Effect of thermal modification on axial compression properties and hardness of laminated bamboo

Vacuum thermal modification is an environmentally friendly method to improve the durability and reduce insect infestation of bamboo, which has the advantages of preventing deformation, preventing oxidation of the bamboo surface, and uniformity of thermal conductivity compared to previous studies that used mediums such as nitrogen, air, or oil for heating. This study thermally modified laminated bamboo materials for 1 to 5 h under vacuum conditions ranging from 170 to 230 °C and evaluated their microstructure, surface hardness, water absorption, and axial compression mechanical properties. The results showed that compared with untreated laminated bamboo timber, the colour gradually deepened as the temperature and time of heat treatment increased, while the equilibrium moisture content of heat-treated laminated bamboo timber was reduced by about 4–27% and the density by about 11–18%. This phenomenon indicates that the cell walls of laminated bamboo become denser with increasing temperature, which leads to a significant decrease in the hygroscopicity of laminated bamboo. The dimensional stability of the material was improved. Regarding the macroscopic axial compression test, the results showed that the thermal modification treatment significantly increased the axial compression strength of laminated bamboo, and this improvement in mechanical properties was inversely related to the heat treatment temperature. In addition, through the nanoindentation test, we found that the laminated bamboo's surface hardness and modulus of elasticity showed an inflexion point when the treatment temperature reached 210 °C, indicating that the material properties changed significantly in this temperature range. Investigating the response mechanism between the temperature and time of vacuum thermal heat treatment and the mechanical properties of bamboo will contribute to the development or optimization of the thermal modification process, leading to materials with excellent properties and broadening the application areas.