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Progress in Polymer Science 37 (2012) 15521596

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Progress in Polymer Science

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Biocomposites reinforced with natural fibers: 20002010

Omar Faruk a,d,, Andrzej K. Bledzki a,c, Hans-Peter Fink b, Mohini Sain d

a Institut frWerkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Moenchebergstrasse 3, D-34109 Kassel, Germanyb Fraunhofer Institute for AppliedPolymer Research, Geiselbergstrasse 69, D-14476 Potsdam-Golm, Germanyc West Pomeranian University of Technology, Institute of Material Science and Engineering, 19 PiastowAve, 70310 Szczecin, Polandd Centre for Biocomposites and Biomaterials Processing, University of Toronto, 33Willcocks Street, Toronto, ONM5S3B3, Canada

a r t i c l e i n f o

Article history:

Received 3 August 2011Received in revised form 19 April 2012Accepted 25 April 2012Available online 2 May 2012

Keywords:

BiocompositesNatural fiberBiopolymerNanofiberAcetylationAlkylationSilaneMaleated couplingEnzyme treatmentThermoplasticThermosetsInjection moldingCompression moldingExtrusionPultrusionResin transfer moldingSheet molding compoundLFT-D methodTensileFlexuralImpact

a b s t r a c t

Due to environment and sustainability issues, this century has witnessed remarkableachievements in green technology in the field of materials science through the devel-opment of biocomposites. The development of high-performance materials made fromnatural resources is increasing worldwide. The greatest challenge in working with naturalfiber reinforced plastic composites is their large variation in properties and characteristics.A biocomposites properties are influenced by a number of variables, including the fibertype, environmental conditions (where the plant fibers are sourced), processing meth-ods, and any modification of the fiber. It is also known that recently there has been asurge ofinterest in the industrial applications ofcomposites containing biofibers reinforcedwith biopolymers. Biopolymers have seen a tremendous increase in use as a matrix forbiofiber reinforced composites. A comprehensive review ofliterature (from 2000 to 2010)on the mostly readily utilized natural fibers and biopolymers is presented in this paper.

The overall characteristics of reinforcing fibers used in biocomposites, including source,type, structure, composition, as well as mechanical properties, will be reviewed. More-over, the modification methods; physical (corona and plasma treatment) and chemical(silane, alkaline, acetylation, maleated coupling, and enzyme treatment) will be discussed.The most popular matrices in biofiber reinforced composites based on petrochemical andrenewable resources will also be addressed. The wide variety of biocomposite process-ing techniques as well as the factors (moisture content, fiber type and content, couplingagents and their influence on composites properties) affecting these processes will be dis-cussed. Prior to the processing ofbiocomposites, semi-finished product manufacturing isalso vital, which will be illustrated. Processing technologies for biofiber reinforced compos-ites will be discussed based on thermoplastic matrices (compression molding, extrusion,injection molding, LFT-D-method, and thermoforming), and thermosets (resin transfermolding, sheet molding compound). Other implemented processes, i.e., thermoset com-pression molding and pultrusion and their influence on mechanical performance (tensile,flexural and impact properties) will also be evaluated. Finally, the review will conclude withrecent developments and future trends ofbiocomposites as well as key issues that need tobe addressed and resolved.

Crown Copyright 2012 Published by Elsevier Ltd. All rights reserved.

Corresponding author at: Centre for Biocomposites and Biomaterials Processing, University of Toronto, 33 Willcocks Street, Toronto, ON M5S3B3,Canada. Tel.: +1 416 9781615; fax: +1 416 9783834.

E-mail address: [email protected] (O. Faruk).

0079-6700/$ see front matter. Crown Copyright 2012 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.progpolymsci.2012.04.003
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6.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15827. Development and future trends ofbiocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15828. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1584

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1584

1. Introduction

Thereis a growing trend to use biofibers as fillers and/orreinforcers in plastics composites. Their flexibility duringprocessing, highly specific stiffness, and low cost (on a vol-umetric basis) makethem attractive to manufacturers. Thiscenturyhaswitnessedever-increasingdemandsfortheuti-lization of plastics as important raw materials, more than80% of which are thermoplastics. Biofiber reinforced plas-tic composites are gaining more and more acceptance instructural applications.

Technological development connected with consumerdemands and expectations continues to increase demandson global resources, leading to major issues of materialavailability and environmental sustainability. Over the lastfew decades biofiber composites have been undergoing aremarkable transformation. These materials have becomemore and more sufficient as new compositions and pro-cesses have been intensively researched, developed andconsequently applied. The petroleum crisis made biocom-posites significantly important and biocomposites havebecome engineering materials with a very wide range ofproperties. However, like all materials, they are constantlyunder competitive pressure from the global market, whichin turn, necessitates continuous research. The times ofsimply mixing plastics with natural waste fillers and char-acterizing their main properties are gone.

The concept of using bio-based plastics as reinforcedmatrices for biocomposites is gaining more and moreapproval day by day. The developments in emerging bio-based plastics are spectacular from a technological point ofview andmirror their rapid growth in themarket place. Theaverage annual growth rate globally was 38% from 2003 to2007. In the same period, the annual growth rate was ashigh as 48%in Europe.The worldwidecapacityof bio-basedplastics is expected to increase from 0.36 million metricton (2007) to 2.33 million metric ton by 2013 and to 3.45million metric ton in 2020. The main product in terms ofproduction volumes will be starch-based plastics, PLA andPHA [1].

Theincreasingnumber of publications during therecentyearsincludingreviews [29] and books [1014] reflectthegrowing importance of these new biocomposites. Bledzkiand Gassan have reviewed the reinforcement of the mostreadily used natural fibers in polymer composites up until1999 in their review paper [15]. This paper aims to reviewmore current reinforcement of natural fibers in polymercomposites from 2000 to 2010. This paper will not addressnatural fibers from animals (e.g., silk or wool) or cottonor man-made cellulosic fibers. This review also excludeswood fiber or flour. Given the broadscope of this article, itwill inevitably be incomplete, but will hopefully providea sensible overview of the most popular natural fibers inpolymeric composite materials in the last 11 years.

2. Reinforcing fibers

An increased awareness that non-renewable resourcesare becoming scarce and our inevitable dependence onrenewable resources has arisen. This century could becalled the cellulosic century, because more and morerenewable plant resources for products are being discov-ered. It has been generally stated that natural fibers arerenewableand sustainable,but they arein fact, neither. Thelivingplants are renewableandsustainablefrom which thenatural fibers are taken, but not the fibers themselves.

2.1. Fiber source

The plants, which produce natural fibers, are classified

as primary and secondary depending on their utilization.Primaryplantsarethosegrownfortheirfibercontentwhilesecondary plants are plants in which the fibers are pro-duced as a by-product. Jute, hemp, kenaf, and sisal areexamples of primary plants. Pineapple, oil palm and coirare examples of secondary plants. Table 1 shows the mainfibers used commercially in composites, which are nowproduced throughout the world [16].

2.2. Fiber types

There are six basic types of natural fibers. They are clas-sified as follows: bast fibers (jute, flax, hemp, ramie and

kenaf), leaf fibers (abaca, sisal and pineapple), seed fibers(coir, cotton and kapok), core fibers (kenaf, hemp and jute),grass and reed fibers (wheat, corn and rice) and all othertypes (wood and roots).

2.2.1. Flax

Flax, Linum usitatissimum, belongs to the bast fibers. Itis grown in temperate regions and is one of the oldest fibercrops in the world. The bast fiber flax is most frequentlyused in the higher value-added textile markets. Nowadays,it is widely used in the composites area.

The static and dynamic mechanical properties of non-woven based flax fiber reinforced PP composites were

Table 1

Commercially major fiber sources.

Fiber source World production (103 ton)

Bamboo 30,000Jute 2300Kenaf 970Flax 830Sisal 378Hemp 214Coir 100Ramie 100Abaca 70Sugar cane bagasse 75,000

Grass 700


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studied considering the effect of zein coupling agent [17].Zein is a protein, extracted from corn and used as solution.Composites containing zein coupling agent were foundto possess improved mechanical properties. The storagemodulus of composites was found to increase with zeincoupling agent coating due to enhanced interfacial adhe-sion.

The tensile mechanical properties of flax fibers are esti-mated according to their diameter and their location inthe stems [18]. The large scattering of these properties isascribed to the variation of the fiber size along its longitu-dinal axis. The higher values of the mechanical propertiesof the fibers issued from the middle of the stems are associ-ated with the chemical composition of their cell walls. Themechanical properties of unidirectional flax fiber/epoxymatrix composites are studied as a function of their fibercontent. The properties of the composites are lower thanthose expected from single fiber characteristics.

Various investigations of flax fiber/polypropylene com-posites have been completed. These studies focus on manydifferent variables, including: comparison between NMT(natural fiber thermoplastic mat) and GMT (glass fiberthermoplastic mat)[19], the influence of fiber/matrix mod-ification and glass fiberhybridization[20], theeffectoffibertreatment on thermal and crystallization properties [21],the influence of surface treatment on interface by glyceroltriacetate, thermoplastic starch, -methacryl oxypropyltrimethoxy-silane and boiled flax yarn [22], comparison ofmatrices (PP and PLA) on the composite properties [23],the effects of material and processing parameters [24], andthe influence of processing methods [25]. Buttler [26] pre-sented the feasibility of using flax fiber composites in thecoachwork and bus industry.

