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 DOI: 10.1177/0731684412449399

2012 31: 887Journal of Reinforced Plastics and CompositesLibo Yan

reinforced compositesEffect of alkali treatment on vibration characteristics and mechanical properties of natural fabric

  

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Article

Effect of alkali treatment on vibrationcharacteristics and mechanical propertiesof natural fabric reinforced composites

Libo Yan

Abstract

In this article, the effect of alkali treatment (with 5 wt. % sodium hydroxide solution for 30 min) on the compressive,

in-plane shear, impact properties and vibration characteristics of flax- and linen-fabric reinforced epoxy composites was

investigated. Test results show that alkali treatment enhanced the compressive strength and compressive modulus,

in-plane shear strength and shear modulus, and specific impact strength of both flax- and linen-epoxy composites.

However, after the treatment, the impact strength and damping ratio of the flax and linen composites decreased. The

reduction in impact strength and damping ratio is believed to be attributed to the improved fibre/matrix interfacial

adhesion, as analysed by scanning electron microscope.

Keywords

Natural fabrics, composite, mechanical properties, vibration, scanning electron microscope

Introduction

There has been a growing interest in the use of bio-fibres to replace manmade carbon/glass fibres asreinforcement in polymer composites for engineeringapplication.1 The advantages of bio-fibres are they arecost-effective, have low energy consumption, bio-degradability, low density with high specific strengthand stiffness and are readily available.2 In the recentyears, research on nano-composites shows that bio-composites have the potential as the next generationof structural materials.3 Currently, bio-composites aremainly applied in the automotive industry. There wasapproximately 43,000 tonnes of bio-fibres utilized asreinforcement materials of composites in theEuropean Union (EU) in 2003.4 This amount increasedto around 315,000 tonnes in 2010, which accounted for13% of the total reinforcement materials (glass, carbonand natural fibres) in fibre-reinforced composites.5 Theexplosive consumption in bio-composites is an indica-tion of their wider application in the future.

Among the bio-fibres, flax is a promising candidateto replace glass fibre. The tensile strength of flax fibreswere reported up to 1500MPa.6 Physical/mechanicalproperties of some bio-fibres and manmade fibres aregiven in Table 1. Romhany et al. investigated the tensile

fracture and failure behaviour of technical flax fibres.They found that the failure mechanism of flax fibre is acomplex sequence consisting of axial splitting of thetechnical fibre along its elementary constituents,radial cracking of the elementary fibres and multiplefracture of the elementary fibres.7 Bos et al. concludedthat the flax fibre had a complex structure, which con-sisted of cellulose, hemicelluloses, pectin, lignin andother components.8

Flax fibres as composite reinforcement are not con-sidered only in the form of monofilament configur-ation.9 Polymer matrix, reinforced by woven flaxfabric, is the form of composites used commonly instructural applications such as boats. It is reportedthat a 50% (by volume) flax fibre racing boat had com-pleted the France-to-Brazil Transat race in 15th place.10

The success in fabrication of the boat is attributed to

Department of Civil and Environmental Engineering, The University of

Auckland, 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(13) 887–896

! The Author(s) 2012

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DOI: 10.1177/0731684412449399

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the fact that the woven flax fabric allows the control offibre orientation and quality control, good reproduci-bility and high productivity.11 Assarar et al. confirmedthat the tensile stress and strain at failure of flax fabricreinforced polymer composites were 300MPa and1.8%, respectively – putting them close to glass fibrereinforced polymer composites.12 Liu and Hughes stu-died the toughness of flax fabric reinforced epoxy com-posites and concluded that the fibre volume fractiondominates the toughness, rather than the microstruc-tural arrangement of the fibre.13

Bio-composites have been applied in automotive andboat engineering. However, based on the best know-ledge of the author, to date rarely study on bio-composites in civil engineering has been reported. Infact, conventional construction materials such as con-crete and steel reinforcement have some significanteffects on the environment. In the United Kingdom(UK), construction process and building use not onlyconsume the most energy of all sectors and create themost CO2 emissions, they also create the most waste,use most non-energy-related resources and are respon-sible for the most pollution.14 To reduce these negativeenvironmental effects of conventional constructionmaterials, bio-composites as potential constructionmaterial are being investigated.

