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POLYMERPLASTICS TECHNOLOGY AND ENGINEERING Vol. 43, No. 3, pp. 871888, 2004 Wood Fiber Reinforced Polypropylene Composites: Compression and Injection Molding Process Andrzej K. Bledzki * and Omar Faruk Institut fur Werkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Kassel, Germany ABSTRACT Wood fiber reinforced polypropylene composites containing differ- ent types of wood fiber (hard and softwood fiber) were prepared by an injection molding and a compression molding process. Influence of different processing systems and compatibilizer on the composite mechanical properties was investigated. The present study investi- gated the tensile, flexural, charpy impact, and impact properties of wood-fiber reinforced polypropylene composites as a function of processingsystemandcompatibilizer.Fromtheresults,itisobserved that injection molding process showed better tensile and flexural properties comparative with compression molding process, which is about 155% and 60% for tensile strength, and flexural strength, *Correspondence: AndrzejK.Bledzki,InstitutfurWerkstofftechnik,Kunststoff- und Recyclingtechnik, University of Kassel, Monchebergstr.3, D-34109 Kassel, Germany; Fax: 49-561-8043692; E-mail: kutechuni-kassel.de. 871 DOI: 10.1081/PPT-120038068 0360-2559 (Print); 1525-6111 (Online) Copyright Injection molding; Compression molding; Mechanical properties. INTRODUCTION Polypropylene-wood fiber composites are used as substitutes for more expensive and less environmentally friendly materials. Polypropyl- ene is a recyclable polymer and wood fibers derive from a renewable source and are biodegradable. The use of wood fibers in a polypropylene matrix includes many benefits, such as improved dimensional stability of composites, lower processing temperatures, increased heat deflection temperature, improved wood surface appearance, lighter products, low volumetric cost, up to 30% reduced cycle time for injection molded products, and production of good performance materials. 1 Recent progress in compounding technology improves their compe- titiveness against conventional reinforcing agents such as glass fibersand mineral particles. Wood flour fillers are readily available by grinding of wood. As a function of the grinding processes, it is possible to control size, size distribution, shape, andthe aspect ratioof wood flourparticles. Typically wood flour comprises a mixture of broken fibers, partially fibrillated fibers, and fiber bundles. Compounding wood flour together with polypropylene can afford an attractive combination of high specific stiffness and strength, less abrasion during processing, low density, and low price with respect to mineral fillers. Injection molding and extrusion are established processes for manufacturing wood fiber-thermoplastic composites in prismatic or sheet forms. Injection molding requires a polymer with a low molecular weight to maintain low viscosity. Johnson Controls Automotive 2 presented a overview of the state of the art of the use of plastic-natural fiber composite materials for interior car parts and the technologies to 872 Bledzki and FarukORDER REPRINTS producesuchparts(injectionmolding,lowpressureinjectionmolding,and co-injection molding). With emphasis on the research lines performed on severalkinds ofnatural andwood fibers(jute, flax,kenaf,eucalyptus)to be applied to semi-finished products: granules (short natural fiber) for injection molding process. The properties of natural (flax fiber, 3,4 hemp, 5 jute, 6 rice hull, 7 andsisalfiber 8,9 )andwoodfiber 10 reinforcedpolymercompositeswere investigated by the injection molding process. Thermoplastic fiber-reinforced composites are distinguished from thermosetreinforcedcompositesprimarilybyahighelongationatbreak, short cycle times, and the possibility of recycling. The compression moldingtechniqueprovedsuitablefortheproductionofprofileswithany thermoplastic prepreg used. Compression molding brings the thermo- plastic prepreg gently to the required shape without overcompressing the material.The different layerorientations are thus retained aftermolding. Johnson Controls 11 compared new materials and processes for the manufacture of automotive door panels. The material is Fibropur, a natural fiber mat (flax, sisal, hemp, kenaf) sprayed with PU-resin produced by compression molding. Flax fiber, 1214 wood fiber, 15 and jute fiber 16 reinforced compo- sites also were prepared by compression molding process. A recent review report 17 describes the reinforcement of natural and woodfibersintopolymerconsideringdifferentprocessingsystems(extru- sion,injectionmolding,compressionmolding,mixerandexpressprocess). Animportantfeatureofthecompoundingprocessesistheadditionof