Recent Developments in Chemical Modificationand Characterization of Natural Fiber-ReinforcedComposites

Maya Jacob John,1 Rajesh D. Anandjiwala1,21CSIR Materials Science and Manufacturing, Fibers and Textiles Competence Area,Port Elizabeth 6000, South Africa

2Department of Textile Science, Faculty of Science, Nelson Mandela Metropolitan University,Port Elizabeth 6000, South Africa

A critical review of the literature on the various aspectsof natural fibers and biocomposites with a particularreference to chemical modifications is presented inthis paper. A notable disadvantage of natural fibers istheir polarity which makes it incompatible with hydro-phobic matrix. This incompatibility results in poor inter-facial bonding between the fibers and the matrix. Thisin turn leads to impaired mechanical properties of thecomposites. This defect can be remedied by chemicalmodification of fibers so as to make it less hydrophilic.This paper reviews the latest trends in chemical modi-fications and characterizations of natural fibers. Thestructure and properties of natural fibers have beendiscussed. Common chemical modifications and theirmechanisms have also been elaborated. The impor-tance of chemical modifications and the resultantenhancement in the properties of the composites havealso been reviewed. Recent investigations dealing withchemical modifications of natural fiber-reinforced com-posites have also been cited. POLYM. COMPOS., 29:187–207, 2008. ª 2007 Society of Plastics Engineers

INTRODUCTION

Natural fiber-reinforced composites have attracted the

attention of the research community mainly because they

are turning out to be an alternative solution to the ever

depleting petroleum sources. The production of 100% natu-

ral fiber based materials as substitute for petroleum-based

products is not an economical solution. A more viable solu-

tion would be to combine petroleum and bio-based resour-

ces to develop a cost-effective product with diverse appli-

cations. The application of natural fiber-reinforced compo-

sites has been extended to almost all fields.

Natural fibers are hydrophilic in nature as they are

derived from lignocellulose, which contain strongly polar-

ized hydroxyl groups. These fibers, therefore, are inher-

ently incompatible with hydrophobic thermoplastics, such

as polyolefins. The major limitations of using these fibers

as reinforcements in such matrices include poor interfacial

adhesion between polar-hydrophilic fiber and nonpolar-

hydrophobic matrix, and difficulties in mixing due to poor

wetting of the fiber with the matrix. This in turn would

lead to composites with weak interface.

There are many parameters which affect the perform-

ance of a natural fiber-reinforced composite. The degree

and type of adhesion cannot be estimated quantitatively

even though its importance is well recognized. Aspect ratio

has a considerable effect on composite properties, hence it

is important to conserve fiber length as much as possible

during composite processing operations. Fiber aspect ratio

must be in the range of 100–200 for optimum effective-

ness. Fiber orientation has a significant effect on composite

properties. During processing, the fibers tend to orient

along the flow direction causing mechanical properties to

vary in different directions.

Another factor is fiber dispersion. Poor fiber dispersion

results in a loose bundle, embracing an effectively lower

aspect ratio with less reinforcing potential than a single

fiber. In addition, the bundle itself may be low in strength

due to poor adhesion. Both the above factors reduce the

overall strength of the composite. Also fiber bundles act as

barriers to transfer of stress, resulting in impaired proper-

ties. The development of strength in a composite also

depends on the existence of a strong interface. The fiber/

matrix interface in fiber-reinforced composites transfers an

externally applied load to fibers themselves. Load applied

directly to the matrix at the surface of the composites is

transferred to the fibers nearest the surface and continues

from fiber to fiber via matrix and interface. If the interface

Correspondence to: Maya Jacob John; e-mail: mjohn@csir.co.za,

mayajacobkunnel@yahoo.com

DOI 10.1002/pc.20461

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2007 Society of Plastics Engineers

POLYMER COMPOSITES—-2008

is weak, effective load distribution is not achieved and the

mechanical properties of the composites are impaired. On

the other hand, a strong interface can assure that the com-

posite is able to bear load even after several fibers are bro-

ken because the load can be transferred to the intact por-

tions of broken as well as unbroken fibers. A poor interface

is also a drawback in situations other than external me-

chanical loading, e.g. because of differential thermal

expansions of fiber and matrix, premature failure can occur

at a weak interface when the composite is subjected to

thermal stress. Thus, adhesion between fiber and matrix is

a major factor in determining the response of the interface

and its integrity under stress.

Optimization of interfacial adhesion between natural

fibers and polymer matrices (thermoplastic and thermoset)

has been the focus of a large amount of research conducted

during the past two decades. This manuscript attempts to

review the latest advancements in the field of chemical

modification of natural fibers.

NATURAL FIBERS

Structure of Natural Fibers

Natural fibers can be considered as composites of hol-

low cellulose fibrils held together by a lignin and hemicel-

lulose matrix [1]. The cell wall in a fiber is not a homoge-

nous membrane (see Fig. 1) [2]. Each fiber has a complex,

layered structure consisting of a thin primary wall which is

the first layer deposited during cell growth encircling a sec-

ondary wall. The secondary wall is made up of three layers

and the thick middle layer determines the mechanical prop-

erties of the fiber. The middle layer consists of a series of

helically wound cellular microfibrils formed from long

chain cellulose molecules. The angle between the fiber axis

and the microfibrils is called the microfibrillar angle. The

characteristic value of microfibrillar angle varies from one

fiber to another.

Such microfibrils have typically a diameter of about 10–

30 nm and are made up of 30–100 cellulose molecules in

extended chain conformation and provide mechanical

strength to the fiber. The amorphous matrix phase in a cell

wall is very complex and consists of hemicellulose, lignin,

and in some cases pectin. The hemicellulose molecules are

hydrogen bonded to cellulose and act as cementing matrix

between the cellulose microfibrils, forming the cellulose–

hemicellulose network, which is thought to be the main

structural component of the fiber cell. The hydrophobic lig-

nin network affects the properties of other network in a

way that it acts as a coupling agent and increases the stiff-

ness of the cellulose/hemicellulose composite. Some of the

important natural fibers used as reinforcement in compo-

sites are listed in Table 1 [4].

Chemical Composition

The reinforcing efficiency of natural fiber is related to

the nature of cellulose and its crystallinity. The main com-

ponents of natural fibers are cellulose (a-cellulose), hemi-

cellulose, lignin, pectins, and waxes.

Cellulose is a natural polymer consisting of D-anhydro-

glucose (C6H11O5) repeating units joined by b-1,4-glyco-FIG. 1. Structure of natural fiber [2].

TABLE 1. List of important natural fibers [3].

Fiber source Species Origin

Abaca Musa textiles Leaf

Alfa Stippa tenacissima Grass

Bagasse – Grass

Bamboo (>1,250 species) Grass

Banana Musa indica Leaf

Broom root Muhlenbergia macroura Root

Cantala Agave cantala Leaf

Caroa Neoglaziovia variegate Leaf

China jute Abutilon theophrasti Stem

Coir Cocos nucifera Fruit

Cotton Gossypium sp. Seed

Curaua Ananas erectifolius Leaf

Date palm Phoenix dactylifera Leaf

Flax Linum usitatissimum Stem

Hemp Cannabis sativa Stem

Henequen Agave fourcroydes Leaf

Isora Helicteres isora Stem

Istle Samuela carnerosana Leaf

Jute Corchorus capsularis Stem

Kapok Ceiba pentranda Fruit

Kenaf Hibiscus cannabinus Stem

Kudzu Pueraria thunbergiana Stem

Mauritius hemp Furcraea gigantea Leaf

Nettle Urtica dioica Stem

Oil palm Elaeis guineensis Fruit

Piassava Attalea funifera Leaf

Pineapple Ananus comosus Leaf

Phormium Phormium tenas Leaf

Roselle Hibiscus sabdariffa Stem

Ramie Boehmeria nivea Stem

Sansevieria (Bowstring hemp) Sansevieria Leaf

Sisal Agave sisilana Leaf

Sponge gourd Luffa cylinderica Fruit

Straw (Cereal) – Stalk

Sun hemp Crorolaria juncea Stem

Cadillo/Urena Urena lobata Stem

Wood (>10,000 species) Stem

188 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

sidic linkages at C1 and C4 position [5]. The degree of po-

lymerization (DP) is around 10,000. Each repeating unit

contains three hydroxyl groups. These hydroxyl groups and

their ability to hydrogen bond play a major role in directing

the crystalline packing and also govern the physical proper-

ties of cellulose. Solid cellulose forms a microcrystalline

structure with regions of high order, i.e. crystalline regions,

and regions of low order, i.e. amorphous regions. The crys-

tal nature (monoclinic sphenodic) of naturally occurring

cellulose is known as cellulose I. Cellulose is resistant to

strong alkali (17.5 wt%) but is easily hydrolyzed by acid to

water-soluble sugars. Cellulose is relatively resistant to

oxidizing agents.

