Carbon Nanofiber/Polydopamine/Fe ³⁺ Composite Aerogel Coatings as Gas–Solid Fire Barriers for Rigid Polyurethane Foams

Inspired by the hierarchical structures of biological systems and interfacial engineering principles, this study focuses on developing sustainable biobased flame retardants to address rigid polyurethane foam (RPUF)’s intrinsic flammability and poor thermal stability. A hierarchical cellulose nanofibril/polydopamine/Fe3+ aerogel coating was fabricated on RPUF via vacuum freeze-drying, and its effects on RPUF’s thermal stability, flame retardancy, and char-forming behavior were systematically investigated. Thermogravimetric analysis showed the coating significantly improved RPUF’s thermal stability, with char residue at 800 °C increasing by 122% compared with the uncoated sample. The coated RPUF achieved a limiting oxygen index of 60 vol %, indicating greatly enhanced flame retardancy. Cone calorimeter tests confirmed the coating prolonged ignition time, reduced peak heat release rate and total heat release, increased fire performance index, and decreased fire growth index, exhibiting excellent fire protection efficiency. Its good flame retardancy stems from the superior gas–solid synergistic effect: a dense, highly graphitized char layer forms on RPUF composite during combustion to act as a protective barrier, and nonflammable gas dilutes combustibles via condensed-gas phase synergy. This work provides theoretical and experimental support for biobased aerogel flame-retardant design and holds potential in construction fire protection.