Biodegradable composites based onl-polylactide and jute fibres

Composites Science and Technology 63 (2003) 1287–1296
www.elsevier.com/locate/compscitech
Biodegradable composites based on l-polylactide and jute fibres
David Placketta,*, Tom Løgstrup Andersenb, Walther Batsberg Pedersenc, Lotte Nielsenc
a
Danish Polymer Centre, Technical University of Denmark, Building 423, Lyngby 2800, Denmark
b
Materials Research Department, Risø National Laboratory, 4000 Roskilde, Denmark
c
Danish Polymer Centre, Risø National Laboratory, 4000 Roskilde, Denmark
Accepted 21 February 2003
Abstract
Biodegradable polymers can potentially be combined with plant fibres to produce biodegradable composite materials. In our
research, a commercial l-polylactide was converted to film and then used in combination with jute fibre mats to generate composites by a film stacking technique. Composite tensile properties were determined and tensile specimen fracture surfaces were examined using environmental scanning electron microscopy. Degradation of the polylactide during the process was investigated using
size exclusion chromatography. The tensile properties of composites produced at temperatures in the 180–220 C range were significantly higher than those of polylactide alone. Composite samples failed in a brittle fashion under tensile load and showed little
sign of fibre pull-out. Examination of composite fracture surfaces using electron microscopy showed voids occurring between the
jute fibre bundles and the polylactide matrix in some cases. Size exclusion chromatography revealed that only minor changes in the
molecular weight distribution of the polylactide occurred during the process.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Polylactide; A. Fibres; B. Mechanical properties; Electron microscopy; Size exclusion chromatography
1. Introduction
Biodegradable polymers have been the subject of
research and development since the early 1970s and
growing pressure on the world’s resources as well as
concerns about disposal of plastics led to intensified
interest and commercial activity in the 1990s [1]. Biodegradable polymers, or biopolymers as they are sometimes known, are now attracting interest for a wide
range of applications from special high-performance
products in medicine and surgical applications through
to commodity uses in films and packaging [2].
In terms of volume applications, the biopolyester
polylactide or PLA is arguably the biodegradable polymer that has the greatest commercial potential. PLA is a
polymer of lactic acid and is commonly produced by
ring-opening polymerisation of the cyclic lactide dimer
[3]. PLA may also be produced by direct polycondensation of lactic acid and although this route usually yields
fairly low molecular weight species, special procedures
can overcome this problem [4]. Lactic acid, the starting
material for PLA synthesis, can be produced by fer* Corresponding author. Fax: +45-4588-2161.
E-mail address: dp@polymers.dk (D. Plackett).
mentation from a number of different renewable
resources and, for example, a large PLA production
facility based on use of corn starch-derived lactic acid
and with an annual production capacity of 140,000
metric tons is currently being established in North
America by Cargill Dow LLC, a joint venture between
Cargill Corporation and Dow. A recent review article
lists a number of other current manufacturers of PLA
[5].
The commercially attractive features of PLA, for
example as a future commodity packaging material,
include its production from renewable resources as well
as its good mechanical properties. After use, PLA polymers can be recycled or alternatively disposed of by
incineration or by landfilling. Landfill disposal effectively closes the loop in returning the polymer to the soil
where it biodegrades. Obstacles in the path of wider
application of PLA products include the present price of
the polymer relative to commodity thermoplastics and
its brittle character in thicker materials. In addition, a
recent article suggested that biodegradable polymers are
not really as ‘‘green’’ as is generally assumed if total
energy costs are considered [6]. However, PLA is still in its
relative infancy in terms of development for commercial
applications and considerable progress in overcoming
0266-3538/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0266-3538(03)00100-3
1288
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
perceived problems with its use and parallel decreases in
bulk prices might be anticipated in the coming years.
For example, Bogaert and Coszach [7] suggest that the
sales price for PLA could decrease significantly over the
period 2001–2008 while market potential could increase
by a factor of 10.
The use of natural fibres to reinforce thermoplastics
such as polypropylene and polyethylene as an alternative to synthetic or glass fibres has been and continues to be the subject of research and development.
The potential advantages of using natural fibres have
been well documented and are generally based on
environmental friendliness as well as health and safety
factors [8]. Challenges to the use of natural fibres
include their tendency to absorb moisture and their
susceptibility to thermal degradation at processing
temperatures over 200 C, although technical solutions
to these problems are feasible. Wider adoption of natural fibres in polymer reinforcement would also be
assisted by the development of international standards
as well as methods to produce agro-fibres in a way that
gives consistent material from season to season and
from year to year.
