Wood-plastic composites are being examined for a greater number of structural-type applications that may be exposed to different environments, some of them adverse. This paper discusses the influence of moisture absorption on the mechanical properties of wood flour-polypropylene composites. Composites filled with 20% or 40% wood flour (by weight) were placed in different environments, removed periodically, and tested for moisture content and mechanical properties. The composites filled with 20% wood flour (20%WF) absorbed moisture in all the environmental exposures, but no significant degradation of properties was observed. The 40%WF composites absorbed more moisture than did the 20%WF composites. The flexural properties of composites subjected to a water bath and to 90% relative humidity were lower than the flexural properties of the other composites. Tensile properties and notched impact strengths decreased only for composites placed in a water bath. Unnotched impact strengths did not change significantly with absorption of moisture.

This study investigated weathering performance of an HDPE wood plastic composite reinforced with extracted or delignified wood flour (WF). The wood flour was pre-extracted with three different solvents, toluene/ethanol (TE), acetone/water (AW), and hot water (HW), or sodium chlorite/acetic acid. The spectral properties of the composites before and after artificial weathering under accelerated conditions were characterized by Fourier transform infrared (FTIR) spectroscopy, the surface color parameters were analyzed using colorimetry, and the mechanical properties were determined by a flexural test. Weathering of WPC resulted in a surface lightening and a decrease in wood index (wood/HDPE) and flexural strength. WPCs that were reinforced with delignified wood flour showed higher ΔL^* and ΔE^* values, together with lower MOE and MOR retention ratios upon weathering when compared to those with non-extracted control and extracted WF.