Kinetic studies on photo-degradation of thermally-treated spruce wood during natural weathering: Surface performance, lignin and cellulose crystallinity

•Degree of cellulose crystallinity during natural weathering periods can specify the performance of wood.•A kinetic study on surface photo-degradation determine the service time of wood materials in exterior uses.•Thermal treatment can delay color change of wood due to upgraded lignin and crystalline structure of cellulose.•Thermal treatment improved the hydrophobicity and crystallinity index of wood.•Natural weathering changed wettability and morphological properties of the wood surface. The hygroscopic behavior of wood-based materials is undoubtedly influential on the material surfaces exposed to climatic conditions. The paper aims to kinetically investigate the photo-degradation and surface characteristics of thermally-treated spruce wood at 185 °C and un-treated (reference) sample through 2 years of natural weathering. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray diffraction (XRD) were employed to investigate the chemical changes occurred in lignin and crystalline structure of cellulose as a function of outdoor exposure. Detailed measurements of morphological and surface changes were evaluated during exposing periods. Surface wettability and roughness as well as cracks were increased for all samples due to weathering as the exposure time extended. Prominently, the color changes (ΔE) and photo-chemical reactions were found to be negligible in short-time weathering thanks to thermal modification. In the long–term exposing, more OH-bonds caused by lignin leaching out resulted in presence of cracks and the affinity for water on reference sample. Degree of crystallinity significantly decreased by 12.7% in thermally treated wood and 18.3% in reference sample after long-term weathering while, the crystallinity index of thermally-treated wood increased to 4.7% in 6-month weathering. Hence, the amorphous structures and chromophores groups appeared on the reference wood at a faster and more intense rate during exposing time. Thermal treatment delayed photo-yellowing as well as color change of wood due to up-graded lignin and cellulose crystallinity through a photo-stable structure. Accordingly, kinetic studies ascertained the service-time of wood products in exterior uses.