Surface-constrained mineralized intumescent coatings with high adhesion, mechanical robustness, flame retardancy, smoke suppression, and thermal insulation

To enhance the fire safety and thermal insulation performance of timber buildings, this study proposes a strategy for constructing a surface-constrained mineralized flame-retardant protective film. This approach used sodium silicate as the matrix and incorporated ammonium polyphosphate, sorbitol, and sodium hexametaphosphate to construct a silicon-phosphorus intumescent flame-retardant system. Hydroxyapatite (HAP) was modified with a coupling agent and introduced as a mineral-reinforcing filler to prepare an NHA@M-HAP coating. By improving the dispersion of HAP and its interfacial compatibility within the coating, the formation of the Si-O-Si network, phosphate structural units, and Ca-P mineral framework was promoted, thereby reducing the coating’s porosity and interfacial defects. The NHA@M-HAP coating achieved grade 1 adhesion, a pencil hardness of 5 H, and a bending diameter of 1 mm. After applying the coating, the char length of the substrate after open-flame combustion was reduced by 87.3%, and the time to ignition was extended from 16 s to 173 s. The peak heat release rate, total heat release, and total smoke release were reduced to 53.92 kW/m 2 , 30.25 MJ/m 2 , and 47.56 m 2 /m 2 , respectively, and the coated substrate self-extinguished. The residual mass of the coating reached 74.5% at 800 °C. After continuous heating for 80 s, the backside temperature remained as low as 42.2 °C, demonstrating that the coating inhibited heat transfer, oxygen diffusion, and combustible volatiles release. This strategy provides new insights into the structural design of green, high-performance flame-retardant coatings for interior wood building components.