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 DOI: 10.1177/0021998313508995

published online 18 November 2013Journal of Composite MaterialsMS Islam and M Miao

Optimising processing conditions of flax fabric reinforced Acrodur biocomposites  

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JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Optimising processing conditions of flaxfabric reinforced Acrodur biocomposites

MS Islam and M Miao

Abstract

Acrodur solution and dispersion have been developed as alternative wood adhesives to phenolic and urea formaldehyde

resins. They are non-corrosive and do not emit carcinogenic gases. Contrary to most resins used in natural plant fibre

composites, Acrodur has superior tolerance to moisture during composite fabrication and thus drying of the preforms

may be minimised or eliminated. The aim of this study was to produce optimised flax fabric reinforced Acrodur

biocomposites by varying the ratio between Acrodur solution and dispersion, relative humidity, curing time and

temperature. The optimised biocomposites provided a combination of specific tensile strength 57.9 MPa-cm3/g and

specific Young’s modulus 5.5 GPa-cm3/g at a low density 0.91 g/cm3. Thermogravimetric analysis and contact angle

measurement showed that the biocomposites had higher thermal stability and hydrophobicity than the fabrics.

The low loss of tensile properties upon water immersion was explained by tortuous wicking path in the biocomposites.

Keywords

Fabrics/textiles, polymer-matrix composites, thermomechanical properties, environmental degradation

Introduction

The use of natural plant fibres such as flax, hemp,kenaf, jute and wood fibres as reinforcement in boththermoplastics and thermosets has increased dramatic-ally during the last 20 years. These natural fibre bio-composites compare well with glass fibre reinforcedpolymer composites in terms of recyclability whenusing a thermoplastic matrix and energy recoverythrough incineration when using a thermoset matrix,and with traditional structural materials in terms ofspecific mechanical properties.1 Thermoset polymersare particularly attractive as matrix materials for nat-ural plant fibre reinforced biocomposite production asthey generally have reactive functional groups thatmake them compatible with hydrophilic fibre surfaces.1

Flax is one of the strongest and stiffest available nat-urally renewable fibres. The major constituent of flaxfibre is crystalline cellulose (64.1–73.8%) held togetherby hemicellulose (11–16.7%), lignin (2–2.9%) and waxysubstances (1.5%).2–4

Conventional thermoset matrices such as polyesterand vinyl ester have the ability to reinforce naturalfibres. However, polyester and vinyl ester matricesemit styrene not only during processing but also fromthe finished product. Styrene is classified as a possiblecarcinogen by the international agency for research oncancer (IARC) and is also classified as a photochemical

ozone creator. This ozone is highly reactive and isknown to affect human health, crops and forests.Other thermoset resins such as resole type phenolicresins (phenol-formaldehyde) and urea-formaldehyderesins are used extensively in the production of woodbiocomposite board.5,6 Both of these resins possessexceptionally good adhesive properties, high rigidityand dimensional stability. Phenolic resins have excep-tional heat and fire resistance due to their highly cross-linked aromatic structures and urea-formaldehydeshave the ability to glue oily woods. However, bothphenol and urea formaldehydes emit formaldehyde,which is also classified as a carcinogen for humans bythe State of California7 and World HealthOrganization’s IARC.8 Formaldehyde also causes eye,nose and respiratory irritation and can be an asthmatrigger, with children more susceptible than adults9

during composites production by hot pressing andfrom the finished composite boards. Therefore, theuse of these common thermoset resins as binders is

CSIRO Materials Science and Engineering – Geelong, Belmont, Victoria,

Australia

Corresponding author:

MS Islam, Wiley Park, Sydney, New South Wales 2195, Australia.

Email: saifulctp@yahoo.com

Journal of Composite Materials

0(0) 1–12

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DOI: 10.1177/0021998313508995

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not engaging. Researchers around the world are tryingto find alternative binders.

