Comparison on thermally modified beech wood in different mediums: Morphology, chemical change and water-related properties

Thermal modification has been proved to be an effective approach to improve the dimensional stability of wood, while different processes of thermal modification often resulted in various efficacy. In this study, the morphology, chemical structure, crystallinity, pore size distribution, and some water-related properties were investigated in order to explore the effect of different mediums (vacuum, nitrogen, steam and air) on thermally modified wood (TMW). It was found that heating medium plays an important role in thermal modification of wood. At both 180 and 210 C-degrees, the general severity of thermal modification is in the following order: air > steam > nitrogen > vacuum. Accordingly, the highest dimensional stability appeared in air TMW, and then steam, nitrogen, and vacuum. It seemed that the water-related properties were primarily affected by the extent of decomposition of the amorphous polysaccharides, but showed no direct relationship with the pore structure. Lignin changed greatly for TMW treated in steam at 180 C-degrees and all TMW treated at 210 C-degrees, by showing more uniform distribution in all cell wall layers. It might be resulted from the migration of lignin fragments after depolymerization, the formation of pseudo lignin from holocellulose, and the increased relative lignin content in secondary wall. The changes on pore structure and lignin structure might have significant effect on other wood properties.