www.elsevier.com/locate/procbio

Process Biochemistry 42 (2007) 329–334

Composite materials derived from biodegradable

starch polymer and jute strands

F. Vilaseca, J.A. Mendez, A. Pelach *, M. Llop, N. Canigueral,J. Girones, X. Turon, P. Mutje

Group LEPAMAP, Department of Chemical Engineering, University of Girona, Campus Montilivi, Edifici P-I, 17071 Girona, Spain

Received 19 April 2006; received in revised form 31 August 2006; accepted 4 September 2006

Abstract

In this work, starch-based composites reinforced with jute strands were obtained by injection moulding procedure. The influence of the degree

of adhesion at fibre–matrix interface to the mechanical properties of the composites was evaluated. For this purpose, partial delignification of jute

strands by means of NaOH treatment was carried out and the mechanical properties of the corresponding composites determined. The alkali

treatment resulted in an increase of the strength and stiffness of the jute strand/starch composites. The enhancement was analysed in terms of

compatibility and extension of the adhesion at fibre–matrix interface. The application of a blocking reaction to the surface hydroxyl groups of jute

strands corroborated the influence of the mechanical properties to the extension of the interfacial adhesion. Thus, higher extension of the hydrogen

bonds at fibre–matrix interface gave higher strength and stiffness of the jute strand/starch-based composites.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: Starch-based biopolymer; Reinforced biopolymer; Jute strands; Alkali treatment; Blocking reaction; Wettability; Mechanical properties

1. Introduction

The reinforcement of polymer matrices with fibres is a huge

research field under constant development. From the beginning

of the XX century, thermoplastics and thermosetting polymers

have been reinforced to improve the properties of the final

material. Traditionally, mineral and synthetic fibres such as

glass, carbon and aramid fibres have been used to improve both

the stress and stiffness of petroleum-based polymeric matrices

[1–3].

However, the use of non-biodegradable matrices and non-

biodegradable reinforcement is disturbing the control of the

residues at the end of the lifetime of their composites.

Governmental regulations and growing environmental aware-

ness throughout the world have triggered a paradigm shift

towards designing materials compatible with the environment.

Accordingly, the current research is focused to the use of

biodegradable polymers and fibres, which lead to the fabrication

of highly environmental respectful composites [4,5].

* Corresponding author. Tel.: +34 972418438; fax: +34 972418399.

E-mail address: angels.pelach@udg.es (A. Pelach).

1359-5113/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.procbio.2006.09.004

Biocomposite materials are understood as a biodegradable

polymer matrix reinforced with a natural element. When the

development of biocomposites started in the late 1980s, these

kinds of biodegradable materials, not yet satisfying each of the

requirements of biocomposites, were available in the market

[6,7]. One of the most often investigated biopolymers is starch.

The first attempts to employ starch in biodegradable materials

involved its use as filler in synthetic polymers, such as

polyethylene [8] or, in its gelatinized form, as a component of

blends with water soluble or water dispersible polymers [9].

Starch is one of the least expensive biodegradable materials used

for many non-food items such as paper making, cardboard, textile

sizing and adhesives. More recently, starch has been used as the

main polymer in thermoplastic compositions and has been proce-

ssed into eating utensils and as raw material for film production.

Converted to thermoplastic material, starch offers an interesting

alternative for synthetic polymers where long-term durability is

not needed and rapid degradation is an advantage [4,10].

