Epoxy curing in mild and eco-friendly conditions: Towards bisphenol A-free systems

Epoxy resins rank among the most significantly used thermosets, showing high thermal and mechanical properties. Unfortunately, current polymerization processes to reach these properties are energy-intensive, characterized by high temperatures and long processing duration. Addressing this problem, recent years have witnessed the emergence of curing methods under mild and ecofriendly conditions, aligning with societal and ecological challenges. Mild conditions were delineated in this review as a polymerization without solvent and at temperatures not exceeding 80 °C. This work highlights three methods, by focusing on research works from 2015 to date: i) polyadditions via step-growth ring opening polymerization, ii) photopolymerization leading to homopolymerization of bio-based monomers and iii) redox polymerization achieved through the release of cations or acidic protons species, initiating the cationic polymerization. In the context of ecofriendly conditions, the replacement of bisphenol-A present in many epoxy monomers is also a huge challenge to keep both good mechanical properties and fast polymerization kinetics. In this context, this review aims at underlining the increasing importance of epoxy curing under mild conditions, in possible combination with bio-based monomers for bisphenol-A replacement and to guide both researchers and industries to explore and develop new curing systems.