Effects of Thermal Modification on the Axial Compression Properties and Hardness of Laminated Bamboo

Abstract

Vacuum thermal modification is an environmentally friendly method for improving the durability of bamboo, which has the advantages of preventing deformation, increasing hardness, and reducing the risk of insect infestation. The vacuum heat treatment of bamboo could effectively reduce the thermal expansion of bamboo and increase the density of bamboo, which is conducive to reducing the hygroscopic properties of bamboo. In this study, laminated bamboo was thermally modified under vacuum conditions from 170 to 230 °C for 1 to 5 hours, and its microstructure, surface hardness, water absorption and axial compression mechanical properties were evaluated. The results showed that compared with untreated laminated bamboo timber, the color 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 approximately 4–27%, and the density was reduced by approximately 11–18%. This phenomenon indicated that the cell walls of laminated bamboo increased in density with increasing temperature, leading 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 compressive 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 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.

Keywords: thermal treatment, Laminated bamboo, Moisture absorption, Axial compression properties, Nanoindentation