A Quercetin-Derived Polybasic Acid Hardener for Reprocessable and Degradable Epoxy Resins Based on Transesterification

The development of recyclable thermosets with high performance from renewable resources is highly desirable from the concerns of environmental and resource economics. Here, a full biobased carboxyl-containing epoxy hardener (CF-quercetin) was prepared from quercetin and succinic anhydride (starch/sugar derivative) through a mild one-step reaction. The epoxy resin cured by CF-quercetin shows tensile strength (92 MPa) comparable with commercial counterparts and high storage modulus (3800 MPa) at room temperature. Owing to the high content of dynamic ester bonds in the epoxy networks, the cured epoxy resin can be repeatedly reprocessed like thermoplastics, and 91% recovery of tensile strength from the first reprocessing cycle can be achieved. Meanwhile, the glass-transition temperature and thermal stability of the reprocessed epoxy resin remain almost constant compared with the original epoxy resin. Furthermore, the epoxy resin cured by CF-quercetin presents rapid degradation in alkali aqueous solution (NaOH, 1 M) within 6 h at room temperature, but it is fully nondegradable in acidic and neutral solutions over a period of 120 days. The cured epoxy resin also shows acceptable stability in many conventional organic solvents. Taken together, our work provides a simple method to prepare a universal quercetin-based hardener for epoxy resins with reprocessability, degradability, and high mechanical performance.