Phosphorylation treatment for enhanced mechanical strength and flame retardancy of densified wood

Densified wood holds great promise as a high-strength material for future construction; however, its practical application is severely constrained by poor fire resistance. Herein, this paper produced a high-strength and fire-resistant wood-based material by combining delignification, vacuum (phosphorylation) impregnation and hot-press densification techniques. With a density of 1180 kg/m³, densified phosphorylated wood is 2.74 times denser than natural poplar (430 kg/m³) and 1.25 times denser than densified wood (94 kg/m³).The bending strength and tensile strength of modified densified poplar wood reached 273 MPa and 208 MPa, respectively, representing 1.31 times and 1.14 times the values of densified wood, and the impact strength was 2.06 times that of densified wood. Furthermore, phosphorylation endows densified wood with outstanding flame-retardant properties. All modified wood achieved the highest flame-retardant grade (V-0) in the UL 94 vertical burning test. This superior performance is evidenced by a 29% reduction in total heat release (THR), a 96% decrease in total smoke production (TSP), and a limiting oxygen index (LOI) elevated to 42%, which is 1.68 times the value of natural wood. This modified densified wood exhibits favorable mechanical properties alongside its highly effective flame retardancy, demonstrating significant application potential in green building as well as furniture materials.