Recently, there has been interest in evaluating the biological durability of woodfiber-thermoplas-tic composites used in exterior applications. Some evidence suggests that these materials may not be inherently decay resistant, as is commonly assumed. In this paper, the fungal resistance of model materials was investigated using field and laboratory exposure experiments. The effects of manufacturing variables such as wood loading, particle size, wood species, surface abrasion, and preservative loading were evaluated. Durability in field exposure was evaluated by mechanical strength testing. Weight loss was used as an indicator of durability in laboratory exposure. The negative affects of water absorption and subsequent swelling complicated fungal durability evaluation through field exposure, so weight loss results from laboratory soil block tests were used to evaluate the affects of manufacturing variables. Decay susceptibility increased with wood content and parti- cle size. The affects of surface abrasion and wood species were less important. Zinc borate was found to be an effective, leach-resistant preservative at a 1 percent loading, based on the weight of wood in the composite. Woodfiber-thermoplastic composites may be susceptible to fungal attack, depending on manufacturing and installation variables, but incorporation of an appropriate preservative system can prevent fungal damage, at least in laboratory evaluations.
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