Deciphering Online Evolution of Intumescent Char Layers via Piperazine Pyrophosphate for Fire‐Safe Epoxy Resin

ABSTRACT The underlying mechanism of microstructural char layer evolution in piperazine pyrophosphate (PAPP)‐based flame‐retardant systems remains incompletely understood. This study elucidates this mechanism by examining the morphological and compositional evolution of char layers in intumescent flame‐retardant epoxy (EP‐IFR) systems following thermal treatment at various temperatures, and presents a concurrent design approach for fabricating PAPP‐containing epoxy composites that generate in situ char layers with superior barrier properties against mass and heat transfer at high temperatures. With the incorporation of 10% PAPP, the system formed continuous and dense surface char layers, which provided structural integrity to the expanded char. Crucially, the system maintained a steady pyrolysis gas generation rate, thereby preventing the formation of fractured char layers due to excessive gas pressure. Compared to neat epoxy, cone calorimeter tests of EP‐IFRs showed significant reductions in heat release rate (HRR, 65.1%), total heat release (THR, 59.5%), smoke production rate (SPR, 60%), and total smoke production (TSP, 65%), alongside achievement of a UL‐94 V‐0 rating and a limiting oxygen index (LOI) of 29.8%. This work further elucidates the microstructural evolution of the char layers in EP‐IFRs, offering a theoretical foundation for advancing halogen‐free, high‐efficiency flame‐retardant epoxy composites.