biodegradable polymers/bamboo ﬁber biocomposite with bio...

Biodegradable polymers/bamboo fiber biocomposite

with bio-based coupling agent

Seung-Hwan Lee*, Siqun Wang

Tennessee Forest Product Center, 2506 Jacob Drive, University of Tennessee, Knoxville, TN 37996-4563, USA

Received 9 December 2004; revised 28 March 2005; accepted 20 April 2005

Abstract

Effects of lysine-based diisocyanate (LDI) as a coupling agent on the properties of biocomposite from poly (lactic acid) (PLA), poly

(butylene succinate) (PBS) and bamboo fiber (BF) were investigated. Tensile properties, water resistance, and interfacial adhesion of both

PLA/BF and PBS/BF composites were improved by the addition of LDI, whereas thermal flow became somewhat difficult due to cross-

linking between polymer matrix and BF. Crystallization temperature and enthalpy in both composites were increased and decreased with

increasing LDI content, respectively. The heat of fusion in both composites was decreased by addition of LDI, whereas there was no

significant change in melting temperature. Thermal degradation temperature of both composites was lower than those of pure polymer

matrix, but the composites with LDI showed higher degradation temperature than those without LDI. Enzymatic biodegradability of PLA/BF

and PBS/BF composites was investigated by Proteinase K and Lipase PS, respectively. Both composites could be quickly decomposed by

enzyme and the addition of LDI delayed the degradation.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Biocomposite; Poly (lactic acid); Poly (butylene succinate); Bamboo fiber; Lysine diisocyanate; Coupling agent

1. Introduction

In recent years, the development of biocomposites from

biodegradable polymers and natural fibers have attracted

great interests in the composite science, because they could

allow complete degradation in soil or by composting

process and do not emit any toxic or noxious components

[1–16].

Among the biodegradable polymers, in particular, poly

(lactic acid) (PLA) and poly (butylene succinate) (PBS) are

of increasing commercial interest. PLA can be synthesized

by the condensation polymerization of the lactic acid or ring

opening polymerization of the cyclic lactide dimer.

Advanced industrial technologies of polymerization have

been developed to obtain high molecular weight PLA that

leads to a potential for structural materials with enough

lifetime to maintain mechanical properties without rapid

hydrolysis even under humid environment, as well as good

1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesa.2005.04.015

* Corresponding author. Tel.: C1 865 974 4965; fax: C1 865 946 1109.

E-mail address: [email protected] (S.-H. Lee).

compostability. PLA is transparent and crystalline polymer

with relatively high melting point and has brittle properties,

i.e. high strength and low elongation at break [17].

PBS is a white crystalline thermoplastic with melting

point of about 90–120 8C (similar to LDPE), glass transition

temperature of about K45 to K10 8C (between PE and PP),

tensile strength between PE and PP, and stiffness between

LDPE and HDPE. PBS has excellent processing capabilities

and can be processed on polyolefin processing machines at

temperatures of 160–200 8C, into various products, such as

infected, extruded and blown ones [1]. Furthermore, its raw

materials, butanediol and succinic acid, may be soon

available from bio-based renewable resources [18–23].

Natural fibers can be a renewable and cheaper substitute

for synthetic fibers, such as glass and carbon and have

numerous advantages, such as low cost, low density, high

toughness, acceptable specific strength properties, ease of

separation and biodegradability. So, there is much research

on natural fiber-reinforced composites [24–33]. However,

the main drawback of natural fiber may be their hydrophilic

nature, which decreases the compatibility with hydrophobic

polymeric matrix. In these composite fields, therefore, most

of the research has focused on improving interfacial

Composites: Part A 37 (2006) 80–91

www.elsevier.com/locate/compositesa

http://www.elsevier.com/locate/compositesa

S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–91 81

properties between the polymer matrices and natural fillers

in order to enhance the physical and mechanical properties

of the end products.

The purpose of this study was to develop the

biocomposites with designable interfacial properties from

biodegradable polymers, PLA and PBS, and bamboo fiber

by using lysine-diisocyanate as a bio-based coupling

agent.

