Structuration de nanocomposites polyoléfine/argile lamellaire en extrusion bivis corotative

International audienceLa réalisation de nanocomposites par mélange d'argile lamellaire dans une matrice polymère à l'état fondu se fait généralement par extrusion bivis corotative. L'objectif de cette opération est de disperser les agglomérats d'argiles, d'exfolier les feuillets et de distribuer ces charges d'épaisseur nanométrique au sein de la matrice. La forte surface spécifique des argiles permet ainsi de développer largement les interactions polymère/argile. Dans le cas des polyoléfines, un troisième élément doit être ajouté en tant que compatibilisant entre la matrice apolaire et la surface polaire des argiles. Le procédé d'extrusion bivis peut être adapté, selon le résultat recherché, via de nombreux paramètres : profil de vis, vitesse de rotation, débit d'alimentation des doseurs, températures de régulation le long du fourreau. Dans ce travail, nous avons étudié l'effet de ces paramètres procédé sur la structure des nanocomposites aux échelles micrométrique et nanométrique. Une formulation a été choisie en fixant les proportions entre la matrice, le compatibilisant et l'argile. Des échantillons ont été prélevés en sortie de machine et le long des vis pour chaque condition opératoire testée. Le niveau de dispersion de l'argile est évalué en utilisant différentes techniques de caractérisation (MEB, MET, DRX, rhéométrie dynamique). Les résultats montrent que l'augmentation de l'énergie mécanique spécifique (EMS) entraîne une diminution du nombre et de la taille des agglomérats présents au niveau de la microstructure, ainsi qu'un phénomène d'exfoliation plus important au niveau de la nanostructure, jusqu'à atteindre un niveau de saturation de l'état de dispersion de l'argile

Rhéologie de farines plastifiées et morphologies de leurs mélanges avec du PBAT

National audienceDeux farines ont été transformées en mélangeur interne avec l'ajout d'une quantité d'eau constante et deux taux de glycérol. Les farines transformées ont fait l'objet d'une étude rhéologique en mode dynamique. L'une d'elles présente le comportement d'un gel, alors que l'autre s'apparente plus à un fluide viscoélastique. La variation du taux de glycérol ne fait qu'abaisser les viscosités et les modules des farines transformées, mais ne change pas le type de comportement rhéologique. Après cette phase de transformation, une partie de chaque farine a été mélangée avec du polybutylène-adipate-téréphtalate (PBAT). Les morphologies et les comportements rhéologiques des mélanges ont été analysés. Nous essayons de mettre en relation les comportements rhéologiques mesurés et les morphologies observées. Il apparaît que la morphologie du mélange dépend plus du type de comportement rhéologique des farines, gel ou fluide, que de leur niveau de viscosité. Par ailleurs l'évolution des morphologies des mélanges avec le taux de plastifiant est différente en fonction du comportement rhéologique des farines

Comportement rhéologique de nanocomposites à matrice polymère

National audienceDans cette étude, nous avons caractérisé le comportement rhéologique de nanocomposites à base de polypropylène et d'argile lamellaire, compatibilisés avec un polypropylène greffé anhydride maléique. Nous avons étudié l'influence du taux de compatibilisant et du taux d'argile. Le niveau d'exfoliation est quantifié par la valeur du seuil d'écoulement déduit des mesures rhéologiques. Son évolution avec le taux d'argile permet de définir un seuil de percolation de l'ordre de 1,5% en masse

In situ characterisation of dispersion processes of silica in an elastomer matrix under shear, impact of a filler treatment

International audienceThe dispersion of a filler immersed in a polymer flow submitted to shear is the result of the counterbalance between hydrodynamic forces acting on the agglomerate and its cohesive forces. The nature of the filler, its surface treatment or its degree of infiltration by the polymer should impact its cohesivity. To better understand the influence of these parameters on dispersion, direct observations of dispersion mechanisms under shear were carried out on raw silica, silica treated with a covering agent and fully infiltrated silica agglomerates. Dispersion mechanisms and critical stresses for dispersion were determined and found to be different in those three cases

The importance of specific mechanical energy during twin screw extrusion of organoclay based polypropylene nanocomposites

International audienceThis study demonstrates how the specific mechanical energy (SME) can be used to describe the influence of extrusion parameters such as screw rotation speed, feed rate and barrel temperature on clay dispersion in organoclay (OMMT) based polypropylene nanocomposites. These materials were prepared by a melt mixing masterbatch process via twin screw extrusion with a wide range of processing conditions. Maleated polypropylene (PP-g-MA) was used as a compatibilizer to allow clay exfoliation. Characterization of the morphological evolution along the extrusion profile revealed that microscale dispersion primarily happens in the melting zone, whereas continuous exfoliation is observed all along the kneading zones, up to the die exit. The results indicate that exfoliation in the kneading zones is mainly issued from clay tactoids and small aggregates with characteristic size inferior to 10. μm, emphasizing the crucial role of primary microscale dispersion on the final structure and properties of the nanocomposites. Relevant quantitative prediction of the multiscale dispersion state along the extrusion profile was obtained using the melt state SME as unique paramete

Time-evolution of the structure of organoclay/polypropylene nanocomposites and application of the time/temperature superposition principle

International audienceWe investigated the rheological properties of nanocomposites composed of polypropylene, organoclay, and maleic anhydride grafted polypropylene in small amplitude oscillatory shear. Samples were prepared in two steps: a masterbatch was first obtained by melt extrusion and then diluted into polypropylene using an internal mixer. Three formulations were investigated. The measurement of the storage and loss moduli evolution with time showed that these materials were not stable: the nanostructure obtained after steady shear continuously changed with time, due to the disorientation of the clay platelets and the build-up of a 3D network. The kinetics of the structure build-up (followed via the melt yield stress) showed a two-step process. This feature was found to be valid whatever the nanocomposite formulation. Such evolution of the structure is generally assumed to violate the time-temperature superposition principle. We demonstrate in this paper that the time-temperature equivalence always exists if the same nanostructure is probed. This was achieved by using different annealing times for different temperatures or annealing the samples at the highest temperature before measuring at lower values. The time-temperature equivalence evidences that the temperature does not induce any chemical change within the material, whose properties remain governed by the same physical phenomena

New insights in dispersion mechanisms of carbon black in a polymer matrix under shear by rheo-optics

International audienceThe dispersion behavior of different carbon black grades was investigated in a common elastomer matrix under shear using a transparent plate-and-plate counter-rotating shear cell coupled with an optical microscope. The objective was to investigate the effect of carbon black intrinsic characteristics (specific area and structure) on dispersion mechanisms. Shear conditions were selected to study independently erosion and rupture mechanisms. The independent study of rupture and erosion mechanisms brings new findings on the respective effect of the filler intrinsic characteristics on each mechanism: (a) Rupture is a sudden mechanism occurring above a critical shear stress, which depends on the pellet size. The rupture criterion appears not to depend on the carbon black specific area or the structure. (b) Erosion proceeds via the detachment of a fixed eroded volume per strain unit and is driven by the applied shear stress and strain. Erosion is a local mechanism. The erosion rate depends on the carbon black characteristics. Faster erosion was measured for a carbon black with a higher structure at equivalent