Hemicellulose is not a form of cellulose and the name is

a misnomer. They comprise a group of polysaccharides

composed of a combination of 5- and 6-carbon ring sugars.

Hemicellulose differs from cellulose in three aspects.

Firstly, they contain several different sugar units whereas

cellulose contains only 1,4-b-D-glucopyranose units. Sec-

ondly, they exhibit a considerable degree of chain branch-

ing containing pendant side groups giving rise to its non-

crystalline nature, whereas cellulose is a linear polymer.

Thirdly, DP of hemicellulose is around 50–300, whereas

that of native cellulose is 10–100 times higher than that of

hemicellulose. Hemicellulose forms the supportive matrix

for cellulose microfibrils. Hemicellulose is very hydro-

philic, soluble in alkali, and easily hydrolyzed in acids.

Lignin is a complex hydrocarbon polymer with both ali-

phatic and aromatic constituents. They are totally insoluble

in most solvents and cannot be broken down to monomeric

units. Lignin is totally amorphous and hydrophobic in na-

ture. It is the compound that gives rigidity to the plants. It

is thought to be a complex, three-dimensional copolymer

of aliphatic and aromatic constituents with very high mo-

lecular weight. Hydroxyl, methoxyl, and carbonyl groups

have been identified in lignin. Lignin has been found to

contain five hydroxyl and five methoxyl groups per build-

ing unit. It is believed that the structural units of lignin

molecule are derivatives of 4-hydroxy-3-methoxy phenyl-

propane. The main difficulty in lignin chemistry is that no

method has been established by which it is possible to iso-

late lignin in its native state from the fiber. Lignin is con-

sidered to be a thermoplastic polymer exhibiting a glass-

transition temperature of around 908C and melting temper-

ature of around 1708C [6]. It is not hydrolyzed by acids,

but soluble in hot alkali, readily oxidized, and easily con-

densable with phenol [7].

Pectins are a collective name for heteropolysaccarides.

They provide flexibility to plants. Waxes make up the last

part of fibers and they consist of different types of alcohols.

Table 2 presents the chemical composition of various natu-

ral fibers [8–10].

The structure, microfibrillar angle, cell dimensions,

defects, and the chemical composition of fibers are the

most important variables that determine the overall proper-

ties of the fibers [11]. Generally, tensile strength and

Young’s modulus of fibers increase with increasing cellu-

lose content. The microfibrillar angle determines the stiff-

ness of the fibers. Plant fibers are more ductile if the micro-

fibrils have a spiral orientation to the fiber axis. If the

microfibrils are oriented parallel to the fiber axis, the fibers

will be rigid, inflexible, and have high tensile strength. Ta-

ble 3 presents the important physical properties of natural

fibers [12, 13].

The inherently polar and hydrophilic nature of lignocel-

lulosic fibers and the nonpolar characteristics of most ther-

moplastics result in compounding difficulties leading to

nonuniform dispersion of fibers within the matrix, which

impairs the properties of the resultant composite. This is a

major disadvantage of natural fiber-reinforced composites.

Another problem is that the processing temperature of

composites is restricted to 2008C as vegetable fibers

undergo degradation at higher temperatures; this restricts

the choice of matrix material. Another serious drawback is

the high moisture absorption of natural fibers leading to

swelling and presence of voids at the interface, which

results in poor mechanical properties and reduces dimen-

sional stability of composites. Another restriction to the

successful exploitation of natural fibers for durable com-

posite application is low microbial resistance and suscepti-

bility to rotting. These properties pose serious problems

during shipping, storage, and composite processing. The

nonuniformity and variation of dimensions and of their me-

chanical properties (even between individual plants in the

same cultivation) poses another serious problem.

It is quite clear that the advantages outweigh the disad-

vantages and most of the shortcomings have remedial

measures in the form of chemical treatments.

TABLE 2. Chemical composition of various natural fibers [8–10].

Fiber

Cellulose

(wt%)

Hemicellulose

(wt%)

Lignin

(wt%)

Waxes

(wt%)

Abaca 56–63 20–25 7–9 3

Alfa 45.4 38.5 14.9 2

Bagasse 55.2 16.8 25.3 –

Bamboo 26–43 30 21–31 –

Banana 63–64 19 5 –

Coir 32–43 0.15–0.25 40–45 –

Cotton 85–90 5.7 – 0.6

Curaua 73.6 9.9 7.5 –

Flax 71 18.6–20.6 2.2 1.5

Hemp 68 15 10 0.8

Henequen 60 28 8 0.5

Isora 74 – 23 1.09

Jute 61–71 14–20 12–13 0.5

Kenaf 72 20.3 9

Kudzu 33 11.6 14 –

Nettle 86 10 – 4

Oil palm 65 – 29 –

Piassava 28.6 25.8 45 –

Pineapple 81 – 12.7 –

Ramie 68.6–76.2 13–16 0.6–0.7 0.3

Sisal 65 12 9.9 2

Sponge gourd 63 19.4 11.2 3

Straw (Wheat) 38–45 15–31 12–20 –

Sun hemp 41–48 8.3–13 22.7 –

DOI 10.1002/pc POLYMER COMPOSITES—-2008 189

MECHANISMS OF CHEMICAL MODIFICATIONS

An in-depth account of surface modification of cellulosic

fibers has been reported by Belgacem and Gandini [14]. The

authors are of the opinion that the most promising approach

of chemical modification seemed to be the one that gave rise

to continuous covalent bonds between cellulose surface and

matrix. They also looked into a novel strategy which did not

involve a chemical modification but the formation of a phys-

ical sleeve of polymer around the fibers.

Surface modifications include (i) physical treatments,

such as solvent extraction; (ii) physico-chemical treat-

ments, like the use of corona and plasma discharges [15] or

laser, g-ray, and UV bombardment; and (iii) chemical mod-

ifications, both by direct condensation of the coupling

agents onto the cellulose surface and by its grafting by

free-radical or ionic polymerizations. The common cou-

pling agents used are silanes, isocyanates [16, 17], and tita-

nate-based compounds.

In this paper, we will focus on some of the chemical

treatments adopted for natural fibers. A few common modi-

fications are explained below.

Alkali Treatment

Alkali treatment leads to the increase in the amount of

amorphous cellulose at the expense of crystalline cellulose.

The important modification occurring here is the removal

of hydrogen bonding in the network structure.

The following reaction takes place as a result of alkali

treatment:

Fiber � OH þ NaOH ! Fiber � O�Naþ þ H2O (1)

The effect of alkali on cellulose fiber is a swelling reac-

tion, during which the natural crystalline structure of the

cellulose relaxes. Native cellulose (i.e. cellulose as it

occurs in nature) shows a monoclinic crystalline lattice of

cellulose-I, which can be changed into different polymor-

phous forms through chemical or thermal treatments. The

important forms are alkali-cellulose and cellulose-II as

shown in Fig. 2.

The type of alkali (KOH, LiOH, NaOH) and its concen-

tration will influence the degree of swelling, and hence the

degree of lattice transformation into cellulose-II [19]. Stud-

ies have shown that Naþ has got a favorable diameter, able

to widen the smallest pores in between the lattice planes

and penetrate into them. Consequently, sodium hydroxide

(NaOH) treatment results in a higher amount of swelling.

