Development of Degradable Epoxy-Acid Thermosetting Polymers and Recyclable Composites by Dual-functional Liquid Hardeners Derived from Solid Carboxylic Acids

Abstract

Epoxy thermosets are widely used in high-performance applications due to their excellent mechanical strength and thermal stability. A hardener component plays a crucial role in controlling the neat polymerization process and determining the final material properties. However, currently available hardeners are limited to amine, thiol, and anhydride chemistries, thereby limiting the range of materials that can be processed and the properties that can be achieved. To broaden the scope of hardeners from commercially available resources, we explore carboxylic acids as alternative curing agents due to their abundance in bio-derived feedstocks and from upcycled plastics. While multifunctional carboxylic acids are typically not suitable due to their high melting temperatures, we envisioned that eutectic supramolecular complexes could be formed between polymerizable amine compounds and solid dicarboxylic acids. When incorporated into epoxy resin systems, these complexes promoted rapid radical (meth)acrylate polymerization, with 90% conversion after 50 seconds of irradiation at 405nm and a low intensity of 5 mW/cm². A second stage of epoxy-acid polymerization was carried out at 130 °C, resulting in covalently crosslinked network polymers with ultimate flexural strengths for aromatic acids of 34.93 ± 12.2 to 163.71 ± 27.61 MPa for aliphatic dicarboxylic acids. We demonstrated the application of these resins to glass fiber-reinforced composite materials fabricated via vacuum-assisted resin transfer molding. We further highlight their practical potential through proof-of-concept recyclability, in which a whole-weave fabric was recovered, with 99.98 wt% resin washed away, enabling the manufacture of 2nd-life parts with a 4% deviation in ultimate tensile strength.