Investigating the degradation and products of thermo-oxidation of polyimide-based engineering plastics

With the ever-growing concern regarding plastic waste and its impact on the environment, understanding the degradation pathways of polyimide (PI)-based plastics becomes increasingly crucial for developing sustainable waste management strategies. This study investigated the thermo-oxidative degradation of PI-based plastics, including PI, polyamide-imide (PAI), and polyetherimide (PEI). We used techniques including thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy, an in situ heated electron spin resonance spectrometer, and a customized pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) system. The TGA results revealed that the onset degradation temperature of PI-based plastics exceeded 460 °C. The degradation of PI was promoted under air, while the degradation of PAI was delayed. For PEI, the degradation under air coincided with its initial decomposition under N2, followed by the oxidation of the residual solid. Changes in the chemical structures were revealed during the characterization of the samples treated above 400 °C. The residual char, obtained in high yield, notably contained N-rich functional groups. Real-time analysis using Py-GC/MS revealed that the pyrolysis of the plastics produced gases such as NH3, HCN, H2O, CO, and CO2, with NH3 and HCN being converted into NOx under an atmosphere of air. The pyrolysis of PI and PAI generated N-containing compounds, including aromatic amines, nitriles, and phthalimides. In contrast, the pyrolysis of PEI produced various phenolic compounds owing to the presence of the aromatic ether groups. The newly formed p-benzoquinone, CO2, H2O, and NOx were identified under air. This study provides comprehensive insights into the unique thermal and oxidative stabilities of PI-based plastics and holds the potential for recovering high-value N-containing materials from waste PI-based plastics.