Wood-Plastic Composites and the Durability Dilemma: Observations from the Field
M J Manning, F M Ascherl, M E Mankowski
Wood-Plastic Composites (WPCs) used for decking, window & door moldings have experienced dramatic growth in North America over the last several years, with annual production increasing at rates greater than 20% per annum since 1998. A key factor contributing to this growth has been the successfully communicated message that they are "virtually maintenance free". The common perception being that the wood fiber is encapsulated by the thermoplastic resin, minimizing the potential for moisture absorption and inhibiting attack by wood destroying organisms. A number of recent publications have raised concerns about the long-term durability of WPCs – in particular, when used in unprotected, above-ground applications. The fundamental question is whether or not the % Moisture Content (%MC) of the wood component can achieve the minimum level necessary to support fungal decay. Laboratory tests utilized by the WPC industry suggest this is not the case and yield values less than 10%MC - well below the accepted minimum threshold of 25% necessary to initiate and support fungal decay. This paper will describe work which evaluates the %MC of the wood component in WPC samples exposed in various field sites with different climatic Scheffer indices. These field results yield a dramatically different picture and show that the outer portion of the WPC surface can achieve %MC levels in the wood component which are in excess of the 25% threshold. Both macro and microscopic evaluations of untreated WPC material revealed evidence of fungal decay. The use of a preservative treatment such as zinc borate to protect against fungal decay will also be discussed.