Synergistic Fire Protection and Water Resistance of Intumescent Coatings via Surface- Engineered Shell-Derived Biogenic Inorganic Particles

Steel-structure intumescent fire-retardant coatings suffer from poor water resistance and weak high-temperature char layers. Herein, shell-derived biogenic inorganic particles (SBIPs) were surface-functionalized with KH-550 and incorporated as a bio-based synergistic filler into silicone-acrylic/epoxy waterborne intumescent coatings (IFR-EP/SAE). Characterizations including combustion simulation, thermogravimetry, limiting oxygen index (LOI), cone calorimetry, SEM, and Raman spectroscopy were performed to investigate interfacial compatibility, fire protection, thermal stability, water-aging resistance, and char evolution. KH-550 functionalization improved SBIP–matrix compatibility, while a distinct biphasic concentration-dependent effect on flame retardancy was observed, with 3 wt.% SE-SBIPs showing optimal performance. Compared with the blank control, the addition amount of 3 wt. % reduced the peak back-side steel temperature to 289.27°C, increased the LOI to 42.5%, and raised the initial rapid thermal-degradation temperature from 304 to 363°C. After 48 h water immersion, it maintained favorable flame-retardant and smoke-suppression performance, with reduced peak heat-release rate and total smoke release. Raman analysis showed a decreased ID/IG ratio from 1.36 to 1.07, indicating enhanced char graphitization. This work provides a sustainable strategy for developing durable bio-based fire-protective coatings.