http://jcm.sagepub.com/Journal of Composite Materials

http://jcm.sagepub.com/content/48/25/3063The online version of this article can be found at:

 DOI: 10.1177/0021998313506241

2014 48: 3063 originally published online 15 October 2013Journal of Composite MaterialsDiène Ndiaye, Ansou Malang Badji and Adams Tidjani

Physical changes associated with gamma doses on wood/polypropylene composites  

Published by:

http://www.sagepublications.com

On behalf of: 

  American Society for Composites

can be found at:Journal of Composite MaterialsAdditional services and information for    

  http://jcm.sagepub.com/cgi/alertsEmail Alerts:

 

http://jcm.sagepub.com/subscriptionsSubscriptions:  

http://www.sagepub.com/journalsReprints.navReprints:  

http://www.sagepub.com/journalsPermissions.navPermissions:  

http://jcm.sagepub.com/content/48/25/3063.refs.htmlCitations:  

What is This? 

- Oct 15, 2013OnlineFirst Version of Record  

- Sep 27, 2014Version of Record >>

by guest on September 27, 2014jcm.sagepub.comDownloaded from by guest on September 27, 2014jcm.sagepub.comDownloaded from

JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Physical changes associated with gammadoses on wood/polypropylene composites

Diene Ndiaye1, Ansou Malang Badji1 and Adams Tidjani2

Abstract

The effect of gamma radiation on the morphology, thermal behavior and mechanical properties of wood polypropylene

composites has been investigated. Simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry

(DSC) have been performed on wood polymer composite (WPC) samples of 9.5� 0.1 mg. These samples were exposed

to different gamma dose in the range 10–100 kGy. The results indicated that gamma radiation improves the mechanical

properties while the thermal stability is decreased. With gamma radiation, the scanning electron microscopy (SEM) of the

micrographs became smoother and we can notice an improvement of interaction between polymer and wood fibers.

Keywords

Wood polymer composites (WPCs), morphology, gamma radiation, thermal stability, mechanical properties

Introduction

The use of wood fibers as a filler is a choice offering aneconomical solution for the increasing costs of woodenproducts and construction materials. There is a consid-erable commercial interest in thermoplastic compositesfilled with wood fibers, due to the potential opportu-nities. The product has the aesthetic appearance ofwood and the processing capability of thermoplasticsand its performance in humid area. These compositeshave been widely studied. It has been shown that prop-erties of wood polymer composites (WPCs) depend onthe characteristics of matrix and fillers, chemical inter-action between wood fibers and polymer, humidityabsorption and processing conditions. The materialcan be considered as an easily attainable (natural)option; it is competitive as far as price is concernedand convenient for a wide range of applications.Disadvantages associated with the use of naturalfibers as reinforcement in thermoplastics are the resultof a lack of a good interfacial adhesion and a poorresistance to humidity absorption. The first key pointfor the production of acceptable WPC is the compati-bility between fibers and the polymer. In particular, thehydrophobic character of polymeric material (low sur-face tension) which contrasts with the hydrophilic char-acter (high surface tension) of wood fibers.1 Scanningthe literature, one can find different surface treatmentsthat have been experienced to improve wood/polymer

adhesion in composites. Remind that the level of adhe-sion and/or the dispersion state of wood are the keypoints for the improvement of mechanical propertiesof the composites. Indeed, the wood particles whichhave high strength and modulus – with good adhesionand uniform dispersion – can impart better mechanicalproperties to the host polymer in order to obtain acomposite with better properties than those of theunfilled polymer. The main solutions found to improvecompatibility are the use of coupling agents, pretreat-ment of wood fiber and/or the polymer through surfacecoating treatment or graft copolymerization whichimproves mechanical properties, water absorption anddispersion.2,3 The most commonly used compatibilizerin WPC is usually polypropylene (PP) grafted withmaleic anhydride (MAPP).4 It is proven in several stu-dies that the nucleation density on the surface of theMAPP is very significant because of the increase in theinterfacial adhesion between the reinforcement andthe MAPP. The thermal stability of the composite is

1Departement de Physique Appliquee, UFR/SAT, Universite Gaston

Berger de Saint-Louis, Senegal2Departement de Physique, Faculte des Sciences et Techniques,

Universite Cheikh Anta Diop de Dakar, Senegal

Corresponding author:

Diene Ndiaye, Universite Gaston Berger de Saint-Louis, Senegal.

