Heating Rate during Thermal Modification in Steam Atmosphere: Influence on the Properties of Maple and Ash Wood

Abstract

This study aimed to compare two thermal modification (TM) schedules—with short and long heating phases—and their influence on the properties of maple (Acer pseudoplatanus L.) and ash (Fraxinus excelsior L.) wood. Two TM runs were conducted in industrial conditions (open system, steam atmosphere; substantially longer method compared to the processes usually described in the literature), with the same peak phase (200 °C, 3 h), but with different heating rates—slow (1.1 °C/h) and fast (2.5 °C/h). The results revealed that both TMs significantly reduced hygroscopicity and swelling of wood, but the influence of slow heating rate—through prolonged exposure of wood to relatively high temperatures—on dimensional stability was more pronounced. The modulus of elasticity, compressive strength and Brinell hardness remained mostly unchanged after TM (except for fast-modified maple), while the modulus of rupture was strongly reduced by TM in both species. It is assumed—at least in the case of maple wood—that a combination of initial moisture content above 8% and fast heating rate during TM can cause more intensive degradation of wood polymers. Relatively small differences in colour between slow- and fast-modified wood were found. The results confirmed the hypothesis that the heating phase is an important part of the TM schedule, and it can directly affect (together with peak temperature and time) certain wood properties.