In this article, three bio-composites, i.e. flax, linen and bamboo fabric reinforced epoxy resin, were manufactured using a vacuum bagging technique. The influence of alkali treatment (with 5 wt% NaOH solution for 30 min) on tensile properties of flax, linen and bamboo single-strand yarns, surface morphology and mechanical properties (with respect to tensile and flexural properties) of the composites were investigated. It was found that the failure mechanism of single-strand fibres under tension consists of fibre breakage and slippage simultaneously. The alkali treatment had a negative effect on the tensile strength and modulus of the flax, linen and bamboo single-strand yarns. However, after the treatment, the tensile and flexural properties of all the composites increased, e.g. the tensile and flexural strength of the treated flax/epoxy composite increased 21.9% and 16.1%, compared to the untreated one. After the treatment in all the composites, the tensile fractured surfaces exhibited an improvement of fibre/epoxy interfacial adhesion.

1. Journal of Reinforced Plastics and Composites
http://jrp.sagepub.com/
Effect of alkali treatment on vibration characteristics and mechanical properties of natural fabric
reinforced composites
Libo Yan
Journal of Reinforced Plastics and Composites 2012 31: 887
DOI: 10.1177/0731684412449399
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2. Article
Journal of Reinforced Plastics
and Composites
Effect of alkali treatment on vibration 31(13) 887–896
! The Author(s) 2012
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DOI: 10.1177/0731684412449399
of natural fabric reinforced composites jrp.sagepub.com
Libo Yan
Abstract
In this article, the effect of alkali treatment (with 5 wt. % sodium hydroxide solution for 30 min) on the compressive,
in-plane shear, impact properties and vibration characteristics of flax- and linen-fabric reinforced epoxy composites was
investigated. Test results show that alkali treatment enhanced the compressive strength and compressive modulus,
in-plane shear strength and shear modulus, and specific impact strength of both flax- and linen-epoxy composites.
However, after the treatment, the impact strength and damping ratio of the flax and linen composites decreased. The
reduction in impact strength and damping ratio is believed to be attributed to the improved fibre/matrix interfacial
adhesion, as analysed by scanning electron microscope.
Keywords
Natural fabrics, composite, mechanical properties, vibration, scanning electron microscope
fracture and failure behaviour of technical flax fibres.
Introduction They found that the failure mechanism of flax fibre is a
There has been a growing interest in the use of bio- complex sequence consisting of axial splitting of the
fibres to replace manmade carbon/glass fibres as technical fibre along its elementary constituents,
reinforcement in polymer composites for engineering radial cracking of the elementary fibres and multiple
application.1 The advantages of bio-fibres are they are fracture of the elementary fibres.7 Bos et al. concluded
cost-effective, have low energy consumption, bio- that the flax fibre had a complex structure, which con-
degradability, low density with high specific strength sisted of cellulose, hemicelluloses, pectin, lignin and
and stiffness and are readily available.2 In the recent other components.8
years, research on nano-composites shows that bio- Flax fibres as composite reinforcement are not con-
composites have the potential as the next generation sidered only in the form of monofilament configur-
of structural materials.3 Currently, bio-composites are ation.9 Polymer matrix, reinforced by woven flax
mainly applied in the automotive industry. There was fabric, is the form of composites used commonly in
approximately 43,000 tonnes of bio-fibres utilized as structural applications such as boats. It is reported
reinforcement materials of composites in the that a 50% (by volume) flax fibre racing boat had com-
European Union (EU) in 2003.4 This amount increased pleted the France-to-Brazil Transat race in 15th place.10
to around 315,000 tonnes in 2010, which accounted for The success in fabrication of the boat is attributed to
13% of the total reinforcement materials (glass, carbon
and natural fibres) in fibre-reinforced composites.5 The
explosive consumption in bio-composites is an indica- Department of Civil and Environmental Engineering, The University of
tion of their wider application in the future. Auckland, Auckland, New Zealand
Among the bio-fibres, flax is a promising candidate
