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CompositesJournal of Reinforced Plastics and

DOI: 10.1177/07316844070877592008;

2009; 28; 1169 originally published online Nov 20,Journal of Reinforced Plastics and CompositesRamakrishna Malkapuram, Vivek Kumar and Yuvraj Singh Negi

Recent Development in Natural Fiber Reinforced Polypropylene Composites

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Recent Development in Natural Fiber

Reinforced Polypropylene Composites

RAMAKRISHNAMALKAPURAM, VIVEK KUMAR AND YUVRAJ SINGHNEGI*

Polymer Science and Technology Program, Department of Paper Technology

Indian Institute of Technology Roorkee, Saharanpur Campus

Saharanpur, UP, India

ABSTRACT: This review article describes the recent developments of natural fiber reinforcedpolypropylene (PP) composites. Natural fibers are low-cost, recyclable, and eco-friendly materials.

Due to eco-friendly and bio-degradability characteristics of these natural fibers, they are consideredas strong candidates to replace the conventional glass and carbon fibers. The chemical, mechanical,and physical properties of natural fibers have distinct properties; depending upon the cellulosiccontent of the fibers which varies from fiber to fiber. The mechanical properties of composites areinfluenced mainly by the adhesion between matrix and fibers. Chemical and physical modificationmethods were incorporated to improve the fibermatrix adhesion resulting in the enhancement ofmechanical properties of the composites. The most important natural fibers are jute, flax, and coirand their novel processing technics to develop natural fiber reinforced composites are also described.

KEY WORDS: natural fibers, polypropylene, chemical and physical modifications, processing,properties.

INTRODUCTION

IN THE RECENT past considerable research and development have been expanded in

natural fibers as a reinforcement in thermoplastic resin matrix. These reinforced plastics

serve as an inexpensive, biodegradable, renewable, and nontoxic alternative to glass or

carbon fibers. The various advantages of natural fibers over man-made glass and carbon

fibers are low cost, low density, competitive specific mechanical properties, reduced energy

consumption and biodegradability. Thermoplastic materials that currently dominate as

matrices for natural fibers are polypropylene(PP), polyethylene, and poly(vinyl chloride)

while thermosets, such as phenolics and polyesters, are common matrices. With a view to

replacing the wooden fittings, fixtures and furniture, organic matrix resin reinforced with

natural fibers such as jute, kenaf, sisal, coir, straw, hemp, banana, pineapple, rice husk,

bamboo, etc., have been explored in the past two decades.

There is an increasing demand from automotive companies for materials with sound

abatement capability as well as reduced weight for fuel efficiency. Natural fibers possess

excellent sound absorbing efficiency and are more shatter resistant and have better energy

management characteristics than glass fiber reinforced composites. In automotive

parts, such composites not only reduce the mass of the component but also lower the

*Author to whom correspondence should be addressed. E-mail: [email protected]

Journal ofREINFORCED PLASTICS AND COMPOSITES, Vol. 28, No. 10/2009 1169

0731-6844/09/10 116921 $10.00/0 DOI: 10.1177/0731684407087759 SAGE Publications 2009

Los Angeles, London, New Delhi and Singapore

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energy needed for production by 80%. Eco-friendly composites can be made by replacing

glass fibers with various types of ligno-cellulose fibers. However, such composites have a

distinct disadvantage of load-bearing capability compared to glass fiber reinforced

thermoplastics. The variation in the properties of natural fibers is another important

aspect that has to be considered. Demands for natural fibers in plastic composites is forecast

to grow at 1520% annually with a growth rate of 1520% in automotive applications, and

50% or more in selected building applications. Other emerging markets are industrial and

consumer applications such as tiles, flower pots, furniture, and marine piers [1].

Development of new composite products from the easily renewable natural materials

has a strong potential to deliver novel biodegradable and/or readily recyclable materials

suitable for the automotive and packaging industry, thereby replacing not so easily

renewable fossil fuel-based polymers/plastics.

Complete matrix fusion to facilitate thorough fiber impregnation, formation of strong

fiber/matrix interfacial bonding, and matrix-to-fiber stress transfer efficiency are vital

requirements for the manufacture of reliable, eco-friendly natural composites that can

possess better mechanical properties and withstand environmental attacks. Bledzki andGassan [2] have stated that the quality of the fibermatrix interface is significant for the

application of natural fibers as reinforcement fibers for plastics. Physical and chemical

methods can be used to optimize this interface. These modification methods are of

different efficiency for the adhesion between matrix and fiber.

However, the main problem of natural fiber/polymer composites is the incompatibility

between the hydrophilic natural fibers and the hydrophobic thermoplastic matrices. It

necessitates the use of compatibilizers or coupling agents in order to improve the adhesion

between fiber and matrix [3].

Matrix or fiber modification is therefore necessary to improve the compatibility between

fiber and matrix. Maleated polyolefins are used to modify the matrix. Such modification ofthe matrix develops the interactions between the anhydride groups of maleated coupling

agents and the hydroxyl groups of natural fibers. For the purpose of making engineering

parts with a wide study on the effect of different coupling agents, such as silanes, maleic

anhydride grafted polypropylene (PPgMA) or modification by acetylation, has been

reported in the literature.

In the present article an attempt has been made to review the state of the art of

organocellulosic fiber reinforced polyolefin composites. The various types of natural fibers

used for such reinforcement are described. The modifications of fiber and matrix are

discussed and the mechanical properties of composites are described also. Performance of

the composites is summarized.

LIGNOCELLULOSIC FIBERS OR MATERIALS

Several types of lignocellulosic materials have been investigated in PP [412].

Cellulosic fibers from other sources have been reported in polyolefin composites by

several authors. A comprehension review of the cellulosic-based materials in composites

has appeared recently [2]. The lignocellulosic fibers that have been used in polyolefins

include cellulose fiber [412], wood fiber [1322], flax [2329], Cannatis sativa (hemp)

[3035], jute [1,25,33,3537], pinewood fiber [3841], sisal [3,33,4244], rice husk [4548],

saw dust [4850], lugffa sponge fiber [3,51], wheat straw [52], paper sludge [53], coconut

fiber [33,54], kenaf [55,56], kapok/cotton [57], pineapple leaf [58], basalt [59], vetiver [60],bio flour [61], bamboo fiber [62], and date palm fiber [63].

