The radical mechanisms of the thermal degradation of polyamide 66 (PA66) occurring under a vacuum at a temperature range between 80 °C and 240 °C (which includes the temperature of practical applications) were investigated using a spin-trapping electron spin resonance (ST-ESR) technique, as well as FTIR, TG-DTA, and GPC methods. No significant weight loss and no sign of thermal degradation are observed at this temperature range under oxygen-free conditions, but a slight production of secondary amine groups is confirmed by FTIR. GPC analysis shows a small degradation by the main chain scission. ST-ESR analysis reveals two intermediate radicals which are produced in the thermal degradation of PA66: (a) a ●CH2- radical generated by main chain scission and (b) a -●CH- radical generated by hydrogen abstraction from the methylene group of the main chain. The ST-ESR result does not directly confirm that a -NH-●CH- radical is produced, although this reaction has been previously inferred as the initiation reaction of the thermal degradation of PA; however, the presence of -●CH- radicals strongly suggests the occurrence of this initiation reaction, which takes place on the α-carbon next to the NH group. The ST-ESR analysis reveals very small levels of reaction, which cannot be observed by common analytical methods such as FTIR and NMR.
Keywords: 1,3,5-tri-tert-butyl-2-nitrosobenzene (TTBNB); electron spin resonance (ESR); nylon 66; polyamide 6,6 (PA66); radical intermediates; spin-trapping method; thermal degradation.