Pyrolysis kinetics and backbone decomposition mechanisms of nylon 6/6: insights from TG-FTIR-GC/MS and density functional theory

A limited understanding of the pyrolysis mechanisms hinders the efficient thermochemical conversion and recovery of nylon waste. This study investigates the pyrolysis behavior, volatile evolution, and backbone degradation mechanisms of nylon 6/6 by integrating thermogravimetry-Fourier transform infrared spectroscopy-gas chromatography/mass spectrometry (TG-FTIR-GC/MS) and density functional theory (DFT) calculations. According to TG analysis, nylon 6/6 undergoes pyrolysis mainly in the range of 390–515 °C, with a mass loss of 95.86 wt%, and exhibits the most intense degradation at approximately 480 °C. Kinetic analysis using the Coats-Redfern integral and Achar differential methods indicates that nylon 6/6 pyrolysis follows the first-order reaction model with an apparent activation energy of 278 kJ/mol. Online FTIR and offline GC/MS identify the characteristic gaseous products, including CO2, NH3, 1-amino-5-hexene (13.54%), and cyclopentanone (79.64%). Furthermore, DFT calculations elucidate the electrostatic potential (ESP) distribution, Fukui function indices, and bond dissociation energies (BDEs) of the nylon 6/6 model molecule. Results demonstrate that the nucleophilic attack of the terminal amino group and the free radical attack of H· facilitate the cleavage of the amide structure. The generated hexamethylenediamine and adipic acid segments subsequently undergo deamination and cyclization reactions to form 1-amino-5-hexene and cyclopentanone, respectively. By elucidating the backbone degradation mechanisms and product formation pathways during nylon 6/6 pyrolysis, the multi-scale study provides a theoretical basis for the recovery of high-value fuels and chemicals from nylon waste.