to replace glass fibre. The tensile strength of flax fibres Corresponding author:
Libo Yan, Department of Civil and Environmental Engineering, The
were reported up to 1500 MPa.6 Physical/mechanical University of Auckland. Level 11, Engineering Building, 20 Symonds
properties of some bio-fibres and manmade fibres are Street, Auckland, 1001, New Zealand
´
given in Table 1. Romhany et al. investigated the tensile Email: lyan118@aucklanduni.ac.nz
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3. 888 Journal of Reinforced Plastics and Composites 31(13)
Table 1. Properties of natural and manmade fibres6
Fibre Density Elongation (%) Tensile strength (MPa) Elastic modulus (GPa)
Flax 1.5 2.7–3.2 500–1500 27.6
Cotton 1.5–1.6 7.0–8.0 400 5.5–12.6
Jute 1.3 1.5–1.8 393–773 26.5
Hemp 1.47 2.0–4.0 690 70
Sisal 1.5 2.0–2.5 511–635 9.4–22
Coir 1.2 30 593 4.0–6.0
Softwood kraft pulp 1.5 4.4 1000 40
E-glass 2.5 0.5 2000–3500 70
S-glass 2.5 2.8 4570 86
Carbon 1.4 1.4–1.8 4000 230–240
the fact that the woven flax fabric allows the control of Materials and methods
fibre orientation and quality control, good reproduci-
Fibre and epoxy
bility and high productivity.11 Assarar et al. confirmed
that the tensile stress and strain at failure of flax fabric Commercial woven flax and linen fabrics were used
reinforced polymer composites were 300 MPa and because of their wide availability. Flax fabric with
1.8%, respectively – putting them close to glass fibre areal weight of 550 g/m2 was obtained from Libeco,
reinforced polymer composites.12 Liu and Hughes stu- Belgium. Linen fabric with areal weight of 350 g/m2
died the toughness of flax fabric reinforced epoxy com- was obtained from Hemptech, New Zealand. Both
posites and concluded that the fibre volume fraction flax and linen are plain weave fabrics. Flax fabric has
dominates the toughness, rather than the microstruc- count of 7.4 threads/cm in warp and 7.4 threads/cm in
tural arrangement of the fibre.13 the weft direction. Linen fabric has count of 10 threads/
Bio-composites have been applied in automotive and cm in warp and 10 threads/cm in the weft direction. The
boat engineering. However, based on the best know- epoxy used is the SP High Modulus Prime 20LV epoxy
ledge of the author, to date rarely study on bio- system. The fabric structures and details for the resin
composites in civil engineering has been reported. In system could be found in previous study.15
fact, conventional construction materials such as con-
crete and steel reinforcement have some significant
effects on the environment. In the United Kingdom
Alkali treatment
(UK), construction process and building use not only Initially, flax and linen fabrics were cut into a size of
consume the most energy of all sectors and create the 400 Â 300 mm2. For alkali-treated specimens, flax and
most CO2 emissions, they also create the most waste, linen fabrics were washed three times with fresh water
use most non-energy-related resources and are respon- to remove contaminants and then dried at room tem-
sible for the most pollution.14 To reduce these negative perature for 48 h. The dried fabrics were then immersed
environmental effects of conventional construction in 5 wt. % NaOH solution (20 C) for 30 min, followed
materials, bio-composites as potential construction by washing 10 times with fresh water and subsequently
material are being investigated. three times with distilled water, to remove the remain-
This article, as a part of on-going research to study ing NaOH solution. Finally, these fabrics were dried at
the feasibility of bio-composites as construction mater- 80 C in an oven for 24 h.
ial, investigated the vibration characteristics (damping
ratio and natural frequency) and the mechanical prop-
erties (with respect to compressive strength, compres-
Composite fabrication
sive modulus, in-plane shear stress and shear modulus, All the composites were manufactured by vacuum bag-
and the impact strength and specific impact strength) of ging technique. It consists of an initial hand lay-up of a
flax and linen fabric reinforced epoxy composites. In fibre preform and then impregnation of the preform
addition, the effect of alkali treatment (with 5 wt. % with resin in a flexible bag in which negative pressure
sodium hydroxide (NaOH) solution for 30 min) on is generated by a vacuum pump. Next, the composites
the mechanical properties and the vibration character- were cured at room temperature for 24 h and placed
istics of the composites were evaluated. into the Elecfurn oven for curing at 65 C for 7 h.
