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 DOI: 10.1177/0731684412439494

2012 31: 425 originally published online 21 February 2012Journal of Reinforced Plastics and CompositesLibo Yan, Nawawi Chouw and Xiaowen Yuan

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

  

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Article

Improving the mechanical propertiesof natural fibre fabric reinforced epoxycomposites by alkali treatment

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

Introduction

Synthetic fibre reinforced polymer (FRP) compositeswith high strength and stiffness have been widely usedin the last decade in aerospace and automotiveindustries.1 In recent years, the use of bio-fibres toreplace synthetic carbon/glass fibres as reinforcementin polymer composites has gained popularity in engi-neering applications due to increasing environmentalconcern.2,3 The advent and application of nanotechnol-ogy have generated renewed interest in bio-compositeswhich show promising potential as the next generationof structural materials.4

In 2003, around 43,000 tonnes of natural fibres wereused by the European automotive industry ascomposite reinforcement materials.5 In 2010, theamount climbed to about 315,000 tonnes, whichaccounted for 13% of the total reinforcement materials(glass, carbon and natural fibres) in fibre-reinforcedcomposites in European Union.6 The explosivegrowth in bio-composites is indicative of their widerapplication in the future due to the favourablemechanical performance of natural fibres.

Flax, hemp, jute, sisal and bamboo are the mostpopular reinforcement materials in bio-compositesbecause they are cost-effective, have low density withhigh specific strength and stiffness, and are readilyavailable.7,8 Nevertheless, natural fibres also possesssome negative characteristics, i.e. they are highly hydro-philic and their mechanical and physical properties arestrongly dependent on the climate, location andweather; so it is difficult to predict their respectivecomposite properties.9 Natural fibres also have a com-plex structure, consisting of cellulose, hemicelluloses,pectin, lignin and other components.10 Thus, natural

1Department of Civil and Environmental Engineering, The University of

Auckland, New Zealand2Department of Mechanical Engineering, The University of Auckland,

New Zealand

Corresponding author:

Libo Yan, Department of Civil and Environmental Engineering, The

University of Auckland, Level 11, Engineering Building, 20 Symonds

Street, Auckland 1001, New Zealand

Email: lyan118@aucklanduni.ac.nz

Journal of Reinforced Plastics

and Composites

31(6) 425–437

! The Author(s) 2012

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DOI: 10.1177/0731684412439494

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fibres as composite reinforcement are not consideredonly in the form of monofilament configuration.11

Polymer matrix, reinforced by woven fabric, is theform of composites used most commonly in structuralapplications such as aircrafts, boats and automobiles.This is attributed to the fact that the woven fabricallows the control of fibre orientation and quality con-trol, good reproducibility and high productivity.12 Inthese applications, good tensile strength is essentialfor the composite performance.13 The composite tensileproperties are significantly dependent on the interfacialbond between the fabric layer and the matrix material,as well as the fabric structure.

To enhance the mechanical properties of bio-composites by improving the fibre/matrix adhesion,surface modification including alkali, saline and acety-lation has been investigated.7,11,14–16 Among thosetreatments, alkali is widely applied because it is easyto operate and cheap. Studies have shown that alkalitreatment with sodium hydroxide (NaOH) solution cansignificantly increase mechanical strengths of flaxmonofilament fibre reinforced composites14,15 andbamboo monofilament fibre reinforced composites.16

Kushwaha and Kumar analysed the effect of differ-ent NaOH solution concentrations on the tensile, flex-ural and toughness properties of bamboo mat/epoxycomposites. The optimum result was obtained whenbamboo mat was treated with 5wt% (by weight)NaOH solution for 30min.16 Wong et al. conducted astudy on the mechanical properties of bamboo fibres.The results showed that 5wt% NaOH treatment led tobetter tensile properties of bamboo fibres.17 The studyby Wang et al. indicated that 5wt% or 10wt% ofNaOH solution was the appropriate concentration foralkalisation of flax fibres, for improving the mechanicalproperties of flax FRPs.18

In this article, three epoxy composites reinforcedwith flax, linen and bamboo woven fabrics were man-ufactured using a vacuum bagging technique (VBT).

