Adhesive joint formation between porous wood adherends with one-component polyurethane part 2

Performance of an adhesive bond depends on the individual strengths of the adherends, the adhesion of the interface layers, and the cohesion of the adhesive itself. Treating wood with liquids prior to bonding can displace air and impurities from the cell surface, and ensure that moisture-sensitive adhesives adhere properly. To understand the influence of the wetting agent, temperature and airing time on one-component polyurethane bonding, the bond quality was quantitatively assessed using tensile shear tests EN 302–1. Supplemented by light microscopy images of the bond design, it was possible to make statements about the bond geometry in relation to strength. It became clear that wetting has a significant influence on the penetration behaviour of the adhesive and thus has a key effect on the thickness of the glue line. As a result, the bond line thickness increases on average by 19 % when the airing time is extended by two minutes. Deeper penetration and reduction in voids are associated with the forcing of a vapour diffusion flow towards the centre of the board, which is intensified by the increase in the moisture conductivity coefficient with rising temperature. Therefore, the amount of adhesive required depends not only on the wood density, but also on wetting parameters.