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4. Yan 889
Table 2. Physical properties of the composites
Fabric Thickness of Thickness of Fibre volume Density
Composites layers each layer (mm) composites (mm) fraction (%) (g/cm3)
Untreated flax/epoxy 6 0.712 5.049 55.1 1.273
Treated flax/epoxy 6 0.705 5.021 55.9 1.158
Untreated linen/epoxy 8 0.510 4.984 54.8 1.228
Treated linen/epoxy 8 0.498 5.011 55.3 1.130
Accelerometer
Composite cantilever plate
Amplifier
5 mm
225 mm
Natural Data acquisition
FFT software
frequency
Figure 1. Schematic view of vibration test system.
vibration of a structure. Damping of a composite can
Fibre volume fraction
be defined as the decay of the composite in vibrations.
Density of the mixed epoxy given by the supplier was It is interpreted as the dissipation of the vibration energy.
1.08 g/cm3. Composite density was determined by the Damping plays an important role in controlling the
buoyancy method using water as the displacement structure from excessive vibrations due to dynamic load-
medium based on ASTM D792.16 The void contents ings. Therefore, understanding the vibration character-
of the composites were determined according to istic of FRP composite material, like damping, has
ASTM D2734.17 After obtaining the density and void industrial significance. Damping ratio – a dimensionless
content for each composite, the fibre volume fraction measure of damping – is a property of the composite that
for the composite was derived from the fibre/epoxy also depends on its mass and stiffness. Vibration test was
resin weight ratio and the densities of both fibre and conducted by using an accelerometer to detect the
epoxy resin matrix.18 The fibre volume fraction Vf was dynamic characteristics of the composite plates.
calculated using the following equation: Figure 1 gives a schematic view of the vibration test
1 system. Three specimens with a size of 250 Â 25 Â 5 mm3
Vf ¼ 1 À À Vv ð1Þ (length  wide  thickness) for each composite was
1 þ Vf =Vr
clamped in the form of cantilever beams with 225 mm
where Vv is the void content of composite and Vr is the effective length span; the accelerometer was attached
volume of epoxy resin. The calculated fibre volume on the free-end side of each cantilever laminiate, and
fractions of the untreated and alkali-treated composites then stimulated the free vibration. The vibration accel-
are listed in Table 2. It can be seen that the fibre volume eration time histories were recorded by the data acquisi-
fractions and thicknesses of all the composites were tion software with a computer. The logarithmic
approximately 55 % and 5 mm, respectively. decrement is used for calculating the damping ratio
of cantilever laminates from the recorded acceleration
time histories based on the following equation:
Vibration test of composites
As a construction material, the damping of the material 1 gi
¼ ln ð2Þ
is an important parameter related to the study of 2j giþj
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5. 890 Journal of Reinforced Plastics and Composites 31(13)
Figure 2. Vibration time-history: (a) Untreated flax/epoxy composite and (b) alkali-treated flax/epoxy composite.
where gi is the peak acceleration of ith peak, giþj is the (length  wide  thickness) for each composite.19 The
peak acceleration of the peak j cycles after ith peak and cross-head speed was 1.5 mm/min for each test. An
ti is the time instant at i cycle in the peak acceleration, extensometer with a gauge was amounted on the speci-
as shown in Figure 2(a). men for measurement of the strain. For each compos-
With respect to the fast Fourier transformation ite, five specimens were tested at room temperature and
(FFT), the vibration frequency spectrum was obtained the average compressive strength and compressive
from the measured time-histories. The main peak cor- modulus were reported.
responds to the natural frequency of the composite.
The average damping ratio and average natural fre-
quency of each composite tested on three specimens
In-plane shear test of composites
was reported. The in-plane shear test was conducted according to
ASTM D3518 with a size of 250 Â 25 Â 5 mm3
(length  wide  thickness) for each composite.20 The
Compressive test of composites
cross-head speed was 2 mm/min. To register the elong-
The compressive test was carried out according to ation during the test, an extensometer with a gauge was
ASTM D3410 on plates with a size of 125 Â 25 Â 5 mm3 placed on each specimen. For each composite, five
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6. Yan 891
specimens were tested at room temperature and the
average shear strength and shear modulus were
obtained.