So far, several conventional isocyanates, such as

methylene diisocyanate (MDI), toluene diisocyanate

(TDI), 4-4 0-methylenedicyclohexyl diisocyanate (hydro-

genated MDI), and hexamethylene diisocyanate have

been used as a coupling agent [34–38]. For example,

Wang et al. reported the effect of MDI on the properties

of PLA/starch blend. The addition of MDI resulted in

an enhancement of mechanical properties and water

resistance [39–40].

However, these isocyanates have found limited use as a

biocompatible material because their ultimate hydrolysis

products, i.e. their corresponding diamines, such as 4,4 0-

methylenedianiline and 2,4-diaminotoluene have been

found to be a cancer suspect agent or produce hepatitis in

man. So, the use of nontoxic materials should be expected as

a coupling agent, in order to synthesize fully biodegradable

biocomposites without emitting toxic or noxious

components.

Lysine-diisocyanate (LDI) is based on lysine with two

amino groups and one carboxyl group, which is one of

natural amino acids. LDI can react with hydroxyl or

carboxyl groups in PLA or PBS, producing urethane

bonds that can be easily and completely hydrolyzed into

raw materials [41–45]. For example, the polyurethane

that synthesized from LDI, glycerol, and ascorbic acid

can be completely degraded in aqueous solution and

yield the nontoxic breakdown products of lysine,

glycerol, and ascorbic acid [46]. In fact, our interest in

LDI as a bio-based coupling agent stems from these

facts, because ecotoxicity is currently a key point in

biocomposites.

2. Experimental

2.1. Materials

PLA (LACEA H-100J) and PBS (Enpol G5300) were

purchased from Mitsui Chemical, Inc. (Tokyo, Japan) and

Ire Chemical Ltd (Wonju, Korea), respectively. Average

length and diameter of bamboo fiber (BF) used in

this study were approximately 500 and 70 mm, respect-

ively. L-lysine-diisocyanate (LDI) was kindly supplied by

Kyowa Hakko Co., Ltd (Tokyo, Japan). The BF was

oven-dried at 105 8C for 6 h, whereas PLA and PBS were

vacuum-dried at 40 8C for 24 h prior to use. Proteinase K

and Lipase PS were purchased from Nacalai Tesque, INC.

(Kyoto, Japan). All other chemicals were purchased from

commercial sources.

2.2. Compounding and compression molding

The polymers and BF were first mixed as dry solids. The

mixture was placed into a batch mixer (Labo Prostomill,

Toyo Seiki, Japan) rotating at a speed of 30 rpm. After the

addition, the rotation speed was increased to 70 rpm and

kneading was conducted for 5 min. Then LDI was added to

the mixture and kneading was further carried out for 10 min.

The amount of LDI added was expressed by NCO content as

following equation

NCO content ZWeight of NCO group=Weight of composite

!100 ð%Þ ð1Þ

The kneading temperatures were 180 8C and 140 8C in

the cases of PLA and PBS, respectively. The kneaded

samples were compression-molded into sheets under a

pressure of 150 kgf cmK2 at 180 8C (PLA/BF composite)

and 140 8C (PBS/BF composite).

2.3. Tensile properties and water absorption test

Dog-bone-shaped samples (5!0.4!50 mm) were cut

from compression-molded sheets and tensile measurements

were made with a Shimadzu Autograph AG-1 (5kN)

(Kyoto, Japan) machine at a cross head speed of

5 mm minK1.

The samples with dimensions 50!50!0.5 mm were

used to examine water absorption behavior after vacuum

drying at 45 8C for 24 h. The samples were immersed in

deionized water (25 8C) and periodically taken out of the

water. Then, the excess water on the surface was removed

by blotting with tissue paper and specimens were weighed.

The amount of water absorbed (Mt) was calculated as

follows

Mtð%Þ Z ðWt KWoÞ=Wo !100 (2)

where Wt and Wo, are the weights of the specimen before

and after immersion in water respectively.