1170 R. MALKAPURAM ET AL.



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CHEMICAL COMPOSITION OF SOME NATURAL FIBERS

In Table 1 cotton has the maximum content of cellulose ranging from 85 to 95%. The

other fibers have cellulose along with lignin and other components such as hemicellulose,

pectin wax, etc., in varying quantities. The various chemical constituents of a specific

natural fiber also vary considerably.

MECHANICAL PROPERTIES OF LIGNOCELLULOSIC MATERIALS

The structure, chemical composition, microfibrillar angle, and cell dimension defects are

the most important variables that affect the overall properties of the fibers. A fiber is more

ductile if the microfibrils have a spiral orientation to the fiber axis. If the microfibrils are

oriented parallel to fiber axis then the fibers are inflexible and rigid. The natural fibers

exhibit considerable variation in diameter along with the length of individual filaments.

In Table 2 the properties of these fibers are summarized.

Table 1. Chemical composition of some natural fibers.

Fiber

Cellulose

(wt%)

Lignin

(wt%)

Hemicellulose

(wt%)

Pectin

(wt%)

Wax

(wt%)

Moisture

content (wt%)

Jute 6171.5 1213 13.620.4 0.4 0.5 12.6

Hemp 70.274.4 3.75.7 17.922.4 0.9 0.8 10

Kenaf 3139 1519 21.5

Flax 71 2.2 18.620.6 2.3 1.7 10

Ramie 68.676.2 0.60.7 13.116.7 1.9 0.3 8

Sunn 67.8 3.5 16.6 0.3 0.4 10

Sisal 6778 811 10.014.2 10 2.0 11Henquen 77.6 13.1 48

Cotton 82.7 5.7 0.6

Kapok 64 13 23 23

Coir 3643 4145 1020 34 8

Banana 6367.6 5 19 8.7

PALF 7082 512 11.8

Table 2. Mechanical properties of natural fibers.

Fiber

Density

(g/cm3)

Diameter

(km)

Tensile

strength (MPa)

Youngs

modulus (GPa)

Elongation

at break (%)

Jute 1.31.45 25200 393773 1326.5 1.161.5

Hemp 690 1.6

Kenaf 2.7

Flax 1.5 3451100 27.6 2.73.2

Ramie 1. 400938 61.4128 1.23.8

Sunn 1.171.9 5.5

Sisal 1.45 50200 468640 9.422.0 37

Cotton 1.51.6 287800 5.512.6 78

Kapok 1.2

Coir 1.15 100450 131175 46 1540

Banana 1.77.9 1.59.0

PALF 2080 4131627 34.582.5 1.6

Natural Fiber Reinforced Polypropylene Composites 1171



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The properties are affected by the chemical constitutents and complex chemical

structure of natural fibers. The angle between the axis and the fibril of the fiber

(microfibrillar or spiral angle) also affect the strength of the fibers. Mechanical

properties are higher if this is smaller. Least tensile strength is shown by coir fibers

which may be attributed to low cellulose content and considerably high microfibrillar

(41458 for coir, 208 for sisal, 148 for PALF, and 108 for flax while other fibers such as

jute, ramie, and hemp, have values 510). The lignin content of the fibers influence its

structure properties and morphology. The waxy substances of the natural fibers affect

the fiber wettability and adhesion characteristics. In specific strength the natural fibers

can be compared with glass fibers.

SURFACE MODIFICATIONS OF LIGNOCELLULOSIC FIBERS

Physical Methods

Reinforcing fibers can be modified by physical and chemical methods. Physical

methods, such as stretching, calandering, thermotreatment, and the production of hybrid

yarns do not change the chemical composition of the fibers. Physical treatments change

structural and surface properties of the fiber and thereby influence the mechanical

bondings to polymers.

Chemical Methods

Strongly polarized cellulose fibers are inherently incompatible with hydrophobic

polymers when two materials are incompatible; it is often possible to bring about

compatibility by introducing a third material that has properties intermediate between

those of the other two. There are several mechanisms of coupling in materials:

. weak boundary layers coupling agents eliminate weak boundary layers,

. deformable layers coupling agents produce a tough, flexible layer,

. restrained layers coupling agents develop a highly crosslinked interphase region, with

a modulus intermediate between that of substrate and of the polymer,

. wettability coupling agents improve the wetting between polymer and substrate

(critical surface tension factor),

. chemical bonding coupling agents form covalent bonds with both materials, and

. acidbase effect coupling agents alter acidity of substrate surface.The development of a definitive theory for the mechanism of bonding by coupling

agents in composites is a complex problem. The main chemical bonding theory alone is not

sufficient. So the consideration of other concepts appears to be necessary, which include

the morphology of the interphase, the acidbase reactions at the interface, surface energy,

and the wetting phenomena.

Change of Surface Tension

The surface energy of fibers is closely related to the hydrophilicity of the fiber. Some

investigations are concerned with methods to decrease hydrophilicity. The modification ofwoodcellulose fibers with stearic acid hydrophobizes those fibers and improves their




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dispersion in PP. A treatment with poly(vinyl acetate) increases the mechanical properties

and moisture repellence.

Impregnation of Fibers

A better combination of fiber and polymer is achieved by impregnation of the

reinforcing fabrics with polymer matrices compatible to the polymer. For this purpose

polymer solutions or dispersions of low viscosity are used.

Graft Copolymerization of Matrix

An effective method of chemical modification of matrix and natural fibers is graft

copolymerization. This reaction is initiated by free radicals of the cellulose molecule. The

cellulose is treated with an aqueous solution with selected ions and is exposed to a high

energy radiation. Then the cellulose molecule cracks and radicals are formed. Afterward

the radical sites of the cellulose are treated with a suitable solution (compatible with thepolymer matrix), for example vinyl monomer (i.e., acrylonitrile, methyl methacrylate,

polystyrene, etc.). The resulting co-polymer possesses properties characteristic of both

fibrous cellulose and grafted polymer.

For example, the treatment of cellulose fibers with hot polypropylenemaleic anhydride

(MAHPP) copolymers [3] provides covalent bonds across the interface. The mechanism

of reaction can be divided into two steps: activation of the copolymer by heating

(t 1708C) (before fiber treatment), and esterification of cellulose.

