Thermal decomposition of aliphatic nylons

An overview of the literature together with selected authors' data on thermal and thermo-oxidative decomposition of commercial aliphatic nylons (nylon 6, nylon 7, nylon 11, nylon 12, nylon 6.6, nylon 6.10, nylon 6.12) is presented. Despite the high level of research activity and the large number of publications in the field, there is no generally accepted mechanism for the thermal decomposition of aliphatic nylons. Polylactams (nylon 6, nylon 11 and nylon 12) tend to re-equilibrate to monomeric or oligomeric cyclic products. Diacid–diamine type nylons (nylon 6.6, nylon 6.10 and nylon 6.12) produce mostly linear or cyclic oligomeric fragments and monomeric units. Because of the tendency of adipic acid to fragment with elimination of CO and H2O and to undergo cyclization, significant amounts of secondary products from nylon 6.6 are reported in some papers. Many authors have shown that the primary polyamide chain scission occurs either at the peptide C(O)NH or at adjacent bonds, most probably at the alkyl–amide NHCH2 bond which is relatively the weakest in the aliphatic chain. Hydrolysis, homolytic scission, intramolecular CH transfer and cis-elimination (a particular case of CH transfer) are all suggested as possible primary chain-scission mechanisms. There are no convincing results reported which tend to generally support one of these mechanisms relative to the others; rather, it seems that the contribution of each mechanism depends on experimental conditions. This conclusion is also supported by the wide spread of kinetic parameters measured under the different experimental conditions. More uniform results are observed in the literature regarding the mechanism of thermo-oxidative decomposition of aliphatic nylons. Most authors agree that oxygen first attacks the N-vicinal methylene group, which is followed by the scission of alkyl–amide NC or vicinal CC bond. Alternatively, it is suggested that any methylene group which is β-positioned to the amide group methylene can be initially oxidized. There are few mechanisms in the literature which explain discoloration (yellowing) of nylons. UV/visible active chromophores are attributed either to pyrrole type structures, to conjugated acylamides or to conjugated azomethines. Some secondary reactions occurring during the thermal or thermo-oxidative decomposition lead to crosslinking of nylons. Nylon 6.6 crosslinks relatively easily, especially in the presence of air, whereas nylon 11 and nylon 12 crosslink very little. Strong mineral acids, strong bases, and some oxides or salts of transition metals catalyse the thermal decomposition of nylons, but minimize crosslinking. In contrast, many fire retardant additives promote secondary reactions, crosslinking and charring of aliphatic nylons. © 1999 Society of Chemical Industry