The effect of bio-technical fiber modification [27], frac-ture behavior and toughness [28], the influence of alkalinefiber treatment on unidirectional composites [29], theeffect of processing parameters [30] on the consecutivedecortication stages of flax fibers (retting, scutching, andhackling) on the flax fiber reinforced epoxy compositeshave also been evaluated.

Flax fiber composites reinforced with polyester resinhave been evaluated for thermal degradation and fireresistance [31], chemical treatments on surface propertiesand adhesion [32], and the effect of chemical treat-mentson waterabsorption and mechanicalproperties[33].Three resins from soybean oil (methacrylated soybean oil,methacrylic anhydride modified soybean oil, and aceticanhydridemodified soybean oil) were used also as matricesfor flax fiber reinforced biocomposites [34].

2.2.2. Hemp

Another notable bast fiber crop is hemp, which belongsto the Cannabis family. It is an annual plant that growsin temperate climates. Hemp is currently the subject ofa European Union subsidy for non-food agriculture, anda considerable initiative in currently underway for theirfurther development in Europe.

Composites of PP with hemp fibers, which were func-tionalized by meansof meltgraftingreactions withglycidylmethacrylate (GMA) and prepared by batch mixing, wereexamined [35]. ThemodificationoffibersandthePPmatrix,

as well as the addition of various compatibilizers werecarried out to improve the fibermatrix interactions. Com-pared to the unmodified system, a modified compositeshowed improved fiber dispersion in the PP matrix andhigher interfacial adhesion as a consequence of chemicalbonding between the fiber and the polymer (PP/Hemp).Thethermal stability and phase behavior of the compositeswas largely affected by the fiber and matrix modification.Changes in thespherulithic morphologyand crystallizationbehavior of PP were observed in the composites due to thenucleating effect of the hemp fibers. Moreover, a markedincrease in the PP isothermal crystallization rate (in therange120138 C) wasrecorded with increasing content ofmodified hemp. All composites displayed a higher tensilemodulus (about 2.9 GPa) and lower elongation at break ascompared to plain PP; compatibilization with modified PP(10phr) resulted in an increasedstiffness of thecompositesas a result of improved fibermatrix interfacial adhesion.

Pickering and co-workers [3638] investigated theeffects of chelator, white rot fungi, and enzyme treatmentson the separation of hemp fibers from bundles, and theimprovement of the interfacial bonding of the hemp fiberswith the PP matrix. The obtained results showed that theinterfacial shear strength of the treated fiber compositeswas higher than that for untreated fiber composites. Thisverifies that thehemp fiber interfacial bonding with PP wasimproved by the white rot fungi treatment. Compositesconsistingof hempfiberstreated withchelator concentratehad the highest tensile strength of 42MPa, which equalsan increase of 19% compared to composites with untreatedhemp fiber.

HempfiberreinforcedPPcompositesexhibitedinterest-ing recyclability [39]. The obtained results prove that themechanicalpropertiesof hempfiber/PP compositesremainwell preserved, despite the number of reprocessing cycles.The Newtonianviscosity decreases withcycles, indicatingadecrease in molecular weight and chain scissions inducedby reprocessing. The decrease of fiber length with repro-cessing could be another reason for decrease of viscosity.

Epoxy resins, which were used as a matrix for hempfiber reinforced composites, were studied regarding theeffect of fiber architecture on the falling weight impactproperties [40], properties and performances of compos-ites for curved pipes [41], impact load performance of resintransfer molded composites [42], micro-mechanics of thecomposites [43], the influence of hybrid blends made ofsoybean oil and nanoclay [44], and the usefulness of unret-ted hemp as a source of fiber for biocomposites [45].

Kunanopparat et al. [46,47] studied the feasibility ofwheatglutenasamatrixforhempfiberreinforcedcompos-ites regarding their thermal treatment and plasticizationeffect on the mechanical properties.

2.2.3. Jute

Jute is produced from plants of the genus Corchorus,which includes about 100 species. It is one of the cheap-est natural fibers and is currently the bast fiber with thehighest production volume. Bangladesh, India and Chinaprovide the best condition for the growth of jute.

Ray and co-workers [4850] extensively investigatedalkali treated jute fiber reinforced with vinyl ester resin.


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In their studies, they regarded the dynamic, mechanical,thermal, and impact fatigue behavior compared to that ofuntreated jute fibervinyl ester composites. Longer alkalitreatment removed the hemicelluloses and improved thecrystallinity, enabling better fiber dispersion. The dynamic,mechanical, thermal and impact properties were superiorowing to the alkali treatment, comprising treatment time,concentration and conditions.

Mohanty et al. [51] investigated effects and influenceof surface modification on the mechanical and biodegrad-ability of jute/Biopol and jute/PA (Poly Amide) composites.Enhancements in tensile strength of more than 50%, 30%in bending strength and 90% in impact strength wereobserved in the composites and are comparative to val-ues achieved for pure Biopol sheets. Degradation studiesshowed that after 150 days of compost burial more than50% weight loss of the jute/Biopol composites occurs.

The effects of hybridization [52] on the tensile prop-erties of jutecotton woven fabric reinforced polyestercomposites were investigated as functions of the fibercontent, orientation and roving texture. It was observedthat tensile properties along the direction of jute rovingalignment(transverse to cotton roving alignment) increasesteadily with fiber content up to 50% and then show a ten-dency to decrease. The tensile strength of composites with50% fiber content parallel to the jute roving is about 220%higher than pure polyester resin.

Jute fiber reinforced PP composites were evaluatedregarding the effect of matrix modification [53], the influ-ence of gamma radiation [54], the effect of interfacialadhesion on creep and dynamic mechanical behavior [55],theinfluenceof silanecouplingagent [56,57], andtheeffectof natural rubber [58].

The properties of jute/plastic composites were studied,including the thermal stability, crystallinity, modification,trans-esterification, weathering, durability, fiber orien-tation on frictional and wear behavior, eco-design ofautomotive components, and alkylation [5966].

Polyester resin was used as matrix for jute fiber rein-forced composites and the relationship between waterabsorption and dielectric behavior [67], the elastic prop-erties, notched strength and fracture criteria [68], impactdamage characterization [69], weathering and thermalbehavior [70], and effect of silane treatment [71] wereexamined.

2.2.4. Kenaf

Kenafbelongs to the genusHibiscus and there are about300 species. Kenaf is a new crop in the United Statesand shows good potential as a raw material for usagein composite products. Latest advances in decorticationsequipment which separates the core from the bast fibercombined with fiber shortages, have renewed the interestin kenaf as a fiber source.

Thermoforming has proven to enable the successfulfabrication of kenaf fiber reinforced PP sheets into sheetform [72]. The optimal fabrication method found for thesematerials was at the compression molding process, whichutilizes a layered sifting of a micro-fine PP powder andchopped kenaf fibers. The fiber content (30 and 40 wt%)provided adequate reinforcement to increase the strength

of the PP matrix. The kenafPP composites compressionmolded in this study proved to have superior tensile andflexural strength when compared to other compressionmolded natural fiber composites such as other kenaf, sisal,and coir reinforced thermoplastics. With the aid of theelastic modulus data, it was also possible to compare theeconomic benefits of using kenaf composites instead ofother natural fibers and E-glass. The manufactured kenaf-maleated PP composites have a higher modulus/cost and ahigher specific modulus than sisal, coir, and even E-glass.Thus, they provide an option for replacing existing mate-rials with a higher strength, lower cost alternative that isenvironmentally friendly.

Hybrid composites of wood flour/kenaf fiber and PPwere prepared to investigate the hybrid effect on the com-posite properties [73]. The results indicated that whilenon-hybridcompositesofkenaffiberandwoodflourexhib-ited the highest and lowest modulus values respectively,themoduli of hybrid composites were closely related to thefiber to particle ratio of the reinforcements. With the helpof the hybrid mixtures equation it was possible to predictthe elastic modulus of the composites better than whenusing the HalpinTsai equation.

Variations of the mechanical and thermal propertiesinduced by the exposure of kenaf fiber reinforced com-posites with PP to electron beam radiation [74], thecomparison of kenaf/PP composites with feather fiber/PP,recycled kraft pulp fiber/PP, and recycled news pulpfiber/PP composites [75] and, surface modified kenaffibers with modified polyester resin as matrix [76], andinfluence of toughening by natural rubber on kenaf fibers with polyester resin as matrix [77] were alsoevaluated.

2.2.5. Sisal

Sisal is an agave (Agave sisalana) and commercially pro-duced in Brazil and East Africa. Between 19982000 and2010, the global demand for sisal fiber and its productsis expected to decline by an annual rate of 2.3% as agri-cultural twine. The traditional market for fibers continuesto be eroded by synthetic substitutes and by the adop-tion of harvesting technologies that utilizes less or notwine.

Magnesium hydroxide and zinc borate were incorpo-rated into sisal/PP composites as flame retardants [78].Adding flame retardants into sisal/PP composites reducedthe burning rate and increased the thermal stability of thecomposites. No synergistic effect was observed when bothmagnesium hydroxide and zinc borate were incorporatedinto the sisal/PP composites. In addition, the sisal/PP com-positesexhibitedinsignificantdifferencesinshearviscosityat high shear rates indicating that the types of flame retar-dantsusedinthisstudyhadnoimpactontheprocessabilityof the composites. The sisal/PP composites which flameretardants were added to, exhibited tensile and flexuralproperties comparable to those of the sisal/PP composites,which flame retardants had not been added to.

