No abstract

The effects of the concentration of a maleated polypropylene additive (0 to 5 percent by weight) and of extrusion blending temperature (190T 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 polyethylenewood 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.

Full factorial studies were conducted to determine the effects of a coupling agent (a low molecular weight maleated polypropylene (MAPP)) and other composition and processing variables on the mechanical properties of a wood-flour-filled polypropylene (PP) composite. Effects of MAPP on the bonding between PP and wood veneer were also examined. At less than 1 percent by weight, MAPP produced useful increases in strength and modulus properties of the composite, and this effect was somewhat enhanced by small-particle-size wood flour and multiple extrusions. However, MAPP caused small losses in notched impact energy. High extrusion temperature (190°C to 250°C) had little influence on strength, but it decreased notched impact energy. Peel force between PP and wood veneer was increased by pretreatment with MAPP for aspen, but not for birch, aspen being more porous than birch. The effectiveness of MAPP may therefore be related to its ability to penetrate the wood and form a strongly held hydrophobic layer that is attractive to the PP, thereby increasing both the effective bonding area and mechanical interlocking.