Hygrothermal recovery behavior of cellulose-rich gelatinous layer in tension wood studied by viscoelastic vibration measurement

Hygrothermal treatment induces irreversible dimensional changes of green wood i.e. hygrothermal recovery (HTR). To understand what happened to cellulose-rich gelatinous (G-) layer in green tension wood during HTR, changes in vibrational properties of konara oak (Quercus serrata Thunb. ex Murray) tension wood (TW) and normal wood (NW) collected from sapwood after hygrothermal treatment were tested regarding HTR. After this treatment, all specimens were air-dried, and their vibrational properties and dimensions were measured in this dried state. The hygrothermal treatment induced an increase in mechanical loss tangent (tanδ) and a decrease in specific dynamic Young’s modulus (E’/ρ). Changes in vibrational properties due to hygrothermal treatment appeared to depend on treatment time and temperature with higher temperatures and longer treatment durations producing larger increases in tanδ and larger decreases in E’/ρ. In TW with a G-layer, a clear correlation between changes in vibrational properties and HTR strains was identified. Tanδ increased and E’/ρ decreased corresponding to HTR strains. Contraction of the G-layer in TW cell walls due to release of locked-in growth stress by hygrothermal treatment seems to be the most plausible mechanism underlying changes in vibrational properties and generation of HTR. In NW without a G-layer, HTR strain was below the limit of detection, which obscures potential correlations. Differences in the intensities of changes in vibrational properties after 120 min hygrothermal treatments at 60, 80, and 100 °C were not significant after drying; however, the difference in the intensities of HTR strains apparently remained after drying.