carbohydrate polymers volume 71 issue 3 2008 [doi 10.1016_j.carbpol.2007.05.040] maya jacob john;...

7/25/2019 Carbohydrate Polymers Volume 71 Issue 3 2008 [Doi 10.1016_j.carbpol.2007.05.040] Maya Jacob John; Sabu Thomas -- Biofibres and Biocomposites

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Review

Biofibres and biocomposites

Maya Jacob John, Sabu Thomas *

School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O., Kottayam, Kerala 686 560, India

Received 14 February 2007; received in revised form 22 May 2007; accepted 24 May 2007Available online 10 June 2007

Abstract

This review deals with a recent study of the literature on the various aspects of cellulosic fibres and biocomposites. Cellulosic fibrereinforced polymeric composites are finding applications in many fields ranging from construction industry to automotive industry.The pros and cons of using these fibres are enumerated in this review. The classification of composites into green composites, hybridbiocomposites and textile biocomposites are discussed. New developments dealing with cellulose based nanocomposites and electrospin-ning of nanofibres have also been presented. Recent studies pertaining to the above topics have also been cited. Finally, the applicationsof cellulosic fibre reinforced polymeric composites have been highlighted.2007 Elsevier Ltd. All rights reserved.

Keywords: Biocomposite; Biofiber; Green composite; Textile; Hybrid

1. Lignocellulosic fibres/natural fibres

Natural fibres are subdivided based on their origins,coming from plants, animals or minerals. All plant fibresare composed of cellulose while animal fibres consist ofproteins (hair, silk, and wool). Plant fibres include bast(or stem or soft sclerenchyma) fibres, leaf or hard fibres,seed, fruit, wood, cereal straw, and other grass fibres. Overthe last few years, a number of researchers have beeninvolved in investigating the exploitation of natural fibresas load bearing constituents in composite materials. Theuse of such materials in composites has increased due totheir relative cheapness, their ability to recycle and forthe fact that they can compete well in terms of strength

per weight of material. Natural fibres can be consideredas naturally occurring composites consisting mainly of cel-lulose fibrils embedded in lignin matrix. The cellulose fibrilsare aligned along the length of the fibre, which render max-imum tensile and flexural strengths, in addition to provid-ing rigidity. The reinforcing efficiency of natural fibre isrelated to the nature of cellulose and its crystallinity. The

main components of natural fibres are cellulose (a-cellu-lose), hemicellulose, lignin, pectins, and waxes.

Cellulose is a natural polymer consisting of D-anhydro-glucose (C6H11O5) repeating units joined by 1,4-b-D-glyco-sidic linkages at C1 and C4 position (Nevell & Zeronian,1985). The degree of polymerization (DP) is around10,000. Each repeating unit contains three hydroxylgroups. These hydroxyl groups and their ability to hydro-gen bond play a major role in directing the crystalline pack-ing and also govern the physical properties of cellulose.Solid cellulose forms a microcrystalline structure withregions of high order i.e. crystalline regions and regionsof low order i.e. amorphous regions. Cellulose is alsoformed of slender rod like crystalline microfibrils. The crys-

tal nature (monoclinic sphenodic) of naturally occurringcellulose is known as cellulose I. Cellulose is resistant tostrong alkali (17.5 wt%) but is easily hydrolyzed by acidto water-soluble sugars. Cellulose is relatively resistant tooxidizing agents.

Hemicellulose is not a form of cellulose and the name isa misnomer. They comprise a group of polysaccharidescomposed of a combination of 5- and 6-carbon ring sugars.Hemicellulose differs from cellulose in three aspects.Firstly, they contain several different sugar units whereascellulose contains only 1,4b-D-glucopyranose units.

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* Corresponding author. Tel.: +91 481 2730003; fax: +91 481 2561190.E-mail addresses: [email protected], [email protected] (S.

Thomas).

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Secondly, they exhibit a considerable degree of chainbranching containing pendant side groups giving rise toits non crystalline nature, whereas cellulose is a linear poly-mer. Thirdly, the degree of polymerization of native cellu-lose is 10100 times higher than that of hemicellulose. Thedegree of polymerization (DP) of hemicellulose is around

50300. Hemicelluloses form the supportive matrix for cel-lulose microfibrils. Hemicellulose is very hydrophilic, solu-ble in alkali and easily hydrolyzed in acids.

Lignin is a complex hydrocarbon polymer with both ali-phatic and aromatic constituents. They are totally insolu-ble in most solvents and cannot be broken down tomonomeric units. Lignin is totally amorphous and hydro-phobic in nature. It is the compound that gives rigidity tothe plants. It is thought to be a complex, three-dimensionalcopolymer of aliphatic and aromatic constituents with veryhigh molecular weight. Hydroxyl, methoxyl and carbonylgroups have been identified. Lignin has been found to con-tain five hydroxyl and five methoxyl groups per building

unit. It is believed that the structural units of lignin mole-cule are derivatives of 4-hydroxy-3-methoxy phenylpro-pane. The main difficulty in lignin chemistry is that nomethod has been established by which it is possible to iso-late lignin in its native state from the fibre. Lignin is consid-ered to be a thermoplastic polymer exhibiting a glasstransition temperature of around 90 C and melting tem-perature of around 170 C(Olesen & Plackett, 1999). It isnot hydrolyzed by acids, but soluble in hot alkali, readilyoxidized, and easily condensable with phenol (Bismarck,Mishra, & Lampke, 2005).

Pectins are a collective name for heteropolysaccarides.

They give plants flexibility. Waxes make up the last partof fibres and they consist of different types of alcohols.

Biofibres can be considered to be composites of hollowcellulose fibrils held together by a lignin and hemicellulosematrix (Jayaraman, 2003). The cell wall in a fibre is not ahomogenous membrane (Fig. 1). Each fibril has a complex,layered structure consisting of a thin primary wall that isthe first layer deposited during cell growth encircling a sec-ondary wall. The secondary wall is made up of three layersand the thick middle layer determines the mechanical prop-erties of the fibre. The middle layer consists of a series of

helically wound cellular microfibrils formed from longchain cellulose molecules: the angle between the fibre axisand the microfibrils is called the microfibrillar angle. Thecharacteristic value for this parameter varies from one fibreto another.

Such microfibrils have typically a diameter of about 10

30 nm and are made up of 30100 cellulose molecules inextended chain conformation and provide mechanicalstrength to the fibre. The amorphous matrix phase in a cellwall is very complex and consists of hemicellulose, lignin,and in some cases pectin. The hemicellulose molecules arehydrogen bonded to cellulose and act as cementing matrixbetween the cellulose microfibrils, forming the cellulosehemicellulose network, which is thought to be the mainstructural component of the fibre cell. The hydrophobic lig-nin network affects the properties of other network in away that it acts as a coupling agent and increases the stiff-ness of the cellulose/hemicellulose composite.

The structure, microfibrillar angle, cell dimen-

sions,defects, and the chemical composition of fibres arethe most important variables that determine the overallproperties of the fibres (Satyanarayana et al., 1986). Gener-ally, tensile strength and Youngs modulus of fibresincreases with increasing cellulose content. The microfibril-lar angle determines the stiffness of the fibres. Plant fibresare more ductile if the microfibrils have a spiral orientationto the fibre axis. If the microfibrils are oriented parallel tothe fibre axis, the fibres will be rigid, inflexible and havehigh tensile strength. Some of the important biofibres arelisted inTable 1.

1.1. Cellulosic fibres: advantages and disadvantages

The growing interest in lignocellulosic fibres is mainlydue to their economical production with few requirementsfor equipment and low specific weight, which results in ahigher specific strength and stiffness when compared toglass reinforced composites. They also present safer han-dling and working conditions compared to synthetic rein-forcements. Biofibres are nonabrasive to mixing andmolding equipment, which can contribute to significantcost reductions. The most interesting aspect about naturalfibres is their positive environmental impact. Biofibres are arenewable resource with production requiring little energy.They are carbon dioxide neutral i.e. they do not returnexcess carbon dioxide into the atmosphere when they arecomposted or combusted. The processing atmosphere isfriendly with better working conditions and therefore therewill be reduced dermal and respiratory irritation. Biofibrespossess high electrical resistance. Thermal recycling is alsopossible. The hollow cellular structure provides goodacoustic insulating properties. The worldwide availabilityis an additional factor.

The inherent polar and hydrophilic nature of lignocellu-losic fibres and the non-polar characteristics of most ther-moplastics results in compounding difficulties leading to

non-uniform dispersion of fibres within the matrix whichFig. 1. Structure of biofibre.

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impairs the efficiency of the composite. This is a major dis-advantage of natural fibre reinforced composites. Anotherproblem is that the processing temperature of composites isrestricted to 200 C as vegetable fibres undergo degrada-tion at higher temperatures; this restricts the choice ofmatrix material. Another setback is the high moistureabsorption of natural fibres leading to swelling and pres-ence of voids at the interface, which results in poormechanical properties and reduces dimensional stabilityof composites. Another restriction to the successful exploi-tation of biofibres for durable composite application is lowmicrobial resistance and susceptibility to rotting. Theseproperties pose serious problems during shipping, storageand composite processing. The non-uniformity and varia-tion of dimensions and of their mechanical properties (evenbetween individual plants in the same cultivation) posesanother serious problem.

