bamboo fibre thermoplastic composites for transport applications

BAMBOO FIBRE THERMOPLASTIC COMPOSITES FOR
TRANSPORT APPLICATIONS
EDUARDO TRUJILLO 1* , LINA OSORIO 1 , CARLOS FUENTES 1 , AART W. VAN
VUURE 1 & IGNAAS VERPOEST 1
1 Department of Metallurgy and Materials Engineering (MTM), Katholieke Universiteit Leuven,
Kasteelpark Arenberg 44, bus 2450, B-3001 Leuven, Belgium. Composite Materials Group.
*Corresponding author. Tel.: +32-16-321-448; fax: +32-16-321-990.
E-mail address: eduardo.trujillo@mtm.kuleuven.be
SUMMARY:
In this study, the mechanical properties of single bamboo (Guadua angustifolia) fibres
were evaluated. To correct for machine compliance, the evaluation was done at four
different span lengths. The results show that the specific Young’s modulus can
compete with glass fibre and the specific strength is only 10% lower. Strength values
of around 800 MPa were obtained by using a novel green mechanical extraction
process. For UD bamboo fibre thermoplastic composites (with Polypropylene and
Maleic Anhydride grafted Polypropylene), flexural tests with two fibre orientations were
performed. Material performance was reasonable, but further work is necessary to
improve fibre-matrix interfacial strength.
INTRODUCTION
Composites allow light-weight design and are as such environmentally friendly
materials. However, there is an increasing interest in so-called renewable materials, to
which category natural fibres clearly belong. Natural fibres can, if mechanical recycling
into secondary applications is not feasible, be thermally recycled or if a biodegradable
matrix is used, be landfilled for biodegradation. Both last options are largely CO2
neutral. Natural fibres like bamboo also require much less energy for cultivation and
extraction than is used for the synthesis of glass and particularly carbon fibre, which
gives them a very favorable carbon footprint. For these reasons natural fibres are
emerging as good alternative reinforcing materials for the future.
Nowadays exploitation and use of sustainable natural resources are a necessity and
they will play a crucial role in the near future. Bamboo Guadua angustifolia, from
Colombia, appears like one of these valuable resources with a high potential thanks to

its excellent mechanical properties and availability. The use of its fibres has not been
considered so often yet, particularly because of difficulties in extracting high quality,
undamaged fibres.
MATERIALS AND METHODS
Bamboo fibres of the species Guadua angustifolia were obtained from well defined
locations in Colombia. Technical fibres (which will be referred to as fibres in this
paper), were extracted from the bamboo culms using a newly developed and
proprietary extraction process, giving a maximum fibre length between 20 and 35 cm.
Figure 1a shows a group of mechanically extracted fibres whose diameter has a range
between 90 and 250 µm; the main diameter concentration is around 150 µm. Before
tensile testing, the fibres were visually selected in order to verify the absence of
defects along the length of the fibres.
a.
Figure 1. a) Bamboo (Guadua angustifolia) technical fibres after mechanical extraction and b)
unidirectional bamboo fibre thermoplastic prepreg.
After this, the cross sectional area of each individual fibre was determined using both
the apparent density (1,44 gr/cm 3 according to ref. 1) and the weight as well as the
length of the fiber. After this they were glued in between two paper frames to assure a
good gripping and straight position in the test clamps. The opening of the paper frame
determines the gauge length; for this experiment it was set at 5, 10, 25 and 40 mm.
The single fibre tensile tests were performed on a mini tensile testing machine with a
loadcell of 200N where the crosshead speed was set at 1 mm/min; the load and the
displacement are registered during the complete test. Because an extensometer
cannot be used during the test, a theoretical correction for the machine compliance
developed by Defoirdt, N., et al (ref. 2) was applied in order to determine the real
elongation of the specimens. By plotting modulus versus 1/span, extrapolating to
b.

1/span = 0 (infinite fibre length) provides the material modulus for which slip and
machine compliance may be ignored. When thus the real material modulus is known,
an estimation can be made for the machine compliance. Knowing this compliance,
strain values can be corrected. The correction was done for every single experiment.
Thermoplastic composites were prepared by compression moulding of stacks (7
layers) of bamboo prepregs (see Fig. 1b), each consisting of untreated fibres and two
different thermoplastic films (Polypropylene and Maleic Anhydride grafted
Polypropylene (MAPP)). To obtain good impregnation, the temperature used was
about 25 degrees C higher than the melting temperature of the polymer and a
pressure of 15 bars was used. In all cases, the natural fibres and prepregs were dried
overnight at 65°C to prevent problems due to moisture. Fibre volume fraction was set
at 45% by weight measurements. Flexural strength and Young’s modulus for UD
composites with longitudinal disposition of the fibres were evaluated by 3 point
bending tests on a universal testing machine (Instron 4426) based on ASTM D790M.
Interface strength was determined from transverse bending strength according to the
latter standard.
RESULTS AND DISCUSSION
Single Bamboo Fibre
Single fibre tensile tests for untreated bamboo fibres were performed at different
spans lengths. The values for tensile strength and Young’s modulus, after the
machine compliance correction was applied (see above), are shown in Figure 2. Strain
to failure remains around 1.9% on average. The modulus showed an extrapolated
value of 43 GPa. Under tensile load bamboo fibres show a clean fracture instead of
defibrillation or splitting. The clean fracture shows that there is a good bond between
the elementary fibres and the technical fibre matrix (typically consisting of lignin; as is
generally known, a vegetable fibre is in itself a composite). The small variation in
tensile strength at different span lengths (Fig. 2) could be an indication of the low
presence of defects along the length. This means that the mechanical extraction
process that was applied in this case did not significantly affect the fibre quality. With a