After this treatment the surface energy of the fibers is increased to a level much closer to

the surface energy of the matrix. Thus, a better wettability and a higher interfacial

O

O

O

O

O

O

OH2

C C

CH

C COH

OH

O

C C

C C CH3

CH3

C

O

OH

O

C

C

HO

H

H

C

C C

C

C C C+

CH3

H2O

H

H2

PP

chain

PP

chain

Cellulosefibe

r

Cellulosefiber

Cellulosefiber

2 HC

2

HO

OH

OH2C

C C CH3H

C

C

O

OOH

+

Figure 1. Mechanism of interaction of cellulose fibers with maleated PP.




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adhesion is obtained. The PP chain permits segmental crystallization and cohesive

coupling between the modified fiber and the PP matrix.

Modification of Lignocellulosic Fibers with Isocyanates

The reactivity of cellulosic hydroxyl groups with a variety of reagents can resolve the

fibers hydrophobicity such as treatment reduces:

. the number of cellulose hydroxyl groups, which are available for moisture pick-up,

. the hydrophilicity of the fibers surface, and

. the swelling of the fiber, by creating a crosslinked network due to covalent bonding,

between matrix and fiber.

The mechanical properties of composites reinforced with wood fibers and PVC or PS as

resin can be improved by an isocyanate treatment of cellulose fibers [52,61].

Polymethylenepolyphenylisocyanate (PMPPIC) in pure state or solution in plasticizercan be used. Isocyanates are chemically linked to the cellulose matrix through strong

covalent bonds: both PMPPIC and PS contain benzene rings, and their delocalized

electrons provide strong interactions, so that there is an adhesion between PMPPIC and

PS.

The isocyanatic treatment is reported to be more effective than the treatment with

silane. Similar results are obtained, when PMPPIC is used for the modification of the

fibers or polymer matrix.

Triazine Coupling Agents

Triazine derivatives form covalent bonds with cellulose fibers. The triazine derivativesare used to reduce the moisture absorption of cellulose fibers and their composites.

RN=C=O+HOCell RHN C OCell

O

Figure 2. Formation of urethane linkage by reaction of isocyanate and hydroxyl groups of cellulose.

C=O

CH2

CH2

CH2 CH2CH CH CH

CH2 CH2

NH

O O

C=O C=O

NH N H

O

Cellulose+Lignin

Polystyrene

Figure 3. Interaction between PS Matrix and PMPPIC modified cellulose fibers.




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Organosilanes as Coupling Agents

Organosilanes are the main group of coupling agents for glass-fiber reinforced polymers.

They have been developed to couple virtually any polymer to the minerals, which are used

in reinforced composites.

PROCESSING AND PROPERTIES OF NATURAL

FIBER REINFORCED PP COMPOSITES

Khondker et al. [1] have reported that the jute/PP unidirectional composites, specimens

with only 20% of jute fiber (Vf) content, show remarkable improvement in tensile

and bending properties when compared to those of the virgin PP specimens. The

improvements in the mechanical properties are broadly related to various factors, such as

the wettability of resin melts into fiber bundles, interfacial adhesion, orientation, and

uniform distribution of matrix-fibers and the lack of fiber attrition and attenuation during

tubular braiding process.

The water absorption properties of coir, sisal fibers reinforced PP composites in water

at three different temperatures, 23, 50, and 708C, were studied. A decrease in tensile

properties of the composites was demonstrated, showing a great loss in mechanical

properties of the water-saturated samples compared to the dry samples [3].

The hardness and elastic modulus of a cellulose fiber-reinforced PP composite were

investigated by nano-indentation with a continuous stiffness technic. A line of indents was

produced from the fiber to the matrix. There was a gradient of hardness and modulus

across the interphase region. The distinct properties of the transition zone were revealed by

14 indents, depending on nano-indentation depth and spacing [4].

The high-tenacity man-made cellulose filament yarn (rayon tyre cord yarn) reinforcedPP, polyethylene, high impact polystyrene (HIPS), and poly(lactic acid) (PLA) composites.

CI

N

R

R

Cellulose fiber

Cellulose fiber

HN

HN

N N

CI CI

H

O

N

N

N

N CIO

N

CI N CI

H2NR

Figure 4. Modification of cellulose fibers with triazine derivatives.




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The pultrusion compounding method developed for highly homogeneous composites. For

a fiber load of 30 wt%, typical values for tensile strength, modulus, Charpy unnotched,

and notched impact strength are 80 MPa, 3.5 GPa, 85 kJ/m2, and 12 kJ/m2, respectively.

A high impact resistance level is maintained also at low temperatures where the matrix

material becomes brittle. For the other matrix materials, similar reinforcing effects are

observed, except for the impact behavior of HIPS, where the reinforcing fibers interfere

with the impact modification of the matrix polymer. In contrast, the impact characteristics

of PLA are drastically improved increasing the unnotched and notched Charpy strengths

by 380% and 200%, respectively [5].

Amash and Zugenmaier [6] investigated the thermal, morphological and dynamic

mechanical properties of PPcellulose fiber (CF) composites. The effects of drawing on the

structure and physical properties of PPCF composites were also studied. By increasing

draw ratio, the melting peak of PP shifted to higher temperatures suggesting a constrainedmelting, and the uniaxial elastic modulus was considerably enhanced. The biggest

Table 4. Short fiber reinforced PP composites.

Fiber Processing technic

Reference

number

Cellulose fiber Twin-screw extruder and hydraulic press 6

Flax fiber Haake rheomix and injection molding 26

Flax fiber Twin-screw extruder and injection molding 27

Chopped jute fiber Shear K-mixer and injection molding 37

Pinewood fiber Laboratory sigma blade mixer and injection molding 41

Sisal fiber Melt mixing (Haake rheocord mixer), solution mixing

(toluene) followed by extrusion and compression molding

43

Chopped sisal fiber Haake rheocord mixer 44

Kenaf fiber Compression molding through a sheet stacking technic 55

Pineapple leaf fibre Hydraulic press using film stacking technic 58

Vetiver grass Internal mixer and injection molding machine 60

Bamboo fiber Haake rheocord and injection molding 62

Date palm leaves Single-screw extruder and injection molding 63

Table 3. Continuous fiber reinforced PP composites.