This technique is best suited for moulding epoxymatrix-based composites because of the superior flowof epoxy.19 As an alternate to the labour-intensive handlay-up process, VBT offers composites better unifor-mity of lay-up, higher fibre-to-resin ratio and betterstrength-to-weight ratio. In particular, theoretically,there is no limitation on the size of composites withthis technique, which is critical for practical engineeringapplication.

The effect of alkali treatment mainly on monofila-ment flax and bamboo fibres has been investigated bymany researchers.14–18 With regard to fibre yarn prop-erties, only the untreated yarn was considered.20 Todate, the effect of alkali on single fibre yarn has notbeen investigated. This study focuses on the effect ofalkali treatment on the mechanical properties of thethree single-strand yarns and the corresponding com-posites. To study the surface morphology of the yarnsand the composites, scanning electron microscopy(SEM) is used.

Materials and methods

Fibre and epoxy

Commercial woven flax, linen and bamboo fabrics wereused because of their wide availability. The flax fabric(550 g/m2) was obtained from Libeco, Belgium. Thelinen fabric (350 g/m2) and the bamboo fabric (210 g/m2) were obtained from Hemptech, New Zealand. Thestructures of fabrics are displayed in Figure 1. Theepoxy used is the SP High Modulus Prime 20LVepoxy system, which is specifically designed for use ina variety of resin infusion processes (Table 1).

Alkali treatment

Initially, these fabrics were cut into a size of 400� 300mm2. Fibre single-strand yarns were extracted from the

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

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corresponding fabric. For alkali-treated specimens,these fabrics and yarns were washed three times withfresh water to remove contaminants, and then dried atroom temperature for 48 h. The dried fabrics and yarnswere then immersed in 5wt% NaOH solution (20�C)for 30min, followed by washed 10 times with freshwater and subsequently three times with distilledwater, to remove the remaining sodium hydroxide solu-tion. Finally, these fabrics and yarns were dried at 80�Cin an oven for 24 h.

The significance of alkali treatment is the disruptionof hydrogen bonding in the fibre surface, therebyincreasing surface roughness. This treatment removesa certain amount of lignin, wax and oils covering theexternal surface of the fibre cell wall, depolymerises cel-lulose and exposes the crystallites.21 Addition ofsodium hydroxide to natural fibre promotes the ionisa-tion of the hydroxyl group, the alkoxide22

Fibre�OHþNaOH�Fibre�O�Naþ þH2O ð1Þ

The fibre with a higher amount of hydrogen groupswould become more compatible with the epoxy matrix.Thus, alkaline processing directly influences thecellulosic fibril, the degree of polymerisation and theextraction of lignin and hemicellulosic compounds.23

Composite fabrication

All the composites were manufactured by VBT. It con-sists of an initial hand lay-up of a fibre preform and

then impregnation of the preform with resin in aflexible bag in which negative pressure is generated bya vacuum pump. Next, the composites were cured atroom temperature for 24 h and placed into the Elecfurnoven for curing at 65�C for 7 h.

Fibre volume fraction

Density of the mixed epoxy given by the supplier was1.08 g/cm3. Composite density was determined by thebuoyancy method using water as the displacementmedium based on ASTM D792.24 The void contentsof the composites were determined according toASTM D2734.25 After obtaining the density and voidcontent for each composite, the fibre volume fractionfor the composite was derived from the fibre/epoxyresin weight ratio and the densities of both fibre andepoxy resin matrix.26 The fibre volume fraction Vf wascalculated using the following equation

Vf ¼ 1�1

1þ Vf=Vr� Vv ð2Þ

where Vv is the void content of composite and Vr thevolume of epoxy resin. The calculated fibre volumefractions of all the untreated and alkali-treatedcomposites are listed in Table 2. It can be seen thatthe fibre volume fractions and thicknesses of all thecomposites were approximately 55% and 5mm,respectively.

