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Mechanical and Morphological Properties of Chemically Treated Coir-Filled

Polypropylene Composites

Md. Nazrul Islam,* Md. Mominul Haque, and Md. Monimul Huque

Department of Chemistry, Bangladesh UniVersity of Engineering and Technology, Dhaka, Bangladesh

In the present work, coir was chemically treated first with sodium perchlorate and then with 2,4-dinitrophenylhydrazine (DNPH) to improve the mechanical properties of the coir-PP composites. Untreated, oxidized,and DNPH-treated coir samples at different mixing ratios were utilized to prepare the composites. Mechanicalproperties of the composites prepared from both perchlorate and DNPH-treated coir were found to be betterthan those of untreated ones. The tensile strengths of both untreated and treated coir-PP composites decreasedwith an increase in fiber content. However, the values were found to be higher than those of correspondingvalues of untreated ones. Treated coir-PP composites were found to absorb a lower amount of water thanthe untreated ones. To understand why the mechanical properties of composites prepared under differentconditions of coir were different, surface morphologies of the tensile fractured surfaces of the specimenswere recorded using scanning electron microscopy (SEM). The SEM images clearly revealed that there werefewer fiber agglomerations, microvoids, and fiber pull out traces in both perchorate and DNPH-treated coir-PPcomposite than in the untreated one, indicating that better distribution of the fiber into the matrix as well asstronger fiber matrix interfacial adhesion occurred upon treatment of coir.

Introduction

In recent years, significant efforts have been devoted to theuse of agro-based residues as reinforcing fillers in thermoplas-tics. These efforts stem in part from their eco-friendliness,biodegradable nature,1 and worldwide environmental awarenesswith a view to conserve forest resources through reducinguncaring and massive use. From an economic viewpoint, naturalfiber reinforced polymer composites are gaining increasingpopularity as they are low cost, lightweight, require lowprocessing temperature, and reduce wear in processing equip-ment.25 With the new economic dimensions, increasing volumeof agro-based natural fiber has received a wide variety of

industrial uses for manufacturing of housing, interior automotive,and packaging products.68 Other emerging industrial applica-tions of natural fiber reinforced polymer composites includeflower pots, fixtures, furniture, and tiles. Furthermore, sincecomposites prepared by reinforcing natural fibers require lowprocessing temperature, their proper use could unhook theextensive use of fossil fuel, which will consequently reduceenvironmental pollution by lowering the amount of carbondioxide. From the viewpoint of worldwide environmentalawareness, proper use of agro-based natural fiber could reducethe volume of refuse and emission of greenhouse gases,particularly carbon dioxide to the atmosphere that wouldotherwise cause environmental pollution if thrown away or burnt

down.8

The main purpose of incorporation of fillers into thermoplasticpolymer matrixes is to improve the specific physical andmechanical properties of the composites. The factors thatdetermine the physical properties and improve mechanicalstrength of composites are the extent of filler loading, size andshape of the filler, and the filler-matrix interfacial adhesion.9

Therefore, the proper selection of fillers for a particular polymermatrix is an important factor for the improvement of thefiller-matrix interfacial adhesion. Complete fusion of matrix

impregnated with filler through strong interfacial adhesionbetween the two and matrix-to-filler stress transfer efficiencyare the prime requirements for the production of durable andreliable composites having specific mechanical properties thatcan withstand mechanical shocks and dimensional changes dueto moisture absorption.1012 However, the inherent hydrophilicnature of natural fibers does not allow them to couple stronglywith the hydrophobic polymer matrix, resulting in composites

with inferior mechanical properties. To address this drawback,a number of fiber treatment methods have appeared in theliterature.1323 Upon treatment, the hydrophilic nature of thecellulose is significantly reduced, giving better filler-matrix

interfacial adhesion. In this regard, a number of coupling agentspossessing both hydrophilic and hydrophobic parts have alsobeen introduced while preparing composites that can chemicallybridge the hydrophilic filler on one side and facilitate wettingof the hydrophobic matrix with the filler on the other side2

Sanadi et al. reported that the hydrophilic nature of natural fiberreinforced polymer composites can substantially be reduced byacetylation of hydrophilic OH group present in the fiber.2,15

