Heat-treatment effects on wood durability outdoors and performance of polymeric coatings—adhesion and wood-water interactions

The outdoor durability of wood coatings depends not only on the polymeric phase, but also significantly on the physical properties of the wood substrate. This study assesses the exterior durability of coatings applied to heat-treated (185 °C) and untreated spruce wood, emphasizing the role of substrate characteristics under real-world outdoor aging. The properties of heat-treated (HT) wood, in comparison to control (i.e. untreated) samples, were investigated during natural exposure for 6 months: water absorption, surface wettability, air permeability, and equilibrium moisture content (EMC). The adhesion strength (AS) dynamics of solvent-borne polyurethane (PU) coatings to wood substrates was assessed via pull-off testing, considering the effects of heat treatment and natural weathering. A decrease (−15%) in the AS of PU-coating to HT wood was observed, attributed to poor wettability and reduced permeability of wood through thermal modification. However, heat treatment showed better performance in terms of adhesion and coating quality in outdoor conditions, where the AS loss for HT and control wood was −18% and −43%, respectively. The development of amorphous structures and reduction in cellulose degradation temperature (DTG-TMax) due to outdoor weathering, along with the superior performance of HT wood, were confirmed through thermogravimetric analysis (TGA, DTG). Overall, the water-repellency, thermal-stability, low hygroscopicity, and limited permeability of HT wood minimize wood distortion/degradation during aging and ensure the long-term stability of applied coatings in service. Hence, the synergy between heat treatment and protective coating can be considered an effective strategy for wood materials. Nonetheless, a wetting modification to the HT spruce wood surface is recommended prior to the painting or finishing process, to achieve excellent outdoor durability.