Mechanism of thermo-oxidative degradation of polyamide

The thermo-oxidative degradation of polyamide 6 (PA 6) was investigated by means of detailed analysis of the reaction products. The changes in the contents of main groups produced in PA 6 during thermo-oxidation were determined by chemical methods. The volatile products of PA 6 during thermooxidation were collected and characterized by GC-MS. The hydrolysis products of oxidized PA 6 were also characterized by GC-MS. Experimental results showed that the hydroperoxide was the initial oxidation product of PA 6, and through its decomposition and further reactions, various oxidation products were formed, such as keton group and carboxylic group. From the analysis of hydrolysis products of oxidized PA 6, it was shown that a large amount of imide group was formed on the main chain. But at the same time some keton amide group [-CO-NH-(CH2)(4)-CO-CO-NH-] was also formed on the main chain. It means that although hydrogen-abstracting reaction by radicals mainly occurs at the methylene group adjacent to the nitrogen atom, it can occur at the methylene group adjacent to the carbonyl group of amide group as well. From the analysis of the volatile products, of great interest is that a lot of cyclic compounds, which had not been reported yet, were formed during thermo-oxidation of PA 6. It indicates that some cyclization reactions should be included in the oxidation process of PA 6. On the basis of all the obtained experimental results a complete mechanism of PA thermo-oxidative degradation was suggested. The mechanism contains the hydrogen-abstracting reaction by radicals at the methylene group adjacent to nitrogen atom and at the methylene group adjacent to the carbonyl of amide group, a series of radical-addition-cyclization reactions, and radical induced chain-scission reactions. It can well explain the results of the product analysis.