Thermal oxidation kinetics of additive free polyamide 6-6

Abstract

International audienceThermal aging of an additive free PA 6-6 has been elucidated at 90, 100, 120, 140, 150 and 160 C in airventiled ovens by Fourier transform infrared spectrophotometry, viscosimetry in molten state and uniaxial tensile testing. Oxidation of methylene groups starts after a considerably shorter induction period but reaches a lower maximal rate than in additive free PE. Cleavage of CeN bonds constitutes the main source of chain scissions. It leads to the formation of aldehyde chain-ends and a catastrophic decrease in molar mass. Embrittlement occurs at a very low conversion ratio of the oxidation process, in particular when the concentration of aldehyde chain-ends reaches a critical value of [PH¼O]F z 5.6 10 3 mol l 1, corresponding to a critical value of the number average molar mass ofMnFz17 kg mol 1. At this stage, the entanglement network in the amorphous phase is deeply damaged. A non-empirical kinetic model has been derived from the oxidation mechanistic scheme previously established for PE, but improved by adding elementary reactions specific to polyamides such as the rapid decomposition of unstable hydroxylated amide groups. This model describes satisfyingly the main features of the thermal oxidation kinetics of PA 6-6, but also of other types of aliphatic polyamides studied previously in the literature such as: PA 6, PA 12 and PA 4-6, as long as it is not controlled by oxygen diffusion. At the same time, it confirms the existence of an universal character for the thermal oxidation kinetics of aliphatic polyamides whatever their origin, i.e. their initial molar mass, crystallinity ratio, concentration of impurities, structural irregularities, etc

Molecular and macromolecular structure changes in polyamide 11 during thermal oxidation

“…-According to El-Mazry et al [4], the ratio between chain scissions and carbonyls would be close to unity, contrary to the experimental observation by Sagar [5] and later Lemaire et al [6] according to which imides are the main oxidation product. Since imides are formed without chain scission, the ratio of chain scission/carbonyl would be expected significantly lower than 1.…”

“…6, the experimental data are fitted with a simple power law (lines) of exponent 2/3 which is not a kinetic model. A significant feature of polyamide 11 (and other aliphatic polyamides [3,4,19,20,21]) is the absence of induction period that is typical of polyolefins oxidation for instance. Here, oxidation rate is maximal at the beginning of exposure.…”

“…The latter technique [9,10] appears to be relevant in comparison with rheometry measurements [4] where part of the oxidized polymer may undergo thermolysis as the measurements are performed in molten state. Furthermore, SEC is expected to be more informative than viscosimetry (only M W [11]) with the aim of quantifying the dual chain scission/crosslinking process in polyamides.…”

Nouvelles avancées en modélisation cinétique de la thermo-oxydation des matrices époxy-diamine

“…A titre d'information, les paramètres d'Arrhenius et les ordres de grandeur de k 3à 30 • C des principaux types de liaisons méthyléniques C-H sont donnés au Tableau 2. Ces valeurs sont issues d'études antérieures menées au laboratoire [3][4][5][6][7].…”

“…Tout comme les polyesters [5] et les polyamides (PA) aliphatiques [6], les matrices EPO-DA vont donc subir une oxydation beaucoup plus sévère que le polyéthylène (PE …”