Effects of Heat Treatment on Multiscale Structures and Mechanical Properties of Wood:a Review

Heat treatment is an important method to improve the dimensional stability of wood. At the same time, it will cause changes in the multi-scale structure of wood, and then affect the micro and macro mechanical properties of heat-treated wood. In this paper, the effects of heat treatment on multiscale structures and mechanical properties of wood were summarized from three scales of nanometer, micron, and millimeter. At the nanoscale, the degradation of wood hemicellulose during heat treatment resulted in the destruction of matrix properties as an interfacial coupling agent. The hydroxyl groups of cellulose were esterified with the organic acid produced by the degradation of hemicelluloses, and the intermolecular hydrogen bond and the hydrogen bond connected with hemicelluloses were broken in large numbers, which caused the break of molecular chain and weakened the bond strength between hemicelluloses and cellulose. The relative content of lignin changes due to polysaccharide depolymerization, and the self-polycondensation of lignin and the cross-linking reaction with polysaccharide degradation products were important factors for the mechanical properties of the cell wall matrix after heat treatment. At the micron scale, cracks appeared in the intercellular layer of the cell wall of heat-treated wood, and there was a separation between cells. The directional arrangement of the microfibril decreased regularly, the inclination increased, and the mechanical strength of the cell wall broke along the length direction of the microfibril. At the millimeter scale, the early wood and late wood changed from square and round to oval on the transverse section of heat-treated wood, and the early wood cracked more. The cell wall of tracheids and vessels was deformed and cracked on the diameter section. With the increase of heat treatment, the cell wall appeared to vertical axial fracture, pits damage, wood ray cracking, and other phenomena, and a large number of ray parenchyma cells were destroyed, which led to the decrease of the load carrying capacity of wood. The results of this review provide a reference for further study of the mechanism of heat treatment on the structure and mechanical properties of wood. © 2024 Cailiao Daobaoshe/ Materials Review. All rights reserved.