This leads to the formation of new Na–cellulose-I lattice, a

lattice with relatively large distances between the cellulose

molecules, and these spaces are filled with water mole-

cules. In this structure, the OH-groups of the cellulose are

converted into ONa-groups, expanding the dimensions of

molecules. Subsequent rinsing with water will remove the

linked Na-ions and convert the cellulose to a new crystal-

line structure, i.e. cellulose-II, which is thermodynamically

more stable than cellulose-I. NaOH can cause a complete

lattice transformation from cellulose-I to cellulose-II, in

contrast to other alkalis that produce only partial lattice

transformation (see Fig. 2). The alkali solution influences

not only the cellulosic components inside the plant fiber

but also the noncellulosic components (hemicellulose, lig-

nin, and pectin) [18].

Acetylation

Acetylation is a rather attractive method of modifying

the surface of natural fibers and making it more hydropho-

bic. It has been shown to reduce swelling of wood in water

and has been studied more than any other chemical reac-

tion of lignocellulosic materials. The main principle of the

method is to react the hydroxyl groups (OH) of the fiber

with acetyl groups (CH3CO), therefore rendering the fiber

surface more hydrophobic. The hydroxyl groups that react

are those of the minor constituents of the fiber, i.e. lignin

and hemicelluloses, and those of amorphous cellulose [3].

TABLE 3. Physical properties of various natural fibers [8, 9].

Fiber

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Elongation

at break (%)

Density

(g/cm3)

Abaca 400 12 3–10 1.5

Alfa 350 22 5.8 0.89

Bagasse 290 17 – 1.25

Bamboo 140–230 11–17 0.6–1.1

Banana 500 12 5.9 1.35

Coir 175 4–6 30 1.2

Cotton 287–597 5.5–12.6 7–8 1.5–1.6

Curaua 500–1,150 11.8 3.7–4.3 1.4

Date palm 97–196 2.5–5.4 2–4.5 1–1.2

Flax 345–1,035 27.6 2.7–3.2 1.5

Hemp 690 70 1.6 1.48

Henequen 500 6 70 13.2 6 3.1 4.8 6 1.1 1.2

Isora 500–600 – 5–6 1.2–1.3

Jute 393–773 26.5 1.5–1.8 1.3

Kenaf 930 53 1.6 –

Nettle 650 38 1.7 –

Oil palm 248 3.2 25 0.7–1.55

Piassava 134–143 1.07–4.59 21.9–7.8 1.4

Pineapple 1.44 400–627 14.5 0.8–1.6

Ramie 560 24.5 2.5 1.5

Sisal 511–635 9.4–22 2.0–2.5 1.5

FIG. 2. Lattice structures of cellulose I and cellulose II [18].

190 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

The hydroxyl groups in the crystalline regions of the fiber

are closely packed with strong interchain bonding, and are

inaccessible to chemical reagents. The acetylation of the

��OH group in cellulose is represented below.

Fiber � OH þ CH3CO � O � OC � CH3

����!CH3COOH

conc:H2SO4

Fiber � O � CO � CH3 þ CH3COOH (2)

Acetylation has been shown to be beneficial in reducing

moisture absorption of natural fibers. Reduction of about

50% of moisture uptake for acetylated jute fibers and of up

to 65% for acetylated pine fibers has been reported in the

literature [20]. Acetylation has also been found to enhance

the interface in flax/polypropylene composites [21].

Silane Treatment

These chemicals are hydrophilic compounds with differ-

ent groups appended to silicon such that one end will inter-

act with matrix and the other end can react with hydro-

philic fiber, which act as a bridge between them. The

uptake of silane is very much dependent on a number of

factors including hydrolysis time, organofunctionality of

silane, temperature, and pH. Alkoxy silanes are able to

form bonds with hydroxyl groups. Silanes undergo hydro-

lysis, condensation, and the bond formation stage. Silanols

can form polysiloxane structures by reaction with hydroxyl

group of the fibers [22]. The chemical reaction is given in

Fig. 3.

In addition to the self condensation of silanes and the

condensation on the surface of cellulose fiber, amino silane

molecules can interact with the OH groups of cellulose via

their Brønsted basic amino groups.

Herrera-Franco and coworkers [24–26] have studied the

influence of different silane coupling agents on the proper-

ties of henequen fiber-reinforced polymer composites. The

authors used FTIR and XPS spectroscopy to show that the

reaction between silanes and cellulose takes place only at

temperatures above 708C.

CHEMICAL MODIFICATION ANDCHARACTERIZATION OF NATURAL FIBERS

Natural fibers are amenable to chemical modification

due to the presence of hydroxyl groups. The hydroxyl

groups may be involved in the hydrogen bonding within

cellulose molecules, thereby activating these groups or can

introduce new moieties that form effective interlocks

within the system. Surface characteristics, such as wetting,

adhesion, surface tension, or porosity of fibers, can be

improved upon chemical modification. The irregularities of

the fiber surface play an important role in the mechanical

interlocking at the interface. The interfacial properties can

be improved by giving appropriate modifications to the

components, which gives rise to changes in physical and

chemical interactions at the interface. An enormous amount

of work has been conducted in the field of fiber modifica-

tion. Some of the recent studies have been cited below.

FIG. 3. Scheme of interaction of silanes with cellulosic fibers [23].

DOI 10.1002/pc POLYMER COMPOSITES—-2008 191

Aspen Fiber Composites

Xue et al. [27] investigated the influence of the maleic

anhydride grafted polypropylene (MAPP) coupling agent

on the mechanical properties of aspen fiber-reinforced

polypropylene (PP) composites. The coupling agent

improved the adhesion between fiber and matrix and

increased the tensile properties of the composites. The ten-

sile strength of the composites containing compatibilizer

increased by 15% and flexural strength registered an

increase of 40%. The greater interfacial adhesion is also

evident from the SEM photomicrographs of the fractured

specimens as shown in Fig. 4a and b.

In another interesting study, Colom et al. [28] studied

the effect of different chemical modifications on the inter-

facial characteristics of aspen fiber-reinforced high-density

polyethylene (HDPE) composites. The interaction between

aspen fibers and HDPE was improved by the addition of

two coupling agents, namely, maleated polyethylene (epo-

lene C-18) and g-methacryloxy-propyl trimethoxy silane

(silane A-174). Interfacial morphology was studied by

means of Fourier transform infra red (FTIR) spectroscopy.

The authors observed that silane A-174 was a better cou-

pling agent than epolene. This was attributed to the fact

that in composites modified with epolene, adhesion occurs

due to multiple mechanisms of adsorption and interdiffu-

sion, whereas the composites treated with silane showed a

chemical mechanism of adhesion with the formation of

covalent bonds and also hydrogen bridges which would

lead to greater adhesion between the fibers and matrix.

Abaca Fiber Composites

Shibata et al. [29] investigated the effect of surface

treatment on abaca fiber-reinforced polyester composites.

Esterifications using acetic anhydride, butyric anhydride,

alkali treatment, and cyanoethylation were performed as

surface treatments on the abaca fiber. The flexural moduli

of all the fiber-reinforced composites increased with fiber

content. The effect of the surface treatment on the flexural

modulus of the fiber-reinforced composites was not so pro-

nounced.

In an interesting study, Teramoto et al. [30] studied the

biodegradation of aliphatic polyester composites reinforced

by abaca fiber. Abaca fibers, chemically modified with ace-

tic anhydride, showed greater resistance to biodegradation

due to the presence of a protective covering on the fibers.

Bagasse Fiber Composites

Researchers have reported the use of benzoic acid for

chemical modification of bagasse fibers in polyvinyl chlo-

ride composites [31]. It was observed that the tensile

strength of modified composites increased by about 35%,

which indicates better reinforcement. Composites contain-

ing untreated fibers exhibited a tensile strength of 38 MPa,

whereas that of chemically modified composites was

52 MPa.

In another study, Cao et al. [32] investigated the me-

chanical properties of bagasse fiber-reinforced polyester

composites before and after alkali treatment. NaOH solu-

tions of 1, 3, and 5% concentrations were used. Superior

properties were obtained for composites made from 1%

NaOH-treated bagasse fibers. The authors observed a 13%

improvement in tensile strength, 14% in flexural strength,

and 30% in impact resistance, respectively, due to chemi-

cal modification.

