaction through the entire experimenting period (117 days).

Table 2.3 The rates of CO2 evolution and mineralization of each sample in different phases of biodegradation

Reaction rate (Slope) CP PHB PHB/WF composite

0-7d 9-47d 49-117d 0-7d 9-47d 49-117d 0-7d 9-47d 49-117d

CO2(mg/g)/hr0.5 14.9 47.7 16.3 20.0 56.7 27.9 16.0 34.2 15.2

Mineralization(%)/hr0.5 0.76 2.44 0.84 1.02 2.89 1.42 0.82 1.75 0.77

d : days ; mg/g : amount of CO2 evolution per unit weight of material

In the degradation phase, the PHB/WF composite had a lower rate of CO2

evolution than the PHB (34.2 mg/g/hr0.5 vs. 56.7 mg/g/hr0.5). This result could be due

to the complex chemical and physical structure of wood compared with the CP and

PHB. The main chemical components of wood are cellulose, hemi-cellulose, and

  47  

lignin. Each of these chemicals require certain conditions (enzymes, humidity, and

temperature) and time (in the scale of years) to achieve depolymerization or decay

[34][35]. For example, lignin, which ties cellulose microfibers together and forms the

structure of wood and functions as rigid polymers, has complex chemical structures

and anti-enzyme property [4]. Moreover, strong mechanical interlocking occurs when

polymer penetrates into porous wood by high pressure from the injection molding

process [36]. The PHB-rich skin of the composite was caused by the heating

processes. Morphologically, during biodegradation, the PHB located at the surface

was first biodegraded, and then wood flour was exposed to the enzymatic

environment. Another possibility is that in the high humidity condition, after the wood

absorbed moisture, the swelling of the wood flour created gaps between the wood

flour and polymer, which created a gap for moisture and/or for enzyme penetration.

Furthermore, enzymes can depolymerize PHB which is located in wood lumens. A

retarded rate of CO2 evolution and mineralization (Figs. 2.3 and 2.4) was observed

after day 47. The reasons for this retardation may be because the enzymes have

difficultly reacting with PHB that is located in the inmost depth of wood lumens while

simultaneously degrading some wood chemicals. Moreover, after the test ended, some

of the mixture inoculums had a loss of moisture less than 50% (Table 2.4). Since

  48  

moisture plays a role in biodegradation, the fact that mineralization rates of each

sample material decreased may also be influenced by the lower moisture.

Figure 2.3. The accumulative CO2 evolving production of CP, PHB, PHB/WF composite, and control compost material

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 10 20 30 40 50 60

CO

2 (g/

g)

Time (hr0.5)

CP PHB PHB/WF composite Compost

Lag Phase

Biodegradation Phase

  49  

Figure 2.4. The accumulative mineralization of CP, PHB, and PHB/WF composite

Table 2.4 The average moisture content and pH value of control, CP, PHB, and PHB/WF composite

Control CP PHB PHB/WF composite

MC(%) 28.8 (16.0)* 28.8 (9.5) 52.9 (22.0) 41.4 (34.6)

pH value (at 25oC) 8.3 (0.1) 8.7 (0.7) 8.5 (0.1) 8.6 (0.4)

*: standard deviation

As in the discussion of the thermal degradation results, an increased degree of

crystallinity in PHB was indicated by reinforcement with natural fibers, which

function as nucleating points [30][37] for polymer crystallization. With a higher

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50 60

Min

eral

izat

ion

Time (hr0.5)

CP PHB PHB/WF Composite

  50  

degree of crystallinity on PHB/WF composite, a lower rate of depolymerization was

expected. However, in the literature [13][15] it has been noted that if the mixture

inoculums (natural compost) are not isolated on a specific enzymatic condition,

biodegradation could occur simultaneously in the amorphous and crystalline regions.

In accordance with ASTM D 5338 and D 6400, in a natural compost condition, a

polymeric material mineralized over 60% within 180 days can be classified as a

biodegradable material. In this study, ca. 60.2% of the PHB/WF composite was

mineralized after 117 days. Therefore, the PHB/WF composite can be classified as a

biodegradable material.

2.3.3 Thermal analysis

The degree of crystallinity changed on PHB samples by sampling on days 1, 90,

and 117 (shown in Table 2.5) during the composting test. The melting point shifted

down about 3oC from 168.3oC (initial), compared with the PHB collected on days 1

and 117. The enthalpy of melting (ΔHm) and degree of crystallinity increased with

increasing composting time (ca. 17% increase in both enthalpy of melting and degree

of crystallinity). This result could possibly be explained that the amorphous region of

PHB had first biodegraded, followed by the crystalline region. On the PHB/WF

composites, upon day 90 sample collection, most of sample flakes were broken apart

  51  

or difficult to distinguish from the natural compost. After a certain amount of PHB

degraded, the declined mechanical properties of the composite were expected because

wood flour had less adhesion for sticking tighter to form a composite. Furthermore,

the flake shape samples could have been broken into smaller particles during the

mechanical weekly stirring (one requirement in ASTM D 5338 to maintain aeration).

Therefore, the degree of crystallinity on the PHB/WF composite became challenge to

detect after a certain period of testing (90 days in this case).

Table 2.5 Thermal properties of PHB and PHB/WF in different sampling times

Day 1 Day 90 Day 117

PHB

Tm 168.3(0.7) 167.0(2.0) 165.4(0.9)

ΔHm (J/g) 78.7(1.4) 87.6(4.5) 92.1 (1.1)

Crystallinity (%) 53.9(0.9) 60.0(3.1) 63.1(0.8)

PHB/WF composite

Tm 168.0(0.5) - -

ΔHm (J/g) 88.5(0.6) - -

Crystallinity (%) 60.6(0.4) - -

  52  

2.3.4 Fickian diffusion coefficient

As moisture is a key of limitation for operating bioreactions by enzymes that are

produced from microorganisms, moisture penetration is a major influencing factor on

the rate of mineralization. The behavior of small molecules (liquid or gas) transported

throughout solid materials can be numerically presented by the Fickian diffusion.

However, some non-Fickian behavior could be found during the lag phase and also

may show in the alleviated biodegradation phase. The maximum percentages of

mineralization (Msat) were taken at the end of the experiment (day 117), and the

slopes of mineralization over square root of time were taken during the

biodegradation phase (days 9-47). The plot of percentage of mineralization of each

sample material in contrast with the square root of time is shown in Fig. 2.4. The

results of different rates of mineralization in each sample with different periods are

shown in Table 2.3. The PHB/WF composite had a lower Fickian coefficient of

3.69E-5 mm2/hr0.5 compared with PHB (7.72E-5 mm2/hr0.5). The reason for the

PHB/WF composite had a low Fickian coefficient could be discussed by the complex

chemicals of wood and the increased degree of crystallinity induced challenges to

enzyme to degrade the composite.

  53  

2.3.5 Enzymatic Kinetic

The enzymatic kinetic constant, Michaelis constant, Km, and theoretical maximum

reaction velocity, Vmax, were obtained after applying the Lineweaver-Burk plotting

method (Fig. 2.5) with the calculated results from mineralization. The results shown

in Table 2.6 indicate that the PHB had a highest value of Km and Vmax, which

indicates that PHB that had the lowest rate of enzyme binding with PHB, but the

highest enzyme reaction, and produced more CO2 than CP and the PHB/WF

composite. In other words, the enzyme reacted more efficiently on PHB than CP and

the PHB/WF composite. This result also corresponds with the results found in the

mineralization Fickian diffusion. Application of the Michaelis-Menten equation to

analyze the enzyme reaction kinetic during the composting process and can help to

understand the process of composting compostable biocomposites.

  54  

Figure 2.5 Lineweaver-Burk plot of rate of mineralization and substrate content

Table 2.6 Enzyme kinetic constants of different materials

Km Vmax (1/day)

Cellulose paper 562.2 0.291

PHB 776.7 0.494

PHB/WF composite 526.7 0.325

2.3.6 Morphology

The comparison of initial sample and the sample debris after being composted is

shown in Figure 2.6. White rot fungi could be also observed on the composted sample

of cellulose paper and PHB/WF composite. The surface erosion of the tested samples

were observed and studied by SEM. As shown in Fig. 2.7, the marked areas were

y = 163.52x + 3.4382

y = 383.37x + 2.0259

y = 171.28x + 3.0727

-4

0

4

8

12

16

20

24

28

32

-0.04 0 0.04 0.08 0.12 0.16 0.2

1/(m

iner

aliz

atio

n/da

y)

1/(carbon/ash)

CP PHB PHB/WF Composite

  55  

perceived as surface erosion by enzymes and/or microorganisms, in comparison with

the first test and testing after 90 days. On the edge side of the PHB/WF composite,

less evidence was found on the wood flour surface, which confirms our earlier

statement: Wood required more time or was too difficult to biodegrade. Additionally,

the PHB/WF composite samples swelled and the structure loosened under the

hygrothermal conditions and also showed enzyme erosion. The cross section of

PHB/WF composite samples were not able to be prepared by microtone, but ideally,

the longitudinal lumen and the transversal opening (pits) of the wood may possibly

provide a path for enzymes to penetrate into the lumen. However, there is not

sufficient evidence to prove this hypothesis by using SEM technology.

Figure 2.6 Comparison of initial samples (CP, PHB, and PHB/WF composite) and after 90days composted sample debris

  56  

Figure 2.7 SEM Morphology of different sample (CP, PHB, PHB/WF composite) surfaces on day 1 (a) and after 90 days (b)

2.4. Conclusion

A similar amount of theoretical CO2 of CP, PHB, and PHB/WF composite

resulted because the constituted chemical components of these materials are carbon,

oxygen, and hydrogen. The CO2 evolution results showed that the PHB/WF

composite reached over 60% mineralization after 117 days, while the CP and PHB

had reached ca. 70% and 90% mineralization. According to ASTM D 5338, PHB/WF

composite can be classified as a biodegradable and compostable material. During the

biodegradation phase (days 9-47), rates of CO2 evolution in PHB/WF composite and

  57  

PHB were 34.2 mg/g/hr0.5 and 56.7 mg/g/hr0.5. This indicates that more rapid

bioreaction occurred in PHB than in the PHB/WF composite. After mathematically

calculating the percentage of mineralization of each material, the PHB/WF composite

had a lower mineralization rate (ca. 1.75%/hr0.5) than neat PHB or the positive control,

CP (ca. 2.89 and 2.44%/hr0.5, respectively). The complex structure and chemical

structure of wood is believed to have caused the lower mineralization of the PHB/WF

composite than that of the neat PHB. The DSC results indicate that the crystallinity of

the PHB in the composite increased (ca. 10% higher than neat PHB) by adding wood

flour, and further slightly enhanced thermal stability. Under natural compost

conditions, i.e., a condition of unisolated enzymes, the crystalline and amorphous

region of PHB was expected to change by biodegradation, e.g. increased 17% of

crystallinity of PHB. During composting, one limiting factor for microorganisms

operating the biodegradation process is moisture, which becomes a key of operating

biodegradation. After mineralization vs. time fit in the Fickian diffusion equation, the

mineralization diffusion behaviors of the three materials were numerically quantified.

The results of mineralization of Fickian diffusion on PHB and PHB/WF composite

were 7.72 and 3.69 x 10-5 mm2/hr0.5, respectively. By applying the Michaelis-Menten

equation to determine the fundamental enzyme kinetic and understanding, the results

successfully indicated the same results in the mineralization Fickian diffusion: a

  58  

higher enzyme reaction occurred in PHB than for the PHB/WF composite. SEM

morphology showed evidence of erosion on the surface of both the PHB and PHB/WF

composite, but there was less evidence on the wood flour. This result indicates that

wood flour requires more time to fully degrade. To summarize, according to ASTM

D6400, the PHB/WF composites could still be classified as a compostable and

biodegradable material based on the results in this study. The concepts of the Fickian

diffusion and enzyme reaction kinetic, Michaelis-Menten equation were successfully

applied in order to understand fundamental enzyme reactions during composting.

2.5. Acknowledgements

The authors acknowledge to the U.S Department of Energy for supporting this

study by contract NO. DE-AC07-051D14517, and also thank the composting facility

at Washington State University which provided compost materials.

  59  

2.6. References

[1] 「Municipal Solid Waste in the United States: Facts and Figures | Municipal Solid Waste | Wastes | US EPA」. [Online]. Available: http://www.epa.gov/epawaste/nonhaz/municipal/msw99.htm. [Accessed: 06-July-2011].

[2] The BioCycle Guide to the Art & Science of Composting. 1991. Emmaus, Pa: JG Press.

[3] Z.-S. Chen. 2005. Compost production  : a manual for Asian farmers. Taipei Taiwan: Food and Fertilizer Technology Center for the Asian and Pacific Region.

[4] N. Trautmann. 1998. Composting in the classroom  : scientific inquiry for high school students. Dubuque Iowa: Kendall/Hunt Pub.

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[6] S. H. Imam, L. Chen, S. H. Gordon, R. L. Shogren, D. Weisleder, and R. V. Greene. 1998. Biodegradation of Injection Molded Starch-Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) Blends in a Natural Compost Environment. Journal of Polymers and the Environment. 6(2): 91-98

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[10] H. Tsuji and K. Suzuyoshi. 2002. Environmental degradation of biodegradable polyesters 1. Poly(ε-caprolactone), poly[(R)-3-hydroxybutyrate], and poly(L-lactide) films in controlled static seawater . Polymer Degradation and Stability, 75(2):347-355,

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  63  

Chapter 3 A Novel Investigation of Hygrothermal Effects on Properties

Degradation and Propagation of PHB/WF Composites by Using the Continuous

Damage Mechanism

 

Abstract

Wood plastic composites (WPCs) have been developed successfully for use in

outdoor applications due to advantages of low maintenance, durability, and low cost.

However, moisture is one factor that can deteriorate properties of many composite

materials; this situation is known as mechanical properties degradation, especially

under hygro-/hydrothermal conditions. For these reasons, the hygro-/hydrothermal

effects of WPCs need to be investigated. In this study, by using

poly-3-hydroxybutyrate (PHB) to produce PHB/wood flour (WF) composites with

coupling agents included to enhance interfacial bonding, water/moisture resistance

and tensile properties were improved at different levels of moisture content. After

analyzing the results, mechanical degradation of the PHB/WF composites was

observed with changing moisture content. Furthermore, moisture-swollen wood flour

created internal stress and caused damage, such as microcracks, on the PHB matrix.

This result is perceived as damages from creep, which is a viscoelastic behavior of

polymeric material. It is obvious that tensile properties deteriorated with increased

  64  

hygrothermal condition time. However, it is challenging to use a microscope to

observe and quantify the moisture-induced damage. Fortunately, the damage can be

numerically presented by utilizing the fundamental concept of continuous damage

mechanism (CDM), in order to understand the propagation of microcracking. The

results indicated that microcracking was rapidly increased in the initial 10 days, but

slowed after that time. Lastly, by using the results in this investigation, a theoretical

propagation of microcracking is presented in a schematic sketch.

Key Words: WPC, PHB, hygro-/hydrothermal, degradation, microcracks,

propagation, continuous damage mechanism

  65  

3.1 Introduction

Moisture is an environmental effect that changes the physical and mechanical

behavior of fiber reinforced composites. These effects are due to the fact that moisture

consists of small molecules, which can penetrate and diffuse into composite materials

over time based on the theory of diffusion, and further deteriorates properties of the

materials [1]. It is crucial to understand these moisture effects for future design and

selection of structural materials. In residential buildings, a “green” fiber reinforced

composite, i.e. wood plastic composite (WPC), has been introduced into the market in

the last decade due to its low maintenance, low cost, high performance, and high

durability [2][3]. However, the drawback of using wood flour as a reinforcement is

moisture absorption, which is a common characteristic of wood. Furthermore, since

the exterior application means that WPC is highly exposed in an outdoor environment,

behavior of moisture absorption becomes an enormous focus. Many previous

researchers [4][5][6][7][8] have indicated that even when wood flour is embedded in

hydrophobic plastics, moisture can still diffuse and be absorbed over time, and then

cause the properties deterioration. Additionally, such deterioration has been

previously investigated and it was discovered that after moisture was absorbed by

wood four, the dimension became unstable and then swelled. This moisture-induced

swelling of wood flour causes an internal stress to plastic as tensile and/or shear

  66  

stresses and to the wood itself in the form of compressive stress, and also breaks down

interfacial bonding between wood and plastic [5] [7] [9] [10]. Such stress on the

matrix further leads to the formation of defects such as microcracking and interfacial

debonding. The deterioration of mechanical properties, i.e. strength and modulus, can

be seen after the defects have occurred. Both types of defects are widely found in

many cases of synthetic composites, but there are limited studies on WPCs [1]

[5][10][11][12] [13]. These investigators also pointed out that non-Fickian moisture

diffusion (also known as non-linear behavior) resulted due to such defects. The reason

for the phenomenon of anomalous absorption was due to capillary force, which

collected the moisture in the cracks or gaps (microvoids) and/or by the cavitation

mechanism [11] [14].

Given these conclusions, it is now desirable to determine the escalation of

micricracking. Use of a microscope and morphology is the common methodology to

determine the microcracks and microvoids inside of composites. However, one

concern is that during the sample preparation of morphology observation, additional

damage could possibly be generated, resulting in a challenge to detect the true damage

morphology by microtoming the composite samples. Fortunately, utilizing the

concept of continuous damage mechanism (CDM), the propagation of microcracking

growth can be numerically and fundamentally understood [15]. However, as

  67  

mentioned previously, more understanding of the physical and mechanical

degradation of WPC from hygrothermal effects needs to be investigated, because

WPC is targeted to be utilized in variable applications under hygro-/hydrothermal

conditions.

Because sustainability has been a major focus since the early 21st century,

non-petroleum derived bio-polymers are now being used to produce WPC [3][8] to

meet sustainability requirements. Natural fibers have often been studied for

performance effects of biocomposites by blending with one biopolymer,

poly-3-hydroxybutyrate (PHB), due to its low cost and high strength/stiffness of

natural fibers [8][16][17]. However, the discrepancy of surface tension of

hydrophobic bio-plastic and hydrophilic natural fibers causes incompatibility in the

blend system. It has been observed that an interfacial gap between fiber and matrix

causes stress concentration while applying a load [8][18]. After applying loads, the

stresses cannot be transferred properly between fiber and matrix, and then induce low

performance of the biocomposites and WPCs. Hence, it is desirable to modify the

surface adhesion between the two materials, which can be achieved by adding

chemicals such as a compatibilizer, known as a coupling agent and/or interfacial

modifier. The performance of biocomposites can therefore be improved. Research

results indicated that enhanced mechanical properties and reduction of moisture

  68  

absorption were observed by adding compatibilizers to WPCs [4][8][16][17][19][20].

This study is therefore aimed at controlling interfacial bonding by adding

coupling agents to produce PHB/wood flour (WF) composites. The objectives are to:

1) discover the relation of tensile properties and moisture content; 2) find the

hygro-/hydrothermal condition effects on the performance of PHB/wood flour

composites; and 3) apply the concept of CDM to determine the propagation of

microcracking during hygrothermal conditioning by eliminating the preparation error.

3.2 Materials and Methods

3.2.1 Materials

Purified PHB powder was provided by Ningbo Tianan Biologic Material Co.,

Ltd., Ningbo, China. 60-mesh wood flour (Pinus) was provided by American Wood

Fibers, Schofield, WI. Flake form of diphenylmethane Diisocyanate (MDI, Mondur

M) was supplied by Bayer MaterialScience LLC and PHB-g-MA (for preparation and

details, see Appendix A) were functioned as the coupling agent between hydrophobic

PHB and hydrophilic wood flour.

 

 

 

  69  

3.2.2 Methods

3.2.2.1 Composite Extrusion Preparation

3.2.2.1.1 Melt Compounding

The wood flour was first dried in a conventional oven at 103±2oC for 24 hours.

The moisture content was further detected and obtained ca. 1.7%. In this study, 2 wt.

% of two coupling agents, MDI and PHB-g-MA, were placed into plastic bags with a

constant ratio of PHB/wood flour (70/30 at dried wt%). Each of the mixtures in

plastic bags was manually mixed by vigorous shaking and tumbling for 5 minutes.

After being mixed, a co-rotating twin screw extruder (Leistriz ZSE-18) with a screw

diameter of 18 mm and length/diameter (L/D) ratio of 40 equipped with a volumetric

feeder for constant feeding was used to melt and compound the mixture in order to

improve the dispersion and provide energy for the coupling agent to react between the

wood flour and PHB. A declining temperature profile of the extruder was applied

during the extrusion process. The extrusion temperature profile was set at 165, 165,

160, 155, 150, 145, 140, and 135oC, respectively, from the feeding throat to the die

adapter. The screw speed of extruder and feeder was maintained at 150 and 250 rpm.

Under these speeds, the residence time of the materials in the barrel was measured to

be ca. 2 to 2.8 minutes with output rates of 14 to 16.5 grams per minute. The

  70  

extrudate was cooled in a water bath and then pelletized. The different formulations of

pellets were oven dried at 80oC for 24 hours and stored in zip-lock bags.

