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ArticleComparison ofinjection moulded,natural fibre-reinforced composites with PP andPLA as matrices J. P. Mofokeng1, A. S. Luyt1, T. Ta´bi2 and J. Kova´cs2AbstractPoly(lactic acid) (PLA) and polypropylene (PP) were comparatively investigated asmatrices for injection-moulded composites containing small (1–3 wt%) amounts of shortsisal fibre. The morphology, thermal and dynamic mechanical properties, as well asdegradation characteristics were investigated. The scanning electron microscopy (SEM)micrographs of the composites show more intimate contact and better interactionbetween the fibres and PLA, compared to PP. This improved interaction was confirmedby the Fourier-transform infrared (FTIR) spectroscopy results which showed the presence of hydrogen bonding interaction between PLA and the fibres. The thermal stability(as determined through thermogravimetric analysis [TGA]) of both polymers increasedwith increasing fibre content, with a more significant improvement in the case of PP. Thedifferential scanning calorimetry (DSC) results showed a significant influence of thefibres on the cold crystallization and melting behaviour of PLA, even at the low fibre contents of 1–3%. The influence of the fibres on the melting characteristics of the PP wasnegligible, but it had a significant influence on the nonisothermal crystallization temperature range. Both the storage and loss moduli of the PLA decreased with increasing fibrecontent below the glass transition of PLA, but the influence on the loss modulus wasmore significant. The dynamic mechanical analysis (DMA) results clearly show cold crystallization of PLA around 110C, and the presence of fibres gave rise to higher modulusvalues between the cold crystallization and melting of the PLA. The presence of fibresalso had an influence on the dynamic mechanical properties of PP. This article furtherdescribes basic biodegradation observations for the investigated samples.1Department of Chemistry, University of the Free State (Qwaqwa Campus), Phuthaditjhaba, South Africa2Department of Polymer Engineering, Budapest University of Technology and Economics, Budapest, HungaryCorresponding author:A. S. Luyt, Department of Chemistry, University of the Free State (Qwaqwa Campus), Private Bag X13,Phuthaditjhaba 9866, South AfricaEmail: LuytAS@qwa.ufs.ac.zaJournal of Thermoplastic CompositeMaterials25(8) 927–948ª The Author(s) 2011Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0892705711423291jtc.sagepub.comKeywordspoly(lactic acid), polypropylene, sisal, composites, morphology, propertiesIntroductionNatural fibre-reinforced polymer composites have outstanding potential as an alternativefor synthetic fibre composites.1 Due to their structural properties and the fact that theyare relatively cost effective, biodegradable, and lightweight, research is now based onthe utilisation of natural fibres as load-bearing constituents in composite materials.Fibres provide the strength and stiffness and act as reinforcement in a fibre-reinforcedcomposite material.2There have been several publications on PP composites reinforced with short/longsisal fibres (SFs).3–8 It was generally found that SFs enhanced the mechanical propertiesof the composites. The improvement was mainly observed for composites containinglow fibre contents, while higher fibre contents normally gave rise to reduced mechanicalproperties. This was explained as being due to the incompatibility between the fibres andthe matrix, which promoted microcrack formation at the interface as well as nonuniformstress transfer due to fibre agglomeration in the matrix. Reports also indicated anincrease in the thermal stability of the PP in the presence of SF. Other thermal propertiessuch as melting temperature, crystallization temperature, heat of fusion, and percentageof crystallinity were also influenced by the presence of SF, due to the nucleating abilityof these fibres. Although PP/sisal composites are called biocomposites, they are still notcompletely biodegradable.One of the most promising biodegradable polymers is poly(lactic acid) (PLA), andPLA can be manufactured from renewable resources, most commonly from corn.9–12Most of the research on PLA composites