The effects of temperature and moisture exposure on the properties of wood-fiber thermoplastic composites

Wood-fiber thermoplastic composites were prepared with polypropylene (PP) and high-density polyethylene (HDPE) matrix resins employing high shear thermokinetic compounding for fiber dispersion. Wood fibers obtained from recycled newspapers were mixed with the matrix polymers at a level of 30% by weight. In one experiment, sets of six samples were immersed in boiling water for up to 48 hours and subsequently tested for mechanical properties and water absorption. In another experiment, samples were exposed to -40°C, -20°C, 0°C, 23°C, 40°C and 60°C for two hours and then tested at the same temperatures for tensile and flexural properties. Results showed that immersion in boiling water resulted in water absorption of between 3 and 5%, a decrease in tensile and flexural properties, and an increase in impact strength. When the composites were exposed to various temperatures, both strength and modulus decreased significantly with increasing temperatures above ambient (23°C) level for both PP- and HDPE-based composites. The opposite trend was evidenced below freezing.