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Hybridizationeffectonthemechanicalpropertiesofcuraua/glassfibercomposites

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Composites: Part B 55 (2013) 492–497

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Composites: Part B

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Hybridization effect on the mechanical properties of curaua/glass fibercomposites

1359-8368/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.07.014

⇑ Corresponding author. Tel.: +55 51 3308 9416; fax: +55 51 3308 9414.E-mail address: jhsajunior@globomail.com (J.H.S. Almeida Júnior).

José Humberto Santos Almeida Júnior a,⇑, Sandro Campos Amico a, Edson Cocchieri Botelho b,Franco Dani Rico Amado c

a Federal University of Rio Grande do Sul, Materials Engineering Department, Av. Bento Gonçalves, 9500, 91501-970 Porto Alegre, RS, Brazilb São Paulo State University, Department of Materials and Technology, Av. Ariberto Pereira da Cunha, 333, 12516-410 Guaratinguetá, SP, Brazilc State University of Santa Cruz, Exact Science and Technology Department, Ilheus-Itabuna Highway, km 16, 45662-900 Ilheus, BA, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 February 2013Received in revised form 18 May 2013Accepted 10 July 2013Available online 19 July 2013

Keywords:A. HybridC. Micro-mechanicsD. Mechanical testingD. Non-destructive testing

The aim of this paper was to evaluate the effect of hybridizing glass and curaua fibers on the mechanicalproperties of their composites. These composites were produced by hot compression molding, with dis-tinct overall fiber volume fraction, being either pure curaua fiber, pure glass fiber or hybrid. The mechan-ical characterization was performed by tensile, flexural, short beam, Iosipescu and also nondestructivetesting. From the obtained results, it was observed that the tensile strength and modulus increased withglass fiber incorporation and for higher overall fiber volume fraction (%Vf). The short beam strengthincreased up to %Vf of 30 vol.%, evidencing a maximum in terms of overall fiber/matrix interface and com-posite quality. Hybridization has been successfully applied to vegetable/synthetic fiber reinforced poly-ester composites in a way that the various properties responded satisfactorily to the incorporation of athird component.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Glass fiber reinforced polyester composites are largely usedmainly due to a combination of low cost and good mechanicalproperties. However, in the last years, the growing environmentalawareness and new legislation boosted the use of bio-fiber rein-forced polymer composites. These composites have some advanta-ges in relation to glass or other synthetic/mineral fibers, since theyare environmentally friendly, and yield low abrasion in processing,together with low cost and specific gravity.

Sisal, flax, hemp, jute, banana and curaua are all natural fibersused to reinforce polymer composites (thermosets and thermo-plastics) [1]. Curaua is a natural fiber still poorly explored in rela-tion to sisal or jute, even though it shows excellent properties (inrelation to other natural fibers) such as elongation at break, tensilestrength and low density, providing good specific properties. Thecost of curaua fiber is similar to other vegetable fibers but curauais considered odorless as opposed to some other fibers [2]. In appli-cations with less strict mechanical loads such as car interior, furni-ture and acoustic insulation components, the curaua/polyestercomposites can be considered as an alternative to glass fiber com-posites. However, curaua fiber has also disadvantages, such as highmoisture absorption (like all vegetable fibers), inferior mechanical

properties (in relation to synthetic fibers), highly polar surfaces [3]and poor adhesion to most polymeric matrices. Thus, hybridizationof vegetable and glass fibers appears promising.

The factors influencing the final properties of hybrid compositesinclude: matrix nature, overall fiber content, relative fiber volume,fiber length, molding process, fiber–matrix interface, among others[4]. Until now, there are only a few studies in the literature focus-ing on this combination. John and Naidu [5] evaluated the tensilebehavior of sisal/glass/polyester hybrid composites and reportedhigher properties for higher glass fiber incorporation and overall fi-ber volume fraction. Cicala et al. [6] studied some hybrid synthetic/natural composites and reported that hybrid glass/hemp/epoxycomposites showed good tensile properties and lower cost andweight compared to pure glass/epoxy composites. Mishra et al.[7] introduced a small amount of glass in PALF and in sisal polyes-ter composites and obtained improved mechanical performancefor both.