However, biodegradable matrices need to be reinforced to

improve their properties [11–15]. The use of plant fibres, derived

from annually renewable resources, as reinforcing fibres in

composites, provides positive environmental benefits with

respect to ultimate disposability and raw material utilization

[16–18]. Plant fibre composites combine good mechanical

F. Vilaseca et al. / Process Biochemistry 42 (2007) 329–334330

properties with low specific mass. However, their high moisture

absorption, poor wettability and insufficient adhesion between

untreated fibre and polymer matrix lead to debonding at fibre–

matrix interface [19]. To improve the properties of their

composites, natural reinforcing fibres can be modified by

physical and chemical methods [15,20,21]. Plant fibres must

exhibit a compatible surface morphology for the development of

a coherent interface with matrix polymers but because of the

surface impurities present in plant materials the development of a

good fibre–matrix interface is impaired [22,23]. In order to make

use of the good physical and structural characteristics of the

fibres they are treated with chemicals such as caustic soda to

modify surface topography and the fine structure. Partial removal

of lignin and hemicelluloses on the alkali treatment of cellulose

fibres was reported by Sreekala et al. [24]. The removal of

hemicelluloses produces less dense and less rigid interfibrillar

region. As lignin is removed, the middle lamella joining the

ultimate cells is expected to be more plastic, as well as

homogeneous, due to the gradual elimination of microvoids.

According to the literature, mercerization or alkali treatment

greatly improves the resin pick-up or wettability of natural fibres

during composite fabrication [25,26]. Additionally, NaOH

treatment increases the amount of crystalline cellulose and

removes natural and artificial impurities, producing a rough

surface topography. Natural cellulose has cellulose I crystalline

structure, but on alkali treatments it changes to cellulose II, in

which the parallel polymer chains of cellulose I are aligned

antiparallel and higher exposition of OH is observed [27].

In the present contribution, jute strand/starch-based com-

posites will be prepared and characterized. A partial

delignification of jute strands by NaOH treatment will be

carried out and the effect of the extension of fibre–matrix

adhesion on the mechanical properties of composites will be

studied. Finally, an OH-bloking reaction on the jute strand

surface will be realized to validate the dependence of the

mechanical properties to the adhesion degree at interface.

2. Experimental

2.1. Materials

Jute strands were supplied by Celulosa de Levante S.A. (Tortosa, Spain)

with an initial fibre length from 10 to 20 cm. Sodium hydroxide pellets from

Merck (Darmstadt, Germany) were used for the jute strands alkali treatment.

Phenyl isocyanate 98% purity from Fluka (Buchs, Switzerland) was used as

blocking agent for the OH groups onto the jute strands surface. Dibutyltin

dilaureate (DBTL) 95% purity from Sigma–Aldrich Chemie (Steinheim, Ger-

many) was used as catalyst for the surface modification reaction. Carbon

tetrachloride (CCl4) 99.9% purity from Panreac (Castellar del Valles, Barce-

lona, Spain) was used as solvent for the modification reaction. CCl4 was

previously dried with 3 A molecular sieve for at least 12 h before use. The

starch-based biopolymer supplied by Ribawood, S.A (Zaragoza, Spain) was

used as polymer matrix for the composite preparation. The starch-based

biopolymer had a specific gravity of 1.28 g/cm3 and a melt flow index of 5–

6 g/10 min (measured at 180 8C and 5 kg load).

2.2. Washing and alkali treatments

Jute strands were cut down to a nominal length of 10 mm in a blade mill.

Jute strands were washed with tap water at 50 8C for 1 h, with distilled water and

finally dried in an oven at 105 8C for 3 h. After washing down, a portion of jute

strands were kept to be used as reinforcement for the starch-based composites

preparation. Another fraction of jute strands was submitted to alkali treatment

[28]. For this purpose, jute strands were poured into a reaction flask and a

solution of sodium hydroxide solution (20%) was added onto the fibres. The

suspension was held at room temperature for 3 h under stirring. Afterwards, the

reaction medium was filtered and jute strands were thoroughly washed with

distilled water until neutralization. At last, the alkali treated jute strands were

dried in an oven at 105 8C for 3 h and kept to be used as reinforcement for the

starch-based composites preparation.

2.3. Blocking reaction

A chemical modification reaction by means of mono-phenyl isocyanate was

applied onto the jute strands to block OH groups at fibre surface [29]. Phenyl

isocyanate (100 mL) and DBTL (1 mL) were dissolved in dried CCl4. A large

excess of phenyl isocyanate with respect to the surface hydroxyl groups was

used (1000:1). The jute fibres were maintained immersed in the stirring solution

under nitrogen atmosphere at room temperature for 72 h. The treated fibres were

removed and rinsed with the clean solvent. After that, samples were dried

overnight at 80 8C.