ultimately seeks to improve the mechanicalproperties to a level that satisfies the particular applications, where PLA could be areplacement for synthetic polymers like PP.13 PLA has better mechanical properties thanPP, with a tensile strength of 62 MPa and a modulus of 2.7 GPa, in contrast to 36 MPaand 1.2 GPa for PP. Moreover, PLA can be processed by injection moulding, blowmoulding,14 and film extrusion because the glass temperature of PLA is 50–60C and themelting temperature is 168–172C.15 Limited research has been published on the use ofSF as filler or reinforcement in the PLA matrix.7,15–17 Transcrystallinity of PLA on SFwas observed, and it was found that both treated and untreated SFs act as efficientnucleating agents in PLA and that nucleation occurs preferentially along the fibre axis.The results obtained from literature suggests that SF surface modification using alkali orsilane has little or no influence on the nucleation ability of SF in a PLA matrix.18 Noreports could be found on the mechanical and thermal properties for PLA/SF compositesin the available literature.There is limited information on the use of low contents of short natural fibre inpolymer composites.4,19 In these publications, it was reported that there is a reduction inthe mechanical properties of these composites. At low fibre contents (10 wt%), thefibres may act as flaws or have a plasticization effect in the matrix, reducing the tensilestrength of the composite.4928 Journal of Thermoplastic Composite Materials 25(8)In this article, SF-reinforced PLA and PP composites, containing low fibre contentsand prepared through extrusion and injection moulding, were studied. The morphologyand biodegradability, as well as the thermal and thermomechanical properties, of the twosystems were investigated with the aim to understand whether the PLA composites willbe a good substitute for the PP composites in some applications.Materials and methodsSF was obtained from the National Sisal Marketing Committee in Pietermaritzburg,South Africa. It has a diameter range of 100–300 mm, average tensile strength of490 MPa, average modulus of 11,350 MPa, and elongation at break of 5%.Polylactide (PLA) polymer 3051D was supplied by Nature-Works LLC in Europe.The density of the PLA was 1.25 g cm3, melt flow index ([MFI] 210C/2.16 kg) of10–25 g/10 min, relative viscosity of 3.0–3.5, crystalline melting temperature of 150–165C, glass transition temperature of 55–65C, tensile strength at a yield of 48 MPa,and tensile elongation at a yield of 2.5%.Polypropylene (PP) TIPPLEN H 116F homopolymer, as opaque crystalline pellets,was supplied by the Tiszai Chemical Group Plc, Hungary. This is a high melt flowpolymer that was designed for fibres of medium to high spin speeds, and it offers goodhomogeneity, stable extrusion, and excellent processability. It has the following properties: tensile strength at yield of 34.5 MPa, MFI (230C/2.16 kg) of 25 g/10 min, andtensile elongation at yield of 10%.The sisal was cut into 5-mm long fibres and dried in an oven at 130C for 6 h to eliminate the absorbed moisture. The PLA was also dried in an oven at 85C for 6 h before itcould be used. PP was used as supplied without drying. The PLA and PP composites withSF content of 1–3 wt% were extruded at 190C. After the extrusion, the pelletized PLA/sisal composites were placed in the oven at 120C for 3 h to recrystallize. After theannealing process, the PLA samples were dried in the oven at 85C followed by injectionmoulding at 190C. The PP and its composites were injection moulded immediately afterthey were pelletized. An Arburg 320C Allrounder 600-250 injection moulding machinewas used to prepare 2-mm thick and 80 80 mm flat sheet samples, and the injectionmoulding parameters were as follows: Injection volumeSwitchover pointInjection pressureHolding pressureHolding timeCooling timeBack pressureMelt temperatureMould temperature50 cm312 cm3600–700–740 bars (1–2–3 wt% sisal content)600 bars20 s30 s30 bar190C20C Mofokeng et al. 929After processing the fibres were well dispersed in the polymers and the fibre lengthswere in the range of 2–5 mm.A Shimadzu ZU SSX-550 Superscan scanning electron microscope (SEM) was usedto study the fracture surfaces of the samples. The samples