Sorption Characteristic of Thermally Modified Wood at Varying Relative Humidity

Abstract

Thermal modification (TM) is commonly used for improving the performance of wood under varying environmental conditions. The effect of TM on the hygroscopic properties of wood has been studied extensively; however, the sorption mechanism and the states of water of thermally modified wood (TMW) at varying relative humidity (RH) is limited. In this work, Douglas fir was modified at 180 °C, 200 °C and 220 °C for a duration of 2 h and the Hailwood–Horrobin model and NMR relaxometry were used after specimens were conditioned at eight different RHs. The results showed that TM inhibited monolayer and polylayer moisture sorption with increasing modification temperatures in all RHs. The lower slope of the sorption isotherms in TMW decreased compared with the control, indicating that the TM increased the hygroscopical stability of wood. The T2 distribution indicated that no free water was observed in the hygroscopic moisture range. The mobility of water molecules in the cell walls was decreased by TM intensity and increased by increasing RH.