Karmarkar et al.16 studied the wood fiber reinforced PPcomposites and showed that improved mechanical propertiesof the composites can be achieved if a compatibilizer with anisocyanate functional group is introduced between the twocomponents. In our previous reports, we showed that mechanical

properties of natural fiber reinforced polymer composites canbe achieved if the fiber is pretreated with benzene diazoniumsalt21 and post-treated with urotropine.22

In the present study, we endeavored to present the physicaland mechanical properties of chemically treated coir-filledpolypropylene (PP) composites at different treatment conditionsand fiber compositions. Coir is a natural fiber, which is obtainedfrom the husk of matured coconuts. Cellulose is the mainconstituent (43%) of this fiber, which is a hydrophilic glucanpolymer consisting of a linear chain of 1,4-bonded anhydro-glucose unit that contains alcoholic hydroxyl groups.18,19 Here,we have shown a two-step fiber treatment technique to modifythe hydrophilic nature of coir, which is a substantial extension

* To whom correspondence should be addressed. Tel.: +88-01715784778 (extension 7340). Fax: +880-2-863046. E-mail:islammdn@chem.buet.ac.bd.

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to our previous studies.2023 The main objective of chemicaltreatment of coir used in the present study is to improve itsinteraction with the PP matrix. This will consequently improvethe mechanical properties of the composite and reduce the

microvoids in the composites as well as the hydrophilic natureof coir responsible for moisture absorption. Water absorptionbehavior of the composites was characterized to understandthe effect of chemical treatment on the hydrophilicity of thecomposites. To understand why the surface properties of thecomposites prepared under different treatment conditionsare different, SEM images of the tensile fractured surface ofthe samples were also recorded.

Materials and Methods

The thermoplastic polymer, polypropylene (PP), used asmatrix material, was supplied by the Polyolefin Company Private

Ltd., in the form of homopolymer pellets. Its specific gravitywas 0.90-0.91, melt flow index was 10 g/10 min, and meltingtemperature was 165-171 C. The coir, used as reinforcingfiller, was obtained from a local coconut oil factory inBangladesh. The chemicals used to treat coir were NaClO4(Merck) and 2,4-dinitrophenyl hydrazine (DNPH) (Merck).

Treatment of Coir. Coir was first treated with an aqueoussolution of NaClO4to produce cellulose dialdehyde, which wasfurther treated with DNPH to obtain the adduct as shown inScheme 1. Before chemical treatment, coir was cleaned and thendried in an oven at 105 C for 24 h to obtain 1-2% moisturecontent. The dried coir was then kept in a sealed container. Afteroxidation, coir fiber was washed and then dried in air. Forcoupling reaction, DNPH was dissolved in an ethanol/water

mixture in a 1000-mL beaker. The pH of the solution wasadjusted to 3 by adding H2SO4. Coir (500 g) was then immersed

into the solution for about 4 h at 70 C for coupling reactionwith DNPH. After the reaction, coir was taken out of the beaker,washed with distilled water, and finally dried in open air.

Fabrication of Composites and Test Specimens. Coir processed

as mentioned above was initially mixed thoroughly with PPgranules at 10/90, 15/85, 20/80, and 25/75 wt % mixing ratios.The mixture was then passed through an extruder at a constanttemperature of 165 ( 5 C. The extruded composites were cutinto 2-4-cm-long pieces. All the pieces were then crushed intosmaller granules using a grinding machine. The granules weredried in a vacuum oven at 65 C for 1 h and then fed into aninjection molding machine for making specimens. The speci-mens for tensile and flexural tests were prepared from driedgranules using the injection-molding machine at a moldingtemperature of 165 C. Details of experimental procedure andtests of the specimens can be found elsewhere.21

Water Absorption. To measure the water uptake capacityof the composites, rectangular specimens of dimensions 39 mm 10 mm 4.1 mm were prepared. The specimens were driedin an oven at 105 C, cooled in a desiccator using silica gel,and immediately weighed. The water absorption tests werecarried out by immersing the specimens in a water bath for 24 hat room temperature. After immersion, the excess water wasremoved using a piece of soft cloth and final weight of thespecimens was taken. From the difference of the final and initialweights percentage of water uptake was calculated.

