A Study on the Effects of Deep Eutectic Solvent (Chcl-Eg) Pretreatment on the Mechanical and Dimensional Stability Properties of Densified Chinese Fir

Hot-pressing densification is an effective method to improve the physical and mechanical properties of fast-growing wood, but it typically faces challenges such as moisture-induced rebound and brittle fracture of cell walls. This work proposed a strategy for preparing densified Chinese fir (DCF) via deep eutectic solvent (DES) pretreatment. The effects of pretreatment conditions (i.e., DES concentration, treatment temperature, and treatment time) and physical compression ratio on the dimensional stability, mechanical properties, and microstructure of DCF were systematically investigated. The results demonstrated that under the best-performing conditions within the investigated range conditions (10 wt% DES concentration, treatment at 100 °C for 8 h, and 50% compression ratio), the modified fir (viz., DCF) exhibited substantial improvements compared to NW in terms of modulus of rupture (147.8 MPa, a 163.5% increase), modulus of elasticity (8622 MPa, a 71.1% increase), compressive strength along the grain (81.8 MPa, a 138.5% increase), and Shore D hardness (70.6, an increase of 83.9%). Furthermore, the moisture-induced rebound rate was reduced to 0.77%, indicating that the dimensional stability of DCF was effectively enhanced. Microstructural and chemical characterization revealed that DES partially degraded part of the amorphous hemicelluloses, thereby plasticizing the cell wall and enabling cells to undergo flexible folding and dense closure during the hot-pressing process while largely retaining the main cellulose crystalline skeleton. The present work provides a feasible route for converting fast-growing plantation wood into structural materials.