Closed-Loop Recycling of Tough and Flame-Retardant Epoxy Resins

Large-scale applications of flame-retardant epoxy resinshave causedsustainability concerns on the use of renewable resources and end-of-lifewastes. New strategies are required to address chemical recyclingof fire-safe epoxy resins. Here, we demonstrated recyclable flame-retardantepoxy resins (FREPs) using itaconic acid-derived hyperbranched epoxyresin (IA-EHBP) and (1,3,5-hexahydro-s-triazine-1,3,5-triyl)benzyl mercaptan (HT-BM). The unique structure enabled a closed-loopchemical recyclable network that can be degraded into monomers withup to 86% yield of recovered monomers. The hyperbranched topologicalstructure of IA-EHBP also contributed to a significant improvementin the strength and toughness of the FREPs. By using positron annihilationlifetime spectroscopy, the effects of free-volume hole size and relativefractional free-volume on the mechanical performance and dynamic mechanicalproperties of FREPs were studied, and the function relations weretherefore established based on the Williams-Landel-Ferryequation. The incorporation of IA-EHBP effectively promoted the formationof char residue and produced phosphorus- and sulfur-containing freeradicals to prevent the generation of combustible volatiles, thusenhancing the flame retardancy. The FREPs can be recycled and reusedmultiple times without loss of performance. The method reported hereprovides a facile and closed-loop approach for high-efficiency chemicalrecycling and reuse of end-of-life flame-retardant materials.