Composites: Part A 60 (2014) 32–37

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Composites: Part A

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An evaluation of the effects of moisture content on the modulusof elasticity of a unidirectional flax fiber composite

http://dx.doi.org/10.1016/j.compositesa.2014.01.0111359-835X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +33 0546839026.E-mail addresses: djammasseteau@aol.com (B. Masseteau), franck.michaud@

ecoledubois.fr (F. Michaud).

Benjamin Masseteau a,⇑, Franck Michaud b, Mark Irle b, Annette Roy a, Gaëlle Alise a

a CRITT Matériaux Poitou-Charentes ZA Beligon, rue Maurice Mallet, 17300 Rochefort, Franceb LUNAM Université, Ecole Supérieure du Bois, rue Christian Pauc, 44000 Nantes, France

a r t i c l e i n f o

Article history:Received 14 March 2013Received in revised form 2 October 2013Accepted 17 January 2014Available online 25 January 2014

Keywords:A. Polymer–matrix compositesPlant fiberB. Mechanical propertiesMoisture content

a b s t r a c t

Results of tensile tests on ‘‘wet’’ and ‘‘dry’’ flax yarn are presented and these show the large effect thatmoisture content (MC) has on flax fiber modulus of elasticity (MOE). These results are compared to othersfrom tensile tests on flax fiber reinforced epoxy unidirectional composites (FFREUC) made from ‘‘wet’’and ‘‘dry’’ fiber. The homogenized fiber MOEs have been estimated for the composites using the inverserule of mixture. Fiber MOE appears better for dry fiber (by around 20%) for both the yarn and composite. Itis proposed that this difference is the result of changes to the quality of adhesion between matrix andfiber. Adhesion would appear to be better for wet fibers.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Flax fibers have some advantages over glass fibers when used incomposites in that they are derived from a renewable resource,have a low density and good specific mechanical properties. Theyfurthermore cause fewer dermal and respiratory irritations and re-duce tool wear during the machining of the finished composites[1,2]. Their disadvantages include variability and hygroscopicitywhich are the two principal topics of this paper.

The high variability of plant fibers compared to synthetic fiberslike glass and carbon, makes it difficult to predict the mechanicalproperties of flax fiber reinforced epoxy unidirectional composites(FFREUC). Effectively the authors use the law of mixtures in therest of the article. This difficulty may partially explain the rela-tively low uptake of plant fiber composites by industry (only 16%of the flax production goes to composite material [3]).

Flax fiber is an hydrophilic material and so its moisture content(MC) is dependent on the temperature and relative humidity of theair surrounding it [4–7]. The effect of fiber MC on the mechanicalproperties of FFREUC composites has not, however, been thor-oughly investigated. Consequently, this study focuses on the ef-fects caused by varying fiber MC on the modulus of elasticity(MOE) of a FFREUC. Although the composite MOE is not the onlymechanical property affected by the variation of the fiber MC, this

study focuses on MOE prediction because it is less complex thanthe prediction of ultimate properties.

The elastic properties of composites depend on the elastic prop-erties of the fibers and the matrix, but also on the efficiency ofstress transfer between fibers and the matrix [5]. In addition, thequantity of adsorbed water present in a plant fiber composite willaffect: the fiber MOE, the matrix MOE and the fiber–matrix adhe-sion quality.

With regard to fiber MOE, some studies on unitary flax yarn andunitary flax fiber have observed a decrease in MOE and an increasein ultimate stress ru as MC increases from zero [5,7–9]. The differ-ences in the MOE values observed between the studies are proba-bly caused by the different methods used and natural variationwithin fibers and between varieties of flax. Tensile tests on unitaryfiber and yarn are subject to error because sample preparation mayinduce defects. Furthermore, the difficulty of measuring the truecross-sectional area of the fibers causes most researchers to as-sume a constant diameter [10] and to consider the largest diameter(see Fig. 1). A new more reliable method is needed in order to ob-serve the true effects of changing MC on fiber MOE and such amethod is proposed in this paper.

