A novel strategy for high flame retardancy and structural strength of epoxy composites by functionalizing ammonium polyphosphate (APP) using an amine-based hardener

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Designing flame retardant polymeric coatings and fiber reinforced polymer composites that can retain superior structural strength continues to pose a significant challenge in many industrial applications because common flame-retardant fillers significantly degrade mechanical strength. Herein, we present a novel method of functionalizing ammonium polyphosphate (APP) through the cationic ion exchange reaction with NH3+ cations contained in amine-based hardener so that the resulting particles can simultaneously improve structural strength and flame retardancy. This technique maintains the original epoxy’s composition while reducing peak heat release rate, total heat release, and total smoke release by 87 %, 83 %, and 95 % respectively, enabling the polymer to meet the flame-retardant requirements for railway vehicle applications. The incorporation of the hardener-functionalized APP (HF-APP) at 20 wt% into epoxy demonstrates an impressive 92 % retention of the tensile strength of the epoxy. Compared to the original APP, the HF-APP are twice as effective in reducing total heat release and total smoke release, increasing limiting oxygen index, and improving strength retention. Finite element analysis of interface properties between the epoxy and the HF-APP particles reveals that the hybrid particles can separate into filaments at a stress level below the strength of the composite, significantly mitigating their impact on mechanical strength. This novel approach offers new design possibilities for a wide range of industries, such as construction and transportation where it is essential for structures to possess high mechanical strength and flame retardancy.