From epoxy vitrimers to DOPO–based (bio–)epoxy vitrimers: Current state-of-the-art and perspectives

Epoxy thermosets have become indispensable in various applications from household plastics to automotive and aeronautics due to their outstanding features, such as excellent chemical stability and thermo–mechanical properties. During the past two decades, the use of epoxy systems in fire safety applications has significantly increased. Unfortunately, epoxy resins are highly flammable, and their thermosets are neither reprocessable nor recyclable after their lifetime. These drawbacks result in fire hazard and waste accumulation toward environmental pollution, breaching green principles and worldwide laws for sustainable development. The organophosphorus 9,10–dihydro–9–oxa–10–phosphaphenanthrene–10–oxide (DOPO) and its derivatives are some of the most extensively researched flame retardants (FRs) for epoxy resins. DOPO derivatives, either reactive or non–reactive, not only render epoxy systems fireproof but also enhance other properties (e.g., non–oxidative capacity, adhesion, thermo–mechanical stability, and water repellency). With this in mind, the scientific community has focused its research on developing epoxy thermosets that incorporate both fire safety and recyclability. As a solution to the reprocessability/recyclability major issue, in 2011, vitrimers were first reported. These new materials consist of covalent adaptable networks (CANs) which can modify their topology via heat–triggered bond–exchange reactions. This work aims to review the advances in flame retardant (FR) DOPO–based epoxy vitrimers, emphasizing such key aspects as chemistry, flame retardancy, thermo–mechanical features, and sustainability. CANs are summarized and classified via the dynamic covalent bond types. Representative examples in the last five years from both fossil and bio–based DOPO–containing epoxy systems are reviewed and matched within the up–to–date circular economy context to design novel, green, and sustainable materials.