Composites: Part B 42 (2011) 1648–1656

Contents lists available at ScienceDirect

Composites: Part B

journal homepage: www.elsevier .com/locate /composi tesb

Effect of alkali treatment on interfacial and mechanical properties of coir fiberreinforced poly(butylene succinate) biodegradable composites

Tran Huu Nam a,⇑, Shinji Ogihara a, Nguyen Huy Tung b, Satoshi Kobayashi c

a Department of Mechanical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japanb Polymer Center, Hanoi University of Technology, No. 1, Dai Co Viet, Hanoi, Viet Namc Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan

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

Article history:Received 6 January 2011Received in revised form 3 March 2011Accepted 7 April 2011Available online 12 April 2011

Keywords:A. Polymer–matrix composites (PMCs)B. Interface/interphaseB. Mechanical propertiesE. Surface treatments

1359-8368/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.04.001

⇑ Corresponding author. Tel.: +81 (0)4 7124 1501x3E-mail addresses: trannam@rs.noa.tus.ac.jp, thnam

The poly(butylene succinate) (PBS) biodegradable composites reinforced with coir fibers were developed.The effect of alkali treatment on the surface morphology and mechanical properties of coir fibers, inter-facial shear strength (IFSS) and mechanical properties of coir fiber/PBS composites was studied. The effectof fiber mass content varying from 10% to 30% on the mechanical properties of coir fiber/PBS compositeswas also investigated. The coir fibers which are soaked in 5% sodium hydroxide solution at room temper-ature (RT) for 72 h showed the highest IFSS with 55.6% higher than untreated coir fibers. The mechanicalproperties of alkali-treated coir fiber/PBS composites are significantly higher than those of untreatedfibers. The best mechanical properties of alkali-treated coir fiber/PBS composite were achieved at fibermass content of 25% in this study, which showed an increase of tensile strength by 54.5%, tensile modulusby 141.9%, flexural strength by 45.7% and flexural modulus by 97.4% compared to those of pure PBS resin.The fiber surface morphologies and fractured surface of the composites exhibited an improvement ofinterfacial fiber–matrix adhesion in the composites reinforced with alkali-treated coir fibers.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In the past decade, natural fiber composites based on petroleum-based thermoplastics or thermosets matrices have been used invarious industrial sectors, especially in automobile industry suchas door panels, seat backs, headliners, package trays, dashboards,and interior parts [1,2]. However, these natural fiber compositesare not fully environmentally friendly because matrix resins arenon-biodegradable [3]. Therefore, biodegradable composites basedon natural fibers and biodegradable polymeric matrix made fromcellulose, starch, and other natural resources are called ‘‘green com-posites’’ and have been developed because of their environmentallybeneficial properties [4–8]. In general, the research and develop-ment of natural fiber biodegradable composites from renewable re-sources for a wide range of applications is increasing due to theiradvantages, such as eco-friendliness, lightweight, carbon dioxidereduction and biodegradable characteristics.

The commercial natural fibers such as henequen, hemp, jute,kenaf, sisal, flax, bamboo, coir, banana, palm, silk, cotton and woodare renewable resources in many developing countries. Thesefibers offer specific benefits such as low cost, low density, lowpollutant emissions, acceptable specific properties, renewable

ll rights reserved.

917; fax: +81 (0)4 7123 9814..hut@gmail.com (T.H. Nam).

characteristics, enhanced energy recovery, and complete biode-gradability [9–12]. They are considered as strong candidates to re-place the conventional glass fibers due to eco-friendliness, lowcost, renewable resources and biodegradability. Among the naturalfibers, plant fibers which contain strongly polarized hydroxylgroups are hydrophilic in nature [13]. These fibers are inherentlyincompatible with hydrophobic thermoplastics. Furthermore, dueto the presence of pendant hydroxyl and polar groups in variousconstituents of fibers, moisture absorption of fibers is very highand leads to poor interfacial bonding with the hydrophobic matrixpolymers. Therefore, it is necessary to decrease the moistureabsorption and hydrophilic character of fibers by suitable surfacechemical modification [14–17].

Among the plant fibers, coir fibers are nowadays extensivelyused in many industrial applications. Coir is a versatile lignocellu-losic fiber extracted from the tissues surrounding the seed of coco-nut palm (Cocos nucifera). Coir consists of cellulosic fibers withhemicellulose and lignin as the bonding materials for the fibers.Table 1 summarizes several physical, chemical and mechanicalproperties of coir fiber compared with other typical natural fiberssuch as flax, hemp, jute, ramie and sisal [2,11,18]. Coir fiber haslow cellulose and hemicellulose, high lignin content and highmicrofibrillar angle compared with other natural fibers (Table 1).As a result tensile strength and Young’s modulus of coir fiber arelower than those of other plant fibers. Coir fiber has low modulidue to high microfibrillar angle [19]. Besides, elongation at break

Table 1Physical, chemical and mechanical properties of coir fiber compared with other natural fibers [2,11,18].