It is quite clear that the advantages outweigh the disad-vantages and most of the shortcomings have remedial mea-sures in the form of chemical treatments. Thelignocellulosic fibres have an advantage over synthetic ones

since they buckle rather than break during processing and

fabrication. In addition, cellulose possesses a flattened ovalcross section that enhances stress transfer by presenting aneffectively higher aspect ratio.

Researchers (Kozlowski, Muzyczek, & Mieleniak, 2004)at the Institute of Natural Fibres, Poland have developedfire resistant upholstery using fire-retarded flax nonwoven

(LinFR) FT. The non-woven used in the composites playsthe role of fire barrier that reduces the vulnerability of fill-ing material to the development and spread of fire. Thesoftness of the upholstery system is also enhanced. Inanother interesting study, researchers (Zimniewska, Koz-lowski, & Rawluk, 2004) investigated the influence of nat-ural and synthetic fibres which covered the forearmmuscles on the activity of motor units in these muscles.The electrophysiological parameters of motor units weremeasured with electromyographical methods. The resultsindicated that temporary covering of examined muscles inthe forearm with the synthetic clothing changed the patternof motor units activity of muscles. Clothing made of natu-

ral fibre did not evoke this kind of effect.Recently, natural cellulose fibres having properties

between cotton and linen and suitable for various indus-trial applications were extracted from cornhusks (Reddy& Yang, 2005a). High quality cellulosic fibres from corn-husks mean food, clothing and other major industrialproducts from the same source without the need for anyadditional natural resources. Using cornhusks for fibrousapplications would save the land and other naturalresources required to grow fibre crops and will conservethe non-renewable petroleum sources required to producesynthetic fibres. More than 9 million tons of natural cellu-

lose fibres with a potential sale value of $19 billion andwith a value addition of at least $12 billion could be pro-duced from the cornhusks available every year. Utilizingcornhusks as a source for natural cellulose fibres will signif-icantly benefit the agriculture, fibre, food and energy needsof the future and will also benefit the environment. Thestructure and composition of the natural cellulose fibresobtained from cornstalks were found to be different fromthat of the common bast fibres such as flax and kenaf.Cornstalk fibres were found to have relatively lower per-centage of crystallinity but similar microfibrillar angle asthat of the common bast fibres (Reddy & Yang, 2005a,2005b). Another study reported on the production andcharacteristics of natural cellulose fibers obtained fromthe leaves and stems of switchgrass. The composition,structure and properties of fibers obtained from the leavesand stem of switchgrass were studied in comparison to thecommon natural cellulose cotton, linen and kenaf. Theleaves and stems of switchgrass were found to have tensileproperties intriguingly similar to that of linen and cotton,respectively. Fibers obtained from switchgrass leaves pos-sessed crystallinity of 51%, breaking tenacity of 5.5 g perdenier (715 MPa) and breaking elongation of 2.2% whereasthe corresponding values for fibers obtained from switch-grass stems were 46%, 2.7 g per denier and 6.8%, respec-

tively (Reddy & Yang, 2005a, 2005b).Shih (2007)studied

Table 1List of important biofibres

Fibre source Species Origin

Abaca Musa textilis LeafBagasse GrassBamboo (>1250 species) GrassBanana Musa indica Leaf

Broom root Muhlenbergia macroura RootCantala Agave cantala LeafCaroa Neoglaziovia variegata LeafChina jute Abutilon theophrasti StemCoir Cocos nucifera FruitCotton Gossypiumsp. SeedCuraua Ananas erectifolius LeafDate palm Phoenix Dactylifera LeafFlax Linum usitatissimum StemHemp Cannabis sativa StemHenequen Agave fourcroydes LeafIsora Helicteres isora StemIstle Samuela carnerosana LeafJute Corchoruscapsularis StemKapok Ceiba pentranda Fruit

Kenaf Hibiscus cannabinus StemKudzu Pueraria thunbergiana StemMauritius hemp Furcraea gigantea LeafNettle Urtica dioica StemOil palm Elaeis guineensis FruitPiassava Attalea funifera LeafPineapple Ananus comosus LeafPhormium Phormium tenas LeafRoselle Hibiscus sabdariffa StemRamie Boehmeria nivea StemSansevieria (Bowstring hemp) Sansevieria LeafSisal Agave sisilana LeafSponge gourd Luffa cylinderica FruitStraw (Cereal) StalkSun hemp Crorolaria juncea StemCadillo/urena Urena lobata StemWood (>10,000 species) Stem

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the fiber obtained from the water bamboo husks by CP/MAS C13 NMR, elemental analysis, and adiabatic calo-rimeter. The pyrolysis behavior of the fiber was also stud-ied by TGA, TGAGCMS and PyGCMS. The fiberobtained from the water bamboo husks was found to becellulose I.

2. Cellulosic fibre composites

The history of mankind has witnessed several surges inthe field of research and development. The rampant useof petroleum products has created a twin dilemma; deple-tion of petroleum resources and entrapment of plastics inthe food chain and environment. The increasing pollutioncaused by the use of plastics and emissions during inciner-ation is affecting the food we eat, water we drink, air webreathe and threatening the greatest right of human beings,the right to live. The exhaustive use of petroleum basedresources has initiated the efforts to develop biodegradable

plastics. This is based on renewable biobased plant andagricultural products that can compete in the markets cur-rently dominated by petroleum based products. The pro-duction of 100% biobased materials as substitute forpetroleum based products is not an economical solution.A more viable solution would be to combine petroleumand biobased resources to develop a cost-effective producthaving immense applications. Biopolymers or syntheticpolymers reinforced with natural or bio fibres (termed asbiocomposites) are a viable alternative to glass fibre com-posites. Scientists are looking at the various possibilitiesof combining biofibres such as sisal, flax, hemp, jute,

banana, wood and various grasses with polymer matricesfrom non-renewable and renewable resources to form com-posite materials to make the biocomposite revolution areality (Mohanty, Drzal, & Misra, 2002).

Broadly defined, biocomposites are composite materialsmade from natural/bio fibre and petroleum derived non-biodegradable polymers (PP, PE) or biodegradable poly-mers (PLA, PHA). The latter category i.e. biocompositesderived from plant derived fibre (natural/biofibre) andcrop/bioderived plastic (biopolymer/bioplastic) are likelyto be more eco-friendly and such composites are termedas green composites.

The best known renewable resources capable of makingbiodegradable plastics are starch and cellulose (Wilkinson,2001). Starch is one of the least expensive biodegradablematerials available in the world market today. It is a versa-tile polymer with immense potential for use in non-foodindustries. Cellulose from trees and cotton plants is a sub-stitute for petroleum feedstocks to make cellulose plastics.Another aspect that has gained global attention is thedevelopment of biodegradable plastics from vegetable oilslike soybean oil, peanut oil, walnut oil, sesame oil and sun-flower oil. Green composites from soy protein based bio-plastics and natural fibres show potential for rigidpacking and housing and transportation applications

(Tummala, Mohanty, Misra, & Drzal, 2003). Fish oil based

polymers have also attracted the attention of researchersdue to their high degree of unsaturation. Fish oil basedpolymers also possess unique good damping and shapememory properties (Technology International Exchange,1996).

The history of fibre reinforced plastics began in 1908

with cellulose fibre in phenolics, later extending to ureaand melamine and reaching commodity status with glassfibre reinforced plastics. The fibre-reinforced compositesmarket is now a multibillion-dollar business (MaterialThoughts, 2002) (Fig. 2). Though hailed as a miraculousdiscovery long back, plastic products now enjoy anambiguous reputation. Scientists at the BioCompositesCentre at the University of Wales, Bangor are developinghigh-quality packaging of goods, using starch from cornand potatoes to tackle the problem of high cost waste dis-posal (Material Thoughts, 2005a, 2005b). Researchers(Material Thoughts, 2005a, 2005b) are also exploringthe aspects of producing plastics from limonene which is

extracted from citrus. After decades of obscurity, biofibrereinforced composites are being touted as the material ofthe millennium.

2.1. Classification of composites

2.1.1. Green composites

Research efforts are currently being harnessed in devel-oping a new class of fully biodegradable green compositesby combining (natural/bio) fibres with biodegradable resins(Netravali & Chabba, 2003). The major attractions aboutgreen composites are that they are environmentally-

friendly, fully degradable and sustainable, i.e. they are trulygreen in every way. At the end of their life they can be eas-ily disposed of or composted without harming theenvironment.

Fig. 3shows a classification of biodegradable polymersin four families (Averous & Boquillon, 2004). Except thefourth family, which is of fossil origin, most polymers(family 13) are obtained from renewable resources (bio-mass). The first family is agro-polymers (e.g. polysaccha-rides) obtained from biomass by fractionation. Thesecond and third families are polyesters, obtained, respec-tively by fermentation from biomass or from genetically

modified plants (e.g. PHA, polyhydroxyalkanoate) and

Fig. 2. Fibre reinforced plastic composites used in 2002.



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by synthesis from monomers obtained from biomass (e.g.PLA, polylactic acid). The fourth family are polyesters,totally synthesized by the petrochemical process (e.g.PCL, polycaprolactone; PEA, polyesteramide; aliphaticor aromatic copolyesters). A large number of these biode-gradable polymers (biopolymers) are commercially avail-able. They show a large range of properties and they cancompete with non-biodegradable polymers in differentindustrial fields (e.g. packaging).