2.4. Differential scanning calorimeter (DSC) and thermal

gravity analysis (TGA)

DSC measurement was performed on a Perkin-Elmer

Diamond DSC. The samples (about 7 mg) were heated to

200 and 150 8C at 20 8C minK1 in PLA and PBS/BF

composites, respectively, and kept for 5 min to remove the

thermal history. Then, the samples were cooled to 40 8C at

the rate of 2 and 10 8C minK1 in PLA and PBS/BF

composites, respectively, and heated again to 200 and

150 8C at the rate of 20 8C minK1 in PLA and PBS/BF

composites, respectively. The endothermic and exothermic

Table 1

Activation energy for flow of the PLA and PBS/BF (70/30) composites

NCO content (%) Activation energy for flow (kJ/mol)

PLA/BF composite PBS/BF composite

0 193.9 63.4

0.11 195.1 76.5

0.33 195.3 91.5

0.65 197.0 114.1

1.30 211.9 –

S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–9182

peaks were termed as crystallization temperature (Tc) and

melting temperature (Tm), respectively. The crystallization

enthalpy (DHc) and the heat of fusion (DHm) were also

determined from the area of crystallization and melting

peaks, respectively. These values in the composite were

divided by the weight proportion of polymer and the

obtained values were parenthesized in Table 1.

Thermal degradation temperature was measured by a

Perkin-Elmer Pyris 1 TGA. Samples of about 7 mg were

heated from 50 to 600 8C at a rate of 10 8C minK1 under

nitrogen flow (20 mL/min).

2.5. Thermal flow properties

Apparent melt viscosity of composites was measured at

various temperatures with a flow tester (Shimadzu CFT-

500D). The shear stress applied was 2.452!104 Pa and the

orifice diameter and length of die were 1 and 10 mm,

respectively.

Fig. 1. SEM micrographs of interface between matrix and BF in PLA/BF (70/30) c

(70/30) composite without (C), or with LDI (NCO content, 0.65%) (D).

2.6. Enzymatic hydrolysis

The vacuum-dried samples (10!10!0.4 mm3) were

weighed and dipped in a test tube of 5 ml of the Proteinase K

solution (50 mM Tris–HCL buffer, pH 8.6) and Lipase PS

solution (0.1 M phosphate buffer, pH 6.0) in the case of PLA

and PBS/BF composites, respectively. The test tube was

sealed and kept at 38 8C for a predetermined time and

replaced every 48 h (Proteinase K) and 1 week (Lipase PS),

so that enzyme activity remained at a desired level

throughout the experiment duration. The sample was then

removed from the solution, washed thoroughly with

distilled water and ethanol, and then dried in vacuum at

38 8C. The time-course of the weight loss was evaluated and

the remaining samples were observed by SEM.

2.7. Scanning electron microscopy

The morphology of the fractured samples after tensile

testing was examined using a JEOL JSM-5900LV scanning

microscope.

3. Results and discussion

3.1. Interfacial morphology

Many of properties in composite materials would be

affected by their morphology. Fig. 1 shows SEM

omposite without (A), or with LDI (NCO content, 0.65%) (B), and PBS/BF

0

10

20

30

40

50

0 0.5 1 1.5

0 0.5 1 1.5

Ten

sile

ste

ngth

(Mpa

),E

long

atio

n at

bre

ak (

%)

0

10

20

30

40

50

Ten

sile

ste

ngth

(Mpa

),E

long

atio

n at

bre

ak (

%)

2000

2500

3000

3500

You

ng's

mod

ulus

(M

pa)

You

ng's

mod

ulus

(M

pa)

PLA/BF composite

PBS/BF composite

NCO content (%)

1000

1200

1400

1600

Fig. 2. Effect of NCO content on the tensile properties of PLA or PBS/BF

(70/30) composites. C, Tensile strength; &, Young’s modulus; :,

Elongation at break.


micrographs of the tensile fractured surface of PLA and

PBS/BF composite with or without LDI. In the PLA/BF

composite without LDI, two phases can be clearly seen and

many BFs were pulled out from the matrix in the fracture

process, with large voids thereby being created. Also, gaps

between PBS and BF in the PBS/BF composite without LDI

were visible. These findings suggest that the interaction

between matrix and filler was very weak, resulting in less

interfacial adhesion. These features are typical of incompa-

tible polymer composites.