Properties/fibers Coir Flax Hemp Jute Ramie Sisal

Density (g/m3) 1.25–1.5 1.4 1.48 1.45 1.5 1.26–1.33Diameter (lm) 100–450 100 25 60 40–50 100–300Cellulose content (%) 36–43 62–72 67–75 59–71 68–76 74–75.2(B)

60–67(I)

Hemicellulose content (%) 0.2 16–18 16–18 12–13 13–14 10–13.9Lignin content (%) 41–45 2–2.5 2.8–3.3 11.8–12.9 0.6–0.7 8–12(I)

7.6–7.98(B)

Microfibrillar angle (�) 30–45 10 6.2 7–9 7.5–12 10–20Tensile strength (MPa) 105–175(I) 800–1500 550–900 400–800 500–870 600–700

95–118(B)

Young’s modulus (GPa) 4–6(I) 60–80 70 10–30 44 38Elongation at break (%) 17–47(I) 1.2–2.4 1.6 1.16–1.8 1.2 3.64–5.12(I)

23.9–51.4(B) 2–2.5(B)

Moisture absorption (%) 10 7 8 12 12–17 11

(B) – Brazilian; (I) – Indian.

Fig. 1. Chemical structure of PBS used in the present study.

T.H. Nam et al. / Composites: Part B 42 (2011) 1648–1656 1649

of natural fibers increases with increasing microfibrillar angle, thusthe elongation at break of coir is the highest among typical naturalfibers [20]. This property of coir fiber is certainly useful in cushionapplications. An example of the application to seat cushion forautomobiles is reported in [21]. The high lignin content in coir fiberis responsible for other useful properties such as weather, fungal,and bacterial resistance [20]. The lignin content in coir fiber isquite high, so the fiber becomes stiffer and tougher.

Due to hardwearing quality, durability and other advantages,coir is used for marking a wide variety of floor-furnishing materi-als, yarn, rope, etc. However, these traditional coir products con-sume only a small percentage of the potential total worldproduction of coconut husk. According to official website of Inter-national Year for Natural Fibres 2009, about 500,000 tonnes of coirare produced annually, mainly in India and Sri Lanka followed byThailand, Vietnam, the Philippines and Indonesia. Its total valueis estimated at $100 million. Hence, the research and developmentefforts have been underway to find new utilization of coir as a rein-forcement in polymer composites, such as coir-polypropylene andcoir based polyester green composites [15–17,21–25].

A fully biodegradable composite reinforced by natural fibers isusually made from completely biodegradable polymeric matrix.Among the completely biodegradable polymers which have beenfrequently studied as biodegradable polymer matrices in the bio-composites, polylactic acid (PLA) and poly(butylene succinate)(PBS) are increasing commercial interest [26]. However, PBS iscommercially available at lower cost than PLA. PBS can be naturallydegraded into the environment by bacteria and fungi [27,28]. Fur-thermore, PBS has excellent biodegradability in nature, such as insoil, lake, sea, and compost [29]. It can be completely combustibleby fire without evolving toxic gases as described in [30]. It hascomparable mechanical properties with several thermoplasticssuch as polyethylene, polypropylene and polystyrene. Therefore,PBS can be a good candidate material for the matrix of biodegrad-able composites.

The combination of coir fibers and PBS resin can produce theenvironment-friendly biodegradable composite. In the presentwork, tensile properties of untreated and alkali-treated coir fiberswere reported. The effect of alkali treatment on the interfacialshear strength (IFSS) of coir fiber/PBS system was evaluated by sin-gle fiber pull-out test. The PBS biodegradable composites rein-forced with untreated and alkali-treated coir fibers werefabricated by compression molding method. The effect of alkalitreatment and fiber content on mechanical properties of coir fi-ber/PBS biodegradable composites was studied. Coir fiber surfacemorphology and fractured surfaces of untreated and alkali-treatedcoir fiber/PBS composites were investigated by scanning electron

microscope (SEM) providing the information for the evaluation ofinterfacial fiber–matrix adhesion.

2. Experimental

2.1. Materials

Poly(butylene succinate) pellets (PBS, #1001, Showa High Poly-mers, Ltd., Tokyo, Japan) is thermoplastic, aliphatic polyester andalso biodegradable polymer. The melting temperature of the PBSis about 115 �C, the density is 1.26 g/cm3. Fig. 1 depicts the chem-ical structure of PBS used in this study. The golden brown coir fi-bers in the present work were supplied from Betrimex, JSC.,Bentre, Vietnam. It was found that the cross section of coir fiberis not completely circular (Fig. 2), thus fiber cross sectional area(A) is determined approximately by a formula as follows:

A ¼ pab4

ð1Þ

where a and b are dimensions in Fig. 2 measured by an opticalmicroscope MX-7575CS (Hirox Co., Ltd., Tokyo, Japan).

The diameter (d) of coir fiber is calculated approximately asfollows:

d ¼ffiffiffiffiffiffi

abp

ð2Þ

The coir fibers with the length exceeded 100 mm and the diam-eter varying from 100 to 450 lm were selected carefully to be usedin this study.

2.2. Alkali treatment of coir fibers

First of all coir fibers were treated with 5% NaOH solution in aglass beaker for different soaking time (24 h, 48 h, 72 h and 96 h)at room temperature (RT). Next the fibers were taken out of thesolution, then washed several times with fresh water and subse-quently with distilled water. Finally, the coir fibers were air-driedfor more than 2 days. The mean IFSS of 5% alkali-treated coir fibersfor 72 h which will be shown in next section is higher than that ofuntreated and other alkali-treated coir fibers. Therefore, anotherseries of experiments with the same procedure were followed ex-

Fig. 2. Typical cross section of coir fiber.