were fractured after insertionin liquid nitrogen and sputter-coated with gold under an argon gas flow for 20 min. Thecoated samples were left to dry at room temperature for 2 h before SEM could be performed. The SEM analyses were done at 15 kV and at a magnification of 300.A Perkin-Elmer Spectrum 100 Fourier-transform infrared (FTIR) spectroscopefitted with an attenuated total reflection (ATR) detector, equipped with a diamond crystal, and a Perkin Elmer Multiscope FTIR microscope was used to study the chemicalbonding and molecular structure of the composites and polymers. The infrared (IR) spectra were recorded between 4000 and 650 cm1 at a resolution of 8 cm1. A clean, emptydiamond crystal was used for the collection of the background spectrum.A Perkin-Elmer DSC7 differential scanning calorimeter (DSC) from Waltham, MA,USA, was used for the DSC analyses. All the analyses were performed under nitrogenflow (20 ml min1). The PP samples (5–10 mg) were analysed at 25–180C at a rateof 10C min1, while the PLA samples were analysed at 25–160C at the same rate. Thesamples were heated, cooled, and reheated under the same conditions mentioned above.Three samples from each composition were analysed. Only the second heating scan wasused to determine the melting enthalpies and temperatures. The crystallization enthalpiesand temperatures were determined from the first cooling scan.The thermogravimetric analyses (TGAs) were carried out in a Perkin-Elmer TGA7thermogravimetric analyser, from Waltham, MA, USA. Nitrogen was used as the purgegas at a flow rate of 20 ml min1. The samples (5–10 mg) were heated at 30–600C at arate of 10C min1.A Perkin-Elmer Diamond dynamic mechanical analyzer (DMA) was used in determining the thermomechanical properties of all the samples. The samples, with thedimensions 50 12 1 mm, were analysed in the bending mode and the parameterswere as follows: Start temperatureLimit temperature60C170CRateL amplitudeMinimum tensionTension force gainFrequencyForce amplitude5C min110 mm300 mN1.51 Hz1000 mN The biodegradability (degradation through hydrolysis) of the samples was tested bymonitoring the mass loss during insertion in water at 80C, as well as by taking SEMpictures of the samples that were periodically removed from the hot water. The highertemperature was used to shorten the time of exposure in order to speed up the experiment.930 Journal of Thermoplastic Composite Materials 25(8)Results and discussionMorphologyThe SEM micrographs in Figure 1 show that for the PLA composites there are some fibrebreakages and a minimal number of fibre pull-outs. It seems as if some matrix materialmay be covering the fibres. The presence of matrix coverage on the fibre surfaces and thefibre breakages indicate good interaction between the fibre and the matrix, which resultsin better interfacial adhesion. The gaps between the fibre and the PLA matrix in the pictures clearly indicate that the fibre was trying to pull-out but it could not, which also indicates good fibre/matrix interaction. Huda et al.20 investigated the mechanical andthermomechanical properties of wood fibre-reinforced PLA, and compared them withwood fibre-reinforced PP composites which were processed in the same way by amicro-compounding moulding system. They reported that there was a good adhesionbetween the wood fibre and the PLA, and the maleic anhydride-grafted PP (MAPP) coupling agent had no effect on the morphological properties of the PLA/wood fibre composites. This is in line with our own observations.The SEM micrographs in Figure 2 seem to show surprisingly good adhesion betweenthe hydrophilic SF and the hydrophobic PP polymer, which does not reflect in theproperties that will be discussed later in this article. Most of the fibres at the fractureFigure 1. Scanning electron microscopy (SEM) micrograph of the fracture surface of a poly(lacticacid) (PLA)/sisal composite (300 magnification).Mofokeng et al. 931surface are still attached to the PP. The fibre breakages and the seeming lack of fibrepull-outs in all the pictures seem to indicate good matrix/fibre adhesion. The reason forthis may be that, at the high temperature used for composite preparation, the viscosity ofthe PP (because of its high MFI) was low enough for it to