Reducing the thickness swelling of a model wood composite by creating a three-dimensional adhesive network

Wood composites undergo irreversible thickness swelling and strength losses when they are exposed to water, and there is strong interest in developing cost-effective solutions to this problem. We hypothesised that a composite with more adhesive connections in the Z-(thickness)-direction would be less susceptible to thickness swelling. We test this hypothesis using a model composite consisting of perforated veneer that allowed a polyurethane adhesive to create cross-links in the Z-direction. Model composite specimens were submerged in water, air-dried and their irreversible thickness swelling and surface topography were measured. Z-direction cross-links significantly reduced the thickness swelling of both yellow cedar and white spruce specimens, but the inter-connected adhesive network retained its integrity better in spruce than in cedar specimens. Further research is needed to develop practical ways of creating Z-direction cross-links in composites that resemble commercial products, and also to evaluate the effects they have on the mechanical properties of composites.