compatibilizers, which are required to overcome incompatibility between the polar wood and the nonpolar hydrocarbon polymer. Inadequate compatibility frequently is accompanied by significantly reduced impact and tensile strength. The objective of these studies is to compare the mechanical properties of wood-fiber(PP) composites between the injection molding and the compression molding process. EXPERIMENTAL Materials Polymeric Matrix Polypropylene (Stamylan P17M10) was provided as granules by DSM, Gelsenkirchen, Germany. Its melting temperature was 173 Cand Wood Fiber Reinforced Polypropylene Composites 873ORDER REPRINTS melting index was 10.5g/10min at 230 C. Its density at room temperature was 0.905g/cm 3 . Wood Fibers Standard hard-wood fiber (Lignocel HBS 150-500) and soft-wood fiber(LignocelBK40-90)withparticlesizeof150500mm,weresupplied by J. Rettenmaier and Sohne GmbHCo. Germany. It is also notable that fiber structure of hard- and soft-wood fiber is fibrous and cubic, respectively. Compatibilizer A commercially available maleic anhydride-polypropylene copoly- mer (Licomont, AR 504 FG) was used as a compatibilizer for fiber treatment, and it was obtained from Clariant Corp., Frankfurt, Germany. It was used 5% by weight relative to the wood fiber content andwasexpectedtoimprovethecompatibilityandadhesionbetweenthe wood fiber and the PP matrix. Compounding and Sample Preparation Injection Molding Process Polypropylene granules with hard-wood fiber and soft-wood fiber (30% and 50% by weight) were mixed by twin-screw extruder (Haake extruder, Rheomex PTW 25/32) with and without compatibilizer. All the wood fibers were initially dried at 80 C in an air-circulating oven for 24hr before mixing. The extruded granules were dried again 80 C for 24hr(watercontent1%)beforethesamplepreparationbytheinjection molding process. Test samples were prepared from dried granules by the injection molding process at melting temperature 150 C180 C, mold temperature of 80 C100 C, and under a injection pressure 20kN/mm 2 . Compression Molding Process Polypropylene granules were converted into powder and then mixed with wood fibers. The wood fiber and PP powder mixture were placed 874 Bledzki and FarukORDER REPRINTS into a block cylinder compression molding machine under a pressure 20kN/mm 2 till the temperature reached at 190 C. Then the cylinder pressed for 5min under a pressure 20kN/mm 2 , and then it was followed by cooling (10 C/min) in another press equipped with refrigeration facilities. The prepared sheet (7mm) then was placed into a compression molding machine at 180 C for 510min under a pressure 3kN/cm 2 to bringthesheettoa2mmthickness.Rectangularspecimenswerecutfrom the pressed sheets according to a DIN number for various mechanical experiments. Measurements The tensile and flexural strength (Zwick Machine, UPM 1446) were testedatatestspeedof2mm/minaccordingtoENISO527andENISO 178 for different wood fiberPP composites with and without a com- patibilizer in both processes. All the tests were investigated at room temperature (23 C) and at a relative humidity of 50%. A charpy impact test (EN ISO 179) was carried out with 10 unnotched samples. In each series standard deviation (15%) was used to measure charpy impact energy. To measure the impact characteristics values, the specimens were tested by using a low-velocity falling weight impact tester (EN ISO 6603- 2) at room temperature in nonpenetration mode. The impactor had a mass of 0.75kg, and the impact energy was 0.96J. Scanning Election Microscope The morphology of the wood-fiberPP composites prepared in both processes were investigated by using a scanning electron microscope (SEM) (VEGA TESCAN), whereas, fractured surfaces of flexural test samples were studied with SEM after being sputter coated with gold. RESULTS AND DISCUSSION WoodfiberPPcompositeswith30and50wt%offiberloadingwere prepared to investigate the effect of processing systems on mechanical properties, like tensile and flexural strength, flexural E-modulus, charpy impact strength, and impact properties of composites. We have repor- ted 18 earlier that wood-fiberPP composites, containing (MAH)PP Wood Fiber Reinforced Polypropylene Composites 875ORDER REPRINTS compatibilizer showed the best performance in the concentration of 5% (relativetothewood-fibercontent).Thatiswhy,inourpresentwork,the content of MAHPP was used at 5% for all types of wood fiberPP composites in both processes. The various properties of these composites are discussed below. Results of tensile test of the wood-fiberPP composites are shown in Fig. 1 with the variation of wood fiber (hard-wood fiber and soft-wood fiber) and with and without a compatibilizer for both processes. In general, the wood-fiberPP