Enhancing weathering resistance of wood by using bark extractives as natural photostabilizers in polyurethane-acrylate coating

In this study, Chinese fir bark was extracted separately by three solvents including sodium hydroxide, 95% ethanol, and hot water, and the obtained extractives were labelled as NaE, EtE, and HWE correspondingly. The ultraviolet (UV) absorption and antioxidation activity of those bark extractives were confirmed. The aim of this study was to improve the photostabilization of polyurethane-acrylate (PUA) coating by incorporating bark extractives as photostabilizers. Then, the modified coatings were used to coat Scots pine wood samples in order to enhance their weathering resistance. During 960 h of accelerated UV weathering, the changes in wood surface color, gloss, morphology, and surface chemistry were investigated. Meanwhile, the photostabilization mechanisms of bark extractives were also discussed. The results showed that (1) the free radical scavenging ability of extractives was related with their total phenolic contents; (2) wood protected by PUA/EtE coating experienced the least severe surface deterioration and photodegradation during the whole weathering process; (3) the photosatbilization effect of EtE mainly resulted from its phenolic compounds, which transferred into ketones and quniones after weathering.