3.2.2.1.2 Sheet Extrusion

Three different dried PHB/WF composite pellets were extruded by using the same

extruder with a slit die into a sheet with a cross section of 2mm x 40mm, and then

passed through a set of calendaring rolls. The temperature profile was set the same as

that for the melt compounding process, and the attached slit die was heated up to 135

oC. The screw speeds of the extruder and feeder were maintained at 100 and 85 rpm,

respectively. Under these speeds, the output rate was measured to obtain ca. 26 to 35

grams per minute. The gap between the two compressing rollers was set at ca. 1 mm

and the temperature of the rollers was controlled at 60oC. The exit extruded and

cooled sheets of PHB/WF composites were cut into length of ca. 300 cm for storage.

The three PHB/WF composites were coded as control, MA2 (added 2 wt.% of

MA-g-PHB), and ISO2 (added 2 wt.% of MDI) as shown through this entire paper.

 

 

 

 

  71  

3.2.2.2 Testing Specimen Preparation and Hygro-/Hydro-thermal Conditioning

  Sheets of PHB/WF composites were prepared into a dog-bone shape for further

tensile testing in accordance with ASTM D 638 Type I. All of these specimens were

placed in a condition room at 21oC with 50% of relative humidity for a week in order

to release the internal stress during preparation.

To evaluate water absorption behavior and moisture content effects on

mechanical properties of the PHB/WF composites, a series of tensile specimens were

immersed in distilled water, and sampled at 12, 24, 48, and 96 hours to evaluate the

relation between moisture content and properties and rate of water absorption. The

water absorption test was performed by removing specimens from the water and

wiping out the residual water on the surface of specimens, and then recording the

weight change. Furthermore, in order to understand the moisture content effects of

mechanical properties, tensile properties of the PHB/WF composites were detected

after the measurement of weight change. This examination was conducted by six

replicates of each formulation to reduce experimental error.

The other series of specimens was placed in a condition chamber with 58 oC and

95% of relative humidity and conditioned for 45 days. The sampling days were the 1st,

3rd, 6th, 10th, 15th, 21th, 30th, 36th, and 45th day. In order to reduce the moisture effects

and error on further tensile testing and evaluations, six replicates of each formulation

  72  

were sampled out from the conditioning chamber and then replaced into a control

condition room at 21oC and 50% of relative humidity for one week. The moisture

absorption under this hygrothermal condition was carried out by monitoring five

rectangular shaped specimens of each formulation while sampling the tensile testing

specimens. The thickness and weight change were measured at the sampling period.

3.2.2.3 Mechanical Properties and Density Evaluation

A screw driven Instron 4466 equipped with a 10-KN load cell and a pair of

mechanical grips was used for tensile tests. The tensile tests were conducted at a

crosshead speed of 5 mm per minute. The tensile strain was recorded and measured

by an extensometer (MTS model # 634.12E-24). These setups of tensile tests were

conducted in accordance with ASTM standard D638. Six replicates were tested for

each formulation to obtain a statistical mean value and standard deviation. Sample

density was calculated by dividing sample mass by sample volume. All of the samples

(for both mechanical and density testing) were conditioned at 21oC and 50% of

relative humidity for 7 days prior to the tests.

  73  

3.3 Results and Discussion

3.3.1 Moisture Absorption

Water absorption behavior has long been considered as a drawback to reduce

performance of WPCs and limit their application [3][6][10][18]. This finding was also

believed to deteriorate the performance of PHB/WF composites. However, by adding

coupling agents, water resistance can be improved due to controlling the

hydrophilicity of wood flour by coupling the hydroxyl group of wood with PHB [8].

As shown in the results presented in Fig. 3.1, after adding two different coupling

agents, MA-g-PHB and MDI (shown as MA2 and ISO2, respectively, in Figure 3.1), a

reduction of water absorption was observed in two different PHB/WF composites.

After 96 hours immersion, the control PHB/WF composite, PHB/WF composites with

2% of MA-g-PHB, and PHB/WF composites with 2% of MDI resulted in a mean

value of moisture content of 9.33%, 7.41%, and 5.67%, respectively. This indicated

that moisture resistance was improved with 20 to 40% water absorption reduction by

the coupling agents. Notably, the isocyanate coupling agent of MDI had a more

significant function in controlling the water absorption of the PHB/WF composite

than that of MA-g-PHB.

  74  

 

Figure 3.1 Relative water absorption behaviors of three different PHB/WF composites

 

3.3.2 Moisture Effects on Specific Tensile Properties

Given that WPC has been pointed out as having mechanical properties that

declined with increasing moisture content [5][7][10], moisture content plays an

important role in properties of polymeric composite materials. The relation between

moisture content and mechanical properties of the PHB/WF composites needs to be

addressed, since both wood and biopolymers are sensitive to moisture. As shown in

Figure 3.2 and Figure 3.3, it is clear that with increasing moisture content, three of the

PHB/WF composites had different decreased rates of specific strength and modulus.

Furthermore, the two coupling agents improved ca. 35 to 65% in both specific

strength and modulus in comparison with the control PHB/WF composite with

0.0%

2.0%

4.0%

6.0%

8.0%

10.0%

0 20 40 60 80 100

Rel

ativ

e w

ater

abs

orpt

ion

Immersion time (hours)

Control MA2 ISO2

  75  

different moisture content. The isocyanate coupling agent, MDI, resulted in

outstanding performance of PHB/WF composites [8]. This result is believed to be due

to the fact that the interfacial bonding between wood and PHB was improved, with

the result being that applied loads could be transferred properly without stress

concentration, and also because of the change of hydrophilicity of wood in the

composites [4][9][21][22]. The relation of moisture content and specific properties

can be numerically interpreted using statistical regression analysis (see Figures 3.2

and 3.3). As exhibited in Figure 3.4, it is obvious that the higher the moisture content

obtained, the larger the strain at break. This evidence indicates that moisture can be

recognized as a plasticizer to the PHB/WF composites. On the other hand, the results

of normalized specific tensile modulus (see Fig. 3.3) shows that the two composites

with the addition of 2% of coupling agents had higher sensitivity (higher slope) in

changing moisture content than that of the control PHB/WF composites, but a higher

rate of water absorption than the other two (see Fig 3.1).

  76  

 Figure 3.2 Plot of relation of relative moisture content and specific tensile strength of three different PHB/WF composites  

 Figure 3.3 Plot of relation of relative moisture content and specific tensile modulus of three different PHB/WF composites  

y = -59.4x + 18.6 R² = 0.91

y = -70.4x + 23.5 R² = 0.93

y = -71.7x + 26.4 R² = 0.85 0

10

20

30

0.0% 2.0% 4.0% 6.0% 8.0% 10.0%

Spec

ific

Tehn

sile

Str

engt

h (M

Pa)

Relative moisture content (%)

Control

MA2

ISO2

y = -25.2x + 4.44 R² = 0.90

y = -28.9x + 5.12 R² = 0.94

y = -32.9x + 5.15 R² = 0.98

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0% 2.0% 4.0% 6.0% 8.0% 10.0%

Spec

ific

Tens

ile M

odul

us (G

Pa)

Relative moisture content (%)

Control

MA2

ISO2

  77  

 Figure 3.4 Plot of relation of moisture content and strain at break of three different PHB/WF composites

To normalize overall moisture influences on different PHB/WF composites, the

relation can be theoretically described by using an equation with a moisture corrected

factor [23]:

K(σ  !"  !)′ = C(σ  !"  !) ∙ K(σ  !"  !) (equ. 3.1)

and

C(σ  !"  !) = 1− α(σ  !"  !) ∙ MC (equ. 3.2)

where K is the specific tensile strength (Kσ) and modulus (KE), C(α or E) is the moisture

content adjusting factor to the specific strength and modulus, α(α or E) is the slope of the

normalized linear curve, and MC is the moisture content. This model was further used

to evaluate and correct tensile properties with a linear function of normalization with

y = 0.07x + 0.01 R² = 0.87

y = 0.06x + 0.01 R² = 0.66

y = 0.10x + 0.01 R² = 0.95 0.00%

0.50%

1.00%

1.50%

2.00%

0.0% 2.0% 4.0% 6.0% 8.0% 10.0%

Stra

in a

t bre

ak (m

m/m

m)

Relative moistutr content (%)

Control

MA2

ISO2

  78  

the initial of MC of each PHB/WF composite at immersion time of zero. The

calculated results are shown in Figures 3.5 and 3.6 with linear regression curves. The

slope of the normalized linear curve (α) was expressed, showing that by comparing

with three composites the coupling agents had no significant effects on the normalized

specific tensile strength, but that of moisture content did. Thus, it can be concluded

that moisture content is one sensitive function of the tensile properties of PHB/WF

composites.

Figure 3.5 Plot of relation of relative moisture content and strength adjusting factor of three different PHB/WF composites with regression curves

y = -2.47x + 0.94

y = -2.40x + 0.94

y = -2.22x + 0.97

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.0% 2.0% 4.0% 6.0% 8.0% 10.0%

Str

engt

h a

djus

ting

fact

or

Relative moisture content (%)

Control

MA2

ISO2

  79  

Figure 3.6 Plot of relation of relative moisture content and modulus adjusting factor of three different PHB/WF composites with regression curves

3.3.3 Hygrothermal Effects (moisture and mechanical degradation)

Moisture plays a key role in properties deterioration of composites, especially on

WPCs, due to the hydrophilicity of wood flour. After moisture is absorbed from wood

flour, WPCs become dimensionally unstable, which is known as swelling.

Furthermore, the moisture-induced swelling of composites caused a certain degree of

internal stress on either the wood flour and/or polymer matrix. Fatigue and creep

damage of the composites therefore resulted after stresses accumulated [5][10]. This

phenomenon can be understood as viscoelastic behavior of polymeric material, which

is commonly found not only in the synthetic composites, but also significantly in the

WPCs. In the theory of moisture diffusion, known as Fickian diffusion, moisture

y = -3.32x + 0.80

y = -4.69x + 0.92

y = -5.62x + 0.95 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.0% 2.0% 4.0% 6.0% 8.0% 10.0%

Mod

ulus

adj

sutin

g fa

ctor

Relative moisture content (%)

Control

MA2

ISO2

  80  

diffuses slowly from the surface to the core of the composites [1][24]. It has been

found that shear stress is evident at the interface of the wood particle and polymer

matrix [5]. Notably, under hygrothermal conditions, it appears that moisture and

temperature have a coupling effect on moisture diffusion behavior, as proven in

experimental results and by using the Arrhenius equation to describe this effect

[5][25]; the higher the temperature, the faster the moisture penetration occurs. To

understand the hygrothermal effects on PHB/WF composites, a severe condition,

58oC and 95% relative humidity, was set as a condition to accelerate mechanical

degradation. The results, shown in Figures 3.7 and 3.8, show that specific tensile

strength and modulus reduced with conditioning time. The changes of moisture

absorption with time of three different PHB/WF composites are shown in Figure 3.9.

The control PHB/WF composite exhibited higher moisture absorption than those of

the other two composites in a comparison of ca. 5.0% and ca. 3.2% at day 10. This

result indicates that the coupling agents reduced moisture absorption in the PHB/WF

composites. It is interesting that one transition of the strength reduction (ca. 20%)

exhibited after 10 to 15 days of conditioning. After 21 days of conditioning, the three

composites showed ca. 40% properties reduction, and after 30 days of conditioning, it

was a challenge to detect the properties of the specimens by the universal testing

machine. Moreover, different results were exhibited in the specific tensile modulus

  81  

(see Figure 3.8) compared with that of strength. The modulus of three PHB/WF

composites appeared as a 13 to 24% of reduction before 3 days of conditioning, and

after 6 days, it remained in a stable stage without greater deterioration. However,

higher initial specific tensile properties were found in the composite added with MDI

as a coupling agent compared with the control PHB/WF composites (ca. 41% higher

in specific tensile strength and ca. 14% in that of the modulus), but there was higher

deterioration after hygrothermal conditioning. Possibly, with the high temperature

environment and moisture diffusion, the interfacial bonding was weakened by

moisture and debonding, as the time-temperature dependent of creep damages

occurred and remained a gap at the interface between wood and PHB matrix [5]

and/or created microcracks on the matrix [10]. Both debonding and microcracking, as

defects, are a cause of the reduction of the tensile properties. Studies have pointed out

that micro-cracks and/or micro-voids induced a non-Fickian moisture diffusion or

anomalous moisture absorption behaviors [11][14][24][26][27].

  82  

Figure 3.7 Plot of relation of specific tensile strength against conditioning time

Figure 3.8 Plot of relation of specific tensile modulus against conditioning time

0

5

10

15

20

25

30

0 500 1000 1500 2000

Spec

ific

Tens

ile S

tren

gth

(MPa

)

Conditioning time (sec0.5)

Control MA2 ISO2

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 500 1000 1500 2000

Spec

ific

Tens

ile M

odul

us (G

Pa)

Conditioning time (sec0.5)

Control MA2 ISO2

  83  

Figure 3.9 Plot of relation of relative moisture absorption against hygrothermal conditioning time

3.3.4 Microcracking Evaluation by Continuous Damage Mechanism

Since microcracks potentially occur and are distributed across an entire

cross-section by hygrothermal effects, it is a conundrum to quantify the cracking area

due to extra damage that may be produced during the preparation of microscope

observation. Fortunately, it is possible to apply the theory of continuous damage

mechanism (CDM) to obtain the damage, microcracks and/or gap (debond), inside the

PHB/WF composites. One remarkable effect is more obviously on the reduction of

modulus (stiffness) of the composites, which can be further used to define the

accumulated damage [15]. One-dimensional CDM is the simplest concept, but can

only be applied with certain requirements, such as single stress and damage

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

7.0%

0 200 400 600 800 1000 1200 1400 1600 1800

Rel

ativ

e m

oist

ure

abso

rptio

n (%

)

Hygrothemal conditioning time (sec0.5)

Control

MA2

ISO2

  84  

accumulated in a uni-axial direction. Uni-axial tensile testing is therefore proposed to

be used to determine the damage. The determination of the damage involves

theoretical calculation from the CDM. Consider that a damage-free (microcrack-free)

composite rod bar has a nominal cross-section area (A0). When the rod bar is under a

certain level of stress or load, damages (microcracks) will appear in the rod. In CDM,

this damage can be detected by the loss of modulus of the rod and is represented by

damage parameter, ω, as in the following equation:

ω= 1- E/ E0 or E = (1- ω)*E0 (equ 3.3a, b)

where E0 and E are the initial modulus (at the time of zero) and modulus after damage.

To prove this equation conceptually, microcracks result in the same effect as actual

damage, and the damage can be seen as a reduction of area:

ω= 1- A/A0 or A = (1- ω) A0 (equ 3.4a,b)

where A is the remaining normal area.

By hygrothermal conditioning, the composites are damaged (ω is from 0 to 1), and

effective stress, σ, can be expressed as

σ = P/ A = P/ A * A0/A0 = σ0 * A/ A0 = σ0 * (1- ω) (equ 3.5)

when ω =0, the effective stress will be equal to a normal stress. Based on Hooke’s law

(σ = E*ε), the same strain occurs under a tensile stress, and equ 3.3b can be proved by

deriving equation of 3.5 over the strain. Therefore, the relation of the microcrack area

  85  

under a hygrothermal condition could be investigated on the basis of the reduction of

the tensile modulus.

After using the mean value of the results presented in section 3.3.3 to calculate

area of microcracks (damage) via the theoretical concept of one dimension of CDM,

the relation of conditioning time and proportion of damage is obtained, as shown in

Figure 3.10. There is assuming that the perfect bonding condition is occurred at the

ISO2 because of isocyanate coupling agent can significantly improve the interfacial

bonding between wood and PHB [8]. As control formulation was expected to almost

no interfacial bonding, evidence of gaps was discovered by previous study [8].

Furthermore, the evidence was implied by the results of water and moisture

absorption (Figure 3.1 and 3.9), showing that the control formulation absorbed more

water and/or moisture than the other two PHB/WF composites. Therefore, the damage

propagation of different PHB/WF composites was obtained by using initial specific

tensile modulus of ISO as the E0. Notably, after approximately 10 days of

conditioning, the area of microcracks on each formulation was stabilized. Possibly, in

the two PHB/WF composites with coupling agents added, the interfacial bonding was

believe to be weakened under the hygrothermal condition and further debonded by

shear force from swollen fiber [5]. Gaps therefore remained. These observations

proved that the PHB/WF composites damaged area, which consisted of gaps

  86  

(interfacial debonding) and micro-cracks (creep damage in the PHB matrix). It is also

possible that thermal shock (temperature range from 58 down to 21oC) might mildly

accelerate the microcracking. As proposed, the propagation of damage could be used

to illustrate the microcracking behavior of the PHB/WF composites during

hygrothermal conditioning (see Figure 3.10). Therefore, as the calculated resulted

indicated that the coupling agents were successfully improved the hygrothermal

damage resistance by comparing with the control formulation.

Figure 3.10 Plot of relation of theoretical calculated damage parameter (ω) in percentage against conditioning time

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

0 500 1000 1500

Dam

age

para

met

er

Conditioning time (sec0.5)

Control

MA2

ISO2

  87  

3.3.5 Proposed microcracking mechanism under hygrothermal conditions

To conclude what was investigated above, the presumed microcracking

mechanism is shown in Scheme 3.1. This scenario occurs after the PHB/WF

composite has absorbed moisture to its saturation point, an internal stress was created

by swollen wood flour (diameter increased), followed by the induced creep damage

on PHB matrix with time. As the creep damage propagated, the damage extended

from an initial microcrack to a microvoid and/or debonding at the interface of wood

and PHB (see Scheme 3.1). After the PHB/WF composite cooled and slowly

dehydrated, the composite shrank dimensionally and gaps/voids remained from wood

flour and matrix PHB interfacial debonding and/or microcracks from creep damages

in the matrix. Possibly, the thermal and moisture shock induced some extra damage in

the matrix.

Scheme 3.1 Proposed relation of microcracking by internal stress corresponding with changing moisture content

  88  

3.4 Conclusion

Density is one factor that influences the mechanical performance of a composite,

but by comparing with specific mechanical properties of PHB/WF composites, the

error from the density can be eliminated. According to the results shown in this study,

use of a coupling agent can improve mechanical performance and reduce moisture

and water absorption under a hygro-/hydrothermal condition. As expected, the

coupling agent of MDI resulted in ca. 40~55% less water absorption and ca. 40%

improvement in tensile stress. Overall, moisture content influenced tensile properties

with a negative linear relation. It appeared that moisture reacted as a plasticizer to the

PHB/WF composites; the higher the moisture content, the larger the elongation at the

failure (strain at break) occurred. After theoretically normalizing the moisture effects

on the PHB/WF composites, it is clear that moisture influenced the mechanical

properties, especially in the tensile modulus. As temperature and moisture are coupled

to influence the deterioration of tensile properties of PHB/WF composites, it is

necessary to discuss the hygrothermal effects. As a result, it appeared that the tensile

properties declined after a certain period of time of conditioning at 58oC and 95% RH.

The cause of the properties deterioration was believed to have been caused by

interfacial debonding and creep-induced microcracking, both of which are caused by

swollen wood flour. Moreover, theoretical determination of microcracking can be

  89  

achieved by using the continuous damage mechanism (CDM). Reduction of tensile

modulus was successfully used to obtain the proportion of damages during the

hygrothermal conditioning, without physically sectioning the composite materials for

microscope observation. Crossing the investigated results in this study with the

fundamental understanding by using the CDM, it can be concluded that the interfacial

debonding and damages on matrix could be the main reasons for microvoids (i.e.

microcracks and gaps, respectively). These microviods can induce cavitations and

capillary force to absorb more moisture/water in the PHB/WF composites. By using

the investigated results, a proposed moisture-induced microcracking and debonding

can be illustrated by using a schematic sketch. In conclusion, the hygro-/hydrothermal

effects of the PHB/WF composites were successfully investigated with a fundamental

understanding propagation of microcracking by applying the concept of CDM.

  90  

3.5 References

[1] Y. J. Weitsman. 1991. Moisture in Composites: Sorption and Damage in Composite Materials Series 4 - Fatigue of composite materials, K. L. Reifsnider editted, Amsterdam  ;New York: Elsevier. pp. 385-429.

[2] C. Clemons. 2002. Wood-Plastic Composites in the United States The Interfacing of Two Industries. Forest Products Journal. 52(6):10-18

[3] P. M. Smith and M. P. Wolcott. 2006. Opportunities for Wood/Natural Fiber-Plastic Composites in Residential and Industrial Applications. Forest Products Journal. 56(3):4-11

[4] J.-W. Kim, D. Harper, and A. Taylor. 2008. Effect of Wood Species on Water Sorption and Durability of Wood-Plastic Composites. Wood and Fiber Science. 40(4):519-531

[5] S. V. Rangaraj and L. V. Smith. 2000. Effects of Moisture on the Durability of a Wood/Thermoplastic Composite. Journal of Thermoplastic Composite Materials. 13(2):140-161

[6] A. Schirp and M. Wolcott. 2005. Influence of Fungal Decay and Moisture Absorption on Mechanical Properties of Extruded Wood-Plastic Composites. Wood and Fiber Science. 37(4):643-652

[7] N. Stark. 2001. Influence of Moisture Absorption on Mechanical Properties of Wood Flour-Polypropylene Composites. Journal of Thermoplastic Composite Materials. 14(5):421 -432

[8] Scott P. Anderson. 2007. Wood Fiber Reinforced Bacterial Biocomposites: Effects of Interfacial Modifiers and Processing on Mechanical and Physical Properties. Master thesis, Washington State University

[9] K. Joseph, S. Thomas, and C. Pavithran. 1995. Effect of ageing on the physical and mechanical properties of sisal-fiber-reinforced polyethylene composites. Composites Science and Technology. 53(1):99-110

[10] Stark, N. M. and Gardner, D. J. 2008. Outdoor durability of wood-polymer composites in Wood-polymer composites. Cambridge, England : Woodhead Publishing, Ltd. ; Boca Raton, FL : CRC Press. Woodhead Publishing in materials. pp. 142-165.