Bamboo Fiber Composites

Lee and Wang [33] investigated the effect of lysine-

based diisocyanate (LDI) as a coupling agent on the prop-

erties of biocomposites from polylactic 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/BF compo-

sites were improved by the addition of LDI, but thermal

flow [34] was hindered due to cross-linking between poly-

FIG. 4. SEM micrographs showing the cross section of fractured 30 wt% aspen fiber–polypropylene com-

posites: (a) without maleated anhydride polypropylene and (b) with 2 wt% maleated anhydride polypropylene

[27]. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

192 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

mer matrix and BF. Enzymatic biodegradability of PLA/

BF and PBS/BF composites was investigated by proteinase

K and lipase PS, respectively. It was observed that the

composites could be quickly decomposed by enzyme and

the addition of LDI delayed the degradation. Figure 5a–f

shows the SEM micrographs of PLA or PBS/BF (70/30)

composites with LDI degraded for different degradation

time. It can be clearly seen that the matrix of both compo-

sites were reduced as enzymatic degradation proceeded.

The chemical modification of BF with NaOH solutions

of 10, 15, and 20% concentrations was attempted by Das

and Chakraborty [35]. It was found that a lattice transfor-

mation from cellulose-I to cellulose-II occurred during al-

kali treatment. They also observed that the degree of crys-

tallinity and crystallinity index for bamboo strips increased

with the increase in concentration of alkali up to 15% and

thereafter decreased.

Ismail et al. [36] reported the effect of fiber loading and

bonding agents in BF-reinforced natural rubber composites.

The authors observed that the presence of bonding agents

gave shorter curing time and improved mechanical proper-

ties. The authors continued their study to investigate the

effects of a silane coupling agent (Si69) on curing charac-

teristics and mechanical properties and observed that

scorch time and cure time of the composites decreased

with the presence of a silane coupling agent. The mechani-

cal properties of composites were found to improve with

the addition of Si69 [37] (see Fig. 6). It can be observed

from the figure that tensile strength increased from 7 to 12

MPa due to chemical modification.

Thwe and Liao [38] examined the effect of coupling

agent (maleic anhydride polypropylene) on the tensile

and flexural properties of BF-reinforced PP and bamboo-

glass fiber-reinforced PP hybrid composites. It was shown

FIG. 5. SEM micrographs of PLA or PBS/BF composites with LDI (NCO content, 0.65%) after different

enzymatic degradation time. (A) Control of PLA/BF composite, (B) after 7 days, (C) after 9 days, (D) con-

trol of PBS/BF composite, (E) after 2 days, (F) after 4 days [33].

DOI 10.1002/pc POLYMER COMPOSITES—-2008 193

that the presence of coupling agent and hybridization with

synthetic fibers is a viable approach for enhancing the me-

chanical properties and durability of natural fiber compo-

sites.

Banana Fiber Composites

Pothan et al. [23] investigated the influence of chemical

modification on dynamic mechanical properties of banana

fiber-reinforced polyester composites. A number of silane

coupling agents were used to modify the banana fibers.

The damping peaks were found to be dependent on the na-

ture of chemical treatment. Both storage modulus and

damping values were found to be consistent and indicated

the effectiveness of silane A174 coupling agent (c-metha-

cryloxypropyl trimethoxy silane) for improving fiber–ma-

trix adhesion. They characterized the modified fibers using

XPS and observed the presence of an increased concentra-

tion of silicon elements on the surface of fibers [39].

Joseph et al. [40] studied the environmental durability

of chemically modified banana-fiber-reinforced phenol

formaldehyde (PF) composites. The authors observed that

silane, NaOH, and acetylation treatments improved the re-

sistance of the banana/PF composites on outdoor exposure

and soil burial.

Idicula et al. [41] investigated the thermophysical prop-

erties of banana-sisal hybrid-reinforced composites as a

function of chemical modification. Sisal and banana fibers

were subjected to mercerization and polystyrene maleic an-

hydride (MA) treatments. The authors observed that chemi-

cal modification resulted in an increase of 43% in thermal

conductivity when compared with untreated composites.

Coir Fiber Composites

Brahamakumer et al. [42] studied the effect of natural

waxy surface layer of coir fiber-reinforced polyethylene

composites. To assess the effectiveness of waxy layer on

interfacial bonding, coconut fibers with wax-free surface

and surface-modified wax-free fibers obtained by grafting

of an isocyanate derivative of cardanol (CTDIC) were also

used. The natural waxy surface layer of coconut fiber was

found to provide a strong interfacial bonding between the

fiber and the polyethylene matrix. Removal of the waxy

layer resulted in a weak interfacial bonding which

increased the critical fiber length by 100% and decreased

the composite tensile strength and modulus by 40 and

60%, respectively. The waxy layer due to its polymeric na-

ture showed a stronger effect on fiber/matrix bonding than

by grafted layer of a C15 long alkyl chain molecule onto

the wax-free fiber.

Figure 7a and b show the optical fractographs of the as-

received and wax-free fiber composites containing 20-mm-

long coir fibers. Large fiber pullout can be seen for the

wax-free fiber-reinforced composites due to poor interfa-

cial bonding, and this does not occur for the composites

made from as-received coir fibers.

Calado and Barreto [43] reported the effect of surface

treatment on the surface topography of coir fibers. The

chemical treatment comprised of two steps and the first

was the removal of lignin from the surface of the fibers by

treatment with 2% sodium sulphite solution. The second

step was to treat the lignin free fibers with acetic anhy-

FIG. 6. Effect of fiber loading on tensile strength of rubber composites

with and without silane coupling agent [36].

FIG. 7. Optical fractographs of oriented discontinuous fiber composites (ODFC) of (a) as-received and (b)

wax-free coconut fibers showing large fiber pullout for the latter due to poor interfacial bonding [42].

194 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

dride. This treatment was done to reduce the number of

hydroxyl groups on the surface of the fibers. Reduction in

the number of hydroxyl groups reduces the polarity and

improves the compatibility with common thermosetting

matrices. Figure 8a and b show the clear difference found

between the surface of the untreated and treated fibers. The

untreated fibers have an outer surface layer and this layer is

completely removed by the chemical treatment with so-

dium sulphite and acetic anhydride. On removal of this

outer layer, a rougher but more ordered structure is

revealed. The ordered white dots found on the surface of

the treated fibers can be seen in Fig. 8b and were identified

as silica-rich material.

Mohanty et al. [44] studied the influence of surface

modification on the performances of coir–polyester amide

(PEA biocomposites at a fiber content of 50 wt%). Coir

fibers in the form of unidirectional nonwoven mat were

used in this study. The tensile strengths for coir/PEA com-

posites reported were in the range of 29–38 MPa. The sur-

face modification methods involved were alkali treatment,

cyanoethylation, acetylation, and grafting.

Mercerization of coir fibers has been found to change

the surface topography of the fibers and their crystallo-

graphic structure. The physical and chemical behavior of

coir as a result of the chemical modification was character-

ized by FTIR spectra [45]. The influence of delignification

and alkali treatment on the fine structure of coir fibers was

investigated by Sreenivasan et al. [46]. Surface modifica-

tions of coir fibers by graft copolymerization with methyl

methacrylate and acrylonitrile have also been reported

[47].

Rout et al. [48] have reported the scanning electron mi-

croscopy (SEM) studies of the alkali-treated coir fibers.

They have reported on the progressive changes in the sur-

face of coir fibers by SEM due to the removal of tyloses

from the surface as a result of alkali treatment.

Silva et al. [49] investigated the mechanical and thermal

characterization of coir fiber. The effect of alkali treatment

was evaluated and morphological changes were studied by

SEM. Thermogravimetric analysis showed an increase in

water content and a slight decrease in thermal stability due

to alkali treatment. Figure 9 shows the oval cross section

of the coir fibers. In Fig. 9a, a fiber treated for 48 h in

NaOH is shown for which the central lumen was observed

at higher magnification in Fig. 9b. The cellulose helical

spirals emerging from the fractured cross section are shown

in Fig. 9c.

Date Palm Fiber Composites

Kaddami et al. [50] investigated the effect of chemical

modification on date palm fibers. Date palm fibers were

subjected to maleic and acetic anhydride modifications and

used for reinforcement in epoxy and polyester resin. The

dynamic mechanical properties of the composites were

evaluated and it was observed that the glass-transition tem-

perature of modified composites (388C) was slightly higher

in comparison to the unmodified composites (328C). This

was attributed to better interfacial adhesion in treated com-

posites, which restricts the motion of the polymer chains in

the vicinity of the fiber surface and leads to an increase in

glass transition temperature.

