Organic-inorganic hybrid functionalized chitosan/ammonium polyphosphate (APP): A synergistic strategy for flame-retardant and high-mechanical-strength epoxy thermosets

Epoxy resin (EP) is an industrially essential polymer widely used in electronic packaging, adhesives, and construction. Despite its utility, the high flammability of EP poses a significant safety risk. In this study, a novel chitosan-derived organic-inorganic hybrid flame retardant (CPMM) was synthesized, which was then combined with ammonium polyphosphate (APP) and added to epoxy resin (EP) to produce high-performance flame-retardant epoxy thermosets. Samples containing 7.5 wt% CPMM and 7.5 wt% APP reached a limiting oxygen index (LOI) of 30.5 % and a V-0 rating, along with a marked decrease in peak heat release rate from 1447.0 kW/m² to 267.1 kW/m² compared to pure EP. Thermogravimetric analysis indicated that the synergistic effect of CPMM and APP promotes the formation of more complete intumescent char structures, thereby enhancing the flame-retardant properties of the epoxy thermosets. Furthermore, mechanical performance tests showed improvements in both tensile and flexural strengths due to the robust interaction between CPMM and the matrix. This study provides a promising strategy for the design of bio-based organic-inorganic hybrid flame retardants and epoxy thermosets with enhanced performance.