Tensile test of single-strand yarns

The tensile test was conducted on Instron 5567 machineaccording to ASTM D2256 on single-strand yarnspecimen in the straight configuration, in the case ofno conditioning.27 The specimens were 150mm inlength and were handled in a manner to avoid anychange in twist or any stretching of the specimens.Each test was repeated 10 times at the roomtemperature and the average values were reported.

Table 2. Physical properties of composites

Composites Fabric layers

Thickness of

each layer (mm)

Thickness of

composites (mm)

Fibre volume

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 1. Properties of epoxy system

Resin: SP

PRIME 20LV

Hardener:

SP PRIME

20 Slow

Mix ratio by weight 100 26

Viscosity at 20�C (cP) 1010–1070 22–24

Density (g/cm3) 1.123 0.936

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The cross-sectional area of fibre single-strand yarn wasassumed to be circular; the diameter of the yarn wasmeasured with the help of a projector. An EpsonPowerLite� X12 projector and an Epson DC-11 docu-ment camera are used to measure the diameter. Theprojector has Extended Graphics Array (XGA) resolu-tion. The camera has auto-select output resolutionof Super Extended Graphics Array (SXGA), WideExtended Graphics Array (WXGA) and XGA.

Tensile test of composites

The flat coupon tensile test was conducted on theInstron 5567 machine according to ASTM D3039 onplates with a size of 250� 25� 5mm3 for each compos-ite.28 The cross-head speed was 2mm/min. To registerthe elongation during the test, an extensometer with agauge was placed on each specimen. For each compos-ite, five specimens were tested at room temperature andthe average tensile strength and modulus were obtaineddirectly from the machine.

Three-point bending test of composites

The flexural test was carried out on the Instron 1185machine according to ASTMD790 on plates with a sizeof 100� 20� 5mm3 for each composite.29 The cross-head speed was 2.2mm/min for each test. The lengthof support span was 80mm and the overhang length onboth sides was 10mm. For each composite, five speci-mens were tested at room temperature and the averageflexural strength and modulus were obtained directlyfrom the machine.

Scanning electron microscopy

Surface topographies of the untreated and alkali-trea-ted fibre yarn were investigated using an SEM (PhilipsXL30S FEG, Netherlands) at room temperature, oper-ated at 5 kV. The tensile fracture surfaces of the com-posite samples were also analysed. The sample surfaceswere vacuum-coated by evaporation with platinumbefore examination.

Results and discussion

Tensile properties of fibre yarns

The tensile properties of untreated/alkali-treated flax,linen and bamboo yarns are listed in Table 3. Tensileproperties of flax and bamboo monofilament fibresgiven in literature are demonstrated in Table 4.30–32

It is observed that both measured tensile failure stressand modulus of flax, linen and bamboo single-strandyarns are much lower than those of flax and bamboomonofilament fibres in literature. This is attributed tothe different tensile failure mechanisms between fibreyarn and monofilament fibre. For monofilament fibre,the failure mechanism is a complex sequence consistingof axial splitting of the technical fibre along its elemen-tary constituents, radial cracking of the elementaryfibres and multiple fracture of the elementary fibres.33

The tensile failure of textile fibre yarns is a combinationof fibre slippage and fibre breakage, as shown inFigure 2(b), which shows the flax yarn close to failure.This is because when spinning fibres to yarns, a numberof fibre filaments are twisted into a continuous strand

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

Single-strand fibre yarn

Single-strand

diameter (mm)

Density

(g/cm3)

Tensile failure

stress (MPa)

Elongation at

break (%)

Young’s modulus

(GPa)

