Effect of Water Absorption on the Mechanical Properties of Hemp

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Effect of water absorption on the mechanical properties of hempfibre reinforced unsaturated polyester composites

H.N. Dhakal *, Z.Y. Zhang, M.O.W. Richardson

Advanced Polymer and Composites (APC) Research Group, Department of Mechanical and Design Engineering, University of Portsmouth,

Anglesea Road, Anglesea Building, Portsmouth, Hampshire PO1 3DJ, UK

Received 12 April 2006; received in revised form 22 June 2006; accepted 29 June 2006

Abstract

Hemp fibre reinforced unsaturated polyester composites (HFRUPE) were subjected to water immersion tests in order to study theeffects of water absorption on the mechanical properties. HFRUPE composites specimens containing 0, 0.10, 0.15, 0.21 and 0.26 fibrevolume fraction were prepared. Water absorption tests were conducted by immersing specimens in a de-ionised water bath at 25 C and100 C for different time durations. The tensile and flexural properties of water immersed specimens subjected to both aging conditionswere evaluated and compared alongside dry composite specimens. The percentage of moisture uptake increased as the fibre volume frac-tion increased due to the high cellulose content. The tensile and flexural properties of HFRUPE specimens were found to decrease withincrease in percentage moisture uptake. Moisture induced degradation of composite samples was significant at elevated temperature. Thewater absorption pattern of these composites at room temperature was found to follow Fickian behaviour, whereas at elevated temper-atures it exhibited non-Fickian. 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Polymermatrix composites; Natural fibre; B. Mechanical properties; D. Mechanical testing

1. Introduction

The use of natural plant fibres as reinforcement in poly-mer composites for making low cost engineering materialshas generated much interest in recent years. New environ-mental legislation as well as consumer pressure has forcedmanufacturing industries (particularly automotive, con-struction and packaging) to search for new materials that

can substitute for conventional non-renewable reinforcingmaterials such as glass fibre [1]. The advantages of naturalplant fibres over traditional glass fibres are acceptable asgood specific strengths and modulus, economical viability,low density, reduced tool wear, enhanced energy recovery,reduced dermal and respiratory irritation and good biode-gradability [2]. Natural plant fibre reinforced polymericcomposites, also have some disadvantages such as the

incompatibility between the hydrophilic natural fibresand hydrophobic thermoplastic and thermoset matricesrequiring appropriate use of physical and chemical treat-ments to enhance the adhesion between fibre and thematrix [3].

Hemp is also called cannabis sativa. It is an annual her-baceous plant native to Asia and widely cultivated in Eur-ope [4]. Hemp and flax are the only commercial sources of

long natural fibres grown in the UK Plant stems are pro-cessed by various mechanical methods to extract the fibre[5]. Fibres from hemp stems have been widely used in theproduction of cords and clothing, and have potential forreinforcement in polymermatrix composites (PMCs).Recently, car manufacturers have started manufacturingnon-structural components using hemp and flax fibresdue to their higher specific strength and lower price com-pared to conventional reinforcements [6].

All polymer composites absorb moisture in humidatmosphere and when immersed in water. The effect of

0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2006.06.019

* Corresponding author. Tel.: +44 23 9284 2396; fax: +44 23 9284 2351.E-mail address: [email protected] (H.N. Dhakal).

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absorption of moisture leads to the degradation of fibrematrix interface region creating poor stress transfer effi-ciencies resulting in a reduction of mechanical and dimen-sional properties [7]. One of the main concerns for the useof natural fibre reinforced composite materials is their sus-ceptibility to moisture absorption and the effect on physi-

cal, mechanical and thermal properties [8]. It is importanttherefore that this problem is addressed in order that nat-ural fibre may be considered as a viable reinforcement incomposite materials.

Several studies in the use of natural fibre reinforcedpolymeric composites have shown that the sensitivity ofcertain mechanical and thermal properties to moistureuptake can be reduced by the use of coupling agents andfibre surface treatments [9,10].

