Natural Fiber Polymer Composites: A Review

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Advances in Polymer Technology, Vol. 18, No. 4, 351363 (1999)

1999 by John Wiley and Sons, Inc. CCC 0730-6679/99/040351-13

Natural Fiber PolymerComposites: A Review

D. NABI SAHEB and J. P. JOG

Polymer Engineering Group, Chemical Engineering Division, National Chemical Laboratory,Pune 411 008, India

Received: May 3, 1999Accepted: May 19, 1999

ABSTRACT: Natural fiber reinforced composites is an emerging area inpolymer science. These natural fibers are low cost fibers with low density andhigh specific properties. These are biodegradable and non-abrasive. The naturalfiber composites offer specific properties comparable to those of conventionalfiber composites. However, in development of these composites, theincompatibility of the fibers and poor resistance to moisture often reduce thepotential of natural fibers and these draw backs become critical issue. Thisreview presents the reported work on natural fiber reinforced composites with

special reference to the type of fibers, matrix polymers, treatment of fibers andfiber-matrix interface. 1999 John Wiley & Sons, Inc. Adv in Polymer Techn18: 351363, 1999

Correspondence to:J. P. Jog

Introduction

O ver the past few decades, we find that poly-mers have replaced many of the conventionalmetals/materials in various applications. This ispossible because of the advantages polymers offer

over conventional materials. The most importantadvantages of using polymers are the ease of pro-cessing, productivity, and cost reduction. In most ofthese applications, the properties of polymers aremodified using fillers and fibers to suit the highstrength/high modulus requirements. Fiber-rein-

forced polymers offer advantages over other con-ventional materials when specific properties arecompared. These composites are finding applica-tions in diverse fields from appliances to space-crafts.

Natural fibers have recently attracted the atten-tion of scientists and technologists because of theadvantages that these fibers provide over conven-tional reinforcement materials, and the develop-ment of natural fiber composites has been a subjectof interest for the past few years. These natural1 4

fibers are low-cost fibers with low density and highspecific properties. These are biodegradable andnonabrasive, unlike other reinforcing fibers. Also,they are readily available and their specific prop-erties are comparable to those of other fibers used


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352 VOL. 18, NO. 4

NATURAL FIBER POLYMER COMPOSITES

TABLE IMechanical Properties of Natural Fibers

Fiber Specific Gravity

Tensile Strength

(MPa)

Modulus

(GPa)

Specific

Modulus

Jute 1.3 393 55 38

Sisal 1.3 510 28 22

Flax 1.5 344 27 50

Sunhemp 1.07 389 35 32

Pineapple 1.56 170 62 40

Glass Fiber-E 2.5 3400 72 28

for reinforcements. However, certain drawbackssuch as incompatibility with the hydrophobic poly-mer matrix, the tendency to form aggregates duringprocessing, and poor resistance to moisture greatlyreduce the potential of natural fibers to be used asreinforcement in polymers.

In this article, we shall review the reported workon various aspects of natural fiber reinforced com-posites and address some of the basic issues in de-velopment of such composites.

Natural Fibers

Before discussing the methods of the preparationof these composites and their performance, we de-scribe the types of natural fibers, their microstruc-ture, and their chemical composition.

TYPES OF NATURAL FIBERS

Natural fibers are grouped into three types: seedhair, bast fibers, and leaf fibers, depending upon thesource. Some examples are cotton (seed hairs),ramie, jute, and aflax (bast fibers), and sisal and ab-aca (leaf fibers). Of these fibers, jute, ramie, flax, andsisal are the most commonly used fibers for polymercomposites. Natural fibers in the form of wood flourhave also been often used for preparation of naturalfiber composites. The properties of these fibers arepresented in Table I.

As can be seen from Table I, the tensile strengthof glass fibers is substantially higher than that ofnatural fibers even though the modulus is of thesame order. However, when the specific modulus ofnatural fibers (modulus/specific gravity) is consid-ered, the natural fibers show values that are com-parable to or better than those of glass fibers. These

higher specific properties are one of the major ad-vantages of using natural fiber composites for ap-plications wherein the desired properties also in-clude weight reduction.

MICROSTRUCTURE OF THE FIBERS

Natural fibers themselves are cellulose fiber re-inforced materials as they consist of microfibrils inan amorphous matrix of lignin and hemicellulose.These fibers consist of several fibrils that run allalong the length of the fiber. The hydrogen bondsand other linkages provide the necessary strengthand stiffness to the fibers.

CHEMICAL COMPOSITION OF NATURALFIBERS

The chemical composition of natural fibers varies

depending upon the type of fiber. Primarily, fiberscontain cellulose, hemicellulose, pectin, and lignin.The properties of each constituent contribute to theoverall properties of the fiber. Hemicellulose is re-sponsible for the biodegradation, moisture absorp-tion, and thermal degradation of the fiber as itshows least resistance whereas lignin is thermallystable but is responsible for the UV degradation.The percentage composition of each of these com-ponents varies for different fibers. Generally, the fi-

bers contain 60 80% cellulose, 5 20% lignin, andup to 20% moisture.

NATURAL FIBER COMPOSITES

The matrix phase plays a crucial role in the per-formance of polymer composites. Both thermosetsand thermoplastics are attractive as matrix materialsfor composites. In thermoset composites, formula-tion is complex because a large number of compo-


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ADVANCES IN POLYMER TECHNOLOGY 353


nents are involved such as base resin, curing agents,catalysts, flowing agents, and hardeners. Thesecomposite materials are chemically cured to ahighly cross-linked, three-dimensional networkstructure. These cross-linked structures are highlysolvent resistant, tough, and creep resistant. The fi-

ber loading can be as high as 80% and because ofthe alignment of fibers, the enhancement in theproperties is remarkable.

Thermoplastics offer many advantages over ther-moset polymers. One of the advantages of thermo-plastic matrix composites is their low processingcosts. Another is design flexibility and ease of mold-ing complex parts. Simple methods such as extru-sion and injection molding are used for processingof these composites. In thermoplastics most of thework reported so far deals with polymers such aspolyethylene, polypropylene, polystyrene, andpoly(vinyl chloride). This is mainly because the pro-

cessing temperature is restricted to temperatures be-low 200C to avoid thermal degradation of the nat-ural fibers. For thermoplastic composites, thedispersion of the fibers in the composites is also animportant parameter to achieve consistency in theproduct. Thermoplastic composites are flexible andtough and exhibit good mechanical properties.However, the % loading is limited by the process-ability of the composite. The fiber orientation in thecomposites is random and accordingly the propertymodification is not as high as is observed in ther-moset composites.

