Whether water sorption hysteresis exists above 75 °C and the representation of its relationship with wood softening remains unclear. In this study, sorption hysteresis at three temperature levels, namely, 75, 90 and 99.5 °C, was investigated using accurately controlled conditioning chambers using powder samples from seven wood species. The dry mass loss percentage of the samples and proper equilibrium time was evaluated by pretests to ensure reliable sorption isotherms afterwards. The dry mass loss percentage after the entire sorption isotherm running needs to be kept within 0.5% to avoid complications in determining the equilibrium time. The abrupt increase of equilibrium moisture content at high relative humidity regions and the crossover of adsorption isotherms were not observed. Hysteresis was conspicuous at 75 and 90 °C and was statistically confirmed at 99.5 °C. High sample extractives content or unnecessarily long equilibrium time would cause significant sample dry mass loss, which may suppress the desorption isotherm to an extent that leads to unjustified hysteresis disappearance. The hysteresis behavior observed in this study suggests that wood softening does not necessarily result in hysteresis disappearance, and the explanation of hysteresis progression as a function of wood softening temperature must be evaluated with care in the future.
Funding source: Akita Prefectural University
Funding source: Nanjing Forestry University
Award Identifier / Grant number: 163020200
The sharing of the original sorption isotherm data of sikkim spruce (Picea spinulosa Griff.) from Prof. Jinzhen Cao at Beijing Forestry University is greatly appreciated.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: The experimental work was funded by the Institute of Wood Technology at Akita Prefectural University and the preparation of the manuscript was funded by a research startup grant 163020200 from Nanjing Forestry University.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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