Moisture diffusion in polymeric composites has shownto be governed by three different mechanisms [11,12]. Thefirst involves of diffusion of water molecules inside themicro gaps between polymer chains. The second involves

capillary transport into the gaps and flaws at the interfacesbetween fibre and the matrix. This is a result of poor wet-ting and impregnation during the initial manufacturingstage. The third involves transport of microcracks in thematrix arising from the swelling of fibres (particularly inthe case of natural fibre composites). Generally, based onthese mechanisms, diffusion behaviour of polymeric com-posites can further be classified according to the relativemobility of the penetrant and of the polymer segments,which is related to either Fickian, non-Fickian or anoma-lous, and an intermediate behaviour between Fickian andnon-Fickian [13,14]. In general moisture diffusion in a com-

posite depends on factors such as volume fraction of fibre,voids, viscosity of matrix, humidity and temperature [15].The objective of this work was to compare the influenceof both fibre reinforcement and water uptake on mechani-cal properties of hemp fibre reinforcement unsaturatedpolyester composites and the related kinetics and charac-teristics of the water absorption.

2. Experimental procedure

2.1. Materials

The matrix material used in this study was based on acommercially available unsaturated polyester, Trade NameNORPOL 444-M888 supplied by Reichhold UK Ltd.The matrix was mixed with curing catalyst, methyl ethylketone peroxide (MEKP) at a concentration of 0.01 w/wof the matrix for curing. Needle punched randomly ori-ented non-woven hemp fibre, fabric weight 330 g/m2, wasused as the reinforcement and was provided by JB PlantFibres Enterprises Ltd. The typical chemical composition

and the structure of hemp fibre are presented in Table 1[16]. The mechanical and physical properties of the polyes-ter, hemp, and glass fibre used in this study are presented inTable 2 [17].

2.2. Processing

A combination of hand lay-up and compression mould-ing method was used to prepare the HFRUPE compositesamples. Non-woven hemp fibre mat was first dried at100 C to remove storage moisture in a fan-assisted oven.The storage moisture was recorded for hemp mat approx-imately 9%. A measured quantity of unsaturated polyesterresin mixed with a catalyst (MEKP) for rapid curing waspoured on a pre-weighed amount of non-woven hemp fibremat, which was placed in a mould. The mould was coatedwith a semi-permanent, polymer mould release agent,Frekote FRP90-NC. After pouring the resin, each layerwas left for a few minutes to allow the resin to soak into

the fibre mat. Trapped air was gently squeezed out usinga roller. The hemp fibre and polyester resin were then leftfor about 3 min to allow air bubbles to escape from the sur-face of the resin. The mould was closed and the compositepanel was left to cure in a hydraulic press at a temperatureof 22 C and at a compaction pressure of 10 bar for 1.5 h.The fabrication route of the HFRUPE composites isdepicted in Fig. 1. The schematic of a hydraulic press usedto consolidate composite panels is shown in Fig. 2. Afterbeing taken out from the hydraulic press, the panel was leftto cure at a temperature of 22 C for 24 h before beingremoved from the mould. Subsequently, post curing was

carried out at a temperature of 80 C for 3 h. In additionto this non-woven hemp fibre, a randomly orientedchopped strand mat (E-glass fibre 40 w/w%) was used toprepare reference glass fibre composite sample fabricatedusing similar procedure for comparison purpose.

2.3. Water absorption tests

The effect of water absorption on hemp fibre reinforcedunsaturated composites were investigated in accordancewith BS EN ISO 62:1999 [18]. The samples for tensile and

Table 1Typical chemical composition and structure parameters of hemp fibre

Cellulose Hemicellulose Lignin Pectins Wax Cell length (mm) Spiral angle (Deg) Moisture content (%)

74.4 17.9 3.7 0.9 0.8 23.0 6.2 10.8

Table 2Comparative values of physical and mechanical properties of hemp withE-glass fibre

Fibre Density(g/cm3)

Elongation tobreak (%)

Tensile strength(MPa)

Youngsmodulus (GPa)

Hemp 1.14 1.6 690 3060E-glassa 2.50 2.5 20003500 70

a For comparison purpose.

