High-performance bio-based composites with synergistic flame retardancy from schiff base and ammonium polyphosphate

A bio-based epoxy resin (MA-EP) containing a Schiff base structure was synthesized from vanillin, and combined with ammonium polyphosphate (APP) to construct a flame-retardant thermosetting resin system. FT-IR and NMR characterizations confirmed the successful introduction of imine (C = N) linkages into the MA-EP molecule. Thermal analysis revealed that MA-EP exhibits a high glass transition temperature and a high char yield. With the addition of only 5 wt% APP, the MA-EP/APP-5 composite achieved a limiting oxygen index (LOI) of 28.9% and passed the UL-94 V-0 flame retardancy test. The results demonstrate that the Schiff base backbone imparts intrinsic flame retardancy, while the incorporation of APP enhances the synergistic flame-retardant effect in both the gas and condensed phases, leading to the formation of a P-N-C flame-retardant network. As a result, excellent flame resistance and a well-balanced mechanical performance were achieved at a low additive content. This study provides an effective strategy for the development and application of green, low-toxicity, and high-performance epoxy resin materials.