Solventless Dual-Cure Liquid Resins Via Circular Use of Phthalic Anhydride for Recyclable Composite Applications

Fiber-reinforced composites (FRCs) possess a remarkable strength-to-weight ratio, making them ideal light-weighing alternative materials of metals used in automotive, aerospace, and outdoor equipment applications, but their recycling is challenging. Chemically recyclable thermoset polymers can enable fiber recovery and reuse; however, challenges remain in the separation and purification of depolymerized small molecules for efficient polymer recycling. To this end, a series of liquid resins for chemically recyclable polymer networks is designed based on phthalic anhydride, a widely produced and inexpensive chemical. The straightforward sublimation of phthalic anhydride is leveraged to enable a simple and efficient separation process for polymer recycling. To liquefy phthalic anhydride, five mono-acryloyl-phthalates are synthesized to obtain stable liquid resins together with phthalic diglycidyl ester. These liquid resins undergo dual-cure reactions that comprise photopolymerization of acrylate and, subsequently, heat-mediated epoxy-acid polymerization reactions. These liquid resins exhibit moderate viscosities (2600-6400 cP @ 22 °C), fast curing, and robust thermomechanical properties (T_gs from 71 to 116 °C). It is demonstrated that hydrolysis of the dual-cured polymers completes within 2 h at 80 °C, and direct sublimation produces phthalic anhydride with 82% yield. This resin system is expected to provide a cost-competitive, highly efficient platform for recyclable FRCs.