To clarify the effects of humidity during heating on the physical properties of hydrothermally treated wood, Sitka spruce wood was heated in an autoclave at 120 °C and different heating humidity (HRH), and their equilibrium moisture content (EMC), specific dynamic Young’s modulus (E′/ρ), and mechanical loss tangent (tanδ) were measured at 25 °C and 60 % RH prior to and after the hydrothermal treatment. Higher values of HRH resulted in greater loss in weight (WL) because of the acceleration of thermal degradation in the presence of moisture. The time–humidity superposition was applicable to the changes in WL. The EMC was minimized by heating at intermediate HRH (60 %), but it recovered significantly after the wood was moistened at 100 % RH. This fact suggested that the reduction in hygroscopicity due to hydrothermal treatment included both reversible and irreversible effects. The reversible effect was not observed when the wood was heated at 92 % HRH or above. After the moistening, the EMC value of hydrothermally treated wood decreased monotonically with increasing HRH and WL. An irreversible chemical change such as decomposition of hemicelluloses was responsible for the irreversible effect, whereas the reversible effect may have resulted from the annealing of amorphous wood polymers. After hydrothermal treatment at 80 % HRH or lower, E′/ρ increased and tanδ decreased mainly because of the reduction in EMC. On the other hand, high-humidity heating (92 % HRH) resulted in significant decrease in E′/ρ and remarkable increase in tanδ probably because of the depolymerization of hemicelluloses. The color of wood was darkened by the hydrothermal treatment, and the CIELAB color parameters of hydrothermally treated wood were closely connected to the WL.
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