Reference

number

Jute yarn Tubular braiding technic, compression molding 1

Non wovens of flax and sisal fiber EXPRESS processing (Extrusion press processing) 2Uni-directional lyocell fiber Compression molding 4

Hightenacity cellulosefilament yarn

(rayon tyre cord yarn)

Pultrusion technic applied to conventional

co-rotating twin-screw extruder and composite

specimens prepared by injection molding

5

Unidirectional and multidirectional

flax fiber

Heated press in a mold 23

Flax fiber yarn Pultrusion and compression molding 28

Flax fiber yarn Vacuum heating and compression molding 29

Jute fiber yarn Twin-screw extruder and injection molding 36

Sisal fiber yarn Single-screw extruder and injection molding 42

Kapok/cotton fabric Hydraulic press 57




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Table 5. Particulate fiber reinforced PP composites.


Reference

number

Sisal, coir, luff sponge and

cellulose from pulp fiber

Brabender mixer and processed by

compression molding

3

Cellulose whiskers Solvent mixing followed by hot press 7

Rice husk and wood flour fiber Twin-screw extruder and injection molding 8

Alfa, bleached soft wood,

Avicel fibers

Brabender mixer and compression molding 9

Cellulose fibers Melt mixing (plastomill) and compression molding 10

Eucalyptus wood, ground

corncob and spent brewery

grain fibers

Brabender and hydraulic press 11

Cellulose fibers Droplet method 12

Eucalyptus wood fibers Hot press 12

Lignocel fiber High-speed mixer and injection molding 14

Pine free saw dust Single-screw extruder and compression molding 15

Kraft pulp of eucalyptus fiber Twin-screw extruder and injection molding 16

Hard wood dust Mixer and injection molding 17

Aspen pulp fiber Haake rheomix 600 equipped with roller blades

rotor and compression molding

18

Pinewood flour woodtruder and die 19

Liquified wood mill Haake rheomix, single-screw extruder

and injection molding

20

Wood fiber Twin-screw extruder and compression molding 21

Wood fiber Single-screw extruder and compression molding 22

Flax (pulp) fiber Twin-screw extruder and injection molding 24

Chopped hemp fiber Powder impregnation through compression

molding and extrusion followed by injection molding

30

Hemp strands Heated roll mixer and injection molding 31Hemp fiber Brabender internal mixer and compression molding 32

Sisal, kenaf, hemp,

jute and coir

Compression molding using a film stacking method 33

Hemp fiber Twin-screw extruder and injection molding 34

Jute and hemp fiber Single fiber embedded inside two previously

built sheets of PP matrix by

compressing at melting point

35

Bleached soft wood

pine fibers

Two roll mill and injection molding 38

Pinewood saw dust Granules prepared by single-screw extruder 39

Wood material of radiate pine

(Pinus radiate)

Mixer and hot press 40

Rice husk flour filler Single-screw extruder and injection molding 45

Saw dust/Rice husk Kinetic mixer and injection molding 46

Rice husk Twin-screw extruder and injection molding 47

Rice husk Single-screw extruder and injection molding 48

Saw dust Mixer and compression molding 49

Saw dust Haake rheo mixer and compression molding 50

Luffa fiber Haake rheocord mixer and compression molding 51

Wheat straw fibers Brabender mixer and injection molding 52

Paper sludge Single-screw etruder and compression molding 53

Coconut fiber Haake rheocord mixer and compression molding 54

Layer of kenaf Plied up alternatively and pressed in a heated press 56

Rice husk Pellets processed by twin-screw extruder 61




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influence was observed for the samples of PPspun cellulose and the lowest for neat PP.

In addition to the fibrillar structure of the oriented PP, the highly CF orientation and

the efficient compatibilization in composites are responsible for the effects observed in

the drawn samples.

The nanocomposite films of isotactic PP reinforced with cellulose whiskers highly

dispersed with surfactant were prepared for the first time and compared with either bare or

grafted aggregated whiskers. The nanocomposites obtained with the surfactant-modified

whiskers exhibited enhanced ultimate properties when compared to the neat matrix or to

the composites containing the other filler types [7].

The tensile and Izod impact strength properties of rice-husk flour and wood flour as the

reinforcing filler and different compatibilizing agents, by assessing their mechanical

properties and the morphological characteristics of their fracture surfaces. The tensile

properties of the composites made with the twin-screw extruding system were better than

those of the composites made with the single-screw extruding system, due to the improved

dispersion of the filler. The tensile strength and modulus of the lignocellulosic fillerPP

composite made with the twin-screw extruding system were improved in the case of thecomposite made without any compatibilizing agent and significantly improved in the case

of the composite made with the compatibilizing agent, as compared with those made with

the single-screw extruding system. The Izod impact strengths of the composites made

with the two different extruding systems were almost the same, the degree of dispersion

of the fillers might influence the notched impact performance, but the similar impact

strength of both samples with different extruding processes might be due to the fact that

impact test is not discriminating enough to reveal the difference in dispersion status of the

present composites [8].

The mechanical properties of the CFs reinforced polyethylene composites increased

with increasing the average fiber length and the composite materials prepared usingboth matrices and cellulose fibers treated with g-methacryloxypropyltrimethoxy silane

(MPS), and g-mercaptoproyltrimethoxy silanes (MRPS) displayed good mechanical

performances. On the other hand, with hexadecyltrimethoxy-silanes (HDS) bearing

merely aliphatic chain only a modest enhancement on composite properties was

observed [9].

Qiu et al. [10] have reported that PP with higher molecular weight revealed stronger

interfacial interaction with cellulose fibers in the composites, compared with the lower

molecular weight PP; the composites derived from higher molecular weight of PP exhibited

stronger tensile strength at the same cellulose content.

Georgopoulos et al. [11] have investigated that the loading of LDPE with natural fibers

leads to a decrease in tensile strength of the pure polymer. On the other hand, Youngs

modulus increased due to the higher stiffness of the fibers. Although the properties of

some blends are acceptable for some applications, further improvement is necessary, by

optimizing fiberpolymer interface characteristics.

Joly et al. [12] have optimized the fiber treatments for PP/cellulosic-fiber composites.

In one method the grafting of a PP chain by an ester bond capable of co-crystallizing or

entangling with the fibers was used. While the modification of cellulose fiber was done by

isocyanates for improvement in the rupture, initiation strength was observed by PP

modification of fibers resulting in the crack propagation strength.