penetrate the pores on the fibresurfaces.To examine the existence and type of interfacial interaction in the composites,FTIR experiments were performed and compared with those of the neat PP and PLA.The regions of interest for PLA and the composites are 1780 and 1680 cm1 for theC¼O stretch, and 3600–3000 cm1 for the O–H stretch. The peaks at about 1750 and1180 cm1, which belong to the C¼O stretching and the C–O–C stretching of PLA,are clearly visible in all the PLA spectra (Figure 3). The O–H band for the PLA-basedsamples became more pronounced and broader and shifted to slightly lower wavenumbers, as the fibre content was increased. This is probably due to the ‘free’ hydroxyl groups that are now engaged in hydrogen bonding.21 There is a development of asmall peak just below the carbonyl peak at 1650 cm1. This is an O–H peak that originated from bending of the unresolved hydroxyl group of the absorbed water usuallycarried by cellulose.22,23 As expected, the FTIR spectra of the PP and the PP/sisalcomposites did not show any evidence of interfacial interaction between the polymerand the fibre.Figure 2. Scanning electron microscopy (SEM) micrograph of the fracture surface of a polypropylene (PP)/sisal composite (300 magnification).932 Journal of Thermoplastic Composite Materials 25(8)Thermogravimetric analysisThe TGA curves for all the samples are shown in Figures 4 and 5. One of the goals whenincorporating SF into the two polymers was to increase the temperature region in whichPLA and PP can be useful.24 SF loses moisture around 100C. The degradation of the SFis a two-step degradation process as indicated by the two mass loss steps at 285 and357C. The first step is due to the thermal depolymerisation of hemicellulose and theglycosidic linkages of cellulose. The second step is due to the cellulose decompositionwhich produces a relatively high char residue.25,26 Only one degradation step wasobserved for the neat polymer and for the composites, and the thermal stability was betterfor the composites but did not change significantly with increasing fibre content. Thesurrounding polymer seems to have protected the fibre from lower-temperature degradation, and the interaction between the fibre and the polymer seems to have retarded thedegradation process and/or restricted the diffusion of volatile degradation products outof the sample. The degradation of sisal in the composites was not observed as a separatestep (Figure 4), and we assume that this was due to the low fibre contents and the goodinteraction between the fibre and PLA.Pure PP shows a single degradation step, while the composites show two degradationsteps (Figure 5). The low mass loss around 330C is due to the degradation of SF, whileFigure 3. Fourier-transform infrared (FTIR) spectra of poly(lactic acid) (PLA) and the PLA/sisalfibre composites.Mofokeng et al. 933the major step around 440C is due to the degradation of PP. The degradation temperatures of both the fibre and PP in the composites are significantly higher than those ofpure fibre and pure PP. Salemane et al.27 reported the same behaviour when PP was filledwith wood flour (WF) of different WF particle sizes. The fibre probably starteddegrading at higher temperatures because of the thermal protection by the thermallymore stable PP that surrounded the fibre. The increased thermal stability of the PP isprobably because the presence of the fibre might have (i) immobilized the free radicalsformed during the initiation of the degradation of the polymer chains and (ii) inhibitedthe diffusion of volatile degradation products because of the interaction with the fibre.Differential scanning calorimetryThe DSC results of the annealed PLA samples are shown in Figure 6. The glass transition, cold crystallisation, and melting of the samples are clearly observed in the curves.The glass transition of the annealed neat PLA shows a sharp transition called a hysteresispeak. This happens mostly with annealed semicrystalline polymers. The reason for thisendothermic hysteresis peak is not a lowering of the enthalpy of the polymer due toannealing but a change in the kinetics of unfreezing.28 The sharpness and the intensity ofthe hysteresis peak decrease in the presence of the SF. This is an indication