Properties of the fibers, the aspect ratio of the fi-

bers, and the fiber matrix interface govern theproperties of the composites. The surface adhesion

between the fiber and the polymer plays an impor-tant role in the transmission of stress from matrix tothe fiber and thus contributes toward the perform-ance of the composite.

Another important aspect is the thermal stabilityof these fibers. These fibers are lignocellulosic andconsist of mainly lignin, hemicellulose, and cellu-lose. The cell walls of the fibers undergo pyrolysiswith increasing processing temperature and con-tribute to char formation. These charred layers helpto insulate the lignocellulosic from further thermaldegradation. Since most thermoplastics are pro-cessed at high temperatures, the thermal stability ofthe fibers at processing temperatures is important.Thus the key issues in development of natural re-inforced composites are (i) thermal stability of thefibers, (ii) surface adhesion characteristics of the fi-

bers, and (iii) dispersion of the fibers in the case ofthermoplastic composites.

Numerous reports are available on the naturalfiber composites. Table II summarizes the reportedwork on natural fiber composites. As can be seenfrom the table, the majority of the work is on woodflour, with a few reports on other fibers such as jute,sisal, and kenaf.

Major Issues in Development ofComposites

THERMAL STABILITY OF NATURALFIBERS

Natural fibers are complex mixtures of organicmaterials and as a result, thermal treatment leads toa variety of physical and chemical changes. Thethermal stability of natural fibers can be studied bythermogravimetric analysis (TGA). A typical TGAfor jute fibers is shown in Figure 1. As can be seenfrom the figure, the natural fibers start degrading atabout 240C. The thermal degradation of lignocel-lulosic materials has been reviewed by Nguyen

TABLE IIReported Work on Natural Fiber Composites

Fiber Matrix Polymer References

Wood flour/fiber PE

PP

PVC

PS

Polyurethane

512

1328

2931

3234

35

Jute PP

SBR, nitrile rubber

Epoxy

Polyester

Phenolformaldehyde

3640

50, 51

41, 42

4349

52

Sisal PE

Natural rubber

Polyester epoxy

5355

5861

56, 57, 62

Abaca Epoxy 72

Pineapple PE, polyester 6769

Sunhemp Polyester, PP 76

Oil palm Rubber 80

Kenaf PE, PP 6366

Coir Natural rubber 75

Banana Polyester 7374

Flax PP 7071

Wheat straw PP 70

Bamboo Epoxy 78


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FIGURE 1. Typical TGA thermogram of cellulose fiber(jute powder).

et al. in detail for modified and unmodifiedmaterials.81,82

The thermal degradation of natural fibers is atwo-stage process, one in the temperature range220280C and another in the range 280 300C. Thelow-temperature degradation process is associatedwith degradation of hemicellulose whereas thehigh-temperature process is due to lignin. The ap-parent activation energies for the two processes areabout 28 and 35 kcal/mol, which correspond to thedegradation of hemicellulose and lignin, respec-tively. The degradation of natural fibers is a crucialaspect in the development of natural fiber compos-ites and thus has a bearing on the curing tempera-ture in the case of thermosets and extrusion tem-

perature in thermoplastic composites.For improvement of thermal stability, attempts

have been made to coat the fibers and/or to graftthe fibers with monomers. Grafting is possible sincethe lignin can react with the monomers. Mohanty etal. have reported that grafting of acrylonitrile on83

jute improved the thermal stability as evidenced bythe increase in the degradation temperature from170 to 280C. Sabaa has also reported improved84

thermal stability for acrylonitrile-grafted sisal fibersas evidenced by the increased initial degradationtemperature, lowering of the rate of degradation,and the total weight loss. In another study by Yapet al., the polymer wood composites were pre-85

pared by in situ polymerization of various mono-mers and it was observed that the maximum rate ofdegradation was substantially reduced for phos-phonate-treated wood flour.

The degradation of natural fibers leads to poororganoleptic properties such as odor and color andalso deterioration of their mechanical properties.

Sridhar et al. have studied the thermal stability of43

jute fibers at temperatures ranging from 150 to300C both in air and under vacuum. The degra-dation was monitored by measuring the weightloss, change in chemical structure, and mechanicalproperties. It was observed that heating under vac-

uum at 300C for 2 h resulted in 60% decrease in thetensile strength, which was ascribed to the depoly-merization and oxidation of fibers. In actual prac-tice, processing is carried out under atmosphericconditions and the possibility of thermal degrada-tion leading to inferior mechanical properties can-not be ruled out.

Gonzalez and Myers have studied the effect8687

of thermal degradation on the mechanical proper-ties of wood/polymer composites. The temperaturerange of study was from 220 to 260C and the ex-posure time was varied from 4 to 4096 min. It wasobserved that although, in general, the mechanical

properties deteriorate, as a result of thermal degra-dation of wood flour, toughness and bendingstrength were more affected. It was also pointed outthat the changes in the surface chemistry mightcause changes in the wood/polymer bonding thatis responsible for the inferior properties of the com-posites. In another study of PP/wood flour com-posites, similar loss in properties has been reportedafter extrusion at 250C.88

The thermal degradation of the fibers also resultsin production of volatiles at processing tempera-tures 200C. This can lead to porous polymerproducts with lower densities and inferior mechan-

ical properties.

MOISTURE CONTENT OF THE FIBERS

Cellulosic fibers are hydrophilic and absorbmoisture. The moisture content of the fibers canvary between 5 and 10%. This can lead to dimen-sional variations in composites and also affects themechanical properties of the composites. Duringprocessing of composites based on thermoplastics,the moisture content can lead to poor processabilityand porous products. Treatment of natural fiberswith chemicals or grafting of vinyl monomers canreduce the moisture gain.

BIODEGRADATION ANDPHOTODEGRADATION OF NATURALFIBERS

Natural fibers (lignocellulosics) are degraded bybiological organisms since they can recognize the


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carbohydrate polymers in the cell wall. Lignocellu-losics exposed outdoors undergo photochemicaldegradation caused by ultraviolet light. Resistanceto biodegradation and UV radiation can be im-proved by bonding chemicals to the cell wall poly-mers or by adding polymer to the cell matrix.