Acrodur is a polyester of polycarboxylic acid andpolyalcohol and is an aqueous formaldehyde-freebinder system with high heat resistance and excellentstorage stability i.e. while in storage the physical andchemical properties of the resin are maintained over anextended period of time. Acrodur is produced in twoforms (a) Acrodur solutions and (b) Acrodurdispersions.10

Acrodur solutions are made from a polycarboxylicacid with a polyalcohol dissolved in water. Acrodurdispersions are made from a polycarboxylic acid mod-ified with a latex component along with a polyalcoholdissolved in water (see Figure 1).

Acrodur solutions can be mixed with Acrodur dis-persions to produce properties ranging from highlyflexible to highly stable duroplastic. Acrodur bindersare readily miscible with water and conventional poly-mer binders and additives, allowing simple, precise andfine-tuning of the processing parameters to obtaindesired final properties. Examples of the versatilerange of applications for Acrodur include woodenand natural fibre mouldings for the automotive indus-try, abrasives from corn cob and cellulose substrate-based filter paper for air and oil filters.11 Acroduroffers significant benefits such as diversity, ease of pro-cessing and high environmental compatibility due to itsnon-corrosive properties.12,13 It produces only one by-product during thermal curing reaction: water. It haslow density and Acrodur solution can be mixed withAcrodur dispersion to modify its mechanical and waterabsorption properties. Besides these, Acrodur bindersare very easy to handle and can be applied to a widerange of substrates using any of the commonly usedtechniques. The ingredients in Acrodur do not requirespecial filters or environmental protection measures.Moreover, Acrodur is believed to have the ability toform hydrogen and covalent bonds with the hydroxylgroups present in cellulose of natural plant fibresthrough ester groups.

The objective of this study was to investigate the effectof Acrodur solution to dispersion ratio, relative humid-ity of flax fabric and curing temperature and time on themechanical properties of flax fabric/Acrodur biocompo-sites (FAB). The thermal degradation, hydrophobicityof the optimised biocomposites and ageing of the bio-composites upon immersion in water were also studied.

Experimental

Materials

Acrodur solution (Acrodur 950L) with a density of1.06 g/cm3 and molecular weight of ca 80,000 g/mol

and Acrodur dispersion (DS 3515) with a density of1.20 g/cm3, molecular weight of ca 25,000 g/mol andlatex particle size of ca 80 nm were obtained fromBASF, Germany. A linen fabric (10 oz/yd2, 310 g/m2)was obtained from China. The fabric was plain-wovenfrom 6 s count weft and warp yarns with weft density of39 and warp density of 37 threads per inch. It had a1.48 kN tensile breaking force in the weft (cross-machine) direction and a 1.14 kN tensile breakingforce in the warp (machine) direction when tested50mm width strips.

Methods

Differential scanning calorimetry. ADSC 7 differential scan-ning calorimeter from Perkin Elmer was used tomonitorthe exothermic thermograms of isothermal DSC meas-urements. A steady isothermal baseline was establishedat the preset cure temperature using two empty samplealuminium pans. Acrodur solution to Acrodur disper-sion ratio was 3:1 for all the measurements. The speci-mens were enclosed in a closed aluminium container toprevent any evaporation and were scanned while keep-ing a static air flow of 50mL/min. Isothermal scans werecarried out at 160, 180 and 200�C and the curing reac-tions were considered to be completed when the isother-mal DSC thermograms levelled off to the baseline.The weight of each specimen was approximately 10mg.

Fabrication of biocomposites. To be used in the manufac-ture of biocomposites, flax fabrics were cut into30 cm� 30 cm swatches using a hydraulic press cutter(Torielli, Vigevano-Italia) and dried in an oven at 80�Cuntil the weight was stabilised (i.e. evaporation com-pleted). Table 1 gives the method of manufacture ofvarious biocomposites.

The fibre content of all the biocomposites producedwere approximately 75wt%. For all the biocompositeproduction, the hot press was preheated to the requiredcuring temperature, which was maintained till thecuring process was completed.

