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


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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.

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

  1. 1. Journal of Reinforced Plastics and Composites 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 The online version of this article can be found at: Published by: Additional services and information for Journal of Reinforced Plastics and Composites can be found at: Email Alerts: Subscriptions: Reprints: Permissions: Citations: >> Version of Record - Feb 28, 2012 OnlineFirst Version of Record - Feb 21, 2012 What is This? Downloaded from at The University of Auckland Library on March 3, 2012
  2. 2. Article Journal of Reinforced Plastics and CompositesImproving the mechanical properties 31(6) 425–437 ! The Author(s) 2012 Reprints and permissions:of natural fibre fabric reinforced epoxy DOI: 10.1177/0731684412439494composites by alkali treatment jrp.sagepub.comLibo Yan1, Nawawi Chouw1 and Xiaowen Yuan2AbstractIn this article, three bio-composites, i.e. flax, linen and bamboo fabric reinforced epoxy resin, were manufactured using avacuum bagging technique. The influence of alkali treatment (with 5 wt% NaOH solution for 30 min) on tensile propertiesof flax, linen and bamboo single-strand yarns, surface morphology and mechanical properties (with respect to tensile andflexural properties) of the composites were investigated. It was found that the failure mechanism of single-strand fibresunder tension consists of fibre breakage and slippage simultaneously. The alkali treatment had a negative effect on thetensile strength and modulus of the flax, linen and bamboo single-strand yarns. However, after the treatment, the tensileand flexural properties of all the composites increased, e.g. the tensile and flexural strength of the treated flax/epoxycomposite increased 21.9% and 16.1%, compared to the untreated one. After the treatment in all the composites, thetensile fractured surfaces exhibited an improvement of fibre/epoxy interfacial adhesion.Keywordsnatural fabrics, composite, scanning electron microscopy Flax, hemp, jute, sisal and bamboo are the mostIntroduction popular reinforcement materials in bio-compositesSynthetic fibre reinforced polymer (FRP) composites because they are cost-effective, have low density withwith high strength and stiffness have been widely used high specific strength and stiffness, and are readilyin the last decade in aerospace and automotive available.7,8 Nevertheless, natural fibres also possessindustries.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 arein polymer composites has gained popularity in engi- strongly dependent on the climate, location andneering applications due to increasing environmental weather; so it is difficult to predict their respectiveconcern.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, naturalof structural materials.4 In 2003, around 43,000 tonnes of natural fibres wereused by the European automotive industry as 1 Department of Civil and Environmental Engineering, The University ofcomposite reinforcement materials.5 In 2010, the Auckland, New Zealandamount 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-reinforcedcomposites in European Union.6 The explosive Corresponding author: Libo Yan, Department of Civil and Environmental Engineering, Thegrowth in bio-composites is indicative of their wider University of Auckland, Level 11, Engineering Building, 20 Symondsapplication in the future due to the favourable Street, Auckland 1001, New Zealandmechanical performance of natural fibres. Email: Downloaded from at The University of Auckland Library on March 3, 2012
  3. 3. 426 Journal of Reinforced Plastics and Composites 31(6)fibres as composite reinforcement are not considered This technique is best suited for moulding epoxyonly 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 handform 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 betterThis 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 withtrol, good reproducibility and high productivity.12 In this technique, which is critical for practical engineeringthese 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 bybond 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 notcomposites by improving the fibre/matrix adhesion, been investigated. This study focuses on the effect ofsurface modification including alkali, saline and acety- alkali treatment on the mechanical properties of thelation 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 yarnsto operate and cheap. Studies have shown that alkali and the composites, scanning electron microscopytreatment with sodium hydroxide (NaOH) solution can (SEM) is used.significantly increase mechanical strengths of flaxmonofilament fibre reinforced composites14,15 andbamboo 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 epoxyural and toughness properties of bamboo mat/epoxy Commercial woven flax, linen and bamboo fabrics werecomposites. The optimum result was obtained when used because of their wide availability. The flax fabricbamboo mat was treated with 5 wt% (by weight) (550 g/m2) was obtained from Libeco, Belgium. TheNaOH 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. TheThe results showed that 5 wt% NaOH treatment led to structures of fabrics are displayed in Figure 1. Thebetter tensile properties of bamboo fibres.17 The study epoxy used is the SP High Modulus Prime 20LVby Wang et al. indicated that 5 wt% or 10 wt% of epoxy system, which is specifically designed for use inNaOH solution was the appropriate concentration for a variety of resin infusion processes (Table 1).alkalisation of flax fibres, for improving the mechanicalproperties of flax FRPs.18 Alkali treatment In this article, three epoxy composites reinforcedwith flax, linen and bamboo woven fabrics were man- Initially, these fabrics were cut into a size of 400 Â 300ufactured using a vacuum bagging technique (VBT). mm2. Fibre single-strand yarns were extracted from theFigure 1. Structures of flax, linen and bamboo woven fabrics. Downloaded from at The University of Auckland Library on March 3, 2012