On the other hand, the micrographs of both composites

after compounding with LDI of 0.65% showed that BF

appeared to be coated with matrix polymer. This improved

interfacial adhesion may be due to the compatible effect of

graft copolymer with LDI intermediates, which could be

produced through a chemical reaction between the hydroxyl

groups of polymer and BF under kneading conditions of

higher temperatures and pressure [39,40]. In our previous

paper, it was confirmed that isocyanate groups react with

terminal hydroxyl or carboxyl groups of PLA and the

hydroxyl groups of corn starch, producing a graft copolymer

of PLA and CS with urethane linkage that is confirmed by

NMR measurement [15]. The morphology results support

the improvement of tensile properties and water resistance

by adding LDI, which will be discussed below.

3.2. Tensile properties

Fig. 2 shows the effect of isocyanate group (NCO)

content on the tensile properties of the PLA and PBS/BF

(70/30 by wt.) composites. In the case of PLA/BF

composite, as NCO content increased to 0.33%, tensile

strength and Young’s modulus increased rapidly from 29 to

42 MPa and from 2666 to 2964 MPa, respectively, and then

leveled-off. However, there was no significant effect of LDI

addition on the elongation at break, showing the value of

less than 5%. Similarly, the tensile strength of PBS/BF

composites increased steeply from 21 to 34 MPa with NCO

contents of 0.33%, respectively, and then leveled-off.

However, Young’s modulus was not much changed by

the addition of LDI. Elongation at break increased slightly

by addition of LDI, but still showed the value less than

10%. The NCO group content at which the value of

tensile strength plateaued could be considered the critical

interfacial concentration, which is the minimum value

of interfacial saturation for a coupling agent in the

dispersed phase, because the average size of the dispersed

BF will not be changed with the addition of the coupling

agent [15,47–49].

Figs. 3 and 4 show the effect of BF content on the tensile

properties of PLA and PBS/BF composites with or without

LDI. The NCO content was set at 0.65%. In both composites

without LDI, the tensile strength gradually decreased with

the increase of BF content. This may be due to poor

interfacial adhesion between the polymer matrix and BF

filler. This is a general phenomenon in incompatible

composites with different characteristics, such as hydro-

phobicity of the polymer matrix and hydrophilicity of

the filler [50–53]. However, the tensile strength of the PLA/

BF composite with LDI kept the value of about 45 MPa by

increasing BF content up to 30% and then decreased.

These values were higher than those of the composites

without LDI. Furthermore, the tensile strength of PBS/BF

composite with LDI increased by increasing BF content and

this improvement was more pronounced at higher BF

contents. This may be attributed to enhanced interfacial

adhesion between the polymer matrix and BF filler by

adding LDI.

All composites showed the increase of Young’s modulus

as BF content increased. This is common in composites

reinforced with a hard filler. On the other hand, the

composites with LDI showed higher values than the

composite without, indicating that the composite was

further hardened by the addition of LDI.

Elongation at break decreased with increasing BF

content in all composites. This may be due to the increase

in the discontinuity of the polymer matrix with the increase

in the dispersed phase (BF). However, the composites with

LDI showed slightly higher elongation at break than the

composite without LDI.

0

10

20

30

40

50

60T

ensi

le s

teng

th (

Mpa

)

1000

2000

3000

4000

You

ng's

mod

ulus

(M

pa)

0

2

4

6

8

0 10 20 30 40 50

BF content (%)

Elo

ngat

ion

at b

reak

(%

)

Fig. 3. Effect of BF content and the addition of LDI on the tensile properties

of PLA/BF composites. White, with LDI (NCO content 0.65%); black,

without LDI.