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flexural tests containing different fibre volume fractions ofreinforcement were machined to a size of 150 20 3 mm3 and 60 15 3 mm3, respectively. First all the spec-imens were dried in an oven at 50 C and then were allowedthem to cool to room temperature in a desiccator beforeweighing them to the nearest 0.1 mg. This process wasrepeated until the mass of the specimens were reached con-stant. Water absorption tests were conducted by immersingthe HFRUPE specimens in a de-ionised water bath at 25 Cfor different time durations. After immersion for 24 h, thespecimens were taken out from the water and all surfacewater was removed with a clean dry cloth. The specimenswere reweighed to the nearest 0.1 mg within 1 min of

removing them from the water. The specimens were

weighed regularly at 24, 48, 98, 196, 392 up to 888 h expo-sure. Similarly, the specimens were immersed in water at100 C to determine water absorption at a higher tempera-ture. For this test, the specimens were placed in a containerof boiling de-ionised water. After 30 min of immersion, thespecimens were removed from the boiling water, cooled inde-ionised water for 15 min at room temperature thenremoved and weighed to the nearest 0.1 mg. The weightof the samples was measured at different time intervals upto 31 h of exposure until the water content reached satura-tion. The moisture absorption was calculated by the weightdifference. The percentage weight gain of the samples wasmeasured at different time intervals and the moisture con-

tent versus square root of time was plotted.

Resin/catalyst/hemp fibre Hemp fibreCatalyst(MEKP)

Unsaturated

polyester

MixingResin/catalyst mixing/hemp fibre dryingDrying

Time: 1 hr

Temp: 100 dc

Mixture of Resin/catalyst/hemp fibre

Hemp fibre/mixture of resin and catalyst

Processing

Post curing

Composites

Mixture of

resin/catalystHemp fibreMould frame

Hand lay up

Hydraulic press consolidation

Time: 1.5 hrs, Pressure: 10 Bars

Temp: 25 dc

Post curing

Time: 3 hrs, temp:80 dc

Composite

laminate

Fig. 1. Process flow chart showing the applied fabrication route of HFRUPE composites.

Heat and pressure

Heat and pressure

Hot press platen

Hot press platen

conduction plate

conduction plate

Mixture of UPE/catalyst/hemp

in a mould frameTop/bottom mould plates

Fig. 2. Schematic of the composite consolidation.

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2.4. Mechanical testing

2.4.1. Tensile testing

The tensile strength and modulus of the hemp fibre rein-forced composites before and after water immersion werewith a crosshead speed of 10 mm/min in accordance with

BS EN 2747:1998 [19]. Test specimens were individuallycut using a diamond wheel into rectangular beams fromthe laminate slabs fabricated by a hand lay-up process. Thecut edges were then smoothed using 240 Grade SiC paper.

2.4.2. Flexural testing

The flexural strength and modulus of the compositebefore and after water immersion were determined usingthree-point bending test method following BS EN2746:1998 test method [20]. A span of 48 mm, maintaininga span to depth ratio of 16:1, was used in a 30 kN load cell.The load was placed midway between the supports. Thecrosshead speed applied was 2 mm/min. Each sample was

loaded until the core broke and their average is reported.

2.4.3. Scanning electron microscopy

In order to understand the effect of water absorption onthe microstructure of composites the surfaces of the water-immersed specimens were examined using a scanning elec-tron microscope (SEM) JSM 6100.

3. Results and discussion

The results obtained from this experimental study canbe divided into two parts. The first part considers the nat-

ure of the diffusion into the hemp reinforced compositesand the second evaluates the effects of water absorptionat room temperature and at 100 C exposure on themechanical properties.

3.1. Sorption behaviour

The percentage of water absorption in the compositeswas calculated by weight difference between the samplesimmersed in water and the dry samples using the followingequation:

DMt

mt momo

1001

where DM(t) is moisture uptake, Mo and Mt are the massof the specimen before and during aging, respectively.

Different models have been developed in order to describethe moisture absorption behaviour of the materials [21,22].For one-dimensional moisture absorption each sample isexposed, on both sides, to the same environment, the totalmoisture content Gcan be expressed as follows [23,24]:

G m mims mi 1

8

p2

X1j0

1

2j 12 exp 2j 12p2Dxt

h2

" #

2

where mi is the initial weight of the moisture in the materialand ms is the weight moisture in the material when thematerial is fully saturated, in equilibrium with its environ-ment. D is the mass diffusivity in the composite. This is aneffective diffusivity since all the heterogeneities of the com-posites have been neglected. h is thickness of specimen and

t is the time and j is the summation index. The diffusioncoefficient is an important parameter in Ficks law. Solvingthe diffusion equation for the weight of moisture, and rear-ranging in terms of the percent moisture content, the fol-lowing relationship is obtained:

M 4Mmh

t

p

0:5D0:5x 3

where Mm is the equilibrium moisture content of the sam-ple. Using the weight gain data of the material with respectto time, a graph of weight gain versus time is plotted. Thediffusion coefficient can be calculated using the followingformula:

D d2

p2 t70 4

Where d is sample thickness in mm and t70 is time taken toreach 70% saturation in seconds.