Flax Untreated 0.708 1.43� 0.09 145.4� 8.4 2.9� 0.3 16.4� 0.4

Alkali-treated 0.703 1.22� 0.05 118.5� 10.3 3.1� 0.4 13.8� 0.5

Linen Untreated 0.514 1.35� 0.04 129.7� 10.1 4.3� 0.2 12.3� 0.6

Alkali-treated 0.506 1.17� 0.13 108.4� 12.2 4.4� 0.5 10.7� 0.4

Bamboo Untreated 0.303 1.26� 0.10 67.5� 5.7 2.8� 0.2 5.4� 0.4

Alkali-treated 0.298 0.85� 0.09 46.8� 6.4 2.8� 0.1 3.9� 0.3

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

Fibre Density (g/cm3)

Tensile strength

(MPa)

Tensile modulus

(GPa)

Elongation at

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

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producing radial forces which cause movement of someof these filaments relative to others, and leads to acloser packing of all the filaments within any givencross-section. However, the tensile strength of thefibre bundle cannot achieve that of the yarn becauseclose to failure some fibres break and the rest slip(Figure 2(b)). According to Ghosh et al., the tensilefailure of viscose fibre yarn is strongly dependent onthe yarn structure, i.e. the configuration, alignmentand packing of constituent fibres in the yarn cross sec-tion.20 For fabric with loose packing of fibres in theyarns, the yarn failure mechanism is slippage domi-nated, thus the load-bearing capacity of the slippedfibre is reduced drastically and the final yarn strengthis poor.

Table 3 depicts that the tensile failure stress ofuntreated flax single-strand yarn is 12.1% and115.4% larger than those of untreated linen andbamboo yarns, respectively. The elongation at thebreak point of the linen yarn is almost 50% largerthan that of flax and bamboo yarns.

For the alkali-treated counterparts, the tensilestrength and tensile modulus of all the three fibreyarns decreased. Compared to untreated specimens,the alkali-treated flax, linen and bamboo yarns experi-enced 18.5%, 16.4% and 30.7% decrease in tensilestrength and 15.9%, 13.0% and 27.8% decrease in ten-sile modulus, respectively. However, the elongations atbreak of alkali-treated flax and linen yarns increased.A similar result was obtained by Gomes et al.,34

where a single curaua fibre after alkali treatmentwas considered. This fact may attributable to fibre

damage caused by chemical reaction with sodiumhydroxide during the treatment. This damage is consid-ered to be caused by a chemical structural change suchthat cellulose in the fibre partially changes from crys-talline cellulose I into amorphous cellulose II.35

Table 3 also shows that the alkali treatment leads tothe reduction in the diameter and the density of yarnspecimens. However, the reduction in fibre weight isgreater than that in fibre diameter after this treatment.

Surface morphology of fibre yarns

Alkali treatment could influence the inner cellulosiccomponents of the fibre and the non-cellulosiccomponents such as hemicelluloses, lignin and pectinsimultaneously. After alkali treatment, the (partial)hemicelluloses, lignin and surface impurities such aswaxes and oils were removed from the fibre surface.Since both diameter and density of alkali-treated yarnsdecreased (Table 3), it is indicated that the hemicellu-loses, lignin and pectin of the fibres were dissolved bythe alkaline solution. The removal of these cementingconstituents (hemicellulose, lignin and pectin) resultedin the decrease in tensile properties of fibre yarn byreducing the stress transfer between the fibrils.The removal of surface impurities such as waxesand oils leads to a cleaner and rougher fibre surfacethan before, as displayed in Figure 3. This roughersurface facilitates both mechanical interlockingand bonding reaction due to the exposure of thehydroxyl groups to epoxy, thereby increasing thefibre/matrix adhesion.