that theFigure 4. Thermogravimetric analysis (TGA) curves of poly(lactic acid) (PLA) and the PLA/sisalfibre composites.934 Journal of Thermoplastic Composite Materials 25(8)presence of fibre in some way changed the morphology of the amorphous fractions in thepolymer during the cooling process. The glass transition temperature of the PLA did notchange for the fibre composites (Table 1). The reasons for this is that (i) the annealingproduced more, and more perfect, crystals that acted as physical crosslinks and reducedthe mobility of the amorphous fraction in the polymer and (ii) the fibre content was toolow and any anticipated influence that it may have had on the polymer chain mobilitywas overshadowed by the influence of the PLA crystals.The exothermic transition around 110C (Figure 6) is due to the cold crystallization ofPLA, and it is sharp and well resolved for the sample containing 1 wt% of SF. This peakshifts to higher temperatures and also decreases in intensity with the increase in the SFcontent. The fibres can either act as nucleation sites for the crystallization of the polymeror restrict the mobility of the polymer chains. These two actions may always be presentwhen a polymer is cooled in the presence of well-dispersed short fibres. When only 1%of SF was present, the nucleation by the short fibres was probably dominant, giving riseto a large extent of cold crystallization and the formation of two melting peaks, indicative of two crystal fractions (meta-stable and perfected crystals28) formed during theoriginal cooling of the sample and the subsequent cold crystallization. As the fibreFigure 5. Thermogravimetric analysis (TGA) curves of polypropylene (PP) and the PP/sisal fibrecomposites.Mofokeng et al. 935content increased (2 wt%), the cold crystallisation peak intensity decreased and it shiftedto higher temperatures. The intensity of the second melting peak also decreased, and bothmelting peaks shifted to lower temperatures. This indicates that the immobilizationeffect became more dominant. At higher fibre content (3 wt%), the cold crystallisationpeak shifted to even higher temperatures and its intensity decreased significantly, and thesecond melting peak completely disappeared. This shows that the immobilization effectwas even more dominant, but the fact that the melting temperature for this composite wasstill higher than that of neat PLA shows that some fibres still acted as nucleating agents.Figure 6. Differential scanning calorimetry (DSC) heating curves of poly(lactic acid) (PLA) and thePLA/sisal fibre composites.Table 1. DSC results of all the investigated PLA and PLA/sisal composite samplesSample Tg (C) Tm (C)PLA 63.7 + 0.5 153.9 + 0.599/1 (w/w) PLA/sisal 63.4 + 0.2 170.1 + 0.598/2 (w/w) PLA/sisal 63.1 + 0.2 165.4 + 0.897/3 (w/w) PLA/sisal 63.2 + 0.3 156.0 + 1.3DSC: differential scanning calorimetry, Tg: glass transition temperature, PLA: poly(lactic acid), Tm: peaktemperature of melting.936 Journal of Thermoplastic Composite Materials 25(8)Figures 7 and 8 illustrate the DSC heating and cooling curves, and Table 2summarizes the DSC results for PP and the PP/SF composites. Figure 7 and Table 2 showthat the melting temperatures of the composites are higher than that of pure PP, and theTable 2. DSC results of all the investigated PP and PP/sisal composite samplesSample DHexp m (J g1) DHcalc m (J g1) Tm (C)PP 63.2 + 2.3 63.2 165.9 + 1.999/1 (w/w) PP/sisal 65.7 + 7.5 62.6 168.0 + 2.898/2 (w/w) PP/sisal 70.0 + 1.8 61.9 167.7 + 0.897/3 (w/w) PP/sisal 71.3 + 5.1 61.3 167.7 + 1.4DHexpc (J g1) DHcalc c (J g1) Tc (C)PP 91.7 + 0.8 91.7 112.1 + 0.699/1 (w/w) PP/sisal 95.3 + 1.6 90.8 120.6 + 0.298/2 (w/w) PP/sisal 87.5 + 1.3 89.9 120.3 + 0.997/3 (w/w) PP/sisal 89.8 + 1.7 89.0 120.2 + 0.8PP: polypropylene, DSC: differential scanning calorimetry, Tm: peak temperature of melting, DHm: meltingenthalpy, Tc: peak temperature of crystallization, DHc: crystallization enthalpy.Figure 7. Differential scanning calorimetry (DSC) heating curves of polypropylene (PP) and thePP/sisal fibre composites.Mofokeng et al. 937experimental melting enthalpies are higher than the calculated ones, indicating a highertotal crystallinity for the composites. The crystallisation temperature (Tc) values of PP