Sisal /PP composites were investigated regarding theenvironmental effects on the degradation behavior [79],the influence of coupling agents on the abrasive wear


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properties [80], and the effect of ageing on the mechanicalproperties [81].

Zhang et al. [82] developed all plant fiber compositesby converting wood flour into thermoplastics using anappropriatebenzylation treatment and compoundingbothdiscontinuous and continuous sisal fibers to produce com-posites from renewable resources. The degradation testsdemonstrated that the prepared sisal/plasticized woodflour composites were fully biodegradable. To acceleratethe decomposition process, both cellulose and lignin in thecomposites should be considered. The hydrophobicity andflame resistance of composites is important when regard-ing practical applications. Molecular modification and/orincorporation of inorganic additives are proper measuresprovided that the composites biodegradability remainsunchanged.

Many other studies were carried out on sisal fiberreinforced polyester composites regarding their moisture-absorption properties [83], and fiber treatment withadmicellar [84]. Sisal fiber reinforced phenolic resin com-posites were investigated with regard to the chemicalmodification of such with lignins [85], the modificationusing hydroxy terminated polybutadiene rubber [86], theeffect of cure cycles [87], using glyoxal from naturalresources [88], and the effect of alkali treatment [89].Furthermore epoxy resin was used as a matrix for sisalfiber reinforced composites and examined regarding theinfluence of fiber orientation on the electrical properties[90], and the degree of reinforcement [91]. Investigationswere also carried out using cement as a matrix for sisalfiber reinforced composites focusing on their crackingmicro-mechanisms [92], and the effects of accelerated car-bonation on cementitious roofing [93].

Towo et al. [94,95] prepared the treated sisal fiber com-posites with an epoxy and polyester resin matrix. Fatigueevaluation and dynamic thermal analysis tests were per-formed. Composites containing alkali treated fiber bundlesproved to have better mechanical properties than thosewith untreated fiber bundles. Alkali treatment had thegreatest effect on thepolyesterresin matrices.An improve-ment in the fatigue lives of composites was observed forthe alkali treatment of sisal fiber bundles. Constant-life dia-grams of epoxy matrix composites with untreated or alkalitreated fiber bundles show the superiority of the alkalitreated fiber compositesregarding low cyclefatigue. Epoxymatrix composites have a longer fatigue life than polyestermatrix composites. The effect of chemical treatment onthe fatigue life is significantly positive for polyester matrixcomposites but has much less influence on the fatigue lifeof epoxy matrix composites.

Sisal fibers were also investigated with other matrices,such as rubber [96,97], phenol formaldehyde [98], cellu-lose acetate [99], bio polyurethane [100], and polyethylene[101] regarding theirmechanical, morphological,chemical,and Cure characteristics.

2.2.6. Abaca

The abaca/banana fiber, which comes from the bananaplant, is durable and resistant to seawater. Abaca, thestrongest of the commercially available cellulose fibers, isindigenous to the Philippines and is currently produced

0

1

2

3

4

5

6

Jute-PPFlax-PPAbaca-PP

Notche

dcharpyimpact

strength[mJ/mm]

5mm

5mm+MAH-PP

Fig. 1. Comparison of notch Charpy strength of abaca/jute/flax fiberPPcomposites with and without MAHPP.Adopted from [103]. Copyright 2007. By permission from Budapest Uni-versity of Technology.

there and in Ecuador. It was once the preferred cordagefiber for marine applications.

Bledzki et al. [102,103] examined the mechanical prop-erties of abaca fiber reinforced PP composites regardingdifferent fiber lengths (5, 25 and 40mm) and differentcompounding processes (mixer-injection molding, mixer-compression molding and direct compression moldingprocess). It was observed that, with increasing fiber length(540mm), the tensile and flexural properties showed anincreasing tendency though not a significant one. Amongthe three different compounding processes compared,the mixer-injection molding process displayed a bettermechanical performance (tensile strength is around 90%higher) than the other processes.

When abaca fiber PP composites were compared withjute and flax fiber PP composites, abaca fiber compositeshad the best notched Charpy (Fig. 1) and falling weightimpact properties. Abaca fiber composites also showedhigher odor concentration (Fig. 2) compared to jute andflax fiber composites.

A dynamic mechanical analysis of, polarity parame-ters of banana fiber reinforced polyester composites werecarried out by Thomas and co-workers [104,105] with spe-cial reference to the effects of fiber loading, frequencyand temperature. The storage modulus was found to behighest for composites with 40% fiber loading, indicating

Fig. 2. Comparison of odor emission concentration abaca/jute/flaxfiberPP composites.

Adopted from [103]. Copyright 2007. By permission from Budapest Uni-versity of Technology.


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that the incorporation of abaca fiber in the polyestermatrix induces reinforcing effects at higher temperatures.Increased dynamic modulus values and low damping val-ues verified improved interactions between the fiber andthe matrix.

Abaca fibers have been investigated with cement [106],polyurethane [107], aliphatic polyester resin [108], PP[109], urea formaldehyde [110], PE [111], polyester [112],andpolyvinylalcohol [113] asmatrices,inordertoevaluatethe composites properties.

2.2.7. Pineapple leaf fiber

Pineapple (Ananas comosus) is a tropical plant nativeto Brazil. Pineapple leaf fiber is rich in cellulose, rela-tively inexpensive and abundantly available. Furthermore,it has the potential for polymer reinforcement. At presentpineapple leaf fibers are a waste product of pineap-ple cultivation and therefore these relatively inexpensivepineapple fiber can be obtained for industrial purposes.

Pineapple leaf fiber was reinforced with polycarbon-ate to produce functional composites [114]. The silanetreated modified pineapple leaf fibers composite exhibitedthe highest tensile and impact strengths. The thermogravi-metric analysis showed that the thermal stability of thecomposites is lower than that of neat polycarbonate resin.In addition, the thermal stability decreased with increasingpineapple leaf fiber content.

The thermal conductivity and thermal diffusivity ofpineapple leaf fiber reinforced phenol formaldehyde com-posites were studied using the Transient Plane Source(TPS) technique [115]. It is found that the effective ther-mal conductivity and effective thermal diffusivity of thecomposites decrease, as compared with pure phenolformaldehyde as the fraction of fiber loading increases.

The quality enhancement of pineapple leaf fiber hasbeen attempted using different surface modifications likedewaxing, alkali treatment, cyanoethylation and graftingacrylonitrile onto dewaxed fibers [116]. The mechanicalproperties reach an optimum at fiber load of 30wt% andamong all modifications, 10% acrylonitrile grafted fiberreinforced polyester composite exhibited the maximumtensile strength (48.36MPa). However, cyano-ethylatedfiber composites exhibited better flexural and impactstrengths, i.e., 41% and 27% more than the detergentwashed composite respectively.

The influence of surface treatment and fiber content ofpineappleleaf fiber was also investigatedwith PP [117] andnatural rubber as matrix [118].

2.2.8. Ramie

Ramie belongs to the family Urticaceae (Boehmeria),which includes about 100 species. Ramies popularity as atextile fiber has been limited largely by regions of produc-tion and a chemical composition that has required moreextensive pre-treatment than is required of the other com-mercially important bast fibers.

Ramie fiber reinforced PP composites were fabricatedusing a hybrid method of melt-blending and injectionmolding processes [119]. Different ramie fiber/PP com-posites were fabricated by varying the fiber length, fibercontent and method of fiber pre-treatment. The results

exhibited increases in fiber length and fiber content alsoshowincreasedtensilestrength,flexural strengthand com-pression strength noticeably in turn. Yet, they also result innegative influences on the impact strength and elongationbehavior of the composites.

Thermoplastic biodegradable composites consisting oframie fibers and a PLA/PCL matrix were manufacturedusing the in situ polymerization method [120]. The effectsof fiber length and content on the tensile and impactstrengths of this natural-fiber-reinforced biodegradablecomposite were discussed, including the influence of asilane coupling agent for improved interfacial adhesion.The results showed that the tensile strength and impactstrength were highest when a silane coupling agent wasemployed, the ramie fiber length was 56 mm and whenthe fiber content was 45wt%.

Bulletproof panels were made from ramie fiber rein-forced composites by hand lay-up process with epoxy asa matrix [121]. These prototype bulletproof panels werebelieved to be lighter in weight and more economical thanconventional bulletproof panels. Conventional bulletproofpanels are made from ceramic plates, kevlar/aramid com-posites, and steel-based material, which are popular inmilitary standard antiballistic equipment today. The bul-let testing results showed that the panels could resist thepenetration of a high-impact projectile (level II) obtainingonly minimal fractures. Level IV ballistic testing showedthat all prototype panels could not resist the high-impactvelocity of theprojectile. Therefore,testsproved that ramiefibers have sufficient breaking strength and toughness forlevel II bullet testing.

Ramie fibers were also reinforced using polyester[122,123], epoxybioresin [124], soy protein [125,126],epoxy [127] and PP [128] for the matrix.

2.2.9. Coir

Coir husk fibers are located between the husk andthe outer shell of the coconut. As a by-product of theproduction of other coconut products, coir production islargely determined by demand. Abundant quantities ofcoconut husk imply that, given the availability of labor andother inputs, coir producers can adjust relatively rapidly tomarket conditions and prices. It is estimated that approx-imately 10% of all husks are utilized for fiber extraction,satisfying a growing demand for fiber and coir products.

