Hydro and thermal stability of die drawn wood polymer composites in comparison to solid wood

Both isotropic and oriented wood polymer composites (WPC) based on 40% w/w of a softwood powder/hardwood powder and polypropylene (PP), together with solid pieces of wood, were subjected to water immersion and thermal expansion tests. Although generally die drawing increased the amount of water absorbed by the WPC by about 2-fold when compare to isotropic WPC, the oriented WPC exhibited extremely high hydro-dimensional stability. The values of the longitudinal and transverse swelling/shrinkage of the WPC oscillated only between 0 and −2.3% compared to values of between 4 and 14% for the solid woods. Incorporation of soft/hard wood powders into PP also substantially decreased its thermal expansion coefficient α in both the isotropic and the oriented states. This extremely positive effect was enhanced by increasing the draw ratio. In the longitudinal direction, α decreased from about 80 × 10⁻⁶ °C⁻¹ (for the isotropic PP) to 5 × 10⁻⁶ °C⁻¹ for the highly drawn PP filled with softwood.