Hybridization is usually classified into interlaminate and intra-laminate. Interlaminate, or simply laminate, consists in depositinglayers made of different fibers, whereas, in intralaminates, both fi-bers are entangled within a single layer. Silva et al. [8] studiedinterlaminate curaua/glass hybrid composites, whereas AlmeidaJúnior et al. [9] and Amico et al. [10] studied curaua/glass and si-sal/glass intralaminate composites, respectively.

The scope of this study is to evaluate the mechanical behavior ofpure curaua and pure glass fiber composites and to compare them

J.H.S. Almeida Júnior et al. / Composites: Part B 55 (2013) 492–497 493

with intralaminate hybrid curaua/glass fiber composites with dis-tinct fiber curaua/glass content and overall fiber content. Theoret-ical predictions based on micromechanical analysis of hybridswere included for comparison.

2. Materials and methods

2.1. Materials

The following materials were used:

– Curaua fiber (interlaced rope), from a cooperative located at thePara State of Brazil. Curaua composition is, in weight, around74% cellulose, 7.5% lignin, 10% hemicellulose, 1% ashes and 9%moisture [11].

– Glass fiber roving ME 3050 (areal density of 4000 tex), fromOwens Corning.

– Commercial unsaturated and accelerated orthophthalic polyes-ter resin ARAZYN� 13.0 (supplied by Ashland).

– Acetyl acetone peroxide (AAP), as initiator, and degassing agentA515 (BYK).

2.2. Composite molding

The curaua fiber rope was disentangled and classified. Both fi-bers (curaua and glass) were cut to 50 ± 2 mm in length. This isaround 10 times the curaua fiber critical length (in polyester)[12]. The chopped fibers were randomly distributed in a pre-mold(with the same dimensions of the mold) and compacted usinghydraulic press (Marconi – MA 098/A3030) for 30 min at 3 tonand room temperature. All mats, except the pure glass one, weredried in an oven at 105 �C for 30 min prior to molding.

The resin system was prepared by adding 2 wt.% of curing agentand 2 wt.% of degassing agent, in relation to the polyester. Bothwere followed by 2 min of manually stirring for homogenization.The resin was then submitted to ultrasonic bath at room tempera-ture for 5 min. The composites were produced by compressionmolding under 6 ton, at 90 �C for 75 min, followed by post-curingat 80 �C for 120 min in an oven with air circulation. The overallfiber volume fraction (%Vf) was varied within 20–40% and the cur-aua/glass fiber ratio also was varied. Table 1 shows the samplecodes and fiber contents.

2.3. Characterization

The rule of mixtures (Eq. (1)) was used to estimate the densityof the composites:

qc ¼ ½ðCcu � qcuÞ þ ðCg � qgÞ� � Vf þ ðqm � VmÞ ð1Þ

where qcu, qg and qm are the density of curaua (1.38 g cm�3), glass(2.54 g cm�3) and matrix (1.18 g cm�3), respectively [9]; Ccu and Cg

Table 1Codes and composition of all studied composites.

Sample Overall Vf (%) Curaua fiber contentratio – Ccu (vol.%)

Glass fiber contentratio � Cg (vol.%)

20/100/0 20 100 020/30/70 20 30 7020/0/100 20 0 10030/100/0 30 100 030/70/30 30 70 3030/50/50 30 50 5030/30/70 30 30 7030/0/100 30 0 10040/100/0 40 100 040/30/70 40 30 7040/0/100 40 0 100

are curaua and glass fibers relative content; Vf and Vm are the over-all volume fraction of fiber and matrix, respectively.

Tensile tests were carried out using an Instron 3382 universaltesting machine with a 100 kN load cell. Seven samples (dimen-sions: 170 � 25 � 3 mm) were tested at constant speed(2 mm min�1), according to ASTM D3039-08 standard. Three-pointbending flexural tests were carried out in the same equipmentusing eight samples (127 � 12.7 � 3 mm) with span-to-depth ratioof 16:1, at 2 mm min�1, following ASTM D790-10 standard. TheIosipescu shear test was performed in a Shimadzu (AG-X model)testing machine, with load cell of 50 kN and test speed of0.5 mm min�1. Three rectangular specimens (76 � 20 � 3 mm)with two symmetrical centrally located V-notches were tested,according to ASTM D5379-05 standard.