Measurement of composite density and void content.

Biocomposite volume was measured by cutting about20mm� 20mm area from the produced biocompositessheet and multiplying this area by the thickness of thesheet. After measuring the volume, the weight of the cutwas also measured. The density of the biocompositeswas then calculated by dividing the biocomposite cutvolume by its weight. Three replicas for each samplewere taken to measure the average density.

The void content of the biocomposites were deter-mined in accordance with the ASTM D 2734-03Standard Test Methods for Void Content ofReinforced Plastics.

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Scanning electron microscopy. The fracture surfaces of thecontrol and water-immersed biocomposites were exam-ined using a Hitachi S-4000 Field Emission SEM oper-ated at 5 kV. Carbon tape was used to mount thesamples on aluminium stubs. The samples were thensputter coated with platinum and palladium to makethem conductive prior to scanning electron microscopicobservation.

Tensile testing. The biocomposites produced were cut intotensile test specimens in both machine (warp) and cross-machine (weft) direction of the production of flax fabricusing a scroll saw in accordance with ASTM D 638-03standard test method for tensile properties of plasticsand placed in a conditioning chamber at 23�C� 3�Cand 50%� 5% RH for 24 h. The specimens were thentested using an INSTRON 5500R tensile testingmachine fitted with a 100 kN load cell at a rate of5mm/min. Five to six specimens were used for eachtest and tested up to the point of failure. Specific strength(MPa cm3 g�1) was calculated from the tensile strength(MPa) and bulk density (g cm-3) of the composite andspecific modulus was calculated in the same way.

Thermogravimetric analysis. Thermogravimetric analysis(TGA) of flax fabric, cured Acrodur resin and FABwas carried out using a Shimadzu TGA-60 instrument.All the measurements were taken whilst maintaining astatic air flow of 150mL/min with a constant heatingrate of 10�C/min in an open alumina crucible.

The weight of the specimens was around 10mg, witha scanned temperature range from 25 to 600�C.

Contact angle measurement. Contact angle measurementswere conducted using KSV CAM100 contact angle andsurface tension tester. The droplet used was distilledwater and was 180 pixels in size. Five specimensfrom each sample were tested. For each measurement,a 15-mm2 sample was cut from the flax fabric, curedAcrodur resin and FAB, and mounted onto a glass slideusing double-sided tape. Specimens were cut fromrandom areas of the samples.

Water immersion tests. Water immersion testing of thesamples (biocomposites produced with dry flax fabricat Acrodur solution to dispersion ratio of 3:1 and curedat 160�C) were carried out in accordance with ASTMD570-98: Standard Test Method for Water Absorption ofPlastics for up to four weeks. The samples were sub-merged in water at 25�C and removed from the waterevery one-week interval, wiped with a clean dry cloth toremove surface water and wiped with paper towel toremove water from the pores, weighed in a high preci-sion balance to assess the weight change and then re-submerged for continued ageing. Tensile testing of theaged samples was carried out in accordance withASTM standards as described in section Tensile testing.Five specimens of each sample were tested.

Table 1. Method of manufacture of various biocomposites.

Type of FAB produced Method of manufacture

(a) Acrodur solution, Acrodur dis-

persion and a mixture of

Acrodur solution and dispersion

A mixture of Acrodur solution to dispersion at the ratio of 3:1 was prepared. The dried flax

fabrics were then taken out of the oven and immediately hand laid-up with the Acrodur

solution, Acrodur dispersion and mixture of Acrodur solution and dispersion respectively

so that very little or no water could be adsorbed by the fabric from the environment

during hand lay-up process. The flax fabrics (four-layer) laid with Acrodur solution,

Acrodur dispersion and mixture of Acrodur solution and dispersion were then pressed

into biocomposites using a compression mould at a curing temperature of 160�C

for 20 min.

