Aromatic Epoxy from Caffeic Acid: Mechanical Properties and Flame Retardancy

The preparation of biobased epoxy resins with superior mechanical properties and flame retardancy is a significant challenge, and the development of caffeic acid epoxy resins (CFAE) from plant-derived caffeic acid that has a conjugate rigidity group and reactive double carbon and hydroxyl groups represents a promising advancement in this field. The epoxy resins with different cross-linking densities, such as CFAE₁GEFA_n and CFAE₁FD_n, can be obtained from CFAE through Diels–Alder reaction with Glycidyl Ether of Furfuryl Alcohol (GEFA) and 2-difuran disulfide (FD), followed by curing with 4,4′-diaminodiphenylmethane (DDM). The tensile strength of caffeic acid-based epoxy resins ranges from 73.8 to 106.5 MPa, much higher than that of bisphenol A diglycidyl ether-based epoxy/DDM (56.6 MPa). In addition, CFAE/DDM is rated V-0 in the UL-94 test and has a high limiting oxygen index of 31% and a very low peak heat release rate of 83.1 W g^–1, suggesting excellent intrinsic flame-retardant properties. Additionally, the reduced smoke production rate of 0.14 m² s^–1, total smoke production of 12.1 m², and total heat release of 62.2 MJ m^–2 of the CFAE-based epoxy indicate a decreased risk of toxic fumes and massive exothermic heat released during combustion. Overall, the resultant CFAE-based epoxy resins with sustainable and enhanced mechanical and flame-retardant properties have potential applications to electronic packaging, construction, decoration, and aerospace fields.