Effect of particle size on properties of wood-flour reinforced polypropylene composites

Research on wood flour-filled propylene has primarily focused on one species of wood flour and one or two broad particle-size distributions. The effects of species and particle size need to be examined to fully exploit the use of wood flour as a filler. Our study focused on the effects of particle size on the properties of polypropylene filled with 40 percent (by weight) wood flour. Eight wood flours were investigated: four screened wood flours with discrete, narrow particle-size distributions and four commercially available wood flours. Particle size did not affect specific gravity but it did affect other properties. Melt flow index, heat deflection temperature, and notched impact energy increased with increasing particle size, whereas unnotched impact energy decreased. Mold shrinkage decreased for particles smaller than about 0.25 mm. Flexural and tensile modulus and strength increased with increasing particle size.