inthe PP/SF composites are higher than that of pure PP. This indicates that the fibres actedas nucleating agents, hence the PP in the composites started crystallizing at highertemperatures. Normally, upon cooling, the polymer will crystallize at a given temperature, and an effective nucleating agent will cause the Tc of the polymer to increase,because crystallization will start earlier during the cooling process. This behaviour canbe explained by the fibre surfaces acting as nucleation sites for the crystallization of thepolymer matrix.29Dynamic mechanical analysisFigures 9–14 show how the addition of different contents of SF influenced the storagemodulus (E00), loss modulus (E0), and damping factor (tan d) of PLA, PP, and theirrespective composites. The storage modulus (E0) below the glass transition of PLAslightly decreased with increasing fibre content. Mathew et al.30 studied the crystallisation of PLA in the presence of different cellulose-based reinforcements. Theyreported that the heat treatment was not expected to result in 100% crystallinity but thatFigure 8. Differential scanning calorimetry (DSC) cooling curves of polypropylene (PP) and thePP/sisal fibre composites.938 Journal of Thermoplastic Composite Materials 25(8)the crystalline regions might restrict the chain mobility. Therefore, the introduction offibres might decrease the stiffness by possibly acting like plasticizers.The sharp decrease in the storage modulus (around 59–63C for most of the samples)corresponds to the a-relaxation of the amorphous regions in PLA.30 The glass transitiontemperature (Tg) seems to increase with increasing SF content. This shows that thepresence of fibre restricts the segmental motion of the polymer chains. The loss moduluscurves in Figure 10 confirm this slight increase in the glass transition temperature. Thestorage and loss moduli started to increase again at temperatures around 100C, which isthe result of the cold crystallization which is typical for PLA,31 and which was alsoobserved in the DSC curves. The decrease in modulus at temperatures around 140Cindicates the softening of the sample before the onset of melting, which was observedfrom 140C in the DSC curves.A larger area under the a-relaxation peak in the tan d curves of a polymerindicates that the molecular chains exhibit a higher degree of mobility, thus betterdamping properties.32 The area under this peak (at about 67C) for the PLA composites(Figure 11) seems to be slightly larger for the fibre composites and shows no specifictrend with increase in the fibre content. There is very little difference between the peaktemperatures of this peak for the PLA and the composite samples, and there is noincreasing or decreasing trend with increasing fibre content. It therefore seems as if theFigure 9. Dynamic mechanical analysis (DMA) storage modulus curves of poly(lactic acid) (PLA)and the PLA/sisal fibre composites.Mofokeng et al. 939Figure 10. Dynamic mechanical analysis (DMA) loss modulus curves of poly(lactic acid) (PLA) andthe PLA/sisal fibre composites.Figure 11. Dynamic mechanical analysis (DMA) damping factor curves of poly(lactic acid) (PLA)and the PLA/sisal fibre composites.940 Journal of Thermoplastic Composite Materials 25(8)fibre did have some influence on the PLA chain mobility, but that the fibre contentswere too low to conclusively establish such influence. The broad transition (between90 and 130C) for all the samples relates to the cold crystallisation of PLA in thistemperature region.Figure 12 shows the temperature dependence on the E0 for neat PP and its composites. The E0 increased when SF was introduced into PP. The increase in the E0indicates an increase in the stiffness of the polymer, which may be the result of arestriction in the segmental motion. This is confirmed by the observably higher Tg ofthe fibre-containing samples (Figure 13). The PP and PP/sisal composite curves showtwo relaxation peaks at about 15 and 70C (Figure 14). The low-temperature peak isrelated to the b-transition of the amorphous fractions of PP and is considered the glasstransition. The higher temperature peak corresponds to the a-transition related to thePP crystalline fractions.33 The composite b-transitions are observed at a slightlyhigher