1650 T.H. Nam et al. / Composites: Part B 42 (2011) 1648–1656

cept that the coir fibers were soaked in various concentrations ofNaOH solution (3% and 7%) for 72 h in order to select the best alkaliconcentration for the treatment. The series are designated by 3NX,5NX and 7NX in which 3N, 5N and 7N corresponding to the soakingin 3%, 5% and 7% NaOH solution, respectively and X correspondingto the soaking time in hours. The reaction of sodium hydroxidewith coir fiber is described as follows:

Coir—OHþ NaOH! Coir—O�Naþ þH2O ð3Þ

2.3. Coir fiber characterization

The single fiber tensile tests were carried out by a universaltesting machine Instron 4442 (Instron Corp., Canton, MA) with acrosshead speed of 5 mm/min at RT. Individual untreated and alka-li-treated coir fibers were carefully chosen, mounted and glued ona paper tab before testing. Gauge length of single fiber tensile spec-imens is 10 mm. The average tensile properties of untreated andalkali-treated coir fibers were measured at least from thirty suc-cessful specimens.

2.4. Interfacial characterization

A single fiber pull-out test was used to measure the IFSS of un-treated and alkali-treated coir fiber/PBS system. The untreated and

Fig. 3. Schematic representation of single fiber pull-out test.

Fig. 4. Schematic representation of the hot p

alkali-treated coir fibers having length over 120 mm were used forpreparing pull-out test specimens by pressing the fibers betweentwo PBS sheets using a hot press equipment (Imoto Corp., Kyoto,Japan). The fibers were kept straight and oriented by fixing its bothends, extending outside the PBS sheets, on the mold using glue asdescribed in [25]. Specimens with a thickness of 1 mm were re-moved from the mold after quickly cooling in ice water. The fiberembedded length in the PBS matrix was obtained by cutting the fi-ber by punching a hole through the specimen. The schematic rep-resentation of single fiber pull-out test is shown in Fig. 3, in whichd is mean diameter of coir fiber and L is embedded length. Meandiameter of coir fiber was calculated using the formula (2) in whichthe dimensions of a and b were measured at the intersection be-tween coir fiber and PBS matrix. Single fiber pull-out test was per-formed by above universal testing machine Instron 4442 with aload cell of 50N and a crosshead speed of 1 mm/min. A force is ap-plied to the free end of fiber to pull it out of the matrix while theforce is continuously monitored and recorded. IFSS value of un-treated and alkali-treated coir fiber/PBS system was estimatedfrom the maximum debonding force (Fd) using following equation:

s ¼ Fd

p� d� Lð4Þ

The single fiber pull-out test was carried out for untreated,5N24, 5N48, 5N72, 5N96, 3N72 and 7N72 treated coir fibers toinvestigate the effect of soaking time and concentration of sodiumhydroxide on the IFSS of coir fiber/PBS system in order to make aright choice of alkali treatment. The mean IFSSs between coir fibersand PBS matrix were obtained from twenty successful pull-out testspecimens.

2.5. Composite fabrication

To begin with coir fibers were dried at 80 �C in the vacuum ovenfor 24 h. Next the dried coir fibers were slightly stretched outstraight within the elastic region. Then they were cut into the seg-ments with the length of 100 mm. After that coir fibers wereweighed and aligned in a parallel array, glued by adhesive tapeand placed in the mold between the PBS sheets. The compositeplates made from PBS and different untreated and alkali-treatedcoir fiber mass content (10%, 15%, 20%, 25% and 30%) were fabri-cated using above hot press equipment. The pure PBS and compos-ite plates were pressed in a stainless steel mold with a thickness of1 mm under 10 MPa pressure for 10 min at 150 �C. Next the moldwas removed from the press, then clamped securely between twosteel mold plates and last quickly quenched by ice water. Thereseems to be no water uptake and no moisture absorption during

ress used to fabricate composite plates.

T.H. Nam et al. / Composites: Part B 42 (2011) 1648–1656 1651

quenching of the sample. The schematic representation of the hotpress for composite fabrication is shown in Fig. 4. Both PBS sheetsand composite plates were prepared with the same thermalhistory.

2.6. Tensile test

The tensile specimens of 100 � 10 � 1 mm3 were cut out frompure PBS and composite plate by cutting machine AC-300CF (MAR-UTO Testing Machine Co., Tokyo, Japan) and kept in desiccator at25 �C and 35% relative humidity before testing. The both clampedends of the specimens (Fig. 5) were glued by two glass fiber rein-forced plastic (GFRP) tabs. Strain gauges were glued at the centerof the specimens to measure the elongation at break. Tensile prop-erties were measured according to JIS K7113 using a universal test-ing machine Senstar SC-5H (JT Tohsi Inc., Tokyo, Japan). All thetensile tests were carried out at RT with a crosshead speed of0.5 mm/min. Tensile specimens were chosen carefully before test-ing. The mean tensile properties of pure PBS and coir fiber/PBScomposites were obtained from five successful specimens for eachfiber content.