Characterization and utilization of coir fiber in coir/natural rubber composites [129], dynamic mechanicalbehavior of coir/natural rubber composites [130], durabil-ity of coir/cement composites [131], the effect acetylationon the coir/epoxy composites [132], the influence of treated coir fiber on the physico-mechanical properties ofcoir/PP composites [133,134], and the effect of fiber phys-ical, chemical and surface properties on the thermal andmechanical properties of coir/PP composites were investi-gated and evaluated [135].

2.2.10. Bamboo

Bamboo (Bambusa Shreb.) is a perennial plant, whichgrows up to 40 m in height in monsoon climates. Generally,it is used in construction, carpentry, weaving and plaitingetc. Curtains made of bamboo fiber can absorb ultraviolet


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radiation in various wavelengths, making it less harmful tohuman body.

The development of composites for ecological pur-poses(eco-composites) using bamboo fibersand theirbasicmechanical properties were evaluated [136]. The steamexplosion technique was applied to extract bamboo fibersfrom raw bamboo trees. The experimental results showedthat the bamboo fibers (bundles) had a sufficient specificstrength, equivalent to that of conventional glass fibers.The tensile strength and modulus of PP based compositesincreased about 15 and 30% when using steam-explodedfibers. This increase was due to good impregnation and areduction of the number of voids, in comparison to com-posites using fibers that were mechanically extracted.

Themechanical and thermal properties of bamboo fiberreinforced epoxy composites [137], the effects of fiberloading, coupling and bonding agents on the mechani-cal properties of bamboo fiber/natural rubber composites[138,139], the isothermal crystallization kinetics of modi-fied bamboo cellulose/PCL composites [140], the influenceof environmental aging on the mechanical properties ofbamboo/glass fiber reinforced PP hybrid composites [141],and the effect of ceramic fillers on mechanical propertiesof bamboo fiber/epoxy composites [142] were evaluated.

2.2.11. Rice husk

Rice is only one of the large group of cereal grains thatcan be used to produce hull fibers. Nowadays wheat, corn,rye, oats and other cereal crops are used to produce fibersand investigating to reinforce in composites area.

Ismail et al. [143147] studied the mechanical prop-erties of rice husk filled polymer composites and theirrelation to fiber loading, coupling agent, processability,hygrothermal aging, and hybridization effect.

Other studies have focused on: flame retardant prop-erties of rice husk/PE composites [148], the using of ricehusk as filler for rice husk/PP composites [149], the ther-mogravimetric analysis of rice husk filled HDPE and PPcomposites [150], the enhancement of the processabil-ity of rice husk/HDPE composites [151], the effect of thepercentage of rice husk content, hydroxyl groups andsize on the flexural, tensile, and impact properties of ricehusk/polyurethane composites [152], nonlinear viscoelas-tic creep characterization of HDPE-rice husk composites[153], the effect of the rice husk size and composition onthe injection molding processability of rice husk/PE com-posites [154], photocatalytic performance of a carbon/TiO

2composite with rice husk [155], the effect of different con-centrations and sizes of particles of rice husk ash-in themechanical properties of rice husk/PP composites [156],the effect of different coupling agent on rice husk/co-polymer PP composites [157], dimensional properties ofrice husk/unsaturated polyester composites [158], andcar-bon/silica composites fabricated from rice husk by meansof binder-less hot-pressing [159].

2.2.12. Oil palm

Oil palms (Elaeis) comprise two species of theArecaceaeor palm family. Nowadays, oil palm empty fruit bunchfibers possess potential as a reinforcement fiber for plastic.Thomas et al. [160,161] investigated the stress relaxation

behavior of phenol formaldehyderesinsreinforced with oilpalm empty fruit bunch fibers and hybrid composites com-posed of oil palm fibers and glass fibers. The effects of fiberloading, fiber treatment, physical ageingand strainlevel onthe stress relaxation behavior were examined and the rateof relaxation at different time intervals was calculated inordertoexplaingradualchangesinrelaxationmechanisms.

Abu Baker et al. [162,163] investigated the effect of oilpalm empty fruit bunch and acrylic impact modifier onmechanical properties and influence of oil extraction ofoil palm empty fruit bunch on the processability of PVCcomposites.

The effects of oil extraction, compounding techniquesand fiber loading [164], and the effect of matrix modi-fication [165] on the mechanical properties of oil palmempty fruit bunch filled PP composites were examined.In addition, a comparative study of oil palm empty fruitbunch fiber/PP composites and oil palm derived cellu-lose/PP composites [166] were studied. The influence ofchemical modification on oil palm/phenol formaldehydecomposites [167], a comparison of epoxy and polyestermatrixes reinforced with oil palm fibers [168], dielectricrelaxation properties of oil palm/polyester resin compos-ites [169], the effect of palm tree fiber orientation ondynamic electrical properties of palm tree fiber-reinforcedpolyester composites [170] have also been investigated.

2.2.13. Bagasse

Bagasse is the fibrous residue which remains aftersugarcane stalks are crushed to extract their juice. It iscurrently used as a renewable natural fiber for the man-ufacture of composites materials.

The compression and injection molding processes wereperformed in order to evaluate which is the better mix-ing method for fibers (sugarcane bagasse, bagasse celluloseand benzylated bagasse) and PP matrixes [171]. The injec-tion molding process performed under vacuum proved towork best. Composites were obtained with a homogeneousdistribution of fibers and without blisters. Although, thecomposites did not have good adhesion between the fiberand the matrix according to their mechanical properties.

The effect of botanical components of bagasse on thesetting of bagasse/cement composites [172], design andoptimization of PHB/bagasse fiber composites [173], tribo-logical properties of bagasse/polyester composites [174],creep properties of bagasse fiber reinforced PVC and HDPEcomposites [175], bagasse/HDPE composites for radia-tion shielding [176], molecular mobility information ofbagasse fiber and their distribution in bagasse/EVA com-posites [177], silane treated bagasse fiber reinforcementfor cementitious composites [178], using polyurethanefrom castor oil [179], durability of bagasse/PP compositesexposed to rainbow fungus [180], and ecodesign and lifecycle assessment as strategy for automotive componentsfrom bagasse/PP composites [181] were examined.

2.3. Structure and chemical composition

Climatic conditions, age and the degradation processinfluencenotonlythestructureoffibers,butalsothechem-ical composition.The major chemical componentof a living


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Table 2

Chemical composition of some common natural fibers.

Fiber Cellulose (wt%) Hemicellulose (wt%) Lignin (wt%) Waxes (wt%)

Bagasse 55.2 16.8 25.3 Bamboo 2643 30 2131 Flax 71 18.620.6 2.2 1.5Kenaf 72 20.3 9 Jute 6171 1420 1213 0.5

Hemp 68 15 10 0.8Ramie 68.676.2 1316 0.60.7 0.3Abaca 5663 2025 79 3Sisal 65 12 9.9 2Coir 3243 0.150.25 4045 Oil palm 65 29 Pineapple 81 12.7 Curaua 73.6 9.9 7.5 Wheat straw 3845 1531 1220 Rice husk 3545 1925 20 1417Rice straw 4157 33 819 838

tree is water. However, on dry basis, all plant cell wallsconsist mainly of sugar based polymers (cellulose, hemi-

cellulose)thatarecombinedwithligninwithlesseramountof extractives, protein, starch and inorganics. The chemi-cal components are distributed throughout the cell wall,which is composed of primary and secondary wall layers.The chemical composition varies from plant to plant, andwithin different parts of the same plant. Table 2 [182184]shows the range of the average chemical constituents for awide variety of plant types.

2.4. Properties

The properties of natural fibers differ among citedworks, because different fibers were used, different

moisture conditions were present, and different testingmethods were employed. Thenaturalfiber reinforced poly-mer composites performance depends on several factors,including fibers chemical composition, cell dimensions,microfibrillar angle, defects, structure, physical properties,and mechanical properties, and also the interaction of afiber with the polymer. In order to expand the use of natu-ral fibers for composites and improved their performance,it is essential to know the fiber characteristics.

The range of the characteristic values, as one of thedrawbacks for all natural products, is remarkably higherthan those of glass-fibers, which can be explained by

differences in the fiber structure due to the overall envi-ronment conditions during growth. Natural fibers can be

processed in different ways to yield reinforcing elementswith different mechanical properties. Mechanical proper-ties of natural fibers can be influenced by many factors.Such as either fiber bundles or ultimate fiber is beingtested. Table 3 [182,183] presents the important physico-mechanical properties of commonly used natural fibers.

The hydrophilic nature of fibers is a major problem forall cellulose-fibers if used as reinforcement in plastics. Themoisture content of the fibers is dependent on the contentof non-crystalline parts and the void content of the fibers.Overall, the hydrophilic nature of natural fibers influencesthe mechanical properties. Table 4 [185] shows the equi-librium moisture content of some natural fibers.

The moisture content at a given relative humidity canhave a great effect on the biological performance of a com-posite made from natural fibers. For example, a compositemade from abaca fibers would have much greater moisturecontent than would a composite made from flax fibers.

The physical properties of each natural fiber are crit-ical, and include the fiber dimensions, defects, strengthand structure. There are several physical properties thatare important to know about for each natural fiber beforethat fiber can be used to reach its highest potential. Fiberdimensions, defects, strength, variability, crystallinity, andstructure must be taken into consideration.

Table 3Physico-mechanical properties of natural fibers.

