No description available.

Cette thèse porte sur l’étude du comportement au moussage d’un élastomère silicone et de thermoplastiques vulcanisés (TPV) à base de silicone. Le moussage a été réalisé à l’aide des procédés batch et d’extrusion moussage en utilisant le CO2 comme agent moussant. Lamicrostructuration de fluorosilicone, ayant une grande affinité avec le CO2, dans la silicone favorise grandement le moussage par nucléation hétérogène. La morphologie du mélange et des mousses résultantes ainsi que la rhéologie en cisaillement et en écoulement biélongationnelont été améliorés en élaborant sous cisaillement des structures branchées de chaines silicones en présence d’un faible taux de peroxyde. En effet, la modification chimique sous écoulement permet de réduire la taille des nodules de fluorosilicone augmentant ainsi la densité volumique de sites de nucléation potentiels. Elle permet également de créer des structures hyperbranchées générant des propriétés de durcissementdes contraintes en écoulement bi-élongationnel ce qui a pour effet de réduire la croissance cellulaire et la coalescence lors de l’expansion de la mousse.La deuxième partie est dédiée à l’étude du moussage de TPV silicone pour lesquels la phase silicone est partiellement réticulée. Ces TPV ont étonnement le même comportement en rhéologie élongationnelle que la matrice polyéthylène basse densité (PEBD). Bien que les TPVs atteignent des bonnes propriétés en termes d’élongation à rupture à l’état fondu, leur comportement au moussage est très différent. En effet il est principalement lié à leur viscosité sous fort taux de cisaillement ainsi qu’aux conditions d’extrusion i.e. la pression avant filière et la détente en sortie de filière. Avec un taux de réticulation approprié de laphase élastomère, le TPV mousse de manière similaire que le PEBD / This work is devoted to the study of the foaming behavior of a silicone elastomer and thermoplastic vulcanizates (TPV) based silicone. The foaming step was carried out by batch and extrusion foaming processes with CO2 as blowing agent. The foamability of silicone elastomer was improved by a microstructuration of fluorosilicone which is a highly CO2-philic elastomer through heterogeneous nucleation. The foam and blend morphologies as well as the rheology in shear and bi-elongation modes were further fitted by branching silicone chains under shearing conditions in a roll mill with a small amount of peroxide. Indeed, this dynamic chemical modification reduces the size of fluorosilicone droplets which leads to increase the volume density of nucleating sites and consequently lowers the mean bubble size. Furthermore, it allows also the formation of multi-scale branched structures inducing a strain hardening behavior in bi-elongational flow which restrict the cell growth and coalescence during foam expansion.The second part deals with the foaming behavior of TPV silicone in which partially crosslinked silicone nodules are dispersed. Surprisingly, formulations exhibit close rheological behavior in elongation modes that the neat polyethylene (LDPE) matrix and reach prerequisites for foaming applications in terms of elongation at break. However, their foaming behavior are far different and are correlated to the extrusion foaming parameters which are known to control the nucleation i.e. the pressure before the die and the depressurization rate at the die exit. A proper choice of the gel content allows a TPV foamability close to that of LDPE

Ces travaux de thèse portent sur le développement d’un joint silicone pour la connectique dans l’aéronautique. Ce joint silicone doit être résistant aux solvants ainsi qu’aux huiles susceptibles de rentrer en contact avec celui-ci, et posséder de bonnes propriétés thermiques et mécaniques. Pour ce faire, les paramètres influençant ces propriétés ont été étudiés, comme la composition de la matrice, les conditions de sa réticulation et la formulation via différentes charges. Des matériaux silicones tels que le polydiméthylsiloxane (PDMS) et le polytrifluoropropylméthylsiloxane (PTFPMS) ont été sélectionnés pour composer la matrice. Leur flexibilité, leur large plage de température d’utilisation ainsi que leur excellente résistance aux attaques chimiques en font des matériaux de choix pour ce genre d’application. L’étude des mélanges de PDMS et de PTFPMS a démontré que les proportions idéales sont de 70/30 PDMS/PTFPMS. Le type de mélangeur sélectionné est une calandre bi-rouleaux, dont les rouleaux sont chauffés à 40°C. La réticulation de la matrice a été le sujet d’une étude approfondie. La cinétique de réticulation a été étudiée et l’influence des paramètres de réticulation tels que la température de réticulation, la nature et la quantité de peroxyde sur les propriétés finales ont été discutées. Finalement, l’influence de l’ajout de différentes charges sur le gonflement, la résistance thermique et les propriétés mécaniques de l’élastomère a été étudiée afin d’élaborer la formulation du joint silicone. / Polydimethylsiloxane (PDMS) and polytrifluoropropylmethylsiloxane (PTFPMS) elastomers are popular material in the aeronautic and connector fields. Their flexibility, wide service temperature range and chemical resistance make them first-choice materials for such applications. PTFPMS provides oil and apolar solvent resistance to the final material, while PDMS provides resistance to polar solvents, greater thermal resistance than PTFPMS, and cost reduction. Typically, connector seals comprising PDMS and PTFPMS can be composed of blends of homopolymers, of copolymers or of blends of homopolymers and copolymers. This present work deals only with blends of homopolymers. First, commercial PDMS and PTFPMS bases were selected and characterised, the blending process chosen and the PDMS/PTFPMS ratio tuned so as to minimise swelling in acetone and methylcyclohexane while maximising thermal properties. The optimal blend composition comprised 30 wt% PTFPMS. The second part of this work explored the influence of crosslinking conditions on final properties of the cured PDMS/FS blend. Crosslinking parameters, such as the temperature (160 and 180°C), the nature (DCP and DBPH) and the quantity (0.5 and 1 wt%) of peroxide, were varied. It appeared that co vulcanisation between PDMS and PTFPMS, occurs in certain conditions. Swelling as well is influenced by crosslinking conditions but not thermal properties. Finally, the formulation of the ideal elastomer was developed. Fillers, such as TiO2, CaCO3, quartz, CeO, a pigment, Fe2O3 and a platinum compound, were selected and their influence on thermal, mechanical and swelling properties studied. Regarding thermal and solvent properties, a high loading of fillers is a good strategy, however, an increase of permanent set was observed with the augmentation of filler fraction. Final formulations were selected for the compromise they offered between thermal and swelling properties and mechanical behaviour on the lab scale. Morphology observation revealed well dispersed domains, comparable to that of the non additivated blend.