0

10

20

30

40

50

60

Ten

sile

ste

ngth

(M

pa)

0

500

1000

1500

2000

2500

You

ng's

mod

ulus

(M

pa)

0

5

10

15

20

0 10 20 30 40 50

BF content (%)

Elo

ngat

ion

at b

reak

(%

)

Fig. 4. Effect of BF content and the addition of LDI on the tensile properties

of PBS/BF composites. White, with LDI (NCO content, 0.65%); black,

without LDI.


3.3. Water absorption

BF is hydrophilic because it contains an abundance of

hydroxyl groups, but PLA and PBS are hydrophobic polymers.

The effect of the addition of LDI and BF content on the water

absorption behavior of the composites was investigated and

indicated in Fig. 5. The water absorption for all composites

increased greatly during the first 20 h and then leveled-off.

When compared to the composite with and without LDI,

absorption amount and time to reach the plateau were smaller

and longer, respectively, for all composites with LDI than

without, indicating that the addition of LDI makes the

absorption of water difficult. This can be explained by the

improvement in interfacial adhesion between the polymer

matrix and BF filler due to the coupling effect of LDI and the

reaction of LDI with the hydroxyl groups of polymers and BF

resulting in less hydrophilicity.

It was also found that the absorption amount increased in

all composites as BF content increased. Because PLA and

PBS can absorb only the water of about 1%, BF content

will be the major factor affecting the water absorption of

the composites. The increase of BF content caused

1/T (K×10–4)

5

6

7

8

9

10

22 23 24 25

2

3

4

5

6

7

21 21.5 22 22.5 23 23.5

1/T (K×10–4)

Fig. 6. Relationship of logarithm apparent melt viscosity (Ln h) and

reciprocal temperature (1/T) in PLA/BF composite. NCO content: C, 0%;

&, 0.11%; :, 0.33%; %, 0.65%; *, 1.30%.

0

4

8

12

16W

ater

abs

orpt

ion

(%)

PLA/BF composite

0

4

8

12

16

20

0 20 40 60

Time (h)

Wat

er a

bsor

ptio

n (%

)

PBS/BF composite

Fig. 5. Effect of BF content and the addition of LDI on water absorption

of PLA or PBS/BF composites. CB, 90/10; &,, 80/20; :6, 70/30; %,

60/40; *C, 50/50; white and *, (solid line) with LDI (NCO content,

0.65%); black and C, (dash line) without LDI.


the absorption rate to quicken, because of its strong

hydrophilicity.

Table 2

Effect of LDI on crystallization and melting behavior of the PLA and

PBS/BF (70/30) composites

Sample NCO

content

(%)

Tc (8C) DHc (J/g) Tm (8C) DHm (J/g)

PLA – 112.8 33.2 165.3 41.1

PLA/BF

composite

0 115.2 23.5 (33.6) 162.3 27.4 (39.1)

0.11 118.4 21.5 (30.7) 162.7 23.5 (33.6)

0.33 118.7 20.2 (28.9) 161.8 23.6 (33.7)

0.65 119.4 16.4 (23.4) 162.2 23.1 (33.0)

1.30 120.2 14.5 (20.7) 161.6 22.8 (32.6)

PBS – 68.1 60.1 112.1 65.3

PBS/BF

composite

0 74.6 40.8 (58.2) 112.0 42.3 (60.4)

0.11 78.0 37.8 (54.0) 112.1 35.8 (51.1)

0.33 80.2 38.1 (54.4) 112.3 29.4 (42.0)

0.65 82.2 37.3 (53.3) 112.9 24.5 (35.0)

1.30 82.6 31.2 (44.6) 112.1 21.7 (31.0)

( ) Values divided by the weight proportion of polymers.

