Improving the mechanical properties of natural fibre fabric reinforced epoxy composites by alkali treatment

Journal of Reinforced Plastics and Composites
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Improving the mechanical properties of natural fibre fabric reinforced epoxy composites by alkali
treatment
Libo Yan, Nawawi Chouw and Xiaowen Yuan
Journal of Reinforced Plastics and Composites 2012 31: 425 originally published online 21 February 2012
DOI: 10.1177/0731684412439494

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Article
Journal of Reinforced Plastics
and Composites

Improving the mechanical properties 31(6) 425–437
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DOI: 10.1177/0731684412439494
composites by alkali treatment jrp.sagepub.com

Libo Yan1, Nawawi Chouw1 and Xiaowen Yuan2

Abstract
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.

Keywords
natural fabrics, composite, scanning electron microscopy

Flax, hemp, jute, sisal and bamboo are the most
Introduction popular reinforcement materials in bio-composites
Synthetic fibre reinforced polymer (FRP) composites because they are cost-effective, have low density with
with high strength and stiffness have been widely used high specific strength and stiffness, and are readily
in the last decade in aerospace and automotive available.7,8 Nevertheless, natural fibres also possess
industries.1 In recent years, the use of bio-fibres to some negative characteristics, i.e. they are highly hydro-
replace synthetic carbon/glass fibres as reinforcement philic and their mechanical and physical properties are
in polymer composites has gained popularity in engi- strongly dependent on the climate, location and
neering applications due to increasing environmental weather; so it is difficult to predict their respective
concern.2,3 The advent and application of nanotechnol- composite properties.9 Natural fibres also have a com-
ogy have generated renewed interest in bio-composites plex structure, consisting of cellulose, hemicelluloses,
which show promising potential as the next generation pectin, lignin and other components.10 Thus, natural
of structural materials.4
In 2003, around 43,000 tonnes of natural fibres were
used by the European automotive industry as 1
Department of Civil and Environmental Engineering, The University of
composite reinforcement materials.5 In 2010, the Auckland, New Zealand
amount climbed to about 315,000 tonnes, which 2
Department of Mechanical Engineering, The University of Auckland,
accounted for 13% of the total reinforcement materials New Zealand
(glass, carbon and natural fibres) in fibre-reinforced
composites in European Union.6 The explosive Corresponding author:
Libo Yan, Department of Civil and Environmental Engineering, The
growth in bio-composites is indicative of their wider University of Auckland, Level 11, Engineering Building, 20 Symonds
application in the future due to the favourable Street, Auckland 1001, New Zealand
mechanical performance of natural fibres. Email: lyan118@aucklanduni.ac.nz


