Enhancing Recycling of Polyamide 6 and Polyethylene Multilayer Plastics through Sequential Hydrothermal Liquefaction

This study investigates the optimization of hydrothermal liquefaction (HTL) for the chemical recycling of polyamide 6 (PA6) and polyethylene (PE) multilayer plastic films. A two-step HTL process was employed using subcritical water to selectively depolymerize PA6 into caprolactam, followed by supercritical water to convert PE into hydrocarbons. Experiments on raw PA6 yielded up to 88 wt % caprolactam under optimized conditions (325 °C, 30 min). In the two-step process, caprolactam recovery from multilayer films approached 100%. Supercritical HTL produced oil yields of 44–61 wt % from PE. The carbon and nitrogen balances demonstrated efficient recovery of hydrocarbons and nitrogen removal. These results suggest the feasibility of integrating this process into existing plastic waste recycling frameworks.