www.elsevier.com/locate/compositesa

Composites: Part A 38 (2007) 1811–1820

Development and effect of alkali treatment on tensile propertiesof curaua fiber green composites

Alexandre Gomes a,*, Takanori Matsuo a, Koichi Goda b, Junji Ohgi b

a Graduate School of Science and Engineering, Yamaguchi University, Tokiwadai, Ube 755-8611, Japanb Department of Mechanical Engineering, Yamaguchi University, Tokiwadai, Ube 755-8611, Japan

Received 23 August 2006; received in revised form 7 March 2007; accepted 22 April 2007

Abstract

This paper describes development and improvement of mechanical properties of a so-called green composite that was fabricated byreinforcing a cornstarch-based biodegradable resin with high-strength natural fibers extracted from a plant named curaua. Two fabrica-tion methods are proposed, in which stretched slivers of curaua fibers are prepared as reinforcement to increase the composite strength.Moreover, highly concentrated alkali treatment was applied to curaua fibers to improve mechanical properties of green composites. Ten-sile test results showed that alkali-treated fiber composites increased in fracture strain twice to three times more than untreated fiber com-posites, without a considerable decrease in strength. This result proves that appropriate alkali treatment is a key technology forimproving mechanical properties of cellulose-based fiber composites.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Natural fiber composites; B. Fracture toughness; D. Mechanical testing; E. Surface treatment

1. Introduction

Establishment of disposal methods for glass fiber rein-forced plastics (GFRPs) and their recycling laws are impor-tant contemporary subjects because many environmentalproblems have appeared and worsened throughout theworld. As is widely known, GFRPs provide excellent ther-mal and mechanical properties. However, these propertiesmake it difficult to carry out suitable disposal processing.Furthermore, it is necessary to reduce environmentalimpacts, such as global warming, that are generated byconsumption of petroleum, a non-renewable resource.Therefore, increasingly numerous ecologically-aware stud-ies have pointed to practical applications, such as the useof alternative environmentally-friendly materials. The useof natural fiber reinforced plastics represents an attractive

1359-835X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesa.2007.04.010

* Corresponding author.E-mail address: b1280@yamaguchi-u.ac.jp (A. Gomes).

and suitable method for replacing GFRPs [1,2]. Naturalfibers are light and renewable; they are low-cost andhigh-specific-strength resource. For those reasons, naturalfiber composites have already been applied for fabricatingsome products such as furniture and architectural materi-als. Recently, they have gained widespread use in the auto-mobile industry. In their application, synthetic resins, suchas polypropylene and polyethylene, are commonly used asa matrix for natural fiber composites. However, those com-posites often display problems of fiber–matrix compatibil-ity which results in decrease of mechanical properties.Therefore, in order to improve the interaction betweenfiber and matrix, surface treatments are necessary for mod-ifying fibers’ morphology. Treatments using alkaline solu-tions have been applied by several researches [3–6] toimprove mechanical properties and fiber–matrix adhesionof natural fiber reinforced plastics such as polypropylene/flax, epoxy/flax, and polyester/kenaf. During alkali treat-ment, the fibers’ physical structure changes as a result ofalkali’s bleaching action which removes waxy materials,

Fig. 1. Chemical structures of caprolactone, PCL and D-fructose.

1812 A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820

and impurities. This action often leads to improvement ofthe interfacial bonding between fibers and matrix.

Despite the benefits brought from environmentally-friendly composites, it is anticipated however, that in thefuture, reduction of petroleum reserves will provoke adecrease towards the use of petroleum-based resins andprobably the use of plant-based biodegradable resins, suchas polylactic acid (PLA), will increase. Therefore, recently,there has been an increase in studies of natural-fiber fully-

green composites [7–11]. Based on this background, themain purpose our research is the development of fully-green composites with high-strength. This paper deals withfabrication of green composites consisting of cornstarch-based resin matrix and curaua fibers. Curaua fibers are leaffibers extracted from an Amazon-forest plant (Ananas erec-