This article, as a part of on-going research to studythe feasibility of bio-composites as construction mater-ial, investigated the vibration characteristics (dampingratio and natural frequency) and the mechanical prop-erties (with respect to compressive strength, compres-sive modulus, in-plane shear stress and shear modulus,and the impact strength and specific impact strength) offlax and linen fabric reinforced epoxy composites. Inaddition, the effect of alkali treatment (with 5wt. %sodium hydroxide (NaOH) solution for 30min) onthe mechanical properties and the vibration character-istics of the composites were evaluated.

Materials and methods

Fibre and epoxy

Commercial woven flax and linen fabrics were usedbecause of their wide availability. Flax fabric withareal weight of 550 g/m2 was obtained from Libeco,Belgium. Linen fabric with areal weight of 350 g/m2

was obtained from Hemptech, New Zealand. Bothflax and linen are plain weave fabrics. Flax fabric hascount of 7.4 threads/cm in warp and 7.4 threads/cm inthe weft direction. Linen fabric has count of 10 threads/cm in warp and 10 threads/cm in the weft direction. Theepoxy used is the SP High Modulus Prime 20LV epoxysystem. The fabric structures and details for the resinsystem could be found in previous study.15

Alkali treatment

Initially, flax and linen fabrics were cut into a size of400� 300mm2. For alkali-treated specimens, flax andlinen fabrics were washed three times with fresh waterto remove contaminants and then dried at room tem-perature for 48 h. The dried fabrics were then immersedin 5wt. % NaOH solution (20�C) for 30min, followedby washing 10 times with fresh water and subsequentlythree times with distilled water, to remove the remain-ing NaOH solution. Finally, these fabrics were dried at80�C in an oven for 24 h.

Composite fabrication

All the composites were manufactured by vacuum bag-ging technique. It consists of an initial hand lay-up of afibre preform and then impregnation of the preformwith resin in a flexible bag in which negative pressureis generated by a vacuum pump. Next, the compositeswere cured at room temperature for 24 h and placedinto the Elecfurn oven for curing at 65�C for 7 h.

Table 1. Properties of natural and manmade fibres6

Fibre Density Elongation (%) Tensile strength (MPa) Elastic modulus (GPa)

Flax 1.5 2.7–3.2 500–1500 27.6

Cotton 1.5–1.6 7.0–8.0 400 5.5–12.6

Jute 1.3 1.5–1.8 393–773 26.5

Hemp 1.47 2.0–4.0 690 70

Sisal 1.5 2.0–2.5 511–635 9.4–22

Coir 1.2 30 593 4.0–6.0

Softwood kraft pulp 1.5 4.4 1000 40

E-glass 2.5 0.5 2000–3500 70

S-glass 2.5 2.8 4570 86

Carbon 1.4 1.4–1.8 4000 230–240

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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 D792.16 The void contentsof the composites were determined according toASTM D2734.17 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.18 The fibre volume fraction Vfwascalculated using the following equation:

Vf ¼ 1�1

1þ Vf=Vr� Vv ð1Þ

where Vv is the void content of composite and Vr is thevolume of epoxy resin. The calculated fibre volumefractions of the untreated and alkali-treated compositesare listed in Table 2. It can be seen that the fibre volumefractions and thicknesses of all the composites wereapproximately 55 % and 5mm, respectively.