(b) Different relative humidities The dried and cut flax fabric was placed in a humidity chamber set to a constant temperature

of 25�C and required relative humidity (RH) levels for 72 h. The relative humidity levels

used were 40%, 60% and 80%. The dry fabric and the fabrics conditioned to 40%, 60% and

80% RH were hand laid-up with an Acrodur solution/dispersion mixture (ratio 3:1). The

flax fabrics (four-layer) laid with Acrodur mixture were then pressed into biocomposites

using a compression mould at a curing temperature of 160�C for 20 min.

(c) Different curing temperature

and time

The hand lay-ups prepared according to the above procedure were pressed into biocom-

posites at three different curing temperatures 160, 180 and 200�C for 20, 10 and 5 min,

respectively

(d) Different Acrodur solution

to dispersion ratio

Three mixtures of Acrodur solution to dispersion at ratios 3:1, 3:2 and 1:1 were prepared.

The dry flax fabrics and the flax fabrics conditioned at 40% RH (four-layer) laid with three

different Acrodur mixtures respectively were pressed at a curing temperature of 160�C

for 20 min.

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Results and discussion

Differential scanning calorimetry measurementsof Acrodur resin

Figure 2 shows a typical heat flow versus time curve forthe Acrodur mixture (Acrodur solution to dispersionratio of 3:1) cured at 200�C. The Acrodur mixturecurve levels off the base line in around 5min.Acrodur mixture at three different curing temperatures(160, 180 and 200�C) showed similar pattern but differ-ent time scales. The time required to cure the Acrodurmixture at these three different isothermal curing tem-peratures of 160, 180 and 200�C was 20, 10 and 5min,respectively. As expected, the time required for curingwas found to decrease with the increase of temperature.

The density and void content of the biocomposites

The density of the biocomposites produced withAcrodur solution to dispersion ratio of 3:1, curing tem-perature of 160�C and a curing time of 20min wasfound to be 0.91 g/cm3. This low composite density isattributed to the foamy and porous structure of theAcrodur resin. The void content of this biocompositeswere measured and the high void content of approxi-mately 35.3% was in favour of the low biocompositedensity.

Tensile properties of the biocomposites

Figure 3 shows a stress–strain curve obtained from thetensile testing of the FAB. All the composites werefound to have similar stress–strain curves. Since thestress–strain curves of the biocomposites obtainedwere non-linear in nature, the Young’s moduli of the

composites were measured from the initial linear por-tion (as marked by red colour) of the stress–straingraph. This is a common practice for natural fibre rein-forced composites carried out in accordance withASTM D3039/D3039M - 08 Standard Test Methodfor Tensile Properties of Polymer Matrix CompositeMaterials.

FAB produced with Acrodur solution, Acrodur dispersion and

different Acrodur solution to dispersion ratio. Figure 4shows the tensile strength and Young’s modulus ofFAB produced with Acrodur solution, Acrodur disper-sion and three different mixtures of Acrodur solutionand dispersion at the ratios of 3:1, 3:2 and 1:1. All ofthe biocomposites produced here were tested in bothcross-machine (weft) and machine (warp) directions ofthe flax fabric. From the results, it can be seen that the

0

12

24

36

48

60

0 3 6 9 12

Time (min)

Hea

t Flo

w (m

W)

Acrodur Sample

Base line

Figure 2. A typical heat flow versus time curve for the curing

of an Acrodur mixture (Acrodur solution to dispersion ratio of

3:1) at 200�C.

Figure 1. (a) Acrodur solution and (b) Acrodur dispersion formation reactions.