[11] J. Zhou and J. P. Lucas. 1995. The effects of a water environment on anomalous absorption behavior in graphite/epoxy composites. Composites Science and Technology. 53(1):57-64

[12] L. J. Burcham, M. R. Vanlandingham, R. F. Eduljee, and J. W. Gillespie. 1996. Moisture effects on the behavior of graphite/polyimide composites. Polymer Composites. 17(5):682-690

  91  

[13] H. T. Hahn. 1987. Hygrothermal Damage in Graphite/Epoxy Laminates. Journal of Engineering Materials and Technology. 109(1):3-11

[14] C. Shirrell. 1978. Diffusion of Water Vapor in Graphite/Epoxy Composites」, in Advanced Composite Materials—Environmental Effects, J. Vinson edited. 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959: ASTM International, 1978. pp. 21-41.

[15] E. J. Barbero. 2007. Damage Mechanics in Finite element analysis of composite materials, CRC Press. pp. 191 - 206.

[16] L. Jiang et al. 2008. Study of Poly(3-hydroxybutyrate- co -3-hydroxyvalerate) (PHBV)/Bamboo Pulp Fiber Composites: Effects of Nucleation Agent and Compatibilizer. Journal of Polymers and the Environment. 16(2):83-93

[17] A. Gregorova, R. Wimmer, M. Hrabalova, M. Koller, T. Ters, and N. Mundigler. 2009. Effect of surface modification of beech wood flour on mechanical and thermal properties of poly (3-hydroxybutyrate)/wood flour composites. Holzforschung. 63(5):565-570

[18] M. J. A. Chowdhury and M. P. Wolcott. 2007. Compatibilizer selection to improve mechanical and moisture properties of extruded wood-HDPE composites. Forest Products Journal. 57(9):46-53.

[19] H. Dalväg, C. Klason, and H.-E. Strömvall. 1985. The Efficiency of Cellulosic Fillers in Common Thermoplastics. Part II. Filling with Processing Aids and Coupling Agents. International Journal of Polymeric Materials. 11(1):9-38

[20] R. Gauthier, C. Joly, A. C. Coupas, H. Gauthier, and M. Escoubes. 1998. Interfaces in polyolefin/cellulosic fiber composites: Chemical coupling, morphology, correlation with adhesion and aging in moisture. Polymer Composites. 19(3):287-300

[21] E. Petinakis, L. Yu, G. Edward, K. Dean, H. Liu, and A. Scully. 2009. Effect of Matrix–Particle Interfacial Adhesion on the Mechanical Properties of Poly(lactic acid)/Wood-Flour Micro-Composites. Journal of Polymers and the Environment. 17(2):83-94

[22] A. Karmarkar, S. S. Chauhan, J. M. Modak, and M. Chanda. 2007. Mechanical properties of wood-fiber reinforced polypropylene composites: Effect of a novel compatibilizer with isocyanate functional group. Composites Part A: Applied Science and Manufacturing. 38(2):227-233

[23] M. C. Hatch. 2008. Processing, Mechanical, and Environmental Performance of Engineering Polymer Wood-Plastic Composites. Master thesis, Washington State University

[24] L.-W. Cai and Y. Weitsman. 1994. Non-Fickian Moisture Diffusion in Polymeric Composites. Journal of Composite Materials. 28(2):130 -154

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[25] J. P. Komorowski. 1983. Hygrothermal Effects in Continuous Fibre Reinforced Composites. Part I. Thermal and Moisture Diffusion in Composite Materials. Aeronautical note. National Aeroautical Establishment. National Research Council. Ottawa, Ontario, Canada. pp. 1-51

[26] Y. J. Weitsman. 2006. Anomalous fluid sorption in polymeric composites and its relation to fluid-induced damage. Composites Part A: Applied Science and Manufacturing. 37(4):617-623

[27] L. J. Burcham, M. R. Vanlandingham, R. F. Eduljee, and J. W. Gillespie. 1996. Moisture effects on the behavior of graphite/polyimide composites. Polymer Composites. 17(5):682-690

 

  93  

Chapter 4 Investigation of Effects of Coupling Agents on Compostability and

Mineralization Behaviors of Poly-3-hydroxybutyrate/Wood Flour Composites

Abstract

Durability of petroleum-derived plastics is now seen as a drawback to the

environment as indicated in statistical research of municipal solid plastic waste in

accordance with the US Environmental Protection Agency (EPA). One solution is the

use of biodegradable biopolymer, e.g. Poly-3-hydroxybutyrate (PHB), reinforced with

wood flour (WF) to expand its performance and maintain biodegradability. With this

composting technique, which converts solid organic matter to carbon dioxide, one

biocomposite of the PHB/WF composite is seen as an alternative to reduce

environmental impacts. However, other studies have focused on coupling agents that

enhance the performance and water resistance of PHB/WF composites; currently, the

compostability of biodegradable PHB/WF composites remains a deficiency. The

current study focused on coupling agent effects on the compostability and

mineralization of PHB/WF composites. The coupling agents used in this study were

maleic anhydride grafted PHB (PHB-g-MA) and diphenylmethane diisocyanate

(MDI). The results indicated that MDI significantly reduced the compostability of the

PHB/WF composite. An approximately 12% reduction of rate of CO2 evolution and

  94  

mineralization was found and about 35% less material was mineralized at the end of

the test (day 54) compared with the PHB/WF control composite. It was believed that

MDI efficiently coupled and bonded wood flour and PHB and further reduced rate of

mineralization. Notably, wood has a much lower rate of mineralization

(approximately five fold) than PHB. Additionally, the Michaelis-Menten kinetic was

successfully utilized to determine the theoretical enzyme kinetic during the lag phase,

and Fickian diffusion was effectively applied to understand the mineralization

diffusion behavior during the biodegradation phase. It was concluded that by

controlling the interfacial bonding between wood and PHB, compostability of the

PHB/WF composites could also be engineered. Enzymatic surface erosion was

observed by using microscopy. A novel investigated mechanism of enzymatic surface

erosion on the PHB/WF composites was schematically sketched out and the results

corresponded with three phases of biodegradation.

Key words: poly-3-hydroxybutyrate, coupling agent, compostability, mineralization,

Fickian diffusion, enzyme kinetic

 

 

 

  95  

4.1. Introduction

According to the US Environmental Protection Agency (EPA), 243 million tons of

municipal solid waste was generated in 2009 and only about 2 million tons out of 30

million tons of disposed plastic were recovered in the US during 2009[1]. This

statistical result indicates that municipal plastic waste is still a challenge in the US.

Additionally, these plastic wastes contained approximately19 million tons of

non-durable plastic goods including disposable products, containers, and packaging

plastics, and only about 9% of those materials were recycled. Interpretation of these

statistics indicates that about 17.3 million tons of plastic wastes were land-filled in

2009[1]. Utilizing biodegradable biologically derived polymers (biopolymer) that are

now emerging as a biodegradable option to reduce the environmental challenge and

reduce petroleum dependent, biodegradation can be achieved under anaerobic (e.g.

landfill) and aerobic (e.g. composting) conditions. Under anaerobic conditions,

landfill, methane and carbon dioxide (green house gases) are generated as by-products.

Methane can be utilized as an energy source for power and heat. However, if methane

is not been collected properly, it can cause a certain degree of environmental impacts

[2]. Carbon dioxide (CO2), on the other hand, can be absorbed by natural plants and

produce more carbon feedstocks, such as wood and starch. Therefore, to prevent

potential issues from methane, the composting process can aerobically convert the

  96  

organic matter in biodegradable polymeric materials into carbon dioxide efficiently.

With respect to the carbon cycling ecosystem, this process converts organic carbon to

CO2, while remaining organic material as a soil amendment that provides natural

nutrient retention and further reduces the cost of landscaping. Several studies [3][4][5]

have indicated that organic matter can be carried out from the compost mixture and

then used as an energy source for microorganisms during composting. Moreover,

because biopolymers are mainly comprised of carbon, hydrogen and oxygen, they

may also be converted to soil amendments in this process. To evaluate the

compostability or biodegradability of an organic material under aerobic conditions

thus became our focus for further study in order to understand its mineralization

behavior, which can further be used to estimate the amount and rate of CO2 evolution.

One biopolymer, Poly-3-hydroxybutyrate (PHB) has been studied for its

biodegradability under different environments, such as compost [6][7][8][9], seawater

[10], enzymatic conditions [11][12][13][14][15][16], and sewage sludge [17].

However, due to its performance limitation, natural fillers such as wood fiber/flour

(WF) and starch are mainly used to reinforce the biopolymers while maintaining

biodegradability and extend its performance envelope. Unfortunately, the discrepancy

of surface tension leads to the result that in the wood flour and polymer, PHB in this

case, are not compatible, which reduces performance enhancement [18][19]. The

  97  

solution that was investigated in this study is to add a compatibilizer, also referred to

as a coupling agent. A compatibilizer is a solution added to maintain the performance

envelope of the biocomposite. Better filler dispersion and/or chemically interaction

between the filler and the matrix may occur after a compatibilizer is added. Since

wood plastic composites (WPCs) have been developed for decades, researchers have

found that the key to mechanical and physical (moisture) property enhancement is to

modify the interface between wood and plastic. To encourage this performance

enhancement, the compatibilizer effects of WPCs were then studied. Most research

studies have concluded that maleric anhydride (MA) grafted on polyethylene and

polypropylene (MA-PE/MA-PP) are the common potential compatibilizers for the

WPCs [20][21][22]. In these studies, approximately 20 to 30% improvement was

observed on mechanical properties. Water uptakes also received considerable

attention, resulting in about 50% reduction [22]. In biopolymer applications, studies

[18][23] concluded that MA grafted on PHB (MA-PHB) is a suitable compatibilizer

for enhancing natural fiber reinforced PHB composites and about 10 to 30%

difference in strength were observed by different processes. This result is because the

side group of MA-PHB, succinic moieties, can possibly provide hydrogen and/or

chemical bonding with the hydroxyl group (-OH) of wood fiber [20][21][24][25].

Moreover, the isocyanate group (-NCO) of isocyanate adhesives is expected to bond

  98  

chemically on the hydroxyl-rich functional group of wood and partially on PHB

[22][26][27]. A polymeric diphenylmethane diisocyanate (pMDI) can be used as a

compatibilzer to the WPCs. Anderson [18] indicated that about 20~50% variation of

mechanical properties and water uptakes were found on the pMDI added PHB/WF

composites. Also, the pMDI had showed significant improvement compared to the

other compatibilizers. This result could be due to the fact that the functional groups of

isocyanates are highly reacts between wood and PHB. In conclusion, the MA-PHB

and pMDI can provide an interfacial modifying option and create physical and/or

chemical bonding between wood and PHB. However, there is a deficiency of research

of coupling agent effects on biodegradability of PHB/WF composites, as most

previous studies were focused on performance enhancement.

The goal of this study was to demonstrate that the coupling agent affects the

compostability of biodegradable PHB/wood flour (WF) composite by evaluation

under laboratory scale conditions in accordance with ASTM D5338 and that output

data can be further applied based on the Fickian diffusion equation to understand

mineralization behavior. The specific objectives were to: 1) identify the

compostability of PHB/WF composites with coupling agents added by using a

lab-scale apparatus; 2) evaluate the mineralization and morphology change of

PHB/WF composites during composting; and 3) apply the Fickian diffusion equation

  99  

and Michealis-Menten equation to understand mineralization diffusion behavior and

enzyme reaction kinetics.

 

4.2. Materials and methods

4.2.1 Materials

Purified PHB powder was provided by Ningbo Tianan Biologic Material Co.,

Ltd., Ningbo, China. Wood flour (Pinus, 60-mesh) was provided by American Wood

Fibers, Schofield, WI. Maleic anhydride grafted PHB (PHB-g-MA) was prepared by

reaction extrusion process and 0.43% of grafting ratio was detected (for preparation

and details, see Appendix A). Flake form of diphenylmethane diisocyanate (MDI,

Mondur M) was supplied by Bayer MaterialScience LLC. The PHB-g-MA and MDI

functioned as the coupling agent between hydrophobic PHB and hydrophilic wood

flour.

Approximately four-month-matured compost inoculums were provided by the

Washington State University compost facility. The compost was sieved through a

sieve screen with openings of 10 mm diameter during nursing. Proportions of the

compost feedstock [28] are shown in Table 4.1. Moisture content of the compost was

adjusted to approximately 50%.

  100  

Table 4.1 Proportion of compost feedstocks

Manure Ash Compost Food Waste Yard Waste Dirt Wood Chips*

78% 10% 10% 1% 0.5% 0.5% 5%*

* added during the wet season to provide porosity

   

4.2.2 Testing specimen preparation

The wood flour was first dried in a conventional oven at 103±2oC for 24 hours.

The moisture content was further detected to be about 1.7%. Three formulations of

PHB/WF composites of PHB (70 wt.%), i.e. dried wood flour (30 wt.%), and 2 wt. %

of two coupling agents, MDI and PHB-g-MA, were placed into plastic bags with a

constant ratio of PHB/wood flour (70/30 at dried wt.%). The three PHB/WF

composites were coded as control, MA2 (added 2 wt.% of MA-g-PHB), and ISO2

(added 2 wt.% of MDI), as indicated through this chapter. Each of the mixtures in

plastic bags was manually mixed by vigorous shaking and tumbling for 5 minutes.

After being mixed, a co-rotating twin screw extruder (Leistriz ZSE-18) with a screw

diameter of 18 mm and length/diameter (L/D) ratio of 40 equipped with a volumetric

feeder for constant feeding was used to melt and compound the mixture in order to

improve dispersion and provide energy for the coupling agent to react between the

wood flour and PHB. A declining temperature profile of the extruder was applied

  101  

during the extrusion process. The extrusion temperature profile was set at 165, 165,

160, 155, 150, 145, 140, and 135oC, respectively, from the feeding throat to the die

adapter. Screw speed of the extruder and the feeder was maintained at 150 and 250

rpm. Under these speeds, the residence time of the materials in the barrel was

measured at approximately 2 to 2.8 minutes with output rates of 14 to 16.5 grams per

minute. The extrudate was cooled in a water bath and then pelletized. The different

formulations of pellets were oven dried at 80oC for 24 hours and stored in zip-lock

bags.

Three different dried PHB/WF composite pellets were extruded by using the same

extruder with a slit die in a sheet with a cross section of 2mm x 40mm, and then

passed through a set of calendaring rolls. The temperature profile was set to be the

same as that for the melt compounding process, and the attached slit die was heated

up to 135 oC. The screw speeds of the extruder and feeder were maintained at 100 and

85 rpm, respectively. Under these speeds, the output rate was measured to obtain

approximately 26 to 35 grams per minute. The gap between the two compressing

rollers was set at approximately1 mm; the temperature of the rollers was controlled at

60oC. The exit extruded and cooled sheets of PHB/WF composites were chopped into

flake form using a granulator (Nelmor®) with a screen of diameter of approximately

15 mm. This process simulated preparation during composting.

  102  

4.2.3 Composting apparatus and procedure

In this study, the procedures outlined in ASTM D 5338 were used to assess

biodegradation of plastics in a laboratory scale-compost experiment, and the

apparatus set-up was also built in accordance with ASTM D 5338 (see Fig. 4.1). The

system consisted of determining the degree and rate of mineralization of plastic

materials from organic carbon to evolved carbon dioxide. A series of 18 bioreactors

(each in triplicate) containing blank (compost only), PHB flake (sample of PHB

polymer + compost), wood flour (sample of 60 mesh pine wood flour + compost),

PHB/WF composite flake (sample of the control composite + compost),

PHB/WF/MA2 composite flake (sample of the composite with 2% coupling agent of

PHB-g-MA+ compost), and PHB/WF/MDI2 composite flake (sample of the

composite with 2% coupling agent of MDI + compost) were tested via this system.

The ratio of inoculums mixture was controlled at 6:1 (compost:flake samples) in dry

mass. The biodegradation test was carried out under a laboratory-controlled condition:

CO2-free and humidified inlet air with a constant flow of approximately 90 ml/min.,

outlet air after reactors was trapped by 18 CO2 trappers, and 18 bioreactors were

placed in a 58 ± 2oC a water bath throughout the entire experimenting period.

The CO2-free and humidified inlet air was generated by compressing air through a

sodium hydroxide (NaOH) solution to remove CO2 and then passed through deionized

  103  

(DI) water to generate humidify air. To maintain a consistent condition, the

moisturized and CO2-free air was diverted and adjusted by flow meters and further

input to the bottom of each bioreactor. The bioreactor was built using a glass jar of

approximately internal volume 2 L with a screen placed at the bottom of jar to avoid

channeling and provide equally distributed inlet air in the compost. Each mixture of

inoculums was mixed and placed into jars with about 20 mm space from the lid for

outlet air distributed to CO2 trappers. All mixtures of inoculums were stirred weekly

to avoid channeling and to provide proper aeration and contact between sample and

compost. A set of condensed water collectors was placed before each CO2 trapper.

The 400 ml CO2 trappers contained 2.5 M NaOH solution to collect the amount of

CO2 produced by each bioreactor. The following CO2 trapping reaction was discussed,

as stated in ASTM D 5338:

NaOH + CO2 → NaHCO3 --------------------------------------------------- (1)

then

NaHCO3 + NaOH → Na2CO3 + H2O ----------------------------------------- (2)

5 ml aliquots were removed from each trapper and then titrated with 0.5M

hydrochloric acid (HCl) solution 3 times a week, and the CO2 traps were changed

weekly. During titration, CO2 was chemically reacted and removed as follows:

  104  

NaCO3 + HCl → NaHCO3 + NaCl -------------------------------------- (3)

then

NaHCO3 + HCl → NaCl + H2O + CO2 --------------------------------------- (4)

Initially, 5 drops of phenolphthalein indicator (pink color) were added to the 5 ml

aliquots and further titrated until the end-point, as indicated when the color turned to

colorless (as in titration formula 3). In addition, 5 drops of methyl orange indicator

were then added, and then titrated until the color turned from orange to red (as in

titration formula 4). The amounts of HCl consumed during the second titration

(formula 4) were used to calculate and obtain the evolving CO2 in accordance with the

following equation [5]:

CO2 (g) = VHCl (ml) x CHCl (molar) x 44 /1000 ----------------------------- (5)

where VHCl is the volume of HCl consumed in the second titration; CHCl is the

concentration of HCl solution; and 44 is the molar mass of CO2.

Further, the percentage of mineralization can be calculated in accordance with

ASTM D 5338:

Mineralization (%) = [(CO2)sample –(CO2)blank]/ ThCO2 (eq. 4.1)

where (CO2)sample is the cumulative amount of CO2 obtained from the titration result of

each sample mixture inoculum with the time; (CO2)blank is the cumulative amount of

  105  

CO2 obtained from the titration result of each blank inoculum (compost only) with the

time; and ThCO2 is the theoretical quantity of each sample CO2 evolution, which can

be calculated based on the following equation:

ThCO2 = Msample x TOCsample x 44 /12 (eq. 4.2)

where Msample is the total dried mass of the sample material which was introduced to

the compost inoculums; TOCsample is the proportion of total organic carbon (TOC) in

the dried sample material; 44 is molar mass of CO2; and 12 is the atomic mass of

carbon.

Because all of the sample materials used in this study were assumed to be

compostable, the following test was based on an evaluation method on TOC of

compost and/or municipal solid waste (MSW). In compost materials, TOC can be

estimated via a formula of reduction from total organic matter (TOM) on a prediction

equation of MSWs (waste paper based) and city refuse compost experimental results

[29]:

TOC = 1.703 + 0.520 * TOM (eq. 4.3)

According to APHA 2540 D and E, TOM content determines ignition loss of oven

dried samples by using a muffle furnace at 550oC for 30 minutes, and then being

  106  

weighed after cooling in a desiccator. The chemical characteristics of each sample

material are shown in Table 4.2.