The tensile strength of the PP-wood-based composites decreased significantly with

increasing wood fiber content and no significant change in modulus of elasticity was foundfor any weight fraction of wood fiber. Fiber pullout was observed on most of the




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PP composite fracture surfaces examined using SEM. These results indicate a lack of

adhesion between PP and wood fiber [13].

Tensile and flexural tests of foamed composites were investigated as a dependence

of function of density (specific properties) of foamed specimens and compared with

nonfoamed composites based on MAHPP has improved the physico-mechanical

properties up to 80%. Chemical foaming agents have also an effect on surface roughness

of the composites which decreased surface roughness of the foamed composites compared

to the nonfoamed composites. Water absorption and thickness swelling of the composites

were also investigated. The density of microfoamed hard wood fiberPP composites

reduced around 30% and decreased up to 0.741 g/cm3 by using an exothermic chemical

foaming agent. Optical microscopy showed that the cells are round and cell sizes are

affected by chemical foaming agents. Tensile and flexural tests are performed on the

foamed composites to investigate the dependence of these properties on the density

(specific properties) of foamed specimens and compared with nonfoamed composites and

MAHPP has improved the physico-mechanical properties up to 80% [14].

The impact fracture behavior of PP/wood fiber composites modified with maleatedPP as compatibilizer and poly(butadiene styrene) rubber as impact modifier has been

studied by a Charpy impact testing method. The neat PP and unmodified PP/wood

fiber composite exhibit brittle fracture and nearly elastic behavior while the impact

modifier and compatibilizer cause elastic to elasticplastic transition dominated by

unstable crack propagation. Both the impact modifier and the compatibilizer toughen the

PP matrix enhance the total fracture energy of the modified composites. While the crack

initiation energy is mostly only a little material dependent and reflects the matrix behavior,

the crack propagation energy is much more influenced by the morphology [15].

Novel compatibilizer (m-TMI-g-PP) with isocyanate functional group was synthesized

by grafting m-isopropenyl-a,a-dimethylbenzyl-isocyanate (m-TMI) onto isotactic PP in atwin-screw extruder. The effect of filler concentration on the mechanical properties

of bleached kraft pulp of eucalyptus reinforced PP composites, prepared by using

m-TMI-g-PP as the compatibilizer, was investigated. The addition of the compatibilizer

resulted in greater reinforcement of composites, as indicated by the improvement in

mechanical properties. Tensile strength of composites so prepared increased by almost

45%, whereas 85% increase in flexural properties was observed [16].

Costa et al. [18] have evaluated the tensile and flexural performance of PPwood fiber

composites. The effect of these variables on tensile strength, Youngs modulus, elongation

at yield and flexural strength was determined through a 2231 factorial design (where the

variables chosen were: fiber content (F), percentage of MAPP coating on the wood fiber

(C) and type of matrix (M)). The mechanical properties (responses) selected from tensile

data were stress at maximum load (TS), tensile modulus (TM) and elongation at yield (E).

The selected responses of flexural data were stress at maximum load (FS) and flexural

modulus (FM). Using standard least-squares parameter estimation procedures and

statistical analysis, empirical models were built and parameter values and co-variances

were computed. The analysis of variance of the experimental and predicted data shows

that the constructed models provide a fair approximation of actual experimental

measurements. Finally, experimental details regarding the preparation of optimum

composites as predicted by empirical models are discussed.

Jungil Son and his coworkers analyzed the effect of process additives, that is, maleated

PP (MAPP), and a nucleating agent on the viscoelastic properties of different typesof extruded PPwood plastic composites manufactured from either a PP homopolymer,




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a high crystallinity PP, or a PP impact copolymer using dynamic mechanical thermal

analysis. The woodplastic composites were manufactured using 60% pine wood flour and

40% PP on a Davis-Standard Woodtruder. Dynamic mechanical thermal properties,

polymer damping peaks (tan ), storage modulus (E0), and loss modulus (E00) were

measured using a dynamic mechanical thermal analyzer. To analyze the effect of the

frequency on the dynamic mechanical properties of the various composites, DMA tests

were performed over a temperature range of 201008C, at four different frequencies (1, 5,

10, and 25 Hz) and at a heating rate of 58C/min. From these results, the activation energy

of the various composites was measured using an Arrhenius relationship to investigate the

effect of MAPP and the nucleating agent on the measurement of the interphase between

the wood and the plastic of the extruded PP woodplastic composites [19].

Van de Valde and Kiekens [23] have analyzed the unidirectional (UD) and

multidirectional (MD) flax/PP composites at different process times and temperatures.

The UD composites of boiled flax combined with MAA-PP show the best mechanical

properties. Contrary to the UD composites, flax treatment did not lead to the expected

property improvements for MD composites.The composites were compounded with two kinds of flax fibers (natural flax and

flax pulp) and PP. The applied treatments were maleic anhydride (MA), maleic

anhydridepolypropylene copolymer (MAPP) and vinyl trimethoxy silane (VTMO).

The treatment effects on the fibers have been characterized by infrared spectroscopy. Two

technics have been used to determine the surface energy values: the dynamic

contact angle method for the long flax fibers and the capillary rise method for the

irregular pulps. The use of different methods involves a small discordance in

the wettability values. Nevertheless, the three treatments reduce the polar component

of the surface energy of the fiber. Composites containing MAPP showed highest

mechanical properties, whilst the MA and VTMO-treated fiber showed virtually noimprovement compared to untreated ones [24].

Keener et al. [25] used maleated coupling agents to strengthen the composites containing

fillers and fiber reinforcements. The established role of maleated polyolefins results from

two main factors, economical manufacturing and the efficient interaction of maleic

anhydride with the functional surface of fiber reinforcements. Peak performance was

demonstrated in agrofiber PP composites by selecting a maleated coupler that has the

appropriate balance of molecular weight and maleic anhydride content.

Arbelaiz et al. [26] have studied the effect of fiber treatments with maleic anhydride,

maleic anhydride PP copolymer, vinyltrimethoxy silane, and alkalization on thermal

stability of flax fiber and crystallization of flax fiber/PP composites. In order to compare

thermal stability of flax fibers, thermogravimetry (TG) analysis was used. Kinetic

parameters have been determined by the Kissinger method. Results showed that all

treatments improved thermal stability of flax fibers. Addition of flax fiber increased

crystallization rate of PP as revealed by DSC, pressure volume temperature (PVT)

measurements and polarized optical microscopy (POM) technics. Besides depending on

fiber surface treatment and crystallization temperature, flax fiber/PP composites also

showed transcrystallinity.