  4. 4. Yan et al. 427corresponding fabric. For alkali-treated specimens, then impregnation of the preform with resin in athese fabrics and yarns were washed three times with flexible bag in which negative pressure is generated byfresh water to remove contaminants, and then dried at a vacuum pump. Next, the composites were cured atroom temperature for 48 h. The dried fabrics and yarns room temperature for 24 h and placed into the Elecfurnwere 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 freshwater and subsequently three times with distilledwater, to remove the remaining sodium hydroxide solu- Fibre volume fractiontion. Finally, these fabrics and yarns were dried at 80 C Density of the mixed epoxy given by the supplier wasin 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 displacementof hydrogen bonding in the fibre surface, thereby medium based on ASTM D792.24 The void contentsincreasing surface roughness. This treatment removes of the composites were determined according toa certain amount of lignin, wax and oils covering the ASTM D2734.25 After obtaining the density and voidexternal surface of the fibre cell wall, depolymerises cel- content for each composite, the fibre volume fractionlulose and exposes the crystallites.21 Addition of for the composite was derived from the fibre/epoxysodium hydroxide to natural fibre promotes the ionisa- resin weight ratio and the densities of both fibre andtion 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 =Vrwould become more compatible with the epoxy matrix.Thus, alkaline processing directly influences the where Vv is the void content of composite and Vr thecellulosic fibril, the degree of polymerisation and the volume of epoxy resin. The calculated fibre volumeextraction 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 theComposite 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 machineTable 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 anyMix 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 0.936Table 2. Physical properties of composites Thickness of Thickness of Fibre volumeComposites 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 55.9Linen/epoxy Untreated 8 0.510 4.984 54.8 Alkali-treated 8 0.498 5.011 55.3Bamboo/epoxy Untreated 14 0.312 5.085 55.4 Alkali-treated 14 0.304 5.069 54.2 Downloaded from at The University of Auckland Library on March 3, 2012
  5. 5. 428 Journal of Reinforced Plastics and Composites 31(6) 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 Alkali-treated 0.506 1.17 Æ 0.13 108.4 Æ 12.2 4.4 Æ 0.5 10.7 Æ 0.4 Bamboo Untreated 0.303 1.26 Æ 0.10 67.5 Æ 5.7 2.8 Æ 0.2 5.4 Æ 0.4 Alkali-treated 0.298 0.85 Æ 0.09 46.8 Æ 6.4 2.8 Æ 0.1 3.9 Æ 0.3 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.32The cross-sectional area of fibre single-strand yarn wasassumed to be circular; the diameter of the yarn was Scanning electron microscopymeasured 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 (Philipsment 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 surfacesof Super Extended Graphics Array (SXGA), Wide were vacuum-coated by evaporation with platinumExtended Graphics Array (WXGA) and XGA. before examination.Tensile test of composites Results and discussionThe flat coupon tensile test was conducted on the Tensile properties of fibre yarnsInstron 5567 machine according to ASTM D3039 onplates 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. Tensilethe elongation during the test, an extensometer with a properties of flax and bamboo monofilament fibresgauge was placed on each specimen. For each compos- given in literature are demonstrated in Table 4.30–32ite, five specimens were tested at room temperature and It is observed that both measured tensile failure stressthe average tensile strength and modulus were obtained and modulus of flax, linen and bamboo single-stranddirectly 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 fibreThree-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 consistingmachine 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 elementaryhead speed was 2.2 mm/min for each test. The length fibres and multiple fracture of the elementary fibres.33of support span was 80 mm and the overhang length on The tensile failure of textile fibre yarns is a combinationboth sides was 10 mm. For each composite, five speci- of fibre slippage and fibre breakage, as shown inmens 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 numberfrom the machine. of fibre filaments are twisted into a continuous strand Downloaded from at The University of Auckland Library on March 3, 2012