2.7. Flexural test

The flexural properties of pure PBS and coir fiber/PBS biode-gradable composites were measured by a three-point bendingmethod according to JIS K7171 standard using universal testingmachine Autograph AGS-1000A (Shimadzu, Kyoto, Japan). The flex-ural test was carried out at RT with a crosshead speed of 2 mm/min. The dimension of flexural specimens was 50 � 25 � 1 mm3.The ratio between span distance and thickness of pure PBS andcomposite specimens was 16. The flexural strength (rf) and mod-ulus (Ef) were calculated using the following equations:

rf ¼3FLs

2bh2 ð5Þ

Ef ¼L3

s m

4bh3 ð6Þ

Fig. 5. Shape and dimensions of tensile specimen.

Table 2The mean tensile properties of untreated and alkali-treated coir fiber.

Coir fiber Strength (MPa) Modulus (GPa) Failure strain (%)

Untreated 139.67 ± 39.42 2.79 ± 0.54 29.52 ± 6.895N24 treated 218.52 ± 38.38 5.64 ± 0.73 31.40 ± 3.605N48 treated 227.43 ± 40.86 5.68 ± 0.81 32.70 ± 5.105N72 treated 238.26 ± 39.91 5.95 ± 0.79 33.96 ± 5.745N96 treated 210.07 ± 35.09 5.27 ± 0.76 30.00 ± 4.853N72 treated 209.21 + 37.69 4.92 ± 0.84 29.59 ± 4.137N72 treated 228.54 ± 37.15 5.47 ± 0.82 30.53 ± 3.44

where F is the maximal applied force, Ls is the length of supportspan, m is the slope of the force–deflection curve, b and h are thewidth and thickness of the specimen, respectively. Flexural speci-mens were chosen carefully before testing. The mean flexural prop-erties of each composite were obtained from five successful testspecimens.

2.8. Morphological characterization

The coir surface morphologies and fractured surface of the com-posites after tensile tests were examined using SEM (VE-7800, Key-ence Inc., Osaka, Japan).

3. Results and discussion

3.1. Effect of alkali treatment on mechanical properties of coir fiber

Tensile properties of untreated and alkali-treated coir fiberwere presented in Table 2. The mean tensile strength of coir fiberis quite low compared to other natural fiber such as jute, flax,hemp, ramie or sisal fiber. However the strain at failure of coir fiberis quite high compared with other natural and synthetic fibers suchas glass and carbon. As shown in Table 2, alkali treatment of coirfibers improved significantly their tensile properties. It is seen thatat 5% alkali solution when soaking time increases from 24 h to 72 hthe tensile properties of alkali-treated coir fibers increased, butthey decreased beyond 72 h. In addition with the soaking time of72 h tensile properties of coir fiber increased with increasing con-centration of alkali solution up to 5%, but over 5% they decreased.The tensile strength and modulus of coir fibers increased by about71% and 113% when the fibers were soaked in 5% sodium hydrox-ide solution for 72 h, respectively. The increase in tensile strengthand modulus of coir fiber after alkali treatment was explained indetail [15].

3.2. Effect of alkali treatment on coir fiber surface

Alkali treatment improves the fiber–matrix adhesion due to theremoval of natural and artificial impurities from the fiber surfaceas well as changing in the arrangement of units in the cellulosemacromolecule [31]. Alkali treatment increases the surface rough-ness and the amount of cellulose exposed on the fiber surfaceresulting in better mechanical interlocking [16]. Therefore, thedevelopment of a rough surface tomography and enhancement inaspect ratio offer better fiber–matrix interfacial bond resulting inincreasing mechanical properties.

Fig. 6 showed the effect of different alkali treatment on the coirfiber surface. SEM micrograph of untreated coir fiber shows globu-lar particles and cuticles on the fiber surface (Fig. 6a). The globularparticles which cover the pits on the cell walls are embedded in thefiber surface [15]. Some of globular particles were intact but at afew isolated places they were removed creating the pits on 5N48treated coir fiber surface (Fig. 6b). When the soaking time in-creased to 72 h the cell was exposed and a much greater propor-tion of globular particles appeared to be removed (Fig. 6c), thusthe roughness of fiber surface increased. The removal of cuticlelayer will expose lignin on the fiber surface. Lignin, being a pheno-lic natural polymer, should be chemically compatible with PBS re-sin. However, the micrograph in Fig. 6d shows that 5N96 treatedcoir fiber surface is smoother than that of 5N72 fiber surface.Smooth surface of 5N96 treated coir fiber can be explained dueto the removal of all globular particles and cuticles deposited onthe fiber surface. For alkali concentration of 3% (Fig. 6e), nearlyall globular particles on the fiber surface were intact, but the sur-face impurities were removed. With alkali concentration of 7%,

Fig. 6. SEM micrographs of coir fibers: (a) untreated fiber, (b) 5N48 treated fiber, (c) 5N72 treated fiber, (d) 5N96 treated fiber, (e) 3N72 treated fiber, and (f) 7N72 treatedfiber.

1652 T.H. Nam et al. / Composites: Part B 42 (2011) 1648–1656

the micrograph in Fig. 6f depicted the removal of cuticles and glob-ular particles creating the pits on the fiber surface. However, it isobserved that the fiber surface roughness of 7N72 treated coir fiberis lower than that of 5N72 due to the higher alkali concentrationwhich results in the higher removal of fiber surface impurities.

