Effects of heating mediums on microstructure and chemical properties of thermally modified Matoa

Abstract Thermal modification (TM) is considered to be the most environmentally friendly and effective industrial method to reduce the hygroscopicity of wood. However, different heating mediums of TM often results in various performance. In this study, the changes of microstructure, crystallization, chemical composition and equilibrium moisture content (EMC) of thermally modified wood (TMW) were investigated respectively to explore the effects of heating mediums (saturated steam, superheated steam, air), modification temperature and water leaching post-treatment on TMWs. The results showed the general intensity of TM was in the order of: saturated steam > superheated steam > air. Saturated steam induced severer cell wall destruction than the other two mediums. Although the porosity slightly increased with the increasing TM temperature and leaching treatment, superheated steam and air TM still decreased the porosity compared to that of control, whereas saturated steam TM increased the porosity apparently. Although TM increased both relative crystallinity and crystal size of TMWs. The increasing TM temperature slightly increased the relative crystallinity, but decreased the crystal size. Leaching caused opposite changes in crystal size of TMWs with different heating mediums. The highest crystallinity was observed after saturated steam TM. The increase in relative amount of lignin and cellulose due to the hemicellulose degradation was the main chemical changes of TMWs, further lignin condensation reaction was occurred after saturated steam TM. Although saturated steam TM induced increased porosity, its lowest EMC indicated the decrease of hydroxyl groups was the dominate reason for the decreased hygroscopicity than the changes of microstructure.