PROCESSING OF THERMOSETCOMPOSITES

Thermosets are processed by simple processingtechniques such as hand layup and spraying, com-pression, transfer, resin transfer, injection, compres-sion injection, and pressure bag molding operations.A few other methods such as centrifugal casting, coldpress molding, continuous laminating, encapsula-tion, filament winding, pultrusion, reinforced reac-tion injection molding, rotational molding, and vac-uum forming are being used for composites but use

of these methods for natural fiber composites ishardly reported. In thermoset polymers, the fibersare used as unidirectional tapes or mats. These areimpregnated with the thermosetting resins and thenexposed to high temperature for curing to take place.

PROCESSING OF THERMOPLASTICCOMPOSITES

The processing of natural fiber thermoplasticcomposites involves extrusion of the ingredients atmelt temperatures followed by shaping operationssuch as injection molding and thermoforming. Fi-

ber fiber interactions as well as fiber matrix inter-actions play a crucial role in determining the prop-erties of such composites. Many times, it is observedthat these fibers do not function as an effective re-inforcement system due to poor adhesion at the fi-

ber matrix interface. Cellulose fibers also tend toaggregate and therefore the fibers do not dispersewell in a hydrophobic polymer matrix and thuspose difficulties in achieving a uniform distributionof fiber in the matrix. The surface characteristics ofthe reinforcing fiber are important in the transfer-ring of stress from the matrix to the fiber. The pre-treatment of the fiber with suitable additives priorto processing leads to good dispersion and signifi-cantly improved mechanical properties of the com-posites.

The properties of the composites are also influ-enced by the processing parameters in the case ofthermoplastic composites. Takase and Shiraishi88

have reported the effect of processing parameterssuch as mixing time, rpm, and temperature in com-

pounding PP/wood composites. It was observedthat tensile strength varied nonlinearly with rpm,mixing temperature, and time, indicating that fiberlength and dispersion need to be optimum to obtainenhanced properties.

Dispersion of the Fibers in the Matrix

The incorporation of cellulosic fibers in thermo-plastics leads to poor dispersion of the fibers due tostrong interfiber hydrogen bonding, which holdsthe fibers together. Treatment of the fibers and/oruse of external processing aids can reduce this prob-lem.

Various processing aids/coupling agents suchas stearic acid, mineral oil, and maleated ethylenehave been used. The concentration of the addi-tive is approximately 1% by weight of fibers. Thestearic acid is highly effective in dispersing, re-ducing fiber to fiber interaction. Mineral oil func-

tions as a lubricant that is adsorbed by the fibersand this facilitates the disentanglement of individ-ual fibers.

FIBER MATRIX INTERFACE

The incorporation of hydrophilic natural fibers inpolymers leads to heterogeneous systems whoseproperties are inferior due to lack of adhesion be-tween the fibers and the matrix. Thus the treatmentof fibers for improved adhesion is a critical step inthe development of such composites. The treatment

of the fibers may be bleaching, grafting of mono-mers, acetylation, and so on. In addition to the sur-face treatment of fibers, use of a compatibilizer or acoupling agent for effective stress transfer across theinterface can also be explored. The compatibilizercan be polymers with functional groups graftedonto the chain of the polymer. The coupling agentsare tetrafunctional organometallic compounds

based on silicon, titanium, and zirconium and arecommonly known as silane, zirconate, or titanatecoupling agents. Table III presents the structures,functional groups, and applications of a few com-mercial coupling agents. (The coupling mechanism

for the silane coupling agent is illustrated in the Ap-pendix.)

MODIFICATION OF NATURAL FIBERS

Natural fibers are incompatible with the hydro-phobic polymer matrix and have a tendency to formaggregates. These are hydrophilic fibers and thus


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TABLE IIISome Typical Representative Commercial Coupling Agents115

Sl. No. Functional Group Chemical Structure Applicable Polymera

1 Vinyl CH2"CHSiCl3CH2"CHSi(OC2H5)3

Elastomers, polyethylene, silicone elasto-

mers, UP, PE, PP, EPDM, EPR

2 Chloropropyl ClCH2CH2CH2Si(OCH3)3 EP3 Epoxy O

CH2CHCH2O(CH2)3Si(OCH3)3

Elastomers, especially butyl elastomers, ep-

oxy, phenolic and melamine, PC, PVC, UR

4 Methacryl

CH2"C9COO(CH2)3Si(OCH3)3

CH3 Unsaturated polyesters, PE, PP, EPDA,

EPM

5 Amine H2N(CH2)3Si(OC2H5)3HN(CH2)2NH(CH2)3Si(OCH3)3

Unsaturated polyesters, PA, PC, PUR, MF,

PF, PI, MPF

6 Cationic styryl CH2CHC6H4CH2HH2(CH2)3Si(OCH3)3Cl

All polymers

7 Phenyl C6H5Si(OCH3)3 PS, addition to amine silane

8 Mercapto HS(CH2)3Si(OCH3)3HS(CH2)2Si(OC2H5)3

EP, PUR, SBR, EPDM

9 Phosphate(titanate)

O O

C3H7OTi[O9P9O9P(OC8H17)2]3

OH

Polyolefins, ABS, phenolics, polyesters,PVC, polyurethane, styrenics

10 Neoalkoxy

(zirconate)O O

neoalkoxy-Ti[O9P9O9P(OC8H17)2]3

OH

Polyolefins, ABS, phenolics, polyesters,

PVC, polyurethane, styrenics

aAbbreviations according to ASTM 1600.

exhibit poor resistance to moisture. To eliminate the

problems related to high water absorption, treat-ment of fibers with hydrophobic aliphatic and cyclicstructures has been attempted. These structurescontain reactive functional groups that are cap-able of bonding to the reactive groups in the matrixpolymer, e.g., the carboxyl group of the polyesterresin. Thus modification of natural fibers is at-tempted to make the fibers hydrophobic and to im-prove interfacial adhesion between the fiber and thematrix polymer. Chemical treatments such as89103

dewaxing (defatting), delignification, bleaching,acetylation, and chemical grafting are used for mod-ifying the surface properties of the fibers and forenhancing its performance. (The mechanisms ofthese reactions are discussed in detail in the Appen-dix.)

Chemical modification of natural fibers has beenreviewed by Rowell. Table IV summarizes the var-90

ious chemical treatments and coupling agents usedso far for the modification of the fiber surface.

Delignification (dewaxing) is generally carried

out by extracting with alcohol or benzene and treat-

ment with NaOH followed by drying at roomtemperature. Many oxidative bleaching agents93

such as alkaline calcium or sodium hypochloriteand hydrogen peroxide are commercially used.Bleaching generally results in loss of weight andtensile strength. These losses are mainly attributed94

to the action of the bleaching agent or alkali or al-kaline reagent on the noncellulosic constituents offibers such as hemicellulose and lignin.

