Synergistic Enhancement of Flame Retardancy and Mechanical Properties in Vinyl Ester Resins via DOPO-Based Flexible Chain Grafting Modification

Vinyl ester resin (VER) is highly flammable, which limits its use in applications requiring high flame resistance. However, conventional flame-retardant approaches often degrade its mechanical properties. To improve both the flame retardancy and mechanical performance, this work developed a grafting strategy incorporating phosphorus-containing flexible side chains. Three DOPO-based diol derivatives with different chain lengths (DOPE, DOPB, and DOPH) were synthesized and then grafted onto the VER backbone through 4,4′-methylenediphenyl diisocyanate (MDI) to obtain VER/M-DOPx systems. The modified resins showed obvious improvements in both flame retardancy and mechanical properties. VER/M-DOPE and VER/M-DOPB achieved UL-94V-0 ratings, with LOI values of 28.6% and 26.7%, respectively, while VER/M-DOPH reached a V-1 rating and an LOI of 28.9%. VER/M-DOPB showed the best flame-retardant effect, reducing the peak heat release rate (PHRR) and total heat release (THR) by 68.5% and 43.7% compared with VER. The incorporation of flexible DOPO-based diol chains also enhanced toughness and strength; VER/M-DOPE exhibited increases of 4.53%, 26.24%, and 81.35% in tensile, flexural, and impact strengths, respectively. These results indicate the feasibility of enhancing both the flame retardancy and the mechanical properties of VER through grafting phosphorus-containing flexible side chains.