composites show an increasing trend in the mechanical properties with the addition of a compatibilizer. Figure 1 showed that the tensile strength of the composites prepared by the injection molding process is higher compared to the composites prepared by the compression molding process, and it also illustrated that hard- wood-fiber-reinforced PP composites prepared by the injection molding process showed highest tensile strength with the addition of a com- patibilizer, which is nearly at 155% increase to the compression molding process 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 is observed that the flexural strength (Fig. 2) of the composites showed an 0 5 10 15 20 25 30 35 40 WF30% WF30%+MAHPP5% WF50% WF50%+MAH-PP5% HW (injection molding) HW (compression molding) SW (injection molding) SW (compression molding) Tensile strength MPa Figure 1. Tensile strength of hard- and soft-wood-fiberPP composites with and without compatibilizer in both processes. (View this art in color at .) 876 Bledzki and FarukORDER REPRINTS 0 10 20 30 40 50 60 70 WF30% WF30%+MAH-PP5% WF50% WF50%+MAH-PP5% HW (injection molding) HW (compression molding) SW (injection molding) SW ( compression molding) Flexural strength MPa Figure 2. Flexural strength of hard- and soft-wood-fiberPP composites with and without compatibilizer in both processes. (View this art in color at .) 0 1 2 3 4 5 6 WF30% 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-modulusofhard- andsoft-wood-fiberPPcomposites with and without compatibilizer in both processes. (View this art in color at .) Wood Fiber Reinforced Polypropylene Composites 877ORDER REPRINTS increasing tendency with the addition of a compatibilizer. With the comparison between both processing systems, at the 30% wood fiber content (both hard-wood fiber and soft-wood fiber) it is not a very significant difference. But at the 50% wood fiber content the injection molding process showed better flexural strength, with an increase about 60% to compression molding process. Figure 3 showed that the flexural E-modulus of the hard wood fiber andsoft wood-fiberPPcomposites in both processing system followed the same trend as flexural strength. It means 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% wood fiber content, the injection molding process showed better flexural strength,withanincreasingtendencytothecompressionmoldingprocess. 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 of the hardwood fiber and soft-wood-fiberPP composites are found to be more, prepared by the compression molding process than by the injection molding process. With the addition of compatibilizer in com- posites, charpy impact strength increased the maximum in the compres- sionmoldingprocessforhard-wood-fiberPPcomposites,anditisabout 70% at the wood fiber content 30%. Theresultsoftheimpacttestcanbedescribedbytwoseparateissues, described in Fig. 5. They are: (a) Force-deflection curve: the force-deflection curve refers to all the materials behaviors, including the damageinitiation defined by the first significant drop of the force. (b) Characteristic values: loss energy (Wv) as a measure of dissipated energy and strain energy (Ws) as a measure of the stored energy, and the damping index ( *) as a ratio of loss energy to strain energy. Impact resistance of hard-wood fiber and soft-wood-fiberPP composites in both processes is shown in Fig. 6. Figure 6a illustrated the impact resistance of hard-wood-fiberPP composites with and without a compatibilizer in both processes, and impact resistance in the injection molding process shows better performance, where in the compression molding process, a large amount of damage of initiation wasobserved.Butwiththeadditionofacompatibilizer,impactresistance of hard-wood-fiberPP composites shows highest performance in the compression molding process, without having a large amount of damage of initiation. In the case of soft-wood-fiberPP composites (Fig. 6b), it is 878 Bledzki and FarukORDER REPRINTS clearly observed that impact resistance in the injection molding process, shows the better performance comparative to the compression molding process, without having a large amount of damage of initiation, as with the compression molding process. (a) (b) 0 2 4 6 8 10 12 14 Charpy impact strength mJ/mm 2 Injection molding Compression molding 0 2 4 6 8 10 12 14 Charpy impact strength mJ/mm 2 Injection molding Compression molding SW30%+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) and soft-wood-fiberPP composites (b) with and without compatibilizer in both processes. (View this art in color at .) Wood Fiber Reinforced Polypropylene Composites 879ORDER REPRINTS Thedampingindexforallsampleswascalculatedbytakingtheratio of dissipated energy (loss energy) to