3.3. Interfacial shear strength measurement

The typical force–displacement curves obtained from single fi-ber pull-out test for the untreated and alkali-treated coir fiber rein-forced PBS composites were shown in Fig. 7. It can be noted that allthe curves exhibit nonlinear behavior due to the characteristics ofthe ductile matrix. However, once the force reaches its maximumvalue there are clearly significant differences in the way thesecurves drop. In the case of untreated coir, it can be seen that firstthe force increases gradually till it reaches a maximum value, thenthe force suddenly drops to a lower value. Subsequently, the fiberis sliding along the hole-surface until the total embedded length of

Fig. 7. Typical force–displacement curves of single fiber pull-out tests for untreatedand different alkali-treated coir fiber/PBS composite.

the fiber is pulled-out of PBS matrix. This response agrees wellwith that of a poor interface because of the incompatibility be-tween hydrophilic fiber and hydrophobic matrix [31,32]. Thisbehavior shows a small change in the case of 5N24 and 5N48 trea-ted coir fibers due to the removal of cuticles on the fiber surface,thus the fiber–matrix interaction is improved. In the case of5N72 treated coir fiber, the force shows no immediate drop afterit reaches the maximum value. This is due to the higher roughnessof alkali-treated fiber surface leading to better interfacial fiber–ma-trix bond. The force–displacement curve of 5N96 treated coir fiberhas a similar shape compared with 5N72 treated coir fiber.

The mean IFSSs of untreated and alkali-treated coir fiber/PBScomposites were shown in Fig. 8. Mean IFSS of untreated coir fi-ber/PBS system calculated from maximal debonding force of singlefiber pull-out tests is low (2.054 MPa), because of the incompatibil-ity between hydrophilic fiber and hydrophobic matrix and exis-tence of the impurities on the coir fiber surface. Fig. 8 alsoshowed the effect of soaking time on the IFSS between 5% alkali-treated coir fiber and PBS matrix. The IFSS of 5N24, 5N48, 5N72

Fig. 8. IFSS of untreated and 5% alkali-treated coir fiber/PBS composites withdifferent soaking time (mean value and standard deviation).

Fig. 10. Tensile strength of untreated and 5N72 treated coir fiber/PBS biodegrad-able composites (mean value and standard deviation).

T.H. Nam et al. / Composites: Part B 42 (2011) 1648–1656 1653

and 5N96 treated coir fiber/PBS was 2.698 MPa, 2.942 MPa,3.196 MPa and 3.016 MPa, respectively. It is observed that the IFSSof alkali-treated coir fibers reinforced PBS matrix increases withincreasing soaking time from 24 h to 72 h. This can be explaineddue to the removal of cuticle layer on the fiber surface (as seenin Fig. 6) resulting in the increase of interfacial fiber–matrix adhe-sion. Furthermore, the removal of globular particles on the fibersurface during alkali treatment had led to a very rough fiber sur-face with the pits (Fig. 6c). The pits could conveniently increasethe mechanical interlocking between the fiber and PBS resin. How-ever, the IFSS of 5N96 treated coir fiber/PBS is lower than that of5N72 treated fiber. It can be explained that the surface of 5N96treated coir fiber (Fig. 6d) is smoother than that of 5N72 leadingto the less mechanical interlocking between the fiber and PBSresin.

Besides, the effect of NaOH concentration on the IFSS betweenalkali-treated coir fibers for 72 h and PBS matrix was shown inFig. 9. The mean IFSS of 3N72 and 7N72 treated coir fiber/PBS is2.732 MPa and 2.913 MPa, respectively and lower than that of5N72 treated fiber. This can be explained by the fact that the cuti-cles and globular particles still exist on the surface of 3N72 treatedcoir fiber (Fig. 6e) leading to less interfacial fiber–matrix adhesion.The surface of 7N72 treated fiber (Fig. 6f) was treated by high alkaliconcentration resulting in decreasing mechanical interlocking be-tween the fiber and PBS resin. The results show that the higherthe surface roughness leads to higher IFSS.

Fig. 11. Tensile modulus of untreated and 5N72 treated coir fiber/PBS biodegrad-able composites (mean value and standard deviation).

3.4. Tensile properties of the composites

Tensile properties of both untreated and alkali-treated coir/PBSbiodegradable composites with different fiber mass content from0% to 30% were represented in Figs. 10–12. It can be realized thattensile strength and modulus gradually increased with increasingfiber mass content from 0% to 25%, however there was a decreasein the tensile strength and modulus of the composite with 30% fi-ber mass content (as seen in Figs. 10 and 11). Regarding the un-treated coir fiber, the tensile strength and modulus of coir/PBSbiodegradable composites at 15%, 20%, 25% and 30% fiber masscontent were 6.5% and 40.8%, 15.1% and 57.9%, 28.2% and 71.8%,and 14.7% and 69.4% higher than those of pure PBS, respectively.The increase in tensile strength and modulus of the composites isdue to the reinforcement of coir fibers in PBS matrix in the direc-tion of external load, because the strength and modulus of coir fi-ber are higher than those of PBS matrix. Similar results were alsoreported earlier for coir fiber reinforced polyester composites

Fig. 9. Effect of different NaOH concentration on the IFSS between 5% alkali-treatedcoir fibers for 72 h and PBS matrix (mean value and standard deviation).