tifolius) that resembles a pineapple plant. Curaua fibershave low-cost of production and offer a relatively high ten-sile strength level [12,13] which is necessary for practicalapplications. Furthermore, in the past 5 years curaua fibershave gradually gained importance in Brazilian economy.Volkswagen Corp. and Mercedes Benz Corp. have researchcenters which are interacting and cooperating withPOEMA (Poverty and Environment in Amazon) [14] pro-gram for developing and creating new jobs in the Amazo-nian region where curaua fibers are extracted. VolkswagenCorp. has already successfully applied curaua fibers mixedwith rejects from textile industry and polypropylene in theroof liner of vehicles such as VW Fox and VW Polo [15].For these reasons, curaua fibers are highly anticipated asa suitable reinforcing material for environmentally-friendlycomposites. Early in this study [12], we fabricated fully-green unidirectional composites, with low volume fraction(around 30%), by directly hot-pressing slivers of fibers andresin inserted in a metallic mold. This fabrication method isvery simple, low-cost and demands short time; however itis difficult to control the axial arrangement of fibers andthis fact can compromise the tensile strength of unidirec-tional composites. We named this fabrication process asdirect method. Tensile strength of composites was around130 MPa, which is insufficient for replacing some GFRPs.Therefore in the present study, to create green compositeswhich can achieve a higher tensile strength, we increasedfiber volume fraction of composites and also developedtwo new fabrication methods for producing morestraightly arranged fiber composites. In addition, in orderto improve fiber–matrix interface as well as mechanicalproperties of green composites, alkali treatment with10 wt% and 15 wt% sodium hydroxide was performed forcuraua fibers and subsequently alkali-treated compositeswere fabricated. The selection of 10% and 15% solutionswas based on the previous results achieved in this studywhich showed that curaua fibers treated in 10 wt% and15 wt% sodium hydroxide solutions displayed fracturestrain twice to three times higher than that of untreatedfibers.

2. Experimental procedure

2.1. Materials

2.1.1. Matrix

The matrix resin used in this study is a cornstarch-basedbiodegradable resin (Randy CP-300; Miyoshi Oil & FatCo. Ltd., Japan). The biodegradable polymer resin is ther-moplastic, has hydrophilic properties and is made from ablend of polycaprolactone (PCL) and cornstarch. Corn-starch is composed mainly of D-fructose which is a carbo-hydrate isomer to glucose with chemical structureC6H12O6. PCL is a biodegradable polyester with meltingpoint of 60 �C and glass transition temperature of �60 �C[16]. PCL is prepared by polymerization of a monomercalled caprolactone (also known as lactone). Fig. 1 showsthe chemical structures of caprolactone, PCL andD-fructose.

The cornstarch-based resin, which is supplied as anemulsion, comprises micro-order particles of approxi-mately 5 lm in diameter dispersed in a water-based solu-tion [17]. Table 1 shows some physical [18] andmechanical properties of this resin. The mechanical proper-ties of matrix resin were obtained by tensile testing ofmatrix specimens using cross-head speed of 1 mm/minand gage length equals to 50 mm.

2.1.2. Reinforcement

Curaua fibers were used as reinforcement in this study.Curaua fiber is composed of lignin (7.5%), glucan(66.4%), xylan (11.6%) and other materials such as mannan(0.1%), galactan (0.5%) and arabinan (0.5%) [19]. Fig. 2shows SEM photographs of fracture surface (a) andcross-sectional area (b) of curaua fibers. These photos exhi-bit that curaua fiber consists of a bundle of ultimate fibers,

Table 2Mechanical properties of curaua fibers

Density(Mg/m3)

Tensile strength(MPa)

Fracture straina

(%)Young’s modulus(GPa)

1.38 913 3.9 30

a Measured by laser–displacement system.

Table 1Mechanical and physical properties of cornstarch-based matrix

Density(Mg/m3)

Meltingpoint[18] (�C)

Waterabsorption[18] (%)

Adhesivestrengtha

[17] (MPa)

Tensilestrength(MPa)

Fracturestrain(%)

Young’smodulus(GPa)

1.16 58 2 7.2 10.6 6.5 0.531

a Bonded to Japanese cypress.

A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820 1813

with diameter of 9–10 lm, bonded as a result of their envel-opment by some materials such as lignin. Some mechanicalproperties of curaua fibers are shown in Table 2. Slivers ofcuraua fibers used in this study were supplied by POEMAof Para Federal University, Brazil.

2.2. Development of curaua fiber fully-green composites

A press-molding machine with two heaters was used forthe fabrication of the composites. In this study, three fab-rication methods were applied for developing green com-posites. One method, called the direct method (DM) wasapplied in our previous studies; however, the other twomethods, called pre-forming (PF) and prepreg sheet (PS)methods, are newly proposed. In the following lines, wepresent the particularities of those respective fabricationmethods.

(a) Direct method (DM). A sliver of curaua fibers isinserted into a metallic mold and the resin is poureddirectly into them. Thereafter, the material is pressedslightly at 150 �C for 1 h. Subsequently, the heatingprocess is stopped. During the cooling process a pres-sure of 3.27 MPa is applied to it until the temperaturenearly reaches room temperature. This fabricationprocess is schematically shown in Fig. 3a.

