Degradation of Thermal-Mechanically Stable Epoxy Thermosets, Recycling of Carbon Fiber, and Reapplication of the Degraded Products

Conventional epoxy resins can meet the requirement for engineering applications in which high glass transition and thermal stability are essential, but they show no degradability or recyclability. Vitrimers, with covalent adaptable networks, provide reprocessability and degradability. However, the thermally activated bond-exchange reactions prevent the composites from being thermal-mechanically stable at high temperatures. Therefore, developing epoxy/carbon fiber composites with thermal stability and degradability is attractive for industrial applications and environmental sustainability. In this work, we report the degradability of two thermal-mechanically stable epoxy networks derived from self-curing of two eugenol-based epoxy resins: bis(2-methoxy-4-(oxiran-2-ylmethyl)phenyl) succinate (II) and tris(2-methoxy-4-(oxiran-2-ylmethyl)phenyl)benzene-1,3,5-tricarboxylate (III). Degradation can be proceeded through aminolysis of aliphatic esters in the networks at a mild condition. The exact structures of degraded products were precisely analyzed by 1H NMR spectra. An epoxy/carbon fiber composite based on III/carbon fiber was prepared. After degrading the epoxy network, the carbon fiber was recycled. Scanning electron microscopy (SEM) pictures show that the recycled carbon fiber is intact and looks the same as the virgin one. Furthermore, we modified the degraded product of epoxy networks for reapplication. One of the degraded products, oligo(1-oxy-3-(3-methoxy-4-phenylene)propan-2-ol) (IIc), was further reacted with methacrylic anhydride to obtain oligo(1-oxy-3-(3-methoxy-4-phenylene)propan-2-methacrylate) (IId). IId was further thermally cross-linked to a thermoset (IIe) with a Tg value of 170 °C. The successful degradation and reapplication of thermal-mechanically stable epoxy networks and the recycling of carbon fibers are well demonstrated.