A number of researchers and research groups
have already identified the possibilities for new fully
biodegradable composite products through combining
biodegradable polymers with natural fibre reinforcement [9–11]. The review paper by Mohanty et al. [5] also
provides a useful reference point and contains a summary of literature pertaining to biofibre-reinforced biodegradable polymers. As also pointed out by Keller et
al. [12], the application of fibre reinforcement to biopolymers such as PLA should produce composites with
suitable mechanical properties for lightweight construction materials. Given the increasing commercial interest
in natural fibre-reinforced polymer composites, for
example in the building products and automotive sectors in particular, and demands for materials that can
either be recycled or can safely biodegrade in the environment at the end of their service life, the development
of fully biodegradable composites for a wide range of
applications could be an interesting next step in the
evolution of natural fibre composite materials.
This paper describes research in which composites
were prepared from a commercial PLA in combination
with jute fibres. Jute (Corchorus capsularis L.) is an
important tropical crop and grows in India, Bangladesh, China, Thailand, Nepal and Indonesia. In our
research, jute in the form of a non-woven mat was used
as a model system for what might be feasible with a
wide range of plant fibres. The PLA/jute composites
were characterised in terms of mechanical properties
and also examined microscopically. In addition, the
possible degradation of the PLA polymer at high temperature during processing was investigated using size
exclusion chromatography (SEC).
2. Experimental
2.1. Materials and processing
The PLA polymer used in this work was a commercial
l-polylactide granulate identified as L5000 from Biomer
of Krailling, Germany. Jute fibres were used in the form
of a non-woven mat of basis weight 300 g/m2 purchased
from JB Plant Fibres Ltd of the UK.
The PLA granulate was used as received and was
converted into a film approximately 0.2 mm in thickness
using a Haake single-screw extruder. Temperatures in
the three extruder barrel zones were set to 160, 180 and
190 C and the temperature in the die was set at 190 C.
The PLA film was collected on rolls and stored in the
laboratory under ambient conditions prior to use.
Composites containing about 40% jute fibre by
weight were produced using a film-stacking procedure
as shown schematically in Fig. 1. Lay-ups (300 mm120
mm) were prepared in which sections of jute fibre mat
were stacked up with several PLA film layers on either
side. For this procedure, jute mat sections were cut with
the length direction at 90 to the direction of the mat
roll. This was because previous experience had shown
that mat properties varied with direction and that sections cut at 90o provided composites with higher tensile
properties than sections cut in the direction of the roll.
The lay-ups were subjected to rapid press consolidation
using a process involving the following steps: (1) precompression, (2) contact heating under vacuum, (3)
rapid transfer to a press for consolidating and cooling
and (4) removal of the finished part from the press. In a
typical experiment, a lay-up of PLA films and jute fibre
mat sections was placed between two Teflon sheets in a
Fig. 1. Schematic of process for fabricating PLA/jute composites.
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
metal frame, pre-pressed for 15 s at 3.3 MPa and then
rapidly conveyed into the heating stage. The heating
section was sealed automatically as soon as the lay-up
had been transferred and pressure in the section was
then reduced to approximately 400 Pa (four millibar)
within 1–2 min. In this stage, the pre-compressed layups were heated by contact with the platens for a total
time of either 3 or 10 min with the platen temperatures
set at 180, 190, 200, 210 or 220 C. At the end of the
heating stage the frame holding the lay-up was rapidly
transferred back to the press where the assembly was
consolidated under a pressure of 3.3 MPa at 60 C for 1
min.
The quality of the 2 mm-thick pressed composites was
examined by cross-cutting a small section from each
trial panel, wetting the cross-section and looking for
indications that fibres had not been wetted. Fibre wetting appeared to be inadequate when processing at
180 C for 3 min and therefore a heating time of 10 min
was found to be necessary in this case. Heating times of
3 min were used at all other process temperatures.
Reference sample panels without fibre reinforcement
were prepared in a similar manner by laying up a sufficient number of PLA films without inclusion of fibre
mat sections and using a heating time of 10 min at
190 C. In order to determine the influence of jute fibre
moisture content on PLA during processing, a number
of mat sections were vacuum oven dried overnight and
then used to make composites at process temperatures
of 200, 210 and 220 C. Samples for characterisation
studies including tensile and impact property tests and
analysis by size exclusion chromatography were cut
from each panel.