Vibration test of composites

As a construction material, the damping of the materialis an important parameter related to the study of

vibration of a structure. Damping of a composite canbe defined as the decay of the composite in vibrations.It is interpreted as the dissipation of the vibration energy.Damping plays an important role in controlling thestructure from excessive vibrations due to dynamic load-ings. Therefore, understanding the vibration character-istic of FRP composite material, like damping, hasindustrial significance. Damping ratio – a dimensionlessmeasure of damping – is a property of the composite thatalso depends on its mass and stiffness. Vibration test wasconducted by using an accelerometer to detect thedynamic characteristics of the composite plates.Figure 1 gives a schematic view of the vibration testsystem. Three specimens with a size of 250� 25� 5mm3

(length�wide� thickness) for each composite wasclamped in the form of cantilever beams with 225mmeffective length span; the accelerometer was attachedon the free-end side of each cantilever laminiate, andthen stimulated the free vibration. The vibration accel-eration time histories were recorded by the data acquisi-tion software with a computer. The logarithmicdecrement is used for calculating the damping ratio �of cantilever laminates from the recorded accelerationtime histories based on the following equation:

� ¼1

2�jln

gigiþj

ð2Þ

225 mm

5 mm

Composite cantilever plate

Accelerometer

Amplifier

Data acquisitionsoftwareFFT

Naturalfrequency

Figure 1. Schematic view of vibration test system.

Table 2. Physical properties of the composites

Composites

Fabric

layers

Thickness of

each layer (mm)

Thickness of

composites (mm)

Fibre volume

fraction (%)

Density

(g/cm3)

Untreated flax/epoxy 6 0.712 5.049 55.1 1.273

Treated flax/epoxy 6 0.705 5.021 55.9 1.158

Untreated linen/epoxy 8 0.510 4.984 54.8 1.228

Treated linen/epoxy 8 0.498 5.011 55.3 1.130

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where gi is the peak acceleration of ith peak, giþj is thepeak acceleration of the peak j cycles after ith peak andti is the time instant at i cycle in the peak acceleration,as shown in Figure 2(a).

With respect to the fast Fourier transformation(FFT), the vibration frequency spectrum was obtainedfrom the measured time-histories. The main peak cor-responds to the natural frequency of the composite.The average damping ratio and average natural fre-quency of each composite tested on three specimenswas reported.

Compressive test of composites

The compressive test was carried out according toASTM D3410 on plates with a size of 125� 25� 5mm3

(length�wide� thickness) for each composite.19 Thecross-head speed was 1.5mm/min for each test. Anextensometer with a gauge was amounted on the speci-men for measurement of the strain. For each compos-ite, five specimens were tested at room temperature andthe average compressive strength and compressivemodulus were reported.

In-plane shear test of composites

The in-plane shear test was conducted according toASTM D3518 with a size of 250� 25� 5mm3

(length�wide� thickness) for each composite.20 Thecross-head speed was 2mm/min. To register the elong-ation during the test, an extensometer with a gauge wasplaced on each specimen. For each composite, five

Figure 2. Vibration time-history: (a) Untreated flax/epoxy composite and (b) alkali-treated flax/epoxy composite.

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specimens were tested at room temperature and theaverage shear strength and shear modulus wereobtained.

Impact test of composites

The Izod impact test was conducted according toASTM D256 on un-notched plates with a size of65� 12.7� 5 (length�wide� thickness)mm3 for eachcomposite.21 The impact loading was considered witha 25 J-hammer. Impact energy in J/m2 was considered.For each composite, five specimens were tested at roomtemperature and the average impact strength wasobtained.

Scanning electron microscopy

Surface topographies of the untreated and alkali-treated composites were investigated using a scanningelectron microscope (SEM, Philips XL30S FEG,Netherland) at room temperature, operated at 5 kV.The sample surfaces were vacuum coated by evapor-ation with platinum before examination.