Another important biocomposites category is based onagro-polymers matrixes, mainly focussed on starchy mate-

rials. Plasticized starch, the so-called thermoplasticstarch (TPS) is obtained after disruption and plasticiza-tion of native starch, with water and plasticizer (e.g. pol-yol) by applying thermomechanical energy in acontinuous extrusion process. Unfortunately, TPS showssome drawbacks such as a strong hydrophilic character(water sensitive), rather poor mechanical properties com-pared to conventional polymers and an important post-processing variation of the properties. TPS propertiesreach equilibrium only after several weeks. To improvethese material weaknesses, TPS is usually associated withother compounds.

The design and life cycle assessment of green compositeshave been exclusively dealt with by Baillie (2005). Greencomposites have been used effectively in many applicationssuch as mass produced consumer products with short lifecycles or products intended for one time or short timeuse before disposal. Green composites may also be usedfor indoor applications with a useful life of several years.The reinforcement of biofibres in green composites hasbeen highlighted byBismarck et al. (2005).

A number of natural and biodegradable matrices thatare available for use in such green composites (Stevens,2002) are listed inTable 2. The classification of green com-posites based upon the different types of biodegradable

matrices is given below.

2.1.1.1. Biofiberplastic composites. Poly(a-hydroxy acid)such as poly(glycolic acid), PGA, or poly(lactic acid),PLA, are crystalline polymers with relatively high meltingpoint. Recently PLA has been highlighted because of itsavailability from renewable resources like corn. PLA is ahydrophobic polymer because of the incorporation of theCH3 side groups when compared to PGA. Poly(b-hydrox-yalkanoate)s (PHAs), which are synthesized biochemicallyby microbial fermentation and which may be produced inthe future by transgenic plants, represent natural polyes-ters. Bacteria came first and are still the only real sourceof these polyesters; it will still require some more yearsresearch until transgenic plants will be available for pro-duction. Poly(b-hydroxybutyrate) (PHB) (commercialname Biopol) is a biotechnologically produced polyesterthat constitutes a carbon reserve in a wide variety of bacte-ria and has attracted much attention as a biodegradablethermoplastic polyester. It can be degraded to water and

carbon dioxide under environmental conditions by a vari-

Fig. 3. Classification of biodegradable polymers.

Table 2Natural and biodegradable matrices

Biodegradable polymer matrices

Natural Synthetic

1. Polysaccharides 1. Poly(amides)Starch 2. Poly(anhydrides)Cellulose 3. Poly(amide-enamines)Chitin 4. Poly(vinyl alcohol)2. Proteins 5. Poly(vinyl acetate)Collagen/gelatin 6. PolyestersCasein, albumin, fibrogen, Silks 6.1 Poly(glycolic acid)3. Polyesters 6.2 Poly(lactic acid)

Polyhydroxyalkanoates 6.3 Poly(caprolactone)4. Other polymers 6.4 Poly(orthoesters)Lignin 7. Poly(ethylene oxides)Lipids 8. Poly(phosphazines)ShellacNatural rubber



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ety of bacteria and has much potential for applications ofenvironmentally degradable plastics (Mohanty, Misra, &Hinrichsen, 2000).

In an interesting research Plackett, Andersen, Peder-sen, and Nielsen (2003) used commercial L-polylactidewhich was first converted to film and then used in com-

bination with jute fibre mats to generate biodegradablecomposites by a film stacking technique. Degradationof the polylactide during the process was investigatedusing size exclusion chromatography. The tensile proper-ties of composites produced at temperatures in the 180220 C range were significantly higher than those ofpolylactide alone. Examination of composite fracturesurfaces using electron microscopy showed voids occur-ring between the jute fibre bundles and the polylactidematrix in some cases. Size exclusion chromatographyrevealed that only minor changes in the molecularweight distribution of the polylactide occurred duringthe process.

In an innovative study, novel biodegradable films werefabricated from chitosan and poly(lactic acid) bySebastien,Stephane, Copinet, and Coma (2006). It was found thatcomposite films offered a great advantage in preventingthe surface growth of mycotoxinogen strains because oftheir antifungal activity. However, the physico-chemicalproperties of such heterogeneous films, dramatically limittheir further usage as packaging materials.

Lee and Wang (2006) investigated the effects of lysine-based diisocyanate (LDI) as a coupling agent on the prop-erties of biocomposites from poly(lactic acid) (PLA),poly(butylene succinate) (PBS) and bamboo fiber (BF).

They observed that the tensile properties, water resistance,and interfacial adhesion of both PLA/BF and PBS/BFcomposites were improved by the addition of LDI, butthermal flow (Ohkita & Lee, 2006) was hindered due tocross-linking between polymer matrix and BF. Enzymaticbiodegradability of PLA/BF and PBS/BF composites wasinvestigated by Proteinase K and Lipase PS, respectively.It was seen that the composites could be quickly decom-posed by enzyme and the addition of LDI delayed the deg-radation. Fig. 4 shows the SEM micrographs of PLA orPBS/BF (70/30) composites with LDI degraded for differ-ent degradation time. It can be clearly seen that the matrixof both composites became reduced as enzymatic degrada-tion proceeded.

Green composites composed of regenerated cellulose(lyocell) fabric and biodegradable polyesters [poly(3-hydroxybutyrate-co-3-hydroxyvarelate) (PHBV), poly-(butylene succinate) (PBS), and poly(lactic acid) (PLA)]were prepared byShibata, Oyamada, Kobayashi, and Yag-inuma (2004) and properties studied. The tensile moduliand strength of all the biodegradable polyester/lyocell com-posites were found to increase with increasing fiber con-tent. The biodegradation test revealed that the lyocellfabric in PLA/lyocell composite degraded, as shown inFig. 5. Pure PLA film was found to be almost unchanged

after 120 days.

The thermal stability of biodegradable composite filmsprepared from blends of poly(vinyl alcohol), cornstarch,and lignocellulosic fibre was investigated byImam, Cinelli,Gordon, and Chiellini (2005). Thermogravimetric analysis(TGA) indicated the suitability of formulations for meltprocessing, and for application as mulch films in fields at

much higher temperatures. The study also revealed thatboth starch and lignocellulosic fibre degraded much morerapidly than PVA. The addition of fibre to the formula-tions was found to enhance the PVA degradation. Inanother interesting study, the thermal behavior of greencomposites fabricated from bagasse fibre and polyvinylalcohol was investigated byFernandes, Cinelli, and Chiel-lini (2004). They observed an increase of thermal stabilityupon incorporation of bagasse fibre.

In another study biocomposites (Mehta, Mohanty, Mis-ra, & Drzal, 2004) were fabricated using a non-woven fibremat (90% hemp fibre with 10% thermoplastic polyester bin-der) as reinforcement, and unsaturated polyester (UPE)

resin as well as blends of UPE and functionalized vegetableoils as the polymer matrix. All composites were made with30% volume fraction of fibre, which was optimized earlier.The structureproperty relationships of this system as wellas the thermo-mechanical properties of these compositeswere measured. The notched Izod impact strength of bio-composites from biobased resin blends of UPE and func-tionalized vegetable oil and industrial hemp fibre matwere enhanced by 90% as compared to that of the pureUPE-industrial hemp fibre mat composites. The tests alsoshowed an improvement in the tensile properties of thecomposite as a result of the incorporation of the deriva-

tized vegetable oil.Researchers have also developed green composites from

jute fabric and Biopol composites (Mohanty, Khan,Sahoo, & Hinrichsen, 2000). Chemical modification ofthe fabric was carried out to improve interfacial properties.A significant amount of research has been done at the Ger-man Aerospace Centre (DLR) in Braunschweg on biode-gradable plastics and composites (Peijs, 2002). Starch andmodified resins have also been used as matrix to form greencomposites (Goda, 2002).

Biodegradable composites were developed frompoly(propylene carbonate) (PPC) reinforced with granularcornstarch byMa, Yu, and Zhao (2006). The authors alsoinvestigated the effects of succinic anhydride (SA) on themorphology, thermal properties, as well as mechanicalproperties of the composites. The authors are of the opin-ion that SA enhances the properties of PPC/starch com-posites, which was ascribed to the improvement of theinteraction between PPC and granular.

In another study (Ge, Xu, Meng, & Li, 2005) involvingpoly(propylene carbonate) (PPC), starchg-poly(methylacrylate) (Sg-PMA) copolymer was used to reinforcePPC. Starchg-poly(methyl acrylate) (Sg-PMA) copoly-mer was prepared by ceric ammonium nitrate-initiatedpolymerization of methyl acrylate onto corn starch (CS).

Tensile tests showed that the Sg-PMA incorporation



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improved the stiffness and tensile strength of compositessignificantly. TG results indicated that the Sg-PMA addi-tion led to an improvement in the thermal stability ofresulting composites.

Bio-based polymer composite was from plant-derivedkenaf fiber (KF) and biodegradable polyester, poly(L-lac-tide) (PLLA), by melt-mixing technique, by Pan et al.(2007). The effect of the KF weight contents (0, 10, 20,and 30 wt%) on crystallization behavior, composite mor-

phology, mechanical, and dynamic mechanical properties

of PLLA/KF composites were investigated. It was foundthat the incorporation of KF significantly improves thecrystallization rate and tensile and storage modulus. Thecrystallization of PLLA can be completed during the cool-ing process from the melt at 5 C/min with the addition of10 wt% KF. It was also observed that the nucleation den-sity increases dramatically and the spherulite size dropsgreatly in the isothermal crystallization with the presenceof KF. In addition, with the incorporation of 30 wt%

KF, the half times of isothermal crystallization at 120

Fig. 4. SEM micrographs of PLA or PBS/BF composites with LDL (NCO content, 0.65%) after different enzymatic degradation time. (a) Control ofPLA/BF composite, (b) after 7 days, (c) 9 days, (d) control of PBS/BF composite, (e) after 2 days, (f) 4 days.