Acetylation of jute is reported to impart resist-ance to fungal attack and hydrophobicity. Thechange in properties is attributed to the decrease inmoisture sorption in the cell walls and blocking ofthe hydroxyl group of the wall components in sucha way that enzymes of the wood-degrading micro-organisms cannot recognize them as attachable sub-strates. Acetylated jute is considerably more hydro-phobic than unmodified jute. The acetylated jute95

shrinks much less than unmodified jute when ex-posed to water and thus exhibits improved dimen-sional stability.


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Chemical modification through graft copolymer-

ization provides a potential route for significantlyaltering their physical and mechanical properties.Chemical grafting involves attaching to the surfaceof a fiber/filler a suitable polymer with a solubilityparameter similar to that of the polymer matrix,which acts as an interfacial agent and improves the

bonding between the fiber and the matrix. Graft co-polymerization of vinyl monomers such as methylmethacrylate, acrylamide, and acrylonitrile onto cel-lulose, cellulose derivatives, and lignocellulosic fi-

bers has been well established and has been exten-sively studied over the past few decades.9698

Impregnation with monomer followed by its po-

lymerization has also been one of the most commonmethods used for treatment of fibers. Samal and co-workers have reviewed various methods of graft99

copolymerization onto cellulose fibers. Graft copo-lymerization onto cellulose takes place through aninitiation reaction involving attack by macrocellu-losic radicals on the monomer to be grafted. Thegeneration of the macrocellulosic radicals is accom-plished by a variety of methods such as (1) diazo-tization, (2) chain transfer reactions, (3) redox reac-tions, (4) photochemical initiation, and (5)radiation-induced synthesis.

The effect of treatments with other chemicals, forinstance, sodium alginate and sodium hydroxide,has been reported for coir, banana, and sisal fibers

by Mani and Satyanarayan. The treatment re-100

sulted in an increase in debonding stress and thusimproved the ultimate tensile strength up to 30%.Basak et al. have reported that treatment of jute39

with polycondensates such as phenol formalde-hyde, malemine formaldehyde, and cashew nut

shell liquidformaldehyde improved the wettabil-

ity of jute fibers and reduced the water regain prop-erties. Treatment with cardanol formaldehydewas also found to reduce water absorption and im-proved the mechanical properties of a jute/polyole-fin composite. The chemical treatment of jute fibers37

with ethylenediamine and hydrazine results in for-mation of complexes with the hydroxyl group of thecellulose and thus reduces the moisture absorptionof the fiber.46,47

Samal and Ray have studied the chemical mod-102

ification of pineapple leaf fiber using alkali treat-ment, diazo coupling with aniline, and cross-linkingwith formaldehyde andp-phenylenediamine. These

chemical treatments resulted in significant improve-ments in mechanical properties, chemical resistance,and reduced moisture regain. A study by Yap etal. has shown that the treatment of wood with103

vinyl monomers improved termite and funogal re-sistance and they also imparted flame retardancy topolymer wood composites.

Mechanical Properties ofNatural Fiber Composites

The properties of natural fiber reinforced com-posites depend on a number of parameters such asvolume fraction of the fibers, fiber aspect ratio, fi-

ber matrix adhesion, stress transfer at the interface,and orientation. Most of the studies on natural fibercomposites involve study of mechanical propertiesas a function of fiber content, effect of various treat-

TABLE IVChemical Treatments Used for Modification of Natural Fibers

Fiber Chemical Treatments Coupling Agents/Compatibilizers

Wood flour Succinic acid, EHMA, styrene, urea formaldehyde,m-phenyl-

ene bismaleimide, acetic anhydride, maleic anhydride, itaconic

anhydride, polyisocyanate, linoleic acid, abietic acid, oxalic acid,

rosin

Maleated PP, acrylic acid grafted

PP, Silane A-174, Epolene C-18,

Silane A-172, A-174, and A-1100,

PMPPIC, zirconates, titanates

Jute Phenolformaldehyde, malemineformaldehyde, cardanol

formaldehyde

Sisal NaOH, isocyanate, sodium alginate, N-substituted methacrylam-

ide

Pineapple p-phenylene diamine

Banana Sodium alginate

Coir Sodium alginate, sodium carbonate


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ments of fibers, and the use of external couplingagents. Other aspects include the prediction of104108

modulus and strength using some well-establishedmodels for two-phase systems and comparison withexperimental data.104

Both the matrix and fiber properties are impor-

tant in improving mechanical properties of the com-posites. The tensile strength is more sensitive to thematrix properties, whereas the modulus is depen-dent on the fiber properties. To improve the tensilestrength, a strong interface, low stress concentra-tion, fiber orientation is required whereas fiber con-centration, fiber wetting in the matrix phase, andhigh fiber aspect ratio determine tensile modulus.The aspect ratio is very important for determiningthe fracture properties. In short-fiber-reinforcedcomposites, there exists a critical fiber length that isrequired to develop its full stressed condition in thepolymer matrix. Fiber lengths shorter than this crit-

ical length lead to failure due to debonding at theinterface at lower load. On the other hand, for fiberlengths greater than the critical length, the fiber isstressed under applied load and thus results in ahigher strength of the composite.

For, good impact strength, an optimum bondinglevel is necessary. The degree of adhesion, fiberpullout, and a mechanism to absorb energy aresome of the parameters that can influence the im-pact strength of a short-fiber-filled composite. The105

properties mostly vary with composition as per therule of mixtures and increase linearly with compo-sition. However, it has been observed that this linear

dependence on percentage of fiber content does nothold at high percentage (80%) of the fiber, prob-ably due to lack of wetting of the fiber surface bythe polymer.

THERMOSET COMPOSITES

For thermoset composites, the fibers are com-bined with phenolic, epoxy, and polyester resins toform composite materials. These thermoset poly-mers contain reactive groups, which aid the inter-face development. The reported work on thermosetcomposites covers the effect of process parameterssuch as curing temperature and various treatmentson the properties of composites.

