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Industrial Crops and Products 33 (2011) 7–22

Contents lists available at ScienceDirect

Industrial Crops and Products

journa l homepage: www.e lsev ier .com/ locate / indcrop

eview

il palm fiber (OPF) and its composites: A review

. Shinoja, R. Visvanathanb, S. Panigrahic,∗, M. Kochubabua

Directorate of Oil Palm Research, Indian Council of Agricultural Research, Pedavegi, Eluru, Andhra Pradesh 534 450, IndiaDepartment of Food & Agricultural Process Engineering, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu 641 003, IndiaDepartment of Agricultural and Bioresource Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N5A9, Canada

r t i c l e i n f o

rticle history:eceived 14 October 2009eceived in revised form6 September 2010ccepted 17 September 2010

eywords:il palm fibermpty fruit bunchiocomposite

a b s t r a c t

Twenty first century has witnessed remarkable achievements in green technology in material sciencethrough the development of biocomposites. Oil palm fiber (OPF) extracted from the empty fruit bunchesis proven as a good raw material for biocomposites. The cellulose content of OPF is in the range of 43%–65%and lignin content is in the range of 13%–25%. A compilation of the morphology, chemical constituentsand properties of OPF as reported by various researchers are collected and presented in this paper. Thesuitability of OPF in various polymeric matrices such as natural rubber, polypropylene, polyvinyl chloride,phenol formaldehyde, polyurethane, epoxy, polyester, etc. to form biocomposites as reported by variousresearchers in the recent past is compiled. The properties of these composites viz., physical, mechanical,water sorption, thermal, degradation, electrical properties, etc. are summerised. Oil palm fiber loadingin some polymeric matrices improved the strength of the resulting composites whereas less strength

iber propertiesomposite propertiesybrid biocomposite

was observed in some cases. The composites became more hydrophilic upon addition of OPF. Howevertreatments on fiber surface improved the composite properties. Alkali treatment on OPF is preferred forimproving the fiber–matrix adhesion compared to other treatments. The effect of various treatments onthe properties of OPF and that of resulting composites reported by various researchers is compiled in thispaper. The thermal stability, dielectric constant, electrical conductivity, etc. of the composites improved

upon incorporation of OPF. The strength properties reduced upon weathering/degradation. Sisal fiberwas reported as a good combination with OPF in hybrid composites.

© 2010 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82. Oil palm fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1. Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2. Morphology and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3. Surface treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3. Oil palm fiber composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1. Oil palm fiber-natural rubber composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.1.2. Water absorption characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.1.3. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.1.4. Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2. Oil palm fiber-polypropylene (PP) composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.2.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.2.2. Water absorption characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2.3. Degradation/weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3. Oil palm fiber-polyurethane (PU) composites . . . . . . . . . . . . . . . . . . . . .3.3.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3.2. Water absorption characteristics . . . . . . . . . . . . . . . . . . . . . . . . .3.3.3. Degradation/weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +1 306 9665312; fax: +1 306 9665334.E-mail addresses: drrviswanathan@gmail.com (R. Visvanathan), sap382@mail.usask.ca

926-6690/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.indcrop.2010.09.009

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

(S. Panigrahi).

8 S. Shinoj et al. / Industrial Crops and Products 33 (2011) 7–22

3.4. Oil palm fiber-polyvinyl chloride (PVC) composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.4.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.4.2. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.5. Oil palm fiber-polyester composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.5.1. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.5.2. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.5.3. Water absorption characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.5.4. Degradation/weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.6. Oil palm fiber-phenol formaldehyde (PF) composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.6.1. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.6.2. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.6.3. Water absorption characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.6.4. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.6.5. Degradation/weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.7. Oil palm fiber-polystyrene composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.7.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.8. Oil palm fiber-epoxy composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.8.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20. . . . . .

1

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

Oil palm (Elaeis guineensis Jacq.) is the highest yielding edibleil crop in the world. It is cultivated in 42 countries in 11 million haorldwide (Khalil et al., 2008b). West Africa, South east asian coun-

ries like Malaysia and Indonesia, Latin american countries andndia are the major oil palm cultivating countries (Joseph et al.,006). 1 ha oil palm plantation annually produces about 55 ton ofry matter in the form of fibrous biomass while yielding 5.5 ton ofil (Hasamudin and Soom, 2002). From oil palm tree, lignocellu-osic fibers can be extracted from trunk, frond, fruit mesocarp andmpty fruit bunch (EFB). Empty fruit bunch is the fibrous mass leftehind after separating the fruits from sterilized (steam treatmentt 294 kPa for 1 h) fresh fruit bunches (FFB). Among the variousber sources in an oil palm tree, EFB has potential to yield up to3% fibers (Wirjosentono et al., 2004) and hence it is preferable inerms of availability and cost (Rozman et al., 2000). Palm oil indus-ry has to dispose about 1.1 ton of EFB per every ton of oil producedKarina et al., 2008). Some quantity of this highly cellulosic mate-ial is currently used as boiler fuel (Sreekala et al., 1997), in the

reparation of fertilizers or as mulching material (Singh et al., 1982)hereas major portion is left in mill premises itself. When left ineld, these waste materials create great environmental problemsSreekala et al., 1997; Law et al., 2007). A view of EFB wastes piledp for disposal in a palm oil mill in India is shown in Fig. 1. Realiz-

Fig. 1. View of EFB wastes piled up in a palm oil mill premise.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

ing the potential of this natural fiber, a number of research studieshave been undertaken in the recent past on its characterization andutilization, particularly in biocomposite applications.

Biocomposites are defined as the materials made by combiningnatural fiber and petroleum derived non-biodegradable polymer orbiodegradable polymer. Biocomposites derived from natural fiberand crop/bioderived plastic (biopolymer/bioplastic) are likely tobe more eco-friendly and such composites are termed as greencomposites (John and Thomas, 2008). Use of natural fiber as fillerin polymeric matrix offers several advantages over conventionalinorganic fillers with regard to their low energy cost, positive con-tribution to global carbon budget (Hill and Khalil, 2000b), greaterdeformability, biodegradability (Rozman et al., 2003), combustibil-ity, ease of recyclability (Sreekala et al., 2004), good thermal andinsulation properties (Hariharan and Khalil, 2005), lower den-sity, less abrasiveness to processing equipment, environmentallyfriendly nature, lower cost (Raju et al., 2008), renewable nature,non-toxicity, flexible usage, high specific strength (Yousif andTayeb, 2008), good electrical resistance, good acoustic insulationproperty, worldwide availability, etc. Apart from these, natural fiberhas the additional advantage of being composted or the calorificvalue recovered at the end of their life cycle, which is not possiblewith glass fibers (Hill et al., 1998).

There is a growing interest on natural fiber composites in variousfields due to these advantages. Automotive giants such as Daim-ler chrysler use flax–sisal fiber mat embedded in an epoxy matrixfor the door panels of Mercedes benz E-class model (John andThomas, 2008). Coconut fibers bonded with natural rubber latexare being used in seats of the Mercedes benz A-class model. TheCambridge Industry (an automotive industry in MI, USA) is makingflax fiber-reinforced polypropylene for Freightliner century COE C-2 heavy trucks and also rear shelf trim panels of the 2000 modelChevrolet impala. Besides automotive industry, lignocellulosic fibercomposites have also found their application in building and con-struction industries such as for panels, ceilings, and partition boards(Hariharan and Khalil, 2005). Nowadays fiber-reinforced plasticcomposites find applications in fields such as aerospace, automo-tive parts, sports and recreation equipment, boats, office products,machinery, etc. (Sreekala et al., 2002a).

This article is a compilation of the research developments in therecent past on characterization of the fiber extracted from oil palmempty fruit bunch, its utilization in biocomposites and their prop-erties. Various methods to improve compatibility of the fiber andpolymeric matrix and its effect on properties of fibers and result-

ops and Products 33 (2011) 7–22 9

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ng composites are also discussed. The term oil palm fiber is used inlace of oil palm empty fruit bunch fiber throughout this paper foronvenience.

. Oil palm fiber

.1. Extraction

Oil palm fiber (OPF) is extracted from EFB by retting process. Thevailable retting processes are mechanical retting (hammering),hemical retting (boiling with chemicals), steam/vapor/dew ret-ing and water/microbial retting. Water retting is the most popularrocess among these (Raju et al., 2008). Many researchers followedetting method for extraction of the fiber to use in their experi-ents (Sreekala et al., 1997; Sreekala et al., 2005; Joseph et al.,

006). Mechanical extraction is environmentally friendly whereashe other methods pollute water bodies. A machine for extractionf fiber from oil palm EFB was developed by Jayashree et al. (2002).t decorticates EFB, separate pith materials and grade fiber into dif-erent fractions. A sketch of FFB and cross section of EFB showingber arrangement is shown in Fig. 2.

.2. Morphology and properties

Oil palm fiber is hard and tough, which shows similarity to coirbers (Sreekala et al., 1997; Ibrahim et al., 2005). The scanning elec-ron microscopic (SEM) image (Khalil et al., 2008b) of the transverseection of OPF (Fig. 3) shows a lacuna like portion in the middle sur-ounded by porous tubular structures (Sreekala et al., 2002b). Theores on fiber surface have an average diameter of 0.07 �m. Thisorous surface morphology is useful for better mechanical inter-

ocking with matrix resin in composite fabrication (Sreekala et al.,997). However the porous surface structure also facilitates pene-ration of water into the fiber by capillary action, especially whent is exposed to water (Hill and Khalil, 2000b). The vessel elementsn cross sectional view of the fibers are clearly seen in the longitu-inal view also (Fig. 4). Some researchers are of opinion that eachber is a bundle of many fibers (Yousif and Tayeb, 2007; Yousif andayeb, 2008) indicating that these vessel elements are nothing but

ndividual fibers.