The influence of MA-grafted PP (epolene g-3003) as a

coupling agent on the mechanical behavior of date-palm

fiber-reinforced PP composites was recently reported [51].

Utilizing Epolene g-3003 was found to improve the fiber–

matrix adhesion, resulting in a significant improvement in

composite performance. The tensile strength was found to

increase by 5%.

Flax Fiber Composites

A number of studies concerning the chemical modifica-

tion of flax fiber has been reported. Weyenberg et al. [18]

investigated the chemical modification of flax fiber by al-

kali treatment. The study concentrated on optimizing pa-

rameters, such as time and concentration of NaOH, to de-

velop a continuous process for the treatment and fabrica-

FIG. 8. Surface aspects of the coir fibers (a) before and (b) after chemical treatment [43].

DOI 10.1002/pc POLYMER COMPOSITES—-2008 195

tion of unidirectional flax fiber epoxy composites. The

authors observed a clear improvement in the mechanical

properties of the resulting material; treatment with 4%

NaOH solution for 45 s increased the transverse strength of

the composites by up to 30%.

Gouanve et al. [52] investigated the effect of helium,

cold plasma, and autoclave treatments on flax fibers. The

results showed no significant effect after plasma treatment

while an increase in moisture resistance was observed after

autoclave treatment.

Shanks et al. [53] prepared composites from PLA-rein-

forced with flax fibers subjected to interstitial polymeriza-

tion to replace water in the cellulose fibers. The influence

of the interstitial polymerization on the dynamic storage

modulus was found to be significant. The composites of

polymerization treated flax had higher storage moduli than

the unwashed fiber composites, which suggested that the

adhesion between the flax fibers and the matrix was

improved by the treatments.

Wang et al. [54] investigated the effects of different

chemical modifications on the properties of flax fiber-rein-

forced rotationally molded composites. The chemical mod-

ifications carried out were mercerization, peroxide treat-

ment, benzoylation, and peroxide treatment. The modified

fibers were then extruded with the polymer matrix to form

the composite and an improvement in interfacial adhesion

was observed.

Tserki et al. [55] studied the effect of acetylation and pro-

pionylation and the addition of MA-grafted Bionolle 3001

as a compatibilizer on the flax and hemp fiber/polyester

composites. The fiber treatments and the incorporation of

compatibilizer significantly improved the tensile strength of

the composites, whereas an important reduction in the water

absorption was found with the addition of treated fibers. The

use of compatabilizer resulted in the increase in tensile

strength from 25.4 to 33.3 MPa and Young’s modulus from

1,580 to 1,720 MPa. Moreover, incorporation of fibers into

the matrix also increased the rate of biodegradation.

Zafeiropoulos et al. [56] investigated the effect of chemi-

cal modification on the tensile strength of flax fibers. The

authors tried acetylation and stearation and found that the

tensile strength of flax fibers did not exhibit any drastic

improvement. The SEM examination of the fractured surfa-

ces revealed that acetylated fibers exhibited a different mode

of failure from the other fibers, suggesting that the treatment

altered the bulk properties along with the surface properties.

Arbelaiz et al. [57] developed composites by mixing

biodegradable poly(e-caprolactone) thermoplastic (PCL)

with bundles of short flax fibers. In order to improve fiber/

matrix adhesion, poly(e-caprolactone)-g-maleic anhydride

copolymer (PCL-g-MA) was used as compatibilizer. Com-

posites fabricated with bundles of flax fiber and PCL-g-

MA matrix showed an improvement in tensile and flexural

strengths by 54 and 44%, respectively. SEM of fractured

surfaces confirmed the improvement in adhesion between

flax fiber bundles and PCL-g-MA matrix. Results obtained

by thermogravimetric analysis showed that addition of

fibers and modification of matrix slightly reduced the ther-

mal stability of composites.

Recently, Kreze et al. [58] reported on the water sorp-

tion behavior of raw and treated flax fibers. The effects of

conventional pretreatment processes on the sorption ability

of flax fibers were compared with environment-friendly en-

zymatic procedures.

FIG. 9. Scanning electron micrographs (a) and (b) of typical cross sec-

tions of treated coir fiber at two magnifications; (c) a cellulose helical

spiral in a fractured cross section [49].

196 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

Hemp Fiber Composites

Sebe et al. [59] observed a strong interfacial adhesion

between hemp fibers and polyester matrix due to chemical

modification. The modification consisted of esterification

of hemp fibers using methacrylic anhydride which intro-

duced reactive vinylic groups at the surface of the fibers.

Radical copolymerization between the vinylic groups and

the unsaturated bonds of the resin occurred and this

accounted for greater interfacial adhesion.

Mutje et al. [60] investigated the effect of maleated PP

as coupling agent for PP composites reinforced with hemp

fibers. The addition of the coupling agent increased the

ultimate tensile strength and flexural strength by up to 49

and 38%, respectively, in comparison to the composites

produced without coupling agent. The interaction between

the surface of hemp fibers and the coupling agent was

also monitored by FTIR spectroscopy. The efficient inter-

action of MA with the functional surface of hemp fibers

increased the interfacial adhesion and properties of compo-

sites.

Sain et al. [61] studied the interface modification and

mechanical properties of hemp fiber-reinforced polyolefin

composite products. The effect of low-molecular weight

and maleated-type coupling agent on the mechanical prop-

erties of these natural fiber-reinforced PP composites was

also investigated and it was found that the optimum level

of the coupling agent was around 3–4% by weight of the

composites.

Bledzki et al. [62] investigated the influence of chemical

treatment on the properties of hemp fiber-reinforced epoxy

and PP composites. The effects of different mercerization

parameters, such as concentration of alkali (NaOH), tem-

perature, duration time, and tensile stress applied to the

fibers on the structure and properties of hemp fibers were

studied and judged via the cellulose I-II lattice conversion.

It was observed that the mechanical properties of the fibers

can be controlled in a broad range using appropriate pa-

rameters during mercerization treatment.

Rouison et al. [63] studied the water absorption of hemp

fiber-reinforced unsaturated polyester composites by mag-

netic resonance imaging system. The authors observed that

the moisture absorption process followed a diffusion mech-

anism and was found to be more important in the longitudi-

nal than in the transverse direction of composites. Also,

various fiber treatments did not result in a substantial

increase in the resistance to water absorption. According to

the authors, the most effective technique to enhance mois-

ture resistance was to properly encapsulate all the fibers

within the matrix.

Aziz and Ansell [64] investigated the effect of alkali

treatment on the mechanical and thermal properties of

hemp fiber-reinforced polyester composites. The SEM pho-

tomicrograph of the longitudinal surface of untreated fiber

bundles in Fig. 10a shows the presence of wax, oil, and

surface impurities. Waxes and oils provide a protective

layer to the surface of the fibers. The longitudinal view of

the 6% NaOH-treated hemp fiber in Fig. 10b shows a very

clean surface. Its surface topography shows the absence of

surface impurities, which were present in the untreated

fiber. The individual ultimate fibers also show a slight sep-

aration which was not apparent in the untreated hemp fiber

shown in Fig. 10a.

In an innovative study, the characterization of chemi-

cally treated hemp fiber using atomic force microscopy

(AFM) was reported by Pietak et al. [65]. The modifica-

tions employed were steam, alkali, combination of steam–

alkali, enzymatic treatments, and acetylation. The effects

of various treatments on the microstructure are shown from

AFM images in contact mode in Fig. 11. Image from steam

treatment in Fig. 11a suggests changes to, but ineffective

removal of, the primary cell wall. A combination of

steam–alkali treatment in Fig. 11b and c shows smooth,

highly aligned, and well-separated fibrils indicative of the

secondary cell wall. Topography of the enzyme-treated

fibers in Fig. 11d also shows sections which indicate expo-

sure of the secondary cell wall, with cross-banding of the

fibrils that has been observed by other researchers [66],

using enzyme treatments.

Figure 12 shows the adhesion observed on natural fibers

on areas representative of the microstructures shown in

Fig. 11. Consistent with increasing cellulose content and

increasing exposure of the secondary cell wall (indicated

from images of the surface) steam, enzyme, and combina-

tion of steam–alkali-treated fibers show increasing adhe-

sion force, whereas untreated fibers show lower adhesion

force. Further treatment of steam–alkali fibers by acetyla-

tion results in decreased adhesion force, which is consistent

with the expected increase in nonpolarity after the acetyla-

tion treatment.

