A flame-retardant densified wood as robust and fire-safe structural material

Wood is an abundant, sustainable, low-carbon emission engineering material. However, fire safety issues and insufficient mechanical strength limit its applications. Herein, a simple and scalable method is developed to prepare a robust and flame-retardant structural wood that endures high-temperature flame attack. Natural wood undergoes partial delignification, flame-retardant modification, and densification to obtain ammonium dihydrogen phosphate (ADP)-densified wood. ADP is uniformly impregnated to the wood cell walls to form P–O–C bonds with a dense and cross-linked structure. ADP-densified wood presents enhanced thermal stability, flame retardancy, and mechanical robustness. Its mechanical performance is well-preserved from high-temperature flame heating. In particular, its tensile strength and flexural strength are 22.6-fold and 17.8-fold higher than natural wood after being heated for 300 s. The improved performance is due to the synergistic effect of ADP flame retardant, which accelerates the insulating char formation, and the dense and cross-linked wood structure, which leads to the condensed and robust char structure. The insulating and condensed char limit the heat and pyrolyzed combustible gases transfer between the wood surface and the interior, which protects the wood interior from high-temperature flame heating. ADP-densified wood, as a robust and flame-retardant structural material, has great potential in green and energy-saving construction applications.