For jute/polyester composites, an increase inmodulus and strength was reported up to a volumefraction of 0.6 followed by a decrease. Further in-crease in volume fraction resulted in a decrease inthe properties, which was attributed to insufficientwetting of the fiber. In another study of jute/poly-44

ester composites, it was noted that the mechanicalproperties were dependent on the secondary chem-ical bonding between jute fiber and polyester. San-45

adi et al. have studied the mechanical properties76

of sunhemp fiber reinforced polyester. The tensilestrength and modulus increased linearly with in-

creasing fiber content following the rule of mixtures.The improved toughness was ascribed to the fiberpullout mechanism in the composite. In banana/polyester composites, the improvement in the prop-erties was observed only when the fiber weight frac-tion was more than 19%. At 30% fiber content, theflexural strength was 97 MPa and elastic moduluswas 6.5 GPa. Jain et al. have studied bamboo/73 78

epoxy composites up to a volume fraction of 0.85. Itwas noted that the composites with banana fiber ex-hibited better tensile, impact, and flexural strengthcompared with the properties of other natural fibercomposites.

Ismail and Rosnah have studied the curing79characteristics of oil/palm fiber reinforced rubbercomposites. It was noted that the presence of bond-ing agents prolonged the curing time; however, themechanical properties were better due to improvedinterfacial bonding as evidenced by SEM studies.Tobias and Ibarra have studied the effect of cure74

temperature on flexural strength of polyester-basedcomposites. The flexural strength increased with in-creasing cure temperature and maximum strengthwas obtained for abaca/polyester composites com-pared to banana/polyester and rice hull/polyestercomposites. The incorporation of pineapple leaf fi-

bers in polyester resulted in an increase of 2.3 timesin the specific flexural stiffness at 30 wt% content ofthe fiber. The use of Silane A-172 further improved68

the properties of the composites.For sisal/polyester composites, the effect of treat-

ments of fibers with silane, titanate, and zirconatecoupling agents and N-substituted methacrylamidewas investigated by Singh et al. The treatments re-56

sulted in improved strength retention properties ofthe composites and thereby the composites exhib-ited better properties in humid as well as dry con-ditions. Similar improved moisture resistance wasreported by Bisinda et al.62

For natural rubber composites with sisal fibers,treatment with resorcinol and hexamethylenetetra-mine resulted in a better storage modulus. The re-laxation process for the composites was altered andthe composites exhibited a two-stage relaxation pro-cess corresponding to the matrix and the interface.The process was sensitive to the type of bondingagent used.60,61


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THERMOPLASTIC COMPOSITES

Polyethylene-Based Composites

The mechanical properties of thermoplastic com-posites can be improved by improving the compat-

ibility between the fiber and matrix. Compatibiliz-ers such as maleated ethylene, maleated propylene,and a few acrylic-grafted linear polymers are re-ported to enhance the adhesion between the fiberand polymer matrix. For HDPE/cellulose fiber512

composites (at 10 and 30% fiber concentration), thebest improvement in tensile strength and tensilemodulus was achieved with maleated ethylene. Theenhancement in the properties was attributed to thecoupling reaction (ester linkage) between the ma-leated ethylene and the hydroxyl group of cellulose,which improves the bonding between the fiber andmatrix.7

Mechanical properties of HDPE/wood fibercomposites have been studied by Carrasco et al.11

Two types of coupling agentsEpolene C-18 andSilane 174 were evaluated. Use of silane couplingagents resulted in composites with better propertiesthan those of Epolene-treated and untreated woodfiber. The influence of coupling agents on the me-chanical properties of HDPE/wood fibers has also

been studied by Raj et al. It has been observed that4,5

incorporation of wood fibers in HDPE resulted inan increase in the stiffness and decrease in tensilestrength for untreated wood fibers. Treatment ofwood fibers with silane coupling agent and polyi-

socyanate resulted in an increase in tensile strength.Raj et al. have compared the tensile and impact7

properties of LLDPE/wood fiber composites withmica and glass fiber composites and have shownthat the potential advantage of using wood fibers asreinforcement is in terms of material cost and spe-cific properties. Pretreated wood fiber produced asignificant improvement in tensile strength andmodulus. Grafting of aspen chemithermomechani-cal pulp was found to improve the mechanicalproperties of LLDPE composites as noted by Beshayet al.5

The influence of various chemical treatments onthe properties of sisal/PE composites has been in-vestigated by Joseph et al. The chemical treatments55

included treatments with sodium hydroxide, iso-cyanate, and peroxide. The enhancement in theproperties was ascribed to the bonding between si-sal fiber and the PE matrix. Treatment with the car-danol derivative of toluene isocyanate was found to

be better than other treatments as evidenced by the

decrease in the hydrophilic nature of the composite.The composites exhibited better dimensional stabil-ity and retention of properties even after aging,which was ascribed to the improved moistureresistance.54

PP-Based Composites

A systematic study of the effect of surface treat-ments on the properties of PP/cellulose fiber has

been carried out by Bataille et al. The results in-16

dicated an increase in modulus with increase in thefiber content. The addition of coupling agents and/or maleic anhydride PP improved the interfacial ad-hesion, thereby leading to improved properties.Sain et al. have reported that the properties of PP/21

wood fiber composites were very poor due to theabsence of interface modifiers. However, it wasnoted that use of maleated PP, itaconic anhy-

dride, and bismaleimide-modified PP resulted in astable surface and thus improved tensile strength.An increase in tensile and impact strength was re-ported when rosin was used in PP/wood flourcomposites.13

Sun and Hawke have studied the performance24

of wood fiber composites using polyisocyanate as abonding material. Sain et al. have demonstrated20

the use of bismaleimide modification for improvingthe properties of PP/wood fiber composites. Writeand Mathias have reported synergistic reinforce-18

ment of balsa wood composites using ethyl--(hy-droxy methyl)acrylate (EHMA) and styrene. The

significant improvements in the properties were as-cribed to the strong interaction between the fiberand the matrix polymer, confirmed by solid-stateNMR and SEM. The composite also exhibited im-proved dimensional stability. Tibor has reported23

that the properties of electron beam processed PP/wood fiber composites were significantly betterthan those of conventionally processed composites.The improvement in the performance was ex-plained on the basis of creation of active sites in thepolymer as well as the fibers, which resulted in im-proved adhesion.