426 Journal of Reinforced Plastics and Composites 31(6)

fibres as composite reinforcement are not considered This technique is best suited for moulding epoxy
only in the form of monofilament configuration.11 matrix-based composites because of the superior flow
Polymer matrix, reinforced by woven fabric, is the of epoxy.19 As an alternate to the labour-intensive hand
form of composites used most commonly in structural lay-up process, VBT offers composites better unifor-
applications such as aircrafts, boats and automobiles. mity of lay-up, higher fibre-to-resin ratio and better
This is attributed to the fact that the woven fabric strength-to-weight ratio. In particular, theoretically,
allows the control of fibre orientation and quality con- there is no limitation on the size of composites with
trol, good reproducibility and high productivity.12 In this technique, which is critical for practical engineering
these applications, good tensile strength is essential application.
for the composite performance.13 The composite tensile The effect of alkali treatment mainly on monofila-
properties are significantly dependent on the interfacial ment flax and bamboo fibres has been investigated by
bond between the fabric layer and the matrix material, many researchers.14–18 With regard to fibre yarn prop-
as well as the fabric structure. erties, only the untreated yarn was considered.20 To
To enhance the mechanical properties of bio- date, the effect of alkali on single fibre yarn has not
composites by improving the fibre/matrix adhesion, been investigated. This study focuses on the effect of
surface modification including alkali, saline and acety- alkali treatment on the mechanical properties of the
lation has been investigated.7,11,14–16 Among those three single-strand yarns and the corresponding com-
treatments, alkali is widely applied because it is easy posites. To study the surface morphology of the yarns
to operate and cheap. Studies have shown that alkali and the composites, scanning electron microscopy
treatment with sodium hydroxide (NaOH) solution can (SEM) is used.
significantly increase mechanical strengths of flax
monofilament fibre reinforced composites14,15 and
bamboo monofilament fibre reinforced composites.16 Materials and methods
Kushwaha and Kumar analysed the effect of differ-
ent NaOH solution concentrations on the tensile, flex-
Fibre and epoxy
ural and toughness properties of bamboo mat/epoxy Commercial woven flax, linen and bamboo fabrics were
composites. The optimum result was obtained when used because of their wide availability. The flax fabric
bamboo mat was treated with 5 wt% (by weight) (550 g/m2) was obtained from Libeco, Belgium. The
NaOH solution for 30 min.16 Wong et al. conducted a linen fabric (350 g/m2) and the bamboo fabric (210 g/
study on the mechanical properties of bamboo fibres. m2) were obtained from Hemptech, New Zealand. The
The results showed that 5 wt% NaOH treatment led to structures of fabrics are displayed in Figure 1. The
better tensile properties of bamboo fibres.17 The study epoxy used is the SP High Modulus Prime 20LV
by Wang et al. indicated that 5 wt% or 10 wt% of epoxy system, which is specifically designed for use in
NaOH solution was the appropriate concentration for a variety of resin infusion processes (Table 1).
alkalisation of flax fibres, for improving the mechanical
properties of flax FRPs.18
Alkali treatment
In this article, three epoxy composites reinforced
with flax, linen and bamboo woven fabrics were man- Initially, these fabrics were cut into a size of 400 Â 300
ufactured using a vacuum bagging technique (VBT). mm2. Fibre single-strand yarns were extracted from the

Figure 1. Structures of flax, linen and bamboo woven fabrics.


Yan et al. 427

corresponding fabric. For alkali-treated specimens, then impregnation of the preform with resin in a
these fabrics and yarns were washed three times with flexible bag in which negative pressure is generated by
fresh water to remove contaminants, and then dried at a vacuum pump. Next, the composites were cured at
room temperature for 48 h. The dried fabrics and yarns room temperature for 24 h and placed into the Elecfurn
were then immersed in 5 wt% NaOH solution (20 C) oven for curing at 65 C for 7 h.
for 30 min, followed by washed 10 times with fresh
water and subsequently three times with distilled
water, to remove the remaining sodium hydroxide solu-
Fibre volume fraction
tion. Finally, these fabrics and yarns were dried at 80 C Density of the mixed epoxy given by the supplier was
in an oven for 24 h. 1.08 g/cm3. Composite density was determined by the
The significance of alkali treatment is the disruption buoyancy method using water as the displacement
of hydrogen bonding in the fibre surface, thereby medium based on ASTM D792.24 The void contents
increasing surface roughness. This treatment removes of the composites were determined according to
a certain amount of lignin, wax and oils covering the ASTM D2734.25 After obtaining the density and void
external surface of the fibre cell wall, depolymerises cel- content for each composite, the fibre volume fraction
lulose and exposes the crystallites.21 Addition of for the composite was derived from the fibre/epoxy
sodium hydroxide to natural fibre promotes the ionisa- resin weight ratio and the densities of both fibre and
tion of the hydroxyl group, the alkoxide22 epoxy resin matrix.26 The fibre volume fraction Vf was
calculated using the following equation
Fibre À OH þ NaOH Õ Fibre À OÀ Naþ þ H2 O ð1Þ
1
Vf ¼ 1 À À Vv ð2Þ
The fibre with a higher amount of hydrogen groups 1 þ Vf =Vr
would become more compatible with the epoxy matrix.
Thus, alkaline processing directly influences the where Vv is the void content of composite and Vr the
cellulosic fibril, the degree of polymerisation and the volume of epoxy resin. The calculated fibre volume
extraction of lignin and hemicellulosic compounds.23 fractions of all the untreated and alkali-treated
composites are listed in Table 2. It can be seen that
the fibre volume fractions and thicknesses of all the
Composite fabrication
composites were approximately 55% and 5 mm,
All the composites were manufactured by VBT. It con- respectively.
sists of an initial hand lay-up of a fibre preform and
Tensile test of single-strand yarns
The tensile test was conducted on Instron 5567 machine
Table 1. Properties of epoxy system
according to ASTM D2256 on single-strand yarn
Hardener: specimen in the straight configuration, in the case of
Resin: SP SP PRIME no conditioning.27 The specimens were 150 mm in
PRIME 20LV 20 Slow length and were handled in a manner to avoid any
Mix ratio by weight 100 26 change in twist or any stretching of the specimens.
Viscosity at 20 C (cP) 1010–1070 22–24
Each test was repeated 10 times at the room
temperature and the average values were reported.
Density (g/cm3) 1.123 0.936