The matrix MOE is expected to vary with MC as epoxy resins areknown to plasticize on contact with water [11,12]. The water pres-ent in the matrix can catalyze the reaction by bonding withhydroxyls groups generated by the epoxy–amine reaction. If theamount of water is too high, the matrix can plasticize becausewater takes the place of intramolecular hydrogen bonds presentin the epoxy network [13]. In addition, the amount of absorbed

Fig. 1. Difference in real and assumed fiber dimension. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthis article.)

B. Masseteau et al. / Composites: Part A 60 (2014) 32–37 33

water is observed to decrease with the degree of cure [14]. The sig-nificance of this plasticization will be addressed further in thisstudy.

The efficient transfer of shear stress between fibers and matrixis essential for composites that are unidirectionally reinforced bydiscontinuous fibers [2,4,5,8]. The stress transfer could be affectedboth by the presence of water in the fibers and in the matrix[15,16]. Shear stress transfer depends on the quality of adhesionbetween the fiber and the matrix and adhesion is determined bytwo parameters: the energy of adhesion (Wa) and fiber anchoringin the matrix [5,17]. Some authors have shown that the interfacialshear stress is proportional to Wa [18–21]. So an increase in Wa im-plies an improvement in interfacial shear stress transfer and there-fore an improvement of the composite’s MOE. A modification offiber MC may cause a modification of Wa by modifying the polarcomponent of the fiber [5]. The same is true for the matrix if it ad-sorbs water.

Good wetting of the matrix on the fiber will increase the specificsurface area for bonding and will therefore improve the mechani-cal anchoring of the fibers. This will, in turn, improve the MOE ofthe composite [4,17,22]. It has been observed that the wettabilityof flax fibers with epoxy is influenced by fiber MC [5].

All three of the above phenomena could appear simultaneouslyas MC varies. If it is accepted that the changes in matrix MOE withMC are negligible compared to those of fiber MOE and changes inWa, then the remaining two phenomena can be uncoupled. The ap-proach taken in this work is first to study the effects of MC on theMOE of flax yarns and then to determine the effect of fiber MC onthe composite MOE. In the methods section it is shown how it ispossible to uncouple the effects of MC on fiber MOE and on thequality of adhesion by comparing the changes in fiber MOE withthose of the composite MOE from these two tests.

In order to do this the volumetric fraction of fiber, matrix andporosity have to be known accurately. The changes in fiber specificgravity should be taken into account when MC varies as observedin the case of wood [23]. Consequently, volumetric fractions of fi-ber, matrix and porosity must be determined versus MC.

The fiber density must be known accurately because it is used inthe inverse rule of mixture to predict properties and a slight devi-ation in its value causes significant changes in porosity and fibervolume. The usual method (pycnometer [3,4,12]) to measure the fi-ber density does not take into account either the sorption of testliquid by the fiber during the test or the fiber MC. In order to im-prove the accuracy, a method to evaluate true fiber density witha correction for MC is proposed.

2. Materials and methods

2.1. Materials

The epoxy matrix used in this study is a standard DGEBA(DiGlycideEther of Bisphénol A) resin with a tri-amine hardener

provided by SICOMIN� (resin: SR1500 and hardener: SD2503). Cur-ing was conducted at ambient temperature (23 ± 2 �C).

The unidirectional fabric (provided by Libeco Lagae�) is com-posed of flax yarn linked with a few flax yarns in the 90� direction(ground yarn). These fibers came from several varieties and severalharvests in order to homogenize properties which vary with vari-ety, growing conditions and position in the stem [24].

2.2. Composite molding procedure

In order to study the effect of fiber MC on composite MOE, twokinds of composites were made (5 replicates of each): compositesmade from ‘‘wet’’ fibers and composites made from ‘‘dry’’ fibers.Wet fibers were obtained by storing them in a regulated chamber(23 ± 2 �C and 55 ± 5% RH) until equilibrium is reached (at least2 days).

For the entire article, fiber MC is presented on the basis of anhy-drous material by Eq. (1) where mh is the wet mass of the sampleand m0 the dry mass:

MC ¼ mh �m0

m0ð1Þ

Dry fiber was dried in an oven at 103 ± 3 �C to constant mass asdefined by NF B51-004 [25]. In order to avoid temperature effectson wetting, the ‘‘dry’’ fibers were cooled for 1 min and 30 s in a reg-ulated chamber set at 23 �C and 50% RH immediately prior to com-posite lay-up.

