Durability of an extruded HDPE/wood composite

As a part of wood-plastic composite (WPC) material development, this research evaluated the resistance of WPC formulations to fungal decay and biocide leaching. In laboratory tests using small WPC samples exposed directly to brown-rot and white-rot fungi, there were no weight losses caused by decay in most formulations. Formulations with relatively high wood content and not protected by zinc borate exhibited moderate weight losses. Scanning electron microscopy of WPC specimens supporting fungal growth show mycelium concentrated in the interfacial gaps between the wood and thermoplastic component near the specimen surface. The relative influence of each material component on the WPC resistance to decay was analyzed by simplex analysis. Of the various component term effects, the most influential was that relating to composite wood content. Increases in talc concentration also resulted in greater weight losses. An increase in high-density polyethylene (HDPE) in the formulation reduced susceptibility to weight loss during the accelerated decay test. Zinc borate at 2 percent concentration, the lowest level tested, prevented any weight loss. Our conclusion is that either the inclusion of zinc borate or limiting the wood content of WPC can effectively prevent fungal decay. The very low leach rate as determined by laboratory testing appears to be controlled mainly by dissolution of the zinc borate and not by diffusion. The tests show that zinc borate will take at least 20 years to completely dissolve and leach from the material. Results indicate that careful material design can prevent fungal decay of WPC even under severe exposures.