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biocomposites tested along the cross-machine directionare stronger than the biocomposites tested along themachine direction. The stronger biocomposites alongthe cross-machine direction would be expected astheir parent flax fabric was also stronger along thatdirection, as described in section Materials. From theresults of the cross-machine direction samples, it can beseen that the average specific tensile strength is thehighest for the biocomposites produced with anAcrodur solution to dispersion ratio of 3:1 followedby Acrodur solution, Acrodur dispersion, Acrodursolution to dispersion ratio of 3:2 and finally Acrodursolution to dispersion ratio of 1:1. Similar trend wasfound for the specific Young’s modulus of all thecross-machine direction samples, although the specificYoung’s modulus of the Acrodur solution was almost

the same as that of the mixture of Acrodur solution anddispersion at the ratio 3:1. Therefore, it can be con-ferred that the best FAB was produced at Acrodursolution to dispersion ratio of 3:1, which would giveoptimised distribution and dispersion of Acrodur toform hydrogen and covalent bond with acid group ofAcrodur and hydroxyl group of flax fabric. Since mostof the fibre reinforced biocomposite parts require highmechanical properties in the loading (cross-machine)direction and the transverse direction is less important,the rest of the studies will concentrate on biocompositesproduction with weft (cross-machine) direction.

FAB produced at different curing temperature and time. In theinvestigation of the optimization of processingconditions, all samples were produced with the same

y = 4097.6x + 0.0627R2 = 0.9991

0

10

20

30

40

50

0 0.005 0.01 0.015 0.02 0.025 0.03

Strain

Ten

sile

Str

ess

(MP

a)

Figure 3. A typical stress–strain curve obtained from FAB.

0

10

20

30

40

50

60

1:13:23:1Acrodur Dispersion

Acrodur Solution

Sp

ecif

ic t

ensi

le s

tren

gth

(M

Pa-

cm3/g

) Machine Direction Cross-machine Direction

(a)

0

1

2

3

4

5

6

1:13:23:1Acrodur Dispersion

Acrodur Solution

Machine Direction Cross-machine Direction

Sp

ecif

ic Y

ou

ng

's M

od

ulu

s (G

Pa-

cm3 /g

)(b)

Figure 4. Specific tensile properties of FAB produced with Acrodur solution, Acrodur dispersion and three different Acrodur

solution to dispersion ratios. (a) Specific tensile strength at cross-machine direction and machine direction and (b) Young’s modulus at

cross-machine direction and machine direction.

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Acrodur solution to dispersion ratio of 3:1 while curingtemperature and time were varied. The curing time wastaken from the corresponding curing temperatures ofthe DSC scans as described in section Differential scan-ning calorimetry measurements of Acrodur resin. FromFigure 5(a) and (b), it can be seen that both averagetensile strength and Young’s modulus were highest at160�C followed by 180 and 200�C. The decrease of ten-sile properties with the increase of curing temperaturemay be due to occurrence of Acrodur resin decompos-ition to some extent at higher temperatures.14 Severalauthors have investigated the thermal degradation offlax fibres at temperature around 200�C15–18 and theyhave found that initially at low-temperature thermaldegradation is not significant. Untreated flax has beenfound to retain its strength at 170�C for 120min,whereas strength decreases at 210�C by approximately50% over the same time span.19

FAB produced at different relative humidity of flax fabrics. Theoptimised processing conditions as obtained up till thispoint were (1) Acrodur solution to dispersion ratio of3:1, (2) curing temperature of 160�C and (3) curing timeof 20min. In this section, biocomposites were producedat different relative humidity of 0%, 40%, 60% and80% using the optimised processing conditions.

From Figure 6(a), it can be seen that the average ten-sile strength did not change significantly for biocompo-sites produced with 40%, 60% and 80% RH of flaxfabric. Figure 6(b) shows that the average moduluswas not found to reduce significantly for biocompositesproduced with 40% RH of flax fabric, however, aslight reduction was observed for 60% RH and a bitmore reduction was seen for 80% RH. This decrease athigh humidity was relatively small when compared withother binders such as vinylester and unsaturatedpolyester.20,21

0

15

30

45

60

200°C180°C

Ten

sile

Str

eng

th (

MP

a)

160°C0

2

4

6

8

200°C180°C

Yo

un

g's

Mo

du

lus

(GP

a)

160°C

(a)(b)

Figure 5. Tensile properties of FAB produced at different curing temperature and time. (a) Tensile strength and (b) Young’s modulus

at cross-machine direction.