The short-beam shear tests (ILSS) was performed according toASTM D2344-00 standard, using span-to-depth ratio of 4:1 andtest speed of 1 mm min�1, in an EMIC DL3000 (load cell of200 kN) testing machine. Twenty samples were tested for eachfamily and, as per this ASTM standard, sample length is 6 � thick-ness and width is 2 � thickness. The fractured samples were ob-served using scanning electron microscopy (SEM) in a JEOL(Model 6060) equipment.

The elastic properties of non-used Iosipescu specimens werealso evaluated using non-destructive testing based on impulseexcitation of vibration, carried out in Sonelastic� (ATCP EngenhariaFísica) equipment. In this technique, the specimen under testing issubjected to a mechanical tap and reacts emitting an acoustic re-sponse. The elastic moduli are calculated from the natural frequen-cies of vibration extracted from the acoustic response by FastFourier Analyses. The specimen emits an acoustic response whichdepends on its elastic properties, dimensions and mass, and theelastic (E-modulus) [13] can then be obtained using Eq. (2), accord-ing to ASTM E1876-09 standard.

E ¼ 0:9465mf 2

f

w

!Lt

� �3

T and T ¼ 1þ 6:858tL

� �2

ð2Þ

where m is the mass, ff the fundamental resonance frequency inflexure, L the length, w the width, t the thickness and T the correc-tion factor.

3. Results and discussion

3.1. Tensile properties

The stress–strain curves of the 20% %Vf samples are presented inFig. 1. It can be seen that the hybrid samples presented intermedi-ate behavior in relation to the pure composites. The glass fibercomposite (sample 20/0/100) shows higher tensile strength, mod-ulus and elongation at break than the curaua one.

Fig. 2 shows the tensile properties found for all studied compos-ites. The tensile strength of the neat polyester resin was of45.7 MPa and the addition of 20 vol.% of fiber yielded a significantincrease in strength but only if at least some of this fraction wascomprised of glass fiber. This is due to its higher strength and mod-ulus and better adhesion to the matrix in relation to the curaua fi-ber [14].

The composites with 20 vol.% showed high standard deviationin general, probably since the low fiber content leads to resin-richregions in the mats. The samples with 30 vol.% presented an inter-mediate behavior, and samples with 40 vol.% yielded better results,showing lower standard deviation and higher strength, mainly forthe hybrid and the pure glass fiber composites. It is also interestingto see that the 30/30/70 and 30/0/100 samples were in the samerange of strength (considering the deviation), and that the higherthe overall amount of fiber, the more difficult it is for the hybrid

Fig. 1. Stress–strain plots for composites with %Vf of 20%.

494 J.H.S. Almeida Júnior et al. / Composites: Part B 55 (2013) 492–497

to match the pure glass composite since this is a fiber-dominatedproperty.

In Fig. 2, the specific strength shows an increasing trend forhigher glass fiber content, even though this fiber has higher density(meaning specific gravity) than curaua. Indeed, density increaseswith the overall fiber content, and with the relative glass content.In addition, it is interesting to notice that specific strength (i.e.strength/density) increased more clearly with more effective rein-forcement (glass fiber) in each %Vf. Among the hybrid specimens,the 30/30/70 presented higher specific strength.

It is hard to find any papers in the literature on the theoreticalproperties of natural/synthetic random short fiber hybrid compos-ites. For random in-plane short fiber composites, the microme-chanical model developed by Cox and Krenchel [15,16] is one ofthe most commonly used. This model includes a limiting stresstransfer efficiency factor named gL, based on fiber length and otherfactors, and a parameter to account for the type of fiber orientation,named go [17]. Cox [15] introduced the gL factor in the rule of mix-ture aiming to account for a relatively ineffective loading of fibersalong its stress transfer length in short fiber composites (comparedto continuous fibers) [17]. Krenchel [16] introduced go to adjust themodel when the fiber orientation is not exclusively that of theloading, being equal to 0.375 for random in-plane composites.

In this work, the Cox and Krenchel model was adapted to intra-laminate hybrid composites, as follows:

Fig. 2. Tensile strength and specific strength of the neat resin and studiedcomposites.

