Preparation and Characterization of Heat-Treated Douglas Fir Wood with Core–Shell Structure

Wood heat treatment has been generally accepted as an effective wood modification technology as it improves the dimensional stability and biological durability of wood. However, the property improvements are obtained at the cost of reduced mechanical properties. In this study, heat-treated Douglas fir wood with thermally modified shell and unchanged inner core was prepared by surface heating to make possible the broader applications of heat-treated wood. Colour comparison, dynamic vapor sorption and dynamic mechanical analysis were performed to characterize the physico-mechanical performance of the shell and core of the treated wood. The results reveal a darkened, hydrophobic and rigid shell and a barely changed inner core. X-ray photoelectron spectroscopy shows pronounced degradation of polysaccharides in the surface layer. In contrast, the relative contents of different carbon components in the core layer are similar to that of the untreated wood, indicating the reason for the performance difference between the shell and the core of the treated wood. The initial wood moisture content plays an important role in controlling the temperature gap between the core and the shell during treatment and can be used as a key parameter to adjust the structure and performance of the heat-treated wood.