The diffusion properties of composites described byFicks laws was evaluated by weight gain measurementsof pre-dried specimen immersed in water by consideringthe slope of the first part of the weight gain curve versussquare root of time by using the following equation [25].The coefficient of diffusion (D) defined as the slope of thenormalised mass uptake against

pt and has the form:

D p kh4Mm

2 5

where, kis the initial slope of a plot ofM(t) versus t1/2, Mmis the maximum weight gain and h is the thickness of thecomposites.

Fig. 3 shows percentage of weight gain as a function ofsquare root of time for UPE and various loading levels of

0

2

4

6

8

10

12

0 10 20 30 40

Time (Hours)1/2

Weightgain(%)

UPE only UPE/2 Layer hemp UPE/ 3 Layer hemp

UPE/4 Layer hemp UPE/5 Layer hmep UPE/CSM

Fig. 3. Water absorption curves at RT for different specimens.


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UPE/hemp reinforced samples immersed in de-ionisedwater at room temperature (23 C). The maximum percent-age weight gain for UPE, 3, 4 and 5 layers of hemp fibrereinforced specimens, corresponding to 0, 0.15, 0.21 and0.26 fibre volume fractions, respectively, immersed at roomtemperature for 888 h is 0.879, 5.63, 8.16 and 10.97%,

respectively. The water uptake process for all specimensexcept CSM, which hardly absorbs any water, is linear inthe beginning, then slows and approaches saturation afterprolonged time, following a Fickian diffusion process. Boththe initial rate of water absorption and the maximum wateruptake increases for all HFRUPE composites samples asthe fibre volume fraction increases. This phenomenon canbe explained by considering the water uptake characteris-tics of hemp fibre. When the composite is exposed to mois-ture, the hydrophilic hemp fibre swells. As a result of fibreswelling, micro cracking of the brittle thermosetting resin(like unsaturated polyester) occurs. The high cellulose con-tent in hemp fibre (approximately 74%) further contributes

to more water penetrating into the interface through themicro cracks induced by swelling of fibres creating swellingstresses leading to composite failure [26]. As the compositecracks and gets damaged, capillarity and transport viamicro cracks become active. The capillarity mechanisminvolves the flow of water molecules along fibrematrixinterfaces and a process of diffusion through the bulkmatrix. The water molecules are actively attack the inter-face, resulting in debonding of the fibre and the matrix[27]. The SEM evidence in Fig. 4 supports this explanation.

Fig. 5 shows the percentage of weight gain for UPE, 3, 4and 5 layers of hemp specimens immersed in water at

100 C. For UPE, 3, 4 and 5 layer hemp reinforced speci-mens the percentage of moisture absorption is 1.947,7.366, 9.12 and 13.53%, respectively. The effect of fibre vol-ume fraction and temperature on water absorption can beclearly seen. The rate of approach to equilibrium is clearlymore rapid for the 100 C specimens than the samplesimmersed at RT. Higher temperatures seem to acceleratethe moisture uptake behaviour. When the temperature ofimmersion is increased, the moisture saturation time(MST) is greatly shortened. For 5 layer hemp samples atroom temperature, it takes 888 h to reach MST whereasfor 100 C samples, the MST is 31 h. The MST in this casewas shortened by 857 h. This shows that sorption at room

temperature takes far longer period to reach equilibriumthan sorption at elevated temperatures. In addition to theincrease in weight gain percentage, it also shows the weightgain is higher for samples immersed in boiling water thanat room temperature. For 5 layer hemp samples, the weightgain percentage at moisture saturation point at boilingtemperature is approximately 23% higher than at roomtemperature. It is evident that there is a different sorptionbehaviour for immersion at room temperature than for ele-vated temperature indicating different aging mechanisms.The higher and faster weight gain upon exposure to boiling

water may be attributed to the different diffusivity of waterinto the material leading to moisture induced interfacialcracks at an accelerated rate as a result of degradation inthe fibrematrix interface region as well as the state ofwater molecules existing in the HFRUPE composites.Other studies also have reported a similar trend for ageingof polymer composites at elevated temperatures [28].

