Dynamic Weathering Behavior of Heat-Treated Chinese Fir: Surface Properties, Chemical Composition, and Microstructure

Heat-treated wood is widely used for its eco-friendliness and aesthetic appeal. However, it is prone to fading under ultraviolet (UV) radiation, which diminishes its decorative effect and shortens its service life. To clarify the performance evolution and degradation mechanisms of heat-treated wood, Chinese fir, treated at 190 °C under vacuum, was subjected to artificial weathering, and the dynamic changes in surface appearance, chemical composition, and microstructure were monitored. The results show that after artificial weathering, both heat-treated and untreated wood exhibited similar surface color change trends, characterized by darkening, yellowing, and reddening, but heat treatment significantly inhibited surface color changes. After 372 h of weathering, the total color change of heat-treated wood (ΔE = 24.05) was smaller than that of the untreated wood (ΔE = 37.78), and its yellowness index (b* = 58.93%) was also far lower than the untreated group (b* = 119.85%). Additionally, the gloss of heat-treated Chinese fir increased by 17% during weathering. However, as weathering progressed, the protective effect of heat treatment on the appearance gradually weakened, and it could not prevent long-term structural breakdown. The improved color stability is closely linked to condensed lignin and unsaturated phenolic chromophores, while the preferential thermal decomposition of hemicelluloses creates "weathering-vulnerable zones". This study provides both a theoretical basis for understanding the weathering behavior of heat-treated wood and practical guidance for optimizing wood protection technologies.