Effect of Species and Particle Size on Properties of Wood-Flour-Filled Polypropylene Composites

Research on wood-flour-filled polypropylene has primarily focused on one species of wood flour and one or two broad particle-size distributions. To fully exploit the use of wood flour as a filler, the effects of species and particle size need to be examined. This study focused on the effects of both of these variables on the properties of polypropylene filled with wood flour. To examine the effects of species, wood flour derived from four species, ponderosa pine, loblolly pine, maple, and oak, was used as a filler for polypropylene at 20,30,40,50, and 60% by weight wood flour. With increasing wood flour content, flexural and tensile modulus, density, heat deflection temperature, and notched impact energy increased, while flexural and tensile strength, tensile elongation, mold shrinkage, melt flow index, and unnotched impact energy decreased. In general, the hardwoods exhibited slightly better tensile and flexural properties and heat deflection temperatures than did the softwoods. Four commercially available wood flours and four screened wood flours with discrete, narrow particle-size distributions were used to study the effects of particle size on wood-flour-filled polypropylene. In this case, the composite material was 40% by weight wood flour. 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. For particles smaller than about 0.25 mm, mold shrinkage decreased. Flexural and tensile modulus and strength increased with increasing particle size. Introduction The use of scrap-wood-derived fillers for thermoplastics has been accepted by the plastics industry in recent years. Wood-derived fillers have several advantages compared with their inorganic counterparts, such as lower density and lower volumetric cost. They also are less abrasive to processing equipment and are derived from a renewable resource. To facilitate the use of wood-derived fillers, the USDA Forest Service, Forest Products Laboratory (FPL) supplied injection molders with precompounded pellets (1). In an FPL research study, polypropylene (PP) filled with several inorganic fillers commonly used by the plastics industry was compared with PP filled with wood flour or wood fiber derived from demolition wood (2). The results suggested that wood-derived fillers do have a place in the