The mechanical properties (i.e., tensile strength and stiffness) of flax/PP composites are

effectively improved by compatibilization of the fiber/matrix interphase. The most

important factor limiting the properties of the composites lies in the intricate structure of

the fibers themselves, after the interfacial strength is optimized, the internal fiber structurebecomes the weakest point [27].




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Pultrusion technic developed for flax reinforced PP commingled yarn containing

discontinuous flax and PP fibers. Rectangular cross-sectional profiles produced by

using a self-designed pultrusion line. In a series of experiments carried out with yarns of

two different flax fiber contents, the pultrusion parameters were varied. In particular,

the preheating and die temperatures and also the pulling speed, which are the most

relevant parameters regarding the potential future pultrusion of natural fiber composite

profiles at industrial scale. A complete characterization of each profile was conducted

in order to examine the influence of processing parameters on the profile quality.

The mechanical properties were evaluated by performing three-point bending as well as

Charpy impact tests [28].

Unidirectional composites from filament wound nontreated flax yarns and PP

fabricated foils composite have exhibited axial stiffness and strength in the range

2729 and 251321 MPa, respectively. A modified version of the rule-of-mixtures,

supplemented with parameters of composite porosity content and anisotropy of fiber

properties, was developed to improve the prediction of composite tensile properties [29].

The tensile strength of hemp strand/PP composites can be as high as 80% of themechanical properties of glass fiber/PP composites [31].

The composites of isotactic PP with hemp fibers (Cannabis sativa), functionalized

by means of melt grafting reactions with glycidyl methacrylate (GMA) were prepared

by batch mixing. The modification of fibers (hempGMA) and polyolefin

matrix (PP-g-GMA), as well as the addition of various compatibilizers (PP-g-GMA,

SEBS, SEBS-g-GMA), were carried out to improve the fibermatrix interactions.

All composites displayed higher tensile modulus (2.9 GPa) and lower elongation at

break as compared to plain PP; compatibilization with PP-g-GMA (10 phr) resulted in an

increased stiffness of the composites as consequence of an improved fibermatrix

interfacial adhesion [32].Kenaf, hemp, and sisal composites showed comparable tensile strength and modulus

properties but in impact properties hemp appears to outperform kenaf. The tensile

modulus, impact strength and the ultimate tensile stress of kenaf reinforced PP composites

were found to increase with increasing fiber weight fraction. Coir fiber composites

displayed the lowest mechanical properties, but their impact strength was higher than that

of jute and kenaf composites. In most cases the specific properties of the natural fiber

composites were found to compare favorably with those of glass [33].

Pickering et al. [34] investigated the optimization of New Zealand grown hemp fiber

reinforced PP composites. The optimum growing period was found to be 114 days,

producing fibers with an average tensile strength of 857 MPa and a Youngs modulus of

58 GPa. The strongest composite consisted of PP with 40 wt% fiber and 3 wt% MAPP,

and had a tensile strength of 47.2 MPa, and a Youngs modulus of 4.88 GPa.

Doan et al. [36] investigated the effect of maleic anhydride grafted PP (MAPP) coupling

agents on the properties of jute fiber/PP composites.

The addition of 2 wt% MAPP to PP matrices improved the adhesion strength with jute

fibers and the mechanical properties of composites.

Effect of impact modifier on 0, 4, 9, and 14 wt% indicated that both impact and tensile

properties showed increasing trend with the compatibilizer but the reverse was true for the

flexural properties [37].

Girone s et al. [38] studied the PP-based composites, reinforced with surface modified

pine fibers. Tensile modulus increased only by 12% after incorporation of untreated fibersto PP matrix. Treatment with silane led to a significant rise (about 60% compared with




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that of unfilled matrix) in tensile modulus. A higher effect was noted for MRPS, followed

by MPS, HDS, and g-aminopropyltrimethoxysilane (AMPS). Flexural modulus was

amplified by 130% with fiber addition and modestly enhanced after silane treatment

compared with that of untreated fibers, flexural modulus increased by 6, 5, 20, and 8% in

the presence of MPS, HDS, AMPS, and MRPS, respectively.

Borysiak and Doczekalska [39] studied the wood sawdust/PP composites based on

polish timber species pine wood (Pinus silvestris L.) as a softwood species and beech wood

(Fagus silvatica L.) as a hardwood species. The size of wood saw dust ranged from 0.5 to

1.0 mm. Three different types of wood/PP mixture were prepared: (1) PPuntreated wood,

(2) PPNaOH treated wood, and (3) PPesterified wood with maleic anhydride. In this

study, the kinetic parameters of crystallization of PP by differential scanning calorimetry

(DSC) were investigated. It is interesting that crystallization of PP depends on the kind of

wood. The chemical treatment of wood caused changes of crystal conversion and half

crystallization time of PP matrix.

Wechsler and Hiziroglu [40] have studied woodplastic composite (WPC) specimens

having 60 and 80% particle and fiber of radiata pine (Pinus radiata) mixed with PP(plastic), and four different additives, namely Structor TR 016 which is a coupling agent,

CIBA anti-microbial agent (IRGAGUARD F3510) as fungicide, CIBA UV filter coating

(TINUVIN 123S), CIBA blue pigment (Irgalite), and their combinations. Based on the

initial finding of this study static bending properties of the samples enhanced by the above

chemicals were added into both particle and fiber-based specimens. Thickness swelling of

the samples were also improved by the presence of additives in the panels. Micrographs

taken on a scanning electron microscope (SEM) revealed that coupling agent and pigment

resulted in more homogeneous mixture of wood and plastic together. The particle-based

samples had rougher surface characteristics than those of fiber-based ones. No significant

influence of chemicals added in the samples was found on surface roughness values of thesamples manufactured from particle and fiber of radiata pine.

Pickering and Ji [41] studied the effects of poly[methylene (polyphenyl isocyanate)]

(PMPPIC) and MAPP coupling agents, separately and combined, on the strength and

Youngs modulus of New Zealand radiata pine-reinforced PP composites. A modest

improvement compared to the unreinforced matrix strength of 4% and much greater

improvement of 123%, substantially greater than reported elsewhere in the literature, were

obtained using PMPPIC and MAPP, respectively. Youngs modulus improvements

compared to the unreinforced matrix of 77 and 177% were obtained using PMPPIC and

MAPP, respectively.