Email: diene.ndiaye@ugb.edu.sn

Journal of Composite Materials

2014, Vol. 48(25) 3063–3071

! The Author(s) 2013

Reprints and permissions:

sagepub.co.uk/journalsPermissions.nav

DOI: 10.1177/0021998313506241

jcm.sagepub.com

by guest on September 27, 2014jcm.sagepub.comDownloaded from

also improved by the addition of MAPP in the formu-lation compared to composites without MAPP.5 Thecompatibilizer also influences the uptake of water andleads to a decrease in water sensitivity of the compositefrom the same composite without MAPP.6 This reduc-tion can be attributed to an increase in interfacial adhe-sion that reduces water accumulation in the interfacialareas and prevents water from penetrating into thefibers.7 This is what justifies the use of MAPP in thiswork to improve the dimensional stability of the irra-diated material. Another limitation of using woodfibers in WPC outdoor applications is the high sensitiv-ity to UV-light degradation.8 Regardless of formula-tion, the introduction of wood fiber into a PP leads toa change in the morphology of the crystallizing poly-mer. The use of ionizing radiation on polymers con-tinues to draw great attention because it modifiesthem significantly. Polymers exposed to ionizing radi-ation, even at low doses, often undergo structuralchanges accompanied by molecular crosslinking, graft-ing and chain scission reactions.9–11 Using ionizingradiation represents an economic advantage, becauseit reduces the use of additives in the formulas. Withsuitable doses, it is the only technique that uses electro-magnetic energy to generate significant changes.Ionizing radiation (g rays and accelerated electrons g-emitted by a source of 60Co or 137Cs) were developedindustrially since the 1960s for sterilizing medical andsurgical equipment and preservation of food products.They also led to the development of chemistry radi-ation, radical type, primarily applied to polymers. Bytriggering a chemical process of formation of free rad-icals, ionizing radiation can then initiate a number ofchemical reactions such as chains scissions, addition,polymerization, cures, etc. which can lead to variousapplications like: degradation or crosslinking of plastics(including improving recyclability), wood, impregnatedwith resin or grafting on polymers.

The objective of this study is to experience the tech-nique based on the g irradiation of composites as analternative of coupling or compatibilizing agents. It wasused to improve the filler/matrix interface. This tech-nique introduces energy into the material to generatefavorable changes in its morphology. From that point,a competition occurs between scissions and crosslinkingreactions in g-irradiated polymers. The prevailing effectbrings about macroscopic changes in the molecularmass distributions, continuously affecting transitiontemperatures, diffusion coefficients and mechanicalproperties during polymer irradiations. Reyes et al.12

studied the effect of g irradiation on the mechanicalproperties of PP. The carbon–chlorine and carbon–fluorine bonds are well known as radiation-sensitive,this severely limits the use of two industrial polymers,poly (vinyl chloride) (PVC) and poly (tetra fluoro

ethylene) (PTFE) in radiation area. So, in our study,we used PP. The results obtained showed that generalbehavior of the polymer blends was similar, being10–50 kGy the most favorable irradiation range dueto the remarkable improvement in the studied mechan-ical properties experience within it. Tidjani andWatanabe13 examined the gamma-oxidation of linearlow-density polyethylene: the dose–rate effect of irradi-ation on chemical and physical modifications wasobserved. Their results showed that the lower thedose rate, the higher the degree of oxidation in termsof g-product formation, which were very dependent onthe dose rate of initiation.

Experimental

Materials

The basic materials used in this study are listed below:the wood fiber were kindly donated by American Woodfibbers (Schofield, WI) and are constituted predomin-antly with ponderosa pine, maple, oak, spruce, south-ern yellow pine and cedar. The isotactic PP matrix has adensity of 0.900 g/cm3 and a melt flow index of2.5 g/10min, it was provided by Solvay Co. MAPPwith an approximate maleic anhydride (MA) contentof 3wt.% and molecular weight of 40,000 g/mol waspurchased from Aldrich Chemical Company, Inc.(Milwaukee, WI). All ingredients were used as received.