Table 4.2 Chemical characteristics of materials

Compost WF PHB Control MA2 ISO2

TOM (%) 65.4 99.5 99.1 99.4 99.4 99.4

Ash (%) 34.6 0.5 0.9 0.6 0.6 0.6

TOC (%) 35.7 53.4 53.2 53.4 53.4 53.4

ThCO2 284.2 74.4 74.2 74.4 74.4 74.4

Figure 4.1 Schematic of lab-scale material compostability evaluation set-up system

  107  

4.2.4 Fickian diffusion coefficients

During the composting test, moisture was first penetrated into sample materials,

followed by biodegradation reaction. Moisture diffusion behavior plays an important

role in the operation of biodegradation. Moreover, the moisture penetration behavior

of a WPC can be discussed and studied by using the Fickian diffusion equation

[22][30]. If the evolved CO2 vs. time can fit the Fickian diffusion equation, it will

indicate the limiting steps and also explain the mineralization diffusion behavior of

the materials. The Fickian diffusion equation is shown below:

!! =  π !!!!"#

! !!!! !

!     (eq. 4.4)  

where DA is the apparent diffusion constant; Msat is the maximum mineralization

percentage at the end of the test period; dMt/d√t is the slope of mineralization over

square root of time relation; and h is the thickness of the composite flake sample.

4.2.5 Enzymatic kinetic

In composting chemistry, organic matter chemical breakdown is mainly reacted

by enzymes that are produced from microorganisms. Enzymes, in the case of aerobic

microorganisms, can convert organic matter to CO2 and remaining water, energy, and

residuals. In this system, complex organic matters of mixture inoculums are

  108  

depolymerized by the enzymes, and then the microorganisms absorb the breakdowns

as energy sources into their cells and further produce CO2 [3] [4] [5][31]. This

conversion is known as the mineralization process. It can therefore be expected that

the rate of enzyme reaction and rate of mineralization have a positive relation.

The enzyme kinetic is recognized as an accurate and reliable way to determine

enzymatic reaction. It can also provide numerical elementary mechanisms of the

reactions. The concept of the Michaelis-Menten kinetic [32][33][34][35] is widely

used in compost and agriculture science. Since mineralization is driven by bioreaction

(faster bioreaction occurs at a higher rate of mineralization), the results of

mineralization may be used to obtain the kinetic constants from the Michaelis-Menten

equation [33][34][35][36]. This will further facilitate understanding of enzyme

bioreaction behavior on PHB/WF composites, and possibly provide knowledge

concerning the composting behavior on the composite for future research studies. The

equation can be presented as:

v =   !!"#[!]! !  !!

(eq. 4.5) or !!=   !!

!!"#∙   ![!]+   !

!!"# (eq. 4.6)

where Km is Michealis constant, v is production rate (rate of mineralization, %/day),

Vmax is theoretical maximum reaction velocity, and [S] is the substrate (ratio of

converted carbon/ash, g/g).

  109  

Further, the Lineweaver-Burk plotting method can be used to determine the slope,

Km/Vmax, and intercept, 1/Vmax of a plot of equation which incorporates 1/v and 1/[S].

The lower the value of Km, the higher the affinity of enzyme occurs for the substrate

catalyst and/or less efficient of products production rate (CO2 in this case). The higher

the value of Vmax, the faster the rate of reaction occurs during substrate catalyst

[33][34][35].

4.2.6 Morphology

The surface morphology of sample materials was observed by using a scanning

electron microscope (FEI SEM, FEI 200F). The tested samples, removed from

mixture inoculums at day 90, were washed with DI water to remove compost debris

and then dried at 80oC. The control samples were also oven dried to remove moisture,

which could influence the quality of coating. All of the prepared samples were

mounted on aluminum stubs and gold-coated by a sputter coater.

  110  

4.3. Results and discussion

4.3.1 CO2 evolution and mineralization results

In this study, as the observation of the different polymeric materials (i.e . PHB,

PHB/WF composite, PHB/WF/MA2 composite, and PHB/WF/ISO2 composite), a lag

phase of biodegradation occurred during the first week (day 0 to day 5) of compost

experimentation. In this lag phase, theoretically, microorganisms in the compost

inoculums did not fully migrate and react with the test samples. Moreover, low

moisture content (MC) of the samples did not benefit the bioreaction from the

enzymes produced from the microorganisms. Because of these possible reasons, it

resulted in a lower rate of CO2 gas evolution than that in the biodegradation phase

(Table 4.3). After the MC of the samples equilibrated, a rapid bioreaction occurred

within the biodegradation phases, which followed after the lag phases. However,

under high humidity and temperature conditions, hydrolytic cleavage on the polyester

[13][37], PHB (in this case), could possible occur. The PHB polymer chain may have

fractured and reduced to shorter polymer chains, but no CO2 or weight loss occurred

[37][38]. Moreover, the polymer chains of hydrolyzed cleavage possibly provided

more contact surface which could have advanced the bioreactions. Following the lag

phase, a high rate of CO2 evolution appeared until day 21, which was believed to

mark the biodegradation phase. An alleviated biodegradation phase occurred

  111  

beginning on day 24. Due to the similar amount of total theoretical CO2 evolution of

the different materials, the mineralization of each material had the same trend as the

results in the CO2 evolutions. The results shown in Table 4.3 present the rates of CO2

evolution and mineralization of each material in different phases of biodegradation.

Notably, the rate of CO2 evolution (mg/g/hr0.5) had been normalized with the mass of

each sample, which can be further used for product end life cycle. In the other words,

the higher the rate of CO2 evolution, the shorter product end life cycle required. The

curves of accumulative CO2 evolution and mineralization are shown in Fig. 4.2 and

Fig. 4.3. The blank compost had a constant rate of CO2 evolution and mineralization,

which indicates that the compost inoculums had a constant bioreaction throughout the

entire experimenting period.

Table 4.3 Rates of CO2 evolution and mineralization of each sample in different phases of biodegradation

Day Reaction Rate Compost WF PHB Control MA2 ISO2

0-5 CO2* 1.3 7.3 14.2 15.9 13.4 7.6

Mineralization** 0.10% 0.37% 0.73% 0.81% 0.69% 0.39%

7-21 CO2 2.2 6.7 34.8 29.2 28.8 25.8

Mineralization 0.17% 0.34% 1.78% 1.49% 1.47% 1.32%

24-54 CO2 4.7 11.6 14.1 15.2 13.7 11.0

Mineralization 0.36% 0.59% 0.72% 0.78% 0.70% 0.56% * CO2 is amount of CO2 evolution per unit weight of material over square root of hours (mg/g/hr0.5); ** Mineralization is percentage of mineralization over square root of hours (%/hr0.5).

  112  

In the degradation phase (day 7 to 21), in the comparison with PHB, WF and the

PHB/WF (control) composite, wood flour had the lowest rate (6.7 mg/g/hr0.5) of CO2

evolution. Complex chemical compounds, e.g. lignin, are believed to have caused the

difficulty of bioreactions from enzymes with wood [5]. The main chemical

components of wood are cellulose, hemi-cellulose, and lignin. Each of these

chemicals require certain conditions (enzymes, humidity, and temperature) and time

(in the scale of years) to achieve depolymerization or decay [39][40]. For example,

lignin, which ties cellulose microfibers together and forms the structure of wood and

functions as a rigid polymer, has complex chemical structures and anti-enzyme

property [5], which was believed to cause the difficulty of degradation. On the other

hand, the two PHB/WF composites with coupling agents added, MA2 and ISO2, had

lower rate of CO2 evolution than the control (28.8 and 25.8 mg/g/hr0.5 vs. 29.2

mg/g/hr0.5). Studies [18][23] have indicated that coupling agents improved the

interfacial bonding between wood and PHB by changing the wettability (surface

tension) of wood and coupling the functional groups, e.g. hydroxyl group, in wood

with PHB. In this research, the coupled PHB with wood flour can be seen as one

binding component, which resulted the difficulty of bioreaction with enzymes due to

the lignin effects and possible the molecular weight effects. The larger the molecular

weight, the longer the time it takes to degrade. However, when looking at the

  113  

difference between the two coupling agents, there was not enough evidence to

conclude that the PHB/WF composite with PHB-g-MA added had less biodegradation

resistance than MDI because of grafting ratio was only 0.43% compared with the

higher functionality two of MDI.

Morphologically, during biodegradation, the PHB-rich skin of the composites was

first biodegraded, and then wood flour was exposed to the enzymatic environment.

Another possibility is that in the high humidity condition, moisture-induced swelling

of wood flour created defects between the wood flour and polymer as interfacial

debonding and microcracks on surface and inner of the PHB/WF composites.

Furthermore, enzymes can rapidly depolymerize PHB due to the expansion surface

area. A retarded rate of CO2 evolution and mineralization (Figs. 4.2 and 4.3) was

observed after day 21. The reasons for this retardation may be because the enzymes

have difficultly reacting with PHB, which is located in the innermost depth of wood

lumens and/or blocked by the wood flour while simultaneously degrading some wood

chemicals. Further morphology studies are encouraged to focus on this question in

order to understand the composting biodegradation mechanisms and surface erosion

process.

Notably, on the last day (day 54) of this experiment, the mineralization levels of

different polymeric composites and materials were 16.6%, 40.4%, 39.8%, 36.3%, and

  114  

25.7% (WF, PHB, Control, MA2, and ISO2, respectively). The PHB/WF composite

of ISO2 resulted in an approximately 35% and 29%, respectively, less mineralization

than that of the PHB/WF composites of the control and MA2. Only about 9%

difference appeared between the control and MA2. This indicates that using MDI as a

coupling agent can significantly reduce the rate of mineralization during composting.

Figure 4.2 Plot of accumulative CO2 evolving production against square root of time in hours of polymeric composites and materials

0

100

200

300

400

500

600

700

800

900

CO

2 Evo

lutio

n (m

g/g)

Time (hr0.5)

Compost WF PHB Control MA2 ISO2

  115  

Figure 4.3 Plot of accumulative mineralization against square root of time in hours of polymeric composites and materials

4.3.2 Fickian diffusion coefficient

As moisture is a key of limitation for operating bioreactions by enzymes that are

produced from microorganisms, moisture penetration is a major influencing factor on

the rate of mineralization. The behavior of small molecules (liquid or gas) transported

throughout solid materials can be numerically presented by the Fickian diffusion.

Enzymes produced from microorganisms are small molecules and display diffusion

behavior in solid materials [41]. Theoretically, after an enzyme is diffused into a solid

material, the enzyme will react with the substrate and then be consumed by the

microorganism to produce carbon dioxide as its metabolism. For this reason, by

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%

0 5 10 15 20 25 30 35 40

Min

eral

izat

ion

(%)

Time (hr0.5)

Compost WF PHB Control MA2 ISO2

  116  

applying the Fickian diffusion onto mineralization, the mineralization diffusion

process can further be recognized and presented by its diffusion coefficient. However,

non-Fickian behavior, i.e., non-linear relation, could be found during the lag phase

and may also show in the alleviated biodegradation phase. By excluding the lag phase

in this study to understand the enzymatic diffusion behavior, the maximum

percentages of mineralization (Msat) were taken at the end of the experiment (day 54),

and the slopes of mineralization over square root of time were taken during the

biodegradation phase (days 7-21). The results are shown in Table 4.4; the PHB/WF

composite with MDI added had the lowest diffusion coefficient of 4.83E-5 mm2/hr0.5

overall. The diffusion coefficient is approximately 17% and 30% less than that of

MA2 and control PHB/WF composites, respectively. The reason for this difference

could be because the interfacial bonding was improved by the coupling agents and

thus resulted in the difficulty of the enzyme reaction with the PHB-coupled-WF.

Moreover, as mentioned previously, the 0.43% of grafting degree of PHB-g-MA had

limited function on coupling between the functional groups in wood and PHB.

Therefore, the MDI resulted in a significantly reduction of mineralization diffusion.

  117  

Table 4.4 Mineralization diffusion coefficients of different polymeric material and composites

PHB Control MA2 ISO2

Mineralization diffusion

Da (mm2/hr0.5)

5.17E-05 6.94E-05 5.80E-05 4.83E-05

4.3.3 Enzymatic kinetic

To determine the enzymatic kinetic, calculation of the data of mineralization from

the lag phase was used. In biology, an organism starts to expand its population during

the lag phase, and then followed an exponential growth. However, the growth will

meet a population limitation due to limited food sources as a close-to-no or plateau

growing. After it meets the plateau growing state, ideally, the rapidly constant

bioreaction (linear slope) occurs. The enzymatic kinetic of mineralization in this study

was calculated by using the Michaelis-Menten equation, applying the

Lineweaver-Burk plotting method to determine kinetic constants. The enzymatic

kinetic constant, Michaelis constant, Km, and theoretical maximum reaction velocity,

Vmax, were obtained after applying the Lineweaver-Burk plotting method (see in Fig.

4.4) with the calculated results from mineralization. The results shown in Table 4.5

indicate that the PHB/WF composite with MDI added had the lowest value of Km and

Vmax, which indicates that the composite had the lowest enzyme reaction and lowest

rate of CO2 production than the others. In other words, the enzyme reacted more

efficiently on PHB, control and MA2 composites than ISO2 composites by

  118  

comparison with the maximum reaction velocities. This result also corresponds with

the results found in the mineralization Fickian diffusion. Therefore, applying the

Michaelis-Menten equation to analyze the enzyme reaction kinetic during the

composting process can definitely help to fundamentally understand the process of

composting compostable biocomposites.

Figure 4.4 Lineweaver-Burk plot of rate of mineralization and substrate content

Table 4.5 Enzyme kinetic constants of different materials

Km Vmax (1/day)

Wood Flour 0.76 0.023

PHB 25.1 0.24

Control 31.4 0.22

MA2 35.7 0.24

ISO2 21.4 0.13

y = 32.806x + 43.162 y = 103.48x + 4.123 y = 143.48x + 4.5722 y = 150.54x + 4.2133 y = 166.73x + 7.7774

0

10

20

30

40

50

60

70

80

90

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

1/(m

iner

aliz

atio

n/da

y)

1/(carbon/ash)

WF PHB Control MA2 ISO2

  119  

4.3.4 Morphology

Studies indicated that the biodegradation process of biodegradable polymers is a

surface erosion process. By geometrically understand that the surface of the

biocomposites was first exposed with the enzyme and/or microorganisms, and further

result as a consumption process by the bioreactions. This surface erosion is seen as

layer-by-layer erosion process from surface to the core of the biocomposites.

Morphology is one way to determine and understand the surface erosion process by

enzymes on the PHB/WF composites. The results shown in Figures 4.5-A and -B

demonstrate that by comparing before and after composting on PHB samples,

evidence of the eroded surface was found on PHB after being composted. By

increasing magnitude, it appeared that the evidence of the partially eroded sphere of

PHB crystal remained on the sample surface (Figures 4.5-C and -D). This result

shows that the amorphous region of PHB was believed first biodegradated and eroded

by enzymes with our previous results - the degree of crystallinity of PHB was

increased after certain period of time composted. Possibly, a higher density of the

crystal region requires more time to biodegrade. The snow-flake like crystal PHB was

also observed on the sample surface (Figure 4.5-E). Upon observation of cross-section

of PHB (Figure 4.5-F), it is clear that erosion appears on the surface, but not on the

  120  

inner part of the PHB sample. This evidence further to ensures that the enzymatic

biodegradation reacts as a surface erosion process.

Figure 4.5 Morphology of PHB (A) and eroded and/or biodegradated surface of PHB (B – F)

With the observation of PHB/WF composites, evidence of surface erosion is

obviously seen (shown in Figure 4.6). Interestingly, microorganism hypha was also

clear as seen during the morphology observation. Moreover, wood flour was exposed

after the surface of PHB was biodegraded and eroded (arrowed in Figure 4.6-C, -D,

and -E). The surface eroded PHB was also found next to and/or behind wood flour.

The wood flour had no significant biodegradation and/or surface erosion, but it

appeared that wood flour was broken down to finer particles (as seen in Figure 4.6-E

  121  

and -F). This result confirms our earlier statement: wood required more time or was

more difficult to fully biodegrade. Furthermore, after increasing the magnitude in

microscope observation (the results shown in Figures 4.6 and 4.7), evidence of hypha

of microorganism and partially eroded (biodegraded) surface of PHB appears, which

is located underneath of hypha and wood flour (see Figures 4.7-B and -C).

Additionally, it appears that wood flour, here, as an obstacle, detracts hypha from

reaching the inner PHB for further degradation. By connecting this observation and

the results of mineralization, a slow-down phase after biodegradation can be

explained. To conclude this morphology evidence, a surface erosion mechanism can

be sketched out and will be discussed in the next section.

Figure 4.6 Morphology of a PHB/WF composite (A) and eroded and/or biodegradated surfaces of PHB/WF composites (B – F)

  122  

Figure 4.7 Morphology of crystal region of PHB (A) and evidence of hypha (B and C) on the surface of PHB/WF composite

4.3.5 Enzymatic surface erosion mechanism

By combining the results of mineralization and discovery of morphology, the lag,

biodegradation, and slow-down phases can be explained by fundamentally and

theoretically understanding. The proposed schematic sketch of the relation of erosion

mechanism and mineralization is presented in Scheme 4.1 (lag, biodegradation, and

slow-down phase). This presentation is during the lag phase, and the microorganisms

have not been attached or are only partially attached on the surface of the PHB/WF

composite, and in this phase, only some bioreactions occurred. In the biodegradation

phase, the microorganisms are exponentially grown, resulting in rapid bioreactions

and constantly producing carbon dioxide. As to the morphology, the PHB-rich skin

was consumed and the hypha of the microorganisms reached the entire surface and/or

eroded surface (inner) of the composites. At the transition between the biodegradation

phase and a consequent slow-down phase, most of the PHB on the surface was

  123  

consumed and the hypha reached partially decomposed (or broke down) the wood

flour. The wood flour became an obstacle to hypha reaching the PHB inside of wood

lumen or below wood flour. Therefore, a slow-down phase was observed.

Scheme 4.1 Proposed mineralization process and surface erosion mechanism on

PHB/WF composites

  124  

4.4. Conclusion

In this study, three different biodegradation phases during composting were

observed from the results of theoretically calculated carbon dioxide evolution and

mineralization. The phases are lag (day 0 to 5), biodegradation (day 7 to 21), and

slow-down phase (day 21 to 54). The lag phase can also be seen as an adjusting phase

for microorganisms in the compost inoculums. The biodegradation phase is seen as

rapidly bioreaction phased, and the slow-down phase follows after the biodegradation

phase. In comparison with coupling agent effects, MDI resulted in an outstanding

anti-enzymatic reaction, which is implied from lowest rate of carbon dioxide

evolution and lowest rate of mineralization (ca. 12% less rate of CO2 evolution and

rate of mineralization than the PHB/WF composite). In the end of the experiment, the

PHB/WF composite with MDI added resulted in that approximately 35% less material

was mineralized than in the control PHB/WF composites. Moreover, using the

coupling agent of PHB-g-MA resulted in some level of influence on compostability of

the PHB/WF composite, but it was not as significant as MDI. It is believed that the

low grafting ratio of MA was the limitation. During composting, one limiting factor

for microorganisms operating the biodegradation process is moisture, which becomes

a key of operating biodegradation. After fitting the results of mineralization vs. time

in the Fickian diffusion equation, the mineralization diffusion behaviors of the

  125  

PHB/WF composites were numerically quantified. The results of mineralization of

Fickian diffusion on the PHB/WF composites of the control, MA2 and ISO2 were

6.94, 5.80, and 4.83 x 10-5 mm2/hr0.5, respectively. The PHB/WF composite with MDI

added resulted approximately 17% and 30% less than that of MA2 and control

PHB/WF composites, respectively. By applying the Michaelis-Menten equation to

determine the fundamental enzyme kinetic and understanding, the results correspond

with the results found in the mineralization Fickian diffusion: the lowest enzyme

reaction occurred in the PHB/WF composite, with MDI added as compared with the

control and MA2. In the morphology studies, evidence of surface erosion of the PHB

and the PHB/WF composites was found. The sphere crystal structure remained on the

surface of the PHB as well as on some the PHB/WF composites. Additionally, the

hypha of microorganisms and broken wood flour remained on the surface of the

composites. This result indicates and proves that wood flour requires more time to

degrade. Notably, the wood flour was believed to block hypha from reaching the inner

PHB and caused the retardant of biodegradation, which shows in the slow-down

phase. To summarize and reinforce the investigated results in this study, not only

were the Fickian diffusion and the Michealis-Menten kinetic successfully applied to

fundamental understand biodegradation during composting, but also the relation of

surface erosion and mineralization was pointed out by using a schematic sketch which

  126  

provides a state-of-the-art view of mineralization mechanism on the PHB/WF

composites. Lastly, this mechanism could potentially provide valuable information on

the biodegradation of different natural fiber reinforced biocomposites.

4.5. Acknowledgements

The authors acknowledge to the U.S Department of Energy for supporting this

study by contract NO. DE-AC07-051D14517, and also thank the composting facility

at Washington State University which provided compost materials.

