Effects of pressurized N2 and steam as heating medium on the set recovery of surface compressed white poplar (Populus tomentosa)

Permanent fixation of compressive deformation is one critical aspect of wood densification technology. In this study, heated and pressurized nitrogen gas (N₂), steam and their mixture were applied as the heating media to treat surface compressed poplar wood for the compressive deformation fixation. When steam was applied at 0.5 MPa, around 80% of the compressive deformation was permanently fixed. Although heat treatment in N₂ also caused hemicellulose degradation in wood as that caused by heat treatment with steam, yields of acetic acid and aldehyde compounds were significantly lower than that from heat treatment of surface compressed wood (SCW) with steam-N₂ mixture containing more than 50 vol % steam. Heat treatment with steam effectively reduced set recovery of SCW. Set recovery induced from hygroscopicity and water absorption were 2 to 4 times higher than those after treated with steam treatment. Additionally, heat treatment in N₂ resulted in the biggest wood hardness loss. When steam-N₂ volumetric ratio was 1:1, set recovery was over 50% lower than that obtained by heat treatment in N₂, while wood hardness was just slightly decreased. Steam introduction into N₂ for SCW heat treatment significantly reduced the compressive set recovery as well as mass loss. From the perspective of industrial manufacture and application, when the heating medium pressure exceeded 0.3 MPa, steam-N₂ mixture medium with over 50 vol % steam contributed to the permanent fixation of over 50% compressive deformation. Meanwhile, the set recovery of SCW after exposed to high temperature and high humidity was less than 5%, with no excessive hardness loss.