A4 format composites consisting of 6 plies of unidirectionalfabric were laid up by hand using 105 g of DGEBA applied bybrush and then pressed with a vacuum bag (950 mbar). Thepanels were cured in a room regulated at 23 ± 3 �C and 50 ± 5%RH for 14 days. Differential Scanning Calorimetry (DSC) ofseparate samples indicated that curing was complete by theend of this procedure.

2.3. Fiber density evaluation

With the dry mass of fiber, mf0 and the mass of bound water infiber, mwater it can be written:

mwater ¼ MC �mf 0 ð2Þ

Weight fraction of water, Wwater and dry fiber, Wf0 are:

Wwater ¼MC �mf 0

MC �mf 0 þmf 0¼ MC

1þMCand Wf 0

¼ mf 0

MC �mf 0 þmf 0¼ 1

1þMCð3Þ

Therefore fiber cell wall density at a given moisture content,df(MC) can be estimated using the rule of mixture as shownbelow:

df ðMCÞ ¼ df 0Wf 0 þ dwaterWwater ¼MC � dwater þ df 0

1þMCð4Þ

The flax fiber saturation point is considered to be around 15–20% [4,5,10], thus all the water present in the fibers used in thisstudy is bound water. Its density is slightly higher than normalwater because its molecular mobility is reduced due to H-bondingwith hydroxyl groups in the cell wall polymers. In a study on woodfiber [26], it is estimated to be 1.017 at 23 �C. The anhydrous fiberdensity df0 is considered to be 1.54 according to the literature [4,5].Consequently, the density of flax fiber cell wall as a function of MCcan be estimated by equation Eq. (5).

df ðMCÞ ¼ 1:017MC þ 1:541þMC

ð5Þ

34 B. Masseteau et al. / Composites: Part A 60 (2014) 32–37

Several studies show that there is no penetration of the matrix(epoxy or polyester) into the fiber cellular wall [10,27–30]. As aconsequence, when the fiber density is measured by the Archime-des principle, the test liquid must not fill the lumen, otherwise thetest measures the cell wall rather than the apparent density.

2.4. Yarn tensile test procedure

The fiber cross-sectional area is usually calculated by assuminga constant diameter which is estimated by measuring the diameterof a circle that will enclose the fiber cross-section [4,8]. Thisassumption is false on two counts: the diameter is not constantalong the fiber and the effective cross-section is lower than theequivalent fiber section estimated using the largest diameter.

To avoid this problem, a new method is proposed here in whichthe fiber cross-sectional area (S) of a sample containing more than100 yarns is calculated from the sample mass m, the specific grav-ity of the fiber at its current MC(df(MC)) and the free length of thesample L (Eq. (6)). The effect of water on the fiber density df(MC)(see Section 2.3.) must be taken into consideration because if it isnot taken into account, an error would be introduced into the cal-culation of the sample cross-sectional section Seq.

Seq ¼m

df ðMCÞ � L ð6Þ

The samples used were rectangular (150 � 30 mm), with a freelength, L, between the test machine grips of 100 mm. The mass mof the free length is accurately estimated by calculating the gram-mage of the whole sample and assuming that the central 100 mmhas the same grammage. Samples were weighed using a Sartorius�

MSU 125P balance that has a precision of 0.09 mg and measuredwith a digital vernier caliper (±0.1 mm). This is much easier andmore accurate than measuring the free length of unitary fiber[10]. Displacements used in strain calculations were not correctedbecause in a comparison test, errors are the same for each sample(same domain of load). Two kinds of sample were tested: dry andwet fiber, conditioned as described above. Tensile tests were donewith a Zwick Z100 kN test machine in a conditioned room(23 ± 3 �C and 50 ± 5% HR) at a test speed of 1 mm min�1. Nine rep-etitions were tested for each fiber condition.

