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 contin… Show more
“…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].…”
During processing as well as in service life, polymers degrade. For the majority of polymers this is due to oxidative processes. In this review the mechanism of the thermo-oxidative degradation of the most important (unstabilized) polymers as well as the factors determining the degradation rates are discussed.
“…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].…”
During processing as well as in service life, polymers degrade. For the majority of polymers this is due to oxidative processes. In this review the mechanism of the thermo-oxidative degradation of the most important (unstabilized) polymers as well as the factors determining the degradation rates are discussed.
“…[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.…”
“…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.…”
Section: Picl Decay Profiles and Preliminary Kinetic Analysismentioning
Previous studies showed that the decay of photo-induced chemiluminescence (PICL) in polymers does not obey classical first or second order kinetics. This study demonstrates that plotting PICL second-order decay from three fibrous polymers (polypropylene, polyamide 6 and wool keratin) in a modified form gives excellent linear plots. The existence of a significantly less reactive fraction of macroperoxy radicals in heterogeneous polymers can explain the deviation from classical behaviour and this fraction can be quantified from the gradient of the modified plots. This unreactive fraction could be due to either dispersive or diffusive effects in a disordered heterogeneous polymer medium. Both dispersive and diffusive models give very good fits to the PICL decay profiles using three fitting parameters.
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