The processing of sisal fiber reinforced PP composites by injection molding was reported

by Li and his coworkers. The tensile properties for PP, high temperature (HT) and low

temperature (LT) specimens were compared. It was observed that the incorporation of

10 wt% of sisal fibers increases Youngs modulus by about 150%, and the tensile strength

was increased by about 10%. The reinforcement efficiency for LT specimens is slightly

higher than HT samples [42].

Joseph et al. [43,44] prepared sisal fiber reinforced PP composites by melt-mixing and

solution-mixing methods. The methods enhanced the tensile properties of the

composites. The effect of fiber content and chemical treatments on the thermal

properties of sisal/PP composites was also evaluated. It was found that treated fiber

composites show superior properties compared to the untreated system. DSC

measurements exhibited an increase in the crystallization temperature and crystallinity,upon the addition of fibers to the PP matrix. This is attributed to the nucleating effects




16/22

of the fiber surfaces, resulting in the formation of transcrystalline regions. On

increasing the fiber content, the melting peak of the PP component was shifted to

higher temperatures suggesting a constrained melting.

Han-Seung yang et al. [45] prepared a particle-reinforced composite, in that rice-husk

flour as the reinforcing filler and PP as the thermoplastic matrix polymer, and its

mechanical and morphological properties examined as a function of the amount of

compatibilizing agent used. In the sample preparation, four levels of filler loading (10, 20,

30, and 40 wt%) and three levels of compatibilizing agent content (1, 3, and 5 wt%) were

used, and in the tensile test, six test temperatures (30, 0, 20, 50, 80, and 1108C) and five

crosshead speeds (2, 10, 100, 500, and 1500 mm/min) were used. The tensile strengths of

the composites decreased as the filler loading increased, but the tensile properties

were significantly improved with the addition of the compatibilizing agent. Both the

notched and the unnotched Izod impact strengths were almost the same with the addition

of compatibilizing agent.

Sain et al. [46] studied flammability of PP, rice husk filled PP composites, and flame

retarding effect of magnesium hydroxide for these composites by horizontal burningrate and oxygen index tests. Despite the presence of a coupling agent, the addition of

flame-retardants shows a negative impact on the tensile and flexural properties of the

composites and the properties show a reduction of 17 and 15%, respectively, compared to

the composite without flame retardant

Coutinho et al. [49] studied the influence of the amount of MAPP coating of vinyl-tris

(2-methoxy ethoxy) silane treated woodfibers (2 or 10 wt%), type of matrix (PP or MAPP)

and composition (10, 20, or 30 wt%) on the tensile and flexural performance of PPwood

fiber composites was studied. The effect of these variables on tensile strength, Youngs

modulus, elongation at yield, and flexural strength was determined through a 2231

factorial design. The analysis of variance of the experimental and predicted data showthat the constructed models provide a fair approximation of actual experimental

measurements. Finally, experimental details regarding the preparation of optimum

composites as predicted by empirical models were discussed.

Suarez et al. [50] studied the morphological behavior of composites of PP or PP plus

MAPP and saw dust coated with 22.4 wt% MAPP were prepared under fixed processing

conditions (mixing temperature, rate of rotation and mixing time). SEM was performed to

analyze the tensile fracture surfaces and the interfacial fiber/matrix adhesion. The tensile

properties of composites with up to 10% MAPP have not been improved by increasing

MAPP content. This has been attributed to the poor filler/matrix adhesion. The addition

of higher MAPP content to the PP composites produces better adhesion of saw dust to PP

matrix and an increase in the tensile strength.

The effects of coupling agents on the mechanical, morphological, and water sorption

properties of luffa fiber (LF)/(PP) composites were studied. In order to enhance the

interfacial interactions between the PP matrix and the luffa fiber, three different types of

coupling agents, (3-aminopropyl)-triethoxysilane (AS), 3-(trimethoxysilyl)-1-propanethiol

(MS), and maleic anhydride grafted polypropylene (MAPP) were used. The PP composites

containing 215 wt% of LF were prepared in a torque rheometer. The tensile properties of

the untreated and treated composites were determined as a function of filler loading.

Tensile strength and Youngs modulus increased with employment of the coupling agents

accompanied by a decrease in water absorption with treatment due to the better adhesion

between the fiber and the matrix. The maximum improvement in the mechanicalproperties was obtained for the MS treated LF composites. The interfacial interactions




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improved the filler compatibility, mechanical properties, and water resistance of

composites. The improvement in the interfacial interaction was also confirmed by the

Pukanszky model. Good agreement was obtained between the experimental data and the

model prediction [51].

Panthapulakka et al. [52] studied the potential of wheat straw fibers prepared by

mechanical and chemical processes as reinforcing additives for thermoplastics was

investigated. Fibers prepared by mechanical and chemical processes were characterized

with respect to their chemical composition, morphology, and physical, mechanical, and

thermal properties. Composites of PP filled with 30% wheat straw fibers were prepared

and their mechanical properties were also evaluated. The fibers prepared by chemical

process exhibited better mechanical, physical, and thermal properties. Wheat straw fiber

reinforced PP composites exhibited significantly enhanced properties compared to virgin

PP. However, the strength properties of the composites were less for chemically prepared

fiber filled composites. This was due to the poor dispersion of the fibers under the

processing conditions used. These results indicate that wheat straw fibers can be used as

potential reinforcing materials for making thermoplastic composites.Jang and Lee [53] fabricated the paper sludge/PP composites. Flame retardants like

Saytex8010, TPP, Mg(OH)2, and antimony trioxide were introduced into the composites

for improvement of the flame retardancy and was measured with the UL94 test.

Rozman et al. [54] studied the effect and mechanical properties of lignin as a

compatibilizer in the coconut fiberPP. Composites with lignin as a compatibilizer possess

higher flexural properties as compared to the control composites. The results also show

that tensile properties do not improve as lignin is incorporated. SEM results display some

proof of an enhanced compatibility at the interfacial region.

Shibata et al. [56] fabricated the lightweight laminate composites made from kenaf and

PP fibers by press forming. The effects of the number of kenaf layers, heating time, andkenaf weight fraction on the flexural modulus of the composite specimen were

investigated. The flexural modulus increased with increasing number of kenaf layers and

heating time. The increase of the number of kenaf layers contributed to homogeneous PP

dispersion in the composite board. This is because more kenaf layers caused better contact

between kenaf and PP and prevented PP fibers from shrinking by heating. The increase of

heating time contributed to better wetting between kenaf and PP.