2.2. Tensile tests
A total of three or four specimens for tensile testing
were prepared by cutting rectangular samples from each
test panel and then converting these to the final dogbone shape using a computer-controlled routing
machine. Specimens measured 180 mm in length, 25 mm
in maximum width and 15 mm in the narrowest section.
The test specimens were placed in a conditioning room
at 65% relative humidity and 23 C and weighed at
regular intervals until equilibrium had been reached.
Following conditioning, each dogbone specimen was
measured for width and thickness at the mid-point and
at locations 10 mm either side of the mid-point. These
measurements were used to calculate initial sample
cross-section. Samples were then subjected to tensile
testing according to ASTM D 638-99 [13] using an
Instron 6025 100 kN test machine. The machine was
operated at a crosshead speed of 2 mm/min. The longitudinal strain of each specimen was recorded with two
back-to-back extensometers. The average strain was
recorded and stress was calculated as load divided by
1289
initial cross-section. Results were expressed as plots of
stress (MPa) against strain (%). The ultimate tensile
strength or tensile strength at yield, the tensile stiffness,
as determined by the slope of the tangent to the stress–
strain curve drawn through the origin, and elongation
at maximum stress were determined from the stress–
strain plots.
2.3. Impact tests
The impact resistance of a PLA/jute composite in
comparison to a pure PLA material was determined on
unnotched Izod test specimens by a procedure outlined
in ASTM standard D 256-97 [14] and using a Zwick test
machine with a pendulum of 0.46 J energy.
Samples were conditioned at 65% relative humidity
and 23 C until equilibrium moisture content before
testing. A total of 15 samples of PLA and 15 of PLA/
jute composite were tested to determine mean impact
resistance. The PLA/jute composites had been prepared
using a heating time of 3 min at 200 C. The reference
PLA panels were prepared using a heating time of 10
min at 190 C because other time/temperature combinations resulted in unacceptable air pocket entrapment
within the panels.
2.4. Electron microscopy
The fracture surfaces of selected tensile test specimens
were examined using an ElectroScan E3 environmental
scanning electron microscope (ESEM). Samples for
examination were obtained by cutting sections about 2–
3 mm in length from just below the fracture zone.
Tweezers were then used to place sections with fracture
surfaces facing upwards and these were fixed on to a
sample holder by embedding in a layer of wax. The
sample holder was then placed in the ESEM vacuum
chamber and the pressure in the chamber reduced to
approximately 470 Pa (3.5 Torr). A fine mist of water
was sprayed into the ESEM vacuum chamber in order
to enhance sample surface conductivity and to optimise
the clarity of the image. The instrument was operated at
20 kV and various sample surfaces were scanned to
obtain an impression of fibre fracture, fibre distribution
and the appearance of the fibre/polymer interface.
2.5. Size exclusion chromatography
Small samples of PLA granulate, PLA film, PLA
panels or PLA/jute panels, measuring a few mm on
either side, were prepared for SEC analysis by dissolving in tetrahydrofuran (THF) overnight at room temperature. The samples were then heated at 65 C for 6 h
to obtain complete dissolution of PLA. Before injection,
the samples were filtered through a 0.2 mm Millipore filter
(FGLP type). A sample of jute fibre mat was subjected to
1290
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
the same treatment in order to determine whether jute
fibre extracts might interfere with the analyses.
A separation according to molecular size was
obtained by using a pre-column and a HT6E column
from Waters Associates, USA. The eluate from the columns was monitored using both RALS (light scattering)
and RI (refractive index) detectors from Viscotek Corporation, USA. Data was handled by means of Trisec1
software supplied by Viscotek Corporation. Separations
were performed at room temperature using THF as
eluent at a flow rate of 1 ml/min. Four samples were
analysed for each type of material. Analytical results
were presented in the form of cumulative weight fraction or the differential of the cumulative weight fraction
as a function of log (molecular weight).
almost double the tensile strength and more than double
the tensile stiffness of the polylactide by incorporating
jute fibres and using a heating temperature of 210 C. A
statistically equivalent result was obtained when the
temperature in the heating stage was raised to 220 C,
which is well above the recommended processing temperature for the polylactide. The tensile strength and
stiffness values for the 40% jute/PLA composites are
similar to the values obtained for 40% fibreglass/polyropylene composites [15]. It is possible that the reduced
viscosity of the polylactide at the higher temperatures
and hence better flow properties may have helped to
improve fibre wetting and therefore led to the trend of
increasing tensile strength and stiffness as a function of
heating temperature, although other factors such as
changes in PLA crystallinity, composite porosity and
jute fibre surface chemistry may also have contributed.
