Effective strategy for fabricating surface impregnated and unilaterally densified wood with furfuryl alcohol/flame retardants for enhanced mechanical performance and flame retardancy

The widespread utilization of fast-growing plantation timber in furniture and building construction is often hindered by defects such as insufficient mechanical strength, dimensional instability, and flammability. In this work, an effective and simple approach was presented to enhance the performance of fast-growing poplar wood, through surface impregnation with a mixed solution of biomass-derived furfuryl alcohol (FA) and flame retardants ammonium dihydrogen phosphate (ADP), followed by unilateral densification. Benefiting from the compressed surface structure and the addition of ADP, the densified wood impregnated with 8 wt% ADP exhibited superior fire resistance, achieving UL-94 V-0 rating and a high LOI value of 44.3%, and showed remarkably lower heat and smoke release in Cone tests. The incorporation of FA resin significantly enhanced the plasticizing effect of cell wall and the densification degree of surface layer, thereby improving the physical and mechanical properties of wood. The flame-retardant densified wood impregnated with 30 wt% FA and 8 wt% ADP exhibited a surface hardness of 4750.8 N, modulus of rupture of 106.3 MPa, and modulus of elasticity of 8.7 GPa. Moreover, the cross-linked network derived from FA resin effectively reduced the loss of ADP and suppressed the set-recovery of deformed cell walls, thereby enhancing the dimensional stability and durability of densified wood. The surface-impregnated and unilaterally densified wood exhibited exceptional comprehensive properties, showing a promising potential as a high value-added product for furniture and building construction.