Mwaikambo et al. [57] showed that kapok/cotton fabric has been used as reinforcement

for conventional PP and maleic anhydride grafted PP resins. Treating the reinforcement

with acetic anhydride and sodium hydroxide has modified the fabric (fibers). Thermal

and mechanical properties of the composites were investigated. Results show that fiber

modification gives a significant improvement to the thermal properties of the plant fibers,

whereas tests on the mechanical properties of the composites showed poor tensile strength.

Mercerization and weathering were found to impart toughness to the materials, with

acetylation showing slightly less rigidity compared to other treatments on either the fiber

or the composites. The modified PP improved the tensile modulus and had the least

toughness of the kapok/cotton reinforced composites. MAiPP reinforced with the plant

fibers gave better flexural strength and the same flexural modulus at lower fiber content

compared with glass fiber reinforced MAiPP.

Arib et al. [58] investigated the tensile and flexural behaviors of pineapple leaf fiberPP

composites as a function of volume fraction. The tensile modulus and tensile strength of

the composites were found to be increasing with fiber content in accordance with the ruleof mixtures.




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Cziga ny [59] manufactured the basalt fiber reinforced, PP matrix hybrid composites

in the process of carding, needle-punching, and pressing. Hemp, glass, and carbon fibers

were applied besides basalt fiber in these composites. In order to achieve a sufficient

interfacial adhesion, the fibers were treated with the reaction mixture of maleic acid

anhydride and sunflower oil. The hybrid effect in these composites was examined as a

function of fiber content and fiber combination. The strength properties of hybrid

composites improved owing to surface treatment and this was proven by mechanical tests

and microscopic analysis, as well. Acoustic emission methods revealed that there is a

correlation between the physical parameters of sound waves that occurred during failure

and the mechanical properties.

Ruksakulpiwat et al. [60] prepared the injection molded vetiverPP composites at

various ratios of vetiver content and vetiver length. When compared to PP, vetiverPP

composites exhibited higher tensile strength and Youngs modulus but lower elongation

at break and impact strength. An increase in vetiver content led to an increase in

viscosity, heat distortion temperature, crystallization temperature, and Youngs modulus

of the composites. On the other hand, the decomposition temperature, tensilestrength, elongation at break, and impact strength decreased with increasing

vetiver content. The chemical treatment of the vetiver grass improved the mechanical

properties of the composites.

Kim et al. [61] investigated the effect on thermal properties of the addition of

two different compatibilizing agents, maleic anhydride (MA)-grafted polypropylene

(MAPP) and MA-grafted polyethylene (MAPE), to bio-flour-filled, PP, and low

density polyethylene (LDPE) composites. The effect of two different types of MAPE

polymer, MA-grafted high-density polyethylene (HDPE-MA) and MA-grafted linear

LDPE (LLDPE-MA), was also examined. With increasing MAPP and MAPE content,

the thermal stability, storage modulus (E_), tan max peak temperature (glass transitiontemperature: Tg) and loss modulus (Emax) peak temperature ( relaxation) were

slightly increased. The thermal stability, E_ and E__ of MAPE-treated composites were

not significantly affected by the two different MAPE polymers. The melting temperature

(Tm) of the composites was not significantly changed but the crystallinity (Xc) of

MAPP- and MAPE-treated composites was slightly increased with increasing MAPP

and MAPE content. This enhancement of thermal stability and properties could

be attributed to an improvement in the interfacial adhesion and compatibility

between the rice husk flour (RHF) and the matrix due to the treatment of compatibilizing

agent. Attenuated total reflectance (FTIR-ATR) analysis confirmed this result

by demonstrating the changed chemical structures of the composites following

MAPP and MAPE addition.

Liao et al. [62] have studied the resistance of bamboo fiber reinforced PP composite

(BFRP) and bamboo-glass fiber reinforced PP hybrid composite (BGRP) to hygrothermal

aging, and their fatigue behavior under cyclic tensile load, were studied. Injection molded

samples were exposed in water at 258C for up to 6 months and at 758C for up to 3 months.

Tensile strength and elastic modulus of BFRP and BGRP samples have shown moderate

reduction after aging at 258C after 6 months; however, they were reduced considerably

after aging at 758C for 3 months. Moisture absorption and tensile strength degradation are

suppressed by using MAPP as a coupling agent in both types of composite systems. BFRP

and BGRP samples were also loaded cyclically at maximum cyclic load of 35, 50, 65, and

80% of their ultimate tensile stress. Results suggest that BGRP has better fatigueresistance than BFRP at all load levels tested.




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Abu-Sharkh and Hamid [63] studied the degradation characteristics of the date palm

leaves compounded with PP and UV stabilizers. Enhanced stability imparted by the

presence of the fibers in the composites, enhanced interfacial adhesion resulting from

oxidation of the polymer matrix can be the source of retention of mechanical strength.

FUTURE WORK

There is an increasing demand from auto companies for materials with sound abatement

capability as well as reduced weight for fuel efficiency; therefore, the use of natural fibers

over man-made glass and carbon fibers. However, the properties of these natural fiber

reinforced composites are dependent on the properties of the fiber and the adhesion

between fiber and matrix. The enhancement of the properties of the composites can be

achieved by chemical modificatons of matrix and the fiber.

CONCLUSIONS

The chemical, mechanical, and physical properties of natural fibers have distinct

properties; cellulose content of these fibers varies from fiber to fiber. The moisture content

lowers the mechanical properties. The mechanical properties of composites are influenced

mainly by the adhesion between matrix and fibers. As in the case of glass-fibers, the

adhesion properties can be changed by pre-treating the fibers. Novel processing

techniques, chemical and physical modification methods are developed to improve the

fiber-matrix adhesion resulting in the enhancement of mechanical properties of

the composites. Up to now, the most important natural fibers are jute, flax, and coir.

Yet the development of novel processing and modification methods is not finished.

Further improvements can be expected, so that it might become possible to substitutetechnical-fibers in composites quite generally. Natural fibers are low-cost, recyclable and

eco-friendly materials. Eco-friendliness and bio-degradability of these natural fibers may

replace the glass and carbon fibers.

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