Effects of processing method and moisture history on laboratory fungal resistance of wood-HDPE composites

The purpose of this study was to clarify the effects of composite processing and moisture sorption on laboratory fungal resistance of wood-plastic composites. A 2-week water soaking or cyclic boiling-drying procedure was used to infuse moisture into composites made from high-density polyethylene filled with 50 percent wood flour and processed by extrusion, compression molding, or injection molding. Extruded composites absorbed the most moisture; compression-molded composites absorbed less than did extruded composites, and injection-molded composites absorbed the least. Although more moisture was absorbed during water soaking, the cyclic boiling procedure caused more damage to the composites, especially the extruded ones. In neither procedure for infusing water did the composites reach equilibrium. A standard method for determining the durability of structural wood was modified for testing the fungal resistance of composites. Moisture content, flexural properties, and weight loss were measured over a 12-week exposure to the brown-rot fungus Gloeophyllum trabeum. Significant weight losses were found once the composite moisture content reached roughly 12 to 15 percent. This corresponds to an average wood flour moisture content near the fiber saturation point. The greatest weight losses were found for extruded composites that had been preconditioned by boiling. Damage to the composite from moisture sorption complicated the use of flexural performance as a measure of fungal attack. Mechanical performance appeared to be a less sensitive measure of fungal attack than was weight loss. However, flexural modulus loss and strength loss due to fungal attack correlated well with weight loss for individual specimens when the data were plotted together.

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