Hygroscopicity and dimensional stability of wood thermally treated with moist air or low point metal alloy: a comparative study

Abstract Heat treatment is effective in reducing moisture absorption and improving the dimensional stability of wood. Low point metal alloy (LMPA) is characterized by high thermal conductivity and large surface tension and can be used as a liquid heat medium for heat treatment. To investigate the effects of liquid and gaseous heat medium on the hygroscopicity, dimensional stability, and chemical structure of heat-treated wood, LMPA and moist air were used as the heating medium during the heat treatment at 140 °C, 170 °C, and 200 °C for 2 h. The results indicated that LMPA heat treatment effectively improved the dimensional stability of wood by increasing the actual heat treatment temperature and reducing the preheating time of the wood. LMPA heat treatment at 200 °C decreased the radial, tangential, and volumetric swelling coefficients of wood by 62, 55, and 56%, respectively. Compared with moist air heat treatment, the radial, tangential, and volumetric swelling coefficients of the wood were reduced by 37, 27, and 38%, respectively. Chemical analyses via attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray diffraction (XRD) demonstrated that the LMPA heat treatment increased the degradation of hemicellulose and reduced the number of hygroscopic groups, resulting in higher crystallinity and enhanced dimensional stability. Hence, LMPA heat treatment can effectively resolve the poor thermal degradation due to prolonged preheating stage and high energy consumption associated with external power for forced convection during the conventional heat treatment using a gaseous heat medium.