Enhancing the thermo-oxidative stability of polyamide 6 by scalable melt-blending with PEG-grafted glycol lignin

Thermo-oxidative degradation of polyamide 6 (PA6) is a critical problem for automotive or electrical components that are exposed to elevated temperatures. Conventional stabilizers are often toxic and leach out over time, and while natural additives such as lignin are promising alternatives, they tend to have low miscibility with polymers. In this study, blends of polyamide 6 (PA6) and PEG-grafted glycol lignin (GL) were prepared by melt-extrusion and their thermo-oxidative stability was evaluated. GL blends had mechanical and thermal properties similar to neat PA6, but DSC, XRD, and IR analyses indicated that lignin affects the crystal structure by promoting the formation of α-phase. The antioxidant effect of lignin in suppressing degradation was confirmed by isothermal gravimetric analysis. Accelerated aging between 120 and 180 °C resulted in drastically reduced mechanical properties in neat PA6, but the blends with 5–10 wt% GL showed almost no change up to 168 h and master curves were constructed using the time-temperature superposition principle. XRD analysis revealed that lignin regulates aging-induced crystallization and furthermore favors the transition to α-phase in air. These results demonstrate that GL can be used to improve the thermal stability of PA6 for longer service life and can be beneficial for end-of-life mechanical recycling as well as manufacturing processes such as annealing.