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
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.
S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–91 83
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.
S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–9184
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.
S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–91 85
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 propertiesFigs. 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.
S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–9186
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
S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–91 87
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
S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–9188
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%).
S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–91 89
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
S.-H. Lee, S. Wang / Composites: Part A 37 (2006) 80–9190
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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