Fig. 12. Elongation at break of untreated and 5N72 treated coir fiber/PBS biode-gradable composites (mean value and standard deviation).

[22]. The presence of coir fibers in PBS matrix contributes effec-tively to enhance the tensile modulus of PBS resin. It is knownaccording to composite theory that the tensile modulus of a fi-ber-reinforced composite depends on the modulus of the fiberand the matrix, the fiber content and orientation.

Fig. 13. Flexural strength of untreated and 5N72 treated coir fiber/PBS biodegrad-able composites (mean value and standard deviation).

Fig. 14. Flexural modulus of untreated and 5N72 treated coir fiber/PBS biodegrad-able composites (mean value and standard deviation).

1654 T.H. Nam et al. / Composites: Part B 42 (2011) 1648–1656

Besides, the increase in the tensile strength up to 25% fiber con-tent is due to increased wetting of the fiber with the matrix. Thehigh tensile strength at the fiber mass content of 25% might be alsodue to adequate fiber content in composites, which leads to greaterwetting. However, with 10% untreated fiber mass content themean tensile strength of coir/PBS biodegradable composite slightlydecreased. This decrease may be explained by the poor wettabilityleading to a weak interface. Therefore, at lower tensile stress, aweak interface might form cracks, leading to failure similarly asshown in [22]. In addition the decrease in tensile strength at 30%fiber mass content probably resulted from the poor fiber wetting,because the PBS resin content is not sufficient to wet all the fibersurfaces leading to poor interfacial adhesion.

Furthermore, the decrease in elongations at break which wasshown in Fig. 12 is mainly due to the structural integrity of PBSbeing destroyed by the loading of coir fiber, and increasing fibercontent imply poor interfacial fiber–matrix adhesion, leading toquicker fracture than pure PBS [4]. The elongation at break of un-treated coir fiber/PBS composite with 10% fiber mass content sig-nificantly reduced by approximately 35% compared to the one ofPBS resin. After such initial drop, the percent elongation at breakdecreases inconsiderably or nearly remains constant with increas-ing fiber content. This can be explained that the fracture of PBS ma-trix occurred before coir fiber failure, since elongation at break ofcoir fibers are higher than that of PBS resin. This also indicates thatthe ductile nature of PBS resin slightly decreases with the additionof coir fibers.

As demonstrated in Figs. 10–12, alkali treatment of coir fibersimproved the tensile properties of coir fiber reinforced PBS biode-gradable composites. Actually, alkali treatment is mainly a processof surface activation leading to the formation of rough fiber sur-face. As shown above, alkali treatment cleans surface impuritiesand makes the roughness with many pits on the fiber surface. Thiswas well depicted in Fig. 6 by comparing the SEM micrographs ofalkali-treated fiber with the untreated fiber. The formations ofthe pits result in greater mechanical interlocking of the matrixon the fiber surfaces and make the interfacial adhesion stronger.Therefore, the tensile properties of alkali-treated biocompositesare significantly greater than those of the untreated biocomposites.The tensile strength and modulus of 5N72 treated coir fiber/PBScomposites at 10%, 15%, 20%, 25% and 30% fiber mass content were21% and 22.1%, 22.4% and 24.1%, 24.1% and 29.4%, 20.5% and 40.8%,and 14.2% and 25.2% higher than those of untreated coir fiber/PBScomposites, respectively. The increase in tensile properties in caseof 5% alkali-treated fiber composite may be due to greater fiber–matrix interfacial and physical bonding, because physical bondingalso increases after alkali treatment due to the dipolar interactionsbetween fiber–matrix [22]. The experimental results in this studyshow that best tensile properties can be obtained at the fiber masscontent of 25%. Therefore, the results of tensile properties point outthe importance by using the right amount of natural fiber as rein-forcement in the composites.

3.5. Flexural properties of the composites

Effect of alkali treatment on flexural properties of coir/PBS bio-degradable composites with different fiber mass content from 0%to 30% was represented in Figs. 13 and 14. It is found that the flex-ural properties are gradually increased with increasing fiber masscontent from 0% to 25%, but with 30% fiber content they are slightlydecreased or nearly remain constant. The decrease in mean flexuralstrength beyond 25% coir fiber content can be explained due to ashortage of PBS resin to fully wet out between the coir fibers. Inter-estingly, the flexural properties have the same trend as the tensileproperties with the increase of fiber content.

The alkali-treated coir fiber/PBS composites yielded highermean flexural properties compared to pure PBS resin and the un-treated ones. This reflects the contribution of sodium hydroxidein terms of changes of fiber properties and enhancement of fi-ber–matrix adhesion. Compared to pure PBS resin, alkali-treatedcoir fiber/PBS composites at 10%, 15%, 20%, 25% and 30% fiber masscontent exhibited 20.9%, 23.1%, 34%, 45.7% and 42.9% enhancementin flexural strength and 45.6%, 51.1%, 76.6%, 97.4% and 95.6% inflexural modulus, respectively. In this study, the best flexural prop-erties can be obtained at fiber mass content of 25% correspondingwith the tensile properties. Alkali-treated coir fiber/PBS compositereinforced with 25% fiber content showed an increase in mean flex-ural strength by 6% and mean flexural modulus by 16.7% comparedto those of untreated fiber. The results show that surface modifica-tion by alkali treatment has less influence on flexural propertiescompared to tensile properties. This can be explained that the flex-ural failure mode usually shows little or no fiber pull-out [4], be-cause applied force is perpendicular to reinforced fibers of thecomposite specimens in flexural test.