Compounding and processing

Before compounding, the WF was dried in an oven forat least 48 h at 105�C to a moisture content of less than1%. The dried WFs were stored in a sealed plastic con-tainer to prevent the absorption of water vapor. First,the PP was put in the high-intensity mixer (Papenmeier,TGAHK20, Germany), and the WF was added afterthe PP had reached its melting temperature. Themixing process took 10min on average. After blending,the compounded materials were stored in a sealed plas-tic container. Several formulations were produced withvarious contents of PP, WF and MAPP. For the extrac-tion of volatile and harmful gases, the hood was open.For the mechanical property experiments, test speci-mens were molded in a 33-Cincinnati Milacron recipro-cating screw-injection molded (Batavia, OH). Thenozzle temperature was set to 204�C. The extrudate,in the form of strands, was cooled in the air and pelle-tized. The resulting pellets were dried at 105�C for 24 hbefore they were injection-molded into the ASTM testspecimens for flexural, tensile (Type I, ASTM D638)and Izod impact strength test. The dimensions of thespecimens for the flexural tests were 120� 3� 12mm3

(length� thickness�width). The composition of

3064 Journal of Composite Materials 48(25)

by guest on September 27, 2014jcm.sagepub.comDownloaded from

the sample used in our study was in percent (PP/wood/MAPP): 47/50/03. In our previous work,14 it wasproven that the use of MAPP improved the adhesionand dispersion of the reinforcement in the matrix. Inthe current study, the effects of g-irradiated compositeson the interfacial adhesion between the wood fibers andthe PP matrix were studied. Morphological, mechanicaland thermal properties of the composites were alsoinvestigated.

� irradiation

Specimens had been sealed in glass tubes filled withoxygen, prior to being irradiated to integral doses at80�C. The samples were irradiated with g-rays at radi-ation doses of 10, 25, 50, 100 kGy using a dose rate of4.8 kGy/h in air, at room temperature with a 60Cosource. 60Co is the most common gamma emitter usedbecause of its abundance in the market and the oper-ational safety of devices that use it. The experimentalmeasurements were taken after two days of storageunder oxygen. The effect of g irradiation on thethermo-mechanical properties was then monitored.Ionizing radiation penetrated the wood structure andproduced a uniform degree of polymerization. So, itcould create radical sites on the cellulose chains fromwhich branch vinyl polymer can grow to give graftcopolymer.15

Scanning electron microscopy (SEM)

The state of dispersion of the wood inside the polymericmatrix was analyzed using optical microscopy on sam-ples of 100–200 mm thick. The fractured samples werecoated with a thin layer of gold to avoid electrostaticcharging during examination. SEM was used to obtainmicrophotographs of the fracture surfaces of the woodcomposites. These fractures have been performed inliquid nitrogen to avoid any deformation. SEM hasbeen performed using a Fei Quanta 400 microscopeworking at 30 kV. The polymer surface was examinedwith Leica optical microscope working in a transmis-sion mode. Samples were thin enough that no specialpreparation was needed for their observations with theoptical microscope.

Differential scanning calorimetry (DSC)

DSC is widely used to characterize the thermal proper-ties of WPCs. DSC can measure important thermoplas-tic properties, including the melting temperature (Tm),heat of melting, degree of crystallinity � (%), crystal-lization and presence of recyclates/regrinds, nucleatingagents, plasticizers and polymer blends (the presence,composition and compatibility). Thermal analysis of

the WPC samples was carried out on a DSC (PerkinElmer Instruments, Pyris Diamond DSC, and Shelton,CT) with the temperature calibrated with indium. AllDSC measurements were performed with powderedsamples of about 9.5� 0.1mg under a nitrogen atmos-phere with a flow rate of 20mL/min. Three replicateswere run for each specimen. All samples were subjectedto the same thermal history with the following thermalprotocol, which was slightly modified from the onereported by Valentini et al.:16

1. the samples were heated from 40 to 180�C at a heat-ing rate of 20�C/min to erase the thermal history;

2. the samples were cooled from 180 to 40.00�C at acooling rate of 10�C/min to detect the crystallizationtemperature (Tc);

3. the samples were heated from 40 to 180�C at a heat-ing rate of 10�C/min to determine Tm. Tm andthe heat of fusion (�Hm) were obtained from thethermograms during the second heating. Thevalues of �Hm were used to estimate (� (%)),which was adjusted for each sample.

