Application of time–temperature–humidity superposition to the mass loss of wood through hygrothermally accelerated ageing at 95–140 °C and different relative humidity levels

Spruce wood specimens were hygrothermally treated at different temperatures (T, 95–140 °C) and relative humidity during heating (RHh, 0–92%) to measure the loss in dry mass (ML) due to the treatment. The ML was approximated by a power function of the heating duration t, and the plots of ML versus t were successfully superposed as a single master curve by shifting the plots along the t axis using the shift factors aT and aH, representing the effects of T and RHh, respectively. The apparent activation energies calculated from the linear correlation between ln(aT) and the reciprocal of absolute temperature were 109–116 kJ/mol, almost independent of RHh. Plots of aH versus RHh showed sigmoid shapes, suggesting that the aH value depended on the moisture content of the wood. The shift factors allowed the prediction of the ML of wood by hygrothermal treatment at arbitrary T and RHh. The predicted ML values showed good agreement with experimental ML values available in the literature.