The mechanical strength and modulus of coir fiber/PBS biode-gradable composite can show an optimum fiber content. The opti-mum fiber content varies with the nature of both fiber and matrix,fiber aspect ratio, fiber–matrix interfacial adhesion, fiber agglom-eration, processing technique, end, etc. [4]. Similar investigationshave also been reported by Rout et al. [22] for coir/polyester com-posites in which the optimum fiber content is about 17–25%, by

Fig. 15. SEM micrograph of tensile fractured surface of PBS biodegradablecomposite reinforced with 20% mass content of: (a) untreated coir fibers, (b)5N72 treated coir fibers.

T.H. Nam et al. / Composites: Part B 42 (2011) 1648–1656 1655

Brahmakumar et al. [25] for coir/LDPE composites is about 25%,and by Prasad et al. [15] for coir/polyester composites is about30%. In this study, the incorporation of 25% fiber mass contentshowed best mechanical properties of coir fiber/PBS biodegradablecomposites.

3.6. Fractured surface morphologies of the composites

Tensile fractured surface morphologies of untreated and alkali-treated coir fiber/PBS composites were shown in Fig. 15. Fig. 15adepicts several holes that were left after the fibers are pulled-outfrom the matrix. Visible gaps can be found between fiber andPBS matrix in Fig. 15a, suggesting poor interfacial adhesion. How-ever, the gaps are almost disappeared in the case of 5N72 treatedcoir fiber (Fig. 15b), proving good compatibility being formed inPBS composites leading to increase in the interfacial and mechan-ical properties of the composites. It is obvious that untreated coirfiber can be easily pulled-out from the interfacial region with poorcompatibility, resulting in rapid partial-collapse of PBS composite.However, alkali-treated coir fiber having a good adhesion with PBSmatrix can effectively disperse and transfer stress, leading to theimprovement in mechanical properties of coir/PBS biodegradablecomposites. Consequently, the results suggest that alkali treatmentof coir fiber is necessary to enhance the interfacial fiber–matrixadhesion prior to composite processing.

4. Conclusions

Coir fiber/PBS biodegradable composites with different fibercontent have been developed. Effect of alkali treatment on theinterfacial and mechanical properties of coir fiber/PBS biodegrad-able composites has been studied. The following conclusions canbe drawn from this study:

(1) The mechanical properties of investigated coir fibers havebeen measured and evaluated. Alkali treatment of coir fibersimproved significantly their tensile properties.

(2) Alkali treatment of coir fiber increased fiber surface rough-ness leading to the increase of mechanical interlockingbetween the fiber and PBS matrix in the composites. Treat-ment of coir fiber with 5% sodium hydroxide for 72 hresulted in the highest fiber surface roughness.

(3) Alkali treatment of coir fiber enhanced the IFSS of coir fiber/PBS system. The highest IFSS between alkali-treated coirfiber and PBS matrix obtained when coir fibers were soakedin 5% sodium hydroxide for 72 h.

(4) Alkali treatment of coir fibers increased the interfacial bond-ing strength and the wettability of the fibers by PBS resinleading to the enhancement in mechanical properties ofthe composites.

(5) Mean mechanical strength and modulus of the compositesincreased with increasing fiber mass content up to 25%,but over 25% fiber content the tensile strength and modulusof coir fiber/PBS biodegradable composite decreased. Theauthors propose that the 25% coir fiber content reinforcedPBS biodegradable composites have the best tensile proper-ties in this study.

(6) The experimental results in the present work suggest that auseful composite with good strength could be successfullydeveloped using coir fiber as a reinforcing agent for thePBS matrix.

References

[1] Holbery J, Houston D. Natural fiber reinforced polymer composites inautomotive applications. JOM 2006;58(11):80–6.

[2] Cheung HY, Ho MP, Lau KT, Cardona F, Hui D. Natural fibre-reinforcedcomposites for bioengineering and environmental engineering applications.Composites Part B 2009;40:655–63.

[3] Cho D, Lee SG, Park WH, Han SO. Eco-friendly biocomposites materials usingbiofibres. Polym Sci Technol 2002;13(4):460–76.

[4] Liu L, Yu J, Cheng L, Yu W. Mechanical properties of poly(butylene succinate)(PBS) biocomposite reinforced with surface modified jute fibre. CompositesPart A 2009;40:669–74.

[5] Han SO, Lee SM, Park WH, Cho D. Mechanical and thermal properties of wastesilk fiber-reinforced poly(butylene succinate) biocomposites. J Appl Polym Sci2006;100:4972–80.

[6] Cheng S, Lau KT, Liu T, Zhao Y, Lam PM, Yin Y. Mechanical and thermalproperties of chicken feather fiber/PLA green composites. Composites Part B2009;40:650–4.

[7] Ochi S. Development of high strength biodegradable composites using Manilahemp fiber and starch-based biodegradable resin. Composites Part A2006;37:1879–83.