Thermogravimetric analysis (TGA)

This analysis consists to record the weight of sample indynamic conditions at a heating rate of 10�C/minbetween 20 and 700�C under an inert atmosphere ofnitrogen. The experience was performed using Perkinequipment Elmer Pyris-1 TGA. The sample (mass of(10� 1mg)) was placed in a little cup made of alumi-num hanging from a microbalance. The variation of themass of the sample allows drawing the TG (variation ofthe mass in function of the temperature) and TGD(derivative of loss of mass versus the time) thermo-grams. The combination of these two thermogramsgave a clear indication of number of stages of the ther-mal degradation. This method allows determining thedegradation temperature of the materials and thus thethermal stability of the composite.

Mechanical properties

Tensile tests (tensile strength and tensile strain) andthree-point flexural tests (flexural modulus and flexuralstrength) were carried out on an Instron 5585H testingmachine (Norwood, MA) with crosshead rates of 12.5and 1.35mm/min according to the procedures outlinedin ASTM standards D 638 and D 790, respectively.Eight replicates were run to obtain an average valuefor each formulation. Before each test, the films wereconditioned in a 50% relative humidity chamber at23�C for 48 h. The impact strength is defined as theability of a material to resist the fracture under stress

Ndiaye et al. 3065

by guest on September 27, 2014jcm.sagepub.comDownloaded from

applied at a high speed. The impact properties of com-posite materials are directly related to their overalltoughness. The Izod impact strength test is the totalenergy required to break a notched sample. It was mea-sured with an Instron impact pendulum tester (modelPW5) according to ASTM D 256 with acutely notchedspecimens (notch depth 2mm) at room temperature.

Results and discussions

SEM of the composites

Figure 1 shows the SEM micrographs of the fracturesurface of the WP composites made from PP.Unirradiated sample (a) and irradiated ones (b and c)show similar behavior, since irradiation does not influ-ence dispersion, but could affect polymer–filler inter-action. Figure 1(a) shows the pull out effect inuntreated wood fiber/polymer composite, but fromFigure 1(b) and (c) (irradiated samples), there is lesspull. It revealed that there is poor adhesion betweenthe wood fiber and polymer matrix in Figure 1(a) andbetter adhesive force between wood and polymermatrix in the other ones. In the irradiated samples, itcan be seen that the effects of radiation depend on thedose applied to the sample. Moreover, for these sam-ples, a loss of weight (less than 1% after 50 kGy irradi-ation and about 1.25% after the 100 kGy dose) wasobserved and a visible yellowing appeared alreadyafter 50 kGy. These phenomena were most probablysigns of degradation. Smirnov and Deyun17 showedthat the average molecular weight of PP, when sub-jected to g radiation, did not show significant variationswhen the radiation dose was under 10 kGy. The irra-diated samples exhibited a slight improvement of more

smooth surfaces than those unirradiated, probably dueto a reorganization of the matrix chains during irradi-ation. The most evident changes were achieved alreadyafter a dose of 100 kGy. When the irradiation doseincreases the crosslinking density between the resultantcopolymer chains increases. This restricts the expansionof the chains of the prepared copolymer and decreasesthe amorphous phase.

DSC of the composites

Figure 2 presents the thermograms of the second heat-ing of the PP/wood flour blends subjected to differentirradiation doses. The curves were vertically moved forclarity. Only one endothermic peak corresponding toPP can be observed in these figures. As can be seen,these values remain constant at low irradiation doses(<25 kGy) and then tend to increase as dose increases.This behavior is generated by the oxidative degradationof PP when it is submitted to the irradiation dose,which promotes the scission of the polymer chains. gradiation at lower doses (0–10) kGy does not give avisible change in the melting behavior of WPC.

