Thermal decomposition of aliphatic nylons

An overview of the literature together with selected authors' data on therm al and thermooxidative decomposition of commercial aliphatic nylons (nylon 6, nylon 7, nylon 11, nylon 12, nylon 6.6, nylon 6.10, nylon 6.12) is prese nted. Despite the high level of research activity and the large number of p ublications in the field, there is no generally accepted mechanism for the thermal decomposition of aliphatic nylons. Polylactams (nylon 6, nylon 11 a nd nylon 12) tend to re-equilibrate to monomeric or oligomeric cyclic produ cts. Diacid-diamine type nylons (nylon 6.6, nylon 6.10 and nylon 6.12) prod uce mostly linear or cyclic oligomeric fragments and monomeric units. Becau se of the tendency of adipic acid to fragment with elimination of CO and H2 O and to undergo cyclization, significant amounts of secondary products fro m nylon 6.6 are reported in some papers. Many authors have shown that the primary polyamide chain scission occurs ei ther at the peptide C(O)-NH or at adjacent bonds, most probably at the alky l-amide NH-CH2 bond which is relatively the weakest in the aliphatic chain. Hydrolysis, homolytic scission, intramolecular C H transfer and cis-elimin ation (a particular case of C-H transfer) are all suggested as possible pri mary chain-scission mechanisms. There are no convincing results reported wh ich tend to generally support one of these mechanisms relative to the other s; rather, it seems that the contribution of each mechanism depends on expe rimental conditions. This conclusion is also supported by the wide spread o f 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 i s suggested that any methylene group which is beta-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 ac tive chromophores are attributed either to pyrrole type structures, to conj ugated acylamides or to conjugated azomethines. Some secondary reactions occurring during the thermal or thermo-oxidative d ecomposition lead to crosslinking of nylons. Nylon 6.6 crosslinks relativel y 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 prom ote secondary reactions, crosslinking and charring of aliphatic nylons. (C) 1999 Society of Chemical Industry.