The increase in tensile stiffness with increasing process
temperature was not as marked as was the case with the
tensile strength. Average strain at maximum stress was
slightly lower in the composites when compared with
the pure PLA matrix but showed little if any variation
with processing conditions. The increase in tensile
strength and stiffness with addition of jute fibres to PLA
suggests some fibre/polymer surface compatibility and
good stress transfer between the fibres and the matrix. A
decrease in strain at maximum stress with agro-fibre
addition is typical of uncoupled thermoplastic composites and is also seen in coupled systems, although the
decrease in failure strain with increasing fibre amount is
not as severe in these cases.
3. Results and discussion
3.1. Tensile tests
The results of typical weight measurements on dogbone samples during pre-test conditioning are shown in
Fig. 2 and indicate that conditioning to equilibrium at
65% relative humidity and 23 C took between two and
three weeks and resulted in a final moisture uptake of
about 2% on the basis of original sample weight.
Moisture uptake by PLA panels without jute fibre reinforcement was virtually zero over the same period.
Mean tensile strength, tensile stiffness and elongation
values are shown in Table 1. The results, as summarised
for example in Fig. 3, indicate that it was possible to
3.2. Impact resistance
The results of unnotched Izod impact tests are shown
in Table 2 and indicate that there was no statistical difference in mean impact resistance between the PLA
processed at 190 C and the PLA/jute composite processed at 200 C. The literature indicates that the impact
strength of composites is very sensitive to the choice of
fibre and matrix. For example, Chuai et al. [16] found
that reinforcement of polypropylene with untreated
softwood fibres led to a significant decrease in unnotched
Fig. 2. Weight changes in PLA/jute (fabricated at 210 C) at 65%
relative humidity and 23 C.
Table 1
Tensile test results for PLA and PLA/jute composites
Sample type
Process temperature ( C)
Heating time (min)
Tensilea strength (MPa)
Tensilea stiffness (GPa)
Elongationa at max. stress (%)
PLA
PLA/juteb
PLA/juteb
PLA/juteb
PLA/juteb
PLA/juteb
190
180
190
200
210
220
10
10
3
3
3
3
55
72.7
89.3
93.5
100.5
98.5
3.5
8.1
8.5
8.7
9.4
9.5
2.1
1.5
1.8
1.6
1.6
1.5
a
b
Values in parentheses are standard deviations.
Composites containing approximately 40% jute on weight basis.
(0.5)
(2.3)
(3.6)
(1.1)
(0.4)
(3.3)
(0.1)
(0.4)
(0.6)
(0.0)
(0.2)
(0.5)
(0.1)
(0.0)
(0.1)
(0.1)
(0.1)
(0.1)
1291
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
Izod impact strength with increasing fibre content;
however, fibre treatment to improve surface compatibility with polypropylene led to either a much smaller
decrease in impact strength or even an increase depending upon the compatibilising treatment that was used.
Wan et al. [17] studied carbon fibre-reinforced PLA
composites and discovered that unnotched impact
strength increased with fibre reinforcement up to a
maximum value at 30% fibre volume fraction. As noted
by Sanadi et al. [15], the unnotched impact resistance of
thermoplastics generally decreases in the presence of
agro-fibres. The reason for this is that the addition of
fibres creates regions of stress concentration that require
less energy to initiate a crack. Therefore, it is a positive
result from our research that unnotched impact resistance does not decrease while composite tensile strength
and tensile modulus increase when jute is used to
reinforce PLA.
3.3. Electron microscopy
ESEM photomicrographs of PLA/jute tensile specimen fracture surfaces are shown in Figs. 4–7. The brittle
nature of the jute fibre fracture is clearly seen and is
quite different in appearance from cases where, for
example, jute is embedded in polypropylene and much
more fibre pull-out generally occurs. As with the tensile
test results, this is suggestive of a better interface
between fibre and polymer but Figs. 6 and 7 in particular also show that gaps in the fibre/polymer interface
can arise either as a result of manufacturing or during
Table 2
Impact resistance test results for PLA and PLA/jute composites
Fig. 3. Tensile strength of PLA fabricated at 190 C, PLA/jute fabricated at 190 C and PLA/jute fabricated at 210 C.
Sample type
Process
temperature ( C)
Heating
time (min)
Mean impact
resistance (kJ/m2)a
PLA
PLA/juteb
190
200
10
3
15.4 (1.9)
14.3 (2.1)
a
b
Values in parentheses are standard deviations.