Fig. 5. Photographs of PLA (top) artd PLA/lyocell composite (bottom) after the soil burial test: (a) before test, (b) at 60 days, and (c) at 120 days.



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and 140 C were reduced to 46.5% and 28.1% of the purePLLA, respectively. Moreover, the tensile and storagemodulus of the composite are improved by 30% and28%, respectively, by the reinforcement with 30% KF.Scanning electron microscopy observation also showedthat the crystallization rate and mechanical properties

could be further improved by optimizing the interfacialinteraction and compatibility between the KF and PLLAmatrix.

The morphology, mechanical and thermal properties ofcomposites of poly(L-lactide) with hemp fibers was investi-gated by Masirek, Kulinski, Chionna, Piorkowska, andPracella (2007). The thermogravimetric analysis of thecomposites, carried out in both nitrogen and air, showedthat the degradation process of fiber-filled systems startedearlier than that of plain PLA, independently of the pres-ence of the plasticizer. Mechanical tests showed that themodulus of elasticity of the composites markedly increasedwith the hemp content, reaching 5.2 GPa in the case of

crystallized PLA reinforced with 20 wt% hemp, whereasthe elongation and stress at break decreased with anincreasing amount of fiber for all examined systems.

2.1.1.2. Biofibersoy based composites. In US, soybeansprovide over 60% of the fats and oils used for food andthe majority of the feed protein. Soybeans typically containabout 20% oil and 40% protein. Protein levels as high as55% have been observed in soybeans. Soybeans consist ofdiscrete groups of proteins (polypeptides) that span abroad range of molecular sizes and are comprised ofapproximately 38% of non-polar, non-reactive amino acid

residues, while 58% are polar and reactive. Modificationsthat take advantage of water solubility and reactivity areexploited in improving soy protein for use in plastics andother biomaterials (Thames, 1994). Soy protein plasticsof different compositions have been prepared by injectionmoulding (Jane, 1996). Compression moulding is also usedfor soy plastic processing. Dried soy plastics display anextremely high modulus, 50% higher than that of currentlyused epoxy engineering plastics (Wang, Sue, & Jane, 1994).So with proper moisture-barrier, soy protein is a potentialstarting material for engineering plastics.

Researchers recently investigated the effect of stearicacid on tensile and thermal properties of ramie fibre rein-forced soy protein isolate (SPI) resin green composites(Lodha & Netravali, 2005). It was observed that part ofthe stearic acid crystallized in SPI resin and that the crys-tallizability was affected by the addition of glycerol as aplasticizer. The fabricated green composite was found tohave enormous potential for certain indoor applications.

The interfacial adhesion in the composites was apparentfrom SEM photomicrographs of the fracture tensile sur-faces of the ramie/SPI and ramie/modified soy protein iso-late (MSPI) composites tested in the axial direction isshown in Fig. 6a and b, respectively. Some of the fibresat the fracture surface do not show any resin, where as a

few other fibres clearly show resin sticking on the surface.

The presence of resin on some fibre surfaces for the SPIand MSPI resins clearly indicates good interfacial interac-tion between the fibre and the resins.

In an interesting study, researchers (Chabba, Mathews,& Netravali, 2005) modified soy flour (SF) by cross-linkingit with glutaraldehyde (GA). The cross-linked soy flour

(CSF) polymer was characterized for its tensile and thermalproperties. The effect of glycerol on the mechanical proper-ties of the soy flour was also characterized and optimized.CSF polymer showed improved tensile properties and ther-mal stability, compared to unmodified SF resin, for use as aresin to fabricate composites. Unidirectional green com-posites using flax yarn and CSF resin were fabricated andcharacterized for their tensile and flexural properties. Thecomposite specimens exhibited fracture stress and Youngsmodulus of 259.5 MPa and 3.71 GPa, respectively, andflexural strength of 174 MPa, in the longitudinal direction.

Recently green composites were fabricated using pineap-ple leaf fibre and soy based plastic (Liu, Misra, Askeland,

Drzal, & Mohanty, 2005). The addition of compatabilizer(polyester amide grafted glycidyl methacrylate (PEA-g-GMA)) was seen to increase the mechanical properties ofcomposites. In another interesting study involving biocom-posites, the effect of alkali treatment on the thermal prop-erties of Indian grass fibre reinforced soy protein greencomposites was studied by the same group (Liu, Mohanty,Askeland, Drzal, & Misra, 2004).

The characterization of Phytagel modified soy proteinisolate (SPI) resin and unidirectional flax yarn reinforcedgreen composites was undertaken byLodha and Netravali(2005). The incorporation of Phytagel in SPI resin led to

an overall 10-fold increase in the tensile fracture stress anda 9-fold increase in the Youngs moduli of the SPI resinalong with a 7-fold decrease in fracture strain. The dynamicmechanical properties such as storage and loss modulus ofthe modified resin increased and the glass transition tem-perature also increased by about 56 C.

2.1.1.3. Biofibernatural rubber composites. The primaryeffects of bio-fibre reinforcement on the mechanical proper-ties of natural rubber composites include increased modu-lus, increased strength with good bonding at high fibreconcentrations, decreased elongation at failure, greatlyimproved creep resistance over particulate-filled rubber,increased hardness and a substantial improvement in cut,tear and puncture resistance. Biodegradation of vulcanizedrubber material is possible, although it is difficult due to theinterlinkages of the poly(cis-1,4-isoprene) chains, whichresult in reduced water absorption and gas permeabilityof the material (Seal & Morton, 1996). Certain Streptomy-ces strains were seen to degrade rubber samples after 12months of incubation.

The reinforcement of coir fibre in natural rubber hasbeen extensively studied (Geethamma, Joseph, & Thomas,1995). Researchers have also investigated the reinforcementeffects of a leaf fibre sisal fibre in natural rubber (Var-

ghese, Kuriakose, Thomas, & Koshy, 1994).



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Attempts to incorporate oil palm fibre in rubber matrixhave also been successful. The effect of fibre concentrationon the mechanical properties of oil palm reinforced naturalrubber composites was investigated byIsmail, Rosnah, and

Ishiaku (1997). They observed the general trend of reduc-tion in tensile and tear strength with increasing fibreconcentration.

An interesting report on the reinforcement effect of grassfibre bagasse in natural rubber was presented byNas-sar, Ashour, and Washid (1996). Aging experimentsrevealed tensile strength retention of 97%. Scientists havealso developed composites comprising of kenaf fibre andnatural rubber (El Sabbagh et al., 2000). An increase inrheometric and mechanical properties was observed. Pine-apple (Bhattacharya, Biswas, Chaterjee, & Pramanick,1986) and jute fibre (Arumugam, Tamareselvy, Venkata

Rao, & Rajalingam, 1989) have also found their way as apotential reinforcement in natural rubber.

In an interesting study, researchers have used a novelfibre isora fibre in natural rubber (Mathew, Joseph, &Joseph, 2004). Isora fibres are present in the bark of theHelicteres isora plant and are separated by retting process.Isora fibre resembles jute in appearance but surpasses it instrength, durability and lustre. The effects of differentchemical treatments, including mercerisation, acetylation,benzoylation and treatment with toluene diisocyanateand silane coupling agents, on isora fibre properties andmechanical properties were analyzed. Isora fibre was seento have immense potential as reinforcement in natural rub-ber. The variation of tensile strength with chemical modifi-cation is given in Fig. 7.

Researchers have also designed novel rubber biocom-posites by using a combination of leaf and fruit fibre in nat-ural rubber (Haseena & Unnikrishnan, 2005). Theincorporation of sisal and coir fibre in NR was seen toincrease the dielectric constant of the composites. Thesehybrid biocomposites were found to have enormous appli-cations as antistatic agents. In another interesting study,the preparation of composites comprising of waste paperin natural rubber along with boron carbide and paraffinwax, for radiation shielding applications, was investigated

(Madani, Basta, Abdo, & El-Saied, 2004).

In an innovative study, a unique combination of sisaland oil palm fibres in natural rubber has been utilized todesign hybrid biocomposites. It was seen that the incorpo-ration of fibers resulted in increased modulus (Jacob,Varughese, & Thomas, 2004a, 2004b). Chemical modifica-tion of both sisal and oil palm fibres was imperative forincreased interfacial adhesion and resulted in enhancedproperties (Jacob et al., 2004a, 2004b). The viscoelastic(Jacob, Francis, Varughese, & Thomas, 2006), water sorp-tion (Jacob, Varughese, & Thomas, 2005), dielectric(Jacob, Varughese, & Thomas, 2006a, 2006b) and stressrelaxation (Jacob et al., 2006a, Jacob, Varughese, & Tho-mas, 2006b) characteristics were also studied.

A major hurdle to the commercialization of green com-posites is the high cost of biodegradable matrices. Most ofthe biodegradable resins cost significantly more than thecommonly used resins. Inexpensive production of oils forresins through biotechnology would certainly be of helpin expediting their commercialization. The manufacturingcosts can also be reduced by faster, better and more effi-cient processing.

