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The effects of the concentration of a maleated polypropylene additive (0 to 5 percent by weight) and of extrusion blending temperature ( 190°C to 250°C) on the mechanical properties of extruded and injection-molded polypropylene-wood flour composites were investigated. The effects of maleated polypropylene additive on similarly processed polypropylene-wood flour and high density polyethylene-wood flour composites were also compared. Both the additive and the high extrusion temperature led to some wood degradation and to a less polar wood surface. The additive led to greater reinforcement of the composites, as indicated by moderate but useful increases in heat deflection temperature. strength. and modulus. The major portion of those improvements was achieved by adding 1 to 2 percent additive. However. both the additive and the high extrusion temperature decreased impact resistance. presumably as a consequence of increased reinforcement by the filler particles and wood degradation. Heat deflection temperature. strength, and modulus of the polypropylene-wood flour system were marginally better than that of the high density polyethylene-wood flour system; impact resistance was marginally poorer. 17