Modification of Red Phosphorus Flame Retardant via Hydrolytic Microencapsulation: Thermo‐Chemical Behavior Regulation and Flame‐Retardant Mechanism Investigation in Epoxy Composites

To mitigate the limitations of red phosphorus (RP) flame retardants, such as pronounced hygroscopicity and oxidation susceptibility, a high‐efficiency flame retardant (HLE) was synthesized via hydrolytic microencapsulation and incorporated into an epoxy resin (EP) matrix. It was found that the composite with 7 wt% HLE (EP/7‐HLE) exhibited a limiting oxygen index of 34.4% and attained a UL‐94 V‐0 rating, with a self‐extinguishing time of 3 s. Cone calorimetry tests revealed a 43.2% reduction in peak heat release rate (PHRR = 768 kW/m 2 ) and a 33.1% decrease in total heat release (THR = 83 MJ/m 2 ) compared to pure EP. Thermogravimetric analysis confirmed enhanced thermal stability with 23.2% char residue at 800°C, while dynamic mechanical analysis indicated an elevated glass transition temperature of 187°C. Microstructural characterization (SEM/Raman spectroscopy) demonstrated that HLE promotes the formation of a compact, highly graphitized char layer (I D /I G = 1.99), effectively insulating against heat and oxygen transfer. TG‐IR (Thermogravimetric Analysis coupled with Fourier Transform Infrared Spectroscopy) pyrolysis confirmed the significant suppression of harmful gas emissions, including CO 2 and aromatic compounds, by the HLE/EP system. The flame‐retardant mechanism is attributed to a synergistic dual‐phase effect involving gas‐phase actions (radical quenching and gas dilution) and condensed‐phase actions (phosphorus‐catalyzed char formation and physical barrier effects).