A Strategy for Designing Hierarchical Structure Flame Retardant ZM@APP for Improving Flame Retardancy, Mechanical Performance, and Smoke Suppression of Epoxy Resin

Improving flame retardancy without compromising mechanical properties is a significant challenge in epoxy resin (EP) applications. In this study, an inorganic–organic hybrid flame retardant (ZM@APP) with a hierarchical structure was designed by loading the compounds with a shell-core structure ZM (a complex of ZIF-67 and melamine) on the surface of ammonium polyphosphate (APP) through hydrogen bonding. When the loading of ZM@APP was 10.0 wt.%, the tensile strength, flexural strength, and flexural modulus of the EP composites increased by 13.7%, 16.7%, and 87.6%, respectively, compared to pure EP. Surprisingly, modified APP also resulted in the promotion of mechanical performances while APP triggered the decreases in mechanical properties. The 10ZM@APP/EP passed the V-0 rating in the vertical combustion test and achieved the limiting oxygen index of 28.2%. The peak heat release rate and total heat release decreased by 68.4% and 58.5%, respectively; the peak smoke release rate and total smoke production were reduced by 60.0% and 67.5%, respectively, compared to pure EP. The reason was that the triazine ring, cobalt oxide, and pyrophosphoric acid promoted the formation of contact and continuous char. This study offers a new method for preparing EP composites with enhanced flame retardancy, high mechanical properties, and low smoke release.