Dependence of Intrinsic Flame Resistance on the Substitution Pattern of Benzene Dicarboxylate-Based Epoxy Resins

This study investigates the influence of substitution patterns on the thermomechanical properties and intrinsic flame retardancy of benzene dicarboxylate-based epoxy resins. Para- (DG-Te), meta- (DG-Iso), and ortho- (DG-Ph) diglycidylbenzene dicarboxylates were cured with 4,4′-diaminodiphenyl sulfone (DDS) and compared to the conventional DGEBA system. Dynamic mechanical analysis (DMA) reveals that DG-Te achieves a high glass transition temperature ( T g = 236 °C), comparable to DGEBA (238 °C), despite its lower aromatic density. A systematic decrease in T g and gel content is observed from para- to meta- to ortho-substitution, indicating reduced network formation. Thermogravimetric analysis (TGA) demonstrates a significantly enhanced char-forming tendency for DG-Te, with char yields increasing from 18% (DGEBA) to 42% under nitrogen. Elemental analysis reveals increased carbon content and retention of nitrogen and sulfur in the char, indicating enhanced deoxygenation and formation of heteroatom-stabilized, carbon-rich residues in the DG-Te-based system. Cone calorimetry of neat resins shows a substantial reduction in fire load, with the primary peak heat release rate decreasing by 70% and total smoke release by 55% compared to DGEBA. Simultaneously, the residual mass after combustion increases by more than 240%. DG-Te achieves UL94 V-0 classification without additional flame-retardant additives. The transferability of these properties was demonstrated in honeycomb sandwich composites, where DG-Te retains reduced heat release and smoke production. These results establish substitution-pattern-driven molecular design as an effective strategy for intrinsically flame-retardant epoxy systems, enabling competitive fire performance without compromising thermomechanical properties.