Chemistry of wood plastic composite weathering.

摘要

Solid wood has been traditionally used as a decking and siding component in the USA and other parts of the world. However, environmental (because of chemical treatment) and performance (loss of value) issues concerning solid wood in those applications have brought about shift to replace solid wood with WPC. The shift is largely due to the perceived-improved properties, low maintenance, and environmentally benign attributes of WPC. However, a growing issue of concern with these products is their long-term weatherability and color stability. Therefore, retaining WPC color for extended periods during outdoor exposure is of importance. Unfortunately, a fundamental knowledge regarding chemical changes, which influence color changes that occur at the surface of WPC during weathering relative to material compositions are limited. In order to develop WPC with improved weathering performance, the chemical changes of the material compositions that are caused by weathering factors are investigated in this study. The outcomes of such an approach would provide basic understanding of WPC weathering that might likely foster research toward production WPC with improved weathering performance. In this study, the effects of outside and accelerated (UVA and xenon-arc) weathering on the color and chemical changes of wood plastic composites (WPC) produced from different plastics, wood species, and modified wood fiber were investigated. Different analytical techniques were employed to monitor color and chemical changes that occurred during weathering, such colorimetry, Fourier Transform Infrared spectroscopy (STIR), X-ray photoelectron spectroscopy (XPS), Pyrolysis Gas Chromatography - Mass Spectrometry (Py-GC-MS), Gel Permeation Chromatography (GPC), and Differential scanning calorimetry (DSC). The outcomes of weathering trials using high density polyethylene (HDPE), polypropylene (PP) and polyvinyl-chloride (PVC) based WPC with pine wood flour as reinforcement/filler and other additives showed that for both natural and celerated weathering, longer exposure time caused increased color change (and surface lightness), generation of chro ophores, surface oxidation, and wood loss on the weathered WPC surface. Understanding of the basic the and color changes of naturally and accelerated weathered WPC suggest that wood lignin was a contributing factor to WPC color change. Therefore, the outcomes of xenon-arc weathered WPC produced from selected wood species showed that WPC formulation with moderated weathering performance (color stability) could be achieved using wood species of low lignin content (such as poplar). In addition, deligninfied wood fiber based WPC especially from HDPE had less color change compared to untreated wood based WPC. This suggests that cellulose fibers from chemical pulps could be used as a fiber source for weather resistant WPC. In all studies conducted, HDPE-based WPC had the least color change and mostly followed by PP and PVC, which could be associated with thermal history.