Effects of Aluminum Chloride Impregnating Pretreatment on Physical and Mechanical Properties of Heat-Treated Poplar Wood under Mild Temperature

Abstract

The acid formed by thermal degradation of wood can autocatalyze its heat treatment. In this study, exogenous acid was introduced by impregnation into poplar wood to investigate its effect on the physical and mechanical properties of wood. Equilibrium moisture content (EMC), dimensional stability, mass loss (ML), color, modulus of rupture (MOR), and modulus of elasticity (MOE) of heat-treated poplar were tested under mild temperature (130–160 °C) for different pretreatment concentrations of aluminum chloride (AlCl3). The results show that the EMC of the heat-treated wood diminishes by 2.7%–47.8%, and dimensional stability improves significantly after AlCl3 impregnation pretreatment. The samples impregnated with 0.5 mol/L AlCl3 and heat treated at 160 °C achieved the best dimensional stability, which was better than for the samples only heat-treated at 220 °C. The color changed significantly as the impregnating concentration increased, achieving a color effect similar to that of wood only heat-treated at a high temperature such as 200 or 220 °C. Heat-treatment temperature under the same ML of wood samples was reduced, which also mitigated the reduction of MOR. MOE of heat-treated wood with 0.5 mol/L impregnation pretreatment was 11.4%–30.7% more than for samples heat-treated at 160–220 °C. After exogenous acidic AlCl3 impregnation pretreatment, the cell wall structure of the heat-treated wood was found to remain relatively intact. Thus, AlCl3 impregnating pretreatment exerted a substantial and beneficial effect on the physical and mechanical properties of poplar and achieved good performance of poplar wood treated at a mild temperature.