Le développement d'outils numériques de prédiction de la durée de vie des polymères constitue un levier prometteur pour réduire les durées des processus de certification de ces matériaux dans le domaine automobile sans sacrifier leur fiabilité. Cette thèse s'applique à la modélisation de la photo-thermo-oxydation du polypropylène isotactique (iPP), laquelle est responsable de l'altération de ses propriétés mécaniques et d'aspect. L'approche adoptée consiste à coupler la cinétique des réactions de photo- et thermo-oxydation avec des phénomènes physiques, comme le transport du dioxygène et l'atténuation de la lumière UV dans l'épaisseur du matériau, pour décrire l'ensemble des évolutions physico-chimiques. Les propriétés aux échelles supérieures, sur lesquelles seront définis les critères de fin de vie, seront calculées a posteriori en appliquant les relations structure-propriété adéquates. Le principal enjeu était d'étendre le modèle cinétique de vieillissement thermique préexistant au vieillissement photo-thermique en prenant en compte les réactions d'amorçage photolytique. De lourdes campagnes d'essais de vieillissement et de caractérisation menées sur un iPP de référence, ainsi qu'une capitalisation exhaustive des données de la littérature d'autres iPPs, ont permis de mettre au point un modèle cinétique de photo-thermo-oxydation et de le généraliser à l'ensemble de la famille des iPPs dans de larges domaines de pression partielle d'oxygène (de 0.2 à 50 bars), de température (de 40 à 230°C) et d'exposition à la lumière UV (intensités et sources lumineuses variables) décrivant des conditions de vieillissements naturels et accélérés. La validation expérimentale du modèle a permis d'étayer l'approche cinétique et de montrer ses limites, mais aussi de révéler un certain nombre d'enjeux numériques. Le modèle a été conçu pour être un outil numérique évolutif qui permettra, à terme, d'optimiser la représentativité des méthodes d'essais de vieillissement et la performance des formulations commerciales d'iPP. L'ensemble de ces développements théoriques et numériques peut être appliqué à la photo-thermo-dégradation d'autres types de polymères, mais aussi dans d'autres champs d'application de la photochimie macromoléculaire telle que la photo-polymérisation UV.Mots-Clés : Polypropylène, photo-thermo-oxydation, contrôle par la diffusion d'oxygène, effet d'écran, modélisation cinétique, prédiction de durée de vie. / Developing numerical tools for polymer lifetime prediction constitutes a promising opportunity for shortening the duration of material certification procedures in the automotive industry without decreasing their reliability. This PhD thesis aims at modeling the photothermal oxidation of isotactic polypropylene (iPP), which is responsible for the alteration of both its mechanical and aspect properties. The adopted approach consists in coupling the kinetics of photo- and thermo-oxidation reactions with physical phenomena, such as oxygen transport and UV-light attenuation in the material thickness, in order to describe all the physico-chemical changes. Upper-scale properties, from which will be defined the end-of-life criteria, will be calculated afterwards by applying the suitable structure-property relationships. The main challenge was to extend the pre-existing kinetic model of thermal ageing to photothermal ageing by taking into account initiation reactions of photolysis. Heavy campaigns of ageing and characterization tests made on a reference iPP, as well as an exhaustive capitalization of literature data of other iPPs, have allowed elaborating a kinetic model of photothermal oxidation and to generalize it to the whole iPP family in large domains of oxygen partial pressure (from 0.2 to 50 bars), temperature (from 40 to 230°C) and UV-light exposure (variable intensities and light sources) describing both natural and accelerated ageing conditions. The experimental validation of the model has allowed substantiating the kinetic approach and showing its limitations, as well as highlighting some numerical issues. The model has been designed in order to be an upgradable numerical tool which will allow, at term, optimizing the representativeness of the ageing testing devices and the performance of commercial iPP formulations. All these theoretical and numerical developments are prone to be applied to the photothermal degradation of other types of polymer substrates, but also in other application fields of the macromolecular photochemistry such as UV-photopolymerization.Keywords: Polypropylene, photothermal oxidation, oxygen diffusion control, screen effect, kinetic modeling, lifetime prediction.

Developing numerical tools for polymer lifetime prediction constitutes a promising opportunity for shortening the duration of material certification procedures in the automotive industry without decreasing their reliability. This PhD thesis aims at modeling the photothermal oxidation of isotactic polypropylene (iPP), which is responsible for the alteration of both its mechanical and aspect properties. The adopted approach consists in coupling the kinetics of photo- and thermo-oxidation reactions with physical phenomena, such as oxygen transport and UV-light attenuation in the material thickness, in order to describe all the physico-chemical changes. Upper-scale properties, from which will be defined the end-of-life criteria, will be calculated afterwards by applying the suitable structure-property relationships. The main challenge was to extend the pre-existing kinetic model of thermal ageing to photothermal ageing by taking into account initiation reactions of photolysis. Heavy campaigns of ageing and characterization tests made on a reference iPP, as well as an exhaustive capitalization of literature data of other iPPs, have allowed elaborating a kinetic model of photothermal oxidation and to generalize it to the whole iPP family in large domains of oxygen partial pressure (from 0.2 to 50 bars), temperature (from 40 to 230°C) and UV-light exposure (variable intensities and light sources) describing both natural and accelerated ageing conditions. The experimental validation of the model has allowed substantiating the kinetic approach and showing its limitations, as well as highlighting some numerical issues. The model has been designed in order to be an upgradable numerical tool which will allow, at term, optimizing the representativeness of the ageing testing devices and the performance of commercial iPP formulations. All these theoretical and numerical developments are prone to be applied to the photothermal degradation of other types of polymer substrates, but also in other application fields of the macromolecular photochemistry such as UV-photopolymerization.Keywords: Polypropylene, photothermal oxidation, oxygen diffusion control, screen effect, kinetic modeling, lifetime prediction.