Table 3 presents the diffusion coefficients for both roomtemperature and 100 C water-immersed specimens. It canbe seen that the maximum moisture content and the diffu-sion coefficient values increases steadily with an increase infibre volume fraction. The increase is more pronounced forthe specimens immersed at 100 C than those of immersed

Fig. 4. Failure showing (a) matrix cracking, (b) fracture running along the interface and (c) fibrematrix debonding due to attack by water molecules.

0

2

4

6

8

10

12

14

16

0 2 4 6 8

Time (Hours )1/2

Weightgain(%)

UPE only UPE/3 Layer hemp

UPE/4 Layer hemp UPE/5 Layer hemp

Fig. 5. Water absorption curves at boiling temperature for differentspecimens.


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at RT. Higher fibre loaded samples, as would be expected,contain a greater diffusivity due to higher cellulose content.

The moisture uptake at elevated temperatures comparedto RT seems to obey non-Fickian behaviour showing a23% higher moisture uptake for 5 layer hemp fibre rein-forced composites. The moisture uptake results in this

study show Fickian behaviour at room temperature andnon-Fickian at boiling temperature. This is attributed dueto the moist, high temperature environment, and micro-cracks developed on the surface and inside the materials[29]. As the cracks develops material is actually lost, mostlikely in the form of resin particles [30,31] as can be seenin Fig. 6a and b. After the occurrence of damage in thecomposites water transport mechanisms become moreactive [32]. The deviation from Fickian water uptakebehaviour at 100 C is attributed to the development ofmicro cracks in the composites [33].

3.2. Effect of moisture absorption on mechanical properties

3.2.1. Tensile properties

The tensile stresses and strain versus fibre volume frac-tion results for these samples are shown in Figs. 7 and 8.For both dry and water aged samples (exposure time888 h at RT), the stressstrain curves are linear up to thepoint of failure. There is no affect of water absorption ontensile stress for UPE samples. The tensile stress was ratherincreased after water immersion of 888 h. Similarly, for 2layer hemp reinforced samples, the tensile stress is increasedby 22% after immersion in water. This increase in tensile

stress for unreinforced and 2 layer hemp reinforced sampleimplies that further crosslinking or other mechanisms aretaking place enhancing the material strength. The tensilestress however, drops by 38 and 15%, respectively, for 3and 4 layer hemp reinforced specimens. Generally, forhigher fibre volume composites samples immersed in water,it is expected that the relative extent of decrease in tensileproperties is greater compared to dry samples. However,it is interesting to note that for 5 layer hemp reinforced sam-ples, the ultimate tensile stress of wet samples is higher thanthat for dry samples. This could be due to the fact that highamounts of water causes swelling of the fibres, which couldfill the gaps between the fibre and the polymermatrix andeventually could lead to an increase in the mechanical prop-erties of the composites [34]. Similar observations have beenreported for jute fibre reinforced polymer composites where

after 24 h of soaking in water the flexural strength increased

Table 3Moisture uptake of hemp fibre composites immersed in water at RT and100 C

Compositefibre (vol%)

Saturationmoisture uptakeMm (%)

Initial slope ofplot (k) M(t)versus t1/2

Diffusioncoefficient, D,103 (m2/s)

RT 100 C RT 100 C RT 100 C

0 (UPE only) 0.879 1.947 0.102 0.437 5.714 8810 (2L hemp) 3.441 0.102 1.551 15 (3L hemp) 5.639 7.366 0.247 1.178 3.618 4821 (4L hemp) 8.161 9.125 0.346 1.562 3.841 6226 (5L hemp) 10.972 13.533 0.496 2.375 4.367 67

Data in table are means with a sample size of 3 for each specimen group.

Fig. 6. Degradation of composite showing (a) crack development (b) lost

of resin particles due to high accelerated ageing at 100 C.

0

10

20

30

40

50

60

70

80

0.1 0.15 0.21 0.26 UPE

Fibre volume fraction

Tensilestress(MPa)

Stress for samples without moisture absorption

Stress for samples with moisture absorption

Fig. 7. Tensile stress versus fibre volume fraction.