Impact test of composites
The Izod impact test was conducted according to
ASTM D256 on un-notched plates with a size of
65  12.7  5 (length  wide  thickness) mm3 for each
composite.21 The impact loading was considered with
a 25 J-hammer. Impact energy in J/m2 was considered.
For each composite, five specimens were tested at room
temperature and the average impact strength was
obtained.
Scanning electron microscopy
Surface topographies of the untreated and alkali-
treated composites were investigated using a scanning
electron microscope (SEM, Philips XL30S FEG,
Netherland) at room temperature, operated at 5 kV.
The sample surfaces were vacuum coated by evapor-
ation with platinum before examination.
Results and discussion
Vibration characteristics of composites
Figure 2 illustrates the time histories of untreated and
alkali-treated flax/epoxy composites in vibrations. The
average damping ratio and average natural frequency
of all the composites are given in Table 3. It shows that
both flax and linen fabric reinforced polymer compos-
ites exhibit a similar pattern in damping ratio, namely,
Figure 3. Surface morphology of untreated (a) and alkali-
the damping ratio of the untreated composite is larger
treated (b) flax fabric reinforced composites.
than the alkali-treated one. Alkali treatment has a
negative effect on damping ratio of both flax and
linen composites; the decrease in damping ratio of treatment increased the natural frequency of the
flax- and linen-epoxy composite is 7.4% and 9.3%, composites.
respectively (Table 3). For all the considered compos- Damping defines the energy dissipation capability of
ites, the untreated flax-epoxy composite has the largest a material. The damping of fabric reinforced polymer
damping ratio of 1.48 %. With respect to natural composite is believed attributed to the presence of air
frequency, it is observed that both flax and linen voids (e.g. the inherent lumens of the fibres), the visco-
composites possess a smaller natural frequency than elastic characteristics of epoxy matrix and/or the fibre
the corresponding treated one. Compared with the materials and the interphase between the matrix and
untreated composite, the increase in natural frequency the fibre. Interphase is defined as the region adjacent
of the treated composite is believed to be attributed to to fibre surface all along the fibre length.22 Interphase
the fact that the alkali treatment reduced the mass possesses a considerable thickness and its properties are
(a lower density in Table 2) and increased the stiffness different from those of embedded fibres and matrix. It
of the composite. The Young’s modulus of alkali-trea- is true that the mechanical properties (e.g. tensile and
ted composite was larger than that of the untreated flexural properties) of fabric fibre reinforced polymer
one, which was concluded in previous study.15 From composites are highly dependent on the matrix/fibre
the relationship among natural frequency ( f ), mass interphase.15
(m) and stiffness (k) of the composite, namely,
pffiffiffiffiffiffiffiffiffi Fibre/matrix interphases also affect the damping of
f ¼ ð1=2Þ Á k=m, it is easy to derive that the alkali the composites. The decrease in damping ratio of the
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7. 892 Journal of Reinforced Plastics and Composites 31(13)
Table 3. Mechanical properties of treated and untreated compositesa
Specific
Compressive Compressive Shear Shear Impact impact Damping Natural
strength modulus strength modulus strength strength ratio frequency
(MPa) (GPa) (MPa) (GPa) (kJ/m2) (kJ/m2/gÁcm3) (%) (Hz)
Untreated flax/epoxy 90.32 2.18 38.01 2.07 36.53 28.70 1.48 16.02
composite (4.30) (0.13) (2.21) (0.11) (3.24) (–) (0.06) (0.25)
Treated flax/epoxy composite 93.02 2.35 41.11 2.16 33.87 29.25 1.37 16.83
(3.25) (0.20) (2.54) (0.16) (2.96) (–) (0.04) (0.16)
Change due to alkali 3.0 7.8 8.2 4.2 À7.3 1.9 À7.4 5.1
treatment (%)
Untreated linen/epoxy 78.64 1.88 34.06 1.84 30.62 24.93 1.29 16.94
composite (3.45) (0.09) (1.78) (0.12) (2.76) (À) (0.09) (0.12)
Treated linen/epoxy composite 82.28 1.97 35.67 1.93 28.65 25.35 1.17 17.63
(4.02) (0.16) (2.06) (0.20) (2.24) (À) (0.05) (0.28)
Change due to alkali 4.6 4.8 4.7 4.9 À6.4 1.7 À9.3 4.1
treatment (%)
a
Numbers in parentheses are standard deviations.
