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POLYMERPLASTICS TECHNOLOGY AND ENGINEERINGVol. 43, No. 3, pp. 871888, 2004Wood Fiber Reinforced PolypropyleneComposites: Compression and InjectionMolding ProcessAndrzej K. Bledzki*and Omar FarukInstitut fu r Werkstofftechnik, Kunststoff- und Recyclingtechnik,University of Kassel, Kassel, GermanyABSTRACTWood fiber reinforced polypropylene composites containing differ-ent types of wood fiber (hard and softwood fiber) were prepared byan injection molding and a compression molding process. Influenceof different processing systems and compatibilizer on the compositemechanical properties was investigated. The present study investi-gated the tensile, flexural, charpy impact, and impact properties ofwood-fiber reinforced polypropylene composites as a function ofprocessing system and compatibilizer. From the results, it is observedthat injection molding process showed better tensile and flexuralproperties comparative with compression molding process, which isabout 155% and 60% for tensile strength, and flexural strength,*Correspondence: Andrzej K. Bledzki, Institut fu r Werkstofftechnik, Kunststoff-und Recyclingtechnik, University of Kassel, Mo nchebergstr.3, D-34109 Kassel,Germany; Fax: 49-561-8043692; E-mail: kutechuni-kassel.de.871DOI: 10.1081/PPT-1200380680360-2559 (Print); 1525-6111 (Online)Copyright & 2004 by Marcel Dekker, IORDER REPRINTSrespectively. Charpy impact strength increased the maximum incompression molding for hard-wood fiber-PP (PP) composites, and itis about 70% at the wood-fiber content of 30%. Impact resistance ininjection molding process shows better performance comparison tocompression molding process and damping index decreased the mostat about 60% in injection molding process for hard-wood fiber-PPcomposites at the wood-fiber content of 50%. Scanning electronmicroscopy of the composites also is investigated to better under-stand the interaction between wood fiber and polypropylene in bothprocessing systems.Key Words:Woodfiber-PPcomposites;Injectionmolding;Compression molding; Mechanical properties.INTRODUCTIONPolypropylene-wood fiber composites are used as substitutes formore expensive and less environmentally friendly materials. Polypropyl-ene is a recyclable polymer and wood fibers derive from a renewablesource and are biodegradable. The use of wood fibers in a polypropylenematrix includes many benefits, such as improved dimensional stabilityof composites, lower processing temperatures, increased heat deflectiontemperature, improved wood surface appearance, lighter products,low volumetric cost, up to 30% reduced cycle time for injection moldedproducts, and production of good performance materials.1Recent progress in compounding technology improves their compe-titiveness against conventional reinforcing agents such as glass fibers andmineral particles. Wood flour fillers are readily available by grinding ofwood. As a function of the grinding processes, it is possible to controlsize, size distribution, shape, and the aspect ratio of wood flour particles.Typically wood flour comprises a mixture of broken fibers, partiallyfibrillated fibers, and fiber bundles. Compounding wood flour togetherwith polypropylene can afford an attractive combination of high specificstiffness and strength, less abrasion during processing, low density, andlow price with respect to mineral fillers.Injection molding and extrusion are established processes formanufacturing wood fiber-thermoplastic composites in prismatic orsheet forms. Injection molding requires a polymer with a low molecularweight to maintain low viscosity. Johnson Controls Automotive2presented a overview of the state of the art of the use of plastic-naturalfiber composite materials for interior car parts and the technologies to872Bledzki and FarukORDER REPRINTSproducesuchparts(injectionmolding,lowpressureinjectionmolding,andco-injection molding). With emphasis on the research lines performed onseveral kinds of natural and wood fibers (jute, flax, kenaf, eucalyptus) tobe applied to semi-finished products: granules (short natural fiber) forinjection molding process.The properties of natural (flax fiber,3,4hemp,5jute,6rice hull,7and sisal fiber8,9) and wood fiber10reinforced polymer composites wereinvestigated by the injection molding process.Thermoplastic fiber-reinforced