(b) Pre-forming method (PF). The composite is producedby hot-pressing pre-forms of resin-pasted fiber slivers.First, to arrange the fibers better than in DM, curauafiber slivers are wound and stretched around a metal-

Fig. 2. Fracture surface and cross-

lic plate. Next, resin is applied to the slivers using asmall brush. Finally, the pre-forms of fibers embed-ded in resin are dried at 30 �C for 24 h and cut intothe mold dimensions. An example of a fabricatedpre-form is shown in Fig. 4. Afterwards, a pair ofthe dried pre-forms is inserted into the metallic moldand pressed by 6.54 MPa at 150 �C for 1 h. The heat-ing process is then stopped and a pressure of13.1 MPa is applied to it until the temperature nearlyachieves room temperature. This process is depictedschematically in Fig. 3b.

(c) Prepreg sheet method (PS). In general, prepreg sheetsare prepared for fabrication of a laminated compositewith a quasi-isotropic property. This study showsthat such prepreg sheets can also be developed fromnatural fiber slivers. First, to make the arrangementof fibers better than that of pre-forming method, sliv-ers are placed on a metallic plate, stretched and resinis applied into them. Next, thin prepreg sheets areobtained by pressing slightly those resin-pasted sliv-ers one by one at 120 �C. Finally, the fabricated pre-preg sheets are cut to the desired dimensions insidethe metallic mold. The resultant sheet thickness is lessthan 1 mm. Fig. 5 shows before and after cuttingphotographs of a prepreg sheet. Afterwards, a set offive sheets, each with identical fiber orientation, isinserted in the mold and pressed by 3.27 MPa at150 �C for 1 h. Then, the heating process is stoppedand a pressure of 16.9 MPa is applied to the set untilthe temperature approaches room temperature. Thisprocess is shown schematically in Fig. 3c.

sectional area of curaua fiber.

.Prepreg sheets

(a) Direct method (b)Pre-forming method (c) Prepreg sheet method

Resin

Pre-formsCuraua fibers

Ram

Mold

Heating plates

P=3.27 MPa P=13.1 MPa P=16.3 MPa

Fig. 3. Fabrication processes of curaua fiber green composites.

Fig. 4. A pre-form of curaua fibers.

1814 A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820

The composites obtained from the above methods are100 mm long and 15 mm wide; their thickness varied from1 to 1.5 mm dependent on the fiber content. The fiber vol-ume fraction of all fabricated composites was calculatedusing the following equation:

V f ¼ 1� W � W f

qmVð1Þ

where W and V are, respectively, represent the weight andvolume of the fabricated composite. Wf is the weight of

Fig. 5. A prepreg shee

curaua fibers included in the composite and qm is the den-sity of the biodegradable resin. The average volume frac-tions of the fabricated composites were 0.693, 0.670 and0.699, respectively, for DM, PF and PS.

2.3. Tensile test of composites

Axial tensile tests were carried out for all fabricatedcomposites. To avoid stress concentration, before theexperiment, aluminum plates with edges angled at 45� wereattached with epoxy adhesive on both ends of all compos-ites. The gage length of the composite specimens was50 mm. A strain gage was fixed at the center of each spec-imen for measuring uniaxial strain and calculating Young’smodulus. Tensile tests were carried out for the compositespecimens using an Instron-type testing machine (Auto-graph IS-500; Shimadzu Co.). Cross-head speed of the test-ing machine was 1 mm/min, based on the tensile test

t of curaua fibers.

Fig. 6. Shape and dimensions of tensile test specimens.

Fig. 7. Surface of green composites fabricated by each method.

A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820 1815

method for carbon fiber reinforced plastics (JIS K 7073;Japan Industrial Standards). Fig. 6 shows the specimens’dimensions.

2.4. Development of alkali treated fiber fully-green

composites

In this study, alkali treatment was applied to curauafibers for improving the composites’ toughness and interfa-cial bonding between fibers and matrix. The treatment wascarried out by dipping slivers of fibers into a 10 wt% con-centrated sodium hydroxide (NaOH) solution for 2 h atroom temperature. After alkali treatment, the fibers werewashed for a few minutes using a 1 wt% acetic acid solu-tion. Finally, the fibers were washed with water and driedat room temperature for 24 h. The treated fibers were alsoused as reinforcement for the composites. These compos-ites were fabricated by PF and PS methods and also havebeen prepared as tensile test specimens with dimensionsidentical as those shown in Fig. 6. These specimens weretested in the same conditions described above.

