The effect of manufacturing variables, fungal exposure, and moisture cycling on the durability of wood fiber/thermoplastic composites

Woodfiber/thermoplastic composites (WPC) are a new class of wood-based composites that are currently finding use in outdoor exposures. They are generally regarded as decay resistant materials because much of the wood is assumed to be encapsulated in a continuous thermoplastic phase. The purpose of this work was to conduct a systematic investigation of the fungal durability of WPC in which methods of evaluating durability were investigated, along with the effects of wood component variables on fungal durability.; Model composites were produced by compression-molding physically blended wood flour and plastic into panels. Samples for soil block decay and flexural testing were cut from the panels, along with field stakes. The wood content of the composites ranged from 30 to 70%, in 10% increments. Additional composites that contained 50% wood, but with a range of particle sizes and 2 different wood species were also produced. Preservative-treated samples containing zinc borate (ZB) at loadings of 1, 3, or 5% of the wood weight in 50% wood-content composites were used to evaluate the affect of a fungicide treatment.; Soil block decay tests demonstrated that decay susceptibility increased with wood loading. Wood particle size also affected the decay susceptibility of the WPC; composites produced with larger particles lost more weight due to fungal decay than composites produced using small wood particles. Zinc borate prevented weight loss at ZB loadings as low as 1% and was leach resistant under laboratory conditions. WPC field stakes were installed in ground contact near Hilo, Hawaii for up to one year. A set was retrieved at three month intervals, rated visually for decay, and cut up for strength testing. Strength loss appears to have been driven by moisture content cycling, so it was not possible to draw specific conclusions about decay resistance without explicitly accounting for moisture-related effects. The ZB-treated samples also lost large amounts of strength in the field, despite preventing decay in the lab, further indicating that moisture content cycling was mostly responsible for the measured strength loss. However, fungi were isolated from the cores of all stakes (untreated and preservative-treated), so it was also not possible to rule out decay in the stakes either. (Abstract shortened by UMI.)...