density of bamboo fibre of 1.4, specific mechanical properties are very close to these
of glass fibre
Figure 2. Tensile single fibre properties of bamboo fibres (Guadua angustifolia), after machine
compliance correction.
Thermoplastic Bamboo Fibre Composites (BFC)
For unidirectional bamboo fibre thermoplastic composites loaded in the longitudinal
direction of the fibres, it is marked that for both Polypropylene and MAPP the
consolidation temperature has a clear effect on the final behaviour of the composite.
Strength (MPa)
Flexural strength for UD bamboo fibre/thermoplastic composites
220
200
180
160
140
120
100
80
60
40
20
0
Tensile Strength (MPa)
1000
900
800
700
600
500
400
300
200
100
0
Tensile properties of single bamboo fibres
5 10 25 40
Gauge length (mm)
0 0,5 1 1,5 2 2,5 3
Strain at maximum strength (%)
PP/BF 175°C
PP/BF 185°C
MAPP/BF 170°C
MAPP/BF 180°c
Figure 3. Stress strain behavior of bamboo fibre thermoplastic composites.
For both cases, when the processing temperature is raised by 10 degrees C, the
strength of the composites is shifted to the left part of the curve (see Fig. 3). There is
no clear difference between PP and MAPP, as all samples are roughly situated on one
50
45
40
35
30
25
20
15
10
5
0
Young’s modulus (GPa)

line as a function of processing temperature. Also, the Young’s modulus for PP
composites goes from 15.6 to 19.3 GPa and for MAPP composites from 11.8 to 16.4
GPa, improving 24 and 39% respectively. This increase can be explained because the
higher the temperature, the lower the viscosity of the polymer, so it will flow around the
fibres more easily. Young’s modulus is particularly indicative of the quality of
impregnation. Theoretically, at 45% fibre volume fraction, one would expect a
longitudinal modulus of about 20 GPa. Thus, it can be hypothesised that for PP at
185°C (20° above the melting temperature), the wetting is probably quite good. The
theoretical longitudinal strength is about 360 MPa. Whereas, for epoxy resins these
strength values have been approached, for PP and MAPP the strength remains
relatively low, indicating that probably significant improvements in interfacial bond
strength are still possible and desirable.
Transverse flexural strength BFC
Table 1. Results for transverse flexural strength of bamboo fibre/thermoplastic composites
From transverse properties (Table 1), it is clear that even with an improvement in
wetting (see above), the interface remains relatively weak, since the transverse
strength values are lower than the strength values of the pure polymers. There is also
very little difference between the results with PP and MAPP. We have indications that
the surface of our bamboo fibres is covered with lignin, which is apparently not highly
compatible with either PP or MAPP.
Consolidation temperature
165 °C 170°C 175°C 180°C
Polypropylene matrix (MPa) 17.3 ± 1.8 19.7 ± 0.5 18.6 ± 2.2 17.6 ± 1
MAPP matrix (MPa) 16.1 ± 1.4 19.3 ± 0.8 18.7 ± 0.8 20.4 ± 0.3
Flexural properties of bamboo fibre composites are compared with similar natural fibre
composites (Figure 4). This graph is included, just to give a rough idea of comparable
systems, although fibre volume fractions vary strongly. It can e.g. be seen that in case
of flax (with a flax fibre strength of about 700 MPa), good results have been obtained
with MAPP (strength as anticipated higher than 200 MPa). We have seen in previous
research that this situation is reverse for epoxy (bamboo composite strength
significantly higher than flax composite strength). It seems clear that compatibility is

the key in these systems, to allow transfer of fibre properties into composite
properties.
Figure 4. Flexural properties for unidirectional natural fibre/thermoplastic composites (ref. 3-5)
CONCLUSIONS
Technical bamboo fibre is difficult to extract undamaged from the culm, but once a
good extraction process is used, as developed at K.U.Leuven, fibres are obtained with
specific mechanical properties comparable to glass fibre. To reach the potential of
bamboo fibre thermoplastic composites, further work is needed to improve the
compatibility.
ACKNOWLEDGEMENTS
This research is sponsored by the Belgian Science Policy Office, in a cooperative
project with Vietnam.
REFERENCES
Flexural strength (MPa)
400
350
300
250
200
150
100
50
0
Flax / PP (Vf: 18%)
Flexural strength and flexural stiffness for UD natural
fibre/thermoplastic composites
80
Jute / PP (Vf: 50%)
Flexural strength
Flexural stiffness
Jute / PP (Vf: 21%)
Flax / PP (Vf: 27%)
Flax / MAPP (Vf: 28%)
Bamboo / PP (Vf: 45%)
Flax / MAPP (Vf: 32%)
Bamboo / MAPP (Vf: 45%)
1) L. MORENO, L. OSORIO and E. TRUJILLO, Scientia et Technica 34 (2007), p. 613.
2) N. DEFOIRDT et al, accepted paper to Composites, Part A (2010).
3) O. KHONDKER et al, J. Polymers and the Environment 13 N° 2 (2005), p. 115.
4) K. VAN DE VELDE and P. KIEKENS, J. Composite Structures 62 (2003), p. 443.
5) O. KHONDKER, et al, Comp. Part A: App. Sci. and Manuf. 37 (2006), p. 2274.
70
60
50
40
30
20
10
0
Flexural stiffness (GPa)