Microwave-Assisted Acid Hydrolysis of PA6 Wastes in PA6 Process: Kinetics, Activation Energies, and Monomer Recovery

Efficient recycling of polyamide 6 (PA6) requires selective depolymerization routes that recover monomers under moderate conditions. This study investigates microwave-assisted acid hydrolysis of four PA6 waste streams, two oligomer-rich residues (WS-13, WS-24), an industrial fiber (C-fiber), and a commercial resin (C-resin) to elucidate degradation kinetics, activation energies, and product yields. Thermogravimetric analysis revealed multi-step solid-state decomposition, while microwave hydrolysis (125–200 °C, 15–60 min, 400 W) demonstrated strong dependence on acid type. HCl achieved complete conversion, whereas phosphoric and formic acids exceeded 95%. Kinetic analysis under H3PO4 followed pseudo-first-order behavior, with rate constants (0.015–0.141 min−1 at 200 °C) and activation energies reflecting feedstock structure: 53.1 kJ mol−1 (WS-13), 56.5 kJ mol−1 (WS-24), 87.1 kJ mol−1 (C-resin), and 99.9 kJ mol−1 (C-fiber). Monomer yields varied by substrate: WS-13 achieved 62.4% at 200 °C and 45 min (ACA 46%, CPL 16%), WS-24 yielded 62.0% (primarily ACA), C-fiber reached 69.7% (ACA-dominant), and C-resin produced 53.8%. These results show that oligomer-rich wastes are kinetically favored for rapid hydrolysis at lower energy cost, while C-fiber maximizes aminocaproic acid recovery. Overall, microwave-assisted hydrolysis provides a selective, energy-efficient pathway for PA6 circularity, offering design parameters for reactor operation and process optimization.