Results and discussion

Vibration characteristics of composites

Figure 2 illustrates the time histories of untreated andalkali-treated flax/epoxy composites in vibrations. Theaverage damping ratio and average natural frequencyof all the composites are given in Table 3. It shows thatboth flax and linen fabric reinforced polymer compos-ites exhibit a similar pattern in damping ratio, namely,the damping ratio of the untreated composite is largerthan the alkali-treated one. Alkali treatment has anegative effect on damping ratio of both flax andlinen composites; the decrease in damping ratio offlax- and linen-epoxy composite is 7.4% and 9.3%,respectively (Table 3). For all the considered compos-ites, the untreated flax-epoxy composite has the largestdamping ratio of 1.48 %. With respect to naturalfrequency, it is observed that both flax and linencomposites possess a smaller natural frequency thanthe corresponding treated one. Compared with theuntreated composite, the increase in natural frequencyof the treated composite is believed to be attributed tothe fact that the alkali treatment reduced the mass(a lower density in Table 2) and increased the stiffnessof the composite. The Young’s modulus of alkali-trea-ted composite was larger than that of the untreatedone, which was concluded in previous study.15 Fromthe relationship among natural frequency ( f ), mass(m) and stiffness (k) of the composite, namely,f ¼ ð1=2�Þ �

ffiffiffiffiffiffiffiffiffik=m

p, it is easy to derive that the alkali

treatment increased the natural frequency of thecomposites.

Damping defines the energy dissipation capability ofa material. The damping of fabric reinforced polymercomposite is believed attributed to the presence of airvoids (e.g. the inherent lumens of the fibres), the visco-elastic characteristics of epoxy matrix and/or the fibrematerials and the interphase between the matrix andthe fibre. Interphase is defined as the region adjacentto fibre surface all along the fibre length.22 Interphasepossesses a considerable thickness and its properties aredifferent from those of embedded fibres and matrix. Itis true that the mechanical properties (e.g. tensile andflexural properties) of fabric fibre reinforced polymercomposites are highly dependent on the matrix/fibreinterphase.15

Fibre/matrix interphases also affect the damping ofthe composites. The decrease in damping ratio of the

Figure 3. Surface morphology of untreated (a) and alkali-

treated (b) flax fabric reinforced composites.

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treated composites may be attributed to the fact thatalkali treatment leads to better fibre/matrix interfaces.For untreated composites, there are more voids or gapsat the fibre/matrix interfaces. In the vibration, moreenergy has been dissipated due to the internal frictionbetween the fibres and the matrices where more fibre/matrix interfaces are involved, and thereby leads to alarger damping ratio of the composites. After alkalitreatment, the fibre/matrix interfacial adhesion wasimproved. Consequently, the gaps at the fibre/matrixinterfaces were narrowed and resulted in less energydissipation in the vibration. SEM micrographs of theuntreated and treated flax composites are shown inFigure 3. For the untreated composite, there are notice-able gaps between the adjacent fibres and the matrices;this indicates a poor fibre/matrix interfacial adhesion.These noticeable gaps are responsible for dissipatingenergy by fibre/matrix friction during the vibration.The insignificant gaps between the fibre and thematrix indicate the improved interfacial adhesion, asshown in Figure 3(b).

Compressive properties of composites

A comparison of compressive strength and compressivemodulus between pure epoxy and the composites is dis-played in Figure 4. The ultimate compressive strengthsof all the untreated and alkali-treated composites arehighly dependent on the strength of the epoxy matrix,as shown in Figure 4(a). The compressive strength ofuntreated flax- and linen-epoxy composite is90.32MPa and 78.64MPa, respectively, comparedwith the pure epoxy (68MPa). For compressive modu-lus, it can be seen that the stiffness of all untreated/

treated composites mainly depends on the fibres, as thecompressive modulus of the epoxy is 1.13GPa(Figure 4(b)). Compared with the untreated composites,both alkali-treated flax and linen composites have anincrease in compressive strength and compressive modu-lus; the increase in strength is 3.0% and 4.6%, respect-ively. The increase in modulus is 7.8% and 4.8%,respectively (Table 3). The enhancement in compressiveproperties of flax- and linen-epoxy composites by alkalitreatment is possibly due to the improved fibre/matrixinterfacial adhesion, since alkali treatment removes thehydrophilic nature of the cellulose fibre and thusimproves the interfacial bonding.