Table 2. Physical properties of composites

Thickness of Thickness of Fibre volume
Composites Fabric layers each layer (mm) composites (mm) fraction (%)

Flax/epoxy Untreated 6 0.712 5.049 55.1
Alkali-treated 6 0.705 5.021 55.9
Linen/epoxy Untreated 8 0.510 4.984 54.8
Alkali-treated 8 0.498 5.011 55.3
Bamboo/epoxy Untreated 14 0.312 5.085 55.4
Alkali-treated 14 0.304 5.069 54.2



Table 3. Tensile properties of untreated/alkali-treated flax, linen and bamboo single-strand yarns

Single-strand Density Tensile failure Elongation at Young’s modulus
Single-strand fibre yarn diameter (mm) (g/cm3) stress (MPa) break (%) (GPa)

Flax Untreated 0.708 1.43 Æ 0.09 145.4 Æ 8.4 2.9 Æ 0.3 16.4 Æ 0.4
Alkali-treated 0.703 1.22 Æ 0.05 118.5 Æ 10.3 3.1 Æ 0.4 13.8 Æ 0.5
Linen Untreated 0.514 1.35 Æ 0.04 129.7 Æ 10.1 4.3 Æ 0.2 12.3 Æ 0.6
Bamboo Untreated 0.303 1.26 Æ 0.10 67.5 Æ 5.7 2.8 Æ 0.2 5.4 Æ 0.4

Table 4. Properties of flax and bamboo monofilament fibres in literature

Tensile strength Tensile modulus Elongation at
Fibre Density (g/cm3) (MPa) (GPa) break (%) References

Flax 1.40 400–1800 50–70 2–3 Kessler et al.30 and
Bos et al.31
Bamboo 1.38–1.40 140–800 11–35 1.3–3.6 Defoirdt et al.32

The cross-sectional area of fibre single-strand yarn was
assumed to be circular; the diameter of the yarn was
Scanning electron microscopy
measured with the help of a projector. An Epson Surface topographies of the untreated and alkali-trea-
PowerLiteÕ X12 projector and an Epson DC-11 docu- ted fibre yarn were investigated using an SEM (Philips
ment camera are used to measure the diameter. The XL30S FEG, Netherlands) at room temperature, oper-
projector has Extended Graphics Array (XGA) resolu- ated at 5 kV. The tensile fracture surfaces of the com-
tion. The camera has auto-select output resolution posite samples were also analysed. The sample surfaces
of Super Extended Graphics Array (SXGA), Wide were vacuum-coated by evaporation with platinum
Extended Graphics Array (WXGA) and XGA. before examination.