Granules of starch are found in the interior of the vascular bun-le as shown in Fig. 4 (Law et al., 2007). Silica bodies are alsoound in great number on the fiber strand. They attach them-elves to circular craters which are spread uniformly over the fiber

Fig. 3. SEM images of transverse sectio

Fig. 2. Sketch of (A) oil palm FFB (B) Cross section of EFB showing fiber arrangement.

surface. The silica bodies, though hard, can be dislodged mechani-cally, leaving behind perforated silica-crater, which would enhancepenetration of matrix in composite fabrication. This results bet-ter fiber–matrix interfacial adhesion. The chemical composition

and physico-mechanical properties of OPF as reported by variousresearchers are summerised in Tables 1 and 2, respectively.

High cellulose content (Sreekala et al., 2004) and high tough-ness value (John et al., 2008) of OPF make it suitable for compositeapplications. However presence of hydroxyl group makes the fibers

ns of OPF (4×). F: fiber; L: lacuna.

10 S. Shinoj et al. / Industrial Crops an

Fig. 4. SEM image of longitudinal section of OPF (750×).

Table 1Chemical composition of OPF.

Constituents Range

Cellulose (%) 42.7–65 Khoo and Lee (199(2008b)

Lignin (%) 13.2–25.31 Ismail et al. (1997AbuBakar et al. (2

Hemicellulose (%) 17.1–33.5 Hill and Khalil (20Holocellulose (%) 68.3–86.3 Khoo and Lee (199

(2007), Rozman eAsh content (%) 1.3–6.04 Ismail et al. (1997

AbuBakar et al. (2(2008b)

Extractives in hot water (100 ◦C) (%) 2.8–14.79 Khoo and Lee (199Wirjosentono et a

Solubles in cold water (30 ◦C) (%) 8–11.46 Sreekala et al. (19Alkali soluble (%) 14.5–31.17 Khoo and Lee (199

Wirjosentono et aAlfa-cellulose (%) 41.9–60.6 Ismail et al. (1997Alcohol–benzene solubility (%) 2.7–12 Ismail et al. (1997Pentosan (%) 17.8–20.3 Ismail et al. (1997Arabinose (%) 2.5 Law et al. (2007)Xylose (%) 33.1Mannose (%) 1.3Galactose (%) 1.0Glucose (%) 66.4Silica (EDAX) (%) 1.8Copper (g/g) 0.8Calcium (g/g) 2.8Manganese (g/g) 7.4Iron (g/g) 10.0Sodium (g/g) 11.0

Table 2Physico-mechanical properties of OPF.

Property Range

Diameter (�m) 150–500 Sreekala and ThomMicrofibrillar angle (◦) 46 Bismarck et al. (20Density (g/cm3) 0.7–1.55 Sreekala and ThomTensile strength (MPa) 50–400 Sreekala et al. (20

Tayeb (2008)Young’s modulus (GPa) 0.57–9 Sreekala et al. (19

al. (2007a), YousifElongation at break (%) 4–18 Bismarck et al. (20

(2007a), Yousif anTensile strain (%) 13.71 Rao and Rao (2007Length-weighted fiber length (mm) 0.99 Law et al. (2007)Cell-wall thickness (�m) 3.38Fiber coarseness (mg/m) 1.37Fines (<0.2 mm) (%) 27.6Rigidity index, (T/D)3 × 10−4 55.43

d Products 33 (2011) 7–22

hydrophilic, causing poor interfacial adhesion with hydrophobicpolymer matrices during composite fabrication. This may lead topoor physical and mechanical properties of the composite (Raju etal., 2008). Oil palm fibers contain 4.5% of residual oil (AbuBakar etal., 2006). The fiber–matrix compatibility is adversely affected byoil residues, the ester components of which may affect couplingefficiency between fiber and polymer matrix as well as the inter-action between fiber and coupling agents (Rozman et al., 2001c).The fiber properties can be improved substantially through surfacemodifications. Chemical treatments decrease hydrophilic propertyof the fibers and also significantly increase wettability with poly-mer matrix (Khalid et al., 2008b). There are number of treatmentmethods on OPF to improve its properties and make it compatiblewith polymeric matrices.

2.3. Surface treatments

The treatments to improve fiber–matrix adhesion in compositesinclude chemical modification of fiber (using anhydrides, epoxies,isocyanates, etc.), grafting of polymers into lignocellulosic and use

References

1), Sreekala et al. (1997), Hill and Khalil (2000b), Law et al. (2007), Khalil et al.

), Sreekala et al. (1997), Hill and Khalil (2000b), Wirjosentono et al. (2004),006), Khalil et al. (2007a), Rozman et al. (2007)00b), Khalil et al. (2007a), Law et al. (2007), Khalid et al. (2008a)1), Ismail et al. (1997), Law and Jiang (2001), AbuBakar et al. (2006), Law et al.

t al. (2007), Khalid et al. (2008a), Khalil et al. (2008b)), Sreekala et al. (1997), Law and Jiang (2001), Wirjosentono et al. (2004),006), Khalil et al. (2007a), Law et al. (2007), Rozman et al. (2007), Khalil et al.

1), Ismail et al. (1997), Sreekala et al. (1997), Law and Jiang (2001),l. (2004), Law et al. (2007), Khalid et al. (2008a)97), Wirjosentono et al. (2004)1), Ismail et al. (1997), Sreekala et al. (1997), Law and Jiang (2001),

l. (2004), Law et al. (2007)), AbuBakar et al. (2006), Khalid et al. (2008a)), Sreekala et al. (1997), Wirjosentono et al. (2004), Khalid et al. (2008a)), Khalil et al. (2007a)

References

as (2003), Bismarck et al. (2005), Jacob et al. (2006b)05)as (2003), Kalam et al. (2005), Khalil et al. (2007a), Rao and Rao (2007)

04), Bismarck et al. (2005), Kalam et al. (2005), AbuBakar et al. (2006), Yousif and

97), Bismarck et al. (2005), AbuBakar et al. (2006), Jacob et al. (2006b), Khalil etand Tayeb (2007)05), Kalam et al. (2005), AbuBakar et al. (2006), Jacob et al. (2006b), Khalil et al.d Tayeb (2007))

S. Shinoj et al. / Industrial Crops and Products 33 (2011) 7–22 11

Table 3Effect of surface treatments on properties of OPF.

Treatment Effect on OPF

Mercerization Amorphous waxy cuticle layer leaches out.Latex coating Partially masks the pores on the fiber surface.� irradiation Partially eliminates the porous structure of the fiber and causes microlevel disintegration. It degrades mechanical

properties considerably.Silane treatment Imparts a coating on fiber surfaceToluene diisocyanate (TDI) treatment Makes fiber surface irregular as particles are adhered to surface.Acetylation Removes waxy layer from the surface and makes the fiber hydrophobic.Peroxide treatment Fibrillation is observed due to leaching out of waxes, gums and pectic substances.Permanganate treatment Changes the colour and makes fibers soft. Porous structure is observed after treatment.Acrylation Imparts a coating on fiber surface and removes pits containing silica bodies and keeps surface irregular. It improves

mechanical properties of fibers.Silane treatment Keeps the fiber surface undulating and improves mechanical propertiesTitanate treatment Smoothens fiber surface.Alkali treatment Makes the surface pores wider and fiber become thinner due to dissolution of natural and artificial impurities.Benzoylation Imparts a rough surface to the fibers and makes pores prominent, which helps improving the mechanical interlocking

with matrix resin.r. Rem

C Rozm(

otcr(ccm

mutwaimbsipwsIwtafii

sisy(wmfiidta

edo

Oil extraction Imparts bright colour to the fibe

ompiled from: Sreekala et al. (1997), Hill and Khalil (2000b), Sreekala et al. (2000),2006).

f compatabilizers and coupling agents (Khalil et al., 2001). Variousreatments on some lignocellulosic fibers like physical, physico-hemical, chemical grafting, etc. to improve their properties wereeported by Belgacem and Gandini (2005). John and Anandjiwala2008) reviewed the developments in chemical modification andharacterization of natural fiber–polymeric composites and theyoncluded that alkali treatment is the most common and efficientethod of chemical modification to treat natural fibers.Alkali treatment reduces OPF weight by 22%, while silane treat-

ent reduces weight by 6%. The fiber diameter also decreasedpon chemical treatment (Sreekala et al., 1997). Thermal degrada-ion of treated OPF revealed that initial degradation temperatureas higher for alkali treated fibers (350 ◦C), whereas untreated

nd acetylated fibers degraded at 325 ◦C and silane treatmentncreased the degradation temperature to 365 ◦C. Chemical treat-

ent reduces mechanical strength of fibers; however, strain toreak of the fibers increased upon all chemical treatments exceptilane treatment. Young’s modulus of OPF increased with mercer-zation and silane treatments (Sreekala and Thomas, 2003). Thehysico-mechanical properties of OPF improved to a large extenthen treated with bulk monomer allyl methacrylate (AMA) and

ubsequently cured under ultraviolet radiation (Ashraf et al., 2008).t was also observed that treatment of OPF with urea in combination

ith AMA improved both soil and water weathering characteris-ics. The response of individual fibers to applied stress is importants load applied to composite material is transferred from matrix tober. The stress-deformation behaviour of treated OPF was found

ntermediate between brittle and amorphous (Sreekala et al., 2000).Chemical modifications lead to major changes in the fibrillar

tructure of fibers and remove the amorphous components caus-ng change in the deformation behaviour. Brittleness of the fiber isubstantially reduced upon treatments. Thermo-gravimetric anal-sis of grafted OPF indicated that grafting induces thermal stabilityRaju et al., 2007). Water sorption characteristics of treated OPFere studied in detail by Sreekala and Thomas (2003). Equilibriumole percent uptake of water at 30 ◦C was 13.37% for untreated

bers, which reduced to 7.26% (mercerization), 7.65% (latex coat-ng), 7.36% (� irradiation), 8.51% (silane treatment), 6.94% (tolueneiisocyanate treatment), 7.48% (acetylation) and 7.44% (peroxidereatment). Diffusion coefficient, sorption coefficient and perme-

bility coefficient also decreased.

