Abstract:

Objective: The variations of water vapor sorption behavior of wood was investigated during cyclic adsorption-desorption processes, aiming at clarifying the response mechanism of wood cell wall and its adsorbed water to the cyclical humidity variation, and providing theoretical guidance and data reference for the actual usage and rational development of wood. Method: The moisture adsorption/desorption isotherms of Chinese fir (Cunninghamia lanceolata) and masson pine (Pinus massoniana) were measured synchronously by Dynamic vapor sorption analysis during three or five adsorption-desorption cycles among 0% to 95% to 0%. The equilibrium moisture content (EMC) data of samples in the adsorption process were nonlinearly fitted by H-H model. Result: 1) Before and after the sorption cycles, the maximum EMC of samples showed a decreasing trend, and the difference value was ranged from 0.21% to 1.76%. The EMC in the initial cycle was lower than that in the subsequent cycles, especially between 40% and 95% relative humidity. 2) With the increase of the number of sorption cycles, the time required for one single sorption cycle of samples decreased, and the reduction degree also became obviously smaller. Compared with the solid samples, the single sorption cycle of the powder samples required less time. 3) The hygroscopic hysteresis of samples in the initial cycle was more obvious than that in the subsequent cycles. The maximum value of absolute hysteresis decreased and the mean hysteresis coefficient increased with the increase of the number of sorption cycles. 4) The H-H model could be used to describe the water adsorption isotherms of samples during the sorption cycles with R² above 0.996. With the increase of the number of sorption cycles, the parameter W which means the apparent molecular mass of the absolute dry wood per sorption sites decreased, the maximum amount of monolayer molecules water increased, and while the maximum amount of polylayer molecules water and the total amount of adsorbed water decreased. Conclusion: With the increase of the number of sorption cycles, the response of wood cell wall to cyclical humidity variation of samples were weakened: the hygroscopicity of samples decreased, and the maximum EMC showed a decreasing trend; the time required for one single sorption cycle was reduced; and the hygroscopic hysteresis of samples decreased.

Key words: Chinese fir, masson pine, cyclical humidity variation, water adsorption, H-H model