2.5. Composite volumetric fraction

Volumetric fraction of fiber Vf, matrix Vm and porosity V0 haveto be known in order to evaluate fiber MOE variation from tensiletests on composites and by the rule of mixture. Accurately measur-ing the thickness of a composite is difficult because its faces are un-likely to be perfectly coplanar or flat. Consequently, the compositevolume vc is not calculated by its dimensions as usual, but from itsdensity, dc, and its mass, mc (Eq. (7)). The fiber volume, vf, is calcu-lated from the fiber weight, mf, and its density, df. The fabric massmf was calculated from the sample area Sc and the surface weightof the fabric Gr. The actual specimens used were not oven dry andso df in Eq. (7) is replaced by df(MC) as described by Eq. (5).

Vf ¼v f

vc¼

mf

df

mcdc

¼Gr Sc

df

mcdc

¼ GrScdc

df mcð7Þ

Matrix fraction, Vm is usually calculated by the differenceVm = 1 � Vf, assuming an absence of porosity in the composite.But for flax fiber reinforced composites, porosity can be high(>5%, [10]) compared to carbon or glass fiber composites (less than1%, [31]). This must be taken into account in order to avoidapplying matrix mechanical properties to void volume. So the

composition of the composite would be 1 = Vf + Vm + V0. The voidvolume, V0, in the composite can be calculate from the rule ofmixture as in Eq. (8).

dc ¼ df Vf þ dmVm þ d0V0 () V0 ¼dc � dm þ Vf ðdm � df Þ

d0 � dmð8Þ

The matrix density used for all calculations, was 1.15 as deter-mined by a pycnometric method (five samples: 1.15 ± 0.013). Inaccordance with Apicella et al. [32], the matrix specific gravity de-pends on the amount of absorbed water. A simple rule of mixturecan be used to evaluate the effect of the matrix MC. By consideringa water weight gain of 1.85 ± 0.3% during a humid curing (see be-low) and a water specific gravity of 1.0, the humid matrix specificdensity can be calculated to be 1.147. The slight difference with theinitial value (1.15 ± 0.013) will not be considered in this study be-cause the standard error of measurement is greater.

Composite densities were determined by the same method. Theair density, d0, used was1.19 � 10�3 [33] for air with a 50% HR at23 �C and an atmospheric pressure of 1013 mbar.

To uncouple phenomena, the effect of water on the matrix MOEneeds to be studied. The properties of five matrix samples thatwere polymerized in a wet atmosphere of 86 ± 2% HR at 23 ± 2 �Cfor 14 days were compared to five samples cured at 50 ± 5% HRat 23 ± 2 �C.

2.6. Estimation of fiber MOE from the composite MOE

The simple rule of mixture (Eq. (9)) can accurately predict theMOE of composites made with glass and carbon fibers from tensiletest results, [34]. This is because these composites are non-porousand have near perfect fiber/matrix adhesion. Although FFREUCcomposites have comparatively high porosities, the simple rule ofmixture still provides useful indications. Consequently the rule ofmixture for MOE gives:

Ec ¼ Ef Vf þ EmVm ¼ Ef Vf þ Emð1� Vf � V0Þ with 1

¼ Vf þ V0 þ Vm ð9Þ

By inversing Eq. (9), the homogenized fiber MOEfh can be calculatedfrom the composites’ tensile MOE results and their volumetriccompositions:

MOEfh ¼Ec � Emð1� Vf � V0Þ

Vfð10Þ

Composite and matrix tensile MOE measurements were doneusing a standard Zwick Z250 kN test machine following the NFEN ISO 527-4 standard. Strain was measured by an Epsilon� tech-nology model 3542-100 M extensometer (50 mm). Five samples(250 � 25 mm) were tested for each type of composite in a regu-lated chamber (23 �C, HR 50%). The MOEs were calculated between0.025% and 0.05% strain.

3. Results and discussion

Fig. 2 shows typical stress–strain curves of tensile tests from adry and a wet sample. Dry fibers had a MC of 1.5 ± 0.35% whereasthe wet ones had a MC of 7.6 ± 0.3%. The curves are similar to thoseobserved during the loading zone of cyclic tensile tests made onunitary flax fibers [4].

At low strains (<0.5%) the MOE initially increased, which is dueto structural changes in the sample at three different levels: thefabric, the yarn and the fiber scale. At the fabric level, the increasein MOE observed is due to the presence of yarns of slightly differentlengths; as each yarn takes up strain the MOE is observed to in-crease. Whereas at the yarn and fiber level their helical structures

Fig. 2. Typical tensile stress–strain curves, wet and dry yarn.