Efforts are on the anvil for the development ofadvanced green composites made out of high strengthprotein fibres (spider silk) and biodegradable matrices. Bio-

technology is being used to increase the yield of specific tri-

Fig. 6. (a, b) SEM photomicrographs of the fracture tensile surfaces of untreated and treated composites.

Fig. 7. Variation of tensile strength with chemical modification.



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glycerides and oils in beans for producing resins. These res-ins will be inexpensive compared to those available todayand if suitably modified, could be biodegradable. Researchis also being conducted to develop new pathways to synthe-size inexpensive biodegradable resins (Allen, Moore, Lob-kovsky, & Coates, 2002) with better mechanical

properties and thermal stability using nanotechnology(Maiti & Giannelis, 2002).

2.1.2. Hybrid composites

The incorporation of several different types of fibres intoa single matrix has led to the development of hybrid bio-composites. The behavior of hybrid composites is aweighed sum of the individual components in which thereis a more favorable balance between the inherent advanta-ges and disadvantages. Also, using a hybrid composite thatcontains two or more types of fibre, the advantages of onetype of fibre could complement with what are lacking in theother. As a consequence, a balance in cost and perfor-mance could be achieved through proper material design(Thwe & Liao, 2003). The properties of a hybrid compositemainly depend upon the fibre content, length of individualfibres, orientation, extent of intermingling of fibres, fibre tomatrix bonding and arrangement of both the fibres. Thestrength of the hybrid composite is also dependent on thefailure strain of individual fibres. Maximum hybrid resultsare obtained when the fibres are highly strain compatible(Sreekala, George, Kumaran, & Thomas, 2002).

The properties of the hybrid system consisting of twocomponents can be predicted by the rule of mixtures.

PH P1V1 P2V2 ; 1:1:1

where PH is the property to be investigated, P1 the corre-sponding property of the first system and P2 the corre-sponding property of the second system. V1 and V2 arethe relative hybrid volume fractions of the first and secondsystem and

V1 V2 1: 1:1:2

A positive or negative hybrid effect is defined as a positiveor negative deviation of a certain mechanical propertyfrom the rule of hybrid mixture.

The term hybrid effect has been used to describe the phe-nomenon of an apparent synergistic improvement in theproperties of a composite containing two or more typesof fibre. The selection of the components that make upthe hybrid composite is determined by the purpose ofhybridization, requirements imposed on the material orthe construction being designed. The problem of selectingthe type of compatible fibres and the level of their proper-ties is of prime importance when designing and producinghybrid composites. The successful use of hybrid compositesis determined by the chemical, mechanical and physical sta-bility of the fibre/matrix system.

Hybrid biocomposites can be designed by the combina-tion of a synthetic fibre and natural fibre (biofibre) in a

matrix and a combination of two natural fibre/biofibre in

a matrix. Hybridization with glass fibre provides a methodto improve the mechanical properties of natural fibre com-posites and its effect in different modes of stress depends onthe design and construction of the composites. The effect ofhybridization of glass fibre in thermoset biocomposites hasbeen discussed in detail (Ray & Rout, 2005).

The tensile and impact behavior of oil palm fibre-glassfibre-reinforced epoxy resin was investigated by Bakar,Hariharan, and Abdul Khalil (2005). The hybridization ofoil palm fibres with glass fibres increased the tensile strength,Youngs modulus, and elongation at break of the hybridcomposites. A negative hybrid effect was observed for thetensile strength and Youngs modulus while a positivehybrid effect was observed for the elongation at break ofthe hybrid composites. The impact strength of the hybridcomposites increased with the addition of glass fibres.

Cellular biocomposite cores fabricated from industrialhemp or flax fibres with unsaturated polyester were hybrid-ized with woven jute, chopped glass, and unidirectional

carbon fabrics (Burgue~no et al., 2005). Material character-ization showed improved stiffness, strength, and moisture-absorption stability, while flexural tests on laboratory-scaleplates demonstrated enhanced structural behavior. Thesehybrid cellular biofibre-based composites were found toprovide an economic and environmentally friendlier alter-native to entry-level synthetic composites.

Hybridization also has a profound effect on the waterabsorption property of composites. An attempt to studythe moisture uptake characteristics of hybrid systems wasperformed byMishra et al. (2003). The composite systemschosen were sisal/glass and pineapple/glass fibre reinforced

polyester composites. Composites were prepared by vary-ing the concentration of glass fibre and by subjecting thebio-fibres to different chemical treatments. The authorsobserved that water uptake of hybrid composites were lessthan that of unhybridized composites.

A comparative study of the water absorption of the glassfibre (7 wt%)/natural biofibre (13 wt%) with that of non-hybrid composites is given inTable 3where a lowering inwater absorption is evident (Rout, Mishra, Tripathy,Nayak, & Mohanty, 2001).

The novel development of a photofabrication process ofbiofibre composites, based on oil palm empty fruit bunchfibres was recently reported (Kumar, Hee, & Rozman,

Table 3Comparative study of the water absorption of the glass fibre/naturalbiofibre with that of non-hybrid composites

Sample Water absorption (%)

Nonhybrid (coirpolyestercomposite) (20 wt%)

Hybrid [coir/glasspolyester composite]

Untreated 8.53 5.186Alkali- treated (5%) 4.994 3.147PMMA grafted (5%) 3.98 2.663PAN grafted (10%) 4.119 2.997Cyanoethylated 3.6 3.138

Bleached 5.8 3.718



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2005). The process consisted of the following steps: (1) thepreparation of nonwoven mat of biofibre, either alone or incombination with glass and nylon; (2) drying the mat; (3)preparation of photocurable resin matrix, consisting ofvinyl ester and photoinitiator; (4) impregnation of themat by photocurable resin; and (5) irradiation of the

impregnated mat by UV radiation to effect the cure ofthe composite. Oil palm fibre, glass, and nylon fibres weremixed in different proportions. A mixture experimentaldesign was used to generate experimental compositionsof the reinforcing fibres and to model dependency of theresponse variables on the components through mathemat-ical relationships.

The flexural behaviour of hybrid laminated composites(LC) with a hemp fibre/polypropylene core and two glassfibres/polypropylene surface layers at each side of the spec-imen were investigated by Reis, Ferreira, Antunes, andCosta (2007). When compared with full glass fibres rein-forced polypropylene laminates, the hybrid composites were

found to have economical, ecological and recycling advan-tages and also specific fatigue strength benefits. Static andfatigue tests were performed in three point bending for bothlaminates to evaluate flexural strength properties and fati-gue behaviour. The major findings were that laminated com-posites (LC) exhibited an ultimate strength about 4% higherthan the hybrid laminated composites (HLC) associated tochanges in failure mechanisms, while the stiffness moduluswas also about 3.8% higher. Fatigue strength of hybrid lam-inated composites was also about 20% lower than the lami-nated composites as consequence of the change of the failuremechanisms and of the different static strengths.

Scientists (Morye & Wool, 2005) at the Affordable Com-posites from Renewable Resources (ACRES) program atthe University of Delaware investigated the mechanicalproperties of glass/flax hybrid composites based on a novelmodified soybean oil matrix material. Composites with dif-ferent glass/flax ratios and different fibre arrangementswere made using a modified soybean oil matrix material.The fibre arrangement was varied to make symmetric andunsymmetric composites. The latter were tested in differentmodes in flexural tests and drop weight impact tests. Themechanical properties of the composites were found todepend upon the glass/flax ratio and the arrangement offibres in the composite. On proper selection of the arrange-ment of fibres in the composite, the glass fibres and flaxfibres were found to act synergistically resulting in animproved flexural and impact performance.

The mechanical, water absorption, and thermal proper-ties of injection-molded short hemp fiber/glass fiber-rein-forced polypropylene hybrid composites was investigatedbyPanthapulakkal and Sain (2007). The authors observedthat hybridization with glass fiber enhanced the perfor-mance properties. A value of 101 MPa for flexural strengthand 5.5 GPa for the flexural modulus is achieved fromhybrid composite containing 25 wt% of hemp and 15 wt%of glass. Notched Izod impact strength of the hybrid com-

posites exhibited great enhancement (34%) compared to

unhybridized samples. Thermal properties and resistanceto water absorption properties of the hemp fiber compos-ites were improved by hybridization with glass fibers.

The ballistic properties of flax, hemp, and jute fabricsteel hybrid polypropylene composites processed by hotcompression moulding were investigated by Wambua,

Vangrimde, Lomov, and Verpoest (2007). The compositesballistic limit (V50) was determined by subjecting the mate-rial to ballistic impact loading by fragment simulating pro-jectiles (FSPs) using a block manometric cannoninterchangeable (BMCI) gun. Flax composites exhibitedbetter energy absorption than hemp and jute composites.The composites failed by shear cut-out, delamination andfibre fracture. It was found that the ballistic properties ofthe hemp composites increased significantly when a mildsteel plate was used as facing and backing.

The mechanical and thermal properties of hybrid bio-composites from oil palm empty fruit bunch (EFB) fibersand kaolinite in polyurethane was studied by Amin and

Badri (2007). The occurrence of chemical interactionsbetween the hydroxyl terminals in both fillers and the PUsystem was determined via FTIR spectroscopy. Hybridbio-composites showed improved stiffness, strength, andbetter water resistance with the addition of kaolinite toan extent. At 15% of kaolinite loading, maximum flexuraland impact strengths were observed. The interactionbetween kaolinite with PU matrix and EFB fibers enhancedthe mechanical properties of the bio-composites.

