Accelerated weathering of natural fiber-thermoplastic composites : effects of ultraviolet exposure on bending strength and stiffness

Abstract

Mechanical properties of bending stiffness and yield stress were used to evaluate the effects of ultraviolet exposure on natural fiber-thermoplastic composites. Four different specimen formulations were evaluated. Injection molded high density polyethylene (HDPE) served as the polymer base for all formulations. Two lignocellulosic fillers, wood flour and kenaf fiber, were added at 50 percent by weight. Additives consisted of an antioxidant, coupling agent, and ultraviolet (UV) stabilizer. Specimens were exposed in a laboratory weatherometer to high levels of UV radiation and moisture cycling to simulate the effects of sunlight and rain for a 2000-h exposure period. Bending stiffness (modulus of elasticity) and bending yield stress (modulus of rupture) were measured prior to and after specific exposure periods. For the 100 percent HDPE formulation, stiffness increased and yield stress decreased after exposure. For HDPE with additives, no significant changes in mechanical properties occurred after 2,000 hours of exposure. Significant loss in mechanical properties was observed for both fiber-filled formulations. A 42 percent drop in measured stiffness and 24 percent drop in measured bending strength were recorded for the kenaf fiber formulation due to the 2,000-hr. exposure. A less dramatic drop in stiffness (33%) and strength (20%) was recorded for the wood flour formulation. For all formulations except HDPE with no additives exposed for 1,000 hours, the orientation of the degraded surface in bending, whether on the tension or compression side, had no significant effect on stiffness or strength.

Citation

Lundin, Thomas; Falk, Robert H.; Felton, Colin. 2002. Accelerated weathering of natural fiber-thermoplastic composites : effects of ultraviolet exposure on bending strength and stiffness. Sixth International Conference on Woodfiber-Plastic Composites : May 15-16, 2001 ... Madison, Wisconsin. Madison, WI : Forest Products Society, c2002: ISBN: 0892529181: pages 87-93.