3. Experimental results

3.1. Tensile properties of green composites fabricated using

the newly proposed methods

Table 3 shows results of tensile test of untreated-fibercomposites fabricated using DM, PF and PS methods.All values shown there are averages. The values of tensilestrength show that PS composites are the strongest, fol-lowed by PF composites. DM composites show the lowesttensile strength value. Fig. 7 shows the surfaces of compos-ites fabricated using the respective methods. This figure

Table 3Mechanical properties of curaua fiber green composites

Fabricationmethod

Number ofsamples

Volumefraction

Tensile strength(MPa)

Fractustrain

DM 5 0.693 216 1.53PF 0.670 275 1.24PS 0.699 327 1.16

clarifies that fiber arrangement on the surface of DM com-posites is not as good as those of the other two methods.This poor fiber arrangement is probably caused by shrink-age of resin that occurs because of evaporation of waterincluded in the resin. We consider, however, that theshrinkage of resin does not affect the fiber alignment ofPF and PS composites because the pre-forms of resin arecompletely dried in the case of PF composites and the sliv-ers are slightly stretched during the fabrication of PS com-posites. Fig. 8a–c shows micro-optical photographs ofcross-sectional areas of DM, PF and PS composites. Dis-persion of fibers in the cross-sectional area of specimensis quite similar. Therefore, it is inferred that the reasonfor the high tensile strength level achieved by PS compos-ites is mainly related to their improvement on fibers’ longi-tudinal alignment. For that reason, we can stronglyrecommend the PS method for developing a well-orientedfiber green composite. In addition, the Young’s modulusof PS composites was the highest; it achieved impressive36 GPa, which is a value comparable to that of someGFRPs [20] such as Polyester + 50% Glass fibers, whichhas a Young’s modulus of 38 GPa. The fracture strainvalue of DM composites was the highest, followed by thoseof PF and PS composites.

Fig. 9 shows typical stress–strain diagrams of the com-posites. The slope of the stress–strain diagram of PS com-posites is the highest, followed by those of PF and DMcomposites. Furthermore, the diagrams of both PF andPS composites display an almost linear stress–strain rela-tionship. In contrast, the stress–strain diagram of DMcomposites has a particular behavior in which its slopebecomes larger around 0.5% of strain, implying that fibers’alignment of DM composite can be changed toward theaxial direction during tensile test. In other words, it isinferred that fibers which are not oriented to 0� have theirorientation slightly improved by loading process and for

re(%)

Young’s modulus(GPa)

Specific strength(102 m)

Specific modulus(105 m)

13 162 9.629 207 2136 243 26

Fig. 8. Dispersion of fibers in the cross-sectional area of green composites.

0. 5 1 1. 5 2 2.5 3 3.5

100

200

300

400

0

Stre

ss (

MPa

)

Strain (%)

Direct Method (DM) Vf=74%Pre-Forming method (PF) Vf=67%Prepreg Sheets method (PS) Vf=77%

Fig. 9. Stress–strain diagrams of curaua fiber green composites.

1816 A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820

this reason the angle of the stress–strain diagram’ slopeincreases gradually.

The tensile strength of PS composites was 327 MPa,which is one of the highest values of strength even in thefield of natural fiber composites fabricated with syntheticresins, such as polypropylene. Therefore, we conclude thatPS method is a useful fabrication method for increasing thestrength of natural fiber green composites. Despite the highstrength of PS composites, this strength level remainsabout 50% of the tensile strength of some GFRPs suchas Epoxide + 70%Glass fibers [20]. That GFRP has frac-ture strain of 0.0178, which is also greater than the respec-tive 0.0153, 0.0124 and 0.0116 fracture strains of DM, PFand PS composites.

The value of fracture strain achieved by the green com-posites was quite lower in comparison to that of fibers. Thegreat difference between fracture strains of fibers in com-parison with those of the composites may be due to themethods applied for measuring fracture strain of fibers

Table 4Tensile properties of green composites reinforced by alkali-treated fibers

Treatment/fabrication method

Number ofsamples

Volumefraction

Tensile strength(MPa)

Fst

10 wt% NaOH/PF 5 0.631 276 2.10 wt% NaOH/PS 0.727 334 1.15 wt% NaOH/PS 0.759 300 3.