Ec ¼ goVf ðgLgCgEg þ gLcCcuEcuÞ þ ð1� Vf ÞEm ð3Þ

where Ec, Eg, Ecu and Em are the modulus of composite, glass fiber(73 GPa [2]), curaua fiber (36 GPa [3]) and matrix (2.1 GPa [thiswork]), respectively. The parameters gLg

and gLcare the length effi-

ciency factor of the glass and curaua fibers, respectively.Input data for this model include l (fiber length), d (fiber diam-

eter, 16 lm and 72.4 lm for glass and curaua, respectively [thiswork]), Gm (shear modulus of the matrix, 789 MPa, for Poisson’scoefficient of 0.33 [this work]) and Xi, which depends on the geo-metrical arrangement of fibers. The Xi factor was considered equalto 4.0 (the same for square packing), based on Thomason and Vlug[18] and Robinson and Robinson [19]. Full description of this mod-el can be found in the literature [17].

The Halpin–Tsai model, widely used to predict the elastic mod-ulus of composites, was also employed for comparison. The elasticmodulus can be estimated by:

Ec ¼38

E�1 þ58

E�2 ð4Þ

where E�1 and E�2 are the predicted modulus for short unidirectionalfibers in the longitudinal and transverse directions. The full descrip-tion can be found in the literature [20].

The theoretical modulus predictions based on Cox–Krencheland Halpin–Tsai model for isotropic in-plane composites and theobtained experimental results are shown in Fig. 3. It can be ob-served that the same trend was found for all of them, i.e. stiffnessincreases with glass fiber incorporation. Also, the predictions weregenerally close to the experimental results. It is also worth notingthat incorporation of curaua (100% in weight) led to 221% increasein tensile modulus (in relation to the pure resin) and that stiffnessof the 20/30/70 and 30/30/70 hybrid composites were similar tothe respective pure glass fiber composites. Both theoretical modelsyielded the same trend, but experimental data adjusted better tothe Cox–Krenchel theory.

In polymer composites, adding fibers does not always improvethe mechanical properties, because there should be sufficient ma-trix to transfer load to the fibers. Besides, quality of the compositesvaries with %Vf due to experimental conditions (mat productionand composite molding). In the case of this work, the elastic mod-ulus of the composites with 30% overall fiber content showed opti-mized behavior.

The elastic modulus (E) of the hybrid composites were alsoevaluated with nondestructive testing (NDT), and the results

Fig. 3. Experimental elastic modulus and theoretical values from micromechanicsfor the studied composites.

Fig. 5. Short beam strength behavior of all studied composites.

J.H.S. Almeida Júnior et al. / Composites: Part B 55 (2013) 492–497 495

are included in Fig. 3. The elastic modulus presented the samegeneral trend obtained with tensile testing, i.e. the elastic modu-lus increased for higher overall fiber content and for higher glassfiber content. Modulus increased up to %Vf of 30 vol.%, and thehybrid 30/30/70 presented the highest modulus among thehybrids. It is important to mention that the values obtained withimpulse excitation technique may be somewhat different fromthose obtained from tensile tests, since they are based on differ-ent principles.

3.2. Flexural and shear properties

Flexural strength of the composites is shown in Fig. 4a. In theflexural test, some mechanisms act simultaneously such as tension,compression and shearing. In general, it can be seen that the hybridcomposites showed intermediate behavior between the pure cur-aua fiber and the pure glass fiber composites. Some fluctuationsin strength can be credited among others to thickness variationsin the samples.

Flexural modulus (Fig. 4b) showed a similar trend, i.e. an in-crease with glass fiber content. But, in this case, the increase withthe overall fiber content was clearer. The increase in flexural mod-ulus for higher %Vf may be related to good matrix–fiber stresstransfer [14], indicating good interaction between them. It can alsobe noticed that the 40/30/70 hybrid composite showed similarmodulus than the pure glass fiber composite with 30% overall fibercontent. These results are in accordance to those obtained by Idicu-la et al. [1] who obtained higher flexural modulus for higher %Vf

and for higher content of the more rigid fiber in the hybrid com-posite (with sisal and banana fibers).

