Weathering resistance of thermally modified wood finished with coatings of diverse formulations

Thermal modification processes significant affect the chemical configuration of the wood matrix improving moisture-related properties and material durability. In addition, the distinctive dark tones of heat-treated timber increase the economic value of several light-colored species. However, these treatments alter the substrates and it could influence the performance of coating products, necessary to maintain the surface features in certain end-use sectors. Ash wood (Fraxinus excelsior L.) samples industrially treated at 192 °C, 202 °C and 212 °C were coated with decorative waterborne polyurethane and industrial UV-hardened coatings. Afterwards, the samples were subjected to accelerated weathering. The resulting surface topography was assessed with a 3D scanner and a profilometer, revealing slight changes in the texture. Color changes were quantified with hyperspectral imaging by generating surface color maps in CIE Lab coordinates. The results indicate acceptable photostability of thermally modified wood. Changes in wettability, surface free energy, work of adhesion and cohesion were determined by contact angle. Higher hydrophilicity after weathering was observed, but with an acceptable overall performance. At the same time, polar and dispersive shares were redistributed depending on the applied coating. It was demonstrated that heat-treated wood could be an optimal substrate for innovative wood-based products, suitable for use outdoor and exposure to changing weather conditions.