Traditional fluorosilicones contain a siloxane backbone and pendant fluorinated group leading to low temperature ductility and excellent thermal stability. However, acidic or basic catalysts can reduce the thermal stability from a potential 350 °C to 150 °C. The predominant decomposition mechanism is through chain scission and it is hypothesized that preventing this will result in polymers with higher thermal stability. Three approaches were taken to prevent chain scission. First, a series of hybrid fluorosilicones based on (trifluorovinyl)benzene were synthesized through condensation polymerization with initial decomposition temperatures of approximately 240 °C. These were compared to similar aromatic polyethers and removal of the ether oxygen lowered the initial decomposition temperature by approximately 190 °C demonstrating the importance of this oxygen to the stability of polyethers. Second, reverse fluorosilicone (fluorinated backbone and pendant siloxane) terpolymers of chlorotrifluoroethylene (CTFE), vinyl acetate (VAc) and methacryloxypropyl-terminated polydimethylsiloxane (PDMSMA) were synthesized in supercritical CO2 (scCO2) or by emulsion polymerization. Chain scission was prevented as initial decomposition occurred between 231 and 278 °C. In both the emulsion and scCO2 cases, VAc was essential in facilitating cross-propagation between CTFE and PDMSMA and the branching was similar suggesting polymerization media does not affect polymer structure. Emulsion-based polymers had higher molar masses and thermal stability whereas comparable scCO2 polymers had higher yields and incorporated more PDMSMA. Third, a series of homo-, co-, and terpolymers of CTFE, VAc and methacryloxypropyl-terminated silsesquioxane (POSSMA) were synthesized representing the first synthesis of POSSMA containing polymers in scCO2 and demonstrating reverse fluorosilicones can be synthesized without VAc. Chain scission was prevented as initial decomposition occurred from 244 to 296 °C with thermal stability increasing with CTFE content to a limit. Decomposition of the polymers was examined and mechanism elucidated. In air, the copolymers give 40 to 47 wt% char since the silsesquioxane oxidizes to SiO2 while in N2, no residue is seen. In contrast, the terpolymers give a carbonaceous residue of approximately 20 wt% in N2. The flammability and surface properties of the polymers were examined with the terpolymers having flammability similar to p(CTFE) and surface properties comparable to p(POSSMA) giving a low-flammability, hydrophobic polymer.

Traditional fluorosilicones contain a siloxane backbone and pendant fluorinated group leading to low temperature ductility and excellent thermal stability. However, acidic or basic catalysts can reduce the thermal stability from a potential 350 °C to 150 °C. The predominant decomposition mechanism is through chain scission and it is hypothesized that preventing this will result in polymers with higher thermal stability. Three approaches were taken to prevent chain scission. First, a series of hybrid fluorosilicones based on (trifluorovinyl)benzene were synthesized through condensation polymerization with initial decomposition temperatures of approximately 240 °C. These were compared to similar aromatic polyethers and removal of the ether oxygen lowered the initial decomposition temperature by approximately 190 °C demonstrating the importance of this oxygen to the stability of polyethers. Second, reverse fluorosilicone (fluorinated backbone and pendant siloxane) terpolymers of chlorotrifluoroethylene (CTFE), vinyl acetate (VAc) and methacryloxypropyl-terminated polydimethylsiloxane (PDMSMA) were synthesized in supercritical CO2 (scCO2) or by emulsion polymerization. Chain scission was prevented as initial decomposition occurred between 231 and 278 °C. In both the emulsion and scCO2 cases, VAc was essential in facilitating cross-propagation between CTFE and PDMSMA and the branching was similar suggesting polymerization media does not affect polymer structure. Emulsion-based polymers had higher molar masses and thermal stability whereas comparable scCO2 polymers had higher yields and incorporated more PDMSMA. Third, a series of homo-, co-, and terpolymers of CTFE, VAc and methacryloxypropyl-terminated silsesquioxane (POSSMA) were synthesized representing the first synthesis of POSSMA containing polymers in scCO2 and demonstrating reverse fluorosilicones can be synthesized without VAc. Chain scission was prevented as initial decomposition occurred from 244 to 296 °C with thermal stability increasing with CTFE content to a limit. Decomposition of the polymers was examined and mechanism elucidated. In air, the copolymers give 40 to 47 wt% char since the silsesquioxane oxidizes to SiO2 while in N2, no residue is seen. In contrast, the terpolymers give a carbonaceous residue of approximately 20 wt% in N2. The flammability and surface properties of the polymers were examined with the terpolymers having flammability similar to p(CTFE) and surface properties comparable to p(POSSMA) giving a low-flammability, hydrophobic polymer.