Interfacial shear strength (ISS) is one of the important param-ters controlling the toughness and strength of composites. Itepends on fiber surface treatment, modification of matrix andther factors affecting properties of fiber–matrix interface. The

oval of oil layer exposes surface pits and makes surface coarse.

an et al. (2001c), Zakaria and Poh (2002), Agarwal et al. (2003) and AbuBakar et al.

ISS of OPF was 1.15 MPa, 1.5 MPa, 1.8 MPa, 1.8 MPa and 1.5 MPa,respectively for polystyrene, metset (unsaturated polyester with33% styrene content), west system (epoxy), epiglass and crystic(unsaturated polyester with more than 45% styrene content) matrixsystems (Khalil et al., 2001). The acetylation treatment improvedISS of OPF in all these matrices. Improvement in ISS of OPF inpolystyrene matrix upon acetylation was more prominent thanthat of other matrices. Chemical modification of fibers creates moretransactive surface molecules that would readily form bonds withmatrix. These results indicate that ISS of OPF in thermoset matrixwas higher than that in the thermoplastic matrix due to higherwettability of thermosets than thermoplastics. The lignin contentand other main polymerics (hemicellulose, cellulose, etc.) are reac-tive to thermoset than thermoplastic, thereby improving wettingbetween the fiber and the matrix leading to higher ISS values withthermoset matrix.

The effect of some of the treatments on surface properties ofOPF as reported by various researchers is summarized in Table 3.

3. Oil palm fiber composites

Many studies were conducted in the recent past to developOPF filled both thermoset and thermoplastic composites and tocharacterize them for mechanical, physical, electrical, thermal andbiodegradation properties. Most of the studies were focussed onthe behaviour of the composites to different mechanical loadingviz., tensile, flexural, impact and static, which are of great impor-tance in structural and load-bearing applications. Studies on stressrelaxation and creep behaviour for predicting long-term mechani-cal performance, dimensional stability of load-bearing structuresand retention of clamping force were also conducted on com-posites (Sreekala et al., 2001b). Crystallization kinetics of someof the composites developed was also studied for using themin high and low temperature applications. Water absorption andswelling behaviour of composites for applications in packaging,building industry, waste water treatment, etc. have also been stud-ied. Electrical properties of composites for applications in electricalinsulators, electronic and electrical components, etc. have beenattempted. Biodegradation study on OPF composites is importantas the composites need resistance to fungal attack for use in out-

door and to compost the product rather than burning towards endof the life cycle (Khalil and Ismail, 2001).

Most of the researchers followed standard methods for testingthe composites, whereas the test methods were not mentioned insome papers. A compilation of the methods followed for conduct-

12 S. Shinoj et al. / Industrial Crops and Products 33 (2011) 7–22

Table 4Standard methods followed for conducting mechanical testing of OPF composites.

Authors Tensile strength Tear strength Flexural strength Impact strength Hardness

AbuBakar et al. (2005) – – ASTM D790 NS –AbuBakar et al. (2006) – – ASTM D790 NS –Amin and Badri (2007) – – ASTM D 790-86 ASTM D 256-88 –Badri et al. (2005) – BS 4370:Part 1: 1988 – –Badri et al. (2006) – – ASTM D790-86 ASTM D256-88 ASTM D2240Bakar and Baharulrazi (2008) ASTM D638 (Type IV) – – ASTM D256–88 –Hariharan and Khalil (2005) ASTM D638-76 – – ASTM D256 –Hill and Khalil (2000a) BS2782: part 10: 1003: 1977 – BS 2782: part 3: 335A: 1978 – –Hill and Khalil (2000b) BS2782 – BS2782 BS2782 –Ismail et al. (1997) BS 903 part A2 BS 903 part A3 – – ASTM 2240Jacob et al., 2004 ASTM D 412-68 ASTM D 624-54 – – –John et al. (2008) ASTM D412-98 ASTM D624-00 – – ASTM D2240-03Joseph et al., 2006 ASTM D 412-68 – – – –Kalam et al. (2005) NS – – – –Karina et al. (2008) – – ASTM D 790 – –Khalid et al. (2008a) ASTM 1882L – ASTM D790–97 ASTM D235 –Khalid et al. (2008b) ASTM 1822L – ASTM D790-97 ASTM D235 –Khalil and Ismail (2001) BS 2782: Part 10 – – BS 2782: Part 3: 359:1984 –Khalil et al., 2007a ASTMD638 – ASTMD790 ASTMD-256 –Khalil et al. (2000) BS 2782: Part 10: 1003: 1977 – – BS 2782: Part 3: 359; 1984 –Khalil et al. (2007b) JIS A5905 – JIS A5905 JIS A5905 –Khalil et al. (2008a) ASTM D 638 – ASTM D 790 ASTM D 256 –Raju et al. (2008) BS6746 – ASTM D 790-97 ASTM D 256-97. ASTM D2240-89Rozman et al. (2000) ASTM D618 – ASTM D790 ASTM D256 –Rozman et al. (2001a) ASTM D 3039 – – – –Rozman et al. (2001b) ASTM D618 – ASTM D790 – –Rozman et al. (2001c) ASTM D618 – ASTM D790 – –Rozman et al. (2002) – – ASTM D 790 ASTM D 256 –Rozman et al. (2003) – – ASTM D790 – –Rozman et al. (2007) ASTM D 3039 – ASTM D 790 ASTM D 256 –Sreekala et al. (2000) NS – NS NS –Sreekala et al. (2002a) ASTM D 638-76 – ASTM D 790 ASTM D 256 ASTM D 2240

–AS

N

iSpcadd

pectprf

TS

N

Wirjosentono et al. (2004) ASTM D 638-72 type IV –Zakaria and Poh (2002) – –

S—Not specified.

ng mechanical tests on OPF composites is presented in Table 4.tandard methods followed by different researchers for conductinghysical, thermal and degradation properties of OPF composites areompiled and given in Table 5. However the dynamic mechanicalnalysis, abrasion tests, thermal conductivity, thermal diffusivity,ielectric constant and thermo-gravimetric analysis were con-ucted as per no particular standard.

The composite properties are largely dependent on matrixroperties and hence selection of the matrix is based on desirednd properties of the composites. For a particular matrix–fiber

ombination, parameters like fiber content, orientation, size andreatments affect fiber–matrix bonding and end properties. Oilalm fiber has been tried in various matrices including naturalubber (NR), polypropylene (PP), polyvinyl chloride (PVC), phenolormaldehyde (PF), polyurethane (PU), epoxy, polyester, etc.

able 5tandard methods for testing physical, thermal and degradation properties of OPF compo

Authors Water absorption D

Agarwal et al. (2000) – NAmin and Badri (2007) ASTM D 570-8 –Badri et al. (2005) – –1988 (method 2)Badri et al. (2006) ASTM D570-8 –Bakar and Baharulrazi (2008) ASTM D570 AHill and Khalil (2000a) – –Karina et al. (2008) NS –Khalil et al. (2000) BS 2782: Part 4. Method 430A –Khalil et al. (2007a) ASTMD5229 –Khalil et al. (2007b) JIS A5908-1994 –Sreekala et al. (2001a) NS –Sreekala et al. (2002a) – –Sreekala et al. (2004) – –

S: Not specified.

– –TM D790-91 – –

3.1. Oil palm fiber-natural rubber composites

Natural rubber is derived from latex obtained from the sap ofrubber trees. Its use ranges from household articles to industrialproducts. Tire and tube industries are the largest consumers ofrubber and the remaining are taken up by general rubber goods(GRG) sector. A number of research studies have been carried outin the recent past on fabrication and characterization of OPF-NRcomposites.

3.1.1. Mechanical properties3.1.1.1. Effect of fiber loading. The mechanical properties of OPF-sisal-NR and OPF-NR composites reported by Jacob et al. (2004)and Joseph et al. (2006) are summarised in Table 6. Incorporationof OPF in NR matrix decreased tensile strength and elongation at

sites.

SC Degradation Density

S – –– –– BS 4370: Part 1:

– –STM D 3418–82 – –

BS: EN ISO 846: 1997 –– NSNS –– –– –– –– ASTM D 792NS –

S. Shinoj et al. / Industrial Crops an

Table 6Mechanical properties of OPF-NR and OPF-sisal-NR composites at different fibercontents.

Property Fiber content(phr)

Composite

OPF-sisal-NRa

(Jacob et al., 2004)OPF-NR (Josephet al., 2006)

Tensile strength(MPa)

5 – 19.210 1.75 –30 7.5 –50 3.25 7.28

Elongation at break(%)a

0 875 –5 – 1082

10 650 –30 800 49650 – –

Tear strength 0 20 –

bsNhtc

iTawNiTmmahlwtdr

3cAfisamlfireasdso(

3etR

(MPa)b 50 30 –

a OPF and sisal fiber in equal proportions.b The trend was not consistent at incremental fiber loadings.

reak. Natural rubber inherently possesses higher strength due totrain induced crystallization. When fibers are incorporated intoR, regular arrangement of rubber molecules was disrupted andence the crystallization ability decreased causing reduction inensile strength. The tensile strength of sisal fiber-OPF-NR hybridomposite was less than that of pure gum (John et al., 2008).

The stress relaxation rate of OPF-sisal fiber-NR hybrid compos-tes decreased with increase in fiber content (Jacob et al., 2006b).he relaxation in the gum compound was due to rubber moleculeslone and it got hindered due to fiber–rubber interface when fibersere added. The dynamic mechanical analysis of OPF-sisal fiber-R composites indicated that the storage modulus increased with

ncrease in fiber content at all temperatures (Jacob et al., 2006a).he gum compound comprising of rubber phase gives the materialore flexibility resulting in low stiffness and hence low storageodulus. The composite stiffness increases upon fiber addition

nd fibers allow greater stress transfer at the interface resulting inigh storage modulus. The loss modulus also increased with fiber

oading, reaching a maximum of 756 MPa at 50 phr fiber loadinghereas gum has loss modulus of 415 MPa. The damping parame-

er decreased with fiber loading due to lower flexibility and loweregrees of molecular motion caused by incorporation of fibers inubber matrix.