Fiber Tensile strength (MPa) Youngs modulus (GPa) Elongation at break (%) Density [g/cm3]

Abaca 400 12 310 1.5Bagasse 290 17 1.25Bamboo 140230 1117 0.61.1Flax 3451035 27.6 2.73.2 1.5Hemp 690 70 1.6 1.48Jute 393773 26.5 1.51.8 1.3Kenaf 930 53 1.6 Sisal 511635 9.422 2.02.5 1.5Ramie 560 24.5 2.5 1.5Oil palm 248 3.2 25 0.71.55Pineapple 400627 1.44 14.5 0.81.6Coir 175 46 30 1.2Curaua 5001150 11.8 3.74.3 1.4


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Fig. 3. Illustration of a single fiber cell.Adopted from [186] Copyright 2001. By permission from Springer Publishing.

Knowledgeaboutfiberlengthandwidthisimportantforcomparing different kinds of natural fibers. A high aspectratio (length/width) is very important in cellulose basedfiber composites as it give an indication of possible strengthproperties. The fiber strength can be an important factor inselecting a specific natural fiber for a specific application.Changesinthephysicalpropertiescanbeduetodifferencesin fiber morphology. Major differences in structure such asdensity, cell wall thickness, length and diameter result indifferences in physical properties. It is also interesting tonote that the morphology of the land plant fibers is veryfrom different to that of water plant fibers.

In between fiber qualities, the cellulose content andspiral angle is differed from fiber to fiber and also in a sin-gle fiber design. Natural fiber is a composite of the threepolymers (cellulose, hemicelluloses and lignin), in whichtheunidirectionalcellulosemicrofibrils constitute therein-forcing elements in the matrix blend of hemicellulose andlignin. The structure of such a fiber was built as multi-ply construction with layers P, S1, S2 and S3 of cellulosemicrofibrilsatdifferentanglestothefiberaxis(Fig.3) [186].

Depending on the single fiber cell layer, the differentspiral angle of those layers will have a pronounced influ-ence on the properties of the fiber. The elastic modulusdependency on the spiral angle of the fiber is determined

Table 4

Theequilibriummoisture contentof differentnatural fibersat 65%relativehumidity (RH) and 21 C.

Fiber Equilibrium moisture content (%)

Sisal 11Hemp 9.0Jute 12Flax 7Abaca 15Ramie 9Pineapple 13Coir 10Bagasse 8.8

Bamboo 8.9

(Fig. 4) assuming that the fibers (holocellulose fibers) werelignin-free with a cellulosecontent of 65 wt%(which is typ-ical cellulose content for natural fibers [186]). The relativethickness of the different layers were chosen to be P = 8%,S1=8%,S2=76%,andS3=8%.Itisseenthattheelasticmod-ulus decreases with increasing spiral angle.

Crystallinity values of natural fibers vary in differentparts of the plant. The crystallinity tends to decrease asthe plant matures, but the difference between bast andcore fibers is inconclusive. The permeability of kenaf coreis highest and is closely followed by cotton.

The effect of transcrystallinity on the interface in flaxfiber (green flax, dew-retted flax, Duralin-treated flaxand stearic acid-treated flax fiber)/isotactic PP compositeshas been investigated [187,188]. A significant strengthen-ing of the interface was observed when transcrystallinitywas present. Surface modification (stearation) of flaxfiber also induced transcrystallinity. The interfacial shearstrength increased from 12.75 MPa for the samples with-out transcrystallinity to 23.05MPa for the transcrystallizedsamples according to the single fiber fragmentation test.

Fig. 4. Correlation between spiral angle and elastic modulus of a naturalfiber.

Adopted from [186]. Copyright 2001. By permission from Springer Pub-lishing.


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The chemical composition varies from plant to plant,and within different parts of the same plant. It also varieswithin plants from different geographical regions, ages, cli-mate and soil conditions.

The chemical properties are influenced by the fibergrowth time (days after planting), the botanical classi-fication of the fiber and the stalk height. The chemicalcomposition can also vary within the same part of a plant.Both the root and stalk core have a higher lignin contentthan that of the fibers.

2.5. Summary

There are thousands of different fibers in the world andin fact only few of these fibers have been studied. Mostresearch has been carried out to study the potential useof natural fibers for technical applications and this reviewpaper has only covered the most widely studied and usednatural fibers in reinforced composites materials. Amongthe most popular natural fibers; flax, jute, hemp, sisal,ramie, and kenaf fibers were extensively researched andemployed in different applications. But nowadays, abaca,pineapple leaf, coir, oil palm, bagasse, and rice husk fibersare gaining interest and importance in both research andapplications due to their specific properties and availabil-ity.

It should be mentioned that there are also shortcom-ings; a lack of consistency of fiber qualities, high levels ofvariability in fiber properties related to the location andtime of harvest, processing conditions, as well as theirsensitivity to temperature, moisture and UV radiation. Amulti-step manufacturing process is required in order toproduce high quality natural fibers, which contributes tothe cost of high-performance natural fibers as well as theimproved mechanical properties of the composites.

3. Modification of natural fibers

The main disadvantages of natural fibers in reinforce-ment to composites are the poor compatibility betweenfiber and matrix and their relative high moisture absorp-tion. Therefore, natural fiber modifications are consideredin modifying the fiber surface properties to improve theiradhesion with different matrices. An exemplary strengthand stiffness could be achieved with a strong interfacethat is very brittle in nature with easy crack propagationthrough the matrix and fiber. The efficiency of stress trans-fer from the matrix to the fiber could be reduced with aweaker interface.

3.1. Physical method

Physical methods include stretching, calendaring, ther-motreatment, and the production of hybrid yarns for themodification of natural fibers. Physical treatments changestructural and surface properties of the fiber and therebyinfluence the mechanical bonding of polymers. Physicaltreatments do not extensively change the chemical com-position of the fibers. Therefore the interface is generallyenhanced via an increased mechanical bonding betweenthe fiber and the matrix.

3.1.1. Corona treatment

Corona treatment is one of the most interesting tech-niques for surface oxidation activation. This processchanges the surface energy of the cellulose fibers. Coronadischarge treatment on cellulose fiber and hydrophobicmatrix was found to be effective for the improvement ofthe compatibilization between hydrophilic fibers and ahydrophobic matrix.

Tossa jute fibers [189] were corona discharge treated toimprove the mechanical properties of natural fiber/epoxycomposites. Corona-treated fibers exhibited significantlyhigher polar components of free surface energy withincreasing treatment energy output. Owing to difficultiesin effective treatment of three-dimensional objects withcorona discharge, the increased polarity of treated yarns isrelatively small. Furthermore a decrease in the yarn tenac-ity was observed with an increasing corona energy level.

The mechanical properties of corona treated hempfiber/PP composites were characterized by tensile andcompressive stressstrain measurements [190]. The treat-ment of compounds (fibers or matrix) leads to a significantincrease in tensile strength. The modification of cel-lulosic reinforcements rather than PP matrix allowsgreater improvement of the composites properties with anenhancement of 30% of Young modulus. The observationof the fracture surfaces enables discussion about adhesionimprovement.

Flax and Hemp fiber mats were corona treated andthe optimum length of corona treatment to improve thecomposites (with a natural resin matrix) tensile modulusand strength was determined [191]. The composite ten-sile breaking strength reaches a maximum earlier, namelyafter only 5 min of corona treatment and declines rapidlyafterwards. The fiber appearance confirmed that relativelyshort periods of corona treatment are more than suffi-cient to improve adhesion in the fibers. In addition, it wasalso proven that there is no further improvement achievedwhen longer corona treatments are applied, because thesurface opening of the fiber does not improve further after15min of treatment. However, thedegradationof thefibersproceeds further.

3.1.2. Plasma treatment

Plasma treatmentis another physical treatmentmethodand is, similar to corona treatment. The property of plasmais exploited by the method to induce changes on the sur-face of a material. A variety of surface modifications canbe achieved depending on the type and nature of the gasesused. Reactivefree radicals andgroups canbe produced,thesurface energy can be increased or decreased and surfacecross-linking can be introduced.

Polyester composites reinforced with flax fibers, whichwere submitted to helium cold plasma treatment, wereinvestigated by meansof water permeation measurementsand mechanical tests [192]. The analysis of the permeationand mechanical results showed that plasma treatmentimproves the fiber/matrix adhesion and enhances themechanical property stiffness.

Martin et al. [193] prepared sisalHDPE compositesand showed that some improvements of the mechanical


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properties of the composites are achieved due to theplasma treatments.

Jute fibers were subjected to oxygen plasma treatmentasafunctionoftheplasmapowerinordertoenhanceinter-facial adhesion [194]. The interlaminar shear strength ofplasma treated jute fiber/HDPE composites, which wereexposed to plasma powers of 30W and 60W, increasedby approximately 32 and 47%, respectively compared tountreated composites. In turn, the flexural strength ofuntreated composites increases by only 45%. It can be con-cluded that exposure to plasma powers of 60W for 15minin the studied range is the most suitable parameter foroxygen plasma treatment of jute fibers and enables theachievement of the best interfacial adhesion with HDPE.

The influence of plasma treatment on the morphology,wettability, and fine structure of jute fibers and its impacton the interfacial adhesion of jute fibers/unsaturatedpolyester were investigated [195]. A scanning electronmicroscopic micrograph showed the rough surface mor-phology and degradation of fiber due to an etchingmechanism caused by the plasma. Plasma treatmentresulted in the development of hydrophobicity in fibers.However, among all treated fiber composites, the flexu-ral strength of composites prepared with fibers treated for10min with plasma only showed an improved mechanicalstrength of approximately 14% in comparison to raw fibercomposites.