Tensile test of composites Results and discussion
The flat coupon tensile test was conducted on the
Tensile properties of fibre yarns
Instron 5567 machine according to ASTM D3039 on
plates with a size of 250 Â 25 Â 5 mm3 for each compos- The tensile properties of untreated/alkali-treated flax,
ite.28 The cross-head speed was 2 mm/min. To register linen and bamboo yarns are listed in Table 3. Tensile
the elongation during the test, an extensometer with a properties of flax and bamboo monofilament fibres
gauge was placed on each specimen. For each compos- given in literature are demonstrated in Table 4.30–32
ite, five specimens were tested at room temperature and It is observed that both measured tensile failure stress
the average tensile strength and modulus were obtained and modulus of flax, linen and bamboo single-strand
directly from the machine. yarns are much lower than those of flax and bamboo
monofilament fibres in literature. This is attributed to
the different tensile failure mechanisms between fibre
Three-point bending test of composites
yarn and monofilament fibre. For monofilament fibre,
The flexural test was carried out on the Instron 1185 the failure mechanism is a complex sequence consisting
machine according to ASTM D790 on plates with a size of axial splitting of the technical fibre along its elemen-
of 100 Â 20 Â 5 mm3 for each composite.29 The cross- tary constituents, radial cracking of the elementary
head speed was 2.2 mm/min for each test. The length fibres and multiple fracture of the elementary fibres.33
of support span was 80 mm and the overhang length on The tensile failure of textile fibre yarns is a combination
both sides was 10 mm. For each composite, five speci- of fibre slippage and fibre breakage, as shown in
mens were tested at room temperature and the average Figure 2(b), which shows the flax yarn close to failure.
flexural strength and modulus were obtained directly This is because when spinning fibres to yarns, a number
from the machine. of fibre filaments are twisted into a continuous strand


Yan et al. 429

Figure 2. A single-strand flax yarn specimen in tensile test: (a) before loading and (b) close to failure.

producing radial forces which cause movement of some damage caused by chemical reaction with sodium
of these filaments relative to others, and leads to a hydroxide during the treatment. This damage is consid-
closer packing of all the filaments within any given ered to be caused by a chemical structural change such
cross-section. However, the tensile strength of the that cellulose in the fibre partially changes from crys-
fibre bundle cannot achieve that of the yarn because talline cellulose I into amorphous cellulose II.35
close to failure some fibres break and the rest slip Table 3 also shows that the alkali treatment leads to
(Figure 2(b)). According to Ghosh et al., the tensile the reduction in the diameter and the density of yarn
failure of viscose fibre yarn is strongly dependent on specimens. However, the reduction in fibre weight is
the yarn structure, i.e. the configuration, alignment greater than that in fibre diameter after this treatment.
and packing of constituent fibres in the yarn cross sec-
tion.20 For fabric with loose packing of fibres in the
yarns, the yarn failure mechanism is slippage domi-
Surface morphology of fibre yarns
nated, thus the load-bearing capacity of the slipped Alkali treatment could influence the inner cellulosic
fibre is reduced drastically and the final yarn strength components of the fibre and the non-cellulosic
is poor. components such as hemicelluloses, lignin and pectin
Table 3 depicts that the tensile failure stress of simultaneously. After alkali treatment, the (partial)
untreated flax single-strand yarn is 12.1% and hemicelluloses, lignin and surface impurities such as
115.4% larger than those of untreated linen and waxes and oils were removed from the fibre surface.
bamboo yarns, respectively. The elongation at the Since both diameter and density of alkali-treated yarns
break point of the linen yarn is almost 50% larger decreased (Table 3), it is indicated that the hemicellu-
than that of flax and bamboo yarns. loses, lignin and pectin of the fibres were dissolved by
For the alkali-treated counterparts, the tensile the alkaline solution. The removal of these cementing
strength and tensile modulus of all the three fibre constituents (hemicellulose, lignin and pectin) resulted
yarns decreased. Compared to untreated specimens, in the decrease in tensile properties of fibre yarn by
the alkali-treated flax, linen and bamboo yarns experi- reducing the stress transfer between the fibrils.
enced 18.5%, 16.4% and 30.7% decrease in tensile The removal of surface impurities such as waxes
strength and 15.9%, 13.0% and 27.8% decrease in ten- and oils leads to a cleaner and rougher fibre surface
sile modulus, respectively. However, the elongations at than before, as displayed in Figure 3. This rougher
break of alkali-treated flax and linen yarns increased. surface facilitates both mechanical interlocking
A similar result was obtained by Gomes et al.,34 and bonding reaction due to the exposure of the
where a single curaua fibre after alkali treatment hydroxyl groups to epoxy, thereby increasing the
was considered. This fact may attributable to fibre fibre/matrix adhesion.