The short beam test, formerly known as interlaminar shearstrength (ILSS), may be used to evaluate interfacial strength andmay also indirectly address fiber/matrix adhesion and void content[21]. Fig. 5 shows that short beam strength (SBS) of the compositesincreased with glass fiber incorporation within each group. Thisbehavior is due to the more complete fiber wetting and stronger fi-ber/matrix adhesion obtained with the synthetic fiber in relation tothe curaua fiber, which has hydrophilic character, yielding lowercompatibility.

SBS also increased with overall fiber content but only up to30 vol.%, decreasing afterwards. SBS is primarily dependent onthe matrix properties and the fiber/matrix interfacial propertiesrather than the fiber properties [22]. The decrease in SBS for40 vol.% specimens may be due to a large amount of fiber–fiber

Fig. 4. Flexural strength (a) and modu

contact and because poorer quality composites are obtained withthe molding technique used (i.e. compression molding) when highfiber content is employed, which is likely to lead to larger void con-tent. This is expected to have a more pronounced detrimental ef-fect in properties like SBS and in-plane shear.

The failure in short beam testing occured suddenly, with hardlyno prior damage. In most specimens, small interlaminar longitudi-nal cracks were noticed in the mid-plane, differently from a singlemajor crack through the specimen length, as reported for lami-nates [23], as shown in the micrographs of Fig. 6. Fiber length influ-ences SBS and the 50 mm length is adequate for good matrix/curaua fiber stress transfer perhaps due to the extent of fiber bridg-ing effect during crack propagation [24,25]. This is in the samerange of length (45–60 mm) reported by Silva et al. [25] for maxi-mum SBS.

The Iosipescu shear strength of the composites is presented inFig. 7. These results did not show a clear trend, and most of thecomposites presented shear strength within the 20–24 MPa range,with relatively large deviation. Variations in specimen thicknessand experimental difficulties (e.g. related to machining the sym-metrical notches of specific dimensions when vegetable fibers areused) may have negatively influenced these results.

lus (b) of the studied composites.

Fig. 6. Scanning electron microscopy images of short beam fractured samples.

Fig. 7. Iosipescu shear strength of the studied composites.

496 J.H.S. Almeida Júnior et al. / Composites: Part B 55 (2013) 492–497

4. Conclusions

The scope of this study was to evaluate the mechanical proper-ties of natural (curaua) and synthetic (glass) fibers reinforced poly-ester composites with distinct overall fiber volume fraction andcuraua/glass fiber ratio. The tensile strength and elastic modulusincreased upon glass fiber incorporation and for higher %Vf. Thisbehavior is mainly due to the higher strength and stiffness of theglass fiber in relation to curaua and the better adhesion of the for-mer with the polyester resin.

The theoretical predictions based on the micromechanical Cox–Krenchel and Halpin–Tsai models for stiffness of in-plane isotropiccomposites ratified the experimental results. Hybridization wasconsidered successful since replacing 30 vol.% of the glass by cur-aua fiber produced mostly similar results. The specific stiffness re-sults also favored the hybrid composites. The specific tensilestrength increased for higher fiber volume fraction, evidencinggood fiber–matrix load transfer within the studied range of %Vf.The elastic modulus was also evaluated with a nondestructive testbased on vibration excitation technique. The obtained elastic mod-ulus was similar to that of the tensile test, increasing upon incor-poration of the more rigid fiber and for higher %Vf (up to 30 vol.).

Regarding short-beam shear behavior, strength increased up to30 vol.%. This was considered the optimum fiber content, showinggreater fiber/matrix interface and an optimum balance betweenthe two fibers and the resin. Whereas the in-plane shear strength

did not show a clear trend probably due to experimental difficul-ties related to the notching of the specimens.

In all, hybridization was successfully carried out and themechanical properties presented intermediate behavior betweenthe pure glass and pure curaua composites, sometimes close tothe pure glass fiber composites. Among the studied composites,the 30 vol.% overall content with 30% of curaua and 70% glass fibershowed optimum overall performance. Also, the specific propertieshighlighted the fiber hybridization advantage, which is able tocombine lower weight and high mechanical performance, beingalso more environmentally friendly in this case.

Acknowledgments

The authors would like to thank FAPESB, FAPESP, FAPERGS andCNPq for the financial support of this research.

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