Effects of hydroxyl groups and pores on moisture sorption behavior of heat-treated wood and its pore structure prediction

Abstract

Thermal treatment has become an important approach to improve the dimensional stability and durability of wood, owing to its environmental friendliness and sustainability. During thermal treatment, the hydrophilic functional groups and pore structure of wood undergo significant changes, thereby affecting its hygroscopicity; however, the dynamic coupling relationship and quantitative contributions of these factors under varying relative humidity (RH) remain insufficiently clarified, and the characterization of cell wall pores at different RH levels is still challenging. To address these issues, this study refined the Horikawa-Do (H-D) model to quantify the contents of hydroxyl-bound water and pore-confined free-bound water in heat-treated wood under different RH levels, and further developed cell wall pore prediction model based on sorption isotherms. The results showed that both hydroxyl-bound water and pore-confined free-bound water coexist at any RH. At low to medium RH, hydroxyl-bound water adsorption predominates, whereas at high RH, pore-confined free-bound water adsorption dominates. Thermal treatment reduced the hydroxyl content of wood and enhanced hydroxyl binding energy, thereby significantly decreasing hydroxyl-bound water content, while increasing the hygroscopic ratio of pore-confined free-bound water (from 2.02 to 4.15 at 90% RH). This caused the “equilibrium point” between the two hygroscopic mechanisms to shift toward lower RH with increasing treatment temperature, indicating that thermal treatment primarily reduced wood hygroscopicity by decreasing hydroxyl-bound water. In addition, the average size of water clusters associated with moisture sorption was positively correlated with RH but negatively correlated with treatment temperature. The pore size prediction model revealed that the dominant hygroscopic pore size range in wood treated at 230 °C (1.58–2.36 nm) was significantly narrower than that in untreated wood (1.93–3.07 nm), with reductions of 22.2% and 34.1% in the minimum and maximum pore sizes, respectively. These findings systematically elucidate the microscopic mechanisms underlying the improvement of wood dimensional stability by thermal treatment.