2.4. Spectral analysis

The ATR-FTIR spectra of crude and surface-treated jute strands were

recorded using a Mattson Satellite spectrometer equipped with a MKII Golden

Gate Reflection ATR System. Each spectrum was recorded by co-adding 64

scans at 4 cm�1 of optical resolution within the range 600–4000 cm�1. The

ATR-FTIR technique is capable of probing functional groups present both

above and just below the top molecular layer of flat surfaces [30].

2.5. Polarity measurements

The polarity was evaluated through colloidal titration with methylglycol-

chitosan (MGCh). The cationic demand of the fibre suspension (amount of

cationic polymer fixed on the substrate) was determined by the colloidal

titration technique developed by Terayama (1952) [31] and later applied by

various research teams [32–34]. Methylglycolchitosan (MGCh) was used as

cationic polymer while o-toluidine blue was the indicator applied. An excess of

cationic polymer was added to the suspension and, after mixing, measured by

colloidal titration.

2.6. Preparation of starch-based composites

Composite materials from starch-based biopolymer reinforced with hemp

strands were obtained using a heated roll mixer from IQAP LAB SL (Roda de

Ter, Barcelona, Spain) at 80 � 3 8C for 10 min [35–37]. Blends comprising 10,

20 and 30 wt.% of reinforcing material were obtained. After mixing, the blends

were granulated in a blade mill (Agrimsa, St. Adria del Besos, Spain) provided

with a 10 mm mesh and afterwards moulded in a injection-moulding machine

(Type 35-Mateu&Soler SA, Barcelona, Spain). The injection parameters for

moulding non-reinforced biopolymer and composites comprising 10, 20 and

30 wt.% of reinforcement were varied to produce specimens with good surface

appearance and to minimize reduction of the aspect ratio of the reinforcement.

The injection parameters for composites reinforced by 10% of jute fibre were:

Temperatures: 130/140/145 8C; injection time (injection/post-injection/cool-

ing): 0.3/0.09/0.6 s; screw speed: 0.02 m/s; post-pressure: 1.47 � 105 Pa. The

steel mould was according to ASTM D3641 standard. The samples were

conditioned according to ASTM D618 standard before testing. For each

composite blend, ten specimens were tested.

2.7. Mechanical tests

Tensile properties of starch biopolymer and composites were determined

using an INSTRON testing machine (model 1122). Tensile strength measure-

ments and three-point bending tests were carried out following ASTM D638

and ASTM D790 standard methods. Impact strength was measured in an impact

F. Vilaseca et al. / Process Biochemistry 42 (2007) 329–334 331

tester (Charpy) according to ASTM D6110. All the results were taken as the

average of five samples.

2.8. Scanning electron microscopy (SEM)

The scanning electron microphotographs of the samples were taken by

means of a Zeiss DMS 960 SEM microscope. The tensile fracture surfaces of the

composite samples were studied with the microscope operating at 25 kV,

samples being coated with a 10 nm layer of gold.

2.9. Optical microscopy

A DMRXA optical microscope from Leica was used to analyse the fibre

dimensions after processing. For fibre size analysis, the composites were

dissolved in xylene at 137 8C for 3 days and fibres were dried under vacuum

at 80 8C. The fibre size was analysed by means of optical microscopy and Sigma

Scan Pro image analyser.

2.10. Moisture absorption

The moisture absorption of untreated jute strand/starch composites was

determined by placing samples in a sealed glass container under controlled

temperature (20 8C) and relative humidity (98%). The samples were weighted

daily until constant weight was achieved.

3. Results and discussion

The mechanical properties of the starch-based biopolymer

and its composites comprising different percentages of

untreated jute strands, alkali treated jute strands or Ph-NCO

modified jute strands are shown in Table 1. The results for

untreated jute strand composites evidenced sensitive growing

on the tensile strength of composites with progressive

increments of jute strands. With respect to the starch-based

biopolymer, the tensile strength increased by 35, 54 and 68%

for composites comprising 10, 20 and 30% (w/w) respectively.