Composites containing 40% jute on weight basis.
Fig. 4. ESEM photomicrograph of a PLA/jute composite tensile fracture surface showing brittle failure of fibres (PLA/jute composite fabricated
from undried jute at 210 C).
1292
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
the process of tensile testing. The brittle nature of PLA
and its manner of breakage, leaving plate-like areas on
the fracture surface, is shown in Figs. 5 and 6. The
typical surface zone of these composites, largely consisting of polymer in the first 50 to 100 microns, is also
shown at the top of Figs. 6 and 7.
3.4. Size exclusion chromatography
The results of SEC analyses of THF solution extracts
from PLA or PLA/jute composites are shown in Figs. 8
and 9. Analysis of THF extracts from jute fibres alone
showed no contributions that would potentially interfere with the determination of PLA molecular weight
distributions. Fig. 8 shows cumulative weight fraction
and the differential of the cumulative weight fraction as
a function of log(molecular weight) for the PLA granulate, PLA film and PLA/jute composites processed
between 180 and 220 C. A trend to decreasing weightaverage molecular weight is shown going from PLA
granulate through PLA film to the PLA/jute compo-
Table 3
SEC analyses of PLA and PLA/jute composites
Sample type
Process
temperature
( C)
Weight-average
molecular
weight (Mw)a
Polydispersity
(Pd)a
PLA granulate
PLA film
PLA/juteb
PLA/juteb
PLA/juteb
PLA/juteb
PLA/juteb
PLAc
PLAc
PLA/dried juted
PLA/dried juted
PLA/dried juted
N.A.e
N.A.e
180
190
200
210
220
200
220
200
210
220
274,850
263,500
252,850
252,550
247,725
244,150
238,525
239,700
219,300
252,700
251,400
244,650
2.57
2.35
2.32
2.34
2.36
2.34
2.33
2.43
2.37
2.22
2.32
2.34
a
b
c
d
e
Mean of four analyses.
Jute mats used in air-dried form.
PLA converted to test plates without inclusion of fibre mats.
Vacuum oven-dried jute.
N.A.=not applicable.
Fig. 5. ESEM photomicrograph of a PLA/jute tensile fracture showing broken jute fibre bundles and the plate-like fracture of the PLA matrix
(PLA/jute composite fabricated from undried jute at 210 C).
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
1293
Fig. 6. ESEM photomicrograph of a PLA/jute tensile fracture surface showing void spaces between jute fibre bundles and the PLA matrix (PLA/
jute composite fabricated from undried jute at 220 C).
sites. Although differences in weight-average molecular
weight of 10,000 or less are probably not significant in
these experiments, there is clearly a trend towards
decreasing weight-average molecular weight with
increasing process temperature. Fig. 9 illustrates this
more clearly when the cumulative weight fraction is
plotted against log (molecular weight) over a narrower
range on both axes. The overall results in terms of
weight-average molecular weight and polydispersity
[weight-average molecular weight (Mw)/number-average
molecular weight (Mn)] are shown in Table 3. The narrower the molecular weight range, the closer are the values
of Mw and Mn, and the ratio of Mw/Mn may thus be used
as an indication of the breadth of the molecular weight
range in a polymer sample. The trend of decreasing
weight-average molecular weight progressing from raw
PLA granulate through PLA film to PLA composites
incorporating air-dried jute is also clearly shown in
Table 3. Although the Mw values appear higher in cases
where vacuum oven-dried jute has been used, it is unlikely
that these are real differences in statistical terms. The
conclusion that the presence of moisture in the fibres during press consolidation causes only minor degradation of
PLA is also reinforced by the relatively constant polydispersity values shown in Table 3. The stability of PLA
during the process may be a result of processing under
vacuum. Observations during the heating stage of the
process indicated that moisture is removed quite rapidly
and therefore the time during which PLA might react
with water at high temperature would be quite short.
1294
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
Fig. 7. ESEM photomicrograph of a PLA/jute composite tensile fracture surface showing void spaces between fibre bundles and the PLA matrix
(PLA/jute composite fabricated from vacuum oven-dried jute at 210 C).
Fig. 8. SEC analytical results expressed as cumulative weight fraction (Wf) (curve envelope 1) or d(Wf/logMw) (curve envelope 2) as a function of
log molecular weight of PLA. The outer curves on each envelope show results for PLA granulate and PLA/jute (220 C) respectively.