composites are distinguished fromthermoset reinforced composites primarily by a high elongation at break,short cycle times, and the possibility of recycling. The compressionmolding technique proved suitable for the production of profiles with anythermoplastic prepreg used. Compression molding brings the thermo-plastic prepreg gently to the required shape without overcompressing thematerial. The different layer orientations are thus retained after molding.Johnson Controls11compared new materials and processes forthe manufacture of automotive door panels. The material is Fibropur,a natural fiber mat (flax, sisal, hemp, kenaf) sprayed with PU-resinproduced by compression molding.Flax fiber,1214wood fiber,15and jute fiber16reinforced compo-sites also were prepared by compression molding process.A recent review report17describes the reinforcement of natural andwood fibers into polymer considering different processing systems (extru-sion, injection molding, compression molding, mixer and express process).An important feature of the compounding processes is the addition ofcompatibilizers, which are required to overcome incompatibility betweenthe polar wood and the nonpolar hydrocarbon polymer. Inadequatecompatibility frequently is accompanied by significantly reduced impactand tensile strength.The objective of these studies is to compare the mechanicalproperties of wood-fiber(PP) composites between the injection moldingand the compression molding process.EXPERIMENTALMaterialsPolymeric MatrixPolypropylene (Stamylan P17M10) was provided as granules byDSM, Gelsenkirchen, Germany. Its melting temperature was 173?C andWood Fiber Reinforced Polypropylene Composites873ORDER REPRINTSmeltingindexwas10.5g/10minat230?C.Itsdensityatroomtemperature was 0.905g/cm3.Wood FibersStandard hard-wood fiber (Lignocel HBS 150-500) and soft-woodfiber (Lignocel BK 40-90) with particle size of 150500mm, were suppliedby J. Rettenmaier and So hne GmbHCo. Germany. It is also notablethat fiber structure of hard- and soft-wood fiber is fibrous and cubic,respectively.CompatibilizerA commercially available maleic anhydride-polypropylene copoly-mer (Licomont, AR 504 FG) was used as a compatibilizer for fibertreatment, and it was obtained from Clariant Corp., Frankfurt,Germany. It was used 5% by weight relative to the wood fiber contentand was expected to improve the compatibility and adhesion between thewood fiber and the PP matrix.Compounding and Sample PreparationInjection Molding ProcessPolypropylene granules with hard-wood fiber and soft-wood fiber(30% and 50% by weight) were mixed by twin-screw extruder (Haakeextruder, Rheomex PTW 25/32) with and without compatibilizer. All thewood fibers were initially dried at 80?C in an air-circulating oven for24hr before mixing. The extruded granules were dried again 80?C for24hr (water content 1%) before the sample preparation by the injectionmolding process. Test samples were prepared from dried granules by theinjection molding process at melting temperature 150?C180?C, moldtemperature of 80?C100?C, and under a injection pressure 20kN/mm2.Compression Molding ProcessPolypropylene granules were converted into powder and then mixedwith wood fibers. The wood fiber and PP powder mixture were placed874Bledzki and FarukORDER REPRINTSinto a block cylinder compression molding machine under a pressure20kN/mm2till the temperature reached at 190?C. Then the cylinderpressed for 5min under a pressure 20kN/mm2, and then it was followedby cooling (10?C/min) in another press equipped with refrigerationfacilities. The prepared sheet (7mm) then was placed into a compressionmolding machine at 180?C for 510min under a pressure 3kN/cm2tobring the sheet to a 2mm thickness. Rectangular specimens were cut fromthe pressed sheets according to a DIN number for various mechanicalexperiments.MeasurementsThe tensile and flexural strength (Zwick Machine, UPM 1446) weretested at a test speed of 2mm/min according to EN ISO 527 and EN ISO178 for different wood fiberPP composites with and without a com-patibilizer in both processes. All the tests were investigated at roomtemperature (23?C) and at a relative humidity of 50%.A charpy impact test (EN ISO 179) was carried out with 10unnotched samples. In each series standard deviation (15%) was usedto measure charpy impact energy.To measure the impact