The improvement of the strength under flexural conditions was

higher and the same percentages of reinforcement produced

increments of 48, 61 and 89% with respect to the non-

reinforced matrix. Because the enhancements of the maximum

strength are affected by the good dispersion and adhesion at

interface, these results point out that both constituents should

display good wettability and compatibility at fibre–matrix

Table 1

Mechanical properties of starch-based biopolymer, un-treated jute strand/starch com

strand/starch composites

Reinforcement (%) Tensile strength (MPa) Young’s modulus (MPa)

Un-treated jute fibres

0 13.2 � 0.55 600 � 28

10 17.9 � 0.51 1090 � 12

20 20.4 � 0.45 1690 � 19

30 22.2 � 0.47 2470 � 21

NaOH treated jute fibres

10 21.1 � 0.63 1115 � 19

20 23.8 � 0.58 1680 � 25

30 26.3 � 0.55 2490 � 23

Ph-NCO treated jute fibres

30 11.5 � 0.45 1930 � 17

� indicates the standard deviation.

interface. Different interaction mechanisms are accepted to

explain the wettability of the reinforcement in the matrix and

their interfacial adhesion: mechanical anchoring between the

fibre and the matrix, electrostatic attractions, London Van der

Waals interactions, hydrogen bonds and the establishment of

covalent bonds. The existence of one or several mechanisms

involves different wettability and interfacial adhesion, which

might be associated with higher or lower increments on the

tensile strength. Due to the nature of each constituent, a

mechanical anchoring and the presence of hydrogen bonds

between both components could explain the good mechanical

results. Jute strands show a high irregular topography that might

facilitate mechanical interactions between the jute strands and

the matrix during the wetting and mixing process. Moreover,

the high porosity of jute strands might also contribute to the

diffusion phenomenon of the matrix along the reinforcement,

increasing the accessibility of the hydroxylic fibre surface to the

polymer matrix. On the other hand, the existence of hydrogen

bonds at interface is accepted considering the chemical

structure of both fibres and polymer. The effectiveness of the

adhesion at fibre–matrix interface explains the increasing in

tensile and flexural strength for jute strand/starch composites,

which were elevated in spite of the size reduction applied to the

jute strands throughout the processing. Thus, the initial fibre

length of jute strands of 10 mm was reduced to 0.76 mm after

extrusion and to 0.55 mm after the injection process.

Additionally, the diameter of jute strands was also decreased

from 126 to 41 mm after extrusion and to 17.3 mm after

injection. From these results, the aspect ratio (L/D) of jute

strands in the moulded composites was above 30.

In Table 1 the Young’s modulus for starch-based biopolymer

and jute strand composites is also shown. Significant

increments of the Young’s modulus with the percentage of

reinforcement were observed. The Young’s modulus increased

by 80, 180 and 310% for composites reinforced at 10, 20 and

30% (w/w) of untreated jute strands respectively. According to

the literature [38], the increase of composite stiffness is

function to the percentage of reinforcement and its good

dispersion into the matrix but is not function to the good quality

of the interface. Therefore, the intrinsic mechanical properties

posites, NaOH treated jute strand/starch composites and Ph-NCO treated jute

Elongation (%) Flexural strength (MPa) Impact (kJ/m2)

22.6 � 1.3 19.2 � 1.3 81.4 � 2.8

7.7 � 0.7 28.4 � 0.9 15.7 � 0.9

3.2 � 0.4 31.0 � 0.9 11.4 � 0.9

1.8 � 0.3 36.4 � 1 10.5 � 0.8

7.3 � 0.8 32.7 � 1.2 16.9 � 1.2

3.3 � 0.4 35.7 � 0.8 13.2 � 0.9

2.0 � 0.2 41.2 � 0.8 11.9 � 0.9

0.9 � 0.1 20.3 � 0.7 8.4 � 0.7

F. Vilaseca et al. / Process Biochemistry 42 (2007) 329–334332

of the reinforcement itself and the degree of dispersion into the

polymer matrix are the key point for increasing the rigidity of

composite materials. The application of the rule of mixtures

allows the determination of the compatibility factor according

to the following equation (1):