temperature than that of the pure PP, which indicates an immobilization effectof the fibre on the PP chains. The a-relaxation peak is observed for pure PP and all thecomposite samples, indicating that the fibre was more concentrated in the pure amorphous phase of the polymer, because it had very little influence on the interlamellarchain mobility.Figure 12. Dynamic mechanical analysis (DMA) storage modulus curves of polypropylene (PP)and the PP/sisal fibre composites.Mofokeng et al. 941Degradability through hydrolysisThe mass loss results of the PLA samples after being immersed in distilled water at 80Cfor 10 days are shown in Figure 15. The mass loss of neat PLA and its compositesincreased almost linearly with time of immersion. A total mass loss of almost 80% wasobserved for neat PLA after 10 days. At this point, the neat PLA sample was very softand it was difficult to decant water and rinse the sample without losing some material,therefore the experiment was discontinued after 10 days. This mass loss was due to theremoval of PLA from the surface upon biodegradation. In PLA biodegradation, moisturesusceptibility is the primary driving force and involves the following four steps: waterabsorption, ester cleavage forming oligomers, solubilisation of oligomer fractions, anddiffusion of bacteria into soluble oligomers.31,34 The mass loss of the composites followed the same trend and show mass loss values within the same order of magnitude asthose of neat PLA. The observed mass loss is due to the removal of the PLA matrix onthe surface as a result of biodegradation,35 together with the removal of some substancesfrom the SF during the immersion. Chow et al.36 studied the effect of water absorption (at90C) on the mechanical properties of SF/PP (SF/PP) composites. They reported that themass loss of the composites was due to the removal of certain fractions from the SFduring the water immersion. They analysed the samples’ morphology before and afterFigure 13. Dynamic mechanical analysis (DMA) loss modulus curves of polypropylene (PP) andthe PP/sisal fibre composites.942 Journal of Thermoplastic Composite Materials 25(8)immersion using SEM. The results of the tensile broken samples before immersionshowed that the fibre was intact in the PP matrix. For the samples immersed in the hotwater for 72 h, serious debonding between the fibre and the matrix was observed, but theSF did not show any signs of degradation. After 216 h immersion, the interface betweenthe SF and PP was totally removed, a significant portion of the lignin and hemicellulosehad been leached out, and the SF showed microfibrillation. Lee and Wang37 studied thebiodegradation of PLA/bamboo fibre (BF), and they reported that the degradation ofPLA was slow but enhanced in the presence of BF. The SEM results indicated that mostof the matrix was degraded even though the degradation time was short. Shibata et al.38studied the biodegradation of PLA/abaca fibre and reported higher mass loss of theuntreated fibre composites compared to the neat polymer. They could not concludewhether the biodegradation of the matrix polymer was promoted by the presence ofabaca fibre. However, they did not exclude the possibility that the presence of the highlyhydrophilic fibre promoted the biodegradation of the matrix, probably by an action ofhydrolytic depolymerisation.No degradation was anticipated for PP during the immersion in water at 80C. Theneat PP showed no mass loss up to 10 days of immersion, and all the PP/SF compositesshowed less than 1% mass loss after 10 days, which is probably due to limited degradation of the fibre on the sample surface. The fibre in the bulk would have been protectedby the surrounding PP. This mass loss of the PP composites was significantly lower thanthose of the PLA and its composites.The SEM micrographs of the surface of the biodegraded samples immersed in waterat 80C for 2, 8, and 10 days are displayed in Figures 16 and 17. The neat PLA showsFigure 14. Dynamic mechanical analysis (DMA) damping factor curves of polypropylene (PP) andthe PP/sisal fibre composites.Mofokeng et al. 943only some small cracks after 2 days, but the cracks become larger when the SF contentincreases (Figure 16). This shows that the fibre enhanced the