B. Masseteau et al. / Composites: Part A 60 (2014) 32–37 35

unwind during the tensile test until each fiber and each cellulosemicro-fibril is in tension [4,5,8,35,36].

At around 0.5% strain, the curves become linear until fiber fail-ure. For dry yarn, the behavior seems to be less linear than for wetyarn, which may be explained because dry yarn quickly absorbswater during the tensile test (2.0% in 1 min and 30 s) as observedpreviously [5]; thus the MOE decreases during the test.

Fiber MOEs were estimated using the linear part of the curve atstrains between 0.5% and 1.0%.

The values shown in Table 1 are in the same range of those seenin the literature. Slight differences could be explained by three rea-sons: differences in material (size of the yarn, spinning method, fi-ber variety); differences in the test method (test speed, fabricwidth); differences on fiber MC. In one case, the MC was not con-trolled [37], and in the other case [8], the MC used (65% RH) wasdifferent from that one used in this study.

The standard deviations obtained are relatively low comparedto those in the literature for unitary fibers (28–57%, [4,24,38])and similar to those for unitary yarns (4–14%, [8,37]). So as ex-pected, it is possible to significantly decrease the standard devia-tion of results by testing large number of fibers simultaneously.The differences in MOE and in ultimate stress ru between dryand wet fiber are statistically significant at the 99% level of confi-dence. Consequently, the reductions in MOE (�20.9%) and increasein the ultimate stress ru (+19.9%) can be attributed to the effect ofMC and not to artifacts caused by the test procedure. The reductionin MOE observed is similar to the values published previously(�13% to �27%) [5,8,9]. However, the increase in ru is somewhatlower than that observed by others (+28% to +78%) [5,8,9].

It is generally accepted that the changes in MOE and ru arecaused by water plasticization of the hemicelluloses and ligninfound between the cellulose fibrils that make up the fiber [5,39–42].

Studies on wood have shown that the fiber MOE increasesslightly (0.5%) from the oven dry condition and then decreases.The peak is situated between 1% and 4% MC for wood [43–45]and around 6% MC for pure cellulose [46]. It was not possible to

Table 1MOE and ultimate stress ru on flax yarn, dry and wet sample (mean ± standarddeviation and relative standard deviation in brackets).

MC (%) MOE (GPa) ru (MPa)

1.5 ± 0.5 11.5 ± 1.7 (15%) 191.4 ± 32.0 (17%)7.6 ± 0.3 9.1 ± 1.5 (17%) 229.4 ± 16.8 (7%)

Table 2Composites volumetric fraction (from dry and wet fiber).

Fiber MC (%) Vf (%) V0 (%)

Dry 2.0 ± 1.7 32.8 ± 1.3 5.9 ± 1.2Wet 8.7 ± 0.3 37.4 ± 0.7 6.5 ± 0.7

measure the tensile properties of flax fibers with moisture contentsbelow 1.5% in this study so the presence of peak in MOE cannot beconfirmed.

For composites made with ‘‘wet’’ fibers, the curing conditions of50 ± 5% RH at 23 ± 2 �C are the same as fiber storage conditions andso the fiber MC and, therefore, fiber dimensions should not change.However, for the dry fiber composites, the curing conditions mayincrease the fiber MC because it is more humid than the storageconditions. By measuring the composite mass before and after cur-ing, and by assuming that the mass of matrix is not modified bycuring, then it is possible to follow any changes in fiber MC. Forthe five dry fiber composites, the MC at the end of curing was cal-culated to be 2.0 ± 1.7% and this difference from the initial MC isnot statistically significant. As a consequence, it can be consideredthat the curing conditions have no real impact on fiber MC.

To uncouple phenomena, the effect of water on the matrix MOEwas studied. Matrix samples cured in high humidity conditions witha water uptake of 1.85 ± 0.3%, were not found to have significantlydifferent MOE values when standard deviation is taken into account(3.27 ± 0.15 GPa for ambient conditions and 3.41 ± 0.10 GPa for wetconditions). As a consequence, the effect of water on the matrix MOEis considered insignificant in this paper.