The compressive stress–strain curves of all the com-posites are shown in Figure 5. It can be seen that thebehaviour of all the untreated/alkali-treated flax andlinen fabric reinforced epoxy composites under com-pressive loading is non-linear. Three regions could bedefined approximately. In the first region, all the speci-mens show a linear relationship between the stress andstrain. In the second region, the curves exhibit a non-linear pattern before approaching the ultimate stress.The third post-peak curves go down with a continuousincrease in strains; this reveals a ductile behaviour. Thepredominated failure mechanism observed in the com-pression test was fibre micro-buckling. It should benoted here that the strains at break of all theuntreated/alkali-treated flax and linen composites aremore than 8%.

In-plane shear properties of composites

The in-plane shear stress–strain behaviour for bothuntreated and alkali-treated flax- and linen-epoxy

Table 3. Mechanical properties of treated and untreated compositesa

Compressive

strength

(MPa)

Compressive

modulus

(GPa)

Shear

strength

(MPa)

Shear

modulus

(GPa)

Impact

strength

(kJ/m2)

Specific

impact

strength

(kJ/m2/g�cm3)

Damping

ratio

(%)

Natural

frequency

(Hz)

Untreated flax/epoxy

composite

90.32

(4.30)

2.18

(0.13)

38.01

(2.21)

2.07

(0.11)

36.53

(3.24)

28.70

(–)

1.48

(0.06)

16.02

(0.25)

Treated flax/epoxy composite 93.02

(3.25)

2.35

(0.20)

41.11

(2.54)

2.16

(0.16)

33.87

(2.96)

29.25

(–)

1.37

(0.04)

16.83

(0.16)

Change due to alkali

treatment (%)

3.0 7.8 8.2 4.2 �7.3 1.9 �7.4 5.1

Untreated linen/epoxy

composite

78.64

(3.45)

1.88

(0.09)

34.06

(1.78)

1.84

(0.12)

30.62

(2.76)

24.93

(�)

1.29

(0.09)

16.94

(0.12)

Treated linen/epoxy composite 82.28

(4.02)

1.97

(0.16)

35.67

(2.06)

1.93

(0.20)

28.65

(2.24)

25.35

(�)

1.17

(0.05)

17.63

(0.28)

Change due to alkali

treatment (%)

4.6 4.8 4.7 4.9 �6.4 1.7 �9.3 4.1

aNumbers in parentheses are standard deviations.

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composites is shown in Figure 6. The average shearstrength and average shear modulus of all the compos-ites are given in Table 3. The flax/epoxy composite hasa larger shear strength and shear modulus than thelinen-epoxy composite. The shear strength and modu-lus of untreated flax- and linen-epoxy composites is38.0MPa and 2.07GPa, and 34.06MPa and1.84GPa, respectively.

After alkali treatment, the shear strength and shearmodulus of both flax- and linen-epoxy compositesincreased. Compared to the untreated composite, thetreated flax and linen composite experienced 8.2%and 4.7% increase in strength and 4.2% and 4.9%

increase in shear modulus, respectively (Table 3). Thealkali treatment removes the impurities and waxy sub-stances from the fibre surface and creates a roughertopography (Figure 3) which facilitates the mechanicalinterlocking. In addition, the purified fibre surface fur-ther enhances the chemical bonding between the fibreand epoxy matrix because a purified fibre surfaceenables more hydrogen bonds to be formed betweenthe hydroxyl groups of the cellulose at one side andthe epoxy groups at the other side. As a consequenceof the treatment, the fibre/matrix interfacial bondingquality is improved and leads to better in-plane shearproperties of the composites.

Figure 4. Compressive strength and compressive modulus of all the composites.