  127  

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  131  

Chapter 5 Developing PHB/WF/Cell debris Composites: Extrusion

Abstract

Wood plastic composite (WPC) has been promoted as a good example of

engineerable and sustainable materials. To move toward a sustainable society, it is

necessary to use bio-based polymer, i.e. poly-3-hydroxybutyrate (PHB), to produce

WPCs. Production of pure PHB from PHB-laden sterilized bacteria cells requires

solvent-based extraction and purification, both of which have raised concerns of

substantial environmental impacts and energy consumption. One solution for reducing

these impacts is to directly process PHB-laden cells as components in a polymeric

composite. This study investigated using cell debris as a reinforcement to replace

wood flour in order to produce a wood bio-plastic composite. Purified PHB and

PHB-free cell debris were used to simulate PHB-laden microbes in extruding

PHB/wood flour (WF)/cell debris ternary composites (PWCCs) with various wood

flour/cell debris ratios. Torque rheometry was studied to understand the temperature

and viscosity relation, and the results provided information to design temperature

profiles for the extrusion process of different formulations. A commercial formulation

of WPC, high density polyethylene (HDPE)/WF composite, was also extruded as a

comparison. Polymeric methylene-diphnenyl-diisocynate (pMDI) was used as a

  132  

coupling agent to increase the interfacial bonding between the PHB matrix and the

hydrophilic fillers. Mechanical properties and water adsorption of the composites

were investigated in accordance with ASTM standards. The mechanical properties

and moisture resistance of the PWCCs were decreased with the increasing content of

the cell debris. As shown in a morphology study, the low aspect ratio of cell debris

and poor interfacial bonding with PHB were possible reasons causing the

deterioration of performance of PWCCs. However, the performance of the PWCC

formulation of cell debris/wood flour ratio of 3:2 showed that mechanical properties

and water resistance were comparable to a commercial WPC. Moisture diffusion and

activation energy were also evaluated. As a result of this investigation, PWCCs could

potentially replace some of the petroleum derived WPCs in current markets.

Key Words: sustainability, poly-3-hydroxybutyrate (PHB), wood plastic composite

(WPC), torque rheometry, moisture diffusion, flexure properties.

  133  

5.1 Introduction

In recent years, there has been a movement toward the use of sustainable

materials and reducing environmental impacts. Natural resources such as natural fiber

and wood flour are the most common candidates for use as reinforcements to form

polymeric composites because of their low cost coupled with higher specific

performance. For that reason, wood plastic composite (WPC) has been promoted as a

good example of durable, engineerable, and sustainable materials [1]. However, to

lessen dependence on crude oil, WPCs have been investigated by using bio-based

polymers as a matrix. One family group of bio-based polymer, polyhydroxyalkanoates

(PHAs), has been viewed as an alternative to achieve this goal because it has widely

different properties, from stiff to rubbery, or from thermoplastic to elastomer [2] [3].

Many previous studies [4][3][5][6] indicated that in the polymer first discovered in

1925, polyhydroxybutyrate (PHB), French scientist Maurice Lemoigne found that

PHB was an energy reserve and is made up of many different bacteria with

carbohydrate substrate feeding. PHB is a highly crystalline substance with a melting

point of ca. 175oC; its mechanical properties are comparable to those of

polypropylene [4][6]. PHB possesses a similar processability to many commodity

polymers with no issue in using conventional polymer processing techniques and

  134  

equipment. Moreover, PHB [3][5][6][7] is biodegradable and compostable, which

means it can be mineralized to form CO2 with the assistance of microorganisms.

However, the major hurdle preventing PHB use in WPCs is the cost; the cost of

PHB is much higher than that of commodity plastics. A cradle-to-grave analysis [8]

showed that to produce 1kg of PHB from corn would require 65% more fossil fuel to

generate the same quantity of petroleum-based polystyrene. This result indicates that

the methodology of conventional PHA production exhibits a high demand for crude

oil required, along with generating more carbon emissions. On the other hand, if one

considers the costs and procedures from the process of recycling petrochemical

plastics, the overall life cycle costs of PHAs could be more comparable with those of

petrochemical plastics. To solve the energy consumption, one way is to find an

alternative carbohydrate source to produce PHAs because converting from corn to

glucose requires 8129 KJ per kg, and ca. 3.3 kg of glucose is needed to generate 1kg

of PHAs during bacterial fermentation [8]. Moreover, to obtain high purification of

PHAs, the processes of extraction and purification, which are organic solvent and

energy intensive, are needed [3][6]. With the environmental impact of using

crop-based feedstock and the purification process, it is challenging for PHAs to be

represented as a sustainable commodity.

  135  

Recent research [9] has indicated successful utilization of a waste stream that

contained a certain amount of carbohydrate sources from municipal waste water to

generate PHAs. It can be imagined that waste water from forestry, food, and

agriculture industries could be utilized as potential sources. This strategy of utilizing

low cost feed sources from waste water is further seen as a solution to sustainably

produce PHAs while reducing waste water impacts. Furthermore, PHA-based WPCs

would be more economic if the PHA-laden bacterial cells are used directly to produce

the WPCs [10]. With this innovative production process, the PHA extraction and

purification steps are eliminated. To summarize, using municipal wastewater as

feedstock to generate PHAs and directly processing PHA-laden bacterial cells to form

WPCs would bring about significant energy and environmental benefits with ca.

30~40% reduction of total product costs [10].

Although the previous studies [9][10] demonstrated that a PHB-rich cell mass

could be directly used to prepare composites by melt blending and pressure molding,

the study lacks investigation into the microstructure and interface of the composites,

which often critically determines composite properties. The impact of cell mass

content on mechanical properties and water resistance of the composites and the

reasons behind the impact are also not fully identified. Water resistance is an

important property for WPCs due to the environment, to which most WPCs are

  136  

exposed in applications. Current WPC markets are primarily comprised of extruded

WPC products such as deck floors, sidings, railings, etc. The packing pressure

provided by a typical extrusion process is much lower than that in an injection

molding process. Torque rheometry is further needed to study the relation between

temperature and viscosity, which could help to design a suitable process temperature

to increase packing pressure during extrusion processes. In a previous study,

Anderson [11] investigated a coupling agent, polymeric methylene diphenyl

diisocyanates (pMDI), which has shown outstanding performance in enhancing the

interfacial bonding between hydrophobic PHB and hydrophilic wood flour. Bacterial

cell debris has also been classified as a hydrophilic material. By adding the coupling

agent, the performance of WPC, which is mainly integrated with PHA-laden cell

debris and wood flour, can be expected to be improved.

The specific objectives of this study are: 1) to investigate the extrusion

processability of PHB/wood/cell debris composites (PWCCs) by using torque

rheometry; 2) to investigate the cell debris content effect on mechanical properties

and moisture resistance with a comparison of a commercial WPC; and 3) to study the

morphology of microstructure and interface of the PWCCs.

  137  

5.2 Materials & Methods

5.2.1 Materials

Purified PHB powder (Tianan Biologic Material Co., Ltd, Ningbo, China), waste

effluents cell debris (provided by UC-Davis; ca. 88% of particle dimension were less

than 250µm), and wood flour (Pinus,  60-mesh, American Wood Fibers, Schofield, WI)

were the three main components of the PWCCS. Boron nitride (BN) (Carbotherm

PCTF5, Saint Gobain Advanced Ceramics Co., Amherst, NY) was used as a

crystallization agent to promote PHB crystallization. Glycolube WP2200 (Lonza Inc.,

Allendale, NJ) was included as a lubricant to improve processability. Talc (Nicron

403) was provided by Riotinto of Centennial and used for processability and water

resistance improvements. Liquid-form pMDI (Mondur 541, Bayer MaterialScience,

Pittsburgh, PA) contained 31.5 mass % NCO and functioned as the coupling agent

between hydrophobic PHB and hydrophilic wood flour and cell debris. In addition, a

commercial WPC was used as a comparison. The raw materials of the commercial

WPC were high density polyethylene (HDPE, Petrothene® LB010000 from Equistar),

zinc stearate (DLG20, Chemical Distributors, Inc.), and ethylene bis straramide (EBS)

wax (General Electric Specialty Chemicals)

  138  

5.2.2 Methods

5.2.2.1 Particle size distribution analysis of ground cell debris

A hummer mill with a screen size of 0.0312 inches was used to grind the dried

cell debris. The cell debris was ground with the condition of as-received. Particle size

distribution analysis of the ground cell debris was conducted by using a Ro-Tap sieve

analyzer with a series of stacked screens, with the screen size ranging from 20 to

120-mesh (top to bottom). The instrument was allowed to run for about 10 minutes to

separate the cell debris. The particle size distribution of the ground cell debris is

shown in Table 5.1. About 88 wt. % of the cell debris particles were less than

diameter of 250 µm (60-mesh).

Table 5.1 Particle size distribution of cell debris after hammer milling

5.2.2.2 Torque rheometry

Mixing torque rheometry of different formulations was evaluated using a Haake

Rheomix 600p equipped with a 69 ml mixing chamber. 70 vol% of the chamber was

filled with each formulation. The materials were then mixed by a pair of roller blades

Particle

size (µm) <125µm

125~150

µm

150~177

µm

177~250

µm

250~420

µm

420~833

µm

> 833

µm

Weight

percentage

(%)

59.1% 9.3% 7.7% 11.7% 8.9% 2.8% 0.4%

  139  

at 20 rpm under different temperatures for 10 minutes. The torque was continuously

monitored during the mixing process. The torque values obtained under different

mixing conditions were used to optimize the conditions of the extrusion process. The

formulations used for torque rheometry are listed in Table 5.2 without contents of

lubricants, talc, and nucleation agent, since the contents of these chemicals did not

markedly alter the viscosity of the formulations.

Table 5.2 Formulations of PHB/WF control and PHB/WF/Cell debris mixtures for torque rheometry study

Formulation

 

Components

PHB/WF

(PW)

PHB/WF/

20%

Cell debris

(PWC20)

PHB/WF/

40%

Cell debris

(PWC40)

PHB/WF/

60%

Cell debris

(PWC60)

PHB/WF/

80%

Cell debris

(PWC80)

PHB/Cell

debris

(PC)

Cell debris* 0 11.4 22.8 34.2 45.6 57

WF (pine)* 57 45.6 34.2 22.8 11.4 0

PHB 35 35 35 35 35 35

pMDI 4 4 4 4 4 4

*: 60-mesh particle size. WF: Wood flour. All units are in parts.

5.2.2.3 Preparation of composites

60-mech of pine wood flour was dried by a rotary steam tube drier to a moisture

content of about 3%. Ground cell debris powder was dried at 100oC in a convection

oven for 24 hours. Purified PHB, BN, WP2200, and talc were used as-received. PMDI

  140  

was first well dispersed in PHB powder (manually mixed until not a viable

agglomerate), and then blended with all the other components in a drum blender for 5

minutes. Six formulations of PWCCs and one commercial WPC were prepared with

the formulation shown in Table 5.3. Melt compounding was accomplished using a

conical co-rotating twin screw extruder (Cinicinnati Milicron CM 35) with a screw

diameter of 35 mm and a length/diameter (L/D) ratio of 22. To improve melt strength

and reduce material thermal degradation, a declining temperature profile of the

extruder was applied during the extrusion process [12]. The temperature profiles for

all formulations, which had been optimized based on the torque data of torque

rheometry, are shown in Table 5.4. The screw speed was maintained at 20 rpm.

Extrusion residence time was ca. 1 minute. Each formulation was extruded through a

slit die (cross-section 38 X 9.8 mm) and annealed with spray water upon exiting.

Melting pressure was monitored as an indicator of the melt viscosity of the

composites before the materials entered the first die zone.

  141  

Table 5.3 Formulations of HDPE/WF, PHB/WF control, and PHB/WF/cell debris composites

Formulation

Components

HDPE/

WF

(HW)

PHB/

WF

(PW)

PHB/WF/

20%

Cell debris

(PWC 20)

PHB/WF/

40%

Cell debris

(PWC 40)

PHB/WF/

60%

Cell debris

(PWC 60)

PHB/WF/

80%

Cell debris

(PWC 80)

PHB/

Cell debris

(PC)

Cell debris* 0 0 11.4 22.8 34.2 45.6 57 WF (pine)* 58 57 45.6 34.2 22.8 11.4 0 Polymer 32 35 35 35 35 35 35 Talc 7 8 8 8 8 8 8 pMDI 0 4 4 4 4 4 4 Lubricant 2 3 3 3 3 3 3 Wax (EBS) 1 0 0 0 0 0 0 Nucleating agent

(Boron nitride) 0 0.2 0.2 0.2 0.2 0.2 0.2

*: 60 mesh particle size. WF: Wood Flour. Lubricant: Zinc stearate for HW; WP2200 for all others. All units are in parts.

Table 5.4 Temperature profiles used for extrusion of the seven formulations

oC BZ1 BZ2 BZ3 Screw DZ1 DZ2 HW 163 163 163 163 170 170 PW 170 175 165 163 163 160 PWC 20 170 175 165 163 163 160 PWC 40 170 170 165 160 160 160 PWC 60 165 170 165 155 155 155 PWC 80 160 165 160 150 150 150 PC 160 160 155 150 150 150

BZ: barrel zone DZ: die zone

  142  

5.2.2.4 Preparation of testing specimens

The extrudates were cut into 205mm long sections for flexural testing. The

length was 20 times of the sample thickness (9.8 mm) in accordance with ASTM

Standard D 790 requirements. Impact test specimens were prepared following ASTM

Standard D256. The dimensions of the specimens were 12 X 9.8 X 60 mm and the

specimens were notched by a XQZ-I specimen Notch Cutter. Specimens for water

absorption were surfaced by a planer and machined into 11 X 8 X 100 mm.

5.2.2.5 Morphology

Morphology of wood flour and cell debris particles was individually employed

by using scanning electronic microscope (SEM), and as well as interfacial bonding of

particle-polymer. The extruded composite samples were first cut into a small

specimen, and further cryo-microtomed to obtain a non-plastic deformation on the

sectioned surface. The samples were oven dried at 100oC for 24 hours to remove

moisture, which may influence the quality of gold coating. After being cooled down

in a dessicator at room temperature at least 2 hours, all of the dried samples were

mounted on aluminum stubs and gold-coated by a sputter coater. The coated samples

are further observed by using SEM (FEI 200F).

  143  

5.2.2.6 Mechanical properties and sample density

A screw driven Instron 4466 equipped with a 10-KN load cell and a three point

bending test fixture was used for flexural tests. The flexural tests were performed with

a crosshead speed of 3.80 mm/min. Modulus of rupture and modulus of elasticity

were calculated in accordance with ASTM D790. Impact tests were conducted using a

Dynisco BPI tester. ASTM Standards D256 (mode A) and D790 were followed in the

impact and flexural tests, respectively. Statistically, five replicates were tested for

each formulation to obtain a mean value with a standard deviation. Sample density

was calculated by dividing sample mass by sample volume. All of the samples (for

both mechanical and density testing) were conditioned at 21oC and 50% relative

humidity (RH) for 7 days prior to the tests.

5.2.2.7 Moisture resistance

Water absorption of the composites was performed following ASTM D 570.

Samples were immersed in distilled water at room temperature (controlled at 21oC).

The weight and thickness of the samples were measured after different periods of

immersion time. Moisture content (MC) and thickness swelling (TS) were calculated

by the following equations:

MC   % =   !!!!! ×!""!!

(eq. 5.1)

  144  

TS   % =   !!!!! ×!""!!

(eq. 5.2)

where Mt is the mass of the specimens at time t, and M0 is the initial mass of the

specimens; Tt is the thickness of the specimens at time t, and T0 is the initial thickness

of the specimens.

5.3 Results and Discussion

5.3.1 Torque rheometry

In polymer physics, temperature and viscosity of polymer melt are strongly

related, which means that, generally, higher temperature results in a lower viscosity of

polymer melt, non-Newtonian fluid [13]. This can be explained by using the Power

Law model.

η γ = !γ!!! (eq. 5.3)

where η is viscosity, m is a sensitive function of temperature, n is a parameter, γ is

shear rate.

Torque rheometry was used to optimize extrusion temperature profiles of

different formulations. In a study of microscope morphology (see Figures 5.4 and 5.5),

as shown, cell debris had a low aspect ratio (ca. 1.0) and poor interaction with the

PHB matrix. This result could be because the ground cell debris particle had a low

aspect ratio, which did not support an increase in the viscosity of molten mixture, like

  145  

fiber does [14]. It was therefore expected that a decreased viscosity of the PHB/wood

flour/cell debris (PWC) melt could be observed with increasing content of cell debris.

This result led to a lack of high die pressure, which was essential for the packing of

the extrudates during extrusion processes. Insufficient packing of the extrudates

resulted in void-laden low-density products.

In this study, the control formulation (PW) was first melt mixed in the torque

rheometer at different temperatures (160, 165, 170, 175, and 180oC) with a constant

rotation rate of 20 rpm. After 8 minutes of mixing in the rheometer, the control

formulation showed similar torque value of ca. 2.5 Nm when the temperature was set

at 170, 175, and 180 oC (Figure 5.1). At 160 oC, PHB did not melt at all after 8

minutes of mixing because the melting point of the PHB is ca. 165 oC. The mixture

was still in dry powder form with a low torque value. The set of the roller in the

torque rheometer did not create a high shear, but in a twin screw extrusion process,

extra heat can be generated by sharing, and further benefit the melting. At 165 oC, the

molten mixture of polymer and wood flour was found after mixing. Under this

condition, the result showed that it had torque value of about 3.5 Nm after an 8 minute

mix. This result can indicate that the viscosity was slightly higher at a temperature of

165 oC than at 170, 175, and 180oC.

  146  

Figure 5.1 Torque rheometry behavior at different temperatures on the control formulation (PW)

Next, all six formulations were examined to determine their viscosities at 165 oC

by using a torque rheometer. PHB in all of the formulations was well molten and

wood flour was completely wetted with the molten polymer after six minutes of

mixing at this temperature. However, as shown in Figure 5.2, at the 8th minute, the

formulations with high cell debris contents (e.g., PWC60, PWC80, and PC) exhibited

a lower torque (ca. 2 Nm) than the other three formulations (ca. 3.5Nm). This result

was believed to be due to the fact that the low aspect ratio of the cell debris and poor

interaction with the polymer matrix did not contributed to increased viscosity of the

melt mixture. The materials after the mixing also appeared loose for formulations

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8

Torq

ue(N

m)

Time(min)

160C 165C 170C 175C 180C

  147  

PWC80 and PC. To increase the torques for the three low-torque formulations, the

mixing temperature was further decreased and set at 160 oC.

Figure 5.2 Torque rheometry behavior of the six formulations of PWCCs at 165 oC

Figure 5.3 Torque rheometry behavior of six different formulations of PWCCs with processing temperature of 160 and 165 oC

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8

Torq

ue(N

m)

Time(min)

PW

PWC20 PWC40 PWC60

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8

Torq

ue(N

m)

Time(min)

165C PW 165C PWC20 165C PWC40 160C PWC60 160C PWC80 160C PC

  148  

By using this strategy of reducing the mixing temperature, the formulation of

PWC60, PWC80, and PC resulted in an increased torque value of ca. 3.0 Nm at the 6th

minute by comparing with those of the other three formulations (PW, PWC20, and

PWC40) at 165 oC (Figure 5.3). Notably, the formulation of PC exhibited a decreased

torque after 6.5 minutes mixing; this result could possibly be due to the thermal

degradation on cell debris. Moreover, the mixtures after the mixing were as dense as

the other formulations at 165 oC. Therefore, this result could still be due to using 160

oC as the extrusion temperature for the three high cell debris content formulations

(PWC60, PWC80, and PC), and 165 oC for the other three formulations with low cell

debris content. The extrusion temperature profiles for all of the formulations were

further designed based on this investigation and are shown in Table 5.4.

5.3.2 Morphology

As shown in the depiction of microstructure in Figure 5.4, cell debris had an

aspect ratio of ca. 1 with irregular shape; 60-mesh pine wood flour had a fiber shape

to bundle (aspect ratio of ca. 5). The cell debris displayed wide particle size

distribution with a small amount of large particles measuring ca. 300 microns (Table

5.1). Possibly, by intensive shear during extrusion, some wood fiber bundles may be

broken, and further increase the L/D ratios and enhance their reinforcing effect. In

  149  

observation of the microtomed surface of PWCC, the cell debris had a poor interfacial

bonding, which evidenced a gap between cell debris and PHB matrix (Figure 5.5).

Cell debris is a complicated system, comprising mainly protein, polysaccharides,

triglycerides, and inorganic impurities. Pretreatment of cell debris during its

production, impurities of the cell debris, and degradation of cell debris during thermal

processing might be related to limitation of the access of pMDI to the reactive sites of

the cell debris. This finding therefore reduced compatibilization effects and eventually

resulted in poor interfacial bonding between the cell debris and the PHB matrix.

Remarkably, PHB polymer penetrated into wood lumen (Figure 5.5), creating a

mechanical interlock. This result is believed to cause by the high packing pressure

which benefited from the way in which polymer melt combined into the wood lumen.

The high interfacial adhesion between the PHB matrix and wood fiber had been

studied and it was found that the coupling agent, pMDI, is highly reactive with both

hydroxyl and carboxyl groups, and is able to chemically link the two phases [11]. This

result assumes that the high content of cell debris in the PWCCs had declined

mechanical properties caused by the poor interfacial bonding between cell debris and

PHB.

