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The thermal oxidation of polyamide 6 and 6,6 gives rise to azomethine groups (Schiff's bases) as a result of the reaction of aldehyde and ketocarbonyl groups with amino end groups. During the course of degradation, these azomethine groups react with each other in an aldol-type condensation to produce conjugated unsaturated oligoenimine structures that absorb in the UV/VIS range. During aldol condensation, the amino end groups are re-formed, allowing them to react again with the carbonyl groups which are continuously supplied by thermal oxidation (auto-oxidation) to form azomethine groups once more. The concentration and sequence length distribution of the oligoenimine structures can be correlated to the concentration profiles of the amino end groups, carboxyl end groups, and carbonyl groups, in a kinetic model. The concentrations of the individual oligo-enimine species calculated with the aid of the model, are in agreement with the figures found experimentally with UV spectroscopy. Sterically hindered phenols inhibit the formation of carbonyl groups, and thus indirectly counteract the formation of oligoenimine structures (kinetic model for the autoxidation of polymers).

In heat-treated polyamide 6 and polyamide 6.6 fibres UV/VIS-active chromophores are found, which originate from two different sources: one type of chromophore (structured UV spectrum) arises during polycondensation and the spinning of the fibres (molten phase), the other (nonstructured UV spectrum) occurs as a result of a thermo-oxidation of the solid fibre. The chromophore which forms during solid-phase thermo-oxidation is based on conjugated oligoenimines . . .+ A = &A j =N-. . . , as is shown by a combination of UVNIS spectroscopy and fluorescence excitation and emission spectroscopy with chemical analysis. The concentration of the various oligoenimine species depends upon the temperature and duration of the thermooxidation. For a temperature of 160°C and a duration of 2 hours, for example, the following oligoenimine concentrations were found (UV spectroscopy with computer-aided evaluation): for i = 0 44; i = 1: 13; i = 2 4; i = 3: 1; i = 4 0,3 (values expressed in mmol/kg).

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