Wood-plastic composites (WPC) contain wood fiber (or flour), thermoplastics and additives and are exposed to UV light, moisture, and biological deterioration in outdoor installations. Accelerated laboratory tests can help to predict the durability of WPCs, but long term evaluations are needed to validate these results. Field exposed above-ground WPC deck boards (30.5 x 139.7 x 609.6 mm) and in-ground (19 x 19 x 457 mm) stakes were visually evaluated near Saucier, Mississippi and Madison, Wisconsin over 17 years. Four blends were extruded on commercial-scale equipment containing additives including colorant, light stabilizers, lubricant, and the fungicide zinc borate (ZnB). There are some difficulties evaluating the WPC materials compared to solid wood: in-ground stakes snap quite easily when removing them for inspection and decay is more difficult to determine because the WPC does not become soft. Differences in deterioration were seen between each test site: the in-ground deterioration was more severe in Mississippi and the above-ground deterioration was more severe in Wisconsin. The ZnB fungicide provided some decay protection in-ground even at the low level of 1%, and the UV stabilizer package (including colorant) slowed the color change compared to the WPC control above-ground. The blend including both ZnB and UV package provided both decay and UV protection, but mold and stain are still an issue for above-ground. The in-ground results are analogous to the short-term accelerated laboratory evaluations.

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