0

13

26

39

52

65

Ten

sile

Str

eng

th (

MP

a)

0% RH 40% RH 60% RH 80% RH 0% RH 40% RH 60% RH 80% RH0.0

1.5

3.0

4.5

6.0

Yo

un

g's

Mo

du

lus

(GP

a)

(a)(b)

Figure 6. Tensile properties of FAB produced at different relative humidity of flax fabric. (a) Tensile strength and (b) Young’s modulus

at cross-machine direction.

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The reduction in Young’s modulus at high RH wasaccompanied by a large deflection (strain) at break. Thepercentage strain was the highest (8.41%) for the bio-composites at 80% RH, followed by 7.15% at 60% RHand 7.06% at 40% RH.

Curing of acrylic resin in the presence of moisturegenerally activates the bonding by an esterificationreaction between the acid group of acrylic resin andhydroxyl groups of the flax fabric constituents such ascellulose, hemicelluloses and lignin.22 The increase ofaverage tensile strength of the biocomposites producedwith 40% and 60% RH and almost similar tensilestrength at 80% RH of flax fabric when comparedwith dry or 0% RH of flax fabric supports the phenom-enon. Comparison of the SEM micrographs(Figure 7(a) and (b)) of fracture surfaces of biocompo-sites produced with 0% RH (Figure 7(a)) and 40% RH(Figure 7(b)) also supports the increased bonding ofbiocomposites produced with 40% RH of flax fabric.Since from the SEM micrographs, less separated fibres,smaller holes and more fractured fibres can be seen forbiocomposites produced with 40% RH of flax fabricsthan for biocomposites produced with 0% RH.Figure 7(c) shows the SEM micrograph through thethickness of the biocomposites produced at 40% RH.

It can be seen from the micrographs that although thebiocomposite is porous, it is well impregnated with thematrix.

FAB produced at different Acrodur solution to dispersion ratios

of flax fabrics with 40% RH. The optimised Acrodur solu-tion to dispersion ratio for dry flax fabric (0% RH) wasfound to be 3:1. In this section, the tensile properties ofbiocomposites (Figure 8) produced with optimised flaxfabric RH of 40%, curing temperature of 160�C andcuring time of 20min and at different Acrodur solutionto dispersion ratios were studied to further confirm theoptimisation of the processing conditions. It was foundthat the increase of Acrodur solution to dispersion ratiofrom 3:1 to 3:2 and 1:1 decreased the tensile properties.This confirms the optimised bonding between fibre andAcrodur occurred at Acrodur solution to dispersionratio of 3:1.

TGA of the fabric, Acrodur and biocomposites

TGA traces for flax fabric, Acrodur mixture and FABproduced with optimised condition (curing temperatureof 160�C, curing time of 20min, relative humidity of40% and Acrodur solution to dispersion ratio of 3:1)

(a) (b)

(c)

serbifdetarapeSserbifdetarapeS

Larger Holes

Smaller Holes Fractured fibres Fractured fibres

The whole fabric covered with matrix through its thickness Porosity in the biocomposite

Figure 7. SEM micrograph of FAB produced with (a) 0% RH, (b) 40% RH of flax fabric and (c) through thickness cross section of FAB

with 40% RH of flax fabric.

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are shown in Figure 9. It can be seen from the thermo-gram that there are three stages of thermal degradationfor all the samples. For the flax fabric and FAB thedegradation stages are in the temperature range50–220�C (first stage), 220–400�C (second stage) and400–580�C (third stage). On the other hand, forAcrodur mixture the degradation stages are in the tem-perature range of 50–180�C (first stage), 180–400�C(second stage) and 400–580�C (third stage). The threethermal degradation stages are well known in the caseof natural cellulosic fibres, where the first degradationstage is due to evaporation of adsorbed moisture, thesecond degradation stage is due to decomposition ofcellulose leading to the formation of volatile productsand the third degradation stage is due to oxidation ofvolatile and charred products.23 Similarly, the threedegradation stages for Acrodur mixture may be dueto evaporation of adsorbed moisture, decompositionof polyester leading to the formation of volatile