Significant improvement in the properties wasobserved for PP/kenaf fiber composites when ma-leated PP was used for modifying the fibermatrixinterface. Dalvag et al. used maleic anhydride64 66 14

modified propylene to improve the strength andductility of PP/wood fiber and PP/cellulose flourcomposites. Hydrolytic pretreatment of cellulose fi-

bers with oxalic acid was found to improve the ho-mogeneity and mechanical properties of PP, HDPE,


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360 VOL. 18, NO. 4


and PS that contained various amounts of bleachedpulp.15

Improvement in the mechanical strength of woodfiber filled thermoplastic composites has beenachieved with the use of coupling agents. Cellulosefibers treated with vinyl chloride, plasticizers, and

an isocyanate produced better adhesion with silanecoupling agents A-172 and A-174, and A-1100 ex-hibited better bonding between the fibers and PVCmatrix.2931

The physical properties of silane-treated wood/PMMA composites were studied by Elvyet al. As109

a result of treatment, the compressive strength wasfound to increase from 180 to 210% for treated woodfiber composites. The increased values of stress areattributed to the incorporation of polymer into thevoid spaces in the wood fiber.

As discussed earlier, reports on wood fiber com-posites with engineering polymers are scarce. This

is mainly because of the high processing tempera-ture (250C) required for processing of engineer-ing polymers. The influence of the use of jute fiberson the mechanical properties of ABS has been stud-ied by Bawadekar and Jog. It has been observed110

that the ABS/jute fiber composites show remark-able improvement in the flexural modulusfrom2000 to 3500 MPaat 30% jute fiber content. Thecomposites showed marginal changes in the tensilestrength values whereas tensile modulus increasedwith increase in jute fiber content. Surface treatmentof jute using coupling agents was found to result infurther improvement in the modulus.111

Recent Developments

A natural fiber composite with an outstandingcombination of properties is not a dream today. Useof proper processing techniques, fiber treatments,and compatibilizers/coupling agents can lead tocomposites with optimum properties for a particu-lar application.

Recently, there has been increasing interest incommercialization of natural fiber composites andtheir use, especially for interior paneling in the au-tomobile industry. These composites with densityaround 0.9 g/cm , stiffness around 3000 MPa, im-3

pact strength of 25 kJ/m , and good sound absorp-2

tion characteristics are being used by a number ofleading companies. Composites based on polyole-112

fins are now commercially available. It is reportedthat these composites offer advantages of 20% re-duction in processing temperature and 25% reduc-tion in cycle time in addition to a weight reductionof about 30%. The composites provide woodlike113

appearance without requiring the maintenance. The

extruded profiles can be used as a wood substitutein various applications such as window systems anddecking.114

These developments are confined to polymercomposites based on PE, PP, PS, and PVC, for whichthe processing temperature is about 200C. The realchallenge for the scientist is to improve the thermalstability of these fibers so that they can be used withengineering polymers and further the advantage of

both the polymers and the fibers. Thus improvedthermal stability of natural fibers and modificationof fibers for better performance are still an indis-pensable task for the scientist. Such attempts can

widen the applications of natural fiber composites

References

1. Schneider, J. P.; Myers, G. E.; Clemons, C. M.; English,B. W. Eng Plast 1995, 8 (3), 207.

2. Reinforced Plastics 1997, 41(11), 22.

3. Colberg, M.; Sauerbier, M. Kunstst-Plast Europe 1997, 87(12), 9.

4. Schloesser, Th.; Knothe, J. Kunstst-Plast Europe 1997, 87 (9),

25.5. Beshay, A. D.; Kokta, B. V.; Maldas, D.; Daneault, C. Polym

Compos 1985, 6, 261.

6. Maiti, S. N.; Singh, K. J Appl Polym Sci 1986, 32, 4285.

7. Raj, R. G.; Kokta, B. V.; Maldas, D.; Daneault, C. J ApplPolym Sci 1989, 37, 1089.

8. Raj, R. G.; Kokta, B. V.; Grouleau, G.; Daneault, C. PolymPlast Technol Eng 1989, 28 (3), 247.

9. Raj, R. G.; Kokta, B. V.; Grouleau, G.; Daneault, C. PolymPlast Technol Eng 1990, 29 (4), 339.

10. Raj, R. G.; Kokta, B. V.; Daneault, C. J Mater Sci 1990, 25,1851.

11. Carrasco, F.; Saurina, J.; Arnau, J. J.; Pages, P. 6th EuropeanConference on Composite Materials, France, 1993, p 483.

12. Berenbrok, P. A.; Liles, B. E. Special Areas Annual TechnicalConferenceANTEC, Toronto, Conference Proceedings,Vol. 3, pp 29312933; Society of Plastics Engineers, Brook-field, CT, 1997.

13. Lightsey, G.; Short, P. H.; Klasinsley, K. S.; Mann, L. J MissAcad Sci 1979, 24, 76.

14. Dalvag, H.; Klason, C.; Stromvall, H. E. Int J Polym Mater1985, 11, 9.


11/13



15. Boldizer, A.; Klason, C.; Kubat, J. Int J Polym Mater 1987,11, 229.

16. Bataille, P.; Ricard, L.; Sapieha, S. Polym Compos 1989, 10,103.

17. Raj, R. G.; Kokta, B. V.; Dembele, F.; Sanschagrain, B. J ApplPolym Sci 1989, 38, 1987.

18. Wright, J. R.; Mathias, L. J. J Appl Polym Sci 1993, 48,

2241.19. Belgacem, M. N.; Bataille, P.; Sapieha, S. J Appl Polym Sci

1994, 53, 379.

20. Sain, M. M.; Kokta, B. V. J Appl Polym Sci 1994, 54, 1545.

21. Sain, M. M.; Kokta, B. V.; Imbert, C. Polym Plast TechnolPlast Eng 1994, 133, 89.

22. Minqiu, Lu; Collier, J. R.; Collier, B. J. Annual TechnicalConference ANTEC, Conference Proceedings, Vol. 2,p 1433; Society of Plastics Engineers, Brookfield, CT,1995.

23. Czvikovszky, Tibor. Radiat Phys Chem 1996, 47 (3), 425;Mech Eng 1994, 38, 209.

24. Sun, B. C.; Hawke, R. N. J Adv Mater 1996, 27, 45.

25. Gatenholm, P.; Hedenberg, P.; Karlsson, J.; Felix, J. J Eng

Appl Sci 1996, 2, 2302.26. Kazayawoko, M.; Balatinecz, J. J.; Woodhams, R. T.; Law, V.

J Reinf Plast Compos 1997, 16 (15), 1383; Int J Polym Mater1997, 37 (34), 237.

27. Kazayawoko, M.; Balatinecz, J. J.; Woodhams, R. T. J ApplPolym Sci 1997, 66 (6), 1163.

28. Xanthos, M. Plast Rubber Proc Appl. 1983, 3 (3), 223.

29. Kokta, B. V.; Maldas, D.; Daneault, C.; Beland, P. PolymCompos 1990, 11, 84.

30. Maldas, D.; Kokta, B. V. J Vinyl Technol 1993, 15, 38; Kokta,B. V.; Maldas, D.; Daneault, G. C. J. Vinyl Technol 1989, 11,90.

