Effect of heating medium and pressure on the set recovery and physicochemical properties of surface-compressed Chinese fir

Abstract

Pressurized heat treatment is an effective modification method for reducing the deformation recovery of compressed wood. In this study, the set recovery behavior, physicochemical and mechanical properties were studied for Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) subjected to surface compression treatment followed by pressurized heat treatment. The surface compression of wood was conducted in an open hot press with compression ratio of 33% as the first step, followed by pressurized heat treatment at 180 °C with different heating medium (nitrogen, steam, and nitrogen-steam mixed gas) under varying pressures of 0.1 MPa, 0.3 MPa, and 0.5 MPa as the second step. The results showed that the compressed layer density, the modulus of rupture (MOR), and the modulus of elasticity (MOE) of surface-compressed wood increased by about 55.56%, 56.60% and 30.90%, respectively, compared with the uncompressed wood. At higher heating medium pressures of 0.5 MPa with steam, the set recovery of surface-compressed (SC) wood induced by immersion and boiling in water was reduced by 88.03% and 70.11%, respectively, compared to the SC wood without pressurized heat treatment. Under the same medium pressure, SC wood treated with steam exhibited reduced set recovery compared to that treated with nitrogen and a nitrogen-steam mixed gas. It should be noted that incorporating nitrogen as a heating medium during the steam heat treatment process can reduce surface discoloration and improve mechanical properties while permanently fixing more than 70% of the compressive deformation of wood.