.1.1.2. Effect of fiber size. Joseph et al. (2006) introduced the con-ept of critical fiber length in transmitting load from matrix to fiber.t critical fiber length (lc), the load transmittance from matrix tober was maximum. If lc is greater than the length of the fiber,tressed fiber debonded from the matrix and the composite failedt a low load. The tensile properties of OPF-NR composites wereaximum when fiber length was 6 mm and reduced at higher fiber

engths. This was due to the fiber entanglements prevalent at higherber length. The longitudinal orientation of fibers in NR matrixesulted in higher tensile strength (19.2 MPa) than transverse ori-ntation (17.5 MPa). When longitudinally oriented, the fibers wereligned in the direction of strain, causing uniform transmission oftress. When transversely oriented, the fibers were aligned perpen-icular to the direction of the load and they could not take part intress transfer. Elongation at break was also less for the transverselyriented fibers (940%) comparing to longitudinally oriented fibers1082%).

.1.1.3. Effect of fiber treatment. Aqueous alkali treatment atlevated temperatures and various bonding agents improvedhe physical and adhesion properties of OPF-NR composites.esorsinol-formaldehyde: precipitated silica: hexamethylenete-

d Products 33 (2011) 7–22 13

tramine (5:2:5) was found as the best combination of bondingagents for OPF reinforced in NR matrix (Ismail et al., 1997). Alkalitreatment on fiber increased the tensile modulus, tensile strengthand tear strength of OPF-NR composites and the properties werebetter when NaOH of 5% concentration was used (Joseph et al.,2006). At higher alkali concentrations, excessive removal of bind-ing materials such as lignin, hemicellulose, etc. happens causingdegradation of the fiber properties and obviously the compos-ite quality. Alkali and fluro silane treatments on fibers improvedthe fiber–matrix interfacial adhesion in OPF-sisal fiber-NR hybridcomposites (Jacob et al., 2006b). The tensile strength of OPF-sisalfiber-NR hybrid composites was in the range of 6–11 MPa whenvarious treatments were done, while that of pure NR was 14 MPa(John et al., 2008). The shore A hardness of NR was 33 whereas thatfor hybrid composite was in the range of 40–82.

The storage modulus (E′) of OPF-sisal fiber-NR hybrid com-posites increased when chemical treatments were done on fibers(Jacob et al., 2006a). Higher values of E′ were exhibited by thecomposite prepared from fibers treated with 4% NaOH. Improvedinterfacial adhesion and increased surface area of fibers on alkalitreatment lead to more crosslinks within rubber matrix–fiber net-work and thus the E′ increased. The loss modulus increased from634 MPa to 655 MPa when fibers were treated with 0.5% NaOH(both contain 30 phr fibers). The loss modulus further increased to801 MPa when fibers were treated with 4% NaOH. However, treatedfiber composites exhibited low mechanical damping parameter(tan ımax). The strong and rigid fiber–matrix interface due toimproved adhesion reduces molecular mobility in the interfacialzone causing a decrease in tan ı. The presence of bonding agentsresults in more number of crosslinks being formed and alkali treat-ment on fibers leads to strengthening of these crosslinks. As aresult, molecular motion along the rubber macromolecular chainwas severely hindered, leading to low damping characteristics.

3.1.2. Water absorption characteristics3.1.2.1. Effect of fiber loading. The water absorption percentage ofOPF-NR composites increased with increase in fiber loading dueto the hydrophilicity of the fibers (Jacob et al., 2005). The waterabsorption behaviour of NR changed from Fickian to non-Fickianupon addition of OPF due to the presence of microcracks as well asdue to the viscoelastic nature of the polymer. Jacob et al. (2005)also reported that the water uptake of OPF-NR composite waslower than that of OPF-sisal fiber-NR hybrid biocomposite. Theincorporation of sisal fiber containing comparatively more holocel-lulose (23%), which is highly hydrophilic caused more water uptake.Moreover, the lignin content of OPF (19%) was higher than sisal fiber(9%). Lignin being hydrophobic prevents absorption of water.

3.1.2.2. Effect of fiber treatment. The water uptake by OPF-sisalfiber-NR hybrid composites reduced upon mercirization and alkalitreatment on fibers (Jacob et al., 2005). Mercerization improvesthe adhesive characteristics of fiber surface and alkali treatmentleads to fibrillation providing large surface area. This results bet-ter mechanical interlocking between fiber and matrix causing lesswater absorption. Besides, the treatment with NaOH solution pro-motes activation of hydroxyl groups of the cellulose by breakinghydrogen bond. Among three types of silane treatments given tothe fibers viz., fluro silane, amino silane and vinyl silane, the maxi-mum water uptake was exhibited upon vinyl silane treatment andless by flurosilane treatment.

3.1.3. Thermal propertiesGenerally, incorporation of plant fibers into polymeric matrix

increases the thermal stability of the system. In the thermal analy-sis of OPF-sisal fiber-NR hybrid composites also, it was found thataddition of more fibers resulted increase in thermal stability as indi-

14 S. Shinoj et al. / Industrial Crops and Products 33 (2011) 7–22

F

cetmumt

33recqifigpota

dotpeoatovu

oioOomaroa

ig. 5. Variation of dielectric constant with frequency as a function of OPF loading.

ated by the higher peak temperatures in the thermogram (Jacobt al., 2006b). Increased thermal stability was also confirmed byhe decrease in the activation energy of the composites. The ther-

al stability of NR-sisal fiber-OPF hybrid composites improvedpon chemical modification (Jacob et al., 2006b). However chemicalodification resulted in lower activation energy (for decomposi-

ion) values compared to the untreated fiber composite.

.1.4. Electrical properties

.1.4.1. Effect of fiber content. The dielectric constant of fiber-einforced composite system is higher due to the polarizationxerted by incorporation of lignocellulosic fibers. The dielectriconstant of OPF-sisal fiber-NR hybrid composites at all the fre-uencies tested increased with increase in fiber content as shown

n Fig. 5 (Jacob et al., 2006c). The presence of two lignocellulosicbers in NR, which is non-polar leads to the presence of polarroups giving rise to dipole or orientation polarizability. The overallolarizability of a composite is the sum of electronic, atomic andrientation polarization resulting higher dielectric constant. Hencehe dielectric constant increased with increase in fiber loading atll frequencies.

Volume resistivity of OPF-sisal fiber-NR hybrid compositesecreased when the fiber content increased due to the presencef polar groups (Jacob et al., 2006c). In polymers, higher propor-ion of the current flows through the crystalline region whereasassage of current in the amorphous region is due to the pres-nce of moisture. At a frequency of 10 kHz, the volume resistivityf pure gum reduced from 650 × 104 � m to 150 × 104 � m uponddition of 50 phr OPF-sisal fiber combination. At low volume frac-ions, there was a chaotic dispersion of fibers and the orientationf fibers was too random to promote the flow of current. At higholume fractions, the fiber population was adequate to bring aboutniform dispersion, facilitating flow of current.

Dissipation factor or loss tangent (tan ı) is defined as the ratiof electrical power dissipated in a material to total power circulat-ng in a circuit. The dissipation factor of NR matrix at a frequencyf 10 kHz increased from 0.002 to 0.035 upon addition of 50 phrPF-sisal fiber combination (Jacob et al., 2006c). The incorporationf fibers in NR matrix disrupted regular arrangement of rubberolecules leading to loss of crystallization ability. Furthermore,

ddition of fibers enhances flow of current through amorphousegion due to their ability to absorb moisture. Therefore, additionf fibers resulted in higher loss or higher amount of dissipation thatre associated with amorphous phase relaxations.

Fig. 6. Effect of fiber/cellulose loading on tensile strength of PP composites.

3.1.4.2. Effect of fiber treatment. Chemical modification of fibersdecreased the dielectric constant of OPF-sisal fiber-NR hybrid com-posites (Jacob et al., 2006c). This was due to the decrease inorientation polarization of the composites upon treatment. Chem-ical treatment reduced the hydrophilicity of the fibers leading tolowering of orientation polarization and subsequently the dielec-tric constant decreased. Though alkali treatment yielded higherdielectric constant comparing to silane treatment, higher concen-tration of alkali reduced the dielectric constant. Alkali treatmentresulted in unlocking of hydrogen bonds making them more reac-tive. In untreated state, the cellulosic-OH groups are relativelyunreactive as they form strong hydrogen bonds. In addition to this,alkali treatment can lead to fibrillation i.e. breaking down of thefibers into smaller ones. All these factors provide a large surfacearea and give a better mechanical interlocking between the fiberand matrix and lowered the overall polarity and hydrophilicity ofthe system. This resulted in reduction of orientation polarizationand consequently dielectric constant of the composites.

3.2. Oil palm fiber-polypropylene (PP) composites

Polypropylene is an addition polymer made from themonomer—propylene. It is a thermoplastic polymer mainly used inpackaging, stationery, laboratory equipments, automotive compo-nents, etc. Resistance to many chemical solvents, bases and acidsmakes it suitable for a variety of applications. Melt processing ofpolypropylene can be achieved through extrusion and moulding.However, the most common shaping technique followed is injec-tion moulding.