characteristics values, the specimens weretested by using a low-velocity falling weight impact tester (EN ISO 6603-2) at room temperature in nonpenetration mode. The impactor had amass of 0.75kg, and the impact energy was 0.96J.Scanning Election MicroscopeThe morphology of the wood-fiberPP composites prepared in bothprocesses were investigated by using a scanning electron microscope(SEM) (VEGA TESCAN), whereas, fractured surfaces of flexural testsamples were studied with SEM after being sputter coated with gold.RESULTS AND DISCUSSIONWood fiberPP composites with 30 and 50wt% of fiber loading wereprepared to investigate the effect of processing systems on mechanicalproperties, like tensile and flexural strength, flexural E-modulus, charpyimpact strength, and impact properties of composites. We have repor-ted18earlier that wood-fiberPP composites, containing (MAH)PPWood Fiber Reinforced Polypropylene Composites875ORDER REPRINTScompatibilizer showed the best performance in the concentration of 5%(relative to the wood-fiber content). That is why, in our present work, thecontent of MAHPP was used at 5% for all types of wood fiberPPcomposites in both processes. The various properties of these compositesare discussed below.Results of tensile test of the wood-fiberPP composites are shown inFig. 1 with the variation of wood fiber (hard-wood fiber and soft-woodfiber) and with and without a compatibilizer for both processes. Ingeneral, the wood-fiberPP composites show an increasing trend in themechanical properties with the addition of a compatibilizer. Figure 1showed that the tensile strength of the composites prepared by theinjection molding process is higher compared to the composites preparedby the compression molding process, and it also illustrated that hard-wood-fiber-reinforced PP composites prepared by the injection moldingprocess showed highest tensile strength with the addition of a com-patibilizer, which is nearly at 155% increase to the compression moldingprocess at the 50% wood-fiber content.The effect of a processing system on the flexural properties of wood-fiberPP composites can be readily assessed from the Figs. 2 and 3. It isobserved that the flexural strength (Fig. 2) of the composites showed an0510152025303540WF30%WF30%+MAHPP5%WF50%WF50%+MAH-PP5%HW (injection molding)HW (compression molding)SW (injection molding)SW (compression molding)Tensile strength MPaFigure 1.Tensile strength of hard- and soft-wood-fiberPP composites withand without compatibilizer in both processes. (View this art in color at.)876Bledzki and FarukORDER REPRINTS010203040506070WF30%WF30%+MAH-PP5%WF50%WF50%+MAH-PP5%HW (injection molding)HW (compression molding)SW (injection molding)SW ( compression molding)Flexural strength MPaFigure 2.Flexural strength of hard- and soft-wood-fiberPP composites withand without compatibilizer in both processes. (View this art in color at.)0123456WF30%WF30%+MAH-PP5%WF50%WF50%+MAH-PP5%HW (injection molding)HW (compression molding)SW (injection molding)SW ( compression molding)Flexural E-modulus GPa Figure 3.Flexural E-modulus of hard- and soft-wood-fiberPP composites withand without compatibilizer in both processes. (View this art in color at.)Wood Fiber Reinforced Polypropylene Composites877ORDER REPRINTSincreasing tendency with the addition of a compatibilizer. With thecomparison between both processing systems, at the 30% wood fibercontent (both hard-wood fiber and soft-wood fiber) it is not a verysignificant difference. But at the 50% wood fiber content the injectionmolding process showed better flexural strength, with an increase about60% to compression molding process. Figure 3 showed that the flexuralE-modulus of the hard wood fiber and soft wood-fiberPP composites inboth processing system followed the same trend as flexural strength. Itmeans at the 30% wood-fiber content (both hard-wood fiber and soft-wood fiber) it is not very significant in difference. But at the 50% woodfiber content, the injection molding process showed better flexuralstrength, with an increasing tendency to the compression molding process.Figure 4 shows the variation of charpy impact strength of wood-fiberPP composites in both processes with the addition of a com-patibilizer. From the figures, it is seen that the charpy impact strength ofthe hardwood fiber and soft-wood-fiberPP composites are found tobe more, prepared by the compression molding process than by theinjection molding process. With the addition of compatibilizer in com-posites, charpy impact strength increased the maximum in the compres-sion molding process for hard-wood-fiberPP composites, and it is about70% at the wood fiber content 30%.The results of the impact test can be described by two separate issues,described in Fig. 5. They are:(a)Force-deflection curve: the force-deflection curve refers to allthe materials behaviors, including the damage initiation definedby the first significant drop of the force.