treated composites may be attributed to the fact that treated composites mainly depends on the fibres, as the
alkali treatment leads to better fibre/matrix interfaces. compressive modulus of the epoxy is 1.13 GPa
For untreated composites, there are more voids or gaps (Figure 4(b)). Compared with the untreated composites,
at the fibre/matrix interfaces. In the vibration, more both alkali-treated flax and linen composites have an
energy has been dissipated due to the internal friction increase in compressive strength and compressive modu-
between the fibres and the matrices where more fibre/ lus; the increase in strength is 3.0% and 4.6%, respect-
matrix interfaces are involved, and thereby leads to a ively. The increase in modulus is 7.8% and 4.8%,
larger damping ratio of the composites. After alkali respectively (Table 3). The enhancement in compressive
treatment, the fibre/matrix interfacial adhesion was properties of flax- and linen-epoxy composites by alkali
improved. Consequently, the gaps at the fibre/matrix treatment is possibly due to the improved fibre/matrix
interfaces were narrowed and resulted in less energy interfacial adhesion, since alkali treatment removes the
dissipation in the vibration. SEM micrographs of the hydrophilic nature of the cellulose fibre and thus
untreated and treated flax composites are shown in improves the interfacial bonding.
Figure 3. For the untreated composite, there are notice- The compressive stress–strain curves of all the com-
able gaps between the adjacent fibres and the matrices; posites are shown in Figure 5. It can be seen that the
this indicates a poor fibre/matrix interfacial adhesion. behaviour of all the untreated/alkali-treated flax and
These noticeable gaps are responsible for dissipating linen fabric reinforced epoxy composites under com-
energy by fibre/matrix friction during the vibration. pressive loading is non-linear. Three regions could be
The insignificant gaps between the fibre and the defined approximately. In the first region, all the speci-
matrix indicate the improved interfacial adhesion, as mens show a linear relationship between the stress and
shown in Figure 3(b). strain. In the second region, the curves exhibit a non-
linear pattern before approaching the ultimate stress.
The third post-peak curves go down with a continuous
Compressive properties of composites
increase in strains; this reveals a ductile behaviour. The
A comparison of compressive strength and compressive predominated failure mechanism observed in the com-
modulus between pure epoxy and the composites is dis- pression test was fibre micro-buckling. It should be
played in Figure 4. The ultimate compressive strengths noted here that the strains at break of all the
of all the untreated and alkali-treated composites are untreated/alkali-treated flax and linen composites are
highly dependent on the strength of the epoxy matrix, more than 8%.
as shown in Figure 4(a). The compressive strength of
untreated flax- and linen-epoxy composite is
In-plane shear properties of composites
90.32 MPa and 78.64 MPa, respectively, compared
with the pure epoxy (68 MPa). For compressive modu- The in-plane shear stress–strain behaviour for both
lus, it can be seen that the stiffness of all untreated/ untreated and alkali-treated flax- and linen-epoxy
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8. Yan 893
Figure 4. Compressive strength and compressive modulus of all the composites.
composites is shown in Figure 6. The average shear increase in shear modulus, respectively (Table 3). The
strength and average shear modulus of all the compos- alkali treatment removes the impurities and waxy sub-
ites are given in Table 3. The flax/epoxy composite has stances from the fibre surface and creates a rougher
a larger shear strength and shear modulus than the topography (Figure 3) which facilitates the mechanical
linen-epoxy composite. The shear strength and modu- interlocking. In addition, the purified fibre surface fur-
lus of untreated flax- and linen-epoxy composites is ther enhances the chemical bonding between the fibre
38.0 MPa and 2.07 GPa, and 34.06 MPa and and epoxy matrix because a purified fibre surface
1.84 GPa, respectively. enables more hydrogen bonds to be formed between
After alkali treatment, the shear strength and shear the hydroxyl groups of the cellulose at one side and
modulus of both flax- and linen-epoxy composites the epoxy groups at the other side. As a consequence
increased. Compared to the untreated composite, the of the treatment, the fibre/matrix interfacial bonding
treated flax and linen composite experienced 8.2% quality is improved and leads to better in-plane shear
and 4.7% increase in strength and 4.2% and 4.9% properties of the composites.