3.2.1. Mechanical properties3.2.1.1. Effect of fiber loading. Loading OPF in PP matrix reduced itstensile strength as shown in Fig. 6 (Khalid et al., 2008a). The reduc-tion in tensile strength upon fiber loading was due to interruptioncaused by the fiber in transferring the stress along applied forceand this problem was intensified by lack of interfacial adhesionof the fiber and the matrix. It is interesting to note that additionof OPF derived cellulose increased the tensile strength of compos-ites even though the strength lowered at lower cellulose fractions.The tensile strength was equal to that of pure PP at 40% celluloseloading. The flexural modulus increased with increase in filler load-

ing and cellulose composites exhibited higher modulus than fibercomposites. Addition of 50% cellulose to virgin PP increased flexuralmodulus from 1750 MPa to 4250 MPa, whereas OPF composite of50% fiber content increased the modulus up to 2750 MPa only. How-ever, increasing percentage of OPF reduced the flexural strength.

ops an

Icw2fit2iltpvamb

i2fir2tp1g

3ppi(sctpcHPuaa4ipiocafEn(

cui5sttrTfiwr(

S. Shinoj et al. / Industrial Cr

n an attempt to make OPF filled PP composites by reactive pro-essing, maximum tensile strength (21.4 MPa) of the compositesas obtained at 20% weight fraction of fiber (Wirjosentono et al.,

004). However the elongation at break decreased with increase inber loading. The flexural strength, flexural modulus and flexuraloughness of OPF-PP composites at 40% fiber loading were 37 MPa,.13 GPa and 47 kPa, respectively (Rozman et al., 2000). The Izod

mpact strength of pure PP was 32 J/mm, while 10% fiber and cellu-ose loadings reduced it to 25 J/mm thereafter steadily increased upo 37 J/mm when 50% cellulose was added (Khalid et al., 2008a). Oilalm fiber loading of 50% resulted same impact strength as that ofirgin PP. The irregularity in shape and low aspect ratio of OPF mayffect their capabilities to support stress transmitted from the PPatrix (Rozman et al., 2000; Khalid et al., 2008a). The effect would

e amplified if the proportion of OPF is increased.The flexural properties of OPF-glass fiber-PP hybrid compos-

tes reduced upon increasing the proportion of OPF (Rozman et al.,001b). When the whole glass fibers were replaced with OPF (30%ber content), the flexural strength, flexural modulus and flexu-al toughness values reduced from 37MPa to 20 MPa, 3.75 GPa to.75 GPa and 25 kPa to 13 kPa, respectively. The tensile strength,ensile modulus and tensile toughness of OPF-glass fiber-PP com-osites containing 30% fiber reduced, respectively, from 25MPa to3MPa, 850 MPa to 610 MPa and 180 kPa to 105 kPa when the wholelass fiber was replaced with OPF.

.2.1.2. Effect of fiber treatments. The maleic anhydride graftedolypropylene (MAPP) has significant influence on the surfaceroperties of OPF (Khalid et al., 2008b). The OPF-PP compos-

te exhibited good tensile strength (36 MPa) and impact strength38 J/m) upon incorporation of 2% MAPP. However there was aubstantial decrease in mechanical strength with increase in con-entration of MAPP above 2% in the PP matrix. Trimethylolpropaneriacrylate (TMPTA) also significantly influenced the mechanicalroperties of PP-cellulose composite. The toughness of both theomposites was significantly improved by addition of TMPTA.owever, PP-cellulose composite remained the better choice thanP-OPF composite as the tensile strength (43 MPa), flexural mod-lus (3.3 GPa) and impact strength (43 J/m) were better with theddition of 2% TMPTA. Rozman et al. (2003) reported that at Maleicnhydride (MAH) chemical loading of 15%, a flexural strength of0 MPa, flexural modulus of 4 GPa, flexural toughness of 7 kPa and

mpact strength of 80 J/m was exhibited by the OPF-PP compositesrepared with 40 percent of 60 mesh size fibers. The MAH chem-

cal loading increased the flexural strength and flexural modulusf OPF-PP composites due to the increased interfacial adhesionaused by the covalent bonding between MAH and PP. The maleicnhydride treatment has enabled the compatibility between polarunctional groups of OPF and non-polar PP. The coupling agent-43 contributed towards higher tensile strength, flexural tough-ess, flexural modulus and impact strength to OPF-PP compositesRozman et al., 2000).

The tensile strength and toughness of OPF-glass fiber-PP hybridomposites were significantly improved upon removal of resid-al oil from OPF (Rozman et al., 2001c). The tensile strength

ncreased from 7 MPa to 13 MPa and toughness increased from0 kPa to 130 kPa upon oil removal. Oil removal reduced thetress concentrations created by incompatible layer of esters ofhe oil. However, the tensile modulus (700 MPa) and elonga-ion at break (2.3%) did not vary significantly upon residual oilemoval, whereas the flexural properties increased significantly.

he flexural strength, modulus and toughness of unextractedber composite were 13 MPa, 1.9 GPa and 7.5 kPa, respectively,hich increased to 27 MPa, 2.3 GPa and 26 kPa upon residual oil

emoval. Treatment of OPF with coupling agent-polymethylenepolyphenyl isocyanate) (PMPPIC) further increased both ten-

d Products 33 (2011) 7–22 15

sile and flexural properties. Fibers treated with other couplingagents viz., maleic anhydride–modified PP (commercial nameEpolene, E-43) and 3-(trimethoxysilyl)-propylmethacrylate (TPM)also increased the strength properties of the composites inwhich the contribution by E-43 was higher (Rozman et al.,2001b).

3.2.2. Water absorption characteristics3.2.2.1. Effect of fiber loading. The water absorption of OPF-PP com-posites of varying fiber fractions at the end of a six day immersionwas in the range of 4.5–7.5% (Rozman et al., 2000). Compositeswith higher proportion of OPF absorbed more water due to cellu-lose, lignin and hemicellulose, which possess polar hydroxyl groupsleading to formation of hydrogen bonds with water. Absorption ofwater by the cell wall of lignocellulosic materials caused swellingof the cell wall and the thickness of the composite increased.The percent thickness swelling of OPF-PP composites of vary-ing fiber contents after a six day immersion varied from 2.3%to 7%.

3.2.2.2. Effect of fiber treatments. The hydrophobicity of OPF-PPcomposites enhanced upon E-43 treatment to the fiber and theeffect of treatment was further enhanced by chemical loading(Rozman et al., 2000). The increase in hydrophobicity was due to(i) the ability of maleic anhydride residue of E-43 to interact withhydroxyl groups of OPF, blocking the hydroxyl group-water hydro-gen bonding and (ii) the hydrophobicity imparted by the PP chainof E-43.

3.2.3. Degradation/weatheringWeathering decreases strength of the composites. The flex-

ural stress and flexural modulus of OPF-PP composites were41.6 MPa and 3.85 GPa, respectively, which reduced to 27.4 MPaand 2.69 MPa after soil exposure for 12 months (Hill and Khalil,2000a).

3.3. Oil palm fiber-polyurethane (PU) composites

Polyurethane polymers are formed through step-growth poly-merization by reacting a monomer containing at least twoisocyanate functional groups with another monomer contain-ing at least two hydroxyl groups in the presence of a catalyst.Polyurethanes are widely used in high resiliency flexible foamseating, rigid foam insulation panels, microcellular foam seals andgaskets, automotive suspension bushings, carpet underlay, hardplastic parts for electronic instruments, etc. Polyurethane is alsoused for moldings which include door frames, columns, windowheaders, etc. Oil palm fiber acts as a good reinforcement in PUmatrix.

3.3.1. Mechanical properties3.3.1.1. Effect of fiber loading. The mechanical properties of OPF-PU composites at different fiber contents as reported by variousresearchers are compiled in Table 7. Increase in tensile strengthwas due to increase in reaction between lignin and isocyanates informing a three-dimensional network of crosslinkings when thefiber functioned as both filler and reactive component. Modulusalso increased with percent loading of OPF obviously because of itsinherent stiffness. The hydroxyl (OH) groups present on the fiber

surface act as electron donors, while the PU carbonyl groups actas electron acceptors. These interactions can therefore be consid-ered as additional physical crosslinks within the polymer network,which increased the crosslinking capacity and consequently, thehardness of the composite.

16 S. Shinoj et al. / Industrial Crops an

Table 7The mechanical properties of OPF-PU composites at different fiber contents.

Property Fibercontent (%)

Value Reference

Tensile strength (MPa) 30 21 Rozman et al. (2007)30 10 Rozman et al. (2001a)45 30 Rozman et al. (2007)60 35 Rozman et al. (2001a)

Tensile modulus (MPa) 30 3.960 9.5

Tensile toughness (MPa) 30 1360 60

Flexural strength (MPa) 50 75 Rozman et al. (2002)65 11 Badri et al. (2006)75 4.565a 11.2 Amin and Badri (2007)

Flexural modulus (GPa) 65a 2.5550 2.25 Rozman et al. (2002)65 1.45 Badri et al. (2006)75 1.10

Flexural toughness (MPa) 50 6.5 Rozman et al. (2002)Impact strength (J/m) 50 100Impact strength (J/m2) 65a 4600 Amin and Badri (2007)

65 400075 2225 Badri et al. (2006)

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3.4.1.2. Effect of fiber treatments. Benzoylation treatment to thefibers improved the tensile strength, impact strength and stiff-ness of resulting OPF-PVC composite due to improved fiber–matrixadhesion (Bakar and Baharulrazi, 2008). However addition ofacrylic impact modifier caused a negative effect on both modu-

Table 8The mechanical properties of OPF-PVC composites at different fiber contents.

Property Value Fibercontent

Reference

Tensile strength (MPa) 60.0 0% Bakar and Baharulrazi(2008)34.7 40%

Flexural strength (MPa) 80 0% AbuBakar et al. (2005)70 40%72.4 30 phr AbuBakar et al. (2006)74.5 30 phra

Flexural modulus (GPa) 3.5 0% AbuBakar et al. (2005)4.5 40%4.14 30 phr AbuBakar et al. (2006)3.9 30 phra

Shore D hardness 65 7375 55

a The composites were hybridized with 15% (by weight) kaolinite.

.3.1.2. Effect of fiber size. The effect of fiber size on the mechanicalroperties of OPF filled PKO based PU foam was studied by Badri etl. (2005). Higher compressive stress was observed for 45–56 �mber particulate than higher sizes. This was due to the higher sur-

ace area of the fiber in powder form, which may produce betterindrance to stress-impact propagation. Scanning electron micro-copic images indicate that small size fibers got embedded in theatrix well comparing to large size fibers. However the flexural

trength of OPF filled PU composites decreased with decrease inller size (Rozman et al., 2002). Similar trend was observed forensile properties also (Rozman et al., 2001a).