and composites. The fracture strain of fibers was measuredby a laser–displacement system and during tensile test, thedeformation of the adhesive which bonds the fiber to thepaperboard maybe have had a slight influence on the frac-ture strain achieved by single fibers. In other words, weguess that the fracture strain obtained from the experimentwas: fiber’s elongation + adhesive’s elongation. The gagelength used for testing singles fibers was 10 mm, howeverif we increase this gage length, to 50 mm for example, theadhesive’s elongation will not affect the value of fiber’sfracture strain so significantly. On the other hand, the frac-ture strain of composites was measured using a strain gageplaced at the center of each specimen. Therefore, we believethat the strain gage measured quite accurately the value offracture strain of green composites. Based on the fracturestrain achieved by PS composites with high volume frac-tion, we can say that the fracture strain of curaua fibersis some value between 1.1% and 1.5%.

3.2. Tensile properties of alkali-treated fiber composites

Table 4 shows tensile properties of green compositesreinforced by alkali-treated fibers. All values are shownas averages. The tensile strengths of PF and PS compositeswere, respectively, 276 MPa and 334 MPa. These values arealmost equal to that of their untreated-fiber composites.On the other hand, the fracture strain of alkali-treated fibercomposites was much larger than those of untreated-fibercomposites, especially for PF composites which achieved2.78% fracture strain against 1.24% of their untreated-fibercomposites. In general, the area under the stress–strain dia-gram is used for evaluating the degree of toughness. There-fore, it is concluded that 10 wt% NaOH alkali treatmentfor curaua fibers increases the composite toughness with-out any decrease in strength. However, Table 4 shows that

racturerain (%)

Young’smodulus (GPa)

Specific strength(102 m)

Specific modulus(105 m)

78 26 208 2074 32 246 2405 24 217 17

A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820 1817

PS composites did not achieve a satisfactory improvementin fracture strain. According to tensile test results of curauafibers [12], fibers treated in 15 wt% NaOH showed a largerfracture strain than that treated in 10 wt% NaOH solution.Consequently, it is expected that alkali treatment with amore concentrated solution would increase the fracturestrain of the composites. To test that assumption, PS com-posites reinforced by curaua fibers treated in 15 wt%NaOH solution for 2 h were additionally fabricated.Mechanical properties of these composites are also shownin Table 4. Tensile strength of 15 wt% treated fiber com-posites was lower than that of either the untreated fiberor 10 wt% treated fiber composites. This result might beattributable to a chemical structure change in the cellulosethat is inherent in the fibers because cellulose molecularchains in the microfibrils lose their crystalline structurelocally as a result of the alkali treatment. This crystallinestructure loss might be attributable to partial conversionof cellulose I into cellulose II, which occurs during mercer-ization [21,22]. Despite the slight decrease in tensilestrength, the fracture strain of treated fiber compositesincreased to almost three times that of untreated fiber com-posites. That great increase in fracture strain may also berelated to an increase in amorphous cellulose region ofcuraua.

Stress–strain diagrams of alkali-treated fiber compositesfabricated by PF and PS methods are shown respectively inFig. 10a and b, together with that of the untreated fibercomposites. The calculation of the areas below the stress–strain diagrams of PF and PF composites was done, andwe obtained the following results:

(a) The area under the stress–strain diagram of 10%NaOH treated-PF composites is 3.3 times larger thanthat of untreated-PF composites.

(b) The areas under the stress–strain diagrams of 10%and 15% NaOH-PS composites are 1.8 and 2.6 times,respectively, larger than that of untreated-PScomposites.

These results prove that toughness of green compositeswas greatly increased by highly concentrated alkali treat-

(a) Pre-forming - PF

0 0.5 1 1.5 2 2.5 3 3.50

100

200

300

400

Strain (%)

Stre

ss (

MPa

)

St.re

ss (

MPa

)

Untreated (Vf=67%)

10% Treated (Vf=55%)

