Toughness and its mechanisms in epoxy resins

Counting for approximately 3.13 million tons of products, epoxy resins (EPs) have been extensively employed as matrix resins of composites and applied in aerospace, aviation, wind, nuclear power, high-speed rail, as well as many other manufacturing industries. However, certain inherent limitations severely impede further applications of these advanced materials, such as poor fatigue resistance, low impact resistance, hard to recycle, and a ''seesaw'' between toughness and strength. Here, we especially focused on the recent progress in toughening methods and associated mechanisms for these epoxy resins and analytic techniques for characterizing toughness, which highlighted the applicable approaches to generate homogeneous structures. As a typical homogeneous toughness material, hyperbranched polymers, especially hyperbranched epoxy resins (HERs), are ideal candidates to solve the knotty problem of EPs because HERs can homogeneously reinforce and toughen diglycidyl ether of bisphenol-A (DGEBA), promoting the degradation of DGEBA, as well exhibiting strong interfacial interactions among components, due to their excellent compatibility, wettability and low viscosity. Last but not least, we advance the objectives and challenges of epoxy resins in the future. Overall, this review presents an up-to-date overview of toughening methods and mechanisms for EPs, and guidance of emerging research on the sustainable development of EPs in versatile high-tech fields.