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

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Tensile properties of composites

Figure 4 presents the tensile properties of net epoxy resinand untreated/alkali-treated flax, linen and bamboofabric reinforced composites. For untreated specimens(Figure 4(a)), the tensile strengths of flax and linen fabricreinforced composites increased 64.5% and 44.1%,respectively, compared to pure epoxy (73MPa). The ten-sile moduli of flax and linen fabric reinforced compositesare 157.1% and 97.1% higher than that of pure epoxy(3.5GPa), respectively (Figure 4(b)). This indicates thatthe addition of fabrics increases the tensile strength andmodulus of the composites because a uniform stress dis-tribution from the epoxy is transferred to the unidirec-tional fibre. The significant increase in tensile moduli offlax/epoxy and linen/epoxy composites supports the fol-lowing statement derived from the composite matrixtheory that the tensile modulus of fibre-reinforced com-posite is strongly dependent on the modulus of the fibreand the matrix, the fibre content and orientation.However, the addition of bamboo fabric causes a

decrease of the tensile strength of approximately26.4% (Figure 4(a)), and an increase of 25.7% in tensilemodulus compared to the respective values of net epoxy(Figure 4(b)).

The experimental tensile strength and tensilemodulus of the untreated composites are comparedwith their theoretical values obtained from a simplyrule-of-mixture (Table 5). The rule-of-mixture appliedfor continuous fibre composites assumes equal strainin fibre and matrix and a perfect fibre–matrix bond-ing. For tensile modulus, the experimental values ofall the three composites are slightly less than theirtheoretical values. The difference is because therule-of-mixture disregards the fibre/matrix interfacialinteraction, the contribution of the transverseyarns and variations in fibre alignment. The simplyrule-of-mixture overestimates the composite stiffness.With respect to the tensile strength, the experimentalvalues of flax- and linen-epoxy composites are largerwhile that of bamboo-epoxy composite is lower thanthe corresponding predicted value. This comparisonshows that the actual values cannot be obtainedusing the simply rule-of-mixture. This is to beexpected because the measured tensile strengthprovides only one average value. However, the yarntensile strength is very sensitive to the testing condi-tion, e.g. gauge length and strain rates. A differentgauge length and/or strain rate will lead to otheryarn strength, hence resulting in other theoreticalstrength of the composite.

With regard to the tensile strain at failure, only thevalue of linen/epoxy composite of 3.7% is larger thanthat of pure epoxy, at 3.5%. Both flax/epoxy andbamboo/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 oflinen yarn is larger, while those of flax and bambooyarns are lower, compared to the pure epoxy. Thedecrease in tensile strains at failure of the compositesis due to the smaller elongation at break point of fibreyarns compared to that of pure epoxy (Table 3).Additionally, the 14 layers of bamboo fabric in thecomposites (Table 2) may result in the epoxy beinginsufficient to wet the fabrics entirely and lead topoor fibre/matrix interfacial bonding, and thus to thelower tensile properties of the composites.

As shown in Figure 4, the tensile strength andmodulus of all the composites increased due to thetreatment. Compared to the untreated ones, the flax/epoxy, linen/epoxy and bamboo/epoxy compositeshave 21.9%, 18.7% and 32.8% increase in tensilestrength and 13.3%, 8.8% and 13.6% increase in tensilemodulus, respectively.

Figure 5 shows the typical tensile stress–strain rela-tionship of all the composites. The stress–strain curves

Figure 3. Surface morphology of untreated and alkali-treated

single fibre yarns: (a) untreated flax and (b) treated flax.

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can be divided approximately into two zones. The firstzone up to 0.3% strain is a purely elastic behaviour,allowing measurement of the modulus. The secondzone is a non-linear zone until leading to the maximumstrength. When it reaches the maximum tensile strength,the curve is followed by a sudden drop, which indicatesthe occurrence of a brittle failure. This third part is

thought to correspond to the elastic response of thealigned micro-fibrils to the applied strain and the endof the curve represents the ultimate strength which isdue to fibre fraction and fibre pull-out. There is noappreciable plastic deformation in the curves after fail-ure; the crack propagates rapidly without increase in theapplied 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.

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All the specimens failed primarily at a single crosssection in form of a brittle fracture and exhibited pull-out of fibre yarns. It is clear that the fracture crack isperpendicular to the direction of the applied stress andthe failure is almost a strainght line. This indicates thatfailure of the fibre yarns along the load direction,debonding and pull-out, and brittle fracture of thematrix are the main failure mechanisms of the fabric-reinforced composites. This will be further discussed inthe next section.