.3.1.3. Effect of fiber treatments. Both toluene diisocynatereatment and hexamethylene diisocyanate (HMDI) treatmentmproved the tensile strength of OPF-PU composites (Rozman etl., 2007). At fiber loading of 40% and NCO/OH ratio of 1.1, tensiletrengths of 26 MPa, 30 MPa, 28 MPa were exhibited by untreatedber composite, TDI treated fiber composites and HMDI treatedber composites, respectively. The introduction of isocyanates

rom TDI or HMDI has enhanced the interaction between OPF andU matrix.

.3.2. Water absorption characteristics

.3.2.1. Effect of fiber loading. The water absorption by OPF-PUomposites increased sharply on the day of immersion itself andemained constant thereafter (Badri et al., 2006). Composites with5% fiber content absorbed 55% water and composites with 65%ber absorbed 48% water on the first day of immersion. Weak bond-

ng between the matrix and fiber, agglomeration of the fiber andncomplete encapsulation of matrix over OPF are the factors thatead to poor water resistivity of OPF-PU composites.

.3.3. Degradation/weatheringThe dimensional stability of OPF filled PU composites at high and

ow temperature environment was studied by Badri et al. (2005).

he dimensional stability of composite prepared using 45–56 �mber was within the allowable limits. However higher sizes caused

ncreased deformation due to the deterioration of cellular structuref the PU foam at higher fiber sizes.

d Products 33 (2011) 7–22

3.4. Oil palm fiber-polyvinyl chloride (PVC) composites

Polyvinyl chloride is a thermoplastic polymer. It is a vinyl poly-mer constructed of repeating vinyl groups having one of theirhydrogens replaced with a chloride group. Polyvinyl chloride is thethird most widely produced plastic after polyethylene and PP. It isused for sewage pipe lines and other pipe line applications as it isbiologically and chemically resistant. It is also used for windows,door frames and such other building materials by adding impactmodifiers and stabilizers. It becomes flexible on addition of plas-ticizers and can be used in cabling applications as wire insulator.Oil palm fiber is found to be a good combination with PVC to formcomposites.

3.4.1. Mechanical properties3.4.1.1. Effect of fiber loading. The mechanical properties of OPF-PVC composites at various fiber contents reported in literatureare compiled in Table 8. The tensile strength of OPF-PVC compos-ite decreased with fiber loading. The broken ends of short fibersformed during tensile deformation induced cracks in the matrix andlead to a reduction in tensile strength. The inability of OPF to sup-port stress transmitted from the matrix due to its irregular shapeand dispersion problems (agglomerate formation) was another rea-son. The low impact strength at high fiber loading was due toincapability of OPF in dissipating stress through shear yieldingprior to fracture. The reduction in flexural strength was attributedto agglomeration of the filler and its inability to support stressestransferred from the PVC-U matrix.

Raju et al. (2008) reported the reduction in ultimate tensilestrength (UTS) of pure PVC-epoxidized natural rubber (ENR) blendfrom 9 MPa to 6 MPa upon addition of 30% OPF. The reduction in ten-sile strength of composites at higher fiber loading was due to theagglomeration of the filler particles to form a domain that acts like aforeign body. Other mechanical properties viz., tensile modulus andflexural modulus increased with fiber loading, while elongation atbreak and impact strength reduced with fiber loading. There wasonly a marginal increase in hardness with fiber loading beyond 5%level. The elasticity or flexibility of the polymer chain reduced withaddition of fiber, resulting in more rigid composites. The reduc-tion in impact strength of OPF-PVC-ENR composites was due to theincrease in interfacial regions with more fiber content.

Impact strength (kJ/m2) 20 0% Bakar and Baharulrazi(2008)17.4 40%

6.1 30 phr AbuBakar et al. (2006)6.4 30 phra

a OPF was residual oil extracted.

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Property Compositecombination

Value Reference

Tensile strength(MPa)

Polyester 25 Khalil et al. (2007a)

31.5% OPF13.5% glassfiber

48

45% OPF 31

Tensile modulus(GPa)

Polyester 2.6 Khalil et al. (2007a)

31.5% OPF13.5% glass fiber

3.75

Elongation at break Polyester 3% Khalil et al. (2000)55% OPF 3.8%Polyester 2.5 mm Khalil et al. (2007a)31.5% OPF13.5% glass fiber

3.7 mm

Flexural strength(MPa)

Polyester 50 Khalil et al. (2000)

15% 3255% 43Polyester 43.3 Karina et al. (2008)12% glass fiber 165.44.8% OPF7.2% glass fiber

165.4

8.4% OPF3.6% glass fiber

143

12% OPF 36.824.5% OPF10.5% glass fiber

75 Khalil et al. (2007a)

Flexural modulus(GPa)

Polyester 3.2 Hill and Khalil (2000b)

55% OPF 3.7

Impact strength(kJ/m2)

Polyester 6 Khalil et al. (2000)

55% OPF 1824.5% OPF10.5% glass fiber

14.7 Khalil et al. (2007a)

S. Shinoj et al. / Industrial Cr

us and strength (AbuBakar et al., 2005). Oil palm fibers graftedith methyl acrylate (MA) improved the ultimate tensile strength

UTS) of the PVC-ENR blend composites (Raju et al., 2008). At 10%ber loading, untreated fiber composites exhibited UTS of 8 MPa,hich increased to 10 MPa upon grafting. Other mechanical prop-

rties viz., tensile modulus (stiffness), flexural modulus, hardnessnd impact strength also reduced upon grafting, while elongationt break increased. It is interesting to note that impact strengthf the grafted fiber composite was less even though the treatmentmproved interfacial adhesion due to better bonding, which leadso catastrophic brittle failure.

.4.2. Thermal propertiesThe glass transition temperature (Tg) of OPF-PVC-ENR compos-

te increased with fiber content (Ratnam et al., 2008). However thehermal stability was not affected significantly with addition of OPF.enzoylation treatment on fiber reduced the glass transition tem-erature (Tg) of OPF-PVC composites (64 ◦C) than that of pure PVC79.16 ◦C) whereas Tg of untreated OPF composite (80.29 ◦C) andure PVC were not significantly different (Bakar and Baharulrazi,008). Decrease in Tg was due to plasticization effect of fibers thatiffused or dissolved into the PVC matrix.

.5. Oil palm fiber-polyester composites

Polyester is a polymer which contains ester functional group ints main chain. It is used mainly in textiles and packaging industry.olyester is also used to manufacture high strength ropes, threads,oses, sails, floppy disk liners, power belting, etc. Oil palm fiber isgood reinforcement in polyester matrix.

.5.1. Physical properties

.5.1.1. Effect of fiber loading. One of the desirable functions of nat-ral fiber in polymeric matrix is to reduce the composite mass onccount of the inherent low density of the fiber. The density of OPFowder is 1.138 g/cm3 and that of polyester is 1.202 g/cm3. Addi-ion of 55% OPF in polyester matrix reduced composite density to.17 g/cm3 (Hill and Khalil, 2000b). Similarly, the density of glassber-polyester composite can be reduced by substituting glass fiberith OPF. The density of glass fiber is high (2.6 g/cm3) comparing toolyester, which obviously results in higher composite density. Oilalm fiber of 1.14 g/cm3 density used in place of glass fiber resulted

n low density composites (Karina et al., 2008). Incorporation of0% and more OPF in OPF-polyester composites caused reduction

n density from 1.15 to 1.11 g/cm3(Hill and Khalil, 2000b).

.5.2. Mechanical properties

.5.2.1. Effect of fiber loading. The mechanical properties of OPF-olyester and OPF-polyester-glass fiber composites at differentber contents are compiled in Table 9. Oil palm fiber can be a sub-titute for glass fiber in potential applications where it does notequire high load bearing capabilities. The tensile strength of OPF-lass fiber reinforced polyester composites increased up to totalber content of 45% (Khalil et al., 2007a).

The abrasion characteristics of OPF filled polyester compositesere better than pure resin (Yousif and Tayeb, 2008). Oil palmber reinforcement reduced the weight loss on abrasion of pureolyester resin by 50–60%. In addition, the friction coefficient ofPF-polyester composite was less by about 23% comparing to thatf the neat polyester (Yousif and Tayeb, 2007).

.5.2.2. Effect of fiber treatments. Alkali treatment on OPF signifi-antly improved its interfacial shear strength in polyester matrixYousif and Tayeb, 2008). Alkali treatment washed out the outerkin, better exposing fiber to the polyester matrix, leading toroper interaction between their surfaces. In addition, the fine

Rockwell hardness(HRB)

Polyester 113

55% OPF 105

holes created on alkali treatment allowed the polyester to pene-trate into the fiber bundles in a better way. Acetylation treatmentto the fibers improved impact strength of OPF-polyester compos-ites due to improved fiber wettability and resulting fewer voidspaces (Khalil et al., 2000). The tensile stress of OPF-polyestercomposites increased slightly upon both acetylation and silanetreatments and decreased upon titanate treatment (Hill and Khalil,2000b). The flexural modulus of OPF-PP composites also increasedconsiderably upon acetylation treatment on fibers. Similarly theabrasion resistance of OPF-polyester composites was enhancedupon alkali treatment to fibers (Yousif and Tayeb, 2008). Treatedfibers enhanced the adhesion resistance of polyester resin by75–85%, while untreated fibers enhanced the abrasion resistanceonly by 50–60%

3.5.3. Water absorption characteristics3.5.3.1. Effect of fiber loading. The water absorption pattern ofunsaturated OPF-polyester composite followed typical Fickian

behaviour, where the mass of water absorbed increased linearlywith a function of square root of time and then gradually decreaseduntil equilibrium plateau or complete saturation is reached (Khalilet al., 2008a). Increased water absorption of the composites is dueto swelling of the fibers, leading to crack formation in the polyester

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atrix, which act as pathways for the water molecules to dif-use into the composite material. The temperature dependence ofater absorption of OPF-polyester composites was established byhalil et al. (2000). The water absorption characteristics of OPF-lass fiber-polyester hybrid composites were studied by Khalil et al.2007a). Pure polyester resin absorbed 1% water, which increasedo 9% when 45% OPF was incorporated. The absorption percent-ge reduced to 6% when hybridized with glass fibers in the ratiof 70:30. The percentage thickness swelling of pure polyester resinhen submerged in boiling water for 2 h increased from 1.04% to

.2% when 12% glass fibers were incorporated (Karina et al., 2008).hickness swelling further increased to 2.03% when 70% of fiberraction was replaced by OPF and a maximum thickness swellingf 2.46% was observed when 100% of the fiber fraction was replacedith OPF.

