Dual enhancements of flame retardancy and mechanical property of vinyl ester resin composite via using reactive ammonium polyphosphate

Vinyl ester resins (VERs) are widely used in various industries owing to their excellent mechanical properties and chemical resistance. However, their inherent flammability severely restricts further application, and conventional flame retardants often enhance fire safety at the cost of mechanical and thermal performance. Herein, a reactive ammonium polyphosphate (APP) derivative (MDO) with abundant C=C bonds was synthesized by grafting maleic anhydride onto the surface of an amine-modified APP (DO). The resulting MDO not only acts as an efficient flame retardant but also participates in the curing of VER through radical copolymerization. With the addition of only 22 wt% MDO, the 22% MDO/VER composite achieves a vertical burning (UL-94) V-0 rating and a high limiting oxygen index (LOI) of 28.5%, accompanied by remarkable reductions in peak heat release rate (PHRR), total heat release (THR), peak smoke production rate (PSPR) and total smoke production (TSP) compared to the neat VER. More importantly, introducing MDO enhances the tensile strength (49% improvement), flexural strength (56% improvement), and impact strength (19% improvement) of VER while effectively maintaining the glass transition temperature ( T g , 115 °C). The dual enhancement originates from the reactive crosslink sites of MDO that improve the interfacial compatibility and promote condensed-phase charring. This work provides a rational strategy to fabricate high-performance VER composites with superior fire retardancy, outstanding mechanical properties and well-preserved thermal performance.