31. Laurent, M. M.; Park, C. B.; Balatinecz, J. J. J Eng Appl Sci1996, 2, 1900.

32. Sati, M.; Agnelli, J. A. M. J Appl Polym Sci 1989, 37, 1777.33. Maldas, D.; Kokta, B. V. J Adhes Sci Technol 1991, 5, 727.

34. Simonsen, J.; Rials, T. G. J Thermoplast Compos Mater 1996,9, 292.

35. Rials, T. G.; Wolcott, M. P. J Mater Sci Lett 1998, 17 (4), 317.

36. Karmacker, A. C.; Hinrichsen, G. Polym Plast Technol Eng1991, 30 (5), 609.

37. Tan, T. Th. M. Polym Polym Compos 1997, 5, 273.

38. Karmaker, A. C.; Schneider, J. P. J Mater Sci Lett 1996, 15,201.

39. Basak, R. K.; Mitra, B. C.; Sarkar, M. J Appl Polym Sci 1997,67, 103.

40. Gassan, J.; Biedzki, A. K. Compos, Part A: Appl Sci Manuf

1997, 28A, 1001.41. Mohan Ranarajan; Kishor, K. J Reinf Plast Compos 1985,

4, 186.

42. Kumar, P. Indian J Technol 1986, 24, 29.

43. Sridhar, M. K.; Basavarajjappa, G.; Kasturi, S. Sg.; Balsubra-manian, N. Indian J Text Res 1982, 7, 87.

44. Roe, P. J.; Ansell, M. P. J Mater Sci 1985, 20, 4015.

45. Semsarzadeh, M. A. Polym Compos 1986, 7, 23.

46. Pal, S. K.; Muhkhopadhyaya, D.; Sanyal, S. K.; Mukharjea,R. N. J Appl Polym Sci 1988, 35, 973.

47. Pal, S. K.; Sanyal, S. K.; Mukharjea, R. N.; Phani, K. K.J Polym Mater 1994, 1, 69.

48. Varma, I. K.; Ananthakrishanan, S. R.; Krishnamurthy, Z. S.Composites 1989, 20, 383.

49. Mohanty, A. K.; Misra, M. Polym Plast Technol Eng 1995,

34 (5), 729.50. Murthy, V. M.; De, S. K. J Appl Polym Sci 1984, 29, 1355.

51. Bhagwan, S. S.; Tripathy, D. K.; De, S. K. J Appl Polym Sci1987, 33, 1623.

52. Ghosh, A. K.; Dey, S. S. Ceram-Matrix Compos 1993,813.

53. Joseph, K.; Thomas, S.; Pavithran, C.; Brahmakumar, M.J Appl Polym Sci 1993, 47, 1731.

54. Joseph, K.; Thomas, S.; Pavithran, C. Compos Sci Technol1995, 53 (1), 99.

55. Joseph, K.; Thomas, S.; Pavithran, C. Polymer 1996, 37,5139.

56. Singh, B.; Gupta, M.; Verma, A. Polym Compos 1996, 17,910.

57. Joseph, K.; Thomas, S.; Pavithran, C. J Reinf Plast Compos1993, 12, 139.

58. Kumar, R. P.; Amma, P.; Geethakumari, M. L.; Thomas, S.J Appl Polym Sci 1995, 58, 597.

59. Varghese, S.; Kuriakose, B.; Thomas, S.; Premalatha, C. K.;Koshy, A. T. Plast, Rubber Compos Process Appl 1993, 20(2), 93.

60. Varghese, S.; Kuriakose, B.; Thomas, S. J Appl Polym Sci1994, 53, 1051.

61. Varghese, S.; Kuriakose, B.; Thomas, S. J Adhes Sci Technol1994, 8, 235.

62. Bisanda, E. T. N.; Ansell, M. P. Compos Sci Technol 1991,41, 165.

63. Chen, H. L.; Porter, R. S. J Appl Polym Sci 1994, 54,

1781.64. Sanadi, A. R.; Caulfield, D. F.; Rowell, R. M. Plast Eng 1994,

50 (4), 27.

65. Sanadi, A. R.; Caulfield, D. F.; Jacobson, R. E.; Rowell, R. M.Ind Eng Chem Res 1995, 34 (5), 1889.

66. Sanadi, A. R.; Young, A. A.; Clemsons, C.; Rowell, R. M.J Reinf Plast 1994, 13, 54.

67. George, J.; Joseph, K.; Bhagawan, S. S.; Thomas, S. MaterLett 1993, 18 (3), 163.

68. George, J.; Bhagawan, S. S.; Thomas, S. J Therm Anal 1996,47 (4), 1121.

69. Devi, L. U.; Bhagawan, S. S.; Thomas, S. J Appl Polym Sci1997, 64, 1739.

70. Mieck, K. P.; Nechwatal, A.; Knobelsdorf , C. Angew Mak-

romol Chem 1995, 225, 37.71. Hornsby, P. R.; Hinrichsen, E.; Tarverdi, K. J Mater Sci 1997,

32, 1009.

72. Tobias, B. C. Evolving Technologies for the CompetitiveEdge, International SAMPE Symposium and Exhibition(Proceedings) 1997, 42 (2), 996; SAMPE, Covina, CA.

73. Zhu, W. H.; Tobias, B. C.; Coutts, R. S. P. J Mater Sci Lett1995, 14 (7), 508.


12/13

362 VOL. 18, NO. 4


74. Tobias, B. C.; Ibarra, E. Proceedings of the 42nd Interna-tional SAMPE Symposium, p 181, 1997.

75. Geethamma, V. G.; Thomas, K. M.; Lakshminarayanan, R.;Thomas, S. Polymer 1998, 39, 1483.

76. Sanadi, A. R.; Prasad, S. V.; Rohadgi, P. K. J Mater Sci 1986,21, 4299.

77. Fimio Goto Kasahara Yasumasa,PCT Int Appl WO 8810286,

Dec 29, 1988.78. Jain, S.; Kumar, R.; Jindal, U. C. Adv Compos Mater 1990,

135, Oxford IBH Publ Comp Pvt Ltd, New Delhi, India.

