Interaction and adhesion at the interface between wood and paint, glue, lacquer measured with DMTA and SEM

Abstract: The aim of the present thesis was to study the interface between wood (Pinus sylvestis) and synthetic polymers such as glue, lacquer and paint. The weakest part of these interfaces is always the bond. The long-term properties of bonds between wood and an adherend are those which are interesting. If a good bond is achieved, the surface between wood and an adherend would be less susceptible to degradation. To achieve a good bond, the wood polymers and the adhering polymers must be compatible. The contact area between the substrate and an adherend should be as large as possible to give a high quantity of bonds. A large contact area also gives the possibility of adhesion through mechanical interlocking. The interaction between wood and glue and lacquer was studied with Dynamic Mechanical Thermal Analysis. The interaction was correlated with adhesion properties observed with Scanning Electron Microscopy (SEM) fractography. Dynamic mechanical properties of untreated wood in the tangential and radial direction was studied to have a knowledge of the wood polymers behaviour. The properties of the wood polymers was needed for evaluating the change in behaviour when wood was combined with glue and lacquer. It was shown that polymers interacting with the wood polymers experienced a higher free volume in contact with wood, thus decreasing the glass transition temperature of the synthetic polymers. This decrease was considered most probably to be due to the polymers being subjected to tensile forces developed during drying. The high level of interaction was correlated to high adhesion on the wood cell wall level. Two similar commercial polyurethane-alkyd lacquers showed interaction with wood, but a commercial polyvinyl acetate glue showed poor interaction and poor adhesion. The fracture surface between wood and paint in a 50-year-old window was studied with SEM, using backscattered electron image. The method makes it possible to analyse fracture surfaces and determine where the fracture occurrs. It could be used, as an indication of whether the weakest link in different paint/wood systems is the interface.