Effects of Coating on the Dimensional Stability of Wood-Polymer Composites

Wood polymer composites (WPC) are sensitive to moisture because of the hydrophilic nature of the wood fibers. The main objective of this study was to improve the dimensional stability of WPCs by coating. Polypropylene and polylactic acid were reinforced by three pulp fibers (kraft, thermomechanical (TMP), and chemothermomechanical (CTMP)) at three fiber contents (50, 60, and 70% w/w). The resulting WPCs were coated using two commercial coatings, epoxy and acrylic. Kraft fiber WPCs were less sensitive to moisture than TMP and CTMP WPCs. These differences were explained by the crystallinity of the kraft fibers and their better interfacial adhesion to the polymers. The epoxy coating proved to be more effective than the acrylic coating and significantly reduced the water absorption and the thickness swell for all formulations. Negative relationships between the contact angle and water absorption were obtained. These relationships depend on the fiber content and type, the matrix nature, and the coating.