Fig. 10. Stress–strain diagrams of: (a) pre-form

ment. The increase in toughness is directly related toimprovement in fracture strain which is achieved througha chemical change of the fiber’s structure. This changeincreases the amount of cellulose II in the portion of cellu-lose I, and the Young’s modulus of cellulose II [23], whichhas been measured in many studies, is lower than that ofcellulose I, implying that cellulose II deforms much morethan cellulose I. Furthermore, it was reported [24] thatalkali treatment with a high concentration promotes adeformation of individual microfibrils by breakage of inter-locking hydrogen bonds and removal of hemicellulose fromthe fiber. The absence of hemicellulose causes slippage ofcellulose microfibrils, thereby imparting a loosely boundstructure. In addition, the breakage of hydrogen bonds cre-ates many active hydroxyl groups that increase the fiber’shydrophilicity and this fact theoretically leads to improve-ment of water absorption that can cause increase in theplastic deformation of the fiber. Hence, we can understandthe reason for the increase in fracture strain of highly con-centrated alkali-treated fiber composites. Moreover, theYoung’s modulus of alkali-treated fiber compositesdecreased compared to that of untreated fiber composites.That is to say, the 10 wt% alkali-treated fiber PS compos-ites reached 32 GPa, a large value, followed by 10 wt%PF and 15 wt% alkali-treated fiber PS composites, respec-tively, with 26 GPa and 24 GPa. Finally, based on thestress–strain diagrams of PS composites, we can say thathighly concentrated alkali treatment, such as 15 wt%NaOH solution, provides greatly increase in toughnesswith slight decrease in tensile strength and Young’s modu-lus, whereas medium alkali treatment, such as 10 wt%NaOH solution, brings a relatively little improvement intoughness without a decrease in strength.

4. Discussion

Mechanical testing results showed that tensile strengthof 10 wt% alkali treated fiber composites is nearly equalto that of untreated fiber composites. However, the tensilestrength of 10 wt% alkali treated single fibers is much lowerthan that of untreated single fibers [12]. For example,untreated curaua fibers have tensile strength around

(b) Prepreg - PS

0.5 1 1.5 2 2.5 3 3.5

100

200

300

400

0Strain (%)

Untreated (Vf=74%)10% Treated (Vf=74%)15% Treated (Vf=78%)

ing (PF) and (b) prepreg composites (PS).

1818 A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820

900 MPa, whereas curaua fibers treated for 2 h in 10 wt%NaOH solution have tensile strength of 551 MPa. Further-more, taking under consideration the theory of the rule ofmixtures, the strength of untreated composites at 70% fibervolume fraction is about 630 MPa while the experimentaldata showed values around 327 MPa. On the other hand,the theoretical strength of 10 wt% NaOH treated compos-ites is 385 MPa which is a result quite close to 334 MPaobtained from tensile test. In other words, we can say thatthe tensile strength level achieved by untreated and alkali-treated fiber composites is not attributable only to fibers’tensile strength but rather to the following factors:

(1) improvement of interfacial bonding between fibersand matrix;

(2) decrease in the coefficient of variation of alkali-trea-ted fibers’ strength.

Below, these two factors are discussed in detail.

4.1. Improvement of interfacial bonding

Our previous results show that a 20% reduction occursin curaua fiber’s weight during 10 wt% alkali treatment,reflecting the removal of lignin and other materials. Inaddition, alkali treatment theoretically improves fiber’shydrophilicity, which can create a better interfacial interac-tion with the present hydrophilic resin. Furthermore, weobserved using SEM that more fibers were pulled out fromthe matrix of untreated fiber composites than from alkali-

Fig. 11. Fracture surfaces of transverse curaua fiber green composit

Table 5Mechanical properties of transverse curaua fiber green composites

Treatment Numberofsamples

Volumefraction

Tensilestrength(MPa)

Fracturestrain(%)

Young’smodulus(GPa)

Untreated 5 0.428 7.48 1.39 1.1510 wt%

NaOH0.412 8.62 1.75 1.07

treated fiber composites, which suggests a lack of bondingbetween fibers and the matrix of untreated fiber compos-ites. To confirm our assumption of improved interfacialbonding by means of alkali treatment, we carried out trans-verse tensile tests of unidirectional composites reinforcedwith untreated and 10 wt% alkali-treated fibers. Transversetensile test specimens of the composites were obtained bycutting, transversally, a unidirectional 100 mm · 100 mmcomposite fabricated using PF method. The specimens’dimensions were identical to those shown in Fig. 6. Tensiletests were performed in the same conditions mentionedabove. Table 5 shows the results; all values are averages.Table 5 indicates that the tensile strength of alkali-treatedfiber composites is higher than that of untreated fiber com-posites, indicating definitely that interfacial strength of cur-aua fiber green composites was improved by highlyconcentrated alkali treatment. Young’s moduli are almostequal for untreated and treated-fiber composites, which isa completely understandable result, considering that theinitial deformation behavior of the composites must beidentical because no debonding occurs at this stage. Inaddition, Fig. 11 shows a great difference of fracture mor-phologies between untreated (a) and treated fiber compos-ites (b). Apparently, untreated fiber composites have a lackof interfacial strength that gives them a flat fracture surfacecreated by rapid debonding of fibers from the matrix. Onthe other hand, the fracture surface appearance of the trea-ted-fiber composite shows that many fibers are extractedfrom inside of the composite, suggesting that fibers are wellbonded to the matrix. Therefore, fracture strains of treatedfiber composites were higher than those of untreated fibercomposites. Finally, we consider that:

(a) Physically: alkali treatment with highly concentratedsolutions, removed lignin and hemicellulose from thefiber and caused fiber fibrillation which increased thearea available for contact with the matrix. Hence,the resin was able to penetrate much better throughthe fiber’s surface and this fact improved the fiber–matrix interaction at the composite’s interface.

es: (a) untreated and (b) 10 wt% alkali treated fiber composites.

