Wet adhesion of low-VOC coatings on wood: A quantitative analysis

Adhesion of coatings on wood is most critical under wet conditions. However, the mechanisms controlling the adhesion are not well understood. Therefore, a quantitative analysis of coating adhesion on wood is needed. A new technique is presented to measure the wet adhesion quantitatively based on measuring the forces needed to peel the coating from the wood with a tape. Differences in peel forces corresponded to areas in the wood with a lower or a higher degree of coating penetration. Coatings with better penetration and substrate wetting showed an improved adhesion and the better adhering coatings had an increased level of cohesive failure in the coating. Chemical analysis of the air-faced and wood-faced side of the coatings showed differences in chemical composition. The stored strain energy due to hygroscopic stress could reduce the actual level of adhesion significantly. Surface energy effects did not contribute to reduced adhesion after exposure to water. The interfacial work of adhesion between coating and wood was very much lower than the measured force and lacked correlation with measured differences in adhesion. Therefore, it can be concluded that mechanical anchoring plays a dominant role in obtaining good adhesion of a coating on wood.