Benzaldehyde Is the Redox Mediator for Aerobic C–N Cleavage in Epoxy Composite Recycling

Chemical methods are promising avenues to recycle fiber-reinforced polymer waste. Here, we reintroduce tripotassium phosphate-mediated benzyl alcohol conditions for epoxy cleavage. While these conditions were developed in 2008 for anhydride-cured epoxy cleavage and have been applied to amine-linked epoxy cleavage, no account has been made of why these conditions should work with the latter, or why air exposure appears critical for their success. We now show how benzaldehyde, formed in situ from benzyl alcohol, plays the crucial redox role that enables selective C–N bond cleavage: tertiary amines are cleaved by a previously uncharacterized mechanism in which benzaldehyde facilitates iminium formation from the polymer’s C–N linkage. Subsequent hydrolysis and base-promoted elimination cleave the C═N and C–O bonds, respectively, enabling the recovery of bisphenol A and release of the linking diamines. Carbon fiber (CF) and even glass fiber (GF) fabrics are preserved, and recovered CF fabrics are remanufactured into second-generation composites with near-virgin quality and mechanical performance. These mechanistic insights may inform the future development of selective diamine recovery strategies from thermosets, which is a challenge when valorizing composites. Collectively, these results present promising recycling conditions with the potential to valorize matrices and fibers from a wide range of composites, spanning high-performance carbon fiber to widely used fiberglass.