A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820 1819

(b) Chemically: the biodegradable resin used in this studyis fabricated from cornstarch. Cornstarch is com-posed of D-fructose which is a carbohydrate withmolecules possessing straight-chain structure withmany hydroxyl groups which counts for the hydro-philicity of the resin. The untreated fiber is alsohydrophilic because of the OH groups included inthe structure of cellulose. However, after alkali treat-ment the hydrogen bonding network will be brokenand the hydroxyl groups of cellulose will becomemore active and this fact improves the hydrophilicityof the fiber as well as its compatibility with the hydro-philic resin.

4.2. Decreased variation of fibers’ strength

Past reliability strength models of unidirectional com-posites have shown that composites reinforced by fibersthat vary greatly in strength exhibit less strength [25,26].This theoretical remark has been supported throughnumerical simulations [27]. According to our previousresults, the coefficient of variation in strength of curauafibers decreases through alkali treatment. For example,the coefficient of variation in strength of untreated curauafibers was 43.16%, whereas those of fibers treated in10 wt% NaOH for 1 h and 2 h were, respectively, 24.95%and 25.52%. On the other hand, the coefficients of variationin fiber diameter of alkali-treated fibers showed almostequal values to that of untreated fibers. These facts indicatethat the variation in strength of curaua fibers decreases lar-gely through alkali treatment. Consequently, we infer thatthe lower variation in strength of reinforcing fibers isanother reason that contributes for maintaining the tensilestrength of 10 wt% alkali-treated fiber composites at thesame level of that of untreated fiber composites.

5. Conclusions

We developed two new fabrication methods for curauafiber green composites: pre-forming (PF) and prepregsheets (PS) methods. Subsequently, we evaluated theirmechanical properties. The tensile strength of PS compos-ites was the highest; it achieved 327 MPa on average. TheYoung’s modulus of PS composites was 36 MPa, which isa value that is comparable to those of some glass fiber com-posites. In addition, alkali-treated fiber green compositeswere developed using PF and PS methods. Tensile strengthof 10 wt% alkali-treated fiber composites was almost thesame as those of untreated-fiber composites, whereas thefracture strain of alkali-treated PF composites increasedto twice that of untreated-fiber composites. Fracture strainof 15 wt% treated fiber PS composites increased to threetimes that of their untreated-fiber composites without a sig-nificant decrease in strength. These results underscore thathighly concentrated alkali treatment is a suitable methodfor improving fracture strain and toughness of curaua fiber

green composites. Ultimately, based on the young’s modu-lus achieved by PS composites (36 GPa), it is expected thatgreen composites with high modulus will be a strong candi-date for replacement of glass fiber composites. However, inorder to make possible the practical application of fully-green composites in humid environments, some kind offiber treatment (for example, silane treatment [28]) is neces-sary to avoid damages caused by moisture absorption fromthe environment; otherwise the mechanical properties ofthe green composites may decrease.

Acknowledgements

The authors thank Miyoshi Oil & Fat Co. Ltd (Japan)as well as Mr. Wilson Moura from POEMA of Para Fed-eral University (Brazil) that kindly supplied, respectively,the biodegradable resin (Randy CP-300) and curaua fibersused for fabricating the green composites developed in thisstudy at Yamaguchi University, Japan.

References

[1] Wambua P, Ivens J, Verpoest I. Natural fibres: can they replace glassfibre reinforced plastics?. Compos Sci Technol 2003;63:1259–64.

[2] Joshi SV, Drzal LT, Mohanty AK, Arora S. Are natural fibercomposites environment superior to glass fiber reinforced compos-ites?. Compos Part A: Appl Sci Manufact 2004;35:371–6.

[3] Rong MZ, Zhang MQ. The effect of fiber treatment on themechanical properties of unidirectional sisal-reinforced epoxy com-posites. Compos Sci Technol 2001;61:1437–47.