Surface morphology of composites tensile fracturedsurface

Figure 6 depicts a typical fracture zone of untreated flaxfabric-reinforced composites in tension. ‘A’ indicatesthe failure of the fibre due to the tensile stress applied.The fibre pull-out with a considerable length is clearlyvisible (B). ‘C’ points to two large cracks due to brittlefracture of the epoxy matrix adjacent to the fibre as a

result of the brittle nature of the epoxy resin. The gapindicated by ‘D’ between the flax fibre and the matrixrepresents the fibre debonding, which indicates the lossof fibre/matrix interfacial adhesion. Figure 6 clearlyshows that the failure of the fibres in the load direction,debonding and pull-out, and brittle fracture of thematrix have been found to govern the failure of fabricreinforced polymer composites in tension.

SEM micrographs for tensile fractured surfacesof untreated and treated composites are shown inFigure 7. For untreated composites, Figure 7(a), (c)and (e) show some noticeable gaps between the fibresand matrices (indicated by ‘A’, ‘C’ and ‘E’), which arethe evidence of poor fibre/matrix adhesion. In contrast,the fibre/matrix adhesion are enhanced after alkalitreatment (see the locations indicated by ‘B’, ‘D’ and‘F’ in Figure 7(b), (d) and (f), respectively). Compareduntreated (Figure 7(a)) with treated (Figure 7(b)) flaxcomposites, it is clear that the treated fibre surface ismuch rougher than that of untreated flax fibre.

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

Composites

Measured tensile

strength (MPa)

Theoretical tensile

strength (MPa) Change (%)

Measured tensile

modulus (GPa)

Theoretical tensile

modulus (GPa) Change (%)

Flax/epoxy 120.1 112.9 6.4 9.2 10.5 �12.3

Linen/epoxy 105.2 104.1 1.1 7.0 8.3 �15.6

Bamboo/epoxy 53.7 69.5 �22.7 4.5 4.9 �8.2

Changeð %Þ ¼ðMeasuredvalue� TheoreticalvalueÞ

Theoreticalvalue� 100 %

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

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This leads to better bonding at the fibre/matrixinterface because alkali removes the impurities andwaxy substances from the fibre surface and creates arougher topography which facilitates the mechanicalinterlocking. Also, the purified fibre surface furtherenhances the chemical bonding between the fibre andepoxy matrix, because a purified fibre surface enablesmore hydrogen bonds to be formed between the hydro-xyl groups of the cellulose at one side, and the epoxygroups at the other side. In addition, it is clear that fibrepull-out dominates the failure mode as displayed inFigure 7(c). More fibre pull-out in tensile fracturezone indicates the poor fibre/matrix adhesion. As aconsequence of the treatment, the fibre/matrix interfacebonding quality is improved and leads to better tensileproperties of the composites.

Flexural properties of composites

The flexural properties of untreated/alkali-treatedcomposites are illustrated in Figure 8. Compared topure epoxy (82MPa), the flexural strength of theuntreated flax/epoxy composite increased 46.7% andthat of the untreated linen/epoxy composite increased30.6%. The flexural moduli of the untreated flax/epoxy,linen/epoxy and bamboo/epoxy composites increased100%, 57.1% and 14.3%, respectively. The flax, linenand bamboo composites have 20%, 54.3% and 28.6%enhancement in flexural failure strain, compared topure epoxy (Figure 8(c)). This shows that the flexuralstrain at failure of the three fibres are larger than that of

pure epoxy because of the enhancement in flexuralstrain in the composites.

As illustrated in Figure 8, the alkali treatmentenhances the flexural properties of all three fabric rein-forced epoxy composites. Compared to the untreatedcomposites, the flax/epoxy, linen/epoxy and bamboo/epoxy composites experienced 16.1%, 16.7% and13.6% enhancement in flexural strength and 7.2%,9.1% and 6.3% increase in flexural modulus,respectively.

