Effects of heating media on microstructure and chemical composition of heat-treated Pometia pinnata

Changes of microstructure, crystallization, chemical composition, and equilibrium moisture content (EMC) of heat-treated wood (HTW) were investigated to explore the effects of heating media (saturated steam, superheated steam, air) and heat treatment (HT) temperature on HTWs. The results showed that the saturated steam induced more severe cell wall destruction than the other two media. Although the porosity slightly increased with the increasing HT temperature, superheated steam and air HT still decreased the porosity compared to that of control, whereas saturated steam HT increased the porosity. The HT increased both relative crystallinity and crystal size of HTWs. The increasing HT temperature slightly increased the relative crystallinity but decreased the crystal size. The highest crystallinity (55.0%) was observed after saturated steam HT. Leaching led to the increase of crystal size of HTW treated in saturated steam (about 0.15 nm), while those treated in unsaturated steam and air decreased. The increase in relative amount of lignin and cellulose due to the hemicellulose degradation were the main chemical changes of HTWs. Further lignin condensation reaction only occurred after saturated steam HT. Although saturated steam HT induced increased porosity, its lowest EMC (5.91%) indicated the decrease of hydroxyl groups.