[4] Bledzki AK, Fink H-P, Specht K. Unidirectional hemp and flaxEP- and PP-composites: influence of defined fiber treatments. J ApplPolym Sci 2004;93:2150–6.

[5] Van de Weyenberg I, Truong Chi T, Vangrimde B, Verpoest I.Improving the properties of UD flax fibre reinforced composites byapplying an alkaline fibre treatment. Compos Part A: Appl SciManufact 2005. Available online at: www.sciencedirect.com.

[6] Aziz SH, Ansell MP, Clarke SJ, Panteny SR. Modified polyesterresins for natural fibre composites. Compos Sci Technol2005;65:525–35.

[7] Netravali AN, Chabba S. Composites get greener. Mater Today2003:22–9.

[8] Oksman K et al. Natural fibers as reinforcement in polylactic acid(PLA) composites. Compos Sci Technol 2003;63:1317–24.

[9] Lee S, Wang S. Biodegradable polymers/bamboo fiber biocompositewith bio-based coupling agent. Compos Part A: Appl Sci Manufact2006;37:80–91.

[10] Mwaikambo LY, Ansell MP. Hemp fibre reinforced cashew nut shellliquid composites. Compos Sci Technol 2003;63:1297–305.

[11] Nishino T, Hirao K, Kotera M, Nakamae K, Inagaki H. Kenafreinforced biodegradable composite. Compos Sci Technol2003;63:1281–6.

[12] Gomes A, Goda K, Ohgi J. Effects of alkali treatment to reinforce-ment on tensile properties of curaua fiber green composites. JSME IntJ 2004;47:541–6.

[13] Gayer U, Schuh Th. Automotive application of natural fiberscomposite. In: Proceedings of first international symposium onlignocellulosic composites. Brazil: UNESP-Sao Paulo State Univer-sity; 1996.

[14] <http://www.amazonia.org.br/english/guia/detalhes.cfm?id=13597&tipo=6&cat_id=84&subcat_id=410>.

[15] <www.volkswagenag.com/vwag/vwcorp/content/en/sustainability_and_responsibility/achievements/environment/materials.html>.

1820 A. Gomes et al. / Composites: Part A 38 (2007) 1811–1820

[16] Fujii T, Nishino T, Goda K, Okamoto T. Development of environ-mentally friendly composites. High Technol Information 2005:04[in Japanese].

[17] <http://www.ctiweb.co.jp/cti/sinkinou/10_miyoshi.html> [in Japanese].[18] Ochi S, Takagi H, Tanaka H. Mechanical properties of cross-ply

green composites reinforced by manila hemp fibers. In: Proceedingsof first international workshop on green composites. Tokushima,Japan; 2002.

[19] Kelley SS, Rowell RM, Davis M, Jurich CK, Ibach R. Rapid analysisof the chemical composition of agricultural fibers using near infraredspectroscopy and pyrolysis molecular beam mass spectrometry.Biomass Bioenerg 2004;27:77–88.

[20] Mathews FL, Rawlings RD. Composite materials: engineering andscience. WoodHead Publishing Limited; 1999, p. 40.

[21] Okano T, Nishiyama Y. Morphological changes of ramie fiber duringmercerization. J Wood Sci 1998;44:310–3.

[22] George J, Ivens J, Verpoest I. Surface modification to improve theimpact performance of natural fibre composites. In: Proceedings ofICCM12, France; 1999.

[23] Bledzki AK, Gassan J. Composites reinforced with cellulose basedfibres. Prog Polym Sci 1999;24:221–74.

[24] Goda K, Sreekala MS, Gomes A, Kaji T, Ohgi J. Improvement ofplant based natural fibers for toughening green composites – effect ofload application during mercerization of ramie fibers. Compos partA: Appl Sci Manufact 2006;37:2213–20.

[25] Smith R, Phoenix SL, Greenfield MR, Henstenburg RB, Pitt RE.Lower-tail approximations for the probability of failure of three-dimensional fibrous composites with hexagonal geometry. Proc. Roy.Soc. Lond. 1983;A388:353–91.

[26] Goda K. A strength reliability model by Markov process ofunidirectional composites with fibers placed in hexagonal arrays.Int J Solids Struct 2003;40:6813–37.

[27] Goda K, Fukunaga H. Considerations of the reliability of tensilestrength at elevated temperature of unidirectional metal matrixcomposites. Compos Sci Technol 1989;35:181–93.

[28] Bisanda ETN, Ansell MP. The effect of silane treatment on themechanical and physical properties of sisal-epoxy composites. Com-pos Sci Technol 1991;41:165–78.