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

Large-scale applications of flame-retardant epoxy resins have caused sustainability concerns on the use of renewable resources and end-of-life wastes. New strategies are required to address chemical recycling of fire-safe epoxy resins. Here, we demonstrated recyclable flame-retardant epoxy resins (FREPs) using itaconic acid-derived hyperbranched epoxy resin (IA-EHBP) and (1,3,5-hexahydro-s-triazine-1,3,5-triyl) benzyl mercaptan (HT-BM). The unique structure enabled a closed-loop chemical recyclable network that can be degraded into monomers with up to 86% yield of recovered monomers. The hyperbranched topological structure of IA-EHBP also contributed to a significant improvement in the strength and toughness of the FREPs. By using positron annihilation lifetime spectroscopy, the effects of free-volume hole size and relative fractional free-volume on the mechanical performance and dynamic mechanical properties of FREPs were studied, and the function relations were therefore established based on the Williams–Landel–Ferry equation. The incorporation of IA-EHBP effectively promoted the formation of char residue and produced phosphorus- and sulfur-containing free radicals to prevent the generation of combustible volatiles, thus enhancing the flame retardancy. The FREPs can be recycled and reused multiple times without loss of performance. The method reported here provides a facile and closed-loop approach for high-efficiency chemical recycling and reuse of end-of-life flame-retardant materials.