decrease of the tensile strength of approximately
Tensile properties of composites 26.4% (Figure 4(a)), and an increase of 25.7% in tensile
Figure 4 presents the tensile properties of net epoxy resin modulus compared to the respective values of net epoxy
and untreated/alkali-treated flax, linen and bamboo (Figure 4(b)).
fabric reinforced composites. For untreated specimens The experimental tensile strength and tensile
(Figure 4(a)), the tensile strengths of flax and linen fabric modulus of the untreated composites are compared
reinforced composites increased 64.5% and 44.1%, with their theoretical values obtained from a simply
respectively, compared to pure epoxy (73 MPa). The ten- rule-of-mixture (Table 5). The rule-of-mixture applied
sile moduli of flax and linen fabric reinforced composites for continuous fibre composites assumes equal strain
are 157.1% and 97.1% higher than that of pure epoxy in fibre and matrix and a perfect fibre–matrix bond-
(3.5 GPa), respectively (Figure 4(b)). This indicates that ing. For tensile modulus, the experimental values of
the addition of fabrics increases the tensile strength and all the three composites are slightly less than their
modulus of the composites because a uniform stress dis- theoretical values. The difference is because the
tribution from the epoxy is transferred to the unidirec- rule-of-mixture disregards the fibre/matrix interfacial
tional fibre. The significant increase in tensile moduli of interaction, the contribution of the transverse
flax/epoxy and linen/epoxy composites supports the fol- yarns and variations in fibre alignment. The simply
lowing statement derived from the composite matrix rule-of-mixture overestimates the composite stiffness.
theory that the tensile modulus of fibre-reinforced com- With respect to the tensile strength, the experimental
posite is strongly dependent on the modulus of the fibre values of flax- and linen-epoxy composites are larger
and the matrix, the fibre content and orientation. while that of bamboo-epoxy composite is lower than
However, the addition of bamboo fabric causes a the corresponding predicted value. This comparison
shows that the actual values cannot be obtained
using the simply rule-of-mixture. This is to be
expected because the measured tensile strength
provides only one average value. However, the yarn
tensile strength is very sensitive to the testing condi-
tion, e.g. gauge length and strain rates. A different
gauge length and/or strain rate will lead to other
yarn strength, hence resulting in other theoretical
strength of the composite.
With regard to the tensile strain at failure, only the
value of linen/epoxy composite of 3.7% is larger than
that of pure epoxy, at 3.5%. Both flax/epoxy and
bamboo/epoxy composites have less tensile strains,
which is 3.0% and 2.8%, respectively (Figure 5(c)).
This is because the elongation measured at break of
linen yarn is larger, while those of flax and bamboo
yarns are lower, compared to the pure epoxy. The
decrease in tensile strains at failure of the composites
is due to the smaller elongation at break point of fibre
yarns compared to that of pure epoxy (Table 3).
Additionally, the 14 layers of bamboo fabric in the
composites (Table 2) may result in the epoxy being
insufficient to wet the fabrics entirely and lead to
poor fibre/matrix interfacial bonding, and thus to the
lower tensile properties of the composites.
As shown in Figure 4, the tensile strength and
modulus of all the composites increased due to the
treatment. Compared to the untreated ones, the flax/
epoxy, linen/epoxy and bamboo/epoxy composites
have 21.9%, 18.7% and 32.8% increase in tensile
strength and 13.3%, 8.8% and 13.6% increase in tensile
modulus, respectively.
Figure 3. Surface morphology of untreated and alkali-treated Figure 5 shows the typical tensile stress–strain rela-
single fibre yarns: (a) untreated flax and (b) treated flax. tionship of all the composites. The stress–strain curves


Yan et al. 431

can be divided approximately into two zones. The first thought to correspond to the elastic response of the
zone up to 0.3% strain is a purely elastic behaviour, aligned micro-fibrils to the applied strain and the end
allowing measurement of the modulus. The second of the curve represents the ultimate strength which is
zone is a non-linear zone until leading to the maximum due to fibre fraction and fibre pull-out. There is no
strength. When it reaches the maximum tensile strength, appreciable plastic deformation in the curves after fail-
the curve is followed by a sudden drop, which indicates ure; the crack propagates rapidly without increase in the
the occurrence of a brittle failure. This third part is applied stress when it reaches the peak stress.

