Fire and smoke resistant wood achieved via metal phosphate deposition combined with unilateral thermal densification

Wood faces inherent challenges of flammability and toxic smoke emission, limiting its advanced applications. This study constructs a high-performance wood-based material through the synergistic combination of one-side thermal densification and in-situ formed metal phosphate flame retardants (NP@Fe, NP@Cu, NP@Al). The resulting unilaterally densified wood (UW@Fe, UW@Cu, UW@Al) exhibits excellent thermal stability with a high capacity for suppressing ignition, heat release, and smoke generation. Chemical and structural analyses confirm that the insoluble metal phosphates are asymmetrically embedded within the densified surface layer, promoting a dense, thermally stable char barrier during combustion. Compared to untreated wood, UW@Fe achieves a 123.7% increase in limiting oxygen index (LOI), and a 40 s prolongation in time to ignition (TTI) during the Cone calorimetry test (CCT); UW@Al reduces combustion area by 66.2% and mass loss by 91.1% during the macroscopic combustion test (MCT). The CCT reveals substantial reductions in total heat release (THR) (40.1–74.4%) and total smoke production (TSP) (75.3–95.5%) for all treated samples, with UW@Al exhibiting the highest efficiency. Metal phosphates bulking and unilateral densification synergistically generate a more continuous char structure, providing superior surface protection. Scanning electron microscope (SEM) analysis reveals that metalphosphates form porous particle aggregates and filled the pores of the wood residual char. Raman spectroscopy and thermogravimetric analysis (TG) complementarily demonstrate that the char layer exhibits a more graphitic carbon structure and enhanced thermal stability. This work presents a novel strategy for engineering safer, renewable wood-based materials for sustainable construction applications.