Influence of temperature and stabilization on oxygen diffusion limited oxidation profiles of polyamide 6

The oxidative degradation behavior of polymers depends on a combination of chemical and physical factors, with oxygen diffusion being one of the most important, especially when the oxygen consumption rate is larger than its permeability. As a result of diffusion limited oxidation (DLO), at high temperatures the degradation rate of polyamide 6 (PA6) plaques is heterogeneous, with the polymer oxidizing much faster at the surface than in the bulk. Normalized carbonyl index (CI) and UV absorption - depth profiles were found to be mostly degradation time independent, implying equilibrium degradation conditions where oxygen permeability and reaction rates did not change significantly with degradation time. The experimental DLO profiles were described using a basic reactive-diffusion model based on Fickian oxygen diffusion and an oxidation rate being first order in local O2 concentration, as well as by applying an established DLO model based on the basic autoxidation mechanism. Analysis with the second model yielded the best estimation of high temperature oxygen permeability (PO2) data. It also showed some of the limitations in the data analysis when using a simple first order DLO model. It was shown that stabilizers have an influence on the oxidation - depth profiles. Better stabilization results in slower polymer oxidation and the oxidation - depth profiles are therefore less pronounced. At 170 °C it was observed that stabilized plaques (0.5 mm) in the center oxidize faster than unstabilized plaques, which is attributed to the complete consumption of oxygen in the outer layers for the unstabilized plaques. Oxidation rates of differently stabilized samples were also determined by applying the second DLO model.