Hygroscopic behavior, dimensional stability, and chemical structure of bamboo after thermal treatments under dry and wet conditions

Abstract Thermal treatment is a widely used bamboo modification method, but how moisture in the treatment atmosphere influences the hygroscopicity and dimensional stability of thermally modified bamboo is still poorly understood. In this study, the effects of dry thermal (N 2 T), hygrothermal (HGT), hydrothermal (HDT), and combined hydrothermal-dry thermal (HD&N 2 T) treatments on the chemical composition, hygroscopicity, and dimensional stability of Moso bamboo were comparatively investigated. The hygroscopic behavior of bamboo depended not solely on the number of accessible hydroxyl groups, but also on the porous structure of bamboo. The presence of moisture facilitated the degradation of hemicellulose and amorphous cellulose, but HDT treatment with excessive moisture largely deteriorated the dimensional stability of bamboo due to cell softening and collapse. Upon N 2 T treatment, lignin condensed with degradation products of polysaccharides, resulting in the highest crosslinking degree, the fewest sorption sites (7.1 mmol/g), the largest hysteresis value, and a significantly improved dimensional stability with an anti-swelling efficiency (ASE) of 27.3 %. HGT modified bamboo with the lowest hemicellulose content and a moderate crosslinking degree showed the most enhanced dimensional stability, with an ASE of 34.2 %. This study highlights the critical role of moisture in determining the properties of thermally modified bamboo.