Preparation of eugenol-based flame retardant epoxy resin with an ultrahigh glass transition temperature via a dual-curing mechanism

Eugenol epoxy resin, as one of the most promising biobased epoxy resins, still faces problems of insufficient heat resistance, high flammability, and complicated synthesis processes. Based on the principles of the Diels–Alder (D-A) addition reaction and epoxy-amine open-loop crosslinking, the EUEP epoxy monomer (EUEP) was synthesized, and ternary cocuring (EUEP-BDM-DDS) was performed with bismaleimide (BDM) and the high-temperature curing agent 4,4′-diamino-diphenyl sulfone (DDS). The resulting system exhibited an exceptional glass transition temperature (T_g) of 306 °C, surpassing other eugenol epoxies and commercial bisphenol A epoxies. EUEP-BDM-DDS demonstrated superior mechanical properties with high moduli (up to 4.14 GPa for tensile and 4.10 GPa for flexural). Its processing characteristics were also favorable, featuring a long pot-life, low viscosity, and suitable for all operating processes of traditional DGEBA-DDS systems. In addition, the formation of rigid six-membered rings during curing and the higher cross-linking density gave the resin system excellent flame retardant properties, with a limit oxygen index of 33.5 % and passing the V-0 class test of UL-94. The system exhibited significantly lower peak heat release and smoke release rates compared to DGEBA-DDS, indicating enhanced fire safety. And the analysis revealed a coacervated flame retardant mechanism. Moreover, the composite material derived from EUEP-BDM-DDS displayed improved interlaminar shear strength, flexural strength, and high-temperature mechanical properties, outperforming the DGEBA-DDS system. This study paves the way for utilizing biobased eugenol epoxy resins in advanced composite materials, offering enhanced performance and fire safety. It holds significant implications for promoting the application of biobased materials in high-performance composites.