The effect of the various layering patterns on the waterabsorption of the banana-glass hybrid composites was stud-ied byPothan, Cherian, Anandakutty, and Thomas (2007).

It was found that water diffusion occurs in the compositedepending on the layering pattern as well as the tempera-ture. In all the experiments, it was found that compositeswith an intimate mixture of glass and banana show the max-imum water uptake except for temperature of 90 C. At90 C the maximum water uptake is found to be for compos-ites where there is one layer of banana and another layer ofglass. The water uptake follows the same trend as that in allother temperatures till a time span of 4900 min is reached.The kinetics of diffusion was found to be Fickian in nature.

Another innovative approach to hybrid composites isthe incorporation of two natural fibres/biofibre in a matrixsystem. Recently, the mechanical performance of short ran-domly oriented banana and sisal hybrid fibre reinforcedpolyester composites was investigated with reference tothe relative volume fraction of the two fibres at a constanttotal fibre loading of 0.40 volume fraction (Vf), keepingbanana as the skin material and sisal as the core material.A positive hybrid effect was observed in the flexuralstrength (Fig. 8) and flexural modulus of the hybrid com-posites. The tensile strength of the composites showed apositive hybrid effect when the relative volume fraction ofthe two fibres was varied, and maximum tensile strengthwas found to be in the hybrid composite having a ratioof banana and sisal 4:1 (Idicula, Neelakantan, Oommen,

Joseph, & Thomas, 2005).



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As a continuation to the above study the dynamic andstatic mechanical properties of randomly oriented inti-

mately mixed banana and sisal hybrid fibre reinforcedpolyester composites were reported. Maximum stress trans-fer between the fibre and matrix was obtained in compos-ites having volume ratio of banana and sisal as 3:1. Thestorage modulus was found to increase with fibre volumefraction above the glass transition temperature of the com-posites (Idicula, Malhotra, Joseph, & Thomas, 2005).

Idicula et al. (2006) investigated the thermophysicalproperties of banana-sisal hybrid reinforced composites asa function of chemical modification. Sisal and banana fiberswere subjected to mercerization and polystyrene maleicanhydride treatments. The authors observed that chemicalmodification resulted in an increase of 43% in thermal con-ductivity when compared to untreated composites.

The tensile properties of wood flour/kenaf fiber polypro-pylene hybrid composites were investigated byMirbagheri,Tajvidi, Hermanson, and Ghasemi (2007). The hybrideffect on the elastic modulus of the composites was alsoinvestigated using the rule of hybrid mixtures and Hal-pinTsai equations. The relationship between experimentaland predicted values was evaluated and accuracy estima-tion of the models was performed. The results indicatedthat while nonhybrid composites of kenaf fiber and woodflour exhibited the highest and lowest modulus valuesrespectively, the moduli of hybrid composites were closely

related to the fiber to particle ratio of the reinforcements.Rule of hybrid mixtures equation was able to predict theelastic modulus of the composites better than HalpinTsaiequation.

Researchers have also explored the incorporation of afruit and leaf fibre in natural rubber (Haseena, Dasan,Namitha, Unnikrishnan, & Thomas, 2005). The additionof sisal and coir fibres offered good reinforcement andresulted in improvement of properties.

2.1.3. Textile composites

The development of textile technologies such as weaving,

knitting and braiding has resulted in the formation of com-

posites that have superior mechanical properties, as contin-uous orientation of fibres is not restricted at any point.

In applications where more than one fibre orientation isrequired, a fabric combining 0and 90fibre orientations isuseful. Woven fabrics are produced by the interlacing ofwarp (0) fibres and weft (90) fibres in a regular pattern

or weave style. The fabrics integrity is maintained by themechanical interlocking of the fibres. Drape (the abilityof a fabric to conform to a complex surface), surfacesmoothness and stability of a fabric are controlled primar-ily by the weave style.

2.1.4. Common weave architectures

The following is a description of some of the more com-monly found weave styles:

Plain

Each warp fibre passes alternately under and over eachweft fibre. The fabric is symmetrical, with good stability

and reasonable porosity. However, it is the most difficultof the weaves to drape, and the high level of fibre crimpimparts relatively low mechanical properties comparedwith the other weave styles. With large fibres (high tex) thisweave style gives excessive crimp and therefore it is notgenerally used for very heavy fabrics.

Twill

One or more warp fibres alternately weave over andunder two or more weft fibres in a regular repeated manner.This produces the visual effect of a straight or broken diag-onal rib to the fabric. Superior wet out and drape is seen inthe twill weave over the plain weave with only a small reduc-

tion in stability. With reduced crimp, the fabric also has asmoother surface and slightly higher mechanical properties.

Fig. 8. Variation of flexural strength with volume fraction of fibre.



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Satin

Satin weaves are fundamentally twill weaves modified toproduce fewer intersections of warp and weft. The harnessnumber used in the designation (typically 4, 5, and 8) is thetotal number of fibres crossed and passed under, before thefibre repeats the pattern. A crowsfoot weave is a form of

satin weave with a different stagger in the repeat pattern.Satin weaves are very flat, have good wet out and a highdegree of drape. The low crimp gives good mechanicalproperties. Satin weaves allow fibres to be woven in theclosest proximity and can produce fabrics with a closetight weave. However, the styles low stability and asym-metry needs to be considered. The asymmetry causes oneface of the fabric to have fibre running predominantly inthe warp direction while the other face has fibres runningpredominantly in the weft direction.

Basket

Basket weave is fundamentally the same as plain weave

except that two or more warp fibres alternately interlacewith two or more weft fibres. An arrangement of two warpscrossing two wefts is designated 22 basket, but thearrangement of fibre need not be symmetrical. Thereforeit is possible to have 82, 54, etc. Basket weave is flatter,and, though less crimp, stronger than a plain weave, butless stable. It must be used on heavy weight fabrics madewith thick (high tex) fibres to avoid excessive crimping.

Leno

Leno weave improves the stability in open fabrics

which have a low fibre count. A form of plain weave in

which adjacent warp fibres are twisted around consecutiveweft fibres to form a spiral pair, effectively locking eachweft in place. Fabrics in leno weave are normally used incon-junction with other weave styles because if used alonetheir openness could not produce an effective compositecomponent.

Mock leno

A version of plain weave in which occasional warpfibres, at regular intervals but usually several fibres apart,deviate from the alternate under-over interlacing andinstead interlace every two or more fibres. This happenswith similar frequency in the weft direction, and the overalleffect is a fabric with increased thickness, rougher surface,and additional porosity.

Researchers have looked into tensile strength of ramie-cotton hybrid fibre reinforced polyester composites(Junior, Carvalho, Fonseca, Monteiro, & Almeida, 2004).They observed that tensile behaviour was dominated byvolume fraction of ramie fibres aligned in the test direction.The fabric and diameter of the thread did not play any rolein tensile characteristics. Cotton fabric was found to haveminor reinforcement effect due to weak cotton/polyesterinterface. Similar studies were performed by Mwaikamboand Bisanda (1999)on kapok-cotton fibre reinforced poly-ester composites.

Pothen, Thomas, Li, and Mai (2002) conducted tensileand impact studies of woven sisal fabric reinforced polyes-ter composites prepared by RTM technique. It was found

that the weave architecture was a crucial factor in deter-



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mining the response of the composites. Researchers havestudied the micromechanics of moisture diffusion in wovencomposites (Tang, Whitcomb, Li, & Sue, 2005). The weavepattern of the fabric was found to have a profound effecton the water uptake of the composites. They observed thatwoven composites exhibited quicker diffusion than that of

a unidirectional laminate with the same overall fibre vol-ume fraction. The plain weave with a lenticular tow andlarge waviness was seen to exhibit the quickest diffusionprocess.

The mechanical performance of isothalic polyester-based untreated woven jute-fabric composites was investi-gated by Ahmed and Vijayarangan (2007). It was foundthat the tensile strength and tensile modulus of jute-fabriccomposite are 83.96% and 118.97% greater than the ten-sile strength and modulus of unreinforced resin,respectively.

Novolac type phenolic composites reinforced with jute/cotton hybrid woven fabrics were fabricated and its prop-

erties were investigated as a function of fibre orientationand roving/fabric characteristics (De Medeiros, Agnelli,De Carvalho, & Mattoso, 2004). Results showed that thecomposite properties were strongly influenced by test direc-tion and rovings/fabric characteristics. The anisotropydegree was shown to increase with test angle and tostrongly depend on the type of architecture of fabric used,i.e. jute rovings diameter, relative fibre content, etc. Thebest overall mechanical properties were obtained for thecomposites tested along the jute rovings direction. Com-posites tested at 45and 90with respect to the jute rovingdirection exhibited a controlled brittle failure combined

with a successive fibre pullout, while those tested in the lon-gitudinal direction (0) exhibited a catastrophic failuremode. The researchers are of the opinion that jute fibrepromotes a higher reinforcing effect and cotton fibre avoidscatastrophic failure. Therefore, this combination of naturalfibres is suitable to produce composites for lightweightstructural applications.

