The moisture and decay resistance of wood-plastic composites (WPCs) reinforced with extracted or delignified wood flour (WF) was investigated. Three different extractions were preformed: toluene/ethanol (TE), acetone/water (AW), and hot water (HW). Delignification (DL) was performed using a sodium chlorite/acetic acid solution. All WPCs specimens were made with 50% by weight HDPE and WF, first compounded using extrusion followed by injection molding. After preconditioning in water at 70 °C for 5 days, the specimens were exposed for 12 weeks to the brown-rot fungus, Gloeophyllum trabeum in the AWPA D1413 standard soil block test. The WPC made with untreated wood flour had the highest overall weight loss (24.9 ±3.6%), followed by the HW (22.3 ±4.6%) and DL (16.4 ±3%). The TE (7.8 ±0.8%) and AW (7.7 ±2.3%) had the lowest weight losses. WPC moisture content was monitored before and after decay. Cracking was observed in some blends after the preconditioning, which may explain the differences in weight loss.

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