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The stress–strain curves can be divided approxi-mately into two zones. The first zone up to 0.3%strain has a purely elastic behaviour, allowing measure-ment of the modulus. The second zone is a non-linearzone until leading to the maximum shear stress. All thespecimens were failed because of matrix cracking andfibre breakage.

Impact properties of composites

Impact strength of a material is defined as its ability toresist the fracture under stress applied at high speed.The impact behaviour of a composite is significantlyinfluenced by the interfacial bond strength, the matrixand fibre properties. The damage process caused byimpact load energy is dissipated by fibre/matrix

Figure 5. Compressive stress–strain curve of all the composites.

Figure 6. Shear stress–strain behaviour of flax- and linen-epoxy composites.

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debonding, matrix fracture and fibre pull-out and fibrefracture, as displayed in Figure 7. It is observed that theimpact strength of the untreated flax composite(36.53 kJ/m2) is larger than the untreated linen compos-ite (30.62 kJ/m2), as given in Table 3. The difference inimpact strength of flax- and linen-epoxy composites isattributable to the different areal weights of the fabrics.

The alkali treatment reduced the impact strength ofthe composites. The reduction is 7.3 % of flax compos-ite and 6.4 % of linen composite, respectively (Table 3).The decrease in impact strength may be interpreted byassuming that a better fibre/matrix adhesion results inshorter average pull-out lengths of the fibres, asobserved in Figure 8. It is clear that the average fibrepull-out lengths of the untreated flax composite islonger than the alkali-treated flax one.

Specific impact strength is defined as the ratio ofaverage impact strength divided by the density of thecomposite. Table 3 indicates that the alkali treatmentincreased the specific impact strength of the flax andlinen composites. This is because alkali treatment hasa significant reduction in the density of the composites,as shown in Table 2.

Conclusion

Flax and linen fabric reinforced epoxy composites havebeen fabricated using the vacuum bagging technique.The influence of alkali treatment on the vibration char-acteristics, the surface morphologies and mechanicalproperties of the composites were studied. The investi-gation reveals:

1. Alkali treatment with 5wt. % NaOH solutionenhanced the compressive properties, in-planeshear properties of the flax and linen composites.However, the damping ratio and impact strengthof both flax and linen composites decreased due tothe treatment.

2. In vibration, the reduction in damping ratio byalkali treatment is believed to be attributed to theimproved fibre/matrix adhesion resulting in lessenergy dissipation during the vibration, as analysedby SEM.

3. In compression, the ultimate compressive strengthof flax and linen composites is highly dependenton the strength of the epoxy. The stiffness of thefabric reinforced epoxy composite mainly dependson the fibre. The compressive failure of fabric rein-forced epoxy composites exhibits a ductile fracturemode.

4. In in-plane shear test, the stress–strain behaviour ofthe composites exhibits a non-linear manner.

5. The impact strength of the flax composite is superiorto the linen composite. Alkali treatment increased

Figure 7. SEM micrograph of failure modes of flax fabric rein-

forced epoxy composites.

SEM: scanning electron microscopy.

Figure 8. SEM micrographs of impact specimens: (a) Untreated

flax, and (b) alkali-treated flax composites. SEM, scanning elec-

tron microscopy.

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the specific impact strength of the composites, com-pared with their untreated composites.

6. SEM study clearly reveals that the failure offabric reinforced composite under impact is domi-nated by fibre fracture, fibre pull-out and matrixfracture.

This study is part of a research program investigat-ing the feasibility of bio-composites as building mater-ials. Next, flax fabric reinforced epoxy composite in theform of hollow tube as concrete confinement (i.e. flaxFRP tube confined concrete) will be investigated. Thehollow flax FRP tube will act as the permanent form-work for the concrete core and also is expected toincrease compressive strength and ductility of the con-crete as the confinement.

Funding

This research received no specific grant from any funding

agency in the public, commercial, or not-for-profit sectors.

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pp. 592–606.

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