In Figure 3, around 160�C, we observe the presenceof a peak corresponding to the melting of pure PP. Thethermograms of the irradiated composites (25, 50,100 kGy) have the same melting peak shifted slightlybut these thermograms have an additional shoulder ataround 175�C. We can provide two possible explan-ations: the radiation dose is strong enough to changethe crystallization of PP and the reappearance of a newpopulation of smaller crystals which fuse at a lowertemperature. This shoulder comes directly from thereinforcements which are sensitive to g rays. The pres-ence of fibers decreases the thermal stability, radiation

Figure 1. Electron micrographs of WPC: (a) unirradiated, irradiated, (b) 50 kGy and (c) 100 kGy.

WPC: wood polymer composite.

3066 Journal of Composite Materials 48(25)

by guest on September 27, 2014jcm.sagepub.comDownloaded from

in turn causes scissions of chains and all these phenom-ena generate an early fusion and it is more pronouncedin samples without coupling agent. The presence ofMAPP does not significantly modify the crystallizationtemperature but leads to an increase in the degree ofcrystallinity. It is recognized that wood and MAPP actas nucleating agents,14,18 the presence of these twoelements generates the formation of more crystals.However, the higher the radiation dose increases, wesee that the merger tends to begin at an early stage.The fact that Tc is shifted toward higher temperatures

at the maximum dose of (100 kGy) can be explained bya higher irradiation dose acted as a slight precursor ofcrystallization.

TGA of the composites

TGA provides quantitative information on weightchange process while differential thermal gravimetry(DTG) provides the rate of weight loss (dW/dt).

Figures 4 and 5 present, respectively, the TGA andDTG (the first derivative of TGA) curves of irradiatedand unirradiated samples of WPC. The initial mass loss

Figure 2. Effect of gamma radiation on DSC thermograms (cooling) of WPC.

DSC: differential scanning calorimetry; WPC: wood polymer composite.

Figure 3. Effect of gamma radiation on DSC thermograms

(melting) of WPC.

DSC: differential scanning calorimetry; WPC: wood polymer

composite.

Figure 4. TGA curves of WPC with different gamma doses.

TGA: thermogravimetric analysis; WPC: wood polymer

composite.

Ndiaye et al. 3067

by guest on September 27, 2014jcm.sagepub.comDownloaded from

below 100�C is due to the gradual evaporation ofabsorbed moisture. The initial decomposition tempera-ture for the composite is typical of wood, in which pyr-olysis of wood components took place below 250�C.The important degradation process of wood starts ataround 200–220�C and it concerns the three major con-stituents of wood (cellulose, hemicellulose and lignin).Therefore, the temperature profile throughout theextruder screw zones was set from 140 to 185�C, fromthe feed to the metering zone. The mass loss steps ofpure PP occur very slowly under 500�C and above thistemperature, this process grows rapidly. As the gammadose increased, the thermal stability of the compositesdecreased and at the same time, the decomposition tem-peratures of the composites decreased slightly byincreasing the gamma doses. Moreover, the DTGcurve of the irradiated composite showed degradation

of coupling agent around 300�C may be the alcoholproduct during degradation.19 An increase in themass loss rate with gamma dose is also observed inthe first stage of decomposition. High irradiationdoses generate chains scissions into the material inturn contribute to the decrease in thermal stability ofcomposites. For the irradiated composites, it was ver-ified that the maximum degradation rate was shifted toa lower temperature. This indicated that the presence ofthe wood flour lowered the thermal stability of thematerials and g irradiation does not change anything.Because of high inflammability of cellulose, the add-ition of wood makes the composite less thermallystable. The TGA measurements indicate that whenthe polymer is heated, its thermal stability is usuallyappraised from the loss of mass and its molecularweight sharply diminishes due to the degradation ofthe molecular chains.

FT-IR of the composites. To investigate the effect of g-rayirradiation on the mechanical properties of the wood/PP composite, the sample of 50-50 was exposed to g rayat various doses. To clearly observe any change in thechemical integrity of the composite as a function ofradiation dosage, Fourier transform infrared spectros-copy (FT-IR) was employed and the results are shownin Figure 6.