.5.3.2. Effect of fiber treatments. Acetylation treatment on fiberseduced the water absorption (at 100 ◦C) of untreated OPF-olyester composites from 15.8% to 5.7% (Khalil et al., 2000).ood interfacial contact between fiber and matrix and increasedydrophobicity caused by the treatment lead to reduced waterbsorption.

.5.4. Degradation/weathering

.5.4.1. Effect of fiber loading. The loss in tensile properties of OPF-olyester composites upon degradation in soil was quantified byhalil and Ismail (2001). Loss in tensile strength by 8%, 17% and 35%as observed, respectively, after exposure of 3, 6 and 12 months.

imilarly tensile modulus, elongation at break and impact strengthlso reduced upon soil exposure. A loss of impact strength by 6%,8% and 43% was observed, respectively, after 3, 6 and 12 monthsf exposure to soil. Similarly the tensile stress, tensile modulus andlongation at break decreased from 35.1 to 34.6 MPa, 3.29 GPa to.32 MPa and 3.75 to 2.48%, respectively, upon soil burial for 12onths (Hill and Khalil, 2000a).

.5.4.2. Effect of fiber treatment. Chemical treatment to the fiberonsiderably reduced loss of mass of OPF-polyester compos-tes upon weathering (Hill and Khalil, 2000a). The magnitudef mass loss decreased in the order: unmodified fiber > titanatereated > silane treated > acetylated. Chemical treatments alsoould conserve the mechanical properties of OPF-polyester com-osites upon ageing (Khalil et al., 2000). The loss of tensile stress,ensile modulus and elongation at break of OPF-polyester com-osites during ageing in deionized water was more prominentor untreated fiber composites. Though further reduction wasbserved in the subsequent months, an increase in tensile stressnd modulus was observed upon acetylation treatment on fibern the initial 3 months period. The treatments such as acetyla-ion, silane and titanate on fibers conserved the tensile propertiesf OPF-polyester composites even after exposure in soil for manyonths (Khalil and Ismail, 2001). The loss in strength decreased in

he order: unmodified > titanate > silane > acetylated.

.6. Oil palm fiber-phenol formaldehyde (PF) composites

Phenol formaldehyde, a synthetic thermosetting resin obtainedy the reaction of phenols with aldehyde, is used for making ply-ood, particle board, medium density fiber boards and other wood

nd lignocellulose based panel and wood joinery. Phenolic lam-nates are made by impregnating one or more layers of a base

aterial such as paper, fiber or cotton with phenolic resin and lam-nating the resin-saturated base material under heat and pressure.he resin fully polymerises (cures) during this process. A num-er of research reports are available on the properties of OPF-PFomposites.

d Products 33 (2011) 7–22

3.6.1. Physical propertiesThe density of pure PF increased from 1.33 g/cm3 to 1.48 g/cm3

when 40% glass fibers were incorporated. This could be reduced to1.45 g/cm3 when 24% of fiber fraction was replaced with OPF andfurther reduction to 1.2 g/cm3 could be achieved when 96% of thefiber fraction was replaced with OPF (Sreekala et al., 2002a).

3.6.2. Mechanical properties3.6.2.1. Effect of fiber loading. The tensile and flexural strengthproperties of OPF-glass fiber-PF hybrid composite decreased withincrease in fraction of OPF (Sreekala et al., 2002a). In case of com-posite with 40% fiber content (weight fraction), tensile strengthreduced from 80 MPa to 50 MPa, tensile modulus reduced from2.5 GPa to 0.25 GPa, flexural strength reduced from 87 MPa to50 MPa and elongation at break reduced from 6.5% to 4.75%, when25% (volume fraction) of the glass fibers were replaced with OPF.However, the impact strength remained the same (230 kJ/m) evenwith OPF substitution. It was also observed that further fiber load-ing improved the impact strength.

The stress relaxation in OPF-PF composite was studied bySreekala et al. (2001b). Higher relaxation was observed for com-posites with 30% fiber content among the 20%, 30% and 40% fibercontents studied. Stress relaxation mechanism in the OPF rein-forced PF composite exhibited a two step process, wherein thefiber–matrix bond failure contributed to the first step relaxationand the second step relaxation was pre-dominated by the matrixphase relaxation. Hybridization of OPF with glass fiber in the PFmatrix reduced the relaxation rate than both pure glass fiber-PFcomposite and OPF-PF composite. This indicated that OPF withglass fiber resulted in composites of long-term higher mechanicalperformance.

Dynamic mechanical analysis of OPF-PF composite indicatedthat incorporation of OPF increased the modulus and damping char-acteristics of the pure sample (Sreekala et al., 2005). This was dueto the increase in interface area and more energy loss at the inter-face at higher fiber loading. The impact performance of PF resinalso largely improved upon reinforcement with OPF (Sreekala etal., 2000). Sreekala et al. (2005) observed that incorporation of OPFwith glass fiber increased the damping value of the OPF-PF com-posite and notably the damping increased with relative volumefraction of OPF. Both the storage modulus and loss modulus val-ues decreased after the relaxation when the relative OPF volumefraction increased in hybrid composites.

3.6.2.2. Effect of fiber treatments. Oil palm fiber-PF compositeexhibited maximum tensile strength of 40 MPa, tensile modu-lus of 1.3 GPa and elongation at break of 9% when the fiberswere treated, respectively, with permanganate, mercerizationand latex coating (Sreekala et al., 2000). The changes in ten-sile strength and tensile modulus of OPF-PF composites followedthe order: acetylated (highest) > propionylated > extracted > non-extracted (lowest) as reported by Khalil et al. (2007b). The tensilestrength and tensile modulus of acetylated fiber composite was13 MPa and 1.6 GPa, respectively. The composite with acetylatedand propionylated fibers exhibited higher flexural strength thanunmodified fiber composite. Acetylated fiber composite exhibitedflexural strength of 23 N/mm2 whereas raw fiber composite exhib-ited flexural strength of only 9 N/mm2. The stiffness of acetylatedand propionylated fiber composite was higher than unmodifiedfiber composites. This was due to the increased compatibilitybetween the resin and the fiber. The increased fiber–matrix com-

patibility upon modification resulted in formation of a continuousinterfacial region, causing better and efficient stress transfer.

The impact properties of OPF-PF composites varied in the fol-lowing order: non-extracted (lowest) < extracted < propionylated <acetylated (highest). The impact strength of OPF-PF composite

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ncreased from 6 kJ/m2 to 6.8 kJ/m2 upon acetylation treatment onber. In a study by Sreekala et al. (2000), it was observed that impacttrength of pure resin increased from 20 kJ/m2 to 80 kJ/m2 uponddition of 40% untreated OPF. It was also observed that latex coat-ng imparted impact resistance of 190 kJ/m2 followed by acetylated180 kJ/m2), silane (165 kJ/m2) and TDI (155 kJ/m2). Alkali treat-

ent decreased the relaxation rate of OPF-PF composite due totrong interfacial interlocking between fiber and matrix (Sreekalat al., 2001b). Net relaxation in isocyanate treated sample was lesshan that of untreated samples, whereas latex treated fiber com-osite exhibited maximum stress relaxation. The faster relaxation

n latex treated fiber composite was due to low interfacial bondingetween fiber and matrix upon the treatment, causing decrease intrength and stiffness to the composite.

.6.3. Water absorption characteristics

.6.3.1. Effect of fiber loading. Oil palm fiber-PF composite exhib-ted lowest percent water absorption at a fiber content of 40%,mong various fiber contents in the range of 10%–50% (Sreekalat al., 2002b). It is interesting to note that maximum mechanicalroperties were also exhibited at this fiber content. The OPF-glassber-PF hybrid composite was found more hydrophilic than unhy-ridized composite due to the poor compatibility between the OPFnd glass fiber. It was also observed that the water absorptionf OPF-PF composite was diffusion controlled and follows Fickianehaviour, whereas hybridization with glass fiber caused a devia-ion from Fickian behaviour.

.6.3.2. Effect of fiber treatments. The rate of water uptake by OPF-F composite upon different fiber treatments was in the order:xtracted (highest) > non-extracted > propionylated > acetylatedlowest) as reported by Khalil et al. (2007b). The variation betweenhe lowest and highest was 30%. Sreekala et al. (2001a) made annteresting observation that most of the fiber treatments increased

ater absorption of the composites except alkali treatment; how-ver, the treatments reduced water absorption of the fibers. Alkalireatment removes the amorphous waxy cuticle layer of the fibernd activates hydroxyl groups, leading to chemical interactionetween the fiber and matrix. In case of PF, the trend was differents it is hydrophilic whereas most polymers used for compositeabrication are hydrophobic. Therefore the more hydrophobic theber in OPF-PF composite, less the extent of fiber–matrix inter-ction, which facilitates sorption process (Sreekala et al., 2002b).or example, latex coating makes the fibers most hydrophobic andhe OPF-PF composite prepared from latex coated fibers exhibits

aximum water absorption.