products and the third degradation stage is due to oxi-dation of volatile and charred products. The early ter-mination of the initial degradation stage and lower lossof weight for the Acrodur mixture compared to theother two samples indicates low moisture adsorptionof the hydrophobic cured Acrodur mixture. The earlyinitiation of the second degradation stage for the curedAcrodur mixture might be due to the starting of thedecomposition of lower boiling point components (i.e.CO, CO2, CH4, ethylene and acetylene) by polyester at180�C.24

It can be seen from the figure that at the thermaldegradation stage, highest weight loss can be seen forflax fabric followed by the biocomposite and finally forAcrodur mixture. Therefore, it can be conferred thatproduction of biocomposites using Acrodur mixtureas matrix increase the thermal stability of the biocom-posites than that of flax fabric. However, the higherresidual weight for the flax fabric and biocomposites

Figure 9. TGA thermograms of flax, Acrodur and FAB.

0

13

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1:13:2

Ten

sile

Str

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th (

MP

a)

3:10.0

1.5

3.0

4.5

6.0

1:13:2

Yo

un

g's

Mo

du

lus

(GP

a)

3:1

(a) (b)

Figure 8. Tensile properties of FAB produced at different Acrodur solution to dispersion ratios of flax fabric with 40% RH.

(a) Tensile strength and (b) Young’s modulus at cross-machine direction.

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compared to that of Acrodur mixture supports the for-mation of lignicellulose complex at higher temperatureof the natural fibres.25

Contact angle measurement of flax fabric,Acrodur and biocomposites

Figure 10 shows the contact angle photographs of flax,Acrodur and FAB. It can be seen from the contactangle results that the highest contact angle was foundin the FAB, followed by the cured Acrodur mixture andfinally by the flax fabric. The average contact angles ofthe samples were 82.1� for flax fabric, 93.6� for curedAcrodur mixture and 96� for FAB. The increased con-tact angle (hydrophobicity) of the biocomposites maybe attributed to the good fibre-matrix bonding at theinterface.

Water immersion tests of the biocomposites

Figure 11 shows the water uptake of the biocompositeswith immersion time. The percentage water uptakeincreased exponentially with the increase of waterimmersion time although the percentage of wateruptake is only about 3.4% at the end of four-weekimmersion. The low amount of water uptake of the

biocomposites indicates good fibre-matrix adhesion dueto the reasons described below.

Table 2 shows the tensile strength and Young’smodulus of control biocomposite (no water immersion)compared to the biocomposites after water immersionperiods of up to four weeks at room temperature. It canbe seen that after one week of immersion the biocom-posites tensile strength remains almost the same. Thebiocomposite produced was more hydrophobic com-pared to flax fabric and Acrodur mixture alone dueto good fibre matrix bonding as discussed in sectionContact angle measurement of flax fabric, Acrodurand biocomposites. The low bulk density of the com-posites and the optical image of cured Acrodur mixture

Figure 10. Contact angle photographs of (a) flax fabric, (b) cured Acrodur and (c) FAB at optimised processing conditions.

Figure 11. Water uptake versus water immersion time.

Table 2. Tensile properties of FAB with water soaking time.

Sample Tensile strength (MPa) Young’s modulus (GPa)

Control 52.7� 3.68 4.97� 0.31

One week 51.5� 2.67 3.08� 0.25

Two week 47.1� 5.09 2.79� 0.29

Three week 46.4� 3.56 2.67� 0.09

Four week 40.6� 1.86 2.67� 0.43

Figure 12. Foam structure of cured Acrodur mixture.

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in Figure 12 suggest that the composite matrix has afoam structure. The low moisture uptake resultspointed to closed-cell foam matrix.

