Assessment of Nylon-66 Depolymerization for Circular Economy: Kinetic Modeling, Purification, and Sustainable Design

Abstract

Nylon-66 is an important thermoplastic that finds widespread applications in automotive parts, electronics, and textiles. Chemical depolymerization to form nylon-66 monomers, namely, adipic acid (AA) and hexamethylene diamine (HMDA), offers an efficient method to recover value from low-purity and degraded nylon-66 waste. While chemical recycling pathways for step-growth polymers, such as PET and nylon-6, have been extensively investigated, the chemical recycling of nylon-66 is not well understood. This work presents a comprehensive assessment of the academic literature and industrial patents of the three primary types of nylon-66 depolymerization processes: acid hydrolysis, alkaline hydrolysis, and ammonolysis. We use existing data to develop a kinetic model incorporating the activity coefficient for neutral hydrolysis of nylon-66 necessary to reflect the nonideal liquid phase with high water concentration, and we include degradation reactions to model byproducts. We identify purification methods for AA, HDMA, ammonolysis products, and nylon-66 salts and describe how they can be applied to chemical recycling pathways. We present the first process model for the alkaline hydrolysis of nylon-66 leveraging innovations from PET alkaline hydrolysis and demonstrating heat integration and process intensifications, such as mechanical vapor recompression. We demonstrate that the alkaline hydrolysis process for nylon-66 consumes less energy than the comparative PET alkaline hydrolysis process, while producing higher-value products. We use this collective evaluation to provide guidance for future research to further advance nylon-66 chemical recycling.