EC ¼ f cðVFEFÞ þ VMEM (1)

where EC, EF and EM are the Young’s modulus of composite,

reinforcement and matrix, respectively, VF and VM the volume

fraction of reinforcement and matrix, and fc the compatibility

factor. The compatibility factor is a parameter including the

fibre length, aspect ratio, orientation and fibre–matrix adhesion

at interface. As much close to 1 is the compatibility factor,

much better is the fibre–matrix interface and the effort trans-

mission. Because the intrinsic Young’s modulus of jute strands

was 18.1 � 6.5 GPa (calculated from 30 jute strand specimens

at a testing length of 1/2 in.) and the experimental specific

gravity of jute strands was 1.51 g/cm3 the compatibility factor

can be isolated from Eq. (1). The compatibility factor for

composites at 10, 20 and 30% (w/w) was 0.367, 0.393 and

0.438, respectively. According to the Young’s modulus

increases, the elongation of composites decreased dramatically

with respect to the fibre percentage. As expected, the impact

strength also decreased with the percentage of reinforcement.

The high rigidity of composites decreased the capacity of

absorbing energy under impact conditions.

Another aspect to be considered for composite materials

from starch-based biopolymers starch and jute is the moisture

absorption. Fig. 1 shows the evolution of the moisture

absorption for untreated jute strand/starch composites at 10,

20 and 30% (w/w) of reinforcement. After 72 days, the

absorbed humidity was very low. Moreover, the increasing in

the percentage of reinforcement did not affect substantially the

moisture absorption. The experimental values showed a

Fig. 1. Boltzmann parameters and quadratic coefficient of the Boltzmann curve

for the moisture absorption of untreated jute strand/starch composites at 10, 20

and 30 wt.% of reinforcement.

Sigmoidal fit according to the Boltzmann curve as in

Eq. (2), which parameters and quadratic coefficient are also

shown in Fig. 1

y ¼ A2 þðA1 � A2Þ

1þ eðx�x0Þ=dx(2)

where A1 and A2 are the Boltzmann constants.

For the current jute strand/starch based composites, the

existence of hydrogen bonds may be considered the main factor

promoting the good wettability and compatibility constituents

at interface. According to this, the degree of adhesion at fibre–

matrix interface can be enlarged increasing the accessibility of

the surface hydroxyl groups of jute strands, which might

generate more number of hydrogen bonds between the fibres

and the matrix.

Jute strands comprise higher content of lignin than other

strands like hemp or flax [4]. Lignin is a natural polymer

performing the cementation function between the fibres in the

strand. Therefore, a partial delignification by means of an alkali

treatment on the surface of jute strands leaves major number of

hydroxyl groups that will be accessible to the polymer,

increasing the capacity to develop hydrogen bonds at the

interface. The alkali treatment is able to remove, not only the

lignin, but also any volatile component that might be

responsible of giving bad odours from the jute strands. The

results of mechanical properties for composites reinforced with

partially delignified jute strands are also shown in Table 1. Both

tensile and flexural strength improved between 16–19 and 13–

15%, respectively with respect to the composites reinforced

with non-treated jute strands. In relation to impact strength, a

small enhancement was observed. These results confirmed that

a stronger adhesion degree was obtained for composites from

partially delignified jute strands probably due to the major

number of hydrogen bonds between the components. Accord-

ing to some authors [39] the superior strength of alkali treated

jute strand/starch composites may be attributed to the fact that

alkali treatment improves the adhesive characteristics of fibre

surface by removing natural and artificial impurities thereby

producing a rough surface topography. In addition, alkali

treatment leads to fibre fibrillation, i.e. breaking down of the

fibre bundles into smaller fibres, increasing the effective surface

area available for contacting with the polymer matrix. The

development of a rough surface topography and the enhance-

ment of the fibre aspect ratio provide a better fibre–matrix

interface adhesion and an increase in mechanical properties.

The Young’s modulus of NaOH treated jute strand

composites did not show any improvement, which confirms

that the quality of the interface did not affect significantly the

stiffness of the final composite. A slight delignification process

can be a good alternative to improve the interfacial adhesion in

composites and so the maximum stress. However, it has to be

noticed that the cost of the raw material will increase

proportionally to the weight loosed during the delignification

process, and higher residue will be subsequently generated.

