Empowering Synergistic Optimization of Flame Retardancy and Toughness in Epoxy Resins via a Bio‐Based Flexible‐Chain Flame Retardant

ABSTRACT To address the flammability and brittleness of epoxy resin (EP) while meeting green development demands, we developed SiVD, a lignin‐derived vanillin‐based halogen‐free flexible‐chain flame retardant. At 5 wt% SiVD, modified EP achieved a limiting oxygen index of 30.1% and UL‐94 V‐0 rating. Cone calorimetry revealed 24.4% and 28.3% reductions in peak heat release rate and total heat release versus pure EP, along with markedly suppressed smoke production, demonstrating excellent flame retardancy and smoke suppression. The flexible segments of SiVD effectively alleviate EP's inherent brittleness. Even at an ultra‐low 1 wt% loading, impact strength reached 60.31 kJ/m 2 (247.6% higher than pure EP), while tensile and flexural strengths remained comparable, truly realizing synergistic enhancement of flame retardancy and toughness. Mechanistic studies show a dual‐mode flame‐retardant action: in the condensed phase, SiVD promotes a dense protective char layer; in the gas phase, it quenches free radicals and releases inert diluents. For toughening, its flexible moieties absorb impact energy, and excellent interfacial compatibility with EP mitigates mechanical deterioration typically caused by conventional flame retardants. This work offers a novel strategy for developing eco‐friendly, high‐performance flame‐retardant EP materials.