Thermal Degradation of Polyamides
The thermal degradation of polyamides starts with free-radical cleavage of nitrogen–carbon bonds [520]. Degradation of nylon 6 can be illustrated as follows:
When nylon 6 is heated for 100 h at 305C, half of the nitrogen escapes from the polymer and a small amount of carbon dioxide forms [457]. This was postulated by Kamerbeek et al. as being the result of reactions of two terminal amine groups [579]:
In decomposition of nylon 6,6, there is the additional tendency for ring closure by adipic acid [509]:
The cyclization reaction can also occur as follows [509]
or, perhaps, as a bimolecular reaction [509]:
At higher temperatures, the carbamoyl ketone might convert to the Schiff base by a monomolecular process [509]:
The route to the Schiff base may also be through elimination of an isocyanate [509]:
Rather than thermal degradation of nylon 6, it is possible to depolymerize this polyamide. Kamimura and Yamamoto reported that it is possible to depolymerize this nylon back to caprolactam [510]:
The best yield of caprolactam was 86% obtained with N-methyl, N-propyl piperidinium bis (trifluoromethyl-sufonyl) imide as the solvent and N, N-dimethylpyridine as the catalyst. In the past, it was believed that at high temperatures nylons degrade at a faster rate at elevated humidity. This assumption, however, was shown to be erroneous by Bernstein et al. [457].
