One-Pot Green Synthesis of Low-Viscosity Acetal Epoxy Resins for Diluent-Free Industrial Processing and Closed-Loop Recycling of Carbon Fiber Composites

Abstract Recyclable epoxy thermosets and their carbon fiber-reinforced polymer composites (CFRPs) have faced three long-standing bottlenecks: complex synthesis and purification processes, inherently high viscosity incompatible with the industrial vacuum-assisted resin infusion (VARI) process—the dominant technology for manufacturing large wind turbine blades and aerospace composites—and incomplete recovery of individual composite constituents. While dynamic covalent bonds offer a promising route to epoxy recyclability, most existing recyclable epoxy systems fail to simultaneously overcome these hurdles. Herein, we report a green, scalable one-pot synthesis of a novel low-viscosity acetal epoxy resin (BCAE) that simultaneously addresses the synthesis, processing, and end-of-life recycling challenges of epoxy thermosets. This telescoped protocol eliminates intermediate purification steps, delivering BCAE with an ultra-low viscosity of 1.9 Pa·s at 25 °C—6.7-fold lower than commercial bisphenol A diglycidyl ether (DGEBA)—enabling diluent-free VARI fabrication of high-performance CFRPs. The cured BCAE networks exhibit dual recyclability: catalyst-free thermal reprocessability and mild acid-triggered degradation under ambient conditions. Most notably, we demonstrate a closed-loop monomer recycling combined with an open-loop upcycling system with key quantitative breakthroughs: carbon fibers are recovered with 96.7% strength retention in the regenerated composite, the benzaldehyde monomer is recycled to resynthesize BCAE with identical performance, and the tertiary amine polyol degradation product is upcycled as a high-value curing agent for commercial epoxies. Life cycle assessment confirms that closed‑loop recycling reduces CO2‑equivalent emissions by approximately 79% compared with incineration. This work provides a general and industrially viable strategy for sustainable thermoset composites, aligning perfectly with global carbon neutrality goals.