Moisture sorption isotherms and hysteresis of cellulose, hemicelluloses and lignin isolated from birch wood and their effects on wood hygroscopicity

Abstract

Cellulose, hemicelluloses, and lignin in wood have completely different structures and hygroscopicity, which markedly impact the wood moisture adsorption properties. The equilibrium moisture content (EMC), the density of sorption sites, and hysteresis characteristics of cellulose, hemicelluloses, and ball-milled lignin isolated from white birch (Betula platyphylla Suk.) were investigated to determine their influence on the adsorption isotherm and hysteresis of wood. A dynamic vapor sorption apparatus was used to test the sorption isotherm and the rule of mixture was employed to evaluate differences between in-situ and isolated chemical components. The sorption site occupancy (SSO) model was used to calculate the density of sorption sites of birch cellulose, hemicelluloses, and lignin, and the calculated results were then compared with the theoretical hydroxyl content. The results show that the dewaxed birch sample, cellulose, hemicelluloses, and lignin at 95% relative humidity had EMC values of 20.7%, 20.4%, 107.4%, and 11.6%, respectively, their largest relative hysteresis values were 1.60, 1.20, 1.40, and 1.74, respectively. Sorption site density of birch sample, cellulose, hemicelluloses, and lignin calculated by the SSO model were 10.5, 9.2, 17.3, and 6.0 mmol/g, close to that of theoretical hydroxyl content. Lignin had the highest relative hysteresis compared with cellulose and hemicellulose, and interaction and cross-linking between wood chemical components have a great influence on wood hysteresis.