International audienceA global computation model for self-wiping corotating twin screw extruders is proposed. Based on a 1D approximated approach, it has been validated by comparison with experimentation and more sophisticated numerical models. It allows one to obtain, for any screw profile including left-and right-handed screw elements and kneading discs, the profile along the screws of the main flow variables, such as pressure, mean temperature, residence time, and filling ratio. Owing to the approximations made, this model can be easily and rapidly run on a personal computer or a workstation. Important applications may be found in screw profile design, scaleup, compounding or reactive extrusion

International audienceIn the present work, experimental studies of the free-radical-initiated molecular weight degradation of polypropylene in a modular self-wiping corotating twin-screw extruder are investigated. The control of the molecular weight distribution of polypropylene resins by peroxide degradation is widely used in the polymer industry. It allows one to adjust the viscosity of these resins to the level required for processing applications. The purpose of this work was to characterize the influence of peroxide degradation on the rheological behavior of a polypropylene homopolymer and a block polypropylene/polyethylene copolymer, which includes an addition of a low percentage of polyethylene (around 7%). The homopolymer exhibits a classical behavior: When the peroxide amount is increased, we observe a decrease in the viscosity corresponding to a decreasing molecular weight and a pronounced shift toward more Newtonian behavior. The rheological behavior of the copolymer is influenced by the presence of the polyethylene phase which greatly modifies the viscoelastic properties and increases the viscosity when the polypropylene matrix is highly degraded

In this work, experimental and theoretical studies of the free‐radical initiated molecular weight degradation of polypropylene in a modular self‐wiping corotating twin screw extruder have been investigated. Our objective was to build a model that would be able to predict the evolution of the average molecular weight along the screws, in relation to the processing conditions and the geometry of the twin screw extruder. Modeling the process involves resolving interactions occurring between the various flow conditions encountered in the extruder, the kinetics of the reaction and the changes in viscosity with changes in molecular weight. We have studied the influence of operating parameters such as the initial peroxide concentration, the feed rate and the screw speed on the degradation reaction. Good agreement was found between theoretical results and experimental values obtained by size exclusion chromatography measurements.

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The control of the morphology of an immiscible polymer melt is of vital importance for the mastering of the final properties of the product. As polymer blends are produced using corotating twin‐screw extruders, understanding and modeling the changes experienced by the blend during this process is of great interest. In the present study, starting from Ludovic software, developed for computing flow parameters in the twin‐screw extrusion process, we present a computation of the droplet morphology development, based on the basic mechanisms of break‐up and coalescence. Depending on the value of a local capillary number and on local flow conditions, different changes may occur: affine deformation, drop splitting, break‐up by capillary instability, and coalescence. It is thus possible to follow, all along the screws, the changes in morphology, either for a single particle or for a particle distribution. Examples of these different computations are presented and compared with experimental results. Generally speaking, orders of magnitude of droplet size and tendencies when modifying processing conditions are correctly described, but the model still suffers from the absence of descrption of the melting process.

We studied the morphology of a reactive blend, constituted by an EVA/EMA phase dispersed in a polypropylene matrix. The dispersed phase was crosslinked in-situ during the extrusion through a transesterification reaction, catalysed by dibutyltin oxide. Rheological studies allowed to define the changes in rheological behaviour during the crosslinking reaction. Using a flow model developed for twin screw extruders, we computed the reaction extent along the screws and the corresponding blend morphology using a very simple approach. Theoretical results underestimate the extent of the reaction, but explain qualitatively the general tendencies when varying operating parameters.

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For original ovoid shaped artificial ventricles. a biomechanical double sac consisting of a biological sac (porcine pericardium) as the blood contact interface and a synthetic sac (Pebax 3533) as the mechanical support to assume systolic-diastolic dynamic constraints was conceived. The volumetric and mechanical properties were assessed with a three-dimensional modeling of Pebax sacs and computerized simulations of their systolic distortions for both right and left ventricular configurations. The stresses and strains of these sacs were represented as quantitative mappings for a maximum end-systolic state and were below the respective threshold values above which the Pebax material is jeopardized for permanent structure impairment. After fatigue tests applied on Pebax strips under the alleged working conditions of Pebax sacs, the material structure was unchanged and maintained its intrinsic mechanical properties. The theoretical maximum stroke volumes were 74.4 cm3 and 62.4 cm' for the left and right ventricular configurations, respectively. With these mechanical and volumetric features, the biomechanical double sac concept was considered valid and could be provided for a consequent specific total artificial heart. The acknowledged needs for artificial hearts (left ventricular assist devices and total artificial hearts [TAHs]) are induced by the increases in the numbers of patients suffering from end-stage congestive heart failure refractory to medical treatment and by the lack of cardiac grafts for heart transplantations (1-3). However, their common use to satisfy such clinical needs has not yet been possible because of persistent problems despite recent progress. For example, thromboembolic events, infectious complications, anatomical mismatches, and mechanical failures have often been reported for current models and ascribed to their design and/or their biomaterials (4-8) so that it appeared to be appropriate to explore and perfect new designs and biomaterials to resolve these problems (9,lO). For this purpose, a new design of artificial ventricles unique in their truncated ovoid shapes and biomechanical double