Jute fibers were treated indifferent plasma reactors(radio frequency RF and low frequency LF plasma reac-tors) using O2 for different plasmapowers to increase theinterface adhesion between the jute fiber and polyestermatrix. The interlaminar shear strength increased from11.5MPa for the untreated jute fiber/polyester compositeto 19.8 and 26.3MPa for LF and RF oxygen plasma treatedjute fiber/polyester composites, respectively. The tensileand flexural strengths improved of jute fiber/polyestercomposites for both plasma systems. It is clear that oxy-gen plasma treatment of jute fibers by using RF plasmasystem showed greater improvement on the mechanicalproperties of jute/polyester composites compared to usingLF plasma system [196].

3.2. Chemical method

Cellulose fibers, which are strongly polarized, are inher-entlyincompatiblewithhydrophobicpolymersduetotheirhydrophilic nature. In many cases, it is possible to inducecompatibilityin twoincompatible materialsby introducinga third material that has properties intermediate betweenthose of the other two. There are several coupling mecha-nisms in materials (e.g., weak boundary layers, deformablelayers, restrained layers, wettability, chemical bonding,and acidbase effect).

The development of a definite theory for the mecha-nism of bonding using coupling agents in composites is acomplex problem. Themainchemicalbonding theory aloneis not sufficient. So the consideration of other conceptsappears to be necessary. These include the morphologyof the interphase, the acidbase reactions in the interface,surface energy and the wetting phenomena.

Chemical modifications of natural fibers aimed atimproving the adhesion within the polymer matrix usingdifferent chemicals were investigated.

3.2.1. Silane treatment

The surface energy of fibers is closely related to thehydrophilic nature of the fiber. Some investigations areconcerned with methods to decrease hydrophilicity. Silanecoupling agents may contribute hydrophilic properties tothe interface, especially when amino-functional silanes,such as epoxies and urethanes silanes, are used as primersfor reactive polymers. The primer may supply much moreamine functionality than can possibly react with the resinin the interphase. Those amines, which could not react, arehydrophilic and therefore responsible for the poor waterresistance of bonds. An effective way to use hydrophilicsilanes is to blend them with hydrophobic silanes such aspheniltrimethoxysilane. Mixed siloxane primers also havean improved thermal stability, whichis typical for aromaticsilicones.

Thesurfaceofthekenaffiberwasmodifiedusingasilanecoupling agent to promote adhesion with the PS matrix[197]. The fibermatrix adhesion increased through a con-densation reaction between alkoxysilane and hydroxylgroups of kenaf cellulose. Due to the fiber modification,kenaf/PS compositesexhibited higher storage modulus andlower tan than those with untreated fiber indicating agreaterinterfacialinteractionbetweenthematrixresinandthe fiber.

Silane treatment improved the storage modulus ofabaca fiber reinforced polyester composites [198]. Chemi-cal modificationswith silaneA174 (-methacryloxypropyltrimethoxy-silane) and also with NaOH resulted in max-imum increase of the modulus. The hydrogen-bonddonating acidity is found to be the lowest for fibers treatedwith silane A174, after pre-treatment with 0.5% NaOH. Onthe contrary, the highest value was noted for fibers treatedwith silane A151 (vinyl triethoxysilane). The overall polar-ity was found to be at a maximum for fibers treated withsilane A151 (vinyl triethoxysilane).

The effects of different chemical treatments on thefibermatrix compatibility in terms of the surface energyand the mechanical properties of composites werereported [199]. The composites were compounded withtwo kinds of flax fibers (natural flax and flax pulp)and a PP matrix. The applied treatments were vinyltrimethoxy silane (VTMO), maleic anhydride (MA) andmaleic anhydride-polypropylene copolymer (MAPP). Thethree treatments reduced the polar component of thesurface energy of the fiber. Composites containing MAPP-treated fibers possessed the highest mechanicalproperties,while the MA and VTMO-treated fibers obtained similarvalues to those of the untreated ones.

Ismailetal. [200,201] treatedoilpalmemptyfruitbunchand coir fibers with silane coupling agents and found thatwith the addition of silane, the scorch time and cure timeincreased. Furthermore, the tensile strength, tensile mod-ulus, tear strength, fatigue life and hardness of oil palmempty fruit bunch and coir fiber reinforced polyester andnatural rubber composites were enhanced.


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Table 5

Variation of the structure and properties of hemp Yarn (hemp 10/1, Rohemp) due to mercerization (22% NaOH).

Mercerizationtemperature time (C)

Mercerization(min)

Stress at themercerization

Degree of conversioncellI incell II(%)

Tensile strength(cN/tex)

Elongation atbreak (%)

Tension modulus(cN/tex)

Untreated 29.8 3.15 11484 60 Isometric 50 28.5 2.34 1617

10 60 Isometric 50 24.9 2.45 121420 60 Isometric 50 28.2 2.27 1701

10 40 Free shrinkage 70 17.6 6.86 35710 60 Constant load (100 g) 40 29.2 2.60 1499

Note: Distance of clamped support = 500mm, test speed= 250mm/min.

3.2.2. Alkaline treatment

Alkaline treatment or mercerization is one of the mostused chemical method(removesa certainamount of lignin,wax and oils covering the external surface of the fiber cellwall) for natural fibers when used to reinforce thermoplas-tics and thermosets. The important modification achievedwith alkaline treatment is the disruption of the hydrogenbonding in the network structure, thereby increasing thesurface roughness.

The effect of alkali treatment on the wetting ability andcoherence of sisalepoxy composites has been examined[202]. Treatment of sisal fiber in a 0.5 N solution of sodiumhydroxide resulted in more rigid composites with lowerporosity and higher density. The treatment improved theadhesioncharacteristics,duetoimprovedworkofadhesionbecause it increases the surface tension and surface rough-ness. The resulting composites showed improvements inthe compressive strength and water resistance. It was alsodiscovered that the removal of intracrystalline and inter-crystalline lignin and other waxy surface substances by thealkali substantially increases the possibility for mechanicalinterlocking and chemical bonding.

The effects of alkali treatment of pineapple leaf fiber ontheperformance of thepineapple leaf fiber/PLA compositeshave been shown [203]. It was found that alkali-treatedfiber reinforced composite offered superior mechanicalproperties compared to untreated fiber reinforced com-posites. This study also suggested that the appropriatemodification of fiber surfaces significantly contributes toimproving the interfacial properties of the biocomposites.Since improvement of the interfacial adhesion increasesthe impact performance, surface properties of fibers can begreatly modified in terms of different treatments as char-acterized by an increased surface energy, which improveswettability of the fiber when compounding with a PLAmatrix. DMA showed that treated fiber composites have ahigher storage modulus, which indicates greater interfacialbond strength and adhesion between the matrix resin andthe fiber. The alkalized fiber composites have high storagemodulus values, which correspond with their high flexuralmodulus.

The effect of alkylation on the tensile properties oframie fiber was explored [204]. The load application tech-nique was employed during alkali treatment, in order toimprove the mechanical properties of the fiber. The ramiefiber was alkali-treated with a solution containing 15%NaOH and was subjected to applied loads of 0.049 and0.098 N. The results showed that the tensile strength of thetreated ramie fiber was by improved 418% more than for

untreated ramie fibers. The treated fibers Youngs modu-lusdecreased. It should be noted that fracture strains of thetreated ramie fiber drastically increased to 0.0450.072.That equals twice to three times higher than those of theuntreated ramie fiber. It was considered that such prop-erty improvements upon alkylation were correlated witha change of the morphological and chemical structures inmicrofibrils of the fiber.

Jute fibers were subjected to alkali treatment with asolution containing 5% NaOH for 0, 2, 4, 6 and 8h at 30 C[205]. The modulus of the jute fibers improved by 12%, 68%and 79% after 4, 6 and 8h of treatment, respectively. Thetenacity of the fibersimproved by 46% after 6 and 8 h treat-ment and the percentage of breaking strain was reducedby 23% after 8 h treatment. For 35% of the composites withfiber treated for 4 h, the flexural strength improved by 20%from 199.1 to 238.9 MPa. The flexural modulus improvedby 23% from 11.89 to 14.69GPa and the laminar shearstrength increased by 19% from 0.238 to 0.283 MPa.

Ramie fiber [206], green coconut fiber [207], sisal [208],jute [209] and coir [210] fibers were alkalized and theirproperties evaluated.

The variation of mechanical and structural propertiesof hemp yarn upon alkylation in dissimilar conditions wasinvestigated [211]. Alkylation with a rather high alkali con-centration (22%) never led to a complete lattice conversionof cellulose I in cellulose II (Table 5). As expected, the high-est degree of conversion (70%) was reached by a freelyshrinking yarn. Further, the alkylation of hemp yarn underisometric conditions or with constant fiber load (100 p) ata NaOH concentration of 22% results in a crystalline lat-tice conversion of only 4050%. However, this occurs onlywhen combined with the highest strengths and moduli ofthe fiber bundles. It was also observed that only the ten-sile modulus of elasticity exceeds the value of the originalunmodified material.

Table 6

Influence of different fiber treatments on flexural strength of unidirec-tional flaxPP composites.

