Although wood plastic composites (WPCs) offer longer service life and greater rot resistance than natural wood, the fact remains that weather, sun exposure and moisture can diminish product appearance, particularly when some WPC products enter their second decade of use. That’s why manufacturers of these products study the effects of weathering on their WPC’s combination of polymer resin and wood filler.
Such studies use data from a combination of sources. In one recent study, Aspen Research Corp. (White Bear Lake, Minn.) evaluated a Korean manufacturer’s WPC deck board made with high-density polyethylene (HDPE) with 50 percent wood filler, 10 percent calcium carbonate and talc, and small amounts of pigments and antioxidant/anti-weathering additives. The Korea Institute of Construction Materials (KICM) took samples of the WPC and exposed them to filtered ultraviolet (UV) radiation, heat and moisture in xenon arc chambers (see photo) supplied by ATLAS Material Testing Solutions (Chicago, Ill.). Additional samples were exposed to actual outdoor weathering for four months. KICM then asked Aspen, a testing partner, to generate a computer model to help predict service life for WPCs based on the samples.
Aspen conducted a series of physical and chemical tests on the weathered samples and unweathered deck boards to define the WPC system’s chemistry. The system’s resistance to oxidative decomposition was determined using the oxidation induction time (OIT) test, defined by ASTM D3895-04, “Oxidative-Induction Time of Polyolefins by Differential Scanning Calorimetry.” Based on the assumption that UV radiation is the factor most likely to lead to WPC failure, Aspen determined that loss of antioxidant additive content due to UV exposure (measurable by direct chemical analysis, the OIT testing and changes in color and gloss) is a good predictor of service life. “However, antioxidant depletion can be confounded by the interaction of other additives and fillers, which can also affect the degradation rate of WPCs,” says Aspen’s senior development engineer Keith Effertz.
For this group of samples, degradation of the WPC’s structural properties was not a concern but, because its initial antioxidant filler content was very low, Aspen’s model predicted that within five years, surface crumbling would be observed. This outcome was consistent with accelerated testing in the ATLAS chamber.