Thermo-oxydation des polyamides

Some metal components of automotive engine are bound to be replaced by polyamide parts. However, despite their thermal resistance polyamides are sensitive to oxygen leading to thermal oxidation chain reactions responsible for their long-term properties. While durability is critical for polyamide users, only a few studies deal with the elaboration of a kinetic model capable of predicting polyamide lifetime (time to embrittlement) in contrary to polyolefins (especially polyethylene). This PhD thesis is a contribution to the understanding of aliphatic polyamide thermal degradation by considering chemical and physical aspects of oxidation process in order to build a kinetic model. Our approach is based on a multi-scale physicochemical characterization of oxidized PA11 film samples under air between 90 and 165 °C but also under oxygen pressure. The proposed kinetic model coupling oxidation and solid state polymerization is able to simulate the whole experimental data (hydroperoxides, carbonyls and molar mass changes). In a same time, an intrinsic criterion for embrittlement is assessed to predict lifetime whatever the exposure conditions. Finally, the influence of phenols and copper salts on the oxidation kinetic is investigated. A first kinetic model including the phenol stabilizing effect is capable of simulating the main observed trends for stabilized PA11 such as the appearance of the pseudo induction period which contributes to the significant improvement of PA11 durability.Les polyamides sont des thermoplastiques techniques qui entrent dans la conception de pièces destinées à remplacer certains composants métalliques des moteurs automobiles. En dépit de propriétés mécaniques initiales satisfaisantes, leur tenue à long terme est limitée par leur sensibilité à l'oxygène conduisant à des réactions de thermo-oxydation. Ces réactions ont été largement étudiées dans le cas des polyoléfines mais peu dans le cas des polyamides, rendant nécessaire l'élaboration d'un modèle cinétique susceptible de prédire la fragilisation donc la durée de vie des polyamides. Cette thèse est une contribution à la compréhension du processus d'oxydation dans le cas des polyamides aliphatiques et à la construction d'un modèle cinétique. La démarche cinétique réside tout d'abord dans la caractérisation physico-chimique multi-échelle de films de PA11 oxydés dans différentes conditions de températures (90 à 165 °C sous air) et sous différentes pressions partielles d'oxygène. Un modèle cinétique couplant oxydation et post-polycondensation est proposé ici : il permet de simuler les données expérimentales (hydroperoxydes, carbonyles et masse molaires) quelles que soient les conditions d'exposition. Parallèlement, un critère intrinsèque gouvernant la fragilisation du PA11 est identifié afin de prédire cette dernière à partir du modèle cinétique. Enfin, l'influence de l'ajout d'antioxydants phénoliques et des sels de cuivre sur la cinétique d'oxydation est caractérisée. Un premier modèle cinétique prenant en compte la stabilisation du PA11 décrit les tendances spécifiques de la stabilisation du PA11 comme l'apparition de la pseudo-période d'induction contribuant à une augmentation significative de la durée de vie du PA11

Review : Auto-oxidation of aliphatic polyamides

The literature on oxidation kinetics of polyamides and model compounds has been reviewed in order to try to extract suitable information for non-empirical kinetic modeling. Polyamide characteristics are systematically compared to polyolefin ones, these latter being more extensively studied. From kinetic analysis point of view, it is shown that oxidation attacks predominantly a amino methylenes of which C eH bond is considerably weaker than the other methylenes. As a result, propagation by H abstraction is considerably faster in polyamides than in polyethylene for instance. Termination by radical combination is also very fast. Another cause of PA oxidizability is the instability of a amino hydroperoxides linked to the inductive effect of nitrogen. This instability is responsible for many key features of oxidation kinetics especially the absence of induction period. The main stable oxidation products are imides resulting from disproportionation processes meanwhile chain scissions resulting from rearrangements of a amino alkyls by b-scission are also significant process although their yield appears lower than in polyolefins

Molecular and macromolecular structure changes in polyamide 11 during thermal oxidation

The present article reports a study of thermal oxidation of unstabilized polyamide 11 films at several temperatures (90–165 °C) under atmospheric pressure and under various oxygen pressures (up to 1.6 MPa) at 110 °C. The chemical structure changes are monitored by IR spectroscopy (carbonyl groups) and UV–visible spectrophotometry (yellowing). Molar mass changes are determined by size exclusion chromatography (SEC). By investigating the influence of oxygen pressure it is clearly shown that reactions involving P° radicals other than O2 addition cannot be neglected under atmospheric pressure. Under the conditions of this study limited to relatively low oxidation levels, IR and UV measurements indicate that carbonyl groups and chromophores responsible for yellowing have the same relative yield whatever the temperature and oxygen pressure. SEC measurements highlight the significant predominance of random chain scissions over crosslinking events. Crosslinking only appears after an induction time, presumably because it involves reactions between primary oxidation products. The ratio of carbonyl groups over chain scissions is about 7.5 at low conversion and about 2.5 at high conversion, showing that α amino alkoxy radicals are mainly transformed into imides without chain scission