  150  

 Figure 5.4. SEM micrographs of cell debris (left) and pine wood flour (right)

Figure 5.5 Interfacial bonding of wood flour (a) and cell debris (b) with PHB matrix on microtomed surface of PWCC

5.3.3 Density and mechanical properties

The densities of the six extruded PHB formulations and one HDPE/WD

composite are shown in Table 5.5. The values ranged from 1.27 to 1.35 g/cm3, which

reflect that by increasing die pressure for high cell debris content formulations (see in

Table 5.6), the composites were more densely packed and their densities were similar

to the formulations containing less cell debris. The commercial formulation (HW) had

 

 

   

 

 

  151  

the lowest density among all the samples. This was because the density of HDPE (ca.

0.9 g/cm3) was substantially lower than the density of PHB (ca. 1.2 g/cm3).

Table 5.5 Densities of HDPE/wood flour, PHB/WF control and PHB/wood flour/cell debris composites HW PW PWC20 PWC40 PWC60 PWC80 PC Density (g/cm3)

1.18 1.31 1.33 1.32 1.27 1.34 1.35

*: standard deviation less than 0.01 Table 5.6 Actual output date of extrusion processing of the different composites Formulations Die Temperature (oC) Die Pressure (psi) Ampere (A)

HW 170 536 5 PW 163 910 7

PWC 20 166 842 6 PWC 40 164 618 5 PWC 60 160 530 5 PWC 80 156 850 5

PC 153 921 5

Coats [10] pointed out that after being statistical analyzed, one-way analysis of

variance (ANOVA), the mechanical properties of PHB-rich biomass composites

significantly corresponded with density, especially on MOE. In order to

independently consider the effects of cell debris content by using specific MOE,

MOR, and impact strength for the comparison, the error could be limited and a proper

comparison on cell debris effect on mechanical properties could be obtained. After

plotting the cell debris content against mechanical properties, as shown in Figures 5.6,

  152  

5.7, and 5.8, the higher content of cell debris, the lower the specific flexural properties

(moduli) and specific impact strength of the PWCCs. The properties reductions could

be caused by  the low aspect ratio of the cell debris particles [10] and the fact that its

poor interfacial bonding (see Figure 5.5) with the PHB matrix were attributed to this

property deterioration. By plotting with a regression curve on each result (Figure 5.6,

5.7, and 5.8), the relation of the cell debris with each mechanical property can be

displaced by an equation. These equations can further provide for designing product

performance. In the comparison with HW formulation, a commercial WPC

formulation, the ternary PHB composites of containing high cell debris contents

(PWC60 and PWC80) exhibited comparable flexure properties with those of HW

(specific MOR: 21.93±0.71 MPa and specific MOE: 3.22±0.11 GPa). However, the

specific impact strengths of all PWCCs are lower than that of HW (2.17±0.18 KJ/m2).

Considering the PHB yielding rate to integrating PHB-rich biomass to produce

PWCCs, the PHB concentration of the cell debris needs to be contained more than

50% (e.g. PWC60; Table 5.3) to obtain comparable flexure properties (specific MOR:

18.97±0.54 MPa and specific MOE: 3.18±0.09 GPa) with HW. On the other hand, the

more PHB yield in the cell, the better the performance obtained. This result was also

addressed in a previous investigative research [10].

  153  

Figure 5.6 Plot of the relation of cell debris content and specific MOR of the PWCCs

Figure 5.7 Plot of the relation of cell debris content and specific MOE of the PWCCs

y = 155.83x2 - 154.36x + 52.612 R² = 0.97497

0.0

10.0

20.0

30.0

40.0

50.0

60.0

0% 10% 20% 30% 40% 50% 60%

Spec

ific

MO

R (M

Pa)

Cell debris content

y = 5.4155x2 - 7.1511x + 5.1717 R² = 0.96065

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0% 10% 20% 30% 40% 50% 60%

Spec

ific

MO

E (G

Pa)

Cell debris content

  154  

Figure 5.8 Plot of the relation of cell debris content and specific impact strength of the PWCCs

5.3.4 Moisture behavior

Moisture has been seen as a key of deterioration or degradation on properties of

waterfront facilities, which is becoming an enormously increasing expense for

maintenance and materials [15]. To understand the moisture diffusion of WPCs, it is

necessary to understand material development. The seven formulation composites

were immersed for an 8-week period of time; the result of water absorption and

thickness swelling is shown in Figures 5.9 and 5.10. The commercial formulation HW

showed the highest moisture saturation content (Msat, ca. 22%) after 8 weeks

y = 5.4029x2 - 5.8133x + 2.459 R² = 0.95842

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0% 10% 20% 30% 40% 50% 60%

Spci

fic im

pact

sten

gth

(KJ/

m2 )

Cell debris content

  155  

immersion, but it did not exhibit the highest thickness change. The PHB composites

with a high content of cell debris, i.e. PC and PWC 80, showed a larger thickness

change. A very slow rate of moisture absorption occurred on the PW and PWC20

composites. These results indicated that with pMDI to improve interfacial bonding

and suppress the hydrophilicity of wood flour and cell debris, the maximum moisture

absorption at a time of 8 weeks for all of the PHB composites was lower than for the

commercial WPC. For the PC and PWC80 composites, it exhibited a non-Fickian

behavior after ca. 1000 hours of immersion. This result could possibly be explained

by the fact that water soluble material was partially discharged from the material

during the long term immersion [15]. In the dimensional stability, the larger thickness

change of the composites of PC and PWC 80 might be due to the large expansion of

the cell debris or poor interfacial bonding between cell debris and PHB matrix.

Overall, the composite of PWC60 exhibited better moisture resistances than that of

HW composite.

  156  

Figure 5.9 Plot of moisture absorption versus square root of time of seven composites

Figure 5.10 Plot of thickness change versus square root of time of seven composites

0.0

5.0

10.0

15.0

20.0

25.0

0 200 400 600 800 1000 1200 1400

Moi

stur

e ab

sorp

tion

(%)

Time (S0.5)

PW PWC20 PWC40 PWC60 PWC80 PC HW

0.0

5.0

10.0

15.0

20.0

25.0

0 200 400 600 800 1000 1200 1400

Thi

ckne

ss C

hang

e (%

)

Time (S0.5)

PW PWC20 PWC40 PWC60 PWC80 PC HW

  157  

Liquid or gas transportation through a solid material, i.e. WPCs, is often modeled

by Fickian diffusion equation [15][16]. In Fickian diffusion, plotting the amount of

uptakes (moisture in this case) at a given time period against the square root of the

time period produces a linear region followed by a non-linear approach to Msat. The

apparent diffusion constant (DA) can be calculated by:

!! =  ! !!!!"#

! !!!! !

!       (eq. 5.4)  

where h is the thickness of the specimens and dMt/d√t is the slope of the weight gain

versus square root of time. Considering a three-dimensional diffusion, a geometric

edge correction factor can be used to calculate the true diffusion constant (D) [15]:

! =   !!!!! !!

! !! (eq. 5.5)

where l and w are the length and width of the specimens, respectively. The higher true

diffusion constant, the faster moisture, could penetrate through the thickness.

The degree of sample swelling was defined by a swelling coefficient (β) [15],

β =   !"!"#!"!"#

(eq. 5.6)

where TSmax and MCmax are represented as maximum thickness swelling and

maximum moisture absorption, respectively.

As shown in Table 5.7, formulation of PC had the highest true diffusion constant

(1.04E-05 mm2/sec) among the seven formulations, indicating that this composite had

  158  

the highest moisture penetrating rate or the poorest water resistance. To explain this

result geometrically, the gap between cell debris and PHB matrix led to the moisture

penetrated by capillarity into the composites, which increased the rate of diffusion.

Moreover, it was found that the higher content of cell debris resulted in a higher

swelling coefficient of the composites. The internal structure of the composites is also

the main factor affecting the degree of swelling. Chowdhury and Wolcott [17]

indicated that by using a coupling agent, the TSmax and MCmax could be limited

because the strong internal bonding improved dimensional stability. Since the fixed

amount of coupling agent was used on the PWCCs with the result shown in Table 5.7,

the more cell debris that were added, the more moisture was absorbed. This is

believed to be due to the high hydrophilicity (the PC had ca. 7 times faster diffusion

constant than that of the PW) and larger amount of moisture absorption of the cell

debris than wood flour, in the comparison of maximum moisture uptakes of PW and

PC (15.7% vs. 17.9%). Based on the equation of the swelling coefficient, it also

proved that the dimension of high concentration of cell debris was due to the fact that

an unstable or weak interfacial bonding led to the large amount of thickness swelling.

  159  

Table 5.7 Comparison of moisture diffusion parameters of extruded PHB composites and commercial HDPE composite

HW PW PWC20 PWC40 PWC60 PWC80 PC slope* 1.49E-04 6.63E-05 7.21E-05 1.04E-04 1.49E-04 1.63E-04 2.16E-04 MCmax 22.0% 15.7% 15.1% 15.1% 16.3% 15.9% 17.9% TSmax 11.9% 7.9% 9.5% 10.0 % 9.3% 14.8% 23.6% D (mm2/s) 2.16E-06 1.25E-06 1.34E-06 3.34E-06 5.32E-06 8.25E-06 1.04E-05 β 0.541 0.502 0.632 0.660 0.572 0.929 1.316

*: dM!/d t, indication of water absorption rate.

It is possible to express the relation between content of cell debris and true

diffusion constant by an Arrhenius-like relation:

D! = D ∙ exp!!!!∙! (eq. 5.7)

where D is the true diffusion constant of the PW composite, D’ is the true diffusion of

different cell debris content of PWCCs, R is the universal gas constant (8.314

J/K/mol.), T is the temperature of 294oK , Ea is the activation energy (J/mol.). To

obtain the Ea, the equation of 5.7 can also be presented as:

lnD! =    !!!!∙ !!+ lnD (eq. 5.8)

The relation could be obtained by plotting cell debris against true diffusion constant,

resulting in Figure 5.11. By using the regression curve, the activation energy of

moisture diffusion on increasing cell debris content in the PWCCs can be calculated

and obtained as the value of -0.0489 J/mol, which means that by increasing cell debris

content, less energy is required for moisture penetration.

  160  

Figure 5.11 Plot of true moisture diffusion constant versus cell debris content

5.4 Conclusions

The results in this study investigated and demonstrated that using cell debris and

purified PHB to simulate and extrude PHB/wood flour/cell debris products can be

utilized. The results of torque rheometry showed not only an understanding of the

relation of temperature and viscosity, but notably, that it can be successfully used to

design extrusion temperature profiles for formulations with different cell debris

contents. A suitable temperature profile was set for each formulation. A consistent

packing pressure in the die zone occurred in different PWCCs. All of the sample

products extruded at high die pressure were without defects, even at high cell debris

contents. The properties resulted in increasing cell debris content with decreased

y = 2E-05x + 8E-08 R² = 0.95559

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

0 0.1 0.2 0.3 0.4 0.5 0.6

True

diff

usio

n co

nsta

nt (m

m2 /s

ec)

Cell debris content (%)

  161  

properties of the composites, i.e. the mechanical properties and water resistance. The

PHB/wood flour/cell debris composites with a cell debris/wood flour ratio of 3:2

showed mechanical properties and water resistance similar to commercial HDPE

WPC. This result indicates that the more PHB yielded in bacteria cell, the better

performance on the PWCCs with a low wood flour content. In the results of moisture

Fickian diffusion, it appeared that the cell debris was believed to cause the

deterioration of moisture resistance. In conclusion, the cell debris laden biodegradable

PHB/wood flour composites were successfully developed and investigated. In some

of investigated formulations, further potential to replace some of current fossil oil

derived WPCs could be seen.

5.5 Acknowledgements

The authors gratefully acknowledge the financial support provided by the U.S.

Department of Energy, under the grant of Development of Renewable Microbial

Polyesters for Cost Effective and Energy-Efficient Wood-Plastic Composites.

  162  

5.6 References

[1] P. M. Smith and M. P. Wolcott. 2006. Opportunities for Wood/Natural Fiber-Plastic Composites in Residential and Industrial Applications. Forest Products Journal. 56(3): 4-11

[2] M. Flieger, M. Kantorová, A. Prell, T. Řezank, and J. Votruba. 2003. Biodegradable plastics from renewable sources. Folia Microbiologica. 48(1):27-44

[3] S. Y. Lee. 1996. Bacterial polyhydroxyalkanoates. Biotechnology and Bioengineering. 49(1):1-14

[4] P. A. Holmes. 1985. Applications of PHB - a microbially produced biodegradable thermoplastic. Physics in Technology. 16(1):32-36

[5] C. S. K. Reddy, R. Ghai, Rashmi, and V. C. Kalia. 2003. Polyhydroxyalkanoates: an overview. Bioresource Technology. 87(2):137-146

[6] R. A. J. Verlinden, D. J. Hill, M. A. Kenward, C. D. Williams, and I. Radecka. 2007. Bacterial synthesis of biodegradable polyhydroxyalkanoates. Journal of Applied Microbiology. 102(6):1437-1449

[7] U. Pagga, D. B. Beimborn, J. Boelens, and B. De Wilde. 1995. Determination of the aerobic biodegradability of polymeric material in a laboratory controlled composting test. Chemosphere. 31(11-12):4475-4487

[8] T. U. Gerngross. 1999. Can biotechnology move us toward a sustainable society?. Nature Biotechnology. 17(6):541-544

[9] E. R. Coats, F. J. Loge, M. P. Wolcott, K. Englund, and A. G. McDonald. 2007. Synthesis of Polyhydroxyalkanoates in Municipal Wastewater Treatment. Water Environment Research. 79: 2396-2403

[10] E. R. Coats, F. J. Loge, M. P. Wolcott, K. Englund, and A. G. McDonald. 2008. Production of natural fiber reinforced thermoplastic composites through the use of polyhydroxybutyrate-rich biomass. Bioresource Technology. 99(7):2680-2686

[11] Scott P. Anderson. 2007. Wood Fiber Reinforced Bacterial Biocomposites: Effects of Interfacial Modifiers and Processing on Mechanical and Physical Properties. Master thesis, Washington State University

[12] J. Zhang, S. McCarthy, and R. Whitehouse. 2004. Reverse temperature injection molding of BiopolTM and effect on its properties. Journal of Applied Polymer Science. 94(2):483-491

[13] Z. Tadmor and C. G. Gogos. 2006. Principles of polymer processing. John Wiley and Sons.

  163  

[14] S. Migneault, et. al. 2008. Effect of fiber length on processing and properties of extruded wood�fiber/HDPE composites. Journal of Applied Polymer Science. 110(2):1085-1092

[15] S. V. Rangaraj and L. V. Smith. 2000. Effects of Moisture on the Durability of a Wood/Thermoplastic Composite. Journal of Thermoplastic Composite Materials. 13(2):140-161

[16] J.-W. Kim, D. Harper, and A. Taylor. 2008. Effect of Wood Species on Water Sorption and Durability of Wood-Plastic Composites. Wood and Fiber Science. 40(4):519-531

[17] M. J. A. Chowdhury and M. P. Wolcott. 2007. Compatibilizer selection to improve mechanical and moisture properties of extruded wood-HDPE composites. Forest Products Journal. 57(9):46-53

  164  

Chapter 6 Developing PHB/WF/Cell debris Composites: Injection Molding

Abstract

Wood plastic composites (WPCs) have been mainly targeted in the market of

decking or construction materials produced by extrusion techniques. Recently, the

WPC market has been diverted to injection molding applications. Conventionally,

WPCs highly depend on and are made from petroleum-based thermoplastics. Utilizing

biobased thermoplastics for WPCs has been considered as a solution to reduce overall

energy consumption and to lessen the dependence of crude oil with consideration of

sustainability. Poly-3-hydroxybutyrate (PHB) is a biodegradable microbial polyester,

which is biosynthesized by the bioactivities of microbes in carbohydrate-rich waste

effluents. To eliminate the costly step of PHB purification from microbe cells, directly

utilization of unpurified PHB (i.e. PHB-laden microbe cells) to produce WPCs is then

seen to effectively reduce expense and energy consumption. In this study, purified

PHB and PHB-free cell debris were used to simulate PHB laden microbes in

producing PHB/wood flour/cell debris ternary composites (PWCCs) with various

wood flour/cell debris ratios. The PWCCs were prepared by twin screw compounding

followed by injection molding. Polymeric methylene-diphenyl-diisocynate (pMDI)

was used as a coupling agent to increase the interfacial bonding between the PHB

matrix and wood four/cell debris. Morphology, mechanical properties (i.e. tensile,

  165  

flexural, and impact strength), and moisture behavior of the composites were

investigated. The results of the morphology study indicated that PHB penetrated into

wood lumens, where stronger interlocking and interfacial bonding were created with

the PHB matrix than that of cell debris. Through this observation, decreased

mechanical properties and moisture resistance of the composites could be described as

being caused by the weak interfacial bonding between cell debris and PHB while

increasing cell debris content. In comparison with a conventional

polypropylene-wood flour composite, at the replacement ratio of 3:2 (cell

debris:wood flour), the ternary composites exhibited slightly higher mechanical

properties than that of the conventional WPC. After investigating moisture Fickian

diffusion behavior, it was concluded that the cell debris increased moisture absorption.

During the immersion, cracking and dissolving of the composites appeared a

non-Fickain diffusion behavior. Based on what was discovered in morphology studies,

improving the interfacial bonding between cell debris and PHB may provide a better

performance of the ternary composites. In conclusion, to reach sustainability, with the

consideration of energy efficiency and cost effectiveness, the PWCCs were

successfully applied in the injection molding process and their performance was

similar to that of conventional WPC.

  166  

Key words: poly-3-hydroxybutyrate (PHB), wood plastic composite (WPC),

polymeric methylene-diphenyl-diisocynate (pMDI), sustainability, moisture behavior,

waste effluents

6.1 Introduction

Wood plastic composites (WPCs) have shown a rapidly growing rate since

1997[1]. According to a forecasting report from Freedonia Group, Inc. in 2010, WPCs

are expected to reach a market value of ca. 2.3 million dollars in 2013[2]. Recently,

with their high performance in environmental contributions of WPC [1], the market

has been expanded to the use of injection molding applications, such as automotive

parts, toys, consumer goods, etc. in both American and Europe [2]. WPCs are

traditionally highly dependent on polyolefin, e.g. polyethylene (PE), polypropylene

(PP), etc., which is derived from crude oil. In consideration of energy consumption,

the overall energy costs (including recycling, transportation, etc.) of WPCs are much

higher than that of traditional engineered wood products. Utilizing biobased

thermoplastics for WPCs is now seen as a solution to decrease the overall energy

costs and to lessen the dependence of crude oil in a sustainable society.

Unfortunately, renewable microbial polyesters, e.g. polyhydroxyalkanoates

(PHAs), are not currently used in WPCs, primarily because production costs are

  167  

higher than for conventional petrochemical-derived plastics. During extraction and

purification of poly-3-hydroxybutyrate (PHB), one of most common used in the

family of PHAs, more fossil fuel is actually consumed, generating more carbon

dioxide than a polyolefin [3]. Thus, considering the costs and procedures of recycling

the polyolefin, the overall life-cycle costs of the renewable microbial polyesters will

be more comparable. Typically, crop-based carbohydrate feedstock is utilized to

produce PHB, but conversion processes to obtain carbohydrate from corn consumes

energy (8129 KJ per kg) [3]. In this point, Coats et al [4] have successfully

investigated the use of municipal waste effluents that contain a certain level of

carbohydrate to feed microbes and produce PHB. This is seen as the first step to

reduce energy consumption from the purification of PHB and to achieve production of

PHB from waste effluents of municipal, agricultural, and forest products sectors.

Furthermore, it is possible to economically produce WPCs by directly utilizing

unpurified PHB (i.e. PHB-laden microbe cells) to accomplish reduction or elimination

of the most costly step in microbial plastic production - purification of the polymer

from the cell debris [5]. Using unpurified PHB (i.e. sterilized PHB- laden microbe

cells) to produce WPCs can therefore be seen as a significant reduction in raw

material cost and energy consumption.

  168  

Using this strategy, the cost of producing and utilizing these renewable plastics

in WPCs can be greatly reduced. Significant environmental benefits will also be

realized by the wastewater treatment industry in municipal, industrial (pulp mill), and

agricultural sectors through incorporation of waste biosolids (i.e. cell debris, etc.) into

composites. Improved economic competitiveness of the domestic forest products

industry is expected, as the plastic in WPCs comprises on average 52% of formulation

costs and 30% of total product costs [5].

Previous studies [6][7][8] indicated that isocyanate-based adhesives can react

with hydroxyl groups in wood to create a covalent bond (See Figure 6.1), but the

reaction could be influenced by moisture in the wood. Anderson [9] pointed out that

using polymeric diphenylmethane diisocyanate (pMDI) as a coupling agent could

significantly increase the mechanical properties and water resistance of PHB/wood

flour composites in a comparison of four different types of coupling agents. This

result was attributed to the reactions of pMDI with both hydrophilic wood flour and a

hydrophobic PHB matrix, which substantially improves the interfacial bonding

between the two phases. Notably, bacterial cell debris is also categorized as a

hydrophilic material. Therefore, to maintain the sustainability to produce WPCs, by

adding pMDI, performances of a ternary WPC, which integrates with PHB-laden cell

debris (PHB and cell debris) and wood flour, can be expected to be improved.

