Fe-Coordinated Dopamine Derivative-Based Flame Retardant Epoxy Resin: Thermal Stability, Flame Retardancy, and Mechanism

Abstract With the increasing fire safety requirements of epoxy resin (EP), developing sustainable and efficient flame retardants has become highly desirable. In this work, a bio-based phosphorus–nitrogen flame retardant (DA-HCCP) was synthesized using dopamine (DA) and hexachlorocyclotriphosphazene (HCCP), followed by Fe3+ coordination to construct a metal-containing flame-retardant architecture (Fe@DA-HCCP). Subsequently, the as-prepared Fe@DA-HCCP was incorporated into EP to investigate its effects on thermal stability, combustion behavior, and flame-retardant mechanisms. With a loading of only 5 wt %, the EP/Fe@DA-HCCP composites achieved a UL-94 V-0 rating and a limiting oxygen index (LOI) of 33.5%, together with significantly enhanced char formation and thermal stability. Cone calorimetry results revealed that the peak heat release rate (pHRR) and total smoke production (TSP) decreased by 51.7% and 37.2%, respectively, while the time to ignition (TTI) was markedly prolonged, indicating substantially suppressed fire hazard. Mechanistic investigations demonstrated that DA-HCCP primarily exerted flame retardancy through phosphorus–nitrogen synergism by promoting char formation in the condensed phase, while phosphorus-containing active species, including PO• radicals, may provide an auxiliary contribution through gas-phase radical quenching. More importantly, the incorporation of Fe3+ facilitated catalytic dehydration, aromatization, and graphitization during thermal degradation, leading to the evolution of a compact and thermally stable carbonaceous protective layer. Simultaneously, the generation of combustible volatiles and smoke precursors was effectively suppressed. In addition, Fe3+ coordination improved the glass transition behavior and stiffness of the epoxy composite, while maintaining favorable dispersion within the EP matrix. This work provides an effective strategy for constructing bio-based multifunctional flame-retardant systems by integrating phosphazene chemistry with transition-metal catalysis.