biodegradability of thePLA. Figure 17 shows the samples that were immersed for 10 days, and visible lines andFigure 15. Mass loss as function of time for poly(lactic acid) (PLA) and PLA/sisal fibre compositesduring immersion in water at 80C.Figure 16. Scanning electron microscopy (SEM) micrographs (200 magnification) of the biodegraded surfaces of (a) poly(lactic acid) (PLA) and (b) 98/2 (w/w) PLA/sisal after 2 days of immersion.944 Journal of Thermoplastic Composite Materials 25(8)pores were formed on the surface of neat PLA after 10 days of immersion (Figure 17a).The composites with fibre content of 2 and 3 wt% (Figure 17b) show the collapsedsurface of the samples. This indicates that the fibre itself degrades in the process ofimmersion in water at 80C, resulting in further biodegradation of the PLA matrix.ConclusionsPLA and PP were comparatively investigated as matrices for injection-moulded composites containing low (1–3 wt%) short SF contents. The SEM micrographs of the composites show more intimate contact and better interaction between the fibres and PLA,compared to PP, although surprisingly good adhesion was observed for the PP composites. The improved interaction in the case of the PLA composites was confirmed by theFTIR spectroscopy results that showed the presence of hydrogen bonding interactionbetween PLA and the fibres. The thermal stability (as determined through TGA) of bothpolymers increased with increasing fibre content, with a more significant improvementin the case of PP. The DSC results showed a significant influence of the presence of thefibres on the cold crystallization and melting behaviour of PLA, even at the low fibrecontents of 1–3%. The influence of the fibres on the melting characteristics of the PPwas negligible, but it had a significant influence on the nonisothermal crystallizationtemperature range. Both the storage and loss moduli of the PLA decreased withincreasing fibre content below the glass transition of PLA, but the influence on the lossmodulus was more significant. The DMA results clearly show cold crystallization ofPLA around 110C, and the presence of fibres gave rise to higher modulus valuesbetween the cold crystallization and melting of the PLA. The damping properties ofPLA seem to have increased in the presence of SF, but the fibre did not seem to havea significant influence on the Tg of PLA. The presence of fibres increased the Tg of PP,but had almost no influence on the a-relaxation, which is related to the crystalline fraction of PP. Although the presence of SFs had little influence on the mass loss of PLAFigure 17. Scanning electron microscopy (SEM) micrographs (200 magnification) of the biodegraded surfaces of (a) PLA and (b) 98/2 (w/w) PLA/sisal after 10 days of immersion.Mofokeng et al. 945during degradation in water at 80C, the degradation mechanism seems to have beeninfluenced by the presence of fibres.AcknowledgementsThe National Research Foundation and the University of the Free State in South Africaare acknowledged for their financial support of this project. The work reported in thisarticle was also supported by the Ja´nos Bolyai Research Scholarship of the HungarianAcademy of Sciences. The authors would further like to thank Arburg Hungaria Ltd., forthe injection moulding machine. This work is connected to the scientific program of the‘Development of quality-oriented and harmonized RþDþI strategy and functionalmodel at BME’ and was supported by the New Hungary Development Plan (ProjectID: TA´ MOP-4.2.1/B-09/1/KMR-2010-0002).FundingThis research received funding from agencies as mentioned under ‘Acknowledgements’.References1. Mishra S, Mohanty AK, Drzal LT, Misra M and Hindrichsen GA. A review on pineapple leaffibers, sisal fibers and their biocomposites. Macromol Mater Eng 2004; 289: 955–974.2. Jayaraman K. Manufacturing sisal–polypropylene composites with minimum fibre degradation. Compos Sci Technol 2003; 63: 367–374.3. Arzondo LM, Pe´rez CJ and Carella JM. Injection molding of long sisal fiber-reinforcedpolypropylene: effects of compatibilizer concentration and viscosity on fiber adhesion andthermal degradation. Polym Eng Sci 2005; 45: 613–621.4. Joseph PV, Joseph K and Thomas S. 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