  169  

Figure 6.1 Functional group reactions between isocyanate chemicals and

wood/moisture

In this investigative study, a purified microbial polyester, PHB, and PHB-free

cell debris were used to simulate sterilized PHB-laden cell debris to produce

PHB/wood flour/cell debris ternary composites (PWCCs) with variant ratios of wood

flour replacement by cell debris via extrusion compounding, followed by injection

molding processes. Phase morphology, mechanical properties, and moisture behaviors

of the ternary composites were evaluated to understand the relation with the content

of cell debris.

  170  

6.2 Materials and Methods

6.2.1 Materials

The purified PHB powder (provided by Tianan Biologic Material Co., Ltd.,

Ningbo, China), sterilized cell debris harvested from municipal waste effluents

(provided by UC-Davids and ground into 60-mesh particle; see Figure 6.2 for particle

distribution), and wood flour (Pinus, 60-mesh, American Wood Fibers, Schofield, WI)

were the three main components of the PWC composites. As shown in Figure 6.2, the

particle size of cell debris is well distributed from 833 µm to 125 µm, and ca. 38% of

particle smaller than the diameter of 125µm. Boron nitride (BN) (Carbotherm PCTF5,

Saint Gobain Advanced Ceramics Co., Amherst, NY) was used as a crystallization

agent to promote PHB crystallization. Glycolube WP2200 (Lonza Inc., Allendale, NJ)

was included as a lubricant to improve processability. Talc (Nicron 403) was provided

by Riotinto of Centennial and was used for processability and water resistance

improvements. Liquid-form pMDI (Mondur 541, Bayer MaterialScience, Pittsburgh,

PA) contained 31.5 mass % NCO and functioned as the coupling agent between

hydrophobic PHB and hydrophilic wood flour and cell debris.

  171  

Figure 6.2 Ground cell debris particle size distributions

6.2.2 Methods

6.2.2.1 Preparation of composites

60-mech pine wood flour was dried by a rotary steam tube drier. The moisture

content was further detected and obtained ca. 3%. Ground cell debris powder was

dried at 100oC for 24 hours in a convection oven to remove possible moisture and

volatile solids. PHB, BN, WP2200, and talc were used as received. A coupling agent,

pMDI, was first introduced and well dispersed in PHB powder (manually mixed until

not a viable agglomerate). The PHB/pMDI mixture, together with the other

components, were put into a plastic bag and manually mixed by vigorous shaking and

tumbling for 5 minutes. All four formulations of PWCCs and two controls of

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%

45.0%

>0.833 0.42 0.25 0.177 0.15 0.125 <0.125

Freq

uenc

y

Particle size (mm)

  172  

PHB/WF (PW) and PHB/cell debris (PC) are detailed in Table 6.1. In these ternary

composites, a portion of the WF content (20, 40, 60, and 80%, respectively) was

replaced with the same amount of cell debris to produce four different ternary

composite formulations. PW and PC binary composites were also prepared as control

formulations. Each of dried mixed mixtures were following melt compounded by

using a co-rotating twin screw extruder (Leistriz ZSE-18) with a screw diameter of 18

mm and length/diameter (L/D) ratio of 40 equipped with a volumetric feeder. To

improve melt strength and reduce thermal degradation, a declining temperature profile

of the extruder was applied during the extrusion process [10]. The temperatures of the

barrel were set at 170, 175, 170, 165, 164, 163, 162, and 160oC, respectively, from the

feeding throat to the die adapter. The screw speed was maintained at 125 rpm. Under

this speed, the measured residence time of the molten materials in the barrel was ca.

1.5 minutes. The extrudates exiting the die were air cooled and pelletized for injection

molding.

  173  

Table 6.1 Formulations of two controls of PHB/WF (PW) and PHB/cell debris (PC) and PHB/WF/cell debris (PWC) composites

Formulation Components

PW PWC20 PWC40 PWC60 PWC80 PC

Cell debris* 0 11.4 22.8 34.2 45.6 57 Wood flour (pine)* 57 45.6 34.2 22.8 11.4 0 PHB 35 35 35 35 35 35 Talc-Nicron 403 8 8 8 8 8 8 pMDI 4 4 4 4 4 4 Lubricant (WP2200) 3 3 3 3 3 3 Nucleating agent (Boron Nitride) 0.2 0.2 0.2 0.2 0.2 0.2 *: 60 mesh particle size. WF: Wood Flour. All units are in parts.

6.2.2.2 Injection molded testing specimens

Flexural test specimens (12 X 3 X 127 mm) and tensile specimens (ASTM

standard D638 type III) were injection molded by an injection molding machine

(Sumitomo SE 50D). Injection temperatures were controlled at 175, 180, 175, and

170oC from the feeding zone to the nozzle. Mold temperature was set at 60oC. The

filling pressure was set at 1700 kgf/cm2 and the packing pressures were 1250 kgf/cm2

and 1360 kgf/cm2 for the first and second stage, respectively. The true cycling time of

the injection molding process was ca. 75 seconds. Impact test specimens (12 X 3 X 60

mm) were prepared following ASTM D256. All of the impact testing specimens were

notched by a XQZ-I specimen Notch Cutter. The specimens for the water absorption

test were prepared by a milling machine to the size of 11X 2.5 X 125 mm.

  174  

6.2.2.3 Thermal degradation analysis of raw materials

The neat PHB, 60-mesh pine wood flour, ground cell debris was analyzed for

thermal degradation behavior with a thermal gravimetric analysis (TGA), Rheometric

Scientific STA. Thermal degradation (i.e. weight loss) and thermal stability of the raw

materials were performed. Sample size of 8 to 10 mg of each of the raw materials was

placed in an aluminum crucible and exposed to a heating rate of 10oC per minute from

room temperature to 600oC under an air flow rate of 60 ml/min. of inlet inert gas,

nitrogen (N2), which prevents specimen oxidation during the heating process. Sample

weight was monitored continuously during the process.

6.2.2.4 Mechanical properties and sample densities

A screw driven Instron 4466 equipped with a 10-KN load cell and a pair of

mechanical grips was used for tensile tests, and a pair of flexural fixtures was used for

three point bending tests. The tensile tests were conducted at a crosshead speed of 5

mm per minute. The tensile strain was recorded and measured by an extensometer

(MTS model # 634.12E-24). The flexural tests (three point bending) were performed

with a crosshead speed of 1.40 mm per minute. Impact tests were conducted using a

Dynisco Basic Pendulum Impact tester. These setups of tensile, impact, and flexural

tests were conducted in accordance with ASTM standard D638, D256 (mode A), and

  175  

D790, respectively. Five replicates were tested for each formulation to obtain a

statistical mean value and standard deviation. Sample density was calculated by

dividing sample mass by sample volume. All of the samples (for both mechanical and

density testing) were conditioned at 21oC and 50% relative humidity (RH) for 7 days

prior to the tests.

6.2.2.5 Morphology

To investigate wood flour and cell debris distribution in the PHB matrix and

particle-polymer interfacial interaction, tensile fracture surfaces of the composites

were sputter coated with gold and observed by using a Hitachi S-570 scanning

electronic microscope (SEM) with different magnitudes. The morphologies of the

ground cell debris and pine wood flour were also employed by SEM. To understand

the polymer penetration, the injection molded samples were cut into a small specimen,

and followed cryo-microtomed to obtain a non-plastic deformation on the sectioned

surface. All the microtomed specimens were mounted on aluminum stubs and

gold-coated by a sputter coater.

  176  

6.2.2.6 Moisture behavior and diffusion

Composite moisture behaviors were performed in accordance with ASTM D 570

and the immersion period was targeted for twenty weeks. To reduce experience error,

four replicates of each sample composite were carried out. First, because of during the

injection molding process, the higher contain of plastic was found in the skin layer of

the molded specimen, in order to understand the true moisture behavior of the

composites, the plastic-rich skin of specimens was removed from each specimen, and

then immersed in distilled water at room temperature (controlled at 21oC). The weight

and thickness of each specimen was measured at different periods of immersion.

Moisture content (MC) and thickness swelling (TS) were calculated by the following

equations:

MC   % =   !!!!! ×!""!!

(eq. 6.1)

TS   % =   !!!!! ×!""!!

(eq. 6.2)

where Mt is the mass of the specimens at time t, and M0 is the initial mass of the

specimens; Tt is the thickness of the specimens at time t, and T0 is the initial thickness

of the specimens.

  177  

6.3 Results and Discussion

6.3.1 Thermal degradation

TGA was continuously monitoring the weight change of each specimen during

the heating procedure in a furnace at a constant ramp rate of 10oC per minute.

Thermal degradation temperature of each component in the sample can be obtained

from the plot of weight loss (in percentage) versus temperature (Figure 6.3) for neat

wood flour, ground cell debris, and neat PHB powder. It is clear from this figure that

PHB rapidly broke down within a short temperature range (between 250 and 280oC).

The wood flour started to decompose at ca. 200oC, and degradation occurred within a

broader temperature range (lower thermal degradation rate) than in the other two

materials, through a two step degradation process. The cell debris thermal degradation

started at ca. 100oC, but at the lowest degradation rate. About 40% of residue,

possible most are inorganic composition, of cell debris left at the end of the test (600

oC). The low degradation rate and high content of sample residue of the cell debris

was believed to be due to its complex constituents, primarily including various types

of proteins, polysaccharides, triglycerides, and inorganic residue.

  178  

Figure 6.3 Plot of the thermogravimetric curves of pine wood flour, cell debris, and

neat PHB

6.3.2 Material density and mechanical properties

Commonly, in WPCs the density has been considered as a correlation with

mechanical properties [9][11], which means the higher density, the better the

mechanical performance that can be achieved. The densities of composites made by

six different formulations are shown in Table 6.2. Density did not show a significant

change (less than 2.3%) with various cell debris/wood flour ratios. In comparing PW

and PC, the results indicated that the cell debris and wood flour had similar densities.

Possibly, the high pressure applied during the injection molding process benefited

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500

Wei

ght l

oss(

%)

Temperature (oC)

PHB Pine Cell debris

  179  

PHB melt penetration into the wood cell lumen or crushed the cellular structure of

wood flour, also known as densifying wood by the high pressure of molding process.

The results in Table 6.2 list the tensile and flexural strength and modulus of the

six composites for later comparison. The relations of mechanical properties, i.e.

tensile and flexural, and cell debris content are further plotted in Figures 6.4 and 6.5,

respectively. With increasing the content of the cell debris, both of tensile strength

and modulus had a negative result. At the cell debris/wood flour ratio of 3:2

formulation of PWC60, the composites exhibited flexural properties of MOR of

42.8MPa and MOE of 5.88GPa, which were comparable with that of PP/wood flour

composites (MOR of 44MPa; MOE of 3.1GPa [5]). Notably, the properties were

about 55 to 65% of the tensile and flexural properties of the control sample PW (39.7

MPa and 10.2 GPa in tensile strength and modulus; 69.4 MPa and 7.43 GPa in MOR

and MOE). Impact strength of the composites also showed a declining trend; the

higher the cell debris content, the lower the impact strength (Figure 6.6). The reason

for this tread was believed to be poor interfacial bonding between the cell debris and

PHB (discussed later in phase morphology) and degradation (starting at 100oC) of the

cell debris during processing. The degradation of the cell debris produced a gaseous

substance which could cause voids and other sample defects in the composites.

Moreover, the cell debris had a cubic shape or low aspect ratio, which was

  180  

unfavorable to the mechanical properties of the composites compared to the fiber

shape of the wood flour.

Table 6.2 Densities and mechanical properties with standard deviation of four formulations

Formulation Density* Tensile Properties Flexural Properties

Strength (MPa)

Modulus (GPa) MOR (MPa) MOE (GPa)

PW 1.396 39.7(0.94) 10.2(0.30) 69.4(1.25) 9.18(0.06) PWC20 1.388 31.0(0.54) 8.76(0.18) 53.9(1.07) 7.80(0.07) PWC40 1.388 26.4(0.36) 7.83(0.13) 45.2(0.85) 6.66(0.07) PWC60 1.390 23.4(0.51) 6.74(0.17) 42.8(1.73) 5.88(0.09) PWC80 1.368 19.7(0.99) 7.40(0.22) 34.6(1.83) 5.85(0.21) PC 1.365 14.4(0.86) 6.43(0.15) 26.5(2.68) 4.92(0.22)

*Standard deviation is less than 0.01 ( ) Standard deviation

Figure 6.4 Plot of the relation of tensile properties and cell debris contents

y = -40.952x + 37.438 y = 11.968x2 - 12.842x + 10.128

0

2

4

6

8

10

12

14

0

5

10

15

20

25

30

35

40

45

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0%

Mod

ulus

(GPa

)

Stre

ngth

(M

Pa)

Cell debris content

Strength Modulus strength Modulus

  181  

Figure 6.5 Plot of the relation of flexural properties and cell debris contents

Figure 6.6 Plot of the relation of impact strength and cell debris contents

y = -68.872x + 65.029 y = 9.1924x2 - 12.24x + 9.1082

0

2

4

6

8

10

12

0

10

20

30

40

50

60

70

80

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0%

Mod

ulus

(GPa

)

Stre

ngth

(M

Pa)

Cell debris content

MOR MOE strength Modulus

y = 4.768x2 - 4.8055x + 2.6132

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0%

Impa

ct S

tren

gth

(KJ/

m2 )

Cell debris content

  182  

6.3.3 Morphology

The microstructures of cell debris and wood flour are expressed in Figure 6.7.

The cell debris displayed wide particle size distribution with large particles measuring

ca. 300 microns (Figure 6.7a). Cell debris particles exhibited irregular shapes with

low aspect ratio, known as length/diameter (L/D) ratio, or cubic-like shape. On the

other hand, wood flour particles are composed of bundles of wood fibers (Figure

6.7b). During the process of extrusion compounding, intensive shear generated from

two screws could break some of these bundles and increase their aspect ratios, which

could further enhance their reinforcing effect. On the fracture surface (see in Figure

6.8b) of the PHB/wood flour composite, limited interfacial debonding between wood

fiber and PHB matrix was observed, which indicated strong interaction between the

two phases. Because of this strong interaction, many craters were formed by fiber

pulled-out with matrix (see Figure 6.9b) on the fracture surface. In contrast, weak

interaction between the cell debris and PHB could be observed by debonding or

gapping at their interfaces (indicated by the arrows in Figures 6.8c and 6.9c). The cell

debris particles (arrowed) and wood flour (cratered) were found to be homogenously

distributed in the polymer matrix (Figure 6.8c). The difference of the interfacial

properties was distinguished more easily under higher magnification (Figure 6.9). The

high interfacial adhesion between PHB matrix and wood fiber, indicated by the

  183  

arrows in Figure 6.9b, is possibly contributed by the coupling agent, pMDI, which is

highly reactive with both hydroxyl and carboxyl groups [6][7][8], and is able to

chemically link with the two phases. The cell debris is a complicated system,

comprising mainly protein, polysaccharides, triglycerides, and inorganic impurities.

The cell debris pre-treatment during its production, impurities of the cell debris, and

the cell debris degradation during processing might limit the access of pMDI to the

reactive sites of the cell debris and therefore reduce its compatibilization effects,

which could eventually result in weak interfacial bonding between cell debris and

PHB matrix (Figure 6.9c).

In Figure 6.10, the sectioned surfaces of PHB/wood flour/cell debris composites

are displayed. Due to the high pressure from the injection molding process, many

wood cells were crushed or severely deformed (Figure 6.10, deformation indicated by

arrows). The high pressure also forced molten PHB flew into some of the cells. It was

clear that the PHB melt was forced and penetrated into cells through longitudinal and

transverse lumens (Fig. 6.11). This penetration further provided more contact area and

created a strong mechanical interlocking between polymer and wood fiber, which

substantially improved stress transfer between the two phases. In contrast, cell debris

was seemingly not deformed by the high pressure of injection molding process nor

  184  

any PHB penetration observed (Figure 6.10). As a result, they formed a weak

interfacial bond with PHB, resulting in deteriorated properties.

Figure 6.7 SEM micrographs of cell debris (a) and pine wood flour (b)

Figure 6.8. Fracture surfaces of neat PHB (a), PHB/wood flour composite (b), and PHB/wood flour/cell debris composite (c). Magnification: X80

a   b  

a   b   c  

  185  

Figure 6.9. Fracture surfaces of neat PHB (a), PHB/wood flour composite (b), and PHB/wood flour/cell debris composite (c). Magnification: X500

       Figure 6.10 Sectioned surface of PHB/wood flour/cell debris composites

Figure 6.11. Wood lumens filled with polymer

a   b   c  

Cell  debris  

Wood  flour  

wood  cell  

PHB  

PHB  

wood  cell  transverse  lumen  

longitudinal  lumen  

  186  

6.3.4 Moisture diffusion behavior

As shown in Figure 6.12, moisture absorption increased with increasing cell

debris concentration. Moisture absorption rate (initial slope of the curve) also

increased with increasing cell debris concentration. The samples with higher cell

debris concentrations required less time to reach their saturation point. However, it

had been observed that the mass of the specimens were decreased after they reached

their saturation points. For example, the sample of PC reached moisture saturation

content (Msat, ca. 28%) in three days, but a mass reduction occurred after 4 weeks

immersion. This result may be explained by the possibility that water soluble material

was dissolved [12], or perhaps some loosened material on the surface of the immersed

sample fell into the water. In comparison, the control sample (PW) reached its

saturation point of ca. 17% in six weeks. Thickness of six composite samples

increased to a larger extent at a high rate at high content of cell debris (Figure 6.13).

Therefore, it can be concluded that the addition of cell debris increased water

absorption and thickness swelling of the composites. This finding was most probably

due to high hydrophility and dimension instability after absorbed moisture of cell

debris, which could be proved by comparing the saturation point and maximum

thickness swelling of the PC and PW composites (ca. 29.0% vs. ca. 17.7% and ca.

30.2% vs. ca.12.9%) compared to the PW composites. Additionally, cracks and weak

  187  

interfacial bonding between cell debris and PHB allowed faster moisture penetration,

known as capillary absorption.

Figure 6.12 Plot of moisture absorption against square root of time in seconds

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

0 500 1000 1500 2000 2500 3000 3500

Moi

stur

e ab

sorp

tion

Time (s0.5)

PW PW20C PW40C PW60C PW80C

  188  

Figure 6.13 Plot of thickness swelling against square root of time in seconds

Wood flour and cell debris are known as hydrophilic materials. In composites,

the hydrophobic PHB did not absorb moisture, but mainly absorbed by wood and cell

debris. After wood flour absorbed moisture, the dimension of wood flour became

unstable or swelled, which caused internal stress. Based on the viscoelastic behavior

of polymer under a load/stress, continuous damage can occur, which is known as

creep, and create micro-cracks or viable cracks with time goes by. In many studies

[13][14][15][16] it has been indicated that cracks can be found in a composite after a

certain period of time being immersed in water or salt water. In this study, due to a

large dimensional swelling during the immersion, cracks were formed on the surface

of different composite samples. Surface cracks appeared on the third, second, and first

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

0 500 1000 1500 2000 2500 3000 3500

Thi

ckne

ss sw

ellin

g

Time (S0.5)

PW PWC20 PWC40 PWC60 PWC80

  189  

day of immersion on the PWC60, PWC80, and PC composites, respectively. Samples

with a lower concentration of cell debris showed cracks after 4 weeks of immersion.

Liquid or gas transportation through a solid material is often modeled by

Fickian diffusion equation. In Fickian diffusion, plotting the amount of uptake

(moisture in this case) at a given time period against the square root of the time period

produces a linear region followed by a non-linear approach to Msat. The apparent

diffusion constant (DA) [14][17][18] can be calculated by:

!! =  ! !!!!"#

! !!!! !

! (eq. 6.3)

where h is the thickness of the specimens and dMt/d√t is the slope of the weight gain

versus square root of time.

In considering 3-dimensional diffusion, a geometric edge correction factor can

be used to calculate the true diffusion constant (D) [19]:

! =   !!!!! !!

! !! (eq. 6.4)

where L and W are the length and width of the specimens, respectively.

As shown in Table 6.3, the composite of PC had the highest true diffusion

coefficient (4.09E-6 mm2/sec) among the six formulations, which indicates that the

composite had the highest rate of moisture diffusion or the lowest water resistance.

Moreover, plotting the relation of true diffusion constant versus content of cell debris

in Figure 6.14, the higher the cell debris content, the faster the moisture diffusion

  190  

occurred. This can explain that cell debris is a highly hydrophilic material with a poor

interfacial bonding with a PHB matrix, which provided capillary penetration. Because

cracking and material dissolution occurred, most of the moisture absorption curves

exhibited a non-Fickian diffusion behavior after a certain period of immersion time

[20], after which they reached their own saturated points.

Understanding of the relation of sample swelling and moisture absorption can be

achieved by calculating a swelling coefficient (β) [17],

β =   !"!"#!"!"#

(eq. 6.5)

where TSmax and MCmax represent maximum thickness swelling and maximum of

moisture absorption, respectively. The results (Table 6.3) indicated that the higher the

cell debris content, the higher the swelling coefficient of the composites (ranging

from 0.731 to 1.012). Based on equation 6.5, it could be explained that low

dimensional stability (large amount of dimensional swelling) of the composites

caused the high value of swelling coefficient (β).

