Enhanced mechanical and humidity resistance properties of heat-treated wood by impregnation of low-molecular-weight lignin

Heat treatment is admittedly a significant value-adding step in wood processing. However, the decrease in mechanical properties of wood caused by heat treatment has always been an urgent problem to be solved. This study aimed to overcome this barrier by impregnating Balfour spruce (Picea likiangensis var. balfouriana) wood with low-molecular-weight lignin before heat treatment. The low-molecular-weight lignin impregnated more effectively and distributed uniformly in wood cell wall. The equilibrium moisture content of the heat-treated wood with lignin impregnation decreased significantly compared to those without lignin impregnation. The low-molecular-weight lignin impregnation combined with heat treatment improved the dimensional stability and humidity resistance effectively. The impregnated cell walls with lignin were integrated and seldom spoiled during heat treatment. The heat-treated wood impregnated with low-molecular-weight lignin showed an increased MOE, MOR, and compressive strength (CS) of 52, 177, and 25%, respectively, compared with the heat-treated wood without lignin impregnation. The indentation modulus of the secondary walls of heat-treated wood with impregnated low-molecular-weight lignin increased by 300%. These findings provide some insights into the relationship between lignin content and the strength of wood and show a way to enhance the mechanical properties of heat-treated wood without damage to its dimensional stability.