[8] Ogihara S, Okada A, Kobayashi S. Mechanical properties in a bamboo fiber/PBSbiodegradable composite. J Solid Mech Mater Eng 2008;2(3):291–9.

[9] Mohanty K, Misra M, Hinrichsen G. Biofibres, biodegradable polymers andbiocomposites: an overview. Macromol Mater Eng 2000;276/277:1–24.

[10] Monteiro SN, Lopes FPD, Ferreira AS, Nascimento DCO. Natural-fiber polymer–matrix composites: cheaper, tougher, and environmentally friendly. JOM2009;61(1):17–22.

[11] Satyanarayana KG, Arizaga GGC, Wypych F. Biodegradable composites basedon lignocellulosic fibers – an overview. Prog Polym Sci 2009;34(9):982–1021.

[12] Zhang MQ, Rong MZ, Lu X. Fully biodegradable natural fiber composites fromrenewable resources: all plant fiber composites. Compos Sci Technol2005;65:2514–25.

[13] Westerlind BS, Berg JC. Surface energy of untreated and surface-modifiedcellulose fibres. J Appl Polym Sci 1988;36:523–34.

[14] John MJ, Anandjiwala RD. Recent development in chemical modification andcharacterization of natural fiber-reinforced composites. Polym Compos2008;29(2):187–207.

[15] Prasad SV, Pavithran C, Rohatgi PK. Alkali treatment of coir fibers for coir–polyester composites. J Mater Sci 1983;18:1443–54.

[16] Rahman MM, Khan MA. Surface treatment of coir (Cocos nucifera) fibers and itsinfluence on the fibers’ physico-mechanical properties. Compos Sci Technol2007;67:2369–76.

[17] Islam MN, Rahman MR, Haque MM, Huque MM. Physico-mechanicalproperties of chemically treated coir reinforced polypropylene composites.Composites Part A 2010;41:192–8.

1656 T.H. Nam et al. / Composites: Part B 42 (2011) 1648–1656

[18] Satyanarayana KG, Wypych F. Characterization of natural fibers. In: Fakirov S,Bhattacharyya D, editors. Engineering biopolymers homopolymers, blends andcomposites. Munich: Hanser Publishers; 2007. p. 3–48.

[19] Silva GG, De Souza DA, Machado JC, Hourston DJ. Mechanical and thermalcharacterization of native Brazilian coir fiber. J Appl Polym Sci2000;76:1197–206.

[20] Satyanarayana KG, Kulkarni AG, Rohatgi PK. Potential of natural fibres as aresource for industrial material in Kerala. J Sci Ind Res 1981;40:222–37.

[21] Monteiro SN, Terrones LAH, Lopes FPD, d’Almeida JRM. Mechanical strength ofpolyester matrix composites reinforced with coconut fiber wastes. Rev Mater2005;10(4):571–6.

[22] Rout J, Tripathy SS, Nayak SK, Misra M, Mohanty AK. The influence of fibersurface modification on the mechanical properties of coir–polyestercomposites. J Appl Polym Sci 2001;22:468–76.

[23] Asasutjarit C, Charoenvai S, Hirunlabh J, Khedari J. Materials and mechanicalproperties of pretreated coir-based green composites. Composites Part B2009;40:633–7.

[24] Leblanc JL, Furtado CRG, Leite MCAM, Visconte LLY, Ishizaki MH. Investigatingpolypropylene–green coconut fiber composites in the molten and solid statesthrough various techniques. J Appl Polym Sci 2006;102:1922–36.

[25] Brahmakumar R, Pavithran C, Pillai RM. Coconut fibre reinforced polyethylenecomposites: effect of natural waxy surface layer of the fibre on fibre/matrix

interfacial bonding and strength of composites. Compos Sci Technol2005;65:653–69.

[26] Lee SH, Wang A. Biodegradable polymers/bamboo fiber biocomposite withbio-based coupling agent. Composites Part A 2006;37:80–91.

[27] Kim DY, Rhee YH. Biodegradation of microbial and synthetic polyesters byfungi. Appl Microbiol Biotechnol 2003;61(4):300–8.

[28] Jin HJ, Lee BY, Kim MN, Yoon JS. Properties and biodegradation ofpoly(ethylene adipate) and poly(butylene succinate) containing styreneglycol units. Eur Polym 2000;36(12):2693–8.

[29] Hirotsu T, Tsujisaka T, Masuda T, Nakayama K. Plasma surface treatments andbiodegradation of poly (butylene succinate) sheets. J Appl Polym Sci2000;78:1121–9.

[30] Lee SM, Choa D, Park WH, Lee SG, Han SO, Drzal LT. Novel silk/poly(butylenesuccinate) biocomposites: the effect of short fibre content on their mechanicaland thermal properties. Compos Sci Technol 2005;65:647–57.

[31] Valadez-Gonzalez A, Cervantes-Uc JM, Olayo R, Herrera-Franco PJ. Effect offiber surface treatment on the fiber–matrix bond strength of natural fiberreinforced composites. Composites Part B 1999;30:309–20.

[32] Luo S, Netravali AN. Interfacial and mechanical properties of environment-friendly ‘‘green’’ composites made from pineapple fibres andpoly(hydroxybutyrate-co-valerate) resin. J Mater Sci 1999;34:3709–19.