Fiber treatment Flexural strength (N/mm2)

Untreated 77Mercerizeda 115MAHPPb 127Mercerized + MAHPP 149

Note: Lengthwise, fiber content: 36vol%.a

29% NaOHat 20

C, 20min, isometric.b 1% Licomont AR 504 in toluene, 10min.


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Fig. 5. Influence of acetylation of flax fiber on the degree of polymeriza-tion and degree of crystallinity of cellulose.Adopted from [212]. Copyright 2008. By permission from Budapest Uni-versity of Technology.

Table 6 [211] explicitly shows that, besides the signif-icant effects of single alkylation and MAHPP treatments,a combination of the isometric alkylation procedure (e.g.,increasing fiber strength) and coupling agents results in

the highest increase of the composite properties (90%) incomparison to a composite with untreated flax fibers forreinforcement.

3.2.3. Acetylation

Acetylation is another method of modifying the surfaceof natural fibers and making them more hydrophobic. Itdescribes the introduction of an acetyl functional groupinto an organic compound. The main idea of acetylationis to coat the OH groups of fibers which are responsiblefor their hydrophilic character with molecules that have amore hydrophobic nature.

The influence of acetylation on the structure and prop-erties of flax fibers were investigated. Modified flax fiberreinforced PP composites were also prepared [212]. Theeffect of acetylation on the degree of polymerization andcrystallinity of flax fiber is illustrated in Fig. 5.

It was observed that the degree of polymerizationslowly decreased while the degree of acetylation increaseduntil 18%. Once reaching an acetyl content of 18% acetylcontent, the degree of polymerization decreased rapidlydueto vigorousdegradationof cellulose. It wasalso notablethat the degree of crystallinity increased a little bit regard-ing the degree of acetylation, because of the removal oflignin and extractibles. Afterthat, thedegreeof crystallinityof the cellulose decreased with respect to the degreeof acetylation owing to the decomposition of acetylatedamorphous components on the cellulose surface.

The influences of the degree of acetylation of flax fiberson the moisture absorption properties are illustrated inFig. 6 [212]. At 95% RH, the 3.6%, 12%, 18% and 34% degreeof acetylated flax fibers showed about 14%, 18%, 27% and42% lower moisture absorption properties than untreatedflax fiber respectively. The moisture absorption propertiesdecreased proportionally with the increase of acetyl con-tent of fibers due to the reduction of hydrophilicity of thefibers.

Seena et al. investigated the effect of acetylation onabaca fiber reinforced phenol formaldehyde compositesand reported that the tensile strength, tensile modulus and

Fig. 6. Influence of acetylation of flax fibers on the moisture absorptionproperty at 95% relative humidity.Adopted from [212]. Copyright 2008. By permission from Budapest Uni-versity of Technology.

impact strength were found to improve compared to non-treated abaca fiber composites [213].

Tserki and co-workers [214] investigated the acety-lation of flax, hemp and wood fibers. A removal of thenon-crystalline constituents of the fibers, an alteration ofthe characteristics of the surface topography, a change ofthe fiber surface free energy and an improvement of thestress transfer efficiency in the interface took place.

The effects of acetylation on the biological degradationand shear strength of coir and oil palm fiber reinforcedpolyester composites were evaluated [215,216]. It wasreported that acetylated natural fiber reinforced polyestercomposites exhibit higher bio-resistance and less tensilestrength loss compared to composites with silane treatedfiber in biological tests.

Zafeiropoulos and co-workers extensively investigatedthe influence of acetylation on the engineering and char-acterization of the interface in flax fiber/PP composites[217,218]. Furthermore, they also investigated the applica-tion of Weibull statistics [219] and Gaussian statistics [220]to the tensile strength of flax fibers.

3.2.4. Maleated coupling

Nowadays, maleated coupling is widely used tostrengthen natural fiber reinforced composites. The funda-mental difference with other chemical treatments is thatmaleic anhydride is not only used to modify fiber surfacebut also the polymeric matrix to achieve better interfa-cial bonding in between fiber and matrix and improvedmechanical properties in composites. There are numerouspublished studies in which the effect of maleic anhydridegrafting on the mechanical properties of natural fibers hasbeen investigated and it is impossible to list all of themhere. Hence, only a few representative studies are dis-cussed here.

Mohanty et al. [221] used MAPP as a coupling agentfor the surface modification of jute fibers. It was foundthat a fiber loading of 30% with a MAPP concentrationof 0.5% in toluene and 5min of impregnation time with6 mm average fiber lengths resulted in the best possibleoutcomes. An increase in flexural strength of 72.3% wasobserved for the treated composites. In addition to thePP matrix, Mishra et al. [222] reported that maleic anhy-dride treatment reduced the water absorption to a greatextent in abaca, hemp and sisal fiber-reinforced novolaccomposites. Mechanical properties like Youngs modulus,


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Fig. 7. Micrograph of abaca fiber surface morphology (a) unmodified (b) NDS modified (c) fungamix modified.

Adopted from [233]. Copyright 2010. By permission from Elsevier Ltd.

flexural modulus, hardness and impact strength of plantfiber-reinforced composites increased after maleic anhy-dride treatment.

The effects of MAHPP coupling agents on rice-huskflour reinforced PP composites were evaluated [223]. Thetensile strengths of the composites decreased as the fillerloading increased, but the tensile properties were signifi-cantly improved with the addition of the coupling agent.Both the notched and unnotched Izod impact strengthsremained almost the same with the addition of couplingagents. A morphological study revealed the positive effectof a compatibilizing agent on interfacial bonding.

Abaca fiber reinforced composites based onHDPE/Nylon-6 blends were prepared [224]. Maleicanhydride grafted styrene/ethylenebutylene/styrene tri-block polymers (SEBS-g-MA) and maleic anhydride graftedpolyethylene (PE-g-MA) were used to enhance the impactperformance and interfacial bonding between the abacafibers and the resins. By employing SEBS-g-MA, betterstrengths and moduli were obtained for HDPE/Nylon-6based composites than for corresponding HDPE basedcomposites. It was found that the addition of SEBS-g-MAhad a positive influence on the reinforcing effect of theNylon-6 component in the composites. Thermal analysisresults showed that fractionated crystallization of the

Nylon-6 component in the composites was induced by theaddition of both SEBS-g-MA and PE-g-MA. The thermalstability of both composite systems differed slightly,except for an additional decomposition peak, which isrelated to the minor Nylon-6 for the composites from theHDPE/Nylon-6 blends. In combination with SEBS-g-MA,the addition of Nylon-6 and the increased fiber loadinglevel led to an increase in the water absorption value ofthe composites.

The influence of MAHPP on the fibermatrix adhesionin jute fiber-reinforced PP composites and on the materialbehavior under fatigue and impact loadings was inves-tigated [225]. The fibermatrix adhesion was improvedby treating the fibers surface with the coupling agentMAHPP. It was shown that a strong interface is connectedwith a higher dynamic modulus and reduction in stiffnessdegradation with increasing load cycles and applied max-imum stresses. The specific damping capacity resulted inhigher values for the composites with poor bonded fibers.Furthermore, the stronger fibermatrix adhesion reducedthe loss-energy for non-penetration impact tested com-posites about roughly 30%. Tests, which were performedat different temperatures, showed higher loss energies forcold and warm test conditions compared to those car-ried out at room temperature. The post-impact dynamic


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Fig. 8. Tensile and flexural strength of enzyme treated and unmodifiedabacaPP composites.Adopted from [233]. Copyright 2010. By permission from Elsevier Ltd.

modulus roughly 40% after 5 impact events and was 30%lower for composites with poor and good fibermatrixadhesion, respectively.

The effectiveness of MAH as coupling agent has dis-cussed also on thermal and crystallization properties ofsisal fiber/PP composites [226], tensile properties of hilde-gardia fiber/PP composites [227], wetting behavior of flaxfiber/PP composites [228], transcrystallinity of jutefiber/PPcomposites [229], surface properties and water uptakebehavior of flax, hemp fiber reinforced PP composites [230],effects of micro-sized cellulose based corn fibers as rein-forcement agents in PP composites [231], and dynamicmechanical properties of flax and hemp fiber/PP compos-ites [232].

3.2.5. Enzyme treatment

The use of enzyme technology is becoming increasinglysubstantial for the processing of natural fibers. Currently,the use of enzymes in the field of textile and natural fibermodification is also rapidly increasing. A major reason forembracing this technology is the fact that the applicationof enzymes is environmentally friendly. The reactions cat-alyzed are very specific and have a focused performance[233].

Bledzki et al. [233] investigated PP composites withenzyme treated abaca fibers. The surface morphologies ofenzyme treated and untreated abaca fibers are shown inFig. 7. In Fig. 7a, it was observed that the untreated fibersurface is rough, containing waxy and protruding parts.The surface morphology of treated fibers is observed inFig. 7b and c. If the waxy material and cuticle in the treatedsurface are removed, the surface becomes smoother. Fib-rillation is also known to occur when the binding materialsare removed from the surface of the treated fibers. Fibersurface damage was also observed for naturally digestedfibers which occur in natural digestion systems.

The effects of enzyme treatment of abaca fiber on thetensile and flexural strength are shown in Fig. 8. The tensilestrength of enzyme treated abaca composites was found tohave increased 545% due to modification. Natural diges-tionsystem (NDS) modified abacafiber compositesshowedlittle improvement in comparison to unmodified abacafibercomposites.Thetensilestrengthoffungamixmodifiedcomposites increased by 45% as compared to unmodifiedfiber composites. A conventional coupling agent (MAPP)has a positive effect on the tensile strength. It was found

to improve by 40%. Enzymatic treatment was also investi-gated with hemp fiber [234,235], and flax fibers [236].