Table 6.3 Apparent (DA) and true (D) diffusion constants for the six formulations

PW PWC20 PWC40 PWC60 PWC80 PC Slope* 1.08E-04 1.45E-04 2.03E-04 2.27E-04 4.06E-04 5.77E-04 TSmax 12.93 % 13.66 % 15.43 % 20.12 % 21.45 % 30.25 % MCmax 17.75 % 19.09 % 20.39 % 22.16 % 20.97 % 29.05 % DA 7.66E-07 1.35E-06 2.55E-06 2.62E-06 6.11E-06 6.73E-06 D 4.69E-07 8.27E-07 1.56E-06 1.60E-06 3.72E-06 4.09E-06 β 0.731 0.716 0.759 0.908 0.993 1. 012

*: dMt/d√t

  191  

Possibly, the relation between content of cell debris and true diffusion constant

can be expressed by an Arrhenius-like relation:

D! = D ∙ exp!!!!∙! (eq. 6.6)

where D is the true diffusion constant of the PW composite, D’ is the true diffusion of

different cell debris content of PWCCs, R is the universal gas constant (8.314

J/K/mol.), T is the temperature of 294oK , and Ea is the activation energy (J/mol.). To

obtain Ea, the equation of 6.6 can also be presented as:

lnD! =    !!!!∙ !!+ lnD (eq. 6.7)

The relation could be obtained by plotting cell debris against the true diffusion

constant and result in Figure 6.14. By using the regression curve, the activation

energy of moisture diffusion on increasing cell debris content in the PWCCs can be

calculated and obtained as the value of -0.0171 J/mol, which means that by increasing

cell debris content, less energy is required to have moisture penetrated at the same

temperature condition. However, since it was found that the PWC60, PWC80, and PC

had viable cracks occurring in the early step (after less than a week) of immersion, the

moisture diffusion may be influenced because of the cracks are seeing to accelerate

the moisture penetration via capillary force. If only the composites of PW, PWC20,

and PWC40 (observed cracks after 4 weeks immersion) are considered, the activation

energy of moisture diffusion will be -0.0122J/mol.

  192  

Figure 6.14 Plot of true moisture diffusion constant versus cell debris content

6.4 Conclusion

The results of this study investigated and demonstrated that using purified PHB

and PHB-free cell debris were successfully compounded with wood flour by twin

screw extrusion and molded into test samples by injection molding. The

disadvantages of using cell debris, e.g. large and irregular shape, poor interfacial

bonding, and thermal degradation, were believed to contribute to the decline of

performance and moisture resistance of the composites. These properties decreased

with increasing cell debris content. In addition, the formulation of PWC60, the cell

ratio of debris:wood flour is 3:2 showed a comparable mechanical performance with a

HDPE/WF composite. Moisture behavior was investigated using Fickian diffusion

y = 7E-06x + 1E-07

y = 5E-06x + 4E-07

0.00E+00

5.00E-07

1.00E-06

1.50E-06

2.00E-06

2.50E-06

3.00E-06

3.50E-06

4.00E-06

4.50E-06

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0%

True

diff

usio

n co

nsta

nt (m

m2 /s

ec)

Cell debris content

  193  

theory. However, cracking and dissolved materials resulted in composites with a

non-Fickain diffusion behavior after they reached their saturation points. With

evidence from SEM morphology study, improvement of the interfacial bonding

between PHB and cell debris is expected to improve performance of the composite.

Lastly, based on the results from the simulated composites, the PHB laden cell

debris/wood flour composites could possibly be investigated by using extrusion

compounding process followed by injection molding. Other investigated formulations

were further seen as potential replacements for current WPCs, where polymers are

derived from crude oil, or to be utilized in an application with requirements of

biodegradable and “green” aspects.

6.5 Acknowledgement

The authors gratefully acknowledge the financial support provided by the U.S.

Department of Energy, under the grant of Development of Renewable Microbial

Polyesters for Cost Effective and Energy-Efficient Wood-Plastic Composites.

  194  

6.6 References

[1] P. M. Smith and M. P. Wolcott. 2006. Opportunities for Wood/Natural Fiber-Plastic Composites in Residential and Industrial Applications. Forest Products Journal. 56(3):4-11

[2] J. Bowyer. 2010. WOOD-PLASTIC COMPOSITE LUMBER V S. WOOD DECKING  : A COMPARI SON OF PERFORMANCE CHARACTERISTICS AND ENVIRONMENTAL ATTRIBUTES. Minneapolis, MN: Dovetail Partners, Inc., pp. 1-13

[3] T. U. Gerngross. 1999. Can biotechnology move us toward a sustainable society? Nat Biotech. 17(6):541-544

[4] E. R. Coats, F. J. Loge, M. P. Wolcott, K. Englund, and A. G. McDonald. 2007. Synthesis of Polyhydroxyalkanoates in Municipal Wastewater Treatment. Water Environment Research. 79:2396-2403

[5] E. R. Coats, F. J. Loge, M. P. Wolcott, K. Englund, and A. G. McDonald. 2008. Production of natural fiber reinforced thermoplastic composites through the use of polyhydroxybutyrate-rich biomass. Bioresource Technology. 99(7):2680-2686

[6] X. Zhou and C. E. Frazier. 2001. Double labeled isocyanate resins for the solid-state NMR detection of urethane linkages to wood. International Journal of Adhesion and Adhesives. 21(3):259-264

[7] S. Das, M. J. Malmberg, and C. E. Frazier. 2007. Cure chemistry of wood/polymeric isocyanate (PMDI) bonds: Effect of wood species. International Journal of Adhesion and Adhesives. 27(3):250-257

[8] S. L. Wendler and C. E. Frazier. 1996. Effect of moisture content on the isocyanate/wood adhesive bondline by 15N CP/MAS NMR. Journal of Applied Polymer Scienc. 61(5):775-782

[9] Scott P. Anderson. 2007. Wood Fiber Reinforced Bacterial Biocomposites: Effects of Interfacial Modifiers and Processing on Mechanical and Physical Properties. Master thesis, Washington State University

[10] J. Zhang, S. McCarthy, and R. Whitehouse. 2004. Reverse temperature injection molding of BiopolTM and effect on its properties. Journal of Applied Polymer Science. 94(2):483-491

[11] A. Klesov. 2007. Wood-plastic composites. Hoboken N.J.: Wiley-Interscience [12] Y. J. Weitsman. 1995. Effects of Fluids on Polymeric Composites - A Review [13] L. V. Smith and Y. J. Weitsman. 1990. The immersed fatigue response of

polymer composites. International Journal of Fracture. 82(1):31-42

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[14] S. V. Rangaraj and L. V. Smith. 2000. Effects of Moisture on the Durability of a Wood/Thermoplastic Composite. Journal of Thermoplastic Composite Materials. 13(2):140-161

[15] Y. J. Weitsman. 1991. "Moisture in Composites: Sorption and Damage" in Composite Materials Series 4 - Fatigue of composite materials. K. L. Reifsnider editted. Amsterdam  ;New York: Elsevier. pp. 385-429.

[16] A. C. Loos and G. S. Springer. 1979. Moisture Absorption of Graphite-Epoxy Composites Immersed in Liquids and in Humid Air. Journal of Composite Materials. 13(2):131 -147

[17] M. J. A. Chowdhury and M. P. Wolcott. 2007. Compatibilizer selection to improve mechanical and moisture properties of extruded wood-HDPE composites. Forest Products Journal. 57(9):46-53

[18] J.-W. Kim, D. Harper, and A. Taylor. 2008. Effect of Wood Species on Water Sorption and Durability of Wood-Plastic Composites. Wood and Fiber Science. 40(4):519-531

[19] R. M. V. G. K. Rao, N. Balasubramanian, and M. Chanda. 1984. Factors Affecting Moisture Absorption in Polymer Composites Part I: Influence of Internal Factors. Journal of Reinforced Plastics and Composites. 3(3):232 -245

[20] J. Zhou and J. P. Luca. 1995. The effects of a water environment on anomalous absorption behavior in graphite/epoxy composites. Composites Science and Technology. 53(1):57-64

  196  

Chapter 7 Conclusion

Due to the increasing lack of fossil fuel resources, it is an urgent to move toward

to a more sustainable source of energy. Utilizing natural renewable materials such as

wood and biopolymer products, is now becoming an enormous focus in order to

decouple the dependence on fossil fuel and to maintain sustainable requirements. Both

wood and biopolymers, (specifically referred to as PHB in this study), can be renewed

via biological production processes. Blending these two components in order to form

wood biopolymer composites is seen as an alternative solution. Notably, PHB is a

biodegradable and compostable biopolymer, which indicates that PHB can be

biologically converted back to nature. Additionally, wood plastic composite (WPC)

has been developed for decades to satisfy the demands of performance, cost reduction,

and “green” aspects in variable applications, such as extruded and injection-molded

goods. Traditionally, WPC is made with wood flour/fiber (WF) and petroleum-based

polymer, e.g. polyolefin and polyvinyl chlorate. With the sustainable benefit from

wood and biopolymer, PHB, wood bio-polymer composites are seen as a key toward

to sustainability. However, the energy required to purify PHB is a large drawback to

maintaining the sustainable image of biopolymer. Additionally, addition of a coupling

agent to improve the performance of wood biopolymer composites may influence the

compostability of the composite. Therefore, the major objectives of this research are:

  197  

1) evaluating compostability and biodegradation behavior of wood-PHB composites

in lab scale testing; 2) understanding the mechanisms of biodegradation and physical

degradation; and 3) utilizing PHB-laden cell debris to produce wood-PHB composites

through extrusion and injection molding processes.

The results indicated that PHB and PHB/WF composites (50:50 wt%) can be

classified as compostable polymeric materials in accordance with ASTM D5338 and

D6400 via a lab-scale test. The PHB/WF composite had 60% mineralization after 120

days of composting. Different degradation phases were explored, such as lag,

degradation, and slow-down. In accordance with previous studies and results

discussed in this research, the crystallinity of PHB was increased after a certain period

of composting time. This result is explained by the fact that the microorganism

consumes the amorphous region of PHB first, and then the crystalline region. Also, as

small molecular of enzyme, which is produced from microorganisms, it is possible to

penetrate through the PHB/WF composites. After applying the Fickian diffusion

concept on the results of mineralization, the mineralization (biodegradation) behavior

of PHB/WF composite was numerically presented. During composting of

compostable materials, lag phase of the biodegradation was very critical since that is

how the enzymatic bioreaction starts. Microcracking is believed to occur due to the

fact that the viscoelastic behavior of filler reinforced polymeric material under the

  198  

hygrothermal conditions. Under this condition, soften polymer and dimensional

swelled composite is exhibited. After PHB/WF composites with PHB-g-MA and MDI

as coupling agents were placed in a hygrothermal condition chamber, physical

damage and mechanical degradation were observed by using continuous damage

mechanics to theoretically obtain the percentage of crack damage. Using MDI as a

coupling agent in PHB/WF composites resulted outstanding hygrothermal resistance

because of a better interfacial bonding than the control and PHB-g-MA added

formulation. Comparison with mathematically normalized results and moisture effects

on tensile properties proved this statement. After recognized the lag phase, by using a

lab-scale composting setup, the biodegradation of different PHB/WF composites were

performed. The results showed that by improving interfacial bonding, a lower rate of

mineralization was observed on the MDI added PHB/WF composite compared with

PHB-g-MA added and control. Additionally, application of a well-known

Michaelis-Menten enzymatic kinetic concept, enzymatic behavior during the lag

phase can be numerically presented and may provide more fundamental

understanding. Surface morphology observation showed evidence of surface erosion

during the biodegradation. Microscope evidence of hypha and wood flour that

remained on the surface of the PHB/WF composites could represent the slow-down

phase. It could be expected that the microbial resistance from lignin of wood was a

  199  

drawback in influencing the compostability of the PHB/WF composites. Combining

the results of composting and hygrothermal effects, a state-of-the-art view of the

biodegradation mechanism could be schematically sketched to fundamentally

understand the lag, biodegradation, and slow-down phases.

Energy consumption during PHB production and purification are seen as

drawbacks for utilizing PHB. Fortunately, previous research has indicated that PHB

could be generated from waste water treatment plants, which could assist reduction of

energy consumption and demands for raw agricultural materials. Eliminating the

purification step of PHB and directly blending PHB-laden cell debris with wood flour

to produce PHB/WF composites is a possible method to reduce energy consumption.

The results of this study showed that it is possible to use PHB-laden cell debris

to produce PHB/WF composites. The mechanical performance of the composite with

a 3:2 ratio of cell debris and wood flour was comparable with one commercial WPC.

However, the highly hydrophilicity of cell debris deteriorates the moisture resistance

of the PHB/WF composites, even with MDI added as coupling agent.

In conclusion, this innovative research provides much fundamental knowledge

and leads to an understanding of the process of composting biodegradation. Many

theoretically methodologies were successfully applied in order to investigate and

numerically explain the results. From the point of view of sustainability, the PHB/WF

  200  

composite is a suitable candidate to assist the global movement toward a more

sustainable society. This aim could be achieved by estimating the individual and local

product carbon footprint (carbon cycling ecosystem), which may be estimated by

using the mineralization results and reduction of energy consumption from PHB

purification proposed in this research.

  201  

Appendix A Maleation of Poly-3-hydroxybutyrate by Reactive Extrusion

A.1 Introduction

Bio-based polymers have been a focus as an alternative of petroleum derived

polymer, such as polyolefin. Forming a bio-composite, natural organic materials (e.g.

wood flour and starch) are commonly added as reinforcement to these bio-polymers to

expend the performance envelope and cost reduction. However, the discrepancy of

hydrophobic polymer and hydrophilic reinforcements causes a drawback on the

bio-composites. One way to improve the interfacial adhesion is to use functionized

bio-polymers. The functionization can be achieved by using multiple methods that are

mainly to introduce functional groups onto the polymer chain and form internal force

to each other and/or to the functional groups (most are hydroxyl and carboxyl groups)

on the reinforcements. Moad [1] indicated that the most widespread method of

grafting functionality onto polymer chains is by free radical-induced reactive

extrusion. Peroxide is the common use to provide the free radical to initiate reactions

and result a monomer or macro-monomer grafted on the polymer chain. This entire

reaction process is conveyed through an extruder. Maleic anhydride (MA) has widely

been used to graft onto polyolefin and bio-polymers and used as a compatibilizer to

enhance the interfacial adhesion in a biocomposite, such as wood plastic composites

[2][3][4]. Poly-3-hydroxybutyrate (PHB), one of biopolymer, has been introduced to

  202  

produce MA grafted PHB (PHB-g-MA) by reactive extrusion [5], but the degree of

grafting did not be pointed out. Therefore, this following research is using reactive

extrusion to produce PHB-g-MA and identify the degree of grafting and thermal

properties.

A.2 Materials and Methods

A.2.1 Materials

Purified PHB powder was provided by Ningbo Tianan Biologic Material Co.,

Ltd., Ningbo, China. Wood flour (Ponderosa pine, 60-mesh) was provided by

American Wood Fibers, Schofield, WI. Powder form of maleic anhydride (95% of

purity), MA, and liquid form of initiator, 2,5-Dimethyl-2, 5-di-(tert-butylperoxy)

hexane (Luperox® 101) was supplied from Sigma-Aldrich Co. LLC.

A.2.2 Methods

A.2.2.1 Reaction Extrusion of MA-grafting on PHB

Reaction extrusion process has been widely utilized to polymerize or graft

functional groups and/or monomers onto polymers [1][6]. During the reaction process

of extrusion, an initiator is needed to provide a free radical, which is seeing as a

reacting point, onto polymer chain. In this study, the mixture of PHB and additional

  203  

MA (2 parts) and initiator (0.2 parts) was first mixed by manually vigorous shaking

and tumbling for 5 minutes in a plastic bag. The mixture was later thermally reacted

using a co-rotating twin screw extruder (Leistriz ZSE-18) with a screw diameter of 18

mm and length/diameter (L/D) ratio of 40 equipped with a volumetric feeder for a

constant feeding. To improve melt strength and reduce thermal degradation, a

declining temperature profile of the extruder was applied during the extrusion process.

The extrusion temperature profile was set at 165, 175, 170, 165, 160, 155, 150, and

145oC, from the feeding throat to the die adapter. The screw speed of extruder and

feeder was maintained at 55 and 400 rpm. The temperature and pressure of PHB melt

flow before existing die were detected at ca. 131oC and ca. 2.48MPa. Under these

speed, the residence time of the materials in the barrel was measured to be ca. 4

minutes. The extrudate was air cooled and following pelletized into a pellet form. The

pellet of PHB-g-MA was stored in a zip-lock bag.

A.2.2.2 Evaluation of MA-grafting Ratio

Titration was used to determine the degree of maleation for the sample of

PHB-g-MA. The procedure of the titration [6][7] was modified to determine

accurately degree of maleation. The non-reacted MA monomer was evaporated by

heating the PHB-g-MA pellets at ca. 85oC under a conventional oven with a vacuum

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to evolve and discharge air for 24 hours. 4 gram of the pellets were further removed

and dissolved in 75 ml of chloroform, and 20 drops of 1M of hydrochloric solution

was added to hydrolyze anhydride into carboxylic acid at room temperature. The

solution was vigorously stirred for 30 minutes and then purified, known as

precipitation, by dripping into 300ml of acetone in order to remove any residual

homo-/co-polymer of MA. This solution was stored into a refrigerator at ca. 4oC for

24 hours in order to prevent any possible entrapping of homo-/co-polymer of MA and

reduce the settling time. The precipitates were filtered and washed by acetone and

distilled water several times before replace in a conventional oven and venting out

any remaining solvent on the precipitated polymer, PHB-g-MA, at 85oC for 24 hours.

To determine the grafting degree, ca. 0.4 gram of the purified precipitated PHB

polymer was sampled and dissolved in 20 ml of chloroform. The solution was added 5

drops of phenolphthalein as a color indicator, and then titrated using 0.04 M of

potassium hydroxide (KOH) in methanol. The degree of MA grafting was calculated

based on the following equation:

Degree  of  MA  grafting  (%) =  N!"# ∙ V!"#2W!"#$%&

×98.06×100

where NKOH and VKOH are the volume and normality, moles per equivalent, of the

KOH solution; Wsample is the sample weight of PHB-g-MA in grams.

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A.3 Results and Discussion

Reaction extrusion and MA grafting ratio

During the titration process, precipitation, which could result in an experience

error, did not appeare in this study. After titration of the PHB-chloroform solution, the

degree of MA grating on PHB was determined and obtained an average of degree of

0.427% with coefficient of variance (COV) of 5.4%. Notably, the grafting degree is

similar with that of another bio-polymer, polylactide (PLA), which had a degree MA

grafting of ca. 0.45% [6]. PHB is frequently comparing with PLA due to both of them

have ester group in their polymer main chain. This result indicated the MA monomer

was successfully grafted onto the PHB polymer chain. The chemical structure of

PHB-g-MA could be concluded and the proposed mechanism of maleation reaction [6]

is shown in Scheme A.1. The functional group of the grafted MA was believed to

improve the interaction with hydroxyl group of wood flour by ring-opening [2][3][5].

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Scheme A.1 Proposed mechanism of maleation reaction onto a polyester chain

A.4 Conclusion

It can be concluded that by using reactive extrusion can achieve the maleation of

PHB. The titration result indicated that the functional group of MA was successfully

grafted onto the polymer chain of PHB with a mean grafting ratio of 0.43%. As the

maleatic anhydrate is highly reactive with hydroxyl group on wood, this PHB-g-MA

can be further used as a coupling agent to solve the discrepancy of PHB and wood

flour, and further enhance the performance of the PHB/WF composites.

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A.5 References

[1] G. Moad. 1999. The synthesis of polyolefin graft copolymers by reactive extrusion. Progress in Polymer Science. 24(1):81-142

[2] M. J. A. Chowdhury and M. P. Wolcott. 2007. Compatibilizer selection to improve mechanical and moisture properties of extruded wood-HDPE composites. Forest Products Journal. 57(9):46-53

[3] David P. Harper. 2003. A Thermodynamic, Spectroscopic, and Mechanical Characterization of the Wood-Polypropylene Interphase. Master Thesis, Washington State University

[4] R. Gauthier, C. Joly, A. C. Coupas, H. Gauthier, and M. Escoubes. 1998. Interfaces in polyolefin/cellulosic fiber composites: Chemical coupling, morphology, correlation with adhesion and aging in moisture. Polymer Composites. 19(3):287-300

[5] Scott P. Anderson. 2007. Wood Fiber Reinforced Bacterial Biocomposites: Effects of Interfacial Modifiers and Processing on Mechanical and Physical Properties. Master thesis, Washington State University

[6] D. Carlson, L. Nie, R. Narayan, and P. Dubois. 1999. Maleation of polylactide (PLA) by reactive extrusion. Journal of Applied Polymer Science. 72(4):477-485

[7] Y. Nabar, J. M. Raquez, P. Dubois, and R. Narayan. 2005. Production of Starch Foams by Twin-Screw Extrusion:   Effect of Maleated Poly(butylene adipate-co-terephthalate) as a Compatibilizer. Biomacromolecules. 6(2):807-817