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

Abstract

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.