Effects of Impact Modification on the Mechanical Properties of Wood-Fiber Thermoplastic Composites with High Impact Polypropylene (HIPP)

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Abstract
This paper reports on the effects of impact modification on the mechanical properties of wood-fiber thermoplastic composites with high-impact polypropylene (HIPP). Homo polypropylene (PP) was modified through its melt blending with HIPP to obtain an inherently tough matrix resin for PP/wood-fiber composites. The resin modification improved the impact strength of the composites, while it reduced the stiffness and strength properties acquired by the addition of wood-fibers. The requirements of plane strain fracture toughness were not satisfied in this study. In order to compensate the nonplane strain fracture toughness, the specimen strength ratio was used as a comparative measure of fracture toughness. The strength ratio depended on impact modifier levels as well as on wood-fiber concentrations. The work of fracture measured as the area under the load-displacement curve increased with impact modifier levels except at 40% wood-fiber concentration. Microstructural probing showed a degree of wood-fiber alignment in the skin and core of injection-molded specimens and provided some insight into the slight increase in the impact strength of composites.