(b)Characteristic values: loss energy (Wv) as a measure ofdissipated energy and strain energy (Ws) as a measure of thestored energy, and the damping index (?*) as a ratio of lossenergy to strain energy.Impact resistance of hard-wood fiber and soft-wood-fiberPPcomposites in both processes is shown in Fig. 6. Figure 6a illustratedthe impact resistance of hard-wood-fiberPP composites with andwithout a compatibilizer in both processes, and impact resistance in theinjection molding process shows better performance, where in thecompression molding process, a large amount of damage of initiationwas observed. But with the addition of a compatibilizer, impact resistanceof hard-wood-fiberPP composites shows highest performance in thecompression molding process, without having a large amount of damageof initiation. In the case of soft-wood-fiberPP composites (Fig. 6b), it is878Bledzki and FarukORDER REPRINTSclearly observed that impact resistance in the injection molding process,shows the better performance comparative to the compression moldingprocess, without having a large amount of damage of initiation, as withthe compression molding process.(a)(b)02468101214Charpy impact strength mJ/mm2Injection moldingCompression molding02468101214Charpy impact strength mJ/mm2Injection moldingCompression moldingSW30%+PP70%SW30%+PP70%+MAH-PP5%SW50%+PP50%SW50%+PP50%+MAH-PP5%HW30%+PP70%HW30%+PP70%+MAH-PP5%HW50%+PP50%HW50%+PP50%+MAH-PP5%Figure 4.Charpy impact strength of hard-wood-fiberPP composites (a) andsoft-wood-fiberPP composites (b) with and without compatibilizer in bothprocesses. (View this art in color at .)Wood Fiber Reinforced Polypropylene Composites879ORDER REPRINTSThe damping index for all samples was calculated by taking the ratioof dissipated energy (loss energy) to the stored energy (strain energy) tomeasure the impact characteristic values. The loss energy involves energythat is based on irreversible deformations, energy dissipation due to thecreation of matrix cracks and their propagation, delaminations, and,finally, fiber fracture.The damping index of hard- and soft-wood-fiberPP composites as afunction of having a compatibilizer in both processes is shown in Fig. 7.It is seen that the damping index in the injection molding process iscomparatively better than the compression molding process, but thisis not very significant. It is clearly evident that more damping index isdecreased with the addition of a compatibilizer in all cases and it ishighest for hard-wood-fiberPP composites (Fig. 7a) in the injectionmolding process at the wood fiber content 50%, which is nearly 60%.The flexural fractured surface of wood-fiberPP composites in bothinjection and compression molding processes examined with SEM arepresented in Figs. 810. Figure 8a, b shows the hard- and soft-wood-fiberPPcompositescontaining30%woodfibercontentinthecompression molding process. Both Figs. 8a and 8b show the hard-and soft-wood-fiberPP composites in the compression molding process,where present fiber pullout, debonding, fibrillation, and just, like a layerto layer. As we know, these structures (layer to layer) are responsiblefor higher charpy impact strength, and we observed that at Fig. 4where composites made from the compression molding process showedDeflectionForceloss energy (Wv)strain energy (Ws)damage initiationFigure 5.Typical impact force-deflection curve for fiber reinforced polymercomposites including definition of the characteristic values used.880Bledzki and FarukORDER REPRINTSbetter charpy strength in comparison with injection molding processcomposites.But with the addition of a compatibilizer indicates much betterinteraction between the wood fiber and the matrix in both processingsystems, which is represented in Figs. 9 and 