Figure 4. Tensile properties of untreated/alkali-treated flax, linen and bamboo fabric reinforced composites compared to net
epoxy resin.



Table 5. Comparison of experimental with theoretical tensile properties of untreated composites based on rule of mixture

Measured tensile Theoretical tensile Measured tensile Theoretical tensile
Composites strength (MPa) strength (MPa) Change (%) modulus (GPa) modulus (GPa) Change (%)

Flax/epoxy 120.1 112.9 6.4 9.2 10.5 À12.3
Linen/epoxy 105.2 104.1 1.1 7.0 8.3 À15.6
Bamboo/epoxy 53.7 69.5 À22.7 4.5 4.9 À8.2

ðMeasuredvalue À TheoreticalvalueÞ
Changeð %Þ ¼ Â 100 %
Theoreticalvalue

Figure 5. Typical tensile stress–strain curves for untreated/alkali-treated flax, linen and bamboo fabric reinforced composites.

All the specimens failed primarily at a single cross result of the brittle nature of the epoxy resin. The gap
section in form of a brittle fracture and exhibited pull- indicated by ‘D’ between the flax fibre and the matrix
out of fibre yarns. It is clear that the fracture crack is represents the fibre debonding, which indicates the loss
perpendicular to the direction of the applied stress and of fibre/matrix interfacial adhesion. Figure 6 clearly
the failure is almost a strainght line. This indicates that shows that the failure of the fibres in the load direction,
failure of the fibre yarns along the load direction, debonding and pull-out, and brittle fracture of the
debonding and pull-out, and brittle fracture of the matrix have been found to govern the failure of fabric
matrix are the main failure mechanisms of the fabric- reinforced polymer composites in tension.
reinforced composites. This will be further discussed in SEM micrographs for tensile fractured surfaces
the next section. of untreated and treated composites are shown in
Figure 7. For untreated composites, Figure 7(a), (c)
Surface morphology of composites tensile fractured and (e) show some noticeable gaps between the fibres
and matrices (indicated by ‘A’, ‘C’ and ‘E’), which are
surface the evidence of poor fibre/matrix adhesion. In contrast,
Figure 6 depicts a typical fracture zone of untreated flax the fibre/matrix adhesion are enhanced after alkali
fabric-reinforced composites in tension. ‘A’ indicates treatment (see the locations indicated by ‘B’, ‘D’ and
the failure of the fibre due to the tensile stress applied. ‘F’ in Figure 7(b), (d) and (f), respectively). Compared
The fibre pull-out with a considerable length is clearly untreated (Figure 7(a)) with treated (Figure 7(b)) flax
visible (B). ‘C’ points to two large cracks due to brittle composites, it is clear that the treated fibre surface is
fracture of the epoxy matrix adjacent to the fibre as a much rougher than that of untreated flax fibre.