Infrared Spectra. The infrared spectra of raw and treatedcoir were taken on a Shimadzu FT-IR 81001 spectrophotometerwith coaddition of 64 scans at a resolution 4 cm-1 to characterizethe chemical change of coir upon treatment with sodiumperchlorate and DNPH.

Scanning Electron Microscopy (SEM).The morphology ofthe coir-PP composites and interfacial adhesion between the

Scheme 1. Treatment of Coir with Sodium Perchlorate and 2,4-Dinitrophenyl Hydrazine

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filler and the PP matrix was examined using a scanning electronmicroscope (JSM-6701F) supplied by JEOL Company Limited.The samples were viewed perpendicular to the fractured sur-faces. The micrographs were taken at a magnification of 300.

Results and Discussion

Tensile Properties.In the present study, surface modificationof the coir fiber was carried out to achieve better mechanicalproperties of composites and the results were compared withthose of the untreated ones. The presence of hydroxyl groupsof the cellulose in coir is responsible for its inherent hydrophilicnature. As a result, it becomes difficult to compound it withthe hydrophobic polymer matrix, resulting in poor performance

in the mechanical properties as well as dimensional change offurnished products due to moisture absorption of the composite.To overcome these problems, coir was chemically treated firstwith sodium perchlorate and then with DNPH. Scheme 1 showsthe chemical changes of the cellulose in coir upon a two-steptreatment with sodium perchlorate and DNPH. Upon treatmentwith perchlorate, the hydroxyl groups at C2 and C3 and C6 aretransformed into aldehyde, which further undergoes couplingreaction with DNPH. The corresponding IR spectra of raw andchemically treated coir are shown in Figure 1. The IR spectrumof raw coir shows a band in the region near 1646 cm-1, whichis probably due to the CO group of acylester in hemecelluloseor of aldehyde group in lignin24 The IR spectrum of perchlorate-

treated coir shows a band near 1617 cm

-1

, which is maybe dueto the carbonyl groups produced from the oxidation of thehydroxyl groups of cellulose. On the other hand, DNPH-treatedcoir shows a clear band in the region near 1504 cm-1, which ismaybe due to the stretching frequency of nitro group presentin the aromatic ring of DNPH. These results suggest thatchemical modification of the cellulose has occurred upontreatment with perchlorate and DNPH. Figure 2 shows thetensile properties of raw and treated coir-PP composite as afunction of filler loading. It is clear from the figure that valuesof tensile strengths of raw coir-PP composites graduallydecrease with an increase in filler content. With increasing thecomposition of filler in the composite, weak filler-matrixinterfacial area increases, which consequently results in a

decrease in tensile strength.10 Chemical modification of coir hasreduced the hydrophilic nature of coir by reducing the number

of hydroxyl groups in the cellulose. As a result, interfacialadhesion between the filler and matrix has improved. This inturn improved the tensile strengths of both perchlorate andDNPH-treated coir-PP composites. Significant improvementin tensile strengths is found for DNPH-treated coir-PP com-posites. The increase in tensile strength might be due to the

cross-linking network formation between the filler and thematrix. This indicates that fiber treatment can improve the fiber-matrix interfacial adhesion, leading to better stress transferefficiency from the matrix to the filler with consequent improvedmechanical properties of the composites.

Figure 3 shows the Youngs modulus of the composites atdifferent filler loading. As expected, the addition of fiber in-creases the modulus of the composites, resulting from theinclusion of rigid fiber into the soft PP. This observation suggeststhat the incorporation of rigid filler into the soft thermoplasticPP increases the stiffness of the composite. The chemicallytreated coir-PP composites are found to show higher moduluscompared to those of the untreated ones. This indicates thathomogeneous dispersion of coir particles and better filler-matrix

interaction has occurred upon treatment of coir. It has beenreported that crystallites possess higher modulus compared to

Figure 1. Infrared spectra of raw, oxidized, and DNPH-treated coir.

Figure 2. Tensile strength of coir-PP composites: (1) untreated coir, (2)oxidized coir, and (3) DNPH-treated coir.

Figure 3.Effect of fiber loading on the Youngs modulus of (1) raw coir-,(2) oxidized coir-, and (3) DNPH-treated coir-PP composites.

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amorphous substances16 Upon chemical treatment of coir withperoidate and DNPH surface crystallization of coir probablydominates over its bulk nature. Furthermore, incorporation offiber into the polymer matrix reduces the matrix mobility. As aresult, the modulus of the composites upon treatment withperchlorate and DNPH has increased with an increase in fillercontent.

