Durability of wood flour-plastic composites exposed to accelerated freeze-thaw cycling. Part I. Rigid PVC matrix

This study examined the effects of accelerated freezethaw actions on the durability of wood fiber-plastic com-, posites. Rigid PVC formulations filled with various concentrations of wood flour (both pine and maple) were processed in a counterrotating twin-screw extruder and exposed to cyclic freeze-thaw actions according to ASTM Standard D6662. Freeze-thaw cycling was also modified by omitting portions of the test (either the water or freezing) to verify whether or not moisture was the primary cause for property loss. The durability of exposed samples was assessed in terms of flexural properties, density, and dimensional stability. Scanning electron micrographs of unexposed and freeze-thaw-exposed samples were taken to qualitatively evaluate the interfacial adhesion between the wood flour and PVC matrix. The experimental results indicated that the density was not affected by freeze-thaw cycling. The dimensional stability was also relatively unaffected, although greater wood flour content exhibited greater dimensional change. The loss in stiffness of the composites was statistically significant after only two freeze-thaw cycles, regardless of both the wood species and content. Conversely, the strength of the composites was not significantly affected by five freeze-thaw cycles at lower wood flour contents (50 and 75 phr). The deleterious effects of the freeze-thaw actions on the strength of the composites became apparent at higher wood flour content (100 phr) after only two freeze-thaw cycles for maple flour and five freeze-thaw cycles for pine flour. The property loss was attributed primarily to the water portion of the cycling, which appears to have led to the decreased interfacial adhesion between the wood flour and the rigid PVC matrix. (c) 2005 Society of Plastics Engineers.