A Two-Step Chemo-Enzymatic Oxidative Approach for Enhanced Bio-Based Recycling of Epoxy-Based Carbon Fiber-Reinforced Polymers (CFRPs)

Abstract

Carbon fiber-reinforced polymers (CFRPs), particularly epoxy-based CFRPs, have become essential materials in the aerospace, automotive, and wind energy industries due to their exceptional mechanical properties and lightweight characteristics. However, there is a lack of recycling technologies that are environmentally sustainable while also ensuring the recovery of carbon fibers in their original state. Although certain bacterial and fungal strains can colonize epoxy polymers, the identification of potential enzymes capable of efficiently degrading these materials remains elusive. Consequently, there is an urgent need for an effective, sustainable, and biologically inspired solution for eCFRP recycling. In this study, a chemo-enzymatic treatment inspired by natural processes was developed, involving a two-step oxidation of eCFRPs. Three novel bacterial laccases, derived from a metagenome of the European spruce bark beetle (Ips typographus), along with CueO from E. coli and the well-characterized horseradish peroxidase (HRP), were biochemically characterized in detail. These enzymes exhibited a preference for acidic pH conditions and moderate temperatures, with laccase ItL-03 demonstrating notable thermostability. Subsequently, their ability to oxidize three epoxy resin scaffolds containing tertiary amines and representing repeating units of the RTM6 epoxy used in aircraft applications was evaluated. Among them, ItL-03 showed the highest activity, converting over 80% of the substrates within 30 minutes. ItL-03 was therefore selected for application in a chemo-enzymatic treatment approach. For this, organic acids were screened for their effectiveness in recovering carbon fibers from epoxy composites, in comparison to the aggressive standard method using sulfuric acid. Propionic acid combined with hydrogen peroxide emerged as the most promising combination, capable of recovering clean carbon fibers with potential for reuse. Integrating the organic pre-treatment with the enzymatic action on RTM6 (amine-cured epoxy powder) enabled the laccase to partially modify the epoxy and release defined products, which may potentially aid downstream processing. This sequential two-step oxidative treatment offers a more environmentally sustainable alternative, allowing the enzyme to participate in the degradation of recalcitrant polymers. This marks an initial step toward developing a bio-based recycling approach for epoxy-based CFRPs.