In order to study the effect of the adhesion degree at fibre–

matrix interface, an OH blocking reaction onto the jute strands

F. Vilaseca et al. / Process Biochemistry 42 (2007) 329–334 333

Fig. 2. Scheme for the blocking reaction of phenyl isocyanate onto jute strands.

surface was proposed. Therefore, surface chemical modifica-

tion of jute strands was applied using phenyl mono-isocyanate

(Ph-NCO). Fig. 2 illustrates the blocking reaction between jute

hydroxyl groups and phenyl isocyanate generating a urethane

function. The bonded phenyl mono-isocyanate should decrease

the available OH groups and the extension of hydrogen bonds at

fibre–matrix interface. The chemical modification was assessed

by infrared spectroscopy (Fig. 3). A new absorption band at

1540 cm�1 corresponding to the stretching vibration of N–H

groups confirmed the success of the blocking reaction. The

typical stretching vibration of urethane function overlapped to

the absorption band of carboxyl groups of the hemicellulose

Fig. 3. ATR-FTIR spectra for un-treated jute strands and phenyl isocyanate

treated jute strands.

Table 2

Polarity of composites constituents measured as mequiv. of absorbed cationic

polymer per gram of substrate

Material Polarity (mequiv. MGCh g�1)

Crude jute strands 12.16 � 0.01

NaOH treated jute strands 19.18 � 0.01

Ph-NCO modified jute strands 6.75 � 0.01

Starch-based biopolymer 13.26 � 0.01

� indicates the standard deviation.

and lignin included in jute strands. The mechanical results at

Table 1 revealed the high decrease in the tensile and flexural

strengths for Ph-NCO modified jute strand composites, which

were respectively 48 and 44% lower than those obtained with

untreated jute strand composites. The results demonstrated the

great influence of the extension of hydrogen bonds at fibre–

matrix interface, which was also affecting the stiffness of the

final composite. Thus, the Young’s modulus of Ph-NCO treated

jute strand composites was 0.8 times the Young’s modulus of

un-treated jute strand composites. The surface polarity of

composite constituents measured by colloidal titration con-

firmed the different surface polar characteristics of Ph-NCO

modified jute strands with respect to un-treated jute strands

(Table 2). Thus, while the hydrophilic characteristic of un-

treated jute strands was similar to that of starch-based

biopolymer, the Ph-NCO modified jute strands showed lower

polarity. The lower polarity of Ph-NCO modified jute strands

decreased the capacity to created polar interactions between the

constituents resulting in lower mechanical strength and

stiffness of the final composite. On the contrary, the increase

on the ability for producing hydrogen bonds at fibre–matrix

interface, for NaOH treated jute strands, lead to composites

with higher tensile and flexural stresses.

4. Conclusions

The mechanical behaviour of composite materials from

starch-based biopolymer reinforced with jute strands was

studied and the influence of different polar characteristics of

jute strands was considered. The results demonstrated that both

tensile and flexural strength improved with the percentage of

non-modified jute strands by means of a mechanical anchoring

and the formation of hydrogen bonds at fibre–matrix interface.

The Young’s modulus of jute strand composites also increased

with the percentage of reinforcement, in spite of the low

compatibility factor isolated from the Mixture Rule. According

to the stiffening of the composites, the elongation and the

energy to impact decreased.

A partial delignification of jute strands increased the tensile

and flexural strength of composites and the impact strength as

well. A major surface roughness of jute strands, major

accessibility of OH groups on the surface of jute strands and

F. Vilaseca et al. / Process Biochemistry 42 (2007) 329–334334

higher extension of the hydrogen bonds at fibre–matrix

interface were stated to explain the growing of the maximum

stress of composites. The OH blocking reaction showed a

significant decrease of the maximum stress of composites, due

to the decreasing in the availability to form hydrogen bonds at

fibre–matrix interface.

Acknowledgements

This work has been financed by the Spanish Ministry of

Education and Science with the MAT 2002-04299C02-01

project and the Juan de la Cierva Program.

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