Abstract:

Objective: This study aims to investigate the effects of temperature, ultraviolet (UV) wavelength, and water on the color changes of Korean pine under artificial weathering, so as to provide a theoretical basis for the photoaging protection of outdoor wood products. Method: Using weathering tester, four artificial weathering conditions were established based on temperature (30 ℃ and 60 ℃), ultraviolet radiation A wavelength (UVA340 nm and UVA351 nm), and water: T1: 30 ℃ + UVA340; T2: 60 ℃ + UVA340; T3: 60 ℃ + UVA351; T4: 60 ℃ + UVA340 + rain. The structure, chemical composition, and color of Korean pine were analyzed during artificial weathering. The correlation between chemical composition and color was explored as well. Result: The surface roughness of the wood increased with prolonged artificial aging. High temperature, lower UVA wavelength, or the presence of water exacerbated the degradation of the wood surface, leading to high roughness. After 720 h of UVA irradiation, the roughness of samples under conditions T1, T2, T3, and T4 increased by 18.32%, 43.33%, 41.99%, and 193.36%, respectively. Cell structures remained intact under pure UVA irradiation conditions (T1, T2, T3). However, the addition of water (T4) caused significant structural deterioration, including cell wall collapse, failure of intercellular middle lamella, and thinning of wood rays. In terms of appearance, the wood surface exhibited varying degrees of yellowing or browning after artificial weathering. Quantification using the CIE color system showed that under non-water conditions (T1, T2, T3), the L^* value decreased and the a^*, b^* values increased monotonically. The addition of water led to complex changes in L^*, a^* and b^* values. Comparative analysis of different aging conditions indicated that temperature and water significantly affected L^*, a^* and b^*, while UVA wavelength primarily influenced the a^* value. Overall color change (ΔE^*) analysis revealed that the impact of weathering conditions followed the order: rain > temperature > UVA wavelength. The intensity ratios of lignin/carbohydrate R(I_{1 506}/_{1 372}) and carbonyl/carbohydrate R(I_{1 734}/_{1 372}) were used to quantify the relative changes in lignin and carbonyl content, respectively. With prolonged weathering, lignin content decreased while carbonyl content increased. Both elevated temperature and the addition of water accelerated the photochemical reactions, with water having a more pronounced effect. By establishing the correlation between ΔE^* and R(I_{1 506}/_{1 372}) or R(I_{1 734}/_{1 372}), ΔE^* was negatively correlated with lignin content R(I_{1 506}/_{1 372}) and positively with carbonyl content R(I_{1 734}/_{1 372}). Conclusion: The influence of weathering conditions on surface roughness follows the order: rain > temperature > UVA wavelength. Microscopically, the addition of water causes significant degradation of wood cell walls. In terms of surface color, non-water conditions result in darkening, reddening, and yellowing, while the addition of water leads to an overall trend of darkening, greening, and bluing. The impact of aging conditions on ΔE^* follows the order: rain > temperature > UVA wavelength. By establishing quantitative relationships between chemical composition and ΔE^*, it is confirmed that the surface color changes are closely associated with lignin degradation and carbonyl formation.

Key words: artificial weathering, Korean pine, color, chemical composition, structure