It is evident that, the signals associated with the alde-hyde functional groups became stronger by increasingthe radiation dosage. SpeciEcally, the carbonyl stretch-ing at around 1710 cm�1 and the C–H absorption in therange of 2700–2800 cm�1 were identified. In the pres-ence of oxygen, the photo-oxidation of PP leads to the

Figure 6. Infra-red spectra of gamma irradiated WPC in presence of oxygen.

WPC: wood polymer composite.

Figure 5. DTG curves of WPC with different gamma doses.

DTG: differential thermal gravimetry; WPC: wood polymer

composite.

3068 Journal of Composite Materials 48(25)

by guest on September 27, 2014jcm.sagepub.comDownloaded from

formation of hydroxyl (mainly hydro peroxides andalcohols) and carbonyl groups (mainly ketones, estersand acids) which are easily detectable by infrared spec-troscopy, respectively, in the 3200–3600 and 1500–1900 cm�1 areas. The Erst mechanism is free radicalformation along the polymer backbone via chain scis-sion. It is assumed that free radical formation is thedominating event during irradiation at a particulartotal dose. At relatively low total doses, free radicalformation dominates the kinetics of polymer degrad-ation. Once these free radicals are formed, the secondmechanism, cross-linking, becomes feasible. Cross-linking occurs when free radicals recombine. If thecross-link density becomes higher than that of the ori-ginal polymer before irradiation, the free radical mayalso react with oxygen. It is assumed that both themechanisms of chain scission and cross-linking areoccurring randomly and simultaneously.20 The pre-dominance of one mechanism over the other dictatesthe increase or decrease of mechanical properties witheach total dose. The presence of oxygen (air) during girradiation affects the performance of the sample. Uponinitial irradiation in an environment containing oxygen,free radicals are formed within the polymer and readilyreact with free oxygen (O2) according to the followingreaction to form peroxy radicals: RþO2!ROO* Theparticipation of oxygen during the beginning of thisfree radical chain reaction often leads to increased

chain scission activity.21,22 The formation of theseperoxy radicals prevents further cross-linking.The effect of high-energy radiation on organic polymersis to produce ionization and excitation. The polymermay under cleavage or scission (i.e. the polymer mol-ecules may be broken into smaller fragment).Subsequently, rupture of chemical bonds yields frag-ments of the large polymer molecules. The free radicalsthus produced may react to change structure of thepolymer and after the physical properties of the mater-ials. It also may undergo cross-linking (i.e. the mol-ecules may be linked together into largemolecules).23,24 During degradation, there will be lossin strength due to primary bond breakage in the cellu-lose constituents and, therefore, be related to changestaking places in the middle lamella, which reduce theultimate cell.25

Tensile properties of the composites

The effect of gamma radiation on the bending strengthof WPC is summarized in Table 1.

To better visualize the effect of gamma radiation onbending strength, these data served to plot the curve inFigure 7.

The behavior of tensile strength of the unirradiatedand irradiated WPC, with MAPP compatibilizer, isshown in Figure 7. It can be noticed that this propertydecreased when the dose range increased until 50 kGyand then it seemed constant. Beyond a certain dose,irradiation of the polymer causes the breaking of cova-lent bonds of the latter which is accompanied by thecreation of free radicals. This division is also respon-sible for a decrease in molecular weight; these chainscan bind to the initial split point or rearrange by cova-lent cross-links with neighboring chains, leading to

Figure 7. Effect of gamma radiation on tensile strength of WPC.

WPC: wood polymer composite.

Table 1. Effect of gamma radiation on bending strength of

WPC.

Total dose (kGy) 0 10 25 50 100

Bending strength (MPa) 12,35 13,75 14,15 14,5 12,4

WPC: wood polymer composite.