Flexural Properties.The flexural strength and modulus ofboth untreated and treated coir-PP composites are shownin Figures 4 and 5, respectively. As shown in Figure 4, thevalues of flexural strength of both raw and treated coir-PPcomposites initially increased and showed a steady behaviorwith further increases in the filler content. The steadybehavior of flexural strength of the composites could be abalance in the favorable entanglement of the polymer chainwith the filler and opposing weak filler-matrix interfacialadhesion with increasing filler content. It is evident fromFigure 5 that the addition of coir fiber to PP has significantly

increased the modulus of the composites, which is found tobe in agreement with the results of previous reports.16,25 This

observation suggests that addition of coir fiber into thethermoplastic matrix has improved the stiffness of thecomposite. Since coir is a high modulus material, higher fiberconcentration in the composites demands stronger stress forthe same amount of deformation. Consequently, flexuralmodulus of the composites increases with an increase in thefiber content. Chemically treated composites show muchhigher strength and modulus. This could be due to betterfiller-matrix interfacial adhesion and effective stress transferfrom the matrix to the fiber.

Impact Strength. Figure 6 shows the variation of impactstrengths of both raw and chemically treated coir-re-inforced-PP composites at different filler loading. Impact

strength is a measure of the tolerability, when the compositeis subjected to a sudden impact that results in crack pro-pagation through the material. For fiber-reinforced polymericcomposites, it depends on a number of factors, such as thenature of the fiber, polymer matrix, and the polymer-matrixinterfacial bonding.26 Sanadi et al. reported that high fibercontent increases the possibility of fiber agglomeration, whichresults in regions of stress concentration that require lessenergy for crack propagation and that an increase in theresistance of crack propagation occurs if fiber bridges thecrack in the composites.2 As shown in Figure 6, impactstrengths of both treated and untreated coir-PP compositesshow a slight increasing trend with an increase in the fillerloading, indicating that the filler is capable of absorbingenergy because of strong filler-matrix interfacial adhesion.It has been reported that improved interfacial bondingprovides an effective resistance to crack propagation duringimpact tests.11,26 Thus, higher impact strengths of the treatedcoir-PP composites suggest a better interfacial bondingcompared to those of untreated ones. This could be due tobetter kneading of the matrix-filler system during thepreparation of composites, their grinding and then specimenfabrication in injection molding method. Slightly higherimpact strength for perchlorate and DNPH-treated coir-PPcomposites is probably due to the favorable interactionbetween the treated coir and the hydrophobic PP chain ofthe matrix. So-called fiber pullout and fiber agglomeration

could be responsible for lower impact strengths of untreatedcoir composites.

Figure 4. Variation of flexural modulus of PP composites reinforced with(1) raw coir, (2) oxidized coir, and (3) DNPH-treated coir.

Figure 5. Effect of fiber loading on the flexural modulus of (1) raw coir,(2) oxidized coir, and (3) DNPH-treated coir-reinforced-PP composites.

Figure 6. Variation of impact strength of PP composites reinforced with(1) raw coir, (2) oxidized coir, and (3) DNPH-treated coir.

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Hardness. Hardness of a composite material refers to itsresistance to shape changes when force is applied on it. Forcomposites, it depends on the distribution of the filler into thematrix.26 Usually the presence of a more flexible matrix causesthe resultant composites to exhibit lower hardness.15 As shown

in Figure 7, incorporation of both treated and untreated coirinto the PP matrix has reduced the flexibility of the matrix,resulting in more rigid composites. The hardness of both treatedand untreated composites is found to increase with an increasein the filler loading. The incorporation of filler particles intothe PP matrix has reduced the mobility of the polymer chain inthe rigid composites. The treated coir composites seem to havebetter hardness compared to untreated ones. This could beattributed to both better dispersion of the fiber into the matrixwith minimization of voids and stronger interfacial adhesionbetween the matrix and the filler.