  6. 6. Yan et al. 429Figure 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 sodiumof 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 suchcross-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.35close 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 yarnfailure of viscose fibre yarn is strongly dependent on specimens. However, the reduction in fibre weight isthe 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 theyarns, the yarn failure mechanism is slippage domi- Surface morphology of fibre yarnsnated, thus the load-bearing capacity of the slipped Alkali treatment could influence the inner cellulosicfibre is reduced drastically and the final yarn strength components of the fibre and the non-cellulosicis 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 as115.4% 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 yarnsbreak 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 cementingstrength and tensile modulus of all the three fibre constituents (hemicellulose, lignin and pectin) resultedyarns decreased. Compared to untreated specimens, in the decrease in tensile properties of fibre yarn bythe 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 waxesstrength and 15.9%, 13.0% and 27.8% decrease in ten- and oils leads to a cleaner and rougher fibre surfacesile modulus, respectively. However, the elongations at than before, as displayed in Figure 3. This rougherbreak of alkali-treated flax and linen yarns increased. surface facilitates both mechanical interlockingA similar result was obtained by Gomes et al.,34 and bonding reaction due to the exposure of thewhere a single curaua fibre after alkali treatment hydroxyl groups to epoxy, thereby increasing thewas considered. This fact may attributable to fibre fibre/matrix adhesion. Downloaded from at The University of Auckland Library on March 3, 2012
  7. 7. 430 Journal of Reinforced Plastics and Composites 31(6) decrease of the tensile strength of approximatelyTensile properties of composites 26.4% (Figure 4(a)), and an increase of 25.7% in tensileFigure 4 presents the tensile properties of net epoxy resin modulus compared to the respective values of net epoxyand 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 comparedreinforced composites increased 64.5% and 44.1%, with their theoretical values obtained from a simplyrespectively, compared to pure epoxy (73 MPa). The ten- rule-of-mixture (Table 5). The rule-of-mixture appliedsile moduli of flax and linen fabric reinforced composites for continuous fibre composites assumes equal strainare 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 ofthe addition of fabrics increases the tensile strength and all the three composites are slightly less than theirmodulus of the composites because a uniform stress dis- theoretical values. The difference is because thetribution from the epoxy is transferred to the unidirec- rule-of-mixture disregards the fibre/matrix interfacialtional fibre. The significant increase in tensile moduli of interaction, the contribution of the transverseflax/epoxy and linen/epoxy composites supports the fol- yarns and variations in fibre alignment. The simplylowing 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 experimentalposite is strongly dependent on the modulus of the fibre values of flax- and linen-epoxy composites are largerand the matrix, the fibre content and orientation. while that of bamboo-epoxy composite is lower thanHowever, 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 Downloaded from at The University of Auckland Library on March 3, 2012
  8. 8. Yan et al. 431can be divided approximately into two zones. The first thought to correspond to the elastic response of thezone up to 0.3% strain is a purely elastic behaviour, aligned micro-fibrils to the applied strain and the endallowing measurement of the modulus. The second of the curve represents the ultimate strength which iszone is a non-linear zone until leading to the maximum due to fibre fraction and fibre pull-out. There is nostrength. 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 thethe 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 netepoxy resin. Downloaded from at The University of Auckland Library on March 3, 2012
  9. 9. 432 Journal of Reinforced Plastics and Composites 31(6)Table 5. Comparison of experimental with theoretical tensile properties of untreated composites based on rule of mixture Measured tensile Theoretical tensile Measured tensile Theoretical tensileComposites strength (MPa) strength (MPa) Change (%) modulus (GPa) modulus (GPa) Change (%)Flax/epoxy 120.1 112.9 6.4 9.2 10.5 À12.3Linen/epoxy 105.2 104.1 1.1 7.0 8.3 À15.6Bamboo/epoxy 53.7 69.5 À22.7 4.5 4.9 À8.2 ðMeasuredvalue À TheoreticalvalueÞ Changeð %Þ ¼ Â 100 % TheoreticalvalueFigure 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 gapsection in form of a brittle fracture and exhibited pull- indicated by ‘D’ between the flax fibre and the matrixout of fibre yarns. It is clear that the fracture crack is represents the fibre debonding, which indicates the lossperpendicular to the direction of the applied stress and of fibre/matrix interfacial adhesion. Figure 6 clearlythe 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 thedebonding and pull-out, and brittle fracture of the matrix have been found to govern the failure of fabricmatrix 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 surfacesthe 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 aresurface 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 alkalifabric-reinforced composites in tension. ‘A’ indicates treatment (see the locations indicated by ‘B’, ‘D’ andthe failure of the fibre due to the tensile stress applied. ‘F’ in Figure 7(b), (d) and (f), respectively). ComparedThe fibre pull-out with a considerable length is clearly untreated (Figure 7(a)) with treated (Figure 7(b)) flaxvisible (B). ‘C’ points to two large cracks due to brittle composites, it is clear that the treated fibre surface isfracture of the epoxy matrix adjacent to the fibre as a much rougher than that of untreated flax fibre. Downloaded from at The University of Auckland Library on March 3, 2012