The MOEs of the resultant composites were similar regardless ofwhich type of fiber was used (21.4 ± 0.8 GPa for wet fiber compos-ites and 22.9 ± 1.1 GPa for dry fiber composites) but there are dif-ferences in fiber volume fractions (see Table 2). So the simplerule of mixture, as shown in Eq. (10), can be used to estimate theMOEfh of the fibers in the composites. The results of the calcula-tions are shown in Table 3.

The reduction in the calculated fiber MOE is similar to that ob-served when the yarn was tested. However, for a true comparison,the differences in MC between the yarn and composite tests mustbe equivalent in order to compare the fiber MOE differences. Theeffect of MC on fiber MOE is thought to be linear in the MC rangeof these experiments [43]. This assumption was used together withthe inverse rule of mixture for calculating the DMOEfh values givenin Table 4 over the same range of DMC (2.0–8.7%).

If MC has no effect on the quality of adhesion, DMOE from thetwo methods should be very close, if not identical. This is not thecase here and the difference in DMOE obtained from the two meth-ods could be attributed to the changes in the quality of adhesion atthe fiber/matrix interface. The implication is that stress transferbetween the fiber and matrix is better for wet fibers because theMOEfh was not reduced by 22.9%, as it did for the yarn.

For Owens and Wendt [47], the surface free energy of a bodyhas a non-dispersive, cND (polar component), and a dispersive com-ponent, cD (Van der Waals and London bonds). According to theOwens and Wendt theory, the energy of adhesion Wa can be calcu-lated by Eq. (11) where L is the liquid (epoxy matrix) and S the so-lid (flax fiber).

Wa ¼ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffifficND

s cNDL

qþ 2

ffiffiffiffiffiffiffiffiffifficD

s cDL

qð11Þ

A reduction of the polar component (35.9 mJ m�2 to31.2 mJ m�2) has been observed when fibers are dried [48]. By con-sidering that cND and cD are constant (7.6 mJ m�2 and 20.4 mJ m�2

respectively) when the matrix MC varies, and a constant cD of

Table 3Homogenized fiber MOEfh of dry fiber and wet fiber from composites (mean ± stan-dard deviation).

MOEfh (GPa)

MC = 8.7 ± 0.3% 55.3 ± 1.7MC = 2.0 ± 1.7% 67.3 ± 2.0DMOEfh (%) 17.8%

Table 4Fiber MOE increase for 7.3% MC reduction from the two methods.

Test method DMOE (%)

Yarn 22.9Composites 17.7

36 B. Masseteau et al. / Composites: Part A 60 (2014) 32–37

26.6 mJ m�2 for the fiber, Wa would change from 79.7 mJ m�2 to77.4 mJ m�2. This reduction of Wa could cause a reduction in thecomposite MOE. Values used to estimate Wa variation are frommeasurements made on a Krùss K100 tensiometer by the methodof Wilhelmy.

For the matrix, the effect of water absorption on the quality ofadhesion is rarely discussed. As it can absorb water, its polar com-ponent could also increase as for the flax fiber. Thus the efficiencyof stress transfer could improve when there is water at the inter-face by increasing the energy of adhesion.

4. Conclusions

A new method has been developed in order to take into accountthe effect of water on plant fiber density which, in turn, permits amore accurate estimation of the fiber fraction in a composite. Thenew method provides much more consistent results than by test-ing individual flax fibers. Fiber density changes as MC varies, butthis does not seem to have significant consequences on the MOEof the composite.

On the other hand, the MC of flax fibers is shown to affect thetensile properties of the fibers. The reduction in fiber MOE is great-er than that for the composite. It is concluded, therefore, that thequality of adhesion between fiber and the matrix is better in humidcompared to dry environments.

Fortunately, it would appear that it is not necessary to dry flaxfibers to a near oven dry state to obtain good composite properties.Further work is needed to find the optimal MC for flax fibers duringcomposite lay-up.

Acknowledgements

Authors thank SICOMIN� for their material supply (epoxymatrix and flax fiber) and the French National Association for theResearch and Technology (ANRT) for their financial support.

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