Fig. 7. Relationship of logarithm apparent melt viscosity (Ln h) and

reciprocal temperature (1/T) in PBS/BF composite. NCO content: C, 0%;

&, 0.11%; :, 0.33%; %, 0.65%.
3.4. Thermal flow properties
Figs. 6 and 7 show the relationship of logarithm apparent

melt viscosity (Ln h) and reciprocal temperature (1/T) in

PLA and PBS/BF composites, respectively. Under the given

shear stress (4.9!104 Pa), the apparent viscosity of all

composites decreased with increasing temperature. The

relationship between Ln h and 1/T is linear, indicating that

the dependence of h on temperature obeys the Arrhenius

equation:

h Z A expðE=RTÞ (3)

where A is a constant that is related to material properties; E,

the activation energy for flow; R, the universal gas constant;

and T, the absolute temperature.

According to the Arrhenius equation, the activation

energy for flow was calculated and the values are

summarized in Table 1. The composites with LDI in both

composites require higher activation energy for flow

compared to the composite without LDI, showing larger

values than the composite without LDI. It was also found

Fig. 8. TGA thermograms of PLA, PBS, BF and PLA or PBS/BF composites with different NCO content.


that the activation energy increased as LDI content

increased. This suggests that the addition of LDI leads to

an increase in hindrance of molecular motions of polymer

matrix due to crosslinks with BF, making thermal flow

difficult.

3.5. Crystallization and melting behavior

Thermal properties were investigated by DSC and the

obtained results are summarized in Table 2. Pure PLA and

PBS had a crystallization temperature (Tc) of 112.8 8C and

68.1 8C, respectively. In both PLA and PBS/BF composites,

the Tc shifted to high temperature by adding either BF or

LDI, and further increased as LDI content increases. These

higher Tc values of the composites than the pure polymers

indicate that the crystallization rate than the composites

becomes more rapid in nonisothermal processes. The

increase in Tc could be considered to be due to the nucleation

effect of the BF and LDI. In particular, the urethane linkage

between polymer matrix and BF produced by the addition of

LDI might further enhance nucleation of polymer matrix. In

both composites, however, crystallization enthalpy (DHc)

was decreased by increasing LDI content. The molecular

motion of the polymer matrix could be restricted by the

addition of LDI, resulting in a decrease of crystallization

enthalpy.

Melting temperatures (Tm) of pure PLA and PBS were

165.3 and 112.1 8C, respectively. These Tms were not

significantly affected by the addition of either BF or LDI.

However, the heat of fusion (DHc) in both composites was

decreased by addition of BF and LDI. This may be also

attributed to the strong interfacial interaction between

Fig. 9. DTG thermograms of PLA, PBS, BF and PLA or PBS/BF composites with different NCO content.

100


polymer matrix and BF, confining polymer chain

orientation.

0

20

40

60

80

0 5 10 15 20

Degradation time/day

Wei

ght r

emai

ning

/%

Fig. 10. Weight remaining for pure PLA and PLA/BF composites with and

without LDI plotted as a function of enzymatic degradation time. C, PLA;

&, PLA/BF composite without LDI; :, with LDI (NCO content, 0.33%);

%, (0.65%).

3.6. Thermal degradation

Figs. 8 and 9 show the TGA curves and derivative

thermograms (DTG) for PLA, PBS, BF, and the PLA and

PBS/BF composite with different LDI content.

Thermal degradation of PLA and PBS showed com-

pletely in a single stage and occurred at 376 and 405 8C,

respectively. In the case of BF, main three-stage loss of

mass was observed. The first stage with a small hump in the

temperature range from 250 to 300 8C is characteristic of

low molecular weight components, such as hemicellulose

and the second one, appearing at higher temperatures in

the range of 300 and 400 8C, is corresponded to the thermal

degradation of cellulose. And the third one near 420 8C is

due to lignin decomposition.

0

20

40

60

80

100

0 5 10 15 20

Degradation time/day

Wei

ght r

emai

ning

/%

Fig. 11. Weight remaining for pure PBS and PBS/BF composites with and

without LDI plotted as a function of enzymatic degradation time. C, PBS;

&, PBS/BF composite without LDI; :, with LDI (NCO content, 0.33%);

%, (0.65%).