79. Ismail, H. R.; Rosnah, N. Polymer 1997, 38, 4059.

80. Castano, V. M.; Vazquez, P. G.; Amador, M. A.; Garcia,F. Z.; Martinez, E.; Marquez, A.U.; Altmamirano, M.A.

J Reinf Plast Compos 1995, 14 (8), 866.

81. Tinh, N.; Eugene, Z.; Edward, M. B. J Macromol Sci, RevMacromol Chem 1981, C20, 1.

82. Tinh, N.; Eugene, Z.; Edward, M. B. J Macromol Sci, RevMacromol Chem 1981, C21, 1.

83. Mohanty, A. K.; Patnaik, S.; Singh, B. C. J Appl Polym Sci1989, 37, 1171.

84. Sabaa, M. W. Polym Degrad Stab 1991, 32, 209.

85. Yap, M. G. S.; Que, Y. T.; Chia, L. H. L.; Chan, H. S. O. JAppl Polym Sci 1991, 43, 2057.

86. Gonzalez, C.; Myers, G. E. Int J Polym Mater 1993, 23, 67.

87. Mayer, G. E.; Chahyadi, I. S.; Gonzalez, C.; Coberly, C. A.;Ermer, D. S. Int J Polym Mater 1991, 15, 171.

88. Takase, S.; Shiraishi, N. J Appl Polym Sci 1989, 37, 645.

89. Rowell, R. M. ACS Proceedings, Polym Mater Sci Eng 1992,67, 461.

90. Rowell, R. M.; Clemson, C. M. Proceedings of the 26th In-ternational Particleboard/Composites Symposium, WA, p251, 1992.

91. Bledzki, A. K.; Reihmane, S.; Gassan, J. J Appl Polym Sci1996, 59, 1329.

92. Mohanty, A. K.; Singh, B. C. J Appl Polym Sci 1987, 34, 1325.

93. Muzumdar, P.; Sanyal, S.; Dasgupta, B.; Shaw, S. C.; Guha,R. Indian J Fibre Technol 1994, 19, 286.

94. Andersson, M.; Tillman, A. M. J Appl Polym Sci 1989, 37,3437.

95. Sahoo, P. K.; Samantaray, H. S.; Samal, R. K. J Appl PolymSci 1986, 32, 5693.

96. Samal, R. K.; Samantaray, H. S.; Samal, R. N. J Appl PolymSci 1986, 37, 3085.

97. Mannan-Kh., M.; Latifa, B. L. Polymer 1980, 21, 777.

98. Huque, M. M.; Habibuddowla, Md.; Mohamood, A. J.; Jab-bar Mian, A. J Polym Sci, Polym Chem Ed 1980, 18, 1447.

99. Samal, R. K.; Sahoo, P. K.; Samantaray, H. S. J Macromol SciChem, Rev Macromol Chem Phys 1986, C26, 81.

100. Mani, P.; Satyanarayan., K. G. J Adhes Sci Technol 1990, 4,17.

101. Liao, B.; Huang, Y.; Cong, G. J Appl Polym Sci 1997, 66,1561.

102. Samal, R. K.; Ray, M. C. J Polym Mater 1997, 14, 183.

103. Yap, M. G. S.; Chia, L. H. L.; Teoh, S. H. J Wood ChemTechnol 1990, 10, 1.

104. Garcia, Z. F.; Martinez, E.; Alvarez, C. A.; Castano, V. M. JReinf Plast Compos 1995, 14, 641.

105. Tobias, B. C. Proceedings of the International Conferenceon

Advanced Composite Materials; Minerals, Metals & Mate-rials Society (TMS), Warrendale, PA, 1993, p 623.

106. Vollenberg, P. H. Th.; Heiken, D. Polymer 1990, 30, 1652.

107. Felix, J. M.; Gotenholm, P.; Schreiber, H. P. Polym Compos1993, 14, 449.

108. Mukharjea, R. N.; Pal, S. K.; Sanyal, S. K.; Phani, D. K.J Polym Mater 1984, 1, 69.

109. Elvy, S. B.; Dennise, G. R.; Ng, L. T. J Mater Proc Technol1995, 48, 365.

110. Bawadekar, A. A.; Jog, J. P. Private communication.

111. Vasa, D.; Jog, J. P. Private communication.

112. Mod Plast Int, May 1997, p 39.

113. Mod Plast Int, May 1997, p 14.

114. Plast Technol, Jan 98.115. Information collected from Union Carbide and Kenrich

company data sheets.

Appendix

I. Macromolecule of Cellulose

CH2OHn

CH2OH

OHHO

OO

OOH

OH

OHH

HH

H

HH

H

H H

H HH

O

CH2OH

OH

O

OH

HH

H CH2OH

O

O

OH

OH HH

H

H OH


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II. Bleaching Process

In industry, the most common process of bleach-ing the cellulose/jute uses oxidizing agents such assodium hypochlorite, calcium hypochlorite, or hy-drogen peroxide. In the reaction process, nascentoxygen is also reported.93 The nature of the coloringmatter and the reaction mechanism have not beenreported so far.

(a) With sodium hypochlorite

NaOCl H O !: NaOH HOCl2

HOCl !: H OCl

HOCl H Cl !: Cl H O2 2

(b) With H2O2

H O !: HOO H2 2

III. Grafting of Monomer on CelluloseMacromolecule95

(a) Formation of the macromolecule radical

D DCell9H R !: Cell R9H;

D D DR OH or SO4

(b) Grafting the monomerInitiation

D DCell M !: Cell9M

Propagation

Cell9M Cell9M2

Cell9Mnn1

M

MCell9M

Termination

graftD DCell9M Cell9M !:n n

copolymer

Dimerization

Two macrocellulosic radicals might couple toyield a dimerized product

D2Cell !: dimerized polymer

(c) Grafting of monomer on acetylated cellu-

lose92

Cell9O9CR

CH3

O

Cell9O9CRHN

Cell9C9O

CH

M

O

CH2

O

IV. Coupling Agents

Most of the silane/titanate/zirconate couplingagents can be represented as R9(CH2 )9X(OR)n ,where X Si, Ti, or Zr,n 03, OR is the hydro-lyzable alkoxy group, and R and R are the func-tional organic groups (Table III). For example, fortriazine coupling agents, triazine derivatives form acovalent bond with cellulose fibers, schematicallyrepresented as91

H2NpN N

N

cellulose fiber

cellulose fiber

Cl

ClCl

N

R

N

N

HN

R

HN

ClCl

N N

N ClO

Natural Fiber Polymer Composites: A Review

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