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
1295
Fig. 9. SEC analytical results showing cumulative weight fraction (wf) 10 as a function of log molecular weight of PLA. The outer lines on the
envelope show results for PLA granulate and PLA/jute (220 C), respectively.
4. Conclusions
1. The tensile strength and stiffness of PLA can be
approximately doubled when jute fibre reinforcement is used on a 40 wt.% basis. However,
impact resistance as measured by an unnotched
Izod test is not increased.
2. The increase in tensile strength in particular is
dependent on the temperature in the heating
stage of the process, showing a maximum in the
range 210–220 C.
3. Electron microscopy observations show brittle
failure of jute fibres under tension and void
spaces between fibre and polymer matrix indicating that the strength of the jute/PLA interface
could be improved.
4. Limited PLA degradation occurs during rapid
press consolidation under vacuum and this might
be attributable to rapid removal of air and water
during the heating stage.
Acknowledgements
The assistance of the late Palle Vagn Jensen of Risø
National Laboratory, Materials Research Department
in obtaining ESEM photomicrographs of PLA/jute
tensile specimen fracture surfaces is gratefully acknowledged.
References
[1] Chandra R, Rustgi R. Biodegradable polymers. Prog Polym Sci
1998;23:1273–335.
[2] Tournois B. Recent developments in application of raw materials
in bioplastics. In: Derksen JTP, Mogendorff J, Mangan C, Daskaleros T, editors. Proceedings of the symposium on Renewable
Bioproducts—Industrial Outlets and Research for the 21st century. Wageningen, Holland, June 1997. European Commission
Publication EUR 18034; 1997.
[3] Jacobsen S, Fritz H-G, Degee P, Dubois P, Jerome R. New
developments in the ring opening polymerization of polylactide.
Industrial Crops and Products 2000;11:265–75.
[4] Enomoto K, Ajioka K, Yamaguchi A. US Patent 5310865. 1995.
[5] Mohanty AK, Misra M, Hinrichsen G. Biofibres, biodegradable
polymers and biocomposites. Macromol Mater Eng 2000;276/
277:1–24.
[6] Gerngross T, Slater SC. How green are green plastics? Scientific
American 2000;August:24–9.
[7] Bogaert J-C, Coszach P. Poly (lactic acids): a potential solution
to plastic waste dilemma. Macromol Symp 2000;153:287–303.
[8] Eichhorn SJ, Baillie CA, Zafeiropoulos N, Mwaikambo LY,
Ansell MP, Dufresne A, et al. Review—current international
research into cellulosic fibres and composites. Journal of Materials Science 2001;36:2107–31.
[9] Herrmann AS, Nickel J, Riedel U. Construction materials based
upon biologically renewable resources—from components to finished parts. Polymer Degradation and Stability 1998;59:251–61.
[10] Riedel U, Nickel J. Natural fibre-reinforced biopolymers as construction materials—new discoveries. Die Angewandte Makromolekulare Chemie 1999;272:34–40.
[11] Jiang L, Hinrichsen G. Flax and cotton fiber reinforced biodegradable polyester amide composites, 2. Die Angewandte Makromolekulare Chemie 1999;268:18–21.
[12] Keller A, Bruggmann D, Neff A, Muller B, Wintermantel E.
1296
D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296
Degradation kinetics of biodegradable fiber composites. Journal
of Polymers and the Environment 2000;8(2):91–6.
[13] ASTM Standard D 638-99. Standard test method for tensile
properties of plastics. West Conshohocken, PA: American
Society for Testing and Materials; 1999.
[14] ASTM Standard D 256-97. Standard test methods for determining the Izod pendulum impact resistance of plastics. West Conshohocken, PA: American Society for Testing and Materials;
1997.
[15] Sanadi AR, Caulfield DF, Jacobsen RE. Agro-fiber thermo-
plastic composites. In: Rowell RM, Young RA, Rowell JK, editors. Paper and composites from agro-based resources. New
York: CRC Press; 1997 [Chapter 12]..
[16] Chuai C, Almdal K, Poulsen L, Plackett D. Conifer fibers as
reinforcing materials for polypropylene-based composites. Journal of Applied Polymer Science 2001;80:2833–41.
[17] Wan YZ, Wang YL, Li QY, Dong XH. Influence of surface
treatment of carbon fibers on interfacial adhesion strength and
mechanical properties of PLA-based composites. Journal of
Applied Polymer Science 2001;80:367–76.