In an interesting study, concerning the optimization of

hemp fibers, Pickering et al. [67] observed that alkali treat-

ment with 10 wt% NaOH solution at a maximum process-

ing temperature of 1608C with a hold time of 45 min was

found to produce strong fibers with a low lignin content

and good fiber separation. The X-ray diffractogram of

untreated and alkali-treated hemp fibers are shown in

Fig. 13. It can be observed that the major crystalline peak

FIG. 10. SEM micrographs of longitudinal views of (a) untreated hemp

fiber and (b) 6% NaOH-treated hemp fiber [64].

DOI 10.1002/pc POLYMER COMPOSITES—-2008 197

FIG. 11. Morphology of fiber surfaces after treatment obtained from deflection AFM image data. Scale bar

shows 400 nm. Steam treatment is shown in (a), low magnification steam þ alkaline is shown in (b) with

higher magnification in (c), enzyme treatment is shown in (d) [65].

FIG. 12. AFM adhesion forces measured on natural fibers. Estimates of

the water contact angle and surface energy [65].

FIG. 13. X-ray diffraction patterns of NaOH-treated and untreated

hemp fiber [67]. [Color figure can be viewed in the online issue, which

is available at www.interscience.wiley.com.]

198 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

on each pattern occurred at around 2y ¼ 22.58, which rep-

resents the cellulose crystallographic plane (002). The X-

ray diffractogram shows that the intensity of the crystallo-

graphic plane (002) was significantly increased due to al-

kali treatment of the hemp fibers. It was also seen that the

crystallinity index of the hemp fibers increased with alkali

treatment. This was attributed to the better packing and

stress relaxation of cellulose chains as a result of the re-

moval of pectins and other amorphous constituents from

the fiber [68]. The increase in crystallinity obtained by al-

kali treatment of the hemp fibers is thought to be the main

contributing factor for the increase in fiber strength.

Thielemans et al. [69] used lignin to modify hemp fibers

in polyester composites and observed an improvement in

tensile strength of composites. The tensile modulus

increased by 10%, the flexural modulus increased by

22.5%, and the flexural strength increased by 7%. These

increments in the mechanical properties were related to the

amount of lignin covering the fiber.

In another novel approach, the fungal modification of

hemp fiber in polyester composites was carried out by

Gulati and Sain [70]. They used a fungus, Ophiostomaulmi, obtained from elm tree infected with Dutch elm dis-

ease. Treated fibers showed improved acid–base character-

istics and resistance to moisture. Composites produced

from treated fibers showed slightly better mechanical prop-

erties, which was due to improved interfacial adhesion. It

was also observed that the flexural modulus increased from

21.7 to 26.2 MPa due to fungal treatment.

Henequen Fiber Composites

Toledano-Thompson et al. [71] investigated the charac-

terization of henequen fibers treated with an epoxide and

grafted with polyacrylic acid (PAA). The grafting of PAA

onto the henequen fibers was confirmed by FTIR. Gravi-

metric measurements and FTIR characterization indicated

that the amount of grafting depends on the concentration of

the initiator present in the reaction medium. The best con-

ditions for the grafting reaction were found when the initia-

tor was 0.4% with respect to the weight of the fiber.

Herrera-Franco and Valdez-Gonzalez [72] used different

silane coupling agents to increase the extent of adhesion

between henequen fibers and HDPE. The increase in tensile

strength was only found to be possible when the henequen

fibers were treated first with an alkali solution. It was also

shown that the silane treatment produced an increase of

13% in flexural strength while the flexural modulus

remained relatively unaffected.

Isora Fiber Composites

In an interesting study, researchers have used a novel

fiber—isora fiber—in natural rubber [73]. Isora fiber

resembles jute in appearance but surpasses it in strength,

durability, and luster. The effects of different chemical

treatments, including fibers, acetylation, benzoylation,

treatment with toluene diisocyanate (TDI) and silane cou-

pling agents, on the mechanical properties of isora fibers

and composites were analyzed. Isora fiber was seen to have

immense potential as reinforcement in natural rubber. The

variation in tensile strength with chemical modification is

shown in Fig. 14 and it can be observed that treatments

with NaOH and TDI exhibited higher tensile strength

among other chemical modifications.

Jute Fiber Composites

In an interesting study, Ray et al. [74] investigated the

dynamic mechanical properties of vinyl ester composites

reinforced with shellac-treated jute yarn. Shellac is a natu-

ral resin of animal origin and consists of an excretion from

a coccid insect that lives in twigs of certain trees in India.

Shellac is a complex mixture of esters, anhydrides, and lac-

tones of aliphatic and aromatic polyhydroxy carboxylic

acids. A main component found is aleuritic acid which is

9,10,16-trihydroxy palmitic acid. They observed that maxi-

mum flexural properties were exhibited by composites con-

taining jute yarns treated with 1% shellac solution. The

authors suggested that at 1% concentration, the hydrogen

bonding between jute fiber, resin, and deposited shellac

reached an optimum balance which imparted high tensile

modulus to the composites. Figure 15a–d represent the

scanning electron micrographs of the untreated and treated

composites at 1, 2, and 5% concentrations, respectively.

Vilaseca et al. [75] studied the partial delignification of

jute strands by means of NaOH treatment. The alkali treat-

ment resulted in an increase in the strength and stiffness of

the jute strand/starch composites. The enhancement was

analyzed in terms of compatibility and extension of the ad-

hesion at fiber–matrix interface. The authors have sug-

gested that higher extension of the hydrogen bonds at

fiber–matrix interface gave higher strength and stiffness to

the jute strand/starch-based composites.

FIG. 14. Variation of tensile strength with different chemical modifica-

tions [73].

DOI 10.1002/pc POLYMER COMPOSITES—-2008 199

Keener et al. [76] investigated the effect of various

maleated PP coupling agents in jute- and flax-reinforced

PP composites. They observed that tensile strength of flax

fiber-reinforced composites increased by 41% in presence

of compatabilizer. The authors also looked into the effec-

tiveness of oxidized polyethylene and newly developed

maleated polyethylene couplers in wood fiber-reinforced

HDPE composites and found that a 3% loading of compa-

tabilizer could double the tensile strength and triple the

impact properties compared with composites without the

compatabilizer.

Ray et al. [77] conducted another research work by

imparting alkali treatment to jute fibers. The authors sub-

jected jute fibers to chemical treatment with 5% NaOH so-

lution for 4 and 8 h and the mechanical and impact fatigue

properties of jute-reinforced vinyl ester composites were

analyzed. The mechanical properties increased in the case

of composites containing fibers treated with alkali for 4 h.

Composites produced from fibers treated with alkali for 8 h

showed maximum impact fatigue resistance. Interfacial

debonding was also found to be minimal in these compo-

sites.

Kapok Fiber Composites

The influence of chemical modification of kapok–cotton

fabric in PP composites was investigated by Mwaikambo

et al. [78]. Results show that fiber modification gives a sig-

nificant improvement to the thermal properties of the

fibers, whereas tests on the mechanical properties of the

composites showed poor tensile strength. Fibers was found

to impart toughness to the materials, with acetylation

showing slightly less rigidity compared with other treat-

ments on either the fiber or composites.

Kenaf Fiber Composites

The chemical modification of kenaf fibers was carried

out by Edeerozey et al. [79]. Different concentrations of

NaOH were used and the morphological changes were

examined by SEM. The authors observed that treated kenaf

fibers exhibited better mechanical properties than untreated

fibers. Also, the optimum concentration of NaOH was

found to be 6%. Figure 16a and b exhibits the SEM micro-

graphs of untreated and treated kenaf fibers. A decrease in

the amount of surface impurities was observed in the case

of treated fibers. Fiber bundle tests were also performed

and the strength of 6% NaOH-treated fiber bundles was

found to be higher by 13%.