The thermal diffusivity, thermal conductivity and spe-cific heat of jute/cotton, sisal/cotton and ramie/cottonhybrid fabric-reinforced unsaturated polyester compositeswere investigated byAlsina, Carvalho, Filho, and Almeida(2005). These properties were measured both parallel andperpendicular to the plane of the fabrics. The resultsobtained show that higher values were obtained parallelto the plane of the fibres. Sisal/cotton composites showeda particular behavior, with thermal properties very closeto those of the resin matrix. The thermal properties ofthe fabrics, i.e. without any resin, were also evaluatedand were used to predict the properties of the compositesfrom the theoretical series and parallel model equations.The effect of fabric pre-drying on the thermal propertiesof the composites was also evaluated. The results showedthat the drying procedure used did not bring any relevantchange in the properties evaluated.

Textile biocomposites comprising of woven sisal fabric

and natural rubber were fabricated by Jacob et al.

(2006a, 2006b). The mechanical properties of the textilecomposites were found to decrease after chemical modifica-tion which was contrary to the behaviour in short fibercomposites. The solvent sorption (Jacob et al., 2006a,2006b) characteristics of the composites were alsoinvestigated.

2.1.5. Applications of biocomposites

The use of biofibre reinforced composites has extendedto almost all fields. Recently three-layer particleboardswere produced from a mixture of sunflower stalks andpoplar wood at certain ratios utilizing urea-formaldehyde(UF) adhesives. Panels with a density of 0.7 g/cm3 weremanufactured with the ratios of 25, 50, and 75 percentparticles from sunflower stalks or poplar. Panels weresubjected to various tests for physical properties. Resultsshow that all the panels provide properties required bythe normal standards for general purpose-use particle-boards (Bektas, Guler, Kalaycioglu, Mengeloglu, &

Nacar, 2005).Bio-based composite roof structures were successfully

fabricated from soy oil-based resin and cellulose fibresin the form of paper sheets made from recycled card-board boxes. This recycled paper was previously testedin composite sheets and structural unit beams and wasfound to give the required stiffness and strength requiredfor roof construction (Dweib, Hu, Shenton, & Wool,2006).

In a study encompassing many applications, the flameretardancy of biodegradable polymers and biocompositeswas investigated (Matkoet al., 2005). For the comparison,

flame retarded lignocellulosic filler reinforced biocompos-ites were prepared using polypropylene (PP), polyurethane(PUR) and fully biodegradable starch matrices. The com-patibility of wood flake with PP was improved by applica-tion of an alkoxy silane based reactive surfactant. Thesilylation improved not only the compatibility but alsothe thermal stability of the wood flake according to TGmeasurements. Raman spectroscopic analysis of the silylat-ed product showed that the improved thermal stability isthe result of reduced ratio of the amorphous phase of cel-lulose. The phosphorus additives in flame retarded PURbiocomposites, comprising waste bio fillers and recycledpolyol, proved to be very effective because both the matrixand the filler components participate in mechanism offlame retardancy.

Researchers (Hong & Wool, 2004) developed a newlow dielectric constant material suited to electronic mate-rials applications using hollow keratin fibres and chemi-cally modified soybean oil. The unusual low value ofdielectric constant was due to the air present in the hollowmicrocrystalline keratin fibres and the triglyceride mole-cules. The authors are of the opinion that the low costcomposite made from avian sources and plant oil hasthe potential to replace the dielectrics in microchips andcircuit boards in the ever growing electronics materials

field. In an extension of the above study the authors have



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also observed that the coefficient of thermal expansion(CTE) of the composite was low enough for electronicapplications and similar to the value of silicon materialsor polyimides used in printed circuit boards (Hong &Wool, 2005).

Plastic/wood fibre composites are being used in a large

number of applications in decks, docks, window framesand molded panel components (Li & Matuana, 2003). Ithas been reported that 460 million pounds of plastic/wood fibre composites were produced in 1999. Statisticsshow that the production of these composites in 2001has increased to 700 million pounds. Over the last threedecades considerable research has been committed to find-ing an alternative fibre to replace asbestos in fibre cementproducts. Australian research was centred on naturalfibres and ultimately it was a natural fibre, wood pulpfibre, which was responsible for the greatest replacementof asbestos in the beleaguered global fibre cement industry(Coutts, 2005). As these fibres are cheap and readily avail-

able the energy required for the processing of these com-posites is low; also the incorporation of random vegetablefibres in cement matrices requires only a small number oftrained personnel in the construction industry. Recently,researchers (Ghavami, 2005) have explored the use ofbamboo fibre as reinforcement in structural concrete ele-ments. Pulp from eucalyptus waste and residual sisaland coir fibres have also been studied as a replacementfor asbestos in roofing components (Agopyan, Savastano,John, & Cincotto, 2005). The use of cactus pulp as a sta-bilising agent to improve the behaviour of the sisal fibrereinforced soil has been studied (Mattone, 2005).

Chopped barley straw has also been used as a suitablereinforcement for composite soil (Bouhicha, Aouissi, &Kenai, 2005).

Biocomposites also offer immense opportunities for anincreasing role as alternate material, especially as woodsubstitutes in the furniture market. Yet biofibre as con-struction material for buildings were known long before.For centuries, mixtures of straw and loam, dried in thesun, were employed as construction composite in Egypt.Pipes, pultruded profiles and panels with polyester matricesare also quite popular. Large projects have been promotedwhere jute-reinforced polyester resins are used for build-ings, e.g. the Madras House and grain elevators. Today,a renaissance in the use of natural fibres as reinforcementin technical applications is taking place, mainly in the auto-mobile and packaging industries (e.g., egg boxes) (Singh &Gupta, 2005).

In the automotive industry (Suddell & Evans, 2005),cotton fibres embedded in polyester matrix was used inthe body of the East German Trabant car. The useof flax fibres in car disk brakes to replace asbestos fibresis also another example. DaimlerBenz has been exploringthe idea of replacing glass fibres with natural fibres inautomotive components since 1991. A subsidiary of thecompany, Mercedes Benz pioneered this concept with

the Beleem project based in Sao Paolo, Brazil. In this

case, coconut fibres were used in the commercial vehiclesover a 9 year period. Mercedes also used jute-based doorpanels in its E-class vehicles in 1996. In September 2000,Daimler Chrysler began using natural fibres for their vehi-cle production. The bast fibres are primarily used in auto-motive applications because they exhibit greatest strength.

The other advantages of using bast fibres in the automo-tive industry include weight savings of between 10% and30% and corresponding cost savings. Recent studies havealso indicated that hemp-based natural fibre mat thermo-plastics are promising candidates in automotive applica-tions where high specific stiffness is required (Pervaiz &Sain, 2003).

Virtually all the major car manufacturers in Germany(Daimler Chrysler, Mercedes, Volkswagen, Audi Group,BMW, Ford and Opel) now use biocomposites in variousapplications. Interior trim components such as dashboardsand door panels using polypropylene and natural fibres areproduced by Johnson Controls, Inc. for Daimler Chrysler.

In 2000 Audi launched the A2 midrange car in which doortrim panels were made of polyurethane reinforced withmixed flax/sisal mat. DaimlerChrysler has now increasedits research and development in flax reinforced polyestercomposites for exterior applications (Automotive Indus-tries, 2000).

Recent reports by Bledzki, Faruk, and Sperber (2006)point out the importance of biofibers and biocompositesin the automotive industry (Fig. 9a and b). The end of lifevehicle (ELV) directive in Europe states that by 2015, vehi-cles must be constructed of 95% recyclable materials, with85% recoverable through reuse or mechanical recycling and

10% through energy recovery or thermal recycling (Peijs,2003). This will definitely lead to an increased use of biofi-bres. The diverse range of products utilizing biofibres andbiobased resins derived from soy beans is giving life to anew generation of composites for a number of applicationslike hurricane-resistant housing and structures (Wool,2002).

2.2. Designing biocomposites

Although biofibre reinforced polymer composites aregaining interest, the challenge is to replace conventionalglass reinforced plastics with biocomposites that exhibitstructural and functional stability during storage and useand yet are susceptible to environmental degradation upondisposal. An interesting approach in fabricating biocom-posites of superior and desired properties include efficientand cost effective chemical modification of fibre, matrixmodification by functionalizing and blending and efficientprocessing techniques. (Fig. 10)

Another interesting concept is that of engineered natu-ral fibres to obtain superior strength biocomposites(Mohanty et al., 2002). This concept explores the suitableblending of bast (stem) and leaf fibres. The judicious selec-tion of blends of biofibres is based on the fact that the cor-

rect blend achieves optimum balance in mechanical



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properties for e.g., the combination of bast and leaf fibre isexpected to provide a stiffnesstoughness balance in the

resulting biocomposites.

3. Electrospinning (ELSP)

Electrospinning has been recognised as one of the mostefficient techniques for the fabrication of nanopolymerfibres. When the diameter of polymer fibre is shrunk frommicrometer to nanometre range, amazing new propertiesare exhibited in the nanopolymer fibres. The large surfacearea of the fibres contributes to various applications rang-ing from medical prosthesis to composite preparation. Inaddition, the electrospun fibres find applications in variousother fields such as tissue template, protective clothing, cos-metics, drug delivery, conducting nanofibres, sensors, andoptical shutters etc.

Electrospinning has recently drawn strong attention inbiomedical engineering (Yoshimoto, Shin, Terai, & Vacan-ti, 2003), providing a basis for the fabrication of uniquematrices and scaffolds for tissue engineering. ELSP is aspinning method that can produce polymer fibres withdiameters ranging from several micros to 100 nm or lessunder a high-voltage electrostatic field operated betweena metallic nozzle of a syringe and a metallic collector inair. The fibres are typically deposited in the form of a

non-woven fabric onto a target metallic collector through

Fig. 9. (a) Under floor protection trim of Mercedes A class made from banana fibre reinforced composites. (b) Newest Mercedes S class automotive

components made from different bio fibre reinforced composites.

