Cyclical moisture adsorption and desorption analysis of bamboo under effects of sinusoidally changing relative humidity at different temperatures

Bamboo utilization is highly affected by moisture, which continuously changes with daily cyclical relative humidity (RH). In this study, moisture sorption of bamboo under cyclically changing RH at 15 °C, 25 °C, and 40 °C were investigated and compared with poplar wood. The sorption mechanisms were analyzed based on microstructure and composition. Generally, moisture content (MC) changed sinusoidally with time. MC, amplitude and moisture sorption coefficient were higher, while phase lag was lower, in cyclic period of 24 h than in a cyclic period of 1 h, indicating the moisture sorption of bamboo was higher and faster in longer cyclic period. This was because more time was provided to respond, thus more moisture was accumulated and caught up with RH changes. In contrast, temperature had positive effects on amplitude and MSC but inversely correlated with MC and phase lag, mainly due to enhanced potential energy and increased diffusion of water molecules. The sorption amount and sorption speed of bamboo were less than poplar wood, primarily because bamboo had higher density, less voids as water paths and lower cellulose and hemicelluloses. Bamboo presented lower dynamic sorption hysteresis than poplar wood, and the sorption hysteresis ratio of bamboo reached over 0.96, which had no obvious relation to temperature.