Molten salt thermal treatment for carbon fiber composites recovery: Structural effects on thermosetting resin degradation mechanism

Recovering carbon fibers from thermosetting composites is challenging due to stable cross-linked resins and poor heat transfer. Previous research introduced a molten salt thermal treatment method to enhance heat transfer and reactivity for recycling clean and high-strength fibers. However, the diverse resins significantly influence conversion properties, affecting fiber recovery and resin utilization. Expanding on this resins with varying structures were studied, being phenol-rich (EP), ester-rich (PA), and blended (VE). Pyrolysis achieved fiber separation with < 5 % residual resin and < 1 % strength loss for all resins. Despite structural differences, the resins showed similar radical production (hydrogen and methane), but liquid oil compositions varied. VE and PA yielded aromatic hydrocarbons, while EP produced nitrogen compounds and phenols. Specifically, molten salt significantly disrupted the cross-linked structures, as evidenced by increasing NH₃ emissions from amine-cured EP resin. For ester-rich resins, the strong alkalinity of the molten salt effectively promoted decarboxylation, liberating aromatic hydrocarbons. For phenol-rich resin, depolymerization at 400 °C liberated phenolic hydroxyl groups that reacted with the molten salt to form sodium phenolate. Subsequent temperature elevation to 450–500 °C induced polycondensation, yielding char precipitation. These findings facilitate fiber recovery and resin byproduct utilization.