The improvement of flexural properties of treatedfibre composites is possibly due to the removalof outer fibre surface; increase cellulose content andinterfacial adhesion by alkali treatment. However, theresults show that the influence of alkali treatmenton flexural properties is less than that on the tensileproperties (Figures 4 and 8). The possible reason isthat the flexural failure mode shows less fibre pull-out, a consequence of the direction of the appliedstress being perpendicular to the composite laminatein the three-point bending test.

Flexural failure in FRP is characterised by the pres-ence of compressive and tensile stresses. No specimenfailed by typical delamination during loading and thefailure mode shows little fibre pull-out in flax and linencomposites and no fibre pull-out in bamboo compos-ites. As expected, the crack is always initiated on thetensile side of the laminate and propagates in anupward direction to compressive side.

The typical flexural stress–strain curves of theuntreated/alkali-treated composites are shown inFigure 9. Three regions could be defined approxi-mately. All the specimens in the first region show alinear relationship between stress and strain, in whichthe flexural modulus measurement can be performed.In the second region, the curves exhibit a non-linearpattern before approaching the maximum strength.The third region in the curves presents a decreasingtrend after the maximum flexural strength. Thesethird parts of the curves are quite different betweenflax/epoxy, linen/epoxy composites and bamboo/epoxy composites. For both untreated/alkali-treatedbamboo/epoxy composites, the post-peak curves godown very rapidly almost in a straight line withoutincreasing in strains. This indicates that the specimenbreaks into two pieces when the maximum stress isreached, while for untreated/alkali-treated flax andlinen composites, the post-peak curves dip with acontinuous increase in strains; this reveals a ductilebehaviour before fracture of flax and linen compositesin flexure. The possible reason is that although the flax/epoxy and linen/epoxy specimens are broken when themaximum stresses are reached, some fibres are notbroken into two parts; and they still withstand theapplied stress.

Figure 6. SEM micrograph of typical failure modes of untreated

flax fabric reinforced composite in tension. A, failure of fibre;

B, fibre pull-out; C, brittle fracture of epoxy matrix and D,

fibre debonding. SEM, scanning electron microscopy.

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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.

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Conclusions

Flax, linen and bamboo fabric reinforced epoxy com-posites have been manufactured using the VBT. Theinfluence of alkali treatment on the tensile propertiesof single-strand yarns, the surface morphologies and

mechanical properties of the composites were studied.The investigation reveals:

1. Alkali treatment with 5wt% NaOH solution has anegative effect on the tensile strength and modulusof single-strand flax, linen and bamboo yarns.The failure mechanism of natural single-strand

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

epoxy resin.

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fibres under tension is the combination of fibrebreakage and slippage.

2. The alkali treatment significantly increases the ten-sile strength and modulus, flexural strength andmodulus of all the fabric-reinforced composites.However, the tensile strain and flexural strain ofthe composite increased marginally.

3. In tension, the flax, linen and bamboo fabric rein-forced composites exhibit the typical brittle fracturemode. The flax fabric reinforced composite featuresthe largest ultimate tensile strength, and the linenfabric reinforced composites offers the largest tensilefailure strain.

4. In flexure, the bamboo fabric reinforced compositesexhibit the brittle fracture mode while flax and linencomposites possess a ductile behaviour before frac-ture. The flax fabric reinforced composite has thehighest flexural strength at failure, and the linenfabric reinforced composites give the largest failureflexural strain.

5. SEM study clearly reveals that the failure of naturalfibre fabric reinforced composite is dominated by thefailure of fibre yarns along the load direction,debonding and pull-out, brittle fracture of thematrix.

This study is part of a research program investi-gating the feasibility of bio-composites as buildingmaterials. A hybrid composite consisting of bothflax and linen fabric with alkali treatment as rein-forcement may lead to better overall mechanical prop-erties in tension and flexure and will be investigatednext.

Funding

This research received no specific grant from any fundingagency in the public, commercial, or not-for-profit sectors.

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