.6.4. Thermal properties

.6.4.1. Effect of fiber loading. The thermal conductivity and ther-al diffusivity of OPF reinforced PF composite was less than that

f pure resin. The thermal conductivity (�) and thermal diffusiv-ty (�) of untreated OPF-PF composite of 40% fiber loading were.29 W/mK and 0.16 mm2/s, respectively, whereas the thermal con-uctivity and thermal diffusivity of pure PF were 0.348 W/mK and.167 mm2/s, respectively (Singh et al., 2003). The effective thermalonductivity of composite can also be predicted from individualhermal conductivities of fibers and matrix by employing different

odels (Agrawal et al., 1999). The incorporation of OPF in PF matrixesulted in decrease of glass transition temperature (Sreekala etl., 2005). This was due to increased void formation at higher fiber

ontent as voids facilitate the chain mobility at lower temperaturesesulting decrease in Tg value.

.6.4.2. Effect of fiber treatments. The thermal conductivity of fillersn OPF-PF composite increased after chemical treatments such as

d Products 33 (2011) 7–22 19

KMnO4, peroxide treatment, etc., obviously causing increase in con-ductivity of the resulting OPF-PF composite (Agarwal et al., 2003).The cellulose radicals formed during these treatments enhancedthe chemical interlocking at the interface. However silane treatedfiber composite exhibited lower thermal conductivity and ther-mal diffusivity compared to alkali treated fiber composite (Singhet al., 2003). Silane treatment made the fibers less hydrophilic,leading to less adhesion between fibers and hydrophilic phenolicresin. Agarwal et al. (2000) reported increase in thermal stabilityof OPF-PF composite upon fiber treatments. The lignin–cellulosecomplex formed during treatments imparted more stability to thefiber. Activation energy for crystallization and thermal stability ofOPF-PF composite quantified using differential scanning calorime-try indicated that the crystallization energies of treated anduntreated samples were not significantly different. Alkali-treatedsamples had the slowest rate of crystallization and maximumstability.

3.6.5. Degradation/weathering3.6.5.1. Effect of fiber loading. The tensile strength of OPF-PF com-posite reduced from 37 MPa to 16 MPa upon thermal ageingwhereas boiling water ageing reduced the tensile strength only upto 28 MPa (Sreekala et al., 2004). The reduction in tensile strengthwhen the specimens were aged in thermal environment was dueto the decrease in fiber–matrix adhesion owing to shrinkage ofthe fiber in thermal environment. However tensile strength ofcold water aged samples was 38 MPa and biodegraded samples(8 months) was 35 MPa. Increase in strength upon cold waterageing was due to the decrease in void size at the fiber–matrixinterphase during swelling of the fiber. This could exert a radialpressure, leading to higher tensile strength. The flexural strengthof OPF-PF composite also decreased upon ageing. It was alsofound that the Izod impact strength decreased to a very low valueupon � irradiation due to the bond scission and disintegration atthe fiber–matrix interface. However increased stress relaxation ofwater aged OPF-PF composite was observed (Sreekala et al., 2001b).This was due to the changes in interface properties attained duringageing.

3.6.5.2. Effect of fiber treatments. The modulus of OPF-PF compos-ite got enhanced when fibers were given acetylation, isocyanate,acrylate and silane treatments. Peroxide treatment on resin alsocaused increase in modulus upon thermal ageing (Sreekala et al.,2004). Similarly, the flexural properties of peroxide treated, latexmodified and acrylated fiber composite increased upon thermalageing. However, untreated and most of the treated fiber compos-ites exhibited decrease in flexural performance on water ageing. Onthe contrary, the impact strength of OPF-PF composites (41 kJ/m2)increased upon water ageing in untreated, acrylonitrile, peroxideand isocyanate treatment systems. However the impact strength ofthe composite decreased upon thermal ageing, biodegradation andalso upon � irradiation.

3.7. Oil palm fiber-polystyrene composites

Polystyrene is an aromatic polymer made from styrene,an aromatic monomer which is commercially manufacturedfrom petroleum. Polystyrene is commonly injection mouldedor extruded while expanded polystyrene is either extruded ormoulded in a special process. Solid polystyrene is used in disposable

cutlery, plastic models, CD, DVD cases, etc. Foamed polystyrene ismainly used for packaging materials, insulation, foam drink cups,etc. Polystyrene foams are good thermal insulators and thereforeused as building insulation materials such as in structural insulatedpanel building systems. They are also used for non-weight-bearing

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rchitectural structures. The information on OPF-polystyrene com-osites is limited.

.7.1. Mechanical propertiesThe modulus of OPF-polystyrene composite increased with

ncrease in fiber loading up to 30% whereas the maximum strainnd flexural strength decreased (Zakaria and Poh, 2002). The dropn strain was suspected as due to irregular shape of the fibers, whichaused inability of transferring stress from the matrix. The flex-ral properties viz., maximum stress, maximum strain, modulusf elasticity and Young’s modulus of OPF-polystyrene compositest 10% fiber content (300–500 �m size fibers) were reported as6.4 MPa, 0.027 mm, 1665.4 MPa and 2685.8 MPa, respectively. Theexural properties were not affected by fiber size when the sizeas below 300 �m. The flexural properties of polystyrene compos-

te improved upon benzoylation due to better interfacial adhesionnd hydrophobicity of the fibers.

.8. Oil palm fiber-epoxy composites

Epoxy or polyepoxide is a thermosetting polymer formed fromeaction of an epoxide resin with polyamine hardener. Epoxy issed in coatings, adhesives and composite materials. They havexcellent adhesion, chemical and heat resistance, good mechani-al and electrical insulating properties. Epoxies with high thermalnsulation and thermal conductivity combined with high electricalesistance are used for electronics applications. There are only fewtudies conducted on OPF-epoxy composites.

.8.1. Mechanical properties

.8.1.1. Effect of fiber loading. The ultimate tensile strength of OPF-poxy composite decreased from 47.78 MPa to 46.1 MPa when fiberontent increased from 35% to 55%, whereas the ultimate tensiletrength of carbon fiber-epoxy composite and pure epoxy resinere 247 MPa and 62.49 MPa, respectively (Kalam et al., 2005). This

ndicated that OPF failed to act as a reinforcement comparing to car-on fiber in epoxy resin. Oil palm fiber did not contribute towardshe fatigue strength of epoxy as increase in fiber volume ratioesulted in lowering the fatigue resistance of OPF. Oil palm fiber-poxy composite with 10% fiber would be able to support about00 kg load with a maximum deflection of 0.2 mm for a 7 m spansing the T-beam configuration (Bakar et al., 2007). This outcome

ndicated the potential use of OPF to build moderate load support-ng structures, thereby reducing the cost of short span bridge.

The tensile strength of glass fiber-epoxy composite decreasedrom 111 MPa to 24 MPa when glass fiber was replaced with OPFHariharan and Khalil, 2005). This indicated that the hybrid com-osite had intermediate strength characteristics. Oil palm fibers areomposed of individual fibers bonded by strong pectin interface,hich cause individual fibers not loaded uniformly or in some caseo loading at all. Hence the OPF is unable to support the stress trans-

erred from the epoxy matrix successfully. Poor adhesion betweenpoxy matrix and OPF also leads to a weak interfacial bond, result-ng in inefficient stress transfer between epoxy matrix and OPF.

The elongation at break of OPF-epoxy composite (4%) waslightly lower than that of glass fiber-epoxy composite (4.75%);owever, the hybrid composite exhibited higher elongation atreak. This exceptional behaviour of the hybrid composite was dueo the existence of a load sharing mechanism between the plies oflass fiber and OPF. The failed glass fiber plies were able to continueo carry the load while redistributing the remaining load to OPF ply.

he incorporation of glass fibers into OPF composite increased thetiffness of the hybrid composites. Increase in stiffness of the hybridomposite with addition of glass fibers was due to the higher tensileodulus of glass fibers (66–72 GPa) than that of OPF (1–9 GPa). ThePF-epoxy composite exhibited a lower impact strength (18 kJ/m2)

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than glass fiber composite (107 kJ/m2). As OPF composite wassubjected to a high speed impact load, the sudden stress trans-ferred from the matrix to the fiber exceeded the fiber strength,resulting in fracture of OPF at the crack plane without any fiberpullout.

4. Conclusions

The fiber extracted from oil palm empty fruit bunch (EFB) isa byproduct in palm oil mills. Availability of EFB in bulk in palmoil mills and technology available for mechanical extraction offiber from EFB makes industrial level processing of OPF viable. Oilpalm fiber is hard and tough, which shows similarity to coir fibersand its porous surface morphology is useful for better mechan-ical interlocking with matrix resin for composite fabrication. Ithas been used in combination with various polymeric matricesincluding natural rubber (NR), polypropylene (PP), polyvinyl chlo-ride (PVC), phenol formaldehyde (PF), polyurethane (PU), epoxy,polyester, etc. to form biocomposites. Alkali treatment is the mostcommon treatment on OPF to improve fiber–matrix interfacialadhesion. Similarly, removal of residual oil from oil palm fiber sur-face improves the interfacial adhesion between fiber and matrix.Fiber size also has an influence on the properties of the resultingcomposites.

Oil palm fiber loading in some polymeric matrices improved thestrength properties whereas the strength of composite was less insome cases. Glass fiber-OPF hybrid composite is more hydrophilicthan single fiber composite due to incompatibility between both.The thermal stability, dielectric constant, electrical conductivity,etc. were improved upon incorporation of OPF. The strength prop-erties reduced upon weathering/degradation of the composite.Sisal fiber is a good combination with OPF in hybrid composites.

Information on the effect of OPF size on the composite propertiesis limited. Information on thermal properties, electrical resistance,high voltage break down characteristics, degradation/weatheringcharacteristics, deflection in thermal environment, rheology, resis-tance to various chemicals, etc. of the OPF composites is alsolimited and such studies need to be taken up. Oil palm fiber treat-ment strategies to further improve the interfacial adhesion withwide variety of polymeric matrices need to be taken up as thefiber–polymer bonding is crucial in determining the compositeproperties. Completely biodegradable or green composites usingOPF and biodegradable matrices need to be developed in future.The biodegradability of the composites and effect of degradationon various properties also need to be evaluated.

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