Nishino et al. [80] investigated the influence of silane

coupling agent (glycidoxypropyl trimethoxysilane) on

kenaf fiber-reinforced PLA. The stress on the fibers in the

composite under transverse load was monitored in situ

and nondestructively using X-ray diffraction. Figure 17

presents the schematic sketch of a model for the measure-

ment of the strain on kenaf fiber incorporated into PLLA

resin by X-ray diffraction. It was found that the stress on

the composite was effectively transferred to the incorpo-

rated kenaf fiber through the matrix, because of the rela-

tively strong interaction between the kenaf fiber and

FIG. 15. SEM micrographs of (a) untreated, and (b) 1%, (c) 2%, and

(d) 5% shellac-treated jute yarns [74].

200 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

PLLA. In addition, the silane-coupling treatment to the

kenaf fiber was found to be more effective for the rein-

forcement.

Tajvidi et al. [81] investigated the effect of MA modifi-

cation on viscoelastic properties of kenaf fiber-reinforced

PP composites. An increase in storage and loss moduli and

a decrease in the mechanical loss factor were observed for

all treated composites, indicating more elastic behavior of

the composites when compared with the pure PP.

Aziz et al. [82] investigated the effect of modified poly-

ester resins in alkali-treated kenaf fiber composites. Four

types of polyester resin were used in this study. The first

one, type A, with the commercial product name Crystic 2-

406PA, was a conventional unsaturated polyester resin in

styrene monomer. Three more unsaturated polyester resins

were specifically formulated. The second resin type B had

a polymer structure modified to make it more polar in na-

ture, which implied that it was more hydrophilic and there-

fore it could interact or bond better to the OH groups on

the surface of the kenaf fibers. The third resin C contained

an additional monomer, 2,3-epoxypropyl methacrylate,

besides styrene. This monomer contained polar groups

designed to interact or even possibly to bond to the OH

groups on the surface of the natural fibers. The fourth resin

D also contained an additional monomer, 2-hydroxyethyl

methacrylate, which was also designed to interact better

with natural fibers. The resin D was found to possess the

best mechanical properties compared with the other polyes-

ter types (A, B, and C). An increase in flexural strength of

62% was observed for Polyester B matrix composites when

compared with untreated composites.

Luffa Fiber Composites

Demir et al. [83] calculated the influence of chemical

modification on luffa fiber-reinforced PP composites. The

chemical agents used were (3-aminopropyl)-triethoxysilane

(AS), 3-(trimethoxysilyl)-1-propanethiol (MS), and MAPP.

The maximum improvement in the mechanical properties

was obtained for the MS-treated composites. The interfa-

cial interactions improved the fiber compatibility, mechani-

cal properties, and water resistance of composites. The

authors also used atomic force microscope to show that the

surface roughness of fiber decreased with the employment

of silane-coupling agents as shown in Fig. 18. Untreated

and NaOH-treated fibers exhibited a roughness value of

138 and 116 nm, respectively, whereas the AS- and the

MS-treated fibers exhibited a surface roughness of 88 and

85 nm, respectively. This was considered a proof for the

surface coverage of the fibers with a siloxane layer result-

ing in decreased surface roughness.

Boynard et al. [84] studied the flexural properties of

sponge gourd (luffa) fiber-reinforced polyester composites.

Composites were prepared from luffa fibers treated with

NaOH of different concentrations, namely, 5, 10, 20, 40,

and 60% at room temperature. Composites were also pre-

pared with fibers treated with 1 and 5% NaOH at 1008C.

The authors observed that mercerization treatment pro-

duced strong morphological changes on the surface of luffa

fibers. The outer surface of the fibers was completely

FIG. 16. SEM micrograph of (a) an untreated kenaf fiber and (b) 3%

NaOH-treated kenaf fiber [79].

FIG. 17. Schematic model for the measurement of the strain on kenaf

fiber incorporated into PLLA resin by X-ray diffraction [80]. [Color

figure can be viewed in the online issue, which is available at www.

interscience.wiley.com.]

DOI 10.1002/pc POLYMER COMPOSITES—-2008 201

removed exposing the inner fibrillar surface. Composites

prepared from luffa fibers treated with 5% NaOH exhibited

the best flexural properties. Flexural modulus increased by

14% when compared with composites containing untreated

fibers. This increase was attributed to the mechanical inter-

locking due to increase in fiber roughness and contact area

of fibers.

In another interesting study, Tanobe et al. [85] charac-

terized luffa fibers which were chemically modified with

methacrylamide and NaOH. Methacrylamide treatment

severely damaged the fibers, whereas alkali treatment

showed a beneficial effect regarding enhancement of sur-

face area and thermal stability.

Oil Palm Fiber Composites

Sreekala and Thomas [86] investigated the influence of

a number of chemical modifications on oil palm fiber. The

chemical treatments employed were mercerization, latex

coating, g-radiation, acetylation, and peroxide. The afore-

mentioned modifications were found to decrease the me-

chanical properties of fibers but increase the water resist-

ance of fibers. The silane-treated fiber exhibited the highest

tensile strength (273 MPa) among all the treated fibers.

The adhesion of short oil palm fiber with natural rubber

was investigated [87] using various bonding agents like

phenol-formaldehyde, resorcinolformaldehyde–silica, hex-

amethylenetetramine-resorcinolformaldehyde-silica. It was

observed that different bonding agents gave different cure

characteristics and mechanical properties. Better mechani-

cal properties were obtained for the bonding system

RF:Sil:Hex (5:2:5) (see Table 4).

Joseph et al. [88] investigated the influence of alkali

treatment on oil palm fiber-reinforced natural rubber com-

posites. Three different concentrations of alkali were used,

namely 5, 10, and 15%. Composites containing fibers

treated with 5% NaOH exhibited a tensile strength of 9.95

MPa, whereas the composites containing fibers treated with

FIG. 18. Atomic force microscope (AFM) topographic pictures of (a) untreated; (b) NaOH-treated; (c)

NaOH þ CAS silane-treated; and (d) NaOH þ CMS-treated LFs [83].

202 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

10 and 15% NaOH exhibited 9.61 and 8.86 MPa, respec-

tively. This was attributed to degradation of oil palm fiber

when using high concentrations of alkali.

The effect of isocyanate treatment on the properties of

oil palm fiber-reinforced polyurethane composites was

investigated by Rozman et al. [89]. The fibers were treated

with two types of isocyanate: hexamethylene diisocyanate

and TDI. The composites with isocyanate-treated fibers

showed superior tensile and flexural properties than those

without treatment. Tensile modulus of the composites

made from treated fibers increased by three times in com-

parison to composites made from untreated fibers. These

were believed to be attributed to the additional reaction

sites in the form of urethane functional groups produced as

a result of ��NCO reactions with ��OH of oil palm fibers.

In another interesting study, Rozman et al. [90] chemi-

cally modified oil palm empty fruit bunches with MA and

investigated the effect on tensile and dimensional stability

properties of PP composites. Oil palm empty fruit bunch

fiber was ground into small particles and was treated with

MA dissolved in dimethyl formamide. They observed that

composites treated with MA displayed higher tensile and

better dimensional stability properties. It is known that a

lignocellulosic material absorbs water by forming hydro-

gen bonds between water and hydroxyl groups of cellulose,

hemicellulose, and lignin in the cell wall. The authors

found that the composites prepared with treated fiber

exhibited lower water absorption than those with untreated

fibers. This was attributed to the replacement of hydro-

philic OH groups from the empty fruit bunches by more

hydrophobic ester groups. Another reason was due to the

formation of a protective layer on the interfacial zone,

which prevented water molecules from penetrating into the

cell wall.

In an innovative study, a unique combination of sisal

and oil palm fibers in natural rubber was utilized to design

hybrid biocomposites. Chemical modification of both sisal

and oil palm fibers increased the interfacial adhesion and

resulted in enhanced mechanical properties [91]. The influ-

ence of alkali treatment and different silane coupling

agents on the viscoelastic [92, 93], biodegradation [94],

water sorption [95], dielectric [96], and stress relaxation

[97] characteristics were also studied. The authors

observed that chemically modified composites exhibited

lower damping and water uptake properties. They were

also more resistant to degradation and exhibited lower con-

ductivity. This was attributed to the presence of a stronger

interface due to greater adhesion between fibers and

matrix.