0

2

4

6

8

10

12

14

0.1 0.15 0.21 0.26 UPE


Strain(%)

Strain for samples without moisture absorption

Strain for samples with moisture absorption

Fig. 8. Tensile strain versus fibre volume fraction.


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by 28% and after 72 h of water immersion, and flexuralstrength was increased by 45% [35].

The failure tensile strain value for all water-immersedspecimens was found to increase compared to dry speci-mens. The increase in failure strain upon exposure of thesamples to a wet environment can be attributed to the plas-

ticisation of hemp samples caused by moisture absorption.Fig. 9a shows a SEM picture of hemp fibre. At regularintervals along the fibre surface, kinks or nodes can beclearly seen. Fig. 9b shows a HFRUPE composite wherethe effect of kinks or nodes on the surface of the compos-ite laminate reflect the misalignment of fibres. When theseirregularly shaped fibres are placed in composites they donot seem aligned properly leading to fibre entanglement.Fibre alignment factors play a crucial role in the overallproperties of composites. There is always a chance of fibreentanglement with randomly oriented fibre reinforcedcomposites. The random orientation of fibres produceslower mechanical properties compared to long unidirec-

tionally orientated fibres. This fibre entanglement can cre-ate resin rich areas, which can contribute to the formationof voids and porosity (Fig. 10). Voids and porosity canact as stress concentrators leading to failure of compositesamples. Hence, the void content for 2, 3, 4 and 5 layersof hemp fibre composites specimen was found to be 12.56,14.46, 16.60 and 18.64%, respectively. The void content of

composite was calculated using the following standardformula:

Vv 1 qcwf

qf

wmqm

6

where Vv is the volume fraction of voids, qc the density of

composite, wf the weight percent of fibre (%), wm the weightpercent of matrix (%), qf the density of fibre g/cm3 and qm

is the density of matrix g/cm3.As far as voids content in natural fibre composites is

concern, the fabrication techniques are not yet fully devel-oped and the natural origin of the fibre component neces-sarily induces an element of variation in to thecomposites; both factors contribute in creation of voidswhich affects to the overall composite properties. It is evi-dent in this study that as the fibre volume fraction of hempreinforced composite sample increases the void contentalso increases.

3.2.2. Flexural propertiesThe flexural stressstrain versus fibre volume fraction

results for dry and water immersed (exposure time 888 hat RT) HFRUPE composites are shown in Figs. 11 and12. The observations made earlier for the effect of waterabsorption on tensile stress/strain properties are also rele-vant here. The flexural stress drops incrementally as thefibre volume fraction increases hence increased moistureuptake percentage. The decrease in flexural properties afterwater immersion can be related to the weak fibrematrixinterface due to water absorption.

Flexural strain for water-immersed samples has

increases dramatically compared to dry samples. Flexuralstrain for 5 layer hemp reinforced dry samples is 8%whereas after 888 h of water immersion the strain isalmost doubled. HFRUPE composites become more rigiddue to the lower flexibility of the unsaturated polyesterchain. After water aging for 888 h, strain is almost dou-bled compared to dry specimens since natural fibre rein-forced composites tend to be ductile once the loss ofcellulose and integrity has taken place [36]. It has beenreported that water molecules act as a plasticiser agentin the composite material, which normally leads to an

Fig. 9. SEM micrograph of hemp fibre (a) showing kinks or nodes (b)showing fibre misalignment and entanglement.

Fig. 10. Micrograph of water immersed hemp/UPE samples showing effects of voids (a) voids, (b) voids acting as reservoirs and (c) matrix cracking and

delamination after 888 h of immersion.


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increase of the maximum strain for the composites afterwater absorption [37].

The decrease in mechanical properties with increase inmoisture content is may be caused by the formation ofhydrogen bonding between the water molecules and cellu-lose fibre. Natural fibres are hydrophilic with many hydro-xyl groups (OH) in the fibre structure forming a largenumber of hydrogen bonds between the macromolecules

of the cellulose and polymer [38]. With the presence of ahigh OH group percentage, natural fibres such as hemptend to show low moisture resistance. This leads to dimen-sional variation of composites products and poor interfa-cial bonding between the fibre and matrix, causes adecrease in the mechanical properties [39].