  10. 10. 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 appliedflax fabric reinforced composite in tension. A, failure of fibre; stress being perpendicular to the composite laminateB, 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 specimenThis leads to better bonding at the fibre/matrix failed by typical delamination during loading and theinterface because alkali removes the impurities and failure mode shows little fibre pull-out in flax and linenwaxy 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 theinterlocking. Also, the purified fibre surface further tensile side of the laminate and propagates in anenhances 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 themore hydrogen bonds to be formed between the hydro- untreated/alkali-treated composites are shown inxyl 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 apull-out dominates the failure mode as displayed in linear relationship between stress and strain, in whichFigure 7(c). More fibre pull-out in tensile fracture the flexural modulus measurement can be indicates the poor fibre/matrix adhesion. As a In the second region, the curves exhibit a non-linearconsequence 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 decreasingproperties 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-treatedFlexural properties of composites bamboo/epoxy composites, the post-peak curves goThe flexural properties of untreated/alkali-treated down very rapidly almost in a straight line withoutcomposites are illustrated in Figure 8. Compared to increasing in strains. This indicates that the specimenpure epoxy (82 MPa), the flexural strength of the breaks into two pieces when the maximum stress isuntreated flax/epoxy composite increased 46.7% and reached, while for untreated/alkali-treated flax andthat of the untreated linen/epoxy composite increased linen composites, the post-peak curves dip with a30.6%. The flexural moduli of the untreated flax/epoxy, continuous increase in strains; this reveals a ductilelinen/epoxy and bamboo/epoxy composites increased behaviour before fracture of flax and linen composites100%, 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 theenhancement in flexural failure strain, compared to maximum stresses are reached, some fibres are notpure epoxy (Figure 8(c)). This shows that the flexural broken into two parts; and they still withstand thestrain at failure of the three fibres are larger than that of applied stress. Downloaded from at The University of Auckland Library on March 3, 2012
  11. 11. 434 Journal of Reinforced Plastics and Composites 31(6)Figure 7. SEM micrographs of tensile fractured surfaces of untreated/alkali-treated flax, linen and bamboo fabric reinforcedcomposites. ‘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. Downloaded from at The University of Auckland Library on March 3, 2012
  12. 12. Yan et al. 435Figure 8. Flexural properties of untreated/alkali-treated flax, linen and bamboo fabric reinforced composites compared to netepoxy resin. mechanical properties of the composites were studied. The investigation reveals:ConclusionsFlax, linen and bamboo fabric reinforced epoxy com- 1. Alkali treatment with 5 wt% NaOH solution has aposites have been manufactured using the VBT. The negative effect on the tensile strength and modulusinfluence 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 Downloaded from at The University of Auckland Library on March 3, 2012
  13. 13. 436 Journal of Reinforced Plastics and Composites 31(6)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 funding2. 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. References3. In tension, the flax, linen and bamboo fabric rein- 1. Chowdhury F, Hosur M and Jeelani S. Studies on the forced composites exhibit the typical brittle fracture flexural and thermomechanical properties of woven mode. The flax fabric reinforced composite features carbon/nanoclay-epoxy laminates. Mater Sci Eng A the largest ultimate tensile strength, and the linen 2006; 421: 298–306. 2. Assarar M, Scida D, El Mahi A, et al. Influence of water fabric reinforced composites offers the largest tensile ageing on mechanical properties and damage events of two failure strain. reinforced composite materials: flax-fibres and glass-fibres.4. In flexure, the bamboo fabric reinforced composites Mater Des 2011; 32: 788–795. exhibit the brittle fracture mode while flax and linen 3. Bodros E, Pillin I, Montrelay N, et al. Could biopolymers composites possess a ductile behaviour before frac- reinforced by randomly scattered flax fibre be used in ture. The flax fabric reinforced composite has the structural applications? Compos Sci Technol 2007; 67: highest flexural strength at failure, and the linen 462–470. fabric reinforced composites give the largest failure 4. Bordes P, Pollet E and Averous L. Nano-biocomposites: flexural strain. biodegradable polyester/nanoclay systems. Prog Polym Sci5. SEM study clearly reveals that the failure of natural 2009; 34: 125–155. fibre fabric reinforced composite is dominated by the 5. Liu Q, Stuart T, Hughes M, et al. Structural biocomposites failure of fibre yarns along the load direction, from flax –part II: the use of PEG and PVA as interfacial debonding and pull-out, brittle fracture of the compatibilising agents. Composites Part A 2007; 38: matrix. 1403–1413. 6. Carus M and Scholz L. Targets for bio-based composites and natural fibres. Biowerkstoff Report. ISSN 1867-1217, This study is part of a research program investi- Edition 8, March 2011, p.24.gating the feasibility of bio-composites as building 7. Corrales F, Vilaseca F, Llop M, et al. Chemicalmaterials. A hybrid composite consisting of both modification of jute fibers for the production of green-flax and linen fabric with alkali treatment as rein- composites. J Hazard Mater 2007; 144: 730–735.forcement may lead to better overall mechanical prop- 8. Herrerafranco P and Valadezgonzalez A. A study of theerties in tension and flexure and will be investigated mechanical properties of short natural-fiber reinforcednext. composites. Composites Part B 2005; 36: 597–608. Downloaded from at The University of Auckland Library on March 3, 2012
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