Ndiaye et al. 3069

by guest on September 27, 2014jcm.sagepub.comDownloaded from

cross-linking. g irradiation may also remove moisturefrom the composite, which in turn contributes to betterfiber matrix adhesion. With the increase of the radi-ation dose, degradation through the formation ofperoxy and hydroperoxy radicals in the amorphousregions appears to be the primary cause of physicalproperty loss. When sufficient numbers of tie moleculesbetween crystallites are cut through this chain scissionprocess increases the local stress concentration on thecrystals. Therefore, these mechanical properties (tensilestrength and bending strength) are reduced drastic-ally.26 In presence of MAPP, the tensile strength isalso enhanced by forming more chemical bondingbetween the migration of maleated PP and woodfibers since enough fibers provide more active sites onsurface for esterification that is hydroxyl groups. Thus,more ramified structures would be formed as morefibers combined that increase the tensile strength dueto load transfer.27,28 It is observed that tensile proper-ties increased with g radiation up to a certain dose andthen decreased due to the two opposing phenomena,namely, photo cross-linking and photo degradationthat took place simultaneously under g radiation.At lower doses, free radicals are stabilized by a com-bination reaction and, as a result, photo cross-linkingoccurs. The higher the number of active sites generatedon the polymeric substrate, the greater the grafting effi-ciency. But at higher radiation, the main chain may bebroken down and polymer may degrade into frag-ments and, as a result, tensile properties will decreaseafter certain gamma doses. An intense radiation resultsin a loss of tensile strength and a reduced degree ofpolymerization is observed.29 From this investigation,it is clear that g radiation has a strong role on theimprovement of the mechanical properties of thecomposites.

Conclusion

The WPCs were irradiated at different g doses. It hasbeen observed from the SEMmicrographs that g irradi-ation improved the interfacial adhesion between thewood fibers and matrix. In thermal behavior, oxidativedegradation led to chain scission, implying that poly-mer chains fused at lower temperatures. The modifica-tions of the IR spectra of the tested films of WPC upong irradiation in the presence of oxygen are comparableto that of pristine PP, and are mainly characterized byan increase of absorbance in the hydroxyl and carbonylregions. It can be noticed that, to a certain limit of girradiation doses, the mechanical properties of compos-ites such as tensile strength and bending strengthshowed some improvements. We concluded that girradiation could be an appropriate way to improvethe mechanical properties of the WPC and at the

same time, it offers several advantages such as thenon-requirement of chemical reagents and the absenceof residual polluting by-product.

Conflict of interest

None declared.

Funding

This research received no specific grant from any fundingagency in the public, commercial, or not-for-profit sectors.

References

1. Ndiaye D, Fanton E, Morlat ST, et al. Durability of

wood polymer composites: Part 1. Influence of wood

on the photochemical properties. Compos Sci Technol

2008; 68: 2779–2784.

2. Karmarkar A, Chauhan SS, Modak JM, et al.

Mechanical properties of wood fibre reinforced polypro-

pylene composites: effect of a novel compatibilizer withisocyanate functional group. Compos Part A 2006; 38:

227–233.

3. Bledzki AK and Faruk O. Wood fibre reinforced poly-

propylene composites: effect of fibre geometry and cou-

pling agent on physico-mechanical properties. Appl

Compos Mater 2003; 10: 365–379.4. Nourbakhsh A, Kokta BV, Alireza A, et al. Effect of a

novel coupling agent, polybutadiene isocyanate, on

mechanical properties of wood-fiber polypropylene com-

posites. J Reinf Plast Compos 2008; 27(16–17):

1679–1687.5. Tengku FZH, Faisal A and Iqmal T. Effect of maleic

anhydride polypropylene on the properties of coconut

shell filled thermoplastic elastomeric olefin composites.

Indo J Chem 2010; 10(2): 156–161.6. Ndiaye D, Gueye M and Diop B. Characterization, phys-

ical and mechanical properties of polypropylene/

wood-flour composites. Arab J Sci Eng 2013; 38: 59–68.7. Jayanta B and Kale DD. Properties of polypropylene

filled with chemically treated rice husk. J Appl Polym

Sci 2008; 110: 1271–1279.

8. Lundquist L, Marque B and Hagstrand PO. Novel pulpfiber reinforced thermoplastic composites. Compos Sci

Technol 2003; 63: 137–152.

9. Singh A and Bahari KLA. Use of High Energy Radiation

in Polymer Blends Technology. In: Utracki LA (ed.)

