New Approach to Recycling of Epoxy Resins Using Nitric Acid: Regeneration of Decomposed Products through Hydrogenation

Due to the increase in the amount of produced epoxy resin waste, it is desirable to recycle this material to reduce the burden on the environment. Despite many studies focusing on recovering fibers from composites, few studies have focused on recycling the resin itself. In this study, we developed a new approach for recycling the decomposed amine-cured epoxy resin as an amine curing agent. Following the decomposition of the epoxy resin in nitric acid, a nitrated and decomposed resin product was extracted from the nitric acid solution. The resulting extract was then hydrogenated to convert the nitro group into amino group, and the hydrogenated extract was incorporated into the epoxy resin by replacing a portion of the original curing agent. The obtained recycled resin exhibited a higher tensile strength and comparable tensile modulus compared to the virgin resin due to the presence of aromatic amino groups. Moreover, when 20 mass% of the curing agent was replaced with the hydrogenated extract, the recycled resin with the optimized formulation exhibited tensile strength comparable to that of the virgin one. This approach demonstrates a novel recycling method for creating a closed loop of amine-cured epoxy resins.