Pineapple Fiber Composites

Green composites were fabricated using pineapple leaf

fibers and soy-based resin [98]. The addition of polyester

amide grafted glycidyl methacrylate (PEA-g-GMA) as

compatibilizer was seen to increase the mechanical proper-

ties of composites. Tensile and flexural strengths of com-

posites containing treated fibers were found to be 35 and

50 MPa, respectively, whereas those of composites con-

taining untreated fibers were found to be 30 and 45 MPa,

respectively.

Lopattananon et al. [99] investigated the chemical modi-

fication of pineapple leaf fibers in natural rubber. NaOH

solutions (1, 3, 5, and 7%, w/v) and benzoyl peroxide

(BPO) (1, 3, and 5 wt% of fiber) were used to treat the sur-

face of fibers. It was found that all surface modifications

enhanced adhesion and tensile properties. The treatments

with 5% NaOH and 1% BPO provided the best improve-

ment of strength (28 and 57%, respectively) of composites

when compared with that of untreated fiber composites.

Ramie Fiber Composites

The effect of stearic acid on tensile and thermal proper-

ties of ramie fiber-reinforced soy protein isolate (SPI) resin

green composites was investigated by Lodha and Netravali

[100]. It was observed that a part of the stearic acid crystal-

lized in SPI resin and the ability to crystallization was

affected by the addition of glycerol as a plasticizer. The

fabricated green composites were found to have enormous

potential for certain indoor applications. The interfacial ad-

hesion in the composites was apparent from SEM photomi-

crographs of the fractured tensile surfaces of the ramie/SPI

and ramie/modified soy protein isolate (MSPI) composites

tested in the axial direction as shown in Fig. 19a and b,

respectively. It can be seen that some of the fibers at the

fractured surface do not show any resin, whereas a few

other fibers clearly show resin sticking to the surface. The

presence of resin on some fiber surfaces for the SPI and

MSPI resins clearly indicates good interfacial interaction

between the fiber and the resins.

Goda et al. [101] investigated the influence of mercer-

ization on the properties of ramie fibers. The authors

observed that the tensile strength of the treated ramie fibers

was improved by 4–18% while Young’s modulus of the

treated fibers decreased by 65%. The fracture strains of the

treated ramie fibers were found to drastically increase to

three times higher than those of the untreated ramie fibers.

It was considered that such improvement upon merceriza-

TABLE 4. Cure characteristics of oil palm natural rubber composites

with different bonding systems [87].

Cure characteristics

B

(Gum)

H

(U)

I

(T)

J

(U)

K

(T)

L

(U)

M

(T)

Max. torque (Nm) 10 10.4 12.8 12.2 13.4 13.6 14.8

Min. torque (Nm) 0.4 0.6 0.7 0.6 0.7 0.8 0.9

Cure time t90 (min) 17.5 30.6 30.7 26.9 23.9 21.0 21.4

Scorch time t2 (min) 6.1 5 4.4 10.1 9.3 5.5 5.3

B, gum sample; H, phenol-formaldehyde untreated; I, phenol-formal-

dehyde treated; J, resorcinol-formaldehyde-silica (5:2) untreated; K, res-

orcinol-formaldehyde-silica (5:2) treated; L, resorcinol-formaldehyde-

silica-hexa (5:2.5) untreated; M, resorcinol-formaldehyde-silica-hexa

(5:2.5) treated.

DOI 10.1002/pc POLYMER COMPOSITES—-2008 203

tion was related to the changes in morphological and chem-

ical structures in microfibrils of the fibers.

Sisal Fiber Composites

Alvarez and Vazquez [102] investigated the effect of

acetylation and alkali-treatment on the properties of

MaterBi-Y/sisal fiber composites. It was demonstrated that

both treatments changed the morphology of the fibers by

removing cellulosic and cementing materials, creating

voids and producing fibrillation of fibers. These effects

lead to a better adhesion between fibers and matrix. The

authors suggested that the alkali treatment was more effec-

tive than acetylation.

Mwaikambo and Ansell [103] carried out the alkali

treatment of sisal fibers and analyzed the changes with

respect to the diameter and internal structure, such as cellu-

lose content, crystallinity index, and micro-fibril angle.

Alkalization was found to change the internal structure of

sisal fibers which exhibited approximately same specific

stiffness as that of steel. The crystalline nature of the

treated fibers was also found to increase due to alkali treat-

ment. Their results indicated that the structure of sisal fiber

can be chemically modified to attain properties that will

make the fiber useful as a replacement for synthetic fibers.

Martins et al. [104] studied the effect of chemical modi-

fications on the structure and morphology of sisal fibers

by NMR spectroscopy. Mercerization, acetylation, and

resorcinol/hexamethylenetetramine (R/H) treatments were

applied to sisal fibers in order to improve their adhesion in

composites materials. The results showed that the fibers/

acetylation of fibers prior to the treatment with 20/10 g/L

R/H solution for 15 min was the best treatment since the

fibers obtained exhibited the biggest effective surface area

available for chemical interaction and mechanical inter-

locking with the matrix.

Studies dealing with woven sisal fabric-reinforced natu-

ral rubber composites [105] and its moisture uptake charac-

teristics [106] were also reported. Chemical modification

of sisal fabric resulted in lowering of tensile properties.

This was attributed to the fact that the major contribu-

tion to strength in textile composites is the alignment of

yarns in warp and weft direction. Chemical treatment

results in the partial unwinding of yarns (as hemicellulose

dissolves off) and hence the alignment gets antagonized.

This results in lowering of strength of composites. Another

reason is that as sisal fabric is composed of thick strands

and knots, the alkali and silane coupling agents did not

penetrate into the fabric and therefore the interfacial prop-

erties between the sisal fabric and rubber matrix has not

been improved enough.

It can be seen from the above studies that chemical

modification of natural fibers has been exhaustively stud-

ied. Different kinds of natural fibers and different chemi-

cal modification techniques were employed to study the

variation in properties. But most of the studies have not

probed on certain aspects of chemical modification. The

properties of natural fibers have been known to be

affected by varying the concentration and time of

immersion of natural fibers with the coupling agents.

Temperature also has a significant effect on properties of

natural fibers. Most of the studies have been shown to

focus on one parameter only, mainly concentration of

the coupling agent. The variation of processing parame-

ters on the properties of composites is an important req-

uisite in determining the efficiency of the chemical mod-

ification employed.

In the case of thermoplastic composites, it has been

found that though the presence of compatibilizing agents

like (MA-PP) has increased tensile properties, the impact

performance register a decrease. Studies need to be con-

centrated on developing functionalized compatibilizers that

go beyond bridging fiber and matrix. The compatibilizer

must take part in stress and also play a role in absorbing

energy at the interface.

Another point, which is lacking in the above studies, is

that the role of the interface in providing superior mechani-

cal properties has not been explained. Studies must explore

the actual chemistry taking place and follow it up with in-

situ interfacial characterization.

FIG. 19. SEM photomicrographs of the fracture tensile surfaces of untreated and treated composites [100].

204 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

CONCLUSIONS

Chemical modification of natural fibers is necessary for

increased adhesion between the hydrophilic fibers and

hydrophobic matrix. The most promising approach seems

to be one in which covalent bonds are formed between the

fiber and matrix. As apparent from the above studies, one

of the most common and efficient methods of chemical

modification is alkali treatment of fibers and it has been

used to treat almost all the natural fibers with successful

results. Maleated coupling agents are also widely used,

especially with PP matrices. Interactions between the anhy-

dride groups of maleated coupling agents and the hydroxyl

groups of natural fibers can overcome incompatibility prob-

lem between fiber and matrix. Another popular modifica-

tion is the use of silane coupling agents, but reports have

suggested that the coupling is feasible only at high temper-

atures and in the presence of moisture.

Future areas of interest should be focused on developing

coupling agents from natural products. The use of lignin as

a coupling agent in hemp fiber composites has yielded

encouraging results. Shellac has been successfully used for

treating jute fibers. Chitin and chitosan have also been used

as efficient coupling agents for woodflour composites

[107]. Biological coupling agents like fungus have also

been found to increase fiber/matrix adhesion. Zein—pro-

tein from corn—also shows enormous potential to be used

as a coupling agent.

A lot more research has to be conducted to develop

novel methods of modification and investigate the exact

mechanism of reactions occurring at the fiber–matrix inter-

face.

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