Improving the Color Stability of Wood-Plastic Composites Through Fiber Pre-Treatment
Keywords:Wood-plastic composites, weathering, color, extractives, pigments
AbstractWood-plastic composites (WPCs) continue to make inroads into the decking market. One of the main drawbacks from the consumer's perspective is that they can fade during weathering. It has been shown that WPCs fade more when exposed to a combination of light and water spray than when exposed only to light. This suggests that the loss of the color-imparting extractives from the wood could be partly to blame for color fade. Pigments are routinely added to only the plastic matrix to combat WPC color fade despite the fact that change in color could be due primarily to changes in the wood component of WPCs. In this study, we investigated two methods of fiber pretreatment in an attempt to improve the color stability of WPCs. The first was to remove water-soluble extractives from wood flour through a series of washes. Washed wood flour was then compounded with high-density polyethylene and injection-molded into test specimens. The second method involved coloring wood flour with either a water-based dye or an oil-based stain. Colored wood flour was also compounded with high-density polyethylene and injection-molded into test specimens. Pigments were added to half of the composites containing colored wood. The composites underwent accelerated weathering in a xenon-arc weathering apparatus, and were monitored for changes in color. Washing wood flour and coloring wood flour with a water-based dye were ineffective for improving composite color stability during weathering. However, coloring wood flour with an oil-based stain resulted in a more color stable WPC.
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