3.3. Summary

Though natural fiber reinforced composites have devel-oped significantly over the past few years because of their

low cost, low relative density, and high specific strength,the interfacial adhesion between fiber and matrix is still anissue. Extensive research was carried out and reported inliterature and reviewed in this paper, showing the impor-tance of the interface and the influence of various types ofsurfacemodificationsonthephysicalandmechanicalprop-erties of biocomposites. The observed trend lied with thechemical modification compared to physical modification.It has also been shown that maleated and silane treat-ment is becoming a choicemethod dueto beneficialresults.Additive suppliers improved the additives with higheramounts of anhydride functional groups than previousgrades (used in 1980s and 1990s), which create more sites

for chemical links, resulting in significant performanceimprovement at low additive contents. Using couplingagents reduced the water absorption of the compositesbut has not resulted in decreased long term performance.As described above that enzyme technology to modify thenaturalfibersurface,isincreasingsubstantiallyduetoenvi-ronment friendly. In addition, enzyme technology couldbe cost effective, improved product quality compared tomostly use maleated and silane modification.

4. Matrices for biocomposites

The composites shape, surface appearance, environ-

mental tolerance and overall durability are dominatedby the matrix while the fibrous reinforcement carriesmost of the structural loads, thus providing macroscopicstiffness and strength. The polymer market is dominatedby commodity plastics with 80% consuming materialsbased on non-renewable petroleum resources. Govern-ments, companies and scientists are driven to find analternative matrix to the conventional petroleum basedmatrix through public awareness of the environment, cli-mate change and limited fossil fuel resources. Thereforebiobased plastics, which consist of renewable resources,have experienced a renaissance in the past few decades.Fig. 9 shows that the matrices currently used in bio fiber

composites depend on biobased or petroleum based plas-tics and also on their biodegradability.

4.1. Petrochemical based

The effects of the incorporation of natural fibersin petrochemical based thermoplastics and thermosetmatrixes were extensively studied. Polypropylene (PP),polyethylene (PE), polystyrene (PS), and PVC (polyvinylchloride) were used for the thermoplastic matrixes.Polyester, epoxy resin, phenol formaldehyde, and vinylesters were used for the thermoset matrices and arereportedly the most widely used matrices for natural fiberreinforced polymer composites.


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Fig. 9. Current and emerging plastics and their biodegradability.Adopted from [1].

4.1.1. Thermoplastic

4.1.1.1. PE (polyethylene). Several mechanical properties(deformation, fracture, thermal diffusivity, thermal con-ductivity, and specific heat) of flax fiber/HDPE biocom-posites were evaluated [237,238]. Strength and stiffnessimprovement combined with high toughness can beachieved by varying the fiber volume fraction and con-trolling the bonding between layers of the composite.The thermal conductivity, thermal diffusivity, and spe-cific heat of the flax/HDPE composites decreased withincreasing fiber content, but the thermal conductivityand thermal diffusivity did not change significantly attemperatures in the range (170200C) studied. The spe-cific heat of the biocomposites increased gradually withtemperature.

Traditionally the mechanism of moisture absorption isdefined by diffusion theory; but the relationship betweenthe microscopic structure-infinite 3D-network and themoisture absorption could not be explained. Wang et al.

[239] introduced the percolation theory and a percolationmodel was developed to estimate the critical accessiblefiber ratio; the moisture absorption and electrical conduc-tion behavior of composites. At high fiber loading whenfibers are highly connected, the diffusion process is thedominant mechanism; while at low fiber loading close toand below the percolation threshold, the formation of acontinuous network is key. Hence, percolation is the dom-inant mechanism. The model can be used to estimate thethreshold value which can in turn be used to explain mois-ture absorption and electrical conduction behavior. Fig. 10shows the increment of electrical conductivity with theincreasing moisture content of thecompositewith 65% ricehull fiber loading. The composite started showing conduc-tivity after it absorbed approximately 50% of maximummoisture. After this, conductivity increased quickly withfurther moisture absorption. The pattern of the incrementof electrical conductivity suggests a diffusion process ofmoisture absorption.


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Fig. 10. The moisture absorption and electrical conductivity forHDPErice hull composite with 65% fiber content.Adopted from [239]. Copyright 2006. By permission from Elsevier Ltd.

The hygrothermal weathering properties of rice hull

reinforced HDPE composites were studied by Wang et al.[240]. The samples (50% rice hull and 50% HDPE) absorbed4.5% moisture after 2000h of exposure to a relative humid-ity (RH) of 93% at 40 C. The walls of the samples swelledsignificantly (7.1%) in thickness and ultimately developedabout5mmoflongitudinalbowing(Fig. 11) basedon61cmlong boards. Both expansion and bowing were partiallyrecoveredafteranother2000hexposureto20%RHat40 C.

Deformation results in increased moisture contents.Temperature also played a significant role by causing directthermal expansion or contraction and by affecting the rateand the amount of moisture adsorption.

Sisal fiber reinforced PE [241] and HDPE [242,243]

composites were examined regarding their interfa-cial properties, isothermal crystallization behavior andmechanical properties. Sisal fiber treatment with stearicacid increased the interfacial shear strength by 23%compared to untreated fibers in sisal/PE composites. Per-manganate (KMnO4) and dicumyl peroxide (DCP) roughenthe fiber surface and introduced mechanical interlockingwith the HDPE. Sisal fiber reinforced HDPE shows a stablede-bonding process with Permanganate and DCP treat-ment. Silane treated sisal fiber reinforced HDPE exhibits anunstable debonding process. A high crystallization rate andshortcrystallizationhalftimewereobservedforsisal/HDPEcomposites compared to neat HDPE. This demonstrates the

strong nucleating abilities of sisal fibers. With increasing

fiber content, the crystallization activation energy of thecomposites decreased.

Similar investigations (PE as matrix) were carriedout using soya powder [244], curaua [245], rape straw[246,247], hemp [248], rice straw [249], bagasse [250] ricehull fibers [251] and LDPE as matrixes with wheat straw[252], abaca, bagasse and rice straw fibers [253].

4.1.1.2. PP (polypropylene). The thermo-mechanicalbehavior of hemp fibers was studied with focus on man-ufacturing high performance natural fiber reinforced PPcomposites. Hemp fibers subjected to quasi-static tensileexperiments and temperature-controlled harmonic testsrevealed a complex behavior, involving several mecha-nisms [254]. The hemp fiber behavior was affected bytemperature and acted not only as an activation factor, butalso as a degradation factorwith respect to thevisco-elasticproperties of the fibers. It was discovered that PPhempfiber composites with relatively high performance can beachieved with some specific mechanical properties.

The effect of sisal fiber degradation and aging on thePP/sisal fiber composites was investigated. The fiber matproduction method allowed the fibers to avoid much of theprocessing degradation, which is often encountered dur-ing conventional shear mixing processes [255]. The bestpossible mechanical properties for the sisal fiber/PP com-posites were achieved when the fiber length was greaterthan 10mm and the fiber mass fraction was in the range of1535%. The aged sisal fiber showed lower tenacity, break-ing strength and elongation with in comparison to freshsisal fibers. On the contrary, in sisal/PP composites, agedsisal fiber composites exhibited better mechanical proper-ties than fresh sisal fiber/PP composites [256].

Ranaetal.[257] observed an increase in impactstrengthin jute fiber/PP composites as a result of fiber loading. Itwas also found that both the impact and tensile propertiesshowed increasing trends with the addition of compati-bilizers. However, the opposite was true for the flexuralproperties. Improvements of physico-mechanical proper-ties achieved by post-treatment [258], and the interfacialand durability evaluation [259,260] of jute fiber/PP com-posites were studied.

The environmental performance of hemp fiber rein-forced PP composites based on natural fiber mat thermo-plastic (NMT) was evaluated by quantifying carbon storagepotential and CO2 emissions. The results were compared

with commercially available glass fiber composites [261].

Fig.11. Partial recovery of bowing (a) before recovery: 5.00 mm of bowing (b) after recovery: 3.00 mm of bowing.Adopted from [240]. Copyright 2005. By permission from Elsevier Ltd.


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Fig. 12. CO2 emissions per ton composite and reduction in emissions bysubstituting glass fiber with hemp fibers.Adopted from [261]. Copyright 2003. By permission from Elsevier Ltd.

Fig. 12 shows CO2 emissions in ton/ton of composite forboth natural and glass fibers. These values are estimatedby converting energy into CO2 emissions using standardconversion factors for different fossil fuels. Theheat energy

liberated by incineration of hemp fiber and PP is also addedin non-renewable energy requirements to make calcula-tions simpler. The results demonstrate a net reduction inemissionsof3tonCO2/tonofproductifglassfibersaresub-stituted by hemp fibers.

The potential of wheat straw fibers to be used as rein-forcing additives for PP prepared with the influence ofdifferent processes (mechanical and chemical processes)was investigated [262]. Fibers prepared by mechanicaland chemical processes were characterized with respectto their chemical composition, morphology, physical,mechanical and thermal properties. The fibers prepared bychemical processes exhibited better mechanical, physical

and thermal properties.The morph

review biocomposites natural fibers

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