Water absorbed in polymers is generally divided intofree water and bound water. Water molecules (which arecontained in the free volume of polymer and are relativelyfree to travel through the micro voids and holes) are iden-tified as free water. Water molecules that are dispersed inthe polymermatrix and attached to the polar groups of

the polymer are designated as bound water [40]. The char-

acteristics of water immersed specimens are influenced notonly by the nature of the fibre and matrix materials butalso by the relative humidity and manufacturing technique,which determines factors such as porosity and volume frac-tion of fibres. Water uptake can be advantageous for somenatural fibres (such as Duralin fibre) at 66% relative humid-

ity as can fibre plasticising effect as a result of from thepresence of free water [41]. Excessive water absorption,however, leads to an increase in the absorbed bound waterand a decrease in free water. In this situation, water canpenetrate into the cellulose network of the fibre and intothe capillaries and spaces between the fibrils and less boundareas of the fibrils. Water may attach itself by chemicallinks to groups in the cellulose molecules. The rigidity ofthe cellulose structure is destroyed by the water moleculesin the cellulose network structure in which water acts asa plasticiser and it permits cellulose molecules to movefreely. Consequently the mass of the cellulose is softenedand can change the dimensions of the fibre easily with

the application of forces. Observation of the fracture sur-face from the flexural test sample further emphasises theimportance of fibrematrix adhesion on flexural strength.

3.2.3. Influence of moisture on the modulus

Table 4 represents the results of tensile modulus andflexural modulus for both dry and water-immersed speci-mens at RT. It can be seen that moisture absorption causeschange in the modulus as determined by tensile and flexuraltests. The tensile modulus decreases for all hemp reinforcedsamples. The reduction in tensile modulus for 3, 4 and 5layer hemp reinforced specimens compared to dry speci-

mens is 61, 97 and 87%, respectively. A plausible explana-tion for this would be that, the elastic modulus is a fibre-sensitive property in composites and is affected as a resultof moisture absorption. This effect is particularly greaterfor the composites with higher fibre content, in which stresstransfer capability between fibre and matrix interface getssharply reduced due to moisture content.

The flexural modulus, however, is not adversely affectedby moisture absorption. The increase in flexural modulus ismore pronounced with higher fibre content specimens,hence higher moisture content. It would be intuitive toassume that the effect of fibre reinforcement to be less crit-ical for the flexural failure stress than in tensile failure

0

20

40

60

80

100

120

140

UPE 0.1 0.15 0.21 0.26


Flexuralstress(MPa)

Dry sample Water immersed sample

Fig. 11. Flexural stress versus fibre volume fraction.

0

5

10

15

20

25

30

UPE 0.1 0.15 0.21 0.26Fibre volume fraction

Flexuralstrain(%)

Dry sample water immersed sample

Fig. 12. Flexural strain versus fibre volume fraction.

Table 4Tensile and flexural modulus for dry and wet samples

Specimens Fibrevolume(%)

Tensilemodulus (GPa)

Flexuralmodulus (GPa)

Dry Wet Dry Wet

UPE only 0 0.56 0.60 5.51 5.812 Layer hemp 10 0.72 0.64 4.20 5.763 Layer hemp 15 1.0 0.62 5.34 6.084 Layer hemp 21 1.22 0.62 7.30 6.065 Layer hemp 26 1.27 0.68 6.49 8.05

Data in table are means with a sample size of 5 for dry and 3 for wet for

each specimen group.


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mode. This is because the flexural samples fail in combina-tion of compression, shear and tension mode.

4. Conclusions

The effect of water absorption on the mechanical prop-

erties of non-woven hemp fibre reinforced unsaturatedpolyester composites has been studied following immersionat room temperature and boiling temperature. It showsthat moisture uptake increase with fibre volume fractionincreases due to increased voids and cellulose content.The water absorption pattern of these composites at roomtemperature is found to follow Fickian behaviour, whereasat the elevated temperature the absorption behaviour isnon-Fickian. Water uptake behaviour is radically alteredat elevated temperatures due to significant moistureinduced degradation. Exposure to moisture results in sig-nificant drops in tensile and flexural properties due to thedegradation of the fibrematrix interface.

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