Effects of wood fiber characteristics on mechanical properties of wood/polypropylene composites

Commercial wood flour, the most common wood-derived filler for thermoplastics, is produced in a mixture of particle sizes and generally has a lower aspect ratio than wood and other natural fibers. To understand how wood flour and fiber characteristics influence the mechanical properties of polypropylene composites, we first investigated the effect of different sizes of wood flour particles on the mechanical properties of wood-flour-filled polypropylene composites. We then compared the properties of wood-flour-filled composites to those of composites reinforced with refined wood fiber. We also studied the effect of a maleated polypropylene coupling agent on composite properties. Wood flour particles (35, 70, 120, and 235 mesh) were compounded at 40% by weight with polypropylene. Increases in tensile and flexural strength and modulus of the wood flour composites were found to correspond with increases in aspect ratio. Notched impact energy increased with increasing particle size, whereas unnotched impact energy decreased with increasing particle size. Refined wood fiber and 40-mesh wood flour was compounded at 20% and 40% by weight with polypropylene. Wood fiber resulted in higher strengths at both filler levels and higher moduli at the 40% level compared to the strength properties of wood flour composites. The higher aspect ratio of the wood fiber had little effect on impact energy. The maleated polypropylene coupling agent caused greater strength increases in wood fiber composites than in wood flour composites. The coupling agent did not significantly affect tensile or flexural moduli. Our results clearly support the use of higher aspect ratio wood fibers and coupling agents for increasing the strength of wood/plastic composites.