Water sorption in wood: effect of extractives on water sorption and hysteresis behavior

The water sorption and hysteresis behavior of four high extractive content woods —western red cedar, Chinese juniper, tubi, and messmate—were examined at 30, 45, 60, 75, 90, and 99.5 °C by comparing the water sorption isotherms before and after extraction. A significant dry weight loss during water sorption tests was observed for cedar and juniper, complicating their water sorption and hysteresis behavior. Gas adsorption analysis revealed that the cumulative pore volume of dried cell walls in all four species significantly increased after extraction, indicating a comparable extent of likely bulking of extractives within the lignin structure. Across the temperature range of 30 to 99.5 °C, cedar and juniper showed significantly higher equilibrium moisture content values after extraction throughout the entire hygroscopic range. In contrast, only minor differences were observed for tubi and messmate after extraction. This discrepancy may be attributed to variations in the hygroscopicity of extractives deposited in the cell walls. When applying the bulking effect theory, it is important to consider the hygroscopicity of bulked extractives. Tubi shows a distinctly higher magnitude of sorption hysteresis even at 99.5 °C, and no significant differences in the magnitude of hysteresis were observed for either tubi or messmate after extraction.