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9. 894 Journal of Reinforced Plastics and Composites 31(13)
Figure 5. Compressive stress–strain curve of all the composites.
Figure 6. Shear stress–strain behaviour of flax- and linen-epoxy composites.
The stress–strain curves can be divided approxi-
Impact properties of composites
mately into two zones. The first zone up to 0.3% Impact strength of a material is defined as its ability to
strain has a purely elastic behaviour, allowing measure- resist the fracture under stress applied at high speed.
ment of the modulus. The second zone is a non-linear The impact behaviour of a composite is significantly
zone until leading to the maximum shear stress. All the influenced by the interfacial bond strength, the matrix
specimens were failed because of matrix cracking and and fibre properties. The damage process caused by
fibre breakage. impact load energy is dissipated by fibre/matrix
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10. Yan 895
debonding, matrix fracture and fibre pull-out and fibre
fracture, as displayed in Figure 7. It is observed that the
impact strength of the untreated flax composite
(36.53 kJ/m2) is larger than the untreated linen compos-
ite (30.62 kJ/m2), as given in Table 3. The difference in
impact strength of flax- and linen-epoxy composites is
attributable to the different areal weights of the fabrics.
The alkali treatment reduced the impact strength of
the composites. The reduction is 7.3 % of flax compos-
ite and 6.4 % of linen composite, respectively (Table 3).
The decrease in impact strength may be interpreted by
assuming that a better fibre/matrix adhesion results in
shorter average pull-out lengths of the fibres, as
observed in Figure 8. It is clear that the average fibre
pull-out lengths of the untreated flax composite is
longer than the alkali-treated flax one.
Figure 7. SEM micrograph of failure modes of flax fabric rein- Specific impact strength is defined as the ratio of
forced epoxy composites. average impact strength divided by the density of the
SEM: scanning electron microscopy. composite. Table 3 indicates that the alkali treatment
increased the specific impact strength of the flax and
linen composites. This is because alkali treatment has
a significant reduction in the density of the composites,
as shown in Table 2.
Conclusion
Flax and linen fabric reinforced epoxy composites have
been fabricated using the vacuum bagging technique.
The influence of alkali treatment on the vibration char-
acteristics, the surface morphologies and mechanical
properties of the composites were studied. The investi-
gation reveals:
1. Alkali treatment with 5 wt. % NaOH solution
enhanced the compressive properties, in-plane
shear properties of the flax and linen composites.
However, the damping ratio and impact strength
of both flax and linen composites decreased due to
the treatment.
2. In vibration, the reduction in damping ratio by
alkali treatment is believed to be attributed to the
improved fibre/matrix adhesion resulting in less
energy dissipation during the vibration, as analysed
by SEM.
3. In compression, the ultimate compressive strength
of flax and linen composites is highly dependent
on the strength of the epoxy. The stiffness of the
fabric reinforced epoxy composite mainly depends
on the fibre. The compressive failure of fabric rein-
forced epoxy composites exhibits a ductile fracture
mode.
4. In in-plane shear test, the stress–strain behaviour of
Figure 8. SEM micrographs of impact specimens: (a) Untreated the composites exhibits a non-linear manner.
flax, and (b) alkali-treated flax composites. SEM, scanning elec- 5. The impact strength of the flax composite is superior
tron microscopy. to the linen composite. Alkali treatment increased
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11. 896 Journal of Reinforced Plastics and Composites 31(13)
the specific impact strength of the composites, com- 9. Van de Weyenberg I, Ivens J, De Coster A, et al.
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Funding
15. Yan LB, Chouw N and Yuan XW. Improving the
This research received no specific grant from any funding mechancial properties of natural fibre fabric reinforced
agency in the public, commercial, or not-for-profit sectors. epoxy composites by alkali treatment. J Reinf Plast
Compos 2012; 36: 425–437.
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