Fig. 12. SEM micrographs of PLA or PBS/BF composites with LDI (NCO content

composite, (B) after 7 days, (C) 9 days, (D) control of PBS/BF composite, (E) af


In the case of PLA/BF composites, the incorporation of

BF in PLA matrix has significantly affected the thermal

degradation temperature. The composites showed a lower

degradation temperature (more than 50 8C) than that of PLA

and a two-stage loss of mass was mainly observed for all

composites. That is, the degradation in the range of 280–

340 8C is due to PLA degradation and the small hump near

350 8C is due to BF degradation. These lower degradation

temperatures may be attributed to the decrease of molecular

weight of PLA by high kneading temperatures (180 8C). In

the composites, however, thermal degradation temperature

was increased by increasing LDI content. It can be generally

said that the increase of molecular weight by cross-linking

reaction between matrix and BF, or molecular chain-exten-

sion of the matrix itself, could increase the thermal

degradation temperature [54–56].

PBS/BF composites showed an intermediate thermal

stability between those of PBS and BF and the addition of

, 0.65%) after different enzymatic degradation time. (A) control of PLA/BF

ter 2 days, (F) 4 days.

Fig. 13. SEM micrographs of PLA/BF (after 7 days) and PBS/BF (after 7 days) composites with or without LDI after enzymatic degradation. (A) PLA/BF

composite without LDI, (B) with LDI (NCO content, 0.33%), (C) (0.65%), (D) PBS/BF composite without LDI, (E) with (NCO content, 0.33%), (F) (0.65%).


LDI also improved the thermal stability of the composite,

showing higher thermal degradation temperature in the

composite with LDI.

3.7. Enzymatic degradation

The effect of the addition of LDI on biodegradability of

the composites was investigated by enzymatic degradation

using Proteinase K and Lipase PS, which are well known to

degrade easily PLA and PBS, respectively [57–59]. Figs.

10 and 11 show the weight remaining for PLA and/or PBS/

BF composites with and without LDI along with pure PLA

and PBS as a function of elapsing time, respectively. It was

observed that the weight remaining of all samples

decreased almost linearly with elapsing time. The

degradation of pure PLA and PBS was slower than that

of all composites, indicating that BF improved the

degradation of both polymers. As compared between the

composites with and without LDI, all composites with LDI

were more difficult to be degraded than those without. By

increasing LDI content, degradation rate became much

slower. The improved interfacial adhesion between

polymer matrix and BF by coupling effect of LDI will

make the area exposed to enzyme hydrolysis smaller,

resulting in decreasing degradation rate.

Fig. 12 shows the SEM micrographs of PLA or PBS/BF

(70/30) composites with LDI degraded for different

degradation time. It can be clearly seen that the matrix of

both composites became reduced as enzymatic degradation

proceeded. Fig. 13 shows the effect of LDI content on the

degradation at the same period. In the case of the composites

without LDI (A and D), the majority of matrix was degraded

even though degradation time is short. However, the loss of

matrix occurred slower in the composites with LDI than

without LDI and lots of parts remained in the composites with

more LDI content. As mentioned above, stronger interfacial

adhesion will reduce the area exposed to enzyme hydrolysis

and this would result in a longer degradation time.

4. Conclusions

A low concentration of LDI as bio-based coupling agent

was added to environmental-friendly biocomposite during


kneading process. Particularly, tensile properties and water

resistance were appreciably improved by this mechano-

chemical reactive processing, which will be of merit for

industrial applications. These improvements were due to the

enhanced interfacial adhesion between the polymer matrix

and BF. Furthermore, the results of enzymatic degradation

showed that biodegradability could be adjusted by

controlling the degree of interfacial adhesion using LDI.

In areas, where biocompatibility and environmentally

responsible design and construction are required, these

biocomposites have potential for dramatic growth with a

green concept. Primary applications for biocomposites

include toys for children, furniture, flooring, hardware for

electronic products, especially one-way disposable

products, and so on.

Acknowledgements

This work was supported by the USDA Wood Utilization

Research Grant and the Tennessee Agricultural Experiment

Station, Project #83.

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