Mechanism of changes in the mechanical properties of wood due to water adsorption and desorption described by rheological considerations

Abstract This study investigates the rheological mechanisms underlying changes in the mechanical properties of wood during moisture adsorption and desorption. To approximate the net molecular momentum at a specific humidity and time, an index was established by dividing the loss tangent value under equilibrium conditions at each humidity by the loss tangent value at each time point. This approach linked the unstable microstructural behavior associated with adsorption and desorption to the stabilization of the structure over time and molecular mobility at specific conditions. This metric enabled effective tracking of molecular mobility during humidity changes and provided insights into the mechanisms driving changes in mechanical properties. During adsorption, wood transitions from a glassy state far from the rubbery state toward one closer to the rubbery state, while desorption shows the opposite trend. Results also indicated an energy barrier near the medium humidity (30–60 %RH), unrelated to lignin. Instead, the softening of hemicellulose and changes in amorphous polymer states within the cell wall likely play critical roles in these mechanical property changes.