Equol-based epoxy resin as an ideal substitute for the diglycidyl ether of bisphenol A (DGEBA)

The development of high-performance bio-based epoxies with excellent mechanical properties, heat resistance and flame retardant properties is essential to mitigating the resource and environmental burdens caused by the widespread utilization of petroleum-based epoxies. This work focuses on the synthesis of a bifunctional epoxy resin (EEU) with the rigid tetrahydrobenzopyran ring derived from equol. The structure of the EEU epoxy resin was characterized, and using 4,4′ diamino diphenyl sulfone as the curing agent, the rheological properties, curing behavior and comprehensive properties of cured resin were studied. The results demonstrate that EEU-DDS presents the same curing behavior as DGEBA-DDS. The mechanical properties heat resistance and flame retardant properties of EEU-DDS are superior to DGEBA-DDS. The tensile strength, tensile modulus and glass transition temperature reached to 94.0 MPa, 3.62 GPa and 221 °C, respectively, higher than those of DGEBA-DDS (83.2 MPa, 2.81 GPa and 203 °C). EEU-DDS reached the V-1 rating in UL-94 tests with a high limiting oxygen index (LOI) (28 %), and the peak heat release rate and smoke release rate of 631 kW/m² and 0.155 m²/s were significantly lower than that of DGEBA-DDS. The flame retardant mechanism was proven to be gas-coacervated phase synergistic flame retardant. In addition, the interlamellar shear strength (ILSS), bending strength, and impact strength of carbon fiber reinforced resin matrix composites prepared by EEU-DDS reached 97.9 MPa, 1281 MPa, and 81.3 kJ/m², respectively, higher than those of DGEBA-DDS. Therefore, equol-based epoxy resin can serve as an ideal alternative to petroleum-based bisphenol A-type epoxy resin, offering superior comprehensive properties.