Polymer blends handbook. Dordrecht: Kluwer Academic

publishers, 2001.10. Singh A. Irradiation of polymer blends containing a poly-

oleEn. Radiat Phys Chem 2001; 60: 453–459.11. Chipara MI, Constantinescu B, Reyes Romero J, et al.

Conferinta nationala de fizica constanta. Romania, 1998.12. Reyes J, Albano C, Davidson E, et al. In: Proceedings of

8th polychar, TX, USA, 2000.

13. Tidjani A and Watanabe Y. Gamma-oxidation of linear

low-density polyethylene: the dose–rate effect of irradi-

ation on chemical and physical modifications. J Polym

Sci Part A Polym Chem 1995; 33(9): 1455–1460.

3070 Journal of Composite Materials 48(25)

by guest on September 27, 2014jcm.sagepub.comDownloaded from

14. Ndiaye D, Matuana LM, Morlat ST, et al. Thermal andmechanical properties of polypropylene/wood-flour com-posites. J Appl Polym Sci 2011; 119: 3321–3328.

15. Clough R. High energy and polymers: a review of com-mercial processes and emerging applications. NuclInstrum Methods B 2001; 185(1–4): 8–33.

16. Valentini L, Biagiotti J, Kenny JM, et al. Morphological

characterization of single-walled carbon nanotubes-PPcomposites. Compos Sci Technol 2003; 63: 1149–1153.

17. Smirnov LP and Deyun EV. Evolution of the molecular

mass distribution parameters during gamma radiolysis.Polym Sci Serv A 1999; 41(5): 506–511.

18. Peter Z. Materials of cellulose derivatives and fiber-

reinforced cellulose–polypropylene composites: charac-terization and application. Pure Appl Chem 2006;78(10): 1843–1855.

19. Sonia MBN, Graziela SC and Simone MLR. New poly-meric coupling agent for polypropylene/wood flour com-posites. Polym Test 2007; 26: 619–628.

20. Woods RJ and Pikaev AK. Appl Radiat Chem: Radiation

Processing. New York: Wiley & Sons, 1994, ISBN0471544523.

21. Seguchi HO and Ogawa K. Localisation of Ca2þ acti-

vated adenosine triphosphatase activities in the transi-tional epithelium of the rabbit urinary bladder. ActaHistochem Cytochem 1982; 15: 76–89.

22. Baljit S and Nisha S. Mechanistic implications of plasticdegradation. Polym Degrad Stabil 2008; 93: 561–584.

23. Andrzej GC, Mohammad HS and Shamshad A. Progressin radiation processing of polymers in ionizing radiation

& polymers. In: Proceedings of the 6th international sym-posium on ionizing radiation and polymers (IRaP),Namur, Belgium, 2004.

24. Zaman HU, Khan MA and Khan RA. Effect of gammaradiation and bulk monomer on jute fabrics polyethyl-ene/polyvinyl chloride composites. J Polym Eng 2012;32: 301–309.

25. Khan MA, Shehrzade S and Hasan MM. Effect of pre-treatment with gamma-radiation on physical and mech-anical properties of 1, 6, hexanediol diacrylate (HDDA)

jute yarn by UV radiation. Polym Plast Technol Eng2003; 42(5): 795–810.

26. Albanoa C, Reyes J, Ichazo M, et al. Analysis of the

mechanical, thermal and morphological behavior of poly-propylene compounds with sisal fiber and wood flour,irradiated with gamma rays. Polym Degrad Stabil 2002;

76: 191–203.27. Sretenovic A, Muller U and Gindl W. Mechanism of

stress transfer in a single wood fiber-LDPE compositeby means of electronic laser speckle interferometry.

Compos Part A 2006; 37(9): 1406–1412.28. Osswald TA. Fundamental principles of polymer compos-

ites: processing and design. In: Proceedings of the 5th inter-

national conference of wood fiber–plastic composites,Forest Products Society, Madison, WI, 26–27 May 1999.

29. Marcovich NF, Ostrovsky AN, Aranguren MI, et al. In:

Proceedings from the 3rd international symposium on nat-ural polymers and composites-ISNaPol, Sao Pedro, SP,Brazil, 14–17 May 2000.

Ndiaye et al. 3071

by guest on September 27, 2014jcm.sagepub.comDownloaded from