Fig. 10. Tricorner approach in designing of high performancebiocomposites.



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a random projected jet of polymer solution, the so-called

cone or instable jet. The accumulated charges on the poly-mer solution ejected from the nozzle induce the radialcharge repulsion in the electric field, which induces theinstable jet. If the random deposition process of fibres inthe instability jet is appropriately controlled on the meso-scopic scale, the higher-ordered spatial placement of thenano- and microfibres in the ELSP fabrics will be realized.In addition, the high-speed movement of the collector ornozzle may produce oriented meshes and more higher-ordered macroscopic devices.

Researchers recently developed two novel ELSP tech-niques: multilayering ELSP and (multicomponent) mixing

ELSP, both of which are composed of different nano-and microfibres (Kidoaki, Kwon, & Matsuda, 2005). Inthe multilayering ELSP, after electrospinning the first poly-mer, the second polymer is sequentially electrospun on thesame target collector (SeeFig. 11). Such a sequential spin-ning process can produce a multilayered fibre mesh, inwhich a hierarchically ordered structure composed of dif-ferent kinds of polymer mesh could be obtained. For themixing ELSP, two different polymers are simultaneouslyelectrospun from different syringes under special condi-tions. The produced polymer fibres are mixed on the sametarget collector, resulting in the formation of a mixed fibremesh.

Tremendous amount of studies are being undertaken inthe field of electrospinning. The effect of molecular weighton fibrous PVA prepared by electrospinning has beenreported byKoski, Yim, and Shivkumar (2004).Zarkoobet al. (2004) have reported on the structure and propertyof electrospun silk fibres. In the above work, fibres withdiameter 50200 nm could be obtained.

Recently scientists (Theron, Zussmann, & Yarin, 2004)have looked into the governing parameters in the electros-pinning of polymer fibres. Researchers have studied themechanical characterisation of electrospun gelatin nanofi-bres (Huang, Zhang, Ramakrishna, & Lim, 2004). Another

innovative approach has been the preparation of blend bio-

degradable nanofibrous non-woven mats via multi jet elec-

tro spinning (Ding, Kimura, Sato, Fujita, & Shiratori,2004).

Another interesting study was the electrospinning ofchitosan nanofibres from aqueous chitosan solution usingconcentrated acetic acid solution as a solvent. A uniformnanofibrous mat of average fibre diameter of 130 nm wasobtained from 7% chitosan solution in aqueous 90% aceticacid solution in an electric field of 4 kV/cm. An aqueousacetic acid concentration higher than 30% was prerequisitefor chitosan nanofibre formation, because more concen-trated acetic acid in water progressively decreased surfacetension of the chitosan solution and concomitantly

increased charge density of jet without significant effecton solution viscosity (Geng, Kwon, & Jang, 2005).

The fabrication and characterization of poly(vinyl alco-hol)/chitosan blend nanofibers by electrospinning methodwas investigated by Yong-Tang et al. (2007). The resultsshowed that showed that the morphology and diameterof the nanofibers were mainly affected by concentrationof the blend solution, weight ratio of the blend, respec-tively. It was also observed that that there were strongintermolecular hydrogen bonds between the molecules ofchitosan and poly(vinyl alcohol).

Recently researchers reported on the electrospinningand crosslinking of zein nanofibre mats (Yao, Li, & Song,2007). Zein is a protein found in corn. The authorsobserved that crosslinked zein exhibited higher tensileproperties than uncrosslinked zein.

4. Cellulose based nanocomposites

The concept of nanostructured materials design is gain-ing widespread importance among the scientific commu-nity (Berglund, 2005). The strong reinforcement effectsat low volume fraction resulted in a tremendous interestfrom the industry and research circles. With this as aninspiration, the potential of nanoscale cellulose structures

as reinforcement in novel composite materials was extre-

Fig. 11. Proposed electrospinning techniques (a) electrospinning (b) multilayer spinning.



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mely interesting. The concept of cellulose nanocompositesfor load bearing applications is fairly new. Propertyenhancements are expected due to higher Youngs modu-lus of pure cellulose reinforcement and finely distributedreinforcing microfibrils. A major problem in the commer-cial use of cellulose microfibrils in structural materials isthe disintegration of cellulose from plant cell wall at rea-sonable cost and without severe degradation. Another

major problem is dispersion of cellulose microfibrils in apolymer matrix.

A simple model of the cellulose microfibril structure ispresented inFig. 12. Cellulose nanocomposites are usuallyfabricated by utilizing these microfibrils of 1050 nm onwidth as reinforcement in a polymer matrix. Althoughmany studies provide detailed knowledge regarding themorphology and crystallography of different types of cellu-lose, the Youngs modulus of microfibrils from differentsources and subjected to different types of hydrolysis is sel-dom discussed.

The research group at CERMAV-CNRS has presentednumerous studies based on cellulose whiskers reinforcednanocomposites. The whiskers were disintegrated from anedible tunicate, a sea animal. Another area of interest isthat of microfibrillated cellulose (MFC) nanocomposites.MFC based on parenchyma cells (i.e. sugar beet, potatotuber) tend to show small microfibril diameter and thefibrillar structures resemble swirled mats of connected,microfibrils rather than discrete rods (Dufresne, Cavaille,& Vignon, 1997). A review of the recent research into cel-lulosic whiskers, their properties and their application innanocomposite field has been presented by the same group(Samir, Alloin, & Dufresne, 2005).

Recently scientists (Huang, Ichinose, & Kunitake, 2005)

achieved uniform polypyrrole nanocoating on natural cel-

lulose fibres without disrupting the hierarchical networkstructures of individual cellulose fibres by means of poly-merization-induced adsorption (Fig. 13).

High performance composites from low-cost plant pri-mary cell walls were developed byBruce, Hobson, Farrent,and Hepworth (2005). Purified cell wall fragments and cel-

lulose microfibrils were developed from Swede root to formnovel composite materials. The composites fabricated frompurified cell wall fragments and acrylic matrix gave the besttensile properties. Researchers (Bokobza & Chauvin, 2005)have also reinforced natural rubber with nanofibres of sepi-olite. The composites also contained silica particles gener-ated in situ by the sol-gel process. The level ofreinforcement was assessed from mechanical and orienta-tion behaviour.

Algae are also a potential source of cellulose for use incomposite materials. Researchers have concentrated onthe morphology of cellulose powder from Caldophora spat-za algae from the Baltic sea. The effective stiffness of the

algae MFC (which is the combined effect of the microfibrilmodulus and its aspect ratio) was found to be higher thanfor the wheat straw cellulose. Bacterial or microbial cellu-lose (BC) have also found their way as reinforcement incomposites. Cellulose can be produced from some bacteriasuch asAcetobacter xylinum, vinegar or acetic acid. Bacte-rial cellulose is an extracellular product which is excretedinto the culture medium. Acetic acid bacteria are not pho-tosynthetic but can convert glucose, sugar, glycerol to purecellulose. Bacterial cellulose has been frequently studied toclarify the mechanism for biosynthesis (Tokoh, Takabe,Sugiyama, & Fujitha, 2002). Researchers have looked into

the development of cellulose nanocomposites based on bac-terial cellulose and cellulose acetate butyrate (Grunert &Winter, 2002). Though BC composites have evoked wide-spread interest, the industrial use of BC compositesrequires the development of efficient large-scale fermenta-tion technology.

Nanoscale-modifed plant structures such as paper orbast fibres also present a different approach to preparationof nanocomposite structures. An interesting study concen-trated on the preserving the microfibril organization ofwood veneer in composites (Yano, Hirose, Collins, & Yaz-aki, 2001). Pine veneer was used as the starting material.

Fig. 12. Model of the cellulose microfibril structure.

Fig. 13. Scanning electron micrographs of nanocoated cellulose fibres.



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The basic idea was to increase the volume fraction of cellu-lose microfibrils by partially removing the lignin and hemi-cellulose wood polymers and by compressing the veneer.Phenol formaldehyde resin (PF) was used to preserve thecompressed microstructure of the material and to bondmicrofibrils.

The orientation of cellulose nanowhiskers in polyvinylalcohol was investigated by Kvien and Oksman (2007).Cellulose whiskers were incorporated in a polyvinyl alcoholmatrix using solution casting with water as the solvent. Thesuspension was cast and the water was evaporated while ahomogeneous magnetic field of 7 T was applied. Differentmicroscopy investigations of prepared nanocompositesindicated that the cellulose whiskers were oriented perpen-dicular to the direction of the magnetic field. Dynamicmechanical thermal analysis further strengthened the ideaof alignment because the results showed that the dynamicmodulus of the nanocomposite was around 2 GPa higherat room temperature in the aligned direction compared to

the transverse direction.In an interesting study, nanocomposite materials rein-

forced with flax cellulose nanocrystals in waterborne poly-urethane was reported by Xiaodong, Hua, and Chang(2007). The results indicated that the cellulose nanocrystalscould disperse in the polyurethane uniformly and resultedin an improvement of microphase separation between thesoft and hard segments of the WPU matrix. The filmsshowed a significant increase in You

carbohydrate polymers volume 71 issue 3 2008 [doi 10.1016_j.carbpol.2007.05.040] maya jacob john;...

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