Nacre-Inspired Flame-Retardant Coating with a Brick–Mortar Architecture Based on Bio-Based Chitosan/Gelatin and APP/MMT for Flexible Polyurethane Foam

Flexible polyurethane foam (FPUF) suffers from high flammability and environmental concerns, necessitating sustainable fire protection strategies. This work highlights a nacre-inspired, halogen-free flame-retardant coating with a hierarchical multicomponent “brick–mortar” architecture. Ammonium polyphosphate (APP) and sodium montmorillonite (MMT) act as rigid multiscale “bricks”, while Schiff base-modified chitosan (DCS) cross-linked with gelatin (GEL) forms the adhesive “mortar”, constructing a hydrogen-bonded, organic–inorganic hybrid network with robust interfacial compatibility on FPUF. The resulting FPUF/3A@M-D/G composite demonstrates remarkable fire safety, achieving a limiting oxygen index (LOI) of 32.6% and a UL-94 V-0 rating. Cone calorimetry shows significant reductions in pHRR1 (−46.1%) and pHRR2 (−67.7%), and THR (−16.8%), along with prolonged ignition time (from 4 to 6 s) and increased char yield (from 8.2 to 26.3 wt %). Meanwhile, the coating improves compressive strength (from 7.96 to 9.32 kPa) without compromising elasticity. Mechanistic analyses (SEM, FTIR, Raman, XPS, and Py-GC/MS) reveal a dual flame-retardant action of condensed-phase char reinforcement and gas-phase radical inhibition. This scalable bioinspired strategy provides a halogen-free pathway to simultaneously improve flame retardancy and mechanical robustness of FPUF for applications in transportation, furniture, and energy systems.