10. Figures 9a and 9b(a) (b)100010020030040050060070001234567Deflection mmForce NHW30% (Injection molding)Hw30%+MAH-PP5% (Injection molding)HW30% (Compression molding)HW30%+MAH-PP5% (Compressionmolding)100010020030040050060070001234567Deflection mmForce NSW30% (Injection molding)SW30%+MAH-PP5% (Injection molding)SW30% (Compression molding)SW30%+MAH-PP5% (Compression molding)Figure 6.Impact resistance (maximum force) of hard-wood-fiberPP compos-ites and soft-wood-fiberPP composites (b) with and without compatibilizer inboth processes. (View this art in color at .)Wood Fiber Reinforced Polypropylene Composites881ORDER REPRINTS(a)(b)00.511.522.53Damping index -Injection moldingCompression moldingHW30%+PP70%HW30%+PP70%+MAH-PP5%HW50%+PP50%HW50%+PP50%+MAH-PP5%00.511.522.53Damping index -Injection moldingCompression moldingSW30%+PP70%SW30%+PP70%+MAH-PP5%SW50%+PP50%SW50%+PP50%+MAH-PP5%Figure 7.Damping index of hard-wood-fiberPP composites (a) and soft-wood-fiberPP composites (b) with and without compatibilizer in both processes. (Viewthis art in color at .)882Bledzki and FarukORDER REPRINTS(a)(b)Figure 8.SEM micrograph of hard- (a) and soft- (b) wood-fiberPP compositesin compression molding process (wood-fiber content 30%).Wood Fiber Reinforced Polypropylene Composites883ORDER REPRINTS(a)(b)Figure9.SEM micrographsof fracturedsurfaceof soft-wood-fiberPPcomposites in injection molding process (a) without MAHPP 5%, (b) withMAHPP 5%, wood-fiber content 50%.884Bledzki and FarukORDER REPRINTS(a)(b)Figure 10.SEM micrographs of fractured surface of soft-wood-fiberPPcomposites in compression molding process (a) without MAHPP 5%, (b)with MAHPP 5%, wood fiber content 50%.Wood Fiber Reinforced Polypropylene Composites885ORDER REPRINTSrepresent the microstructure of soft-wood-fiberPP composites contain-ing 50% wood-fiber content with and without a compatibilizer preparedby the injection molding process and Figs. 10a and 10b represent thebefore stated composites in compression molding process. It is alsonotable that in comparison between both processes, wood-fiberPPcomposites have a better interaction between wood fiber and the matrixin the injection molding process than the compression molding process;to better understand, the density of the composites prepared by bothprocesses also was measured. It was observed for hard-wood-fiberPPcomposites (30% wood-fiber content). In the injection molding process,composite density was 1.06g/cm3, which is 0.98g/cm3in the compressionmolding process. The lower density of the composites in the compressionmolding process refers to more void content, which represents, finally,poor bonding and interaction between wood fiber and PP. This wasexpectedalsofromthemechanicalpropertiesofwood-fiberPPcomposites in both molding processes.CONCLUSIONSThe influence of the processing systems (injection molding andcompression molding) on the mechanical properties of the hard-wood-and soft-wood-fiber-reinforcedPP composites were investigated in thisstudy. This also was conducted to experimentally determine the effects ofthe addition of a compatibilizer in different processing systems of wood-fiberPP composites. The following points can be drawn from thesestudies.The injection molding process showed better performance thanthe compression molding process (containing same pressure).Tensile and flexural strength increased to about 155% and 60%maximum, respectively, in the injection molding process com-pared with the compression molding process.Charpy impact strength increased the maximum in compressionmolding for hard-wood-fiberPP composites, and it is about 70%at the wood-fiber content 30%.Impactresistanceintheinjectionmoldingprocessshowsbetter performance in comparison to the compression moldingprocess.Damping index decreased by 60% with the addition of MAH-PP5% for hard-wood-fiberPP composites in the injection moldingprocess.886Bledzki and FarukORDER REPRINTSREFERENCES1.American Wood FibersCatalogue; USA, 1997.2.Magurno, A. Vegetable fibers in automotive interior components.Angewandte Makromoleculare Chemie 1999, 272, 99.3.Aurich, T.; Mennig, G. Injection molding of natural fiber reinforcedpolypropylene. Kunststoffe Plast Europe 1999, 89 (3), 6.4.Colberg, M.; Sauerbier, M. Injection molding of natural fiberreinforced plastics. Kun
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