Investigation of polyamide 11 embrittlement during oxidative degradation

Embrittlement processes occurring during thermal oxidation are investigated for stabilized and unstabilized polyamide 11 samples differing by their thicknesses and initial molar masses. Tensile tests were carried out in the temperature range between room temperature and 110 °C in order to investigate the influence of mechanical testing temperature on the embrittlement coordinates. In the same time, molar mass and crystalline morphology are monitored by size exclusion chromatography (SEC) and DSC/SAXS measurements respectively. The experimental results point out the existence of a critical molar mass for ductile-brittle transition M′c about 10 kg mol−1, independent of sample initial molar mass or stabilization, but depending on tensile testing temperature. However, even if oxidation chain scissions are shown to be clearly responsible for the loss of mechanical properties at failure, the structure-property relationships governing ductile-brittle transition require a mixed criterion involving molar mass and crystalline morphology, especially the interlamellar distance. For this purpose, specific molar mass – crystalline morphology relationships are investigate

Thermal stabilization of polyamide 11 by phenolic antioxidants

This paper addresses the effect of hindered phenols (mainly Irganox 1098 with a few comparisons with other phenolic antioxidants) on the stabilization of polyamide 11 aged at several temperatures (90–165 °C). The effect of several phenol concentrations (up to about 0.4%) on kinetic curves for imide build-up, yellowing, and molar mass changes (in association with embrittlement) was investigated. Phenols significantly contribute to yellowing, even at low imide concentrations. When they are used at high concentrations, a post-polycondensation reaction becomes predominant at earlier exposure times, thus increasing molar mass and significantly delaying embrittlement

Quantification of hindered phenols in polyamide 11 during thermal aging

Polyamide 11 films stabilized by Irganox® 1098, Irganox® 1010 or Irganox® 245 were subjected to thermal oxidation at 110°C. The residual phenol content was assessed by comparing three analytical methods:high performance liquid chromatography (HPLC), determination of the Oxidation Induction Time (OIT)and Onset Oxidation Temperature (OOT) by thermal analyses. Both OIT and OOT are reliable for virgin PA11 after a relevant calibration by HPLC measurement. In the case of oxidized samples, OOT measurements have the benefits of being more easily interpretable than OIT and less time-consuming than HPLC measurements.CIFR

Thermal oxidative degradation of polyamides

Les polyamides sont des thermoplastiques techniques qui entrent dans la conception de pièces destinées à remplacer certains composants métalliques des moteurs automobiles. En dépit de propriétés mécaniques initiales satisfaisantes, leur tenue à long terme est limitée par leur sensibilité à l'oxygène conduisant à des réactions de thermo-oxydation. Ces réactions ont été largement étudiées dans le cas des polyoléfines mais peu dans le cas des polyamides, rendant nécessaire l'élaboration d'un modèle cinétique susceptible de prédire la fragilisation donc la durée de vie des polyamides. Cette thèse est une contribution à la compréhension du processus d'oxydation dans le cas des polyamides aliphatiques et à la construction d'un modèle cinétique. La démarche cinétique réside tout d'abord dans la caractérisation physico-chimique multi-échelle de films de PA11 oxydés dans différentes conditions de températures (90 à 165 °C sous air) et sous différentes pressions partielles d'oxygène. Un modèle cinétique couplant oxydation et post-polycondensation est proposé ici : il permet de simuler les données expérimentales (hydroperoxydes, carbonyles et masse molaires) quelles que soient les conditions d'exposition. Parallèlement, un critère intrinsèque gouvernant la fragilisation du PA11 est identifié afin de prédire cette dernière à partir du modèle cinétique. Enfin, l'influence de l'ajout d'antioxydants phénoliques et des sels de cuivre sur la cinétique d'oxydation est caractérisée. Un premier modèle cinétique prenant en compte la stabilisation du PA11 décrit les tendances spécifiques de la stabilisation du PA11 comme l'apparition de la pseudo-période d'induction contribuant à une augmentation significative de la durée de vie du PA11.Some metal components of automotive engine are bound to be replaced by polyamide parts. However, despite their thermal resistance polyamides are sensitive to oxygen leading to thermal oxidation chain reactions responsible for their long-term properties. While durability is critical for polyamide users, only a few studies deal with the elaboration of a kinetic model capable of predicting polyamide lifetime (time to embrittlement) in contrary to polyolefins (especially polyethylene). This PhD thesis is a contribution to the understanding of aliphatic polyamide thermal degradation by considering chemical and physical aspects of oxidation process in order to build a kinetic model. Our approach is based on a multi-scale physicochemical characterization of oxidized PA11 film samples under air between 90 and 165 °C but also under oxygen pressure. The proposed kinetic model coupling oxidation and solid state polymerization is able to simulate the whole experimental data (hydroperoxides, carbonyls and molar mass changes). In a same time, an intrinsic criterion for embrittlement is assessed to predict lifetime whatever the exposure conditions. Finally, the influence of phenols and copper salts on the oxidation kinetic is investigated. A first kinetic model including the phenol stabilizing effect is capable of simulating the main observed trends for stabilized PA11 such as the appearance of the pseudo induction period which contributes to the significant improvement of PA11 durability