Bio-based epoxy resins derived from diphenolic acid via amidation showing enhanced performance and unexpected autocatalytic effect on curing

Bisphenol A (BPA) is the main precursor in the synthesis of epoxy resins. However, because of its toxic and nonrenewable nature, BPA is unsuitable in the sustainable preparation of epoxy resins. In contrast, Diphenolic acid (DPA) is renewable but its carboxyl group limits its applications in bio-based thermosets. In this study, we report a novel amidation route to tailor DPA to produce epoxide monomers that present epoxy resins with a high performance. The diphenolic amides exhibited very low toxicities when compared with that of BPA, and also provided an unexpected autocatalytic effect on the curing reactions when succinic anhydride (SA) was used as the hardener. Dynamic mechanical analyses (DMA) and tensile test results confirmed the remarkable thermomechanical properties of the cured epoxy resins, which may be attributed to the increased cross-linking density and enhanced hydrogen bonding by the amide groups. Among the SA-cured epoxy resins, diglycidyl ether of diphenolic ethylamide exhibited the best performance, which included a high Tg of 114 °C, char yield (CY) of 20%, tensile strength of 60.2 MPa, and toughness value of 185 MPa. In addition, the DPA-derived epoxy resins have great potential in the preparation of bio-based composites because they act as excellent interfaces between the resin and cotton fibers (CoFs). Our study presents a new strategy to prepare bio-based epoxy resins with a high performance, emphasizing the greater potential of amidated DPA derivatives to replace BPA in the epoxy resin market.