Yan et al. 433

pure epoxy because of the enhancement in flexural
strain in the composites.
As illustrated in Figure 8, the alkali treatment
enhances the flexural properties of all three fabric rein-
forced epoxy composites. Compared to the untreated
composites, the flax/epoxy, linen/epoxy and bamboo/
epoxy composites experienced 16.1%, 16.7% and
13.6% enhancement in flexural strength and 7.2%,
9.1% and 6.3% increase in flexural modulus,
respectively.
The improvement of flexural properties of treated
fibre composites is possibly due to the removal
of outer fibre surface; increase cellulose content and
interfacial adhesion by alkali treatment. However, the
results show that the influence of alkali treatment
on flexural properties is less than that on the tensile
properties (Figures 4 and 8). The possible reason is
that the flexural failure mode shows less fibre pull-
Figure 6. SEM micrograph of typical failure modes of untreated out, a consequence of the direction of the applied
flax fabric reinforced composite in tension. A, failure of fibre; stress being perpendicular to the composite laminate
B, fibre pull-out; C, brittle fracture of epoxy matrix and D, in the three-point bending test.
fibre debonding. SEM, scanning electron microscopy.
Flexural failure in FRP is characterised by the pres-
ence of compressive and tensile stresses. No specimen
This leads to better bonding at the fibre/matrix failed by typical delamination during loading and the
interface because alkali removes the impurities and failure mode shows little fibre pull-out in flax and linen
waxy substances from the fibre surface and creates a composites and no fibre pull-out in bamboo compos-
rougher topography which facilitates the mechanical ites. As expected, the crack is always initiated on the
interlocking. Also, the purified fibre surface further tensile side of the laminate and propagates in an
enhances the chemical bonding between the fibre and upward direction to compressive side.
epoxy matrix, because a purified fibre surface enables The typical flexural stress–strain curves of the
more hydrogen bonds to be formed between the hydro- untreated/alkali-treated composites are shown in
xyl groups of the cellulose at one side, and the epoxy Figure 9. Three regions could be defined approxi-
groups at the other side. In addition, it is clear that fibre mately. All the specimens in the first region show a
pull-out dominates the failure mode as displayed in linear relationship between stress and strain, in which
Figure 7(c). More fibre pull-out in tensile fracture the flexural modulus measurement can be performed.
zone indicates the poor fibre/matrix adhesion. As a In the second region, the curves exhibit a non-linear
consequence of the treatment, the fibre/matrix interface pattern before approaching the maximum strength.
bonding quality is improved and leads to better tensile The third region in the curves presents a decreasing
properties of the composites. trend after the maximum flexural strength. These
third parts of the curves are quite different between
flax/epoxy, linen/epoxy composites and bamboo/
epoxy composites. For both untreated/alkali-treated
Flexural properties of composites bamboo/epoxy composites, the post-peak curves go
The flexural properties of untreated/alkali-treated down very rapidly almost in a straight line without
composites are illustrated in Figure 8. Compared to increasing in strains. This indicates that the specimen
pure epoxy (82 MPa), the flexural strength of the breaks into two pieces when the maximum stress is
untreated flax/epoxy composite increased 46.7% and reached, while for untreated/alkali-treated flax and
that of the untreated linen/epoxy composite increased linen composites, the post-peak curves dip with a
30.6%. The flexural moduli of the untreated flax/epoxy, continuous increase in strains; this reveals a ductile
linen/epoxy and bamboo/epoxy composites increased behaviour before fracture of flax and linen composites
100%, 57.1% and 14.3%, respectively. The flax, linen in flexure. The possible reason is that although the flax/
and bamboo composites have 20%, 54.3% and 28.6% epoxy and linen/epoxy specimens are broken when the
enhancement in flexural failure strain, compared to maximum stresses are reached, some fibres are not
pure epoxy (Figure 8(c)). This shows that the flexural broken into two parts; and they still withstand the
strain at failure of the three fibres are larger than that of applied stress.



Figure 7. SEM micrographs of tensile fractured surfaces of untreated/alkali-treated flax, linen and bamboo fabric reinforced
composites. ‘A’, ‘C’ and ‘E’, noticeable gaps between fibres and matrices indicating poor fibre/matrix adhesion, and ‘B’, ‘D’ and ‘F’,
small gaps revealing enhanced fibre/matrix adhesion due to alkali treatment. SEM, scanning electron microscopy.


Yan et al. 435

Figure 8. Flexural properties of untreated/alkali-treated flax, linen and bamboo fabric reinforced composites compared to net
epoxy resin.

mechanical properties of the composites were studied.
The investigation reveals:
Conclusions
Flax, linen and bamboo fabric reinforced epoxy com- 1. Alkali treatment with 5 wt% NaOH solution has a
posites have been manufactured using the VBT. The negative effect on the tensile strength and modulus
influence of alkali treatment on the tensile properties of single-strand flax, linen and bamboo yarns.
of single-strand yarns, the surface morphologies and The failure mechanism of natural single-strand



Figure 9. Typical flexural stress–strain curves for untreated/alkali-treated flax, linen and bamboo fabric reinforced composites.

fibres under tension is the combination of fibre Funding
breakage and slippage.
This research received no specific grant from any funding
2. The alkali treatment significantly increases the ten-
agency in the public, commercial, or not-for-profit sectors.
sile strength and modulus, flexural strength and
modulus of all the fabric-reinforced composites.
However, the tensile strain and flexural strain of
the composite increased marginally. References
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