Water Absorption Behavior. Water absorption character-istics of the composites against filler loading are shown in Figure8. Water absorption (%) increased with an increase in filler

loading. Usually natural fiber-polymer composites withoutcompatibilizer show remarkably water absorption due to the

presence of voids.2,3,27 With an increase in filler loading, thenumber of hydroxyl groups as well microvoids in the compositesincreased, which results in an increase in water absorption.Chemically treated coir-reinforced composites are found to showlower water absorption capacity compared to the untreated ones,indicating that the hydrophilic nature of coir has substantiallydecreased upon chemical treatment with both NaClO4 andDNPH. This can directly be attributed to the decrease in thenumber of hydroxyl groups responsible for the hydrophilic

nature of the cellulose that converted into aldehyde group andsubsequently coupled with DNPH. No dimensional change isobserved upon immersion of the composites in water. Thisindicates that fiber is thoroughly encapsulated in the matrix. Atthe same time, it can also be ascribed that, due to favorableinteraction between the matrix and the treated filler, microvoidsin the composites have substantially minimized, showing lowerwater uptake capacity.

Morphological Study. The morphology of the tensile fracturedsurface gives information as to why mechanical properties ofthe composites prepared under different treatment conditionsare different. The tensile fractured surface morphologies ofuntreated and treated coir-PP composites prepared with 25 wt% coir are shown in Figure 9. The SEM images of the untreatedcoir-PP composite show a number of fiber agglomerations andfiber pullout traces in the composites (image A). These featuressuggest fiber-fiber interaction as well as weak interfacialbonding between the hydrophilic filler and the hydrophobicmatrix. On the other hand, chemically treated coir-PP com-posites show almost uniform dispersion of the filler into thematrix, which results in better interfacial adhesion between thefiller and the matrix with improved mechanical properties. Thisalso implies that hydroxyl groups are being oxidized to aldehydegroup, which upon coupled with DNPH has reduced thehydrophilicity of coir, providing favorable interaction with thePP chain with improved mechanical properties (images B andC). It is evident from images B and C that both fiber pullout

traces and fiber agglomeration as well as the microvoids in thecomposites have significantly reduced in the composite upontreatment of coir with perchlorate and DNPH. This resultsuggests that interfacial bonding between the filler and the matrixhas become much more favorable for treated coir and the matrixcompared to that of the untreated one.

Conclusions

The present work reveals that low cost renewable materialscan be used to prepare useful composites with good me-chanical properties. The tensile strength values of thecomposites of untreated coir showed a decreasing trend with

increasing filler loading. On the other hand, the tensilestrength values of the DNPH-treated composites showed anincreasing trend up to 15% filler loaded composite and thendecreased with further increases in filler content. It isimportant to note here that, at all mixing ratios, the tensilestrengths of the treated coir-PP composites showed highervalues compared to those of the untreated ones. In both cases,the Youngs modulus, flexural strength, flexural modulus,impact strength, and hardness are also found to increase withan increase in filler loading and the values are found to behigher for treated coir-PP composites than those of theuntreated ones. It is concluded that interaction between thefiller and the matrix has become more favorable uponchemical treatment of coir. Water absorption (%) increased

with filler loading; however, treated coir composites showedlower water uptake capacity compared to those prepared from

Figure 7. Hardness of (1) raw coir-, (2) oxidized coir-, and (3) DNPH-treated coir-PP composites at different fiber loading.

Figure 8.Water absorption behavior of PP composites reinforced with (1)raw coir, (2) oxidized coir, and (3) DNPH-treated coir.

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untreated coir, indicating that upon chemical treatment the

number of hydroxyl group in the cellulose of coir has

substantially decreased, giving reduced the hydrophilic nature

of coir. At the same time, it can be said that, due to favorable

interaction between the treated coir and PP, microvoids in

the composites have largely minimized, showing lower water

uptake capacity of the composites. The improved mechanical

properties are supported by SEM images of the fracturedsurfaces that show better dispersion of the filler in the matrix

with almost no fiber pullout traces and agglomeration of thetreated coir-PP composites.

Acknowledgment

We thank the members of the Board of PostgraduateStudies (BPGS) of the Department of Chemistry, BUET forhelpful discussion. The financial assistance (CASR-216/23)approved by the Committee for Advanced Studies and

Research (CASR) BUET for carrying out the present workis highly appreciated.

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ReceiVed for reView May 20, 2009ReVised manuscript receiVedAugust 18, 2009

AcceptedAugust 30, 2009

IE900824C

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