Effect of hygrothermal treatment on wood properties: color changes and kinetic analysis using four softwood and seven hardwood species

Color changes of four softwood and seven hardwood species during hygrothermal treatment were compared among species and kinetically evaluated. Treatment temperature ranged from 70 to 120 °C, and the durations were 5–150 h. Generally, the \(L^{*}\) (lightness) decreased and the total color differences \((\Delta E_{\text{ab}}^{*})\) increased irrespective of the treatment temperature. \(a^{*}\) and \(b^{*}\) (redness and yellowness) values varied spuriously based on the wood species. Kinetic analysis using the time–temperature superposition principle, which uses the whole data set, was successfully applied to the color changes. The apparent activation energies of the color changes calculated from \(\Delta E_{\text{ab}}^{*}\) were 24.3–40.8 kJ/mol for softwood and 32.3–61.3 kJ/mol for hardwood. The average apparent activation energy for hardwood was higher than for softwood. These values were lower than those calculated from other material properties. The obtained results will contribute to assess the color changes during the early stage of kiln drying and hygrothermal modification of wood.