When good fibre-matrix adhesion is achieved, thepath of wicking water molecules is tortuous due tobetter fibre-matrix integrity and hence the water mol-ecules had to change their path of wicking while the

wicking path would be nearly straight for weak fibrematrix bonding. The tortuous path of wicking of watermolecules also delays the degradation of fibre matrixbond integrity.

Figure 13(b) shows that the Young’s modulus of thebiocomposite decreased by about 38% when comparedto the control sample (zero week or no immersion) after

y = –3.34x + 54.75R2 = 0.9275

0

10

20

30

40

50

60

–1 0 1 2 3 4 5

Water Immersion Time (Week)

Ten

sile

Str

eng

th (

MP

a)

y = –0.135x + 3.14R2 = 0.8116

0

1

2

3

4

5

6

–1 0 1 2 3 4 5

Water Immersion Time (Week)

Yo

un

g's

Mo

du

lus

(GP

a)

(a) (b)

Figure 13. Tensile properties of the biocomposites (control and water immersed).

Good fabric-matrix integrity

Loss of fabric- matrix

Figure 14. SEM micrographs of biocomposites showing loss of fibre fibre-matrix structural integrity upon water immersion.

(a) Control, (b) after one week, (c) after two weeks, (d) after three weeks and (e) after four weeks water immersion.

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one-week immersion. This softening could be due toplasticization of the biocomposites by the adsorbedwater molecules. Figure 13(a) shows that the tensilestrength decreased linearly with the increase of thewater immersion time from one week to four weekswith an average decline of 3.34MPa per week. Thedecrease in tensile strength may be due to the loss offibre-matrix integrity by water absorption and subse-quent swelling. The latter can be observed from theSEM micrographs of the control and water immersedbiocomposites in Figure 14. On the other hand, theYoung’s modulus of the biocomposites reduced veryslowly (slope of �0.135) during the period from week1 to week 4. This might indicate an equilibrium state forplasticization by water molecules was reached.

Comparison of the FAB with traditional flaxfabric-polyester composites

Table 3 shows the comparison of tensile properties oftraditional flax fabric-polyester (FPE) composites withflax fabric-Acrodur dispersion produced with approxi-mately 50wt% fabric. It can be seen from the Tablethat for the same fabric content the tensile strength ofthe FPE composites was about 48% higher than flaxfabric-Acrodur dispersion biocomposites while theYoung’s modulus was 14% higher and the elongationat break was 28% higher than the flax fabric-Acrodurdispersion biocomposites. The lower tensile propertiesof the FAB biocomposites when compared with FPEcomposites may be due to the porous nature ofAcrodur resin. However, if the specific tensile proper-ties are compared (see Table 3) for FPE compositeswith flax fabric-Acrodur dispersion biocomposites, theflax fabric-Acrodur dispersion biocomposites weresomewhat higher than the FPE composites in termsof specific Young’s modulus although still lower in spe-cific tensile strength.

Conclusions

FAB produced with Acrodur resin has shown goodspecific tensile properties, thermal resistance andwater resistance. Acrodur is more environmentalfriendly than conventional thermosetting resins as itdoes not emit gases harmful for the living beings.Unlike other conventional resins, Acrodur resin is

tolerable to moisture content in the natural fibre fabricsso that drying of the preforms may be minimised oreven eliminated. Processing variables can be tailoredto produce biocomposites suitable for a wide range ofapplications. The resulting composites showed low lossof tensile properties upon water immersion testing,which was explained by the tortuous path for wickingof water into the biocomposites as a result of goodfibre-matrix interface. The biocomposites producedare expected to be used in the interior household andautomotive applications with increased service life.In the current study, the optimized processing isobtained step by step by isolating the studying param-eters; the coupling effect of different parameters can beinvestigated in the future.

Acknowledgements

The authors are thankful to CRC-ACS, Australia, for sup-porting this project.

Conflict of Interest

None declared.

Funding

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

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