Thermo‐oxidative degradation of polyamide 6 and 6,6. Kinetics of the formation and inhibition of UV/VIS‐active chromophores

Abstract

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).

Review on the thermo-oxidative degradation of polymers during processing and in service

“…Condensations of these aldimines with further aldehydes can lead to unsaturated imines and conjugated double bonds. These structures are postulated as the cause of colour changes due to oxidative degradation [135,136]. Other postulated reasons for this colour change are the formation of pyrrole groups [137], conjugated enals or enons [123] and ketoamide groups formed by oxidation of the methylene groups adjacent to the carbonyls of the amide groups [138].…”

Thermal Oxidation Products of Nylon 6 Determined by MALDI-TOF Mass Spectrometry

“…[1 -3] The formation of azomethine groups has also been proposed to explain the presence of UV/VIS-active chromophores (Scheme 2). [3] Therefore, the MALDI analysis allowed the identification of the species formed in the early stages of the thermal-oxidative degradation of Ny6, although the structural assignments of MALDI peaks to thermo-oxidation products are speculations based on the molecular mass of expected chemical structures.…”

Kinetics of Photo-Induced Chemiluminescence Decay from Polymers

“…In addition to any additives and the TiO 2 delustrant which absorb in this region, PA6 is known to contain traces of UV-absorbing impurities, such as carbonyl and pyrrolyl groups, and also conjugated oligoenimines formed via Schiff bases following exposure to heat during thermal processing. 15,16 Wool keratin absorbs UVA due to the presence of tryptophan Figure 1. Corrected PICL decay profiles of PP, PA6 and wool keratin fabrics following exposure to UVA radiation at various temperatures.…”