Intrinsic Flame Retardancy in Flexible Polyurethane Foams Enabled by a Bio-Based P–N Reactive Polyol and a Silicon-Functionalized Triazine Charring Agent

The development of sustainable and intrinsically flame-retardant polymeric materials remains a critical challenge due to the reliance on migratory, halogen-containing additives. In this work, a bio-based phosphorus–nitrogen–silicon (P–N–Si) synergistic flame-retardant system was rationally designed for flexible polyurethane foams (FPUFs) using renewable feedstocks. A reactive phosphorus–nitrogen polyol (PMESO), derived from epoxidized soybean oil and phytic acid, was chemically incorporated into the polyurethane network to impart intrinsic flame retardancy, while a silicon-functionalized triazine charring agent (KHT) was introduced to reinforce condensed-phase protection. Thermal analysis revealed that PMESO promoted early dehydration and catalytic charring, significantly increasing char yield, whereas KHT further enhanced char compactness and thermal stability under oxidative conditions. Fire performance evaluation demonstrated that the combined system markedly improved flame retardancy, as evidenced by increased limiting oxygen index values, pronounced self-extinguishing behavior, and substantial reductions in heat release and combustion efficiency. Comprehensive char characterization confirmed the formation of a dense, continuous, and P–N–Si–enriched char layer, which effectively acted as a physical barrier against heat and oxygen transfer. In addition, TG–IR and Py–GC/MS analyses indicated the release of nonflammable and flame-inhibiting species into the gas phase, providing auxiliary suppression of flame propagation. Overall, this study demonstrates how a reactive, bio-derived P–N–Si strategy can minimize the use of migratory flame retardants while simultaneously enhancing fire safety and thermal stability. The results highlight a sustainable and chemically informed pathway for designing safer polyurethane materials.