Sustainable Nylon Recycling by Supercritical Depolymerization of Polyamide-6 to ε-Caprolactam in Azeotropic Alcohol/Water Mixtures

This study presents an efficient chemical recycling approach for recycling polyamide-6 (PA6) via supercritical depolymerization into ε-caprolactam (CL) using an azeotropic isopropanol/water mixture. Considering the growing environmental burden of plastic waste and the demand for effective recycling routes, chemical recycling offers a promising solution by converting PA6 back into its monomer, CL, thus enabling potential closed-loop recycling through repolymerization. Depolymerization experiments were conducted with various isopropanol (iPrOH)/water mixtures under supercritical conditions to optimize yield and selectivity. The isopropanol/water azeotrope (12.3 wt % water) afforded a 100% yield of CL, representing the most efficient conversion pathway and demonstrating a greener, low-temperature (300 °C) process for PA6 recovery. Comparative tests using azeotropic ethanol (4.5 wt % water) and sec-butanol (32.0 wt % water) mixtures similarly showed that the azeotropic compositions maximize depolymerization efficiency. Furthermore, anionic repolymerization of the recovered CL confirmed the feasibility of partially incorporating recycled monomer, further advancing the closed-loop lifecycle of PA6.