the stored energy (strain energy) to measurethe impact characteristic values. The lossenergy involves energy that is based on irreversible deformations, energy dissipation due to the creation of matrix cracks and their propagation, delaminations, and, finally, fiber fracture. Thedampingindexofhard-andsoft-wood-fiberPPcompositesasa function of having a compatibilizer in both processes is shown in Fig. 7. It is seen that the damping index in the injection molding process is comparatively better than the compression molding process, but this is not very significant. It is clearly evident that more damping index is decreased with the addition of a compatibilizer in all cases and it is highest for hard-wood-fiberPP composites (Fig. 7a) in the injection molding process at the wood fiber content 50%, which is nearly 60%. The flexural fractured surface of wood-fiberPP composites in both injection and compression molding processes examined with SEM are presented in Figs. 810. Figure 8a, b shows the hard- and soft-wood- fiberPP composites containing 30% wood fiber content in the compression 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 layer to layer. As we know, these structures (layer to layer) are responsible for higher charpy impact strength, and we observed that at Fig. 4 where composites made from the compression molding process showed Deflection Force loss energy (Wv) strain energy (Ws) damage initiation Figure 5. Typical impact force-deflection curve for fiber reinforced polymer composites including definition of the characteristic values used. 880 Bledzki and FarukORDER REPRINTS better charpy strength in comparison with injection molding process composites. But with the addition of a compatibilizer indicates much better interaction between the wood fiber and the matrix in both processing systems, which is represented in Figs. 9 and 10. Figures 9a and 9b (a) (b) 100 0 100 200 300 400 500 600 700 01234567 Deflection mm Force N HW30% (Injection molding) Hw30%+MAH-PP5% (Injection molding) HW30% (Compression molding) HW30%+MAH-PP5% (Compression molding) 100 0 100 200 300 400 500 600 700 0123456 7 Deflection mm Force N SW30% (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 in both processes. (View this art in color at .) Wood Fiber Reinforced Polypropylene Composites 881ORDER REPRINTS (a) (b) 0 0.5 1 1.5 2 2.5 3 Damping index - Injection molding Compression molding HW30%+PP70% HW30%+PP70%+MAH-PP5% HW50%+PP50% HW50%+PP50%+MAH-PP5% 0 0.5 1 1.5 2 2.5 3 Damping index - Injection molding Compression molding SW30%+PP70% SW30%+PP70%+MAH-PP5% SW50%+PP50% SW50%+PP50%+MAH-PP5% Figure 7. Dampingindexofhard-wood-fiberPPcomposites(a)andsoft-wood- fiberPPcomposites(b)withandwithoutcompatibilizerinbothprocesses.(View this art in color at .) 882 Bledzki and FarukORDER REPRINTS (a) (b) Figure 8. SEM micrograph of hard- (a) and soft- (b) wood-fiberPP composites in compression molding process (wood-fiber content 30%). Wood Fiber Reinforced Polypropylene Composites 883ORDER REPRINTS (a) (b) Figure 9. SEM micrographs of fractured surface of soft-wood-fiberPP composites in injection molding process (a) without MAHPP 5%, (b) with MAHPP 5%, wood-fiber content 50%. 884 Bledzki and FarukORDER REPRINTS (a) (b) Figure 10. SEM micrographs of fractured surface of soft-wood-fiberPP composites in compression molding process (a) without MAHPP 5%, (b) with MAHPP 5%, wood fiber content 50%. Wood Fiber Reinforced Polypropylene Composites 885ORDER REPRINTS represent the microstructure of soft-wood-fiberPP composites contain- ing 50% wood-fiber content with and without a compatibilizer prepared by the injection molding process and Figs. 10a and 10b represent the before stated composites in compression molding process. It is also notable that in comparison between both processes, wood-fiberPP composites have a better interaction between wood fiber and the matrix in the injection molding process than the compression molding process; to better understand, the density of the composites prepared by both processes also was measured. It was observed for hard-wood-fiberPP composites (30% wood-fiber content). In the injection molding process, compositedensitywas1.06g/cm 3 ,whichis0.98g/cm 3 inthecompression molding process. The lower density of the composites in the compression molding process refers to more void content, which represents, finally, poor bonding and interaction between wood fiber and PP. This was expected also from the mechanical properties of wood-fiberPP composites in both molding processes. CONCLUSIONS The influence of the processing systems (injection molding and compression molding) on the mechanical properties of the hard-wood- and soft-wood-fiber-reinforcedPP composites were investigated in this