Performance of polypropylene wood composites after outdoor exposure to natural weathering

Abstract

This paper presents a study of the performance of wood flour-polypropylene composites formulated with 60 and 100 mesh and fine pine wood particles, with wood/polypropylene proportions of 6-730, 57-40 and 47-50, with 2 % of coupling agent (maleic anhydride polypropylene) and 1 % of calcium stearate. These composites were exposed to natural weathering during 2 856 hrs (September through January). The effects of the solar radiation and humidity on the change of lightness (ΔL) and surface morphology were assessed with a scanning electron microscope (SEM); the resistance to impact was also evaluated. All composites except 100 % polypropylene experienced an increase in lightness. Regardless of the particle size, the increase in the proportion of wood resulted in a greater modification of the lightness levels. Composites made with 100 mesh or fine particles were observed to have a higher increase of lightness levels. The surface microcracking occurred in 100 % polypropylene and in wood flour-polypropylene composites and was more severe with the larger particles and slightly more evident with 40 % and 50 % wood. Resistance to impact was lost in all the formulations, less so in composites with particles of 60-mesh with 30 % and 40 % wood. With a high proportion of wood particles, the resistance to the impact diminished noticeably. The resistance levels of all the wood-flour-polypropylene formulations after outdoor exposure to weathering surpassed those of pure polypropylene.