The Effect of Different Moderate Thermal Modification Durations on the Wood Properties of American Alder

To investigate the effect of moderate thermal modification (TM) on wood properties, American alder (Alnus rubra) wood was treated at 140 °C for 4 h, 8 h and 13 h, the physical and mechanical properties, dimensional stability and color changes of wood were compared and studied. The results showed that the absolute dry density of moderate-TM wood decreased significantly with time except for the 4 h treatment. Moderate TM can significantly reduce the residual stress of wood up to 90.3%. There were no significant differences in MOR and MOE between most moderate TM wood and the control group; moderate TM decreased the moisture absorption and water up-taking of wood significantly; compared to the control group, the swelling of TM wood for 13 h decreased by 24.2% and 16.0% in the tangential and radial direction, respectively, showing a significant improvement in dimensional stability. There were almost no color changes even when wood endured 140 °C and 13 h TM. The moderate TM at 140 °C for 13 h can efficiently improve wood dimensional stability and retains the natural color of wood while causing almost no damage to the wood’s mechanical strength.