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This work focuses on the extrusion foaming under CO2 of commercial TPV and how the process influences the final morphology of the foam. Moreover, numerical modelling of the cell growth of the extrusion foaming is developed. The results show how a precise control on the saturation pressure, die geometry, temperature and nucleation can provide a homogeneous foam having a low density (<500 kg/m3). This work demonstrates that an optimum of CO2 content must be determined to control the coalescence phenomenon that appears for high levels of CO2. This is explained by longer residence times in the die (time of growth under confinement) and an early nucleation (expansion on the die destabilizes the polymer flow). Finally, this work proposes a model to predict the influence of CO2 on the flow (plasticizing effect) and a global model to simulate the extrusion process and foaming inside and outside the die. For well-chosen nucleation parameters, the model predicts the final mean radius of the cell foam as well as final foam density.

This work focuses on reactive extrusion to design polypropylene chains with long and short branching to obtain a non-linear behaviour for the synthesis of low-density foams (<100kg/m 3 ) using batch scCO2 foaming process. From a maleic anhydride grafted PP (PP-g-MA), the long-branched chains are performed using [MA]/ [OH] reaction (with Sorbitol alcohol). On the other hand, short branched chains are made from hydrolysis/condensation of amino-alkoxysilane (3-Aminopropyl-triethoxysilane, APTES) in situ grafted onto PP chains through the ([MA]/NH2] reaction). Both long and short branching are considered separately and then combine. Our results show the need to combine both systems to reach the optimum of the rheological behaviour to obtain a low-density foam. This also demonstrate that the rheological behaviour under linear and non-linear conditions is relevant to determine the best formulation in terms of critical impact on the foaming behaviour.On the other hand, the foaming behaviour of PP/PA6 polymer blends at T=180°C was also investigated as another way to control the rheology and the foamy behaviour. At this temperature, T=180°C, the PA6 phase behaves as solid dispersed particles in a viscoelastic liquid PP matrix. Finally, this study shows the influence of PA6 concentration on the foaming and foam morphology correlated to a rheological study (shear and extensional) and to the mechanical compressive properties.

The objective of this study is to investigate the ability of thermoplastic vulcanizates (TPVs) (materials based on PP/EPDM blend) to foam under CO2 batch conditions. The EPDM phase, which is dispersed into the PP phase, was dynamically crosslinked either by a phenolic resin (Resol) or by a radical peroxide (dicumyl peroxide). The results show an influence of the crosslinking chemistry on the extensional viscosity of the TPV. Regarding radical chemistry, the peroxide induces polypropylene degradation by β-scission reaction during the dynamic crosslinking process. As a result, the ability of the TPV to deform under extensional flow (Hencky deformation at break <0.5) is greatly reduced. On the contrary, the Resol-based TPV has demonstrated a non-linear viscosity behaviour (strain hardening) and a great ability to deform (Hencky deformation at break >1.5). This unexpected result for a non-homogeneous system can be explained by the confinement of the PP phase between EPDM nodules which gives to the PP chains a gel rheological behaviour. In addition, the influence of the addition of carbon black filler has also been studied. Finally, the relationship between extensional viscosity and physical foaming has been investigated. As for a homogeneous polymer, the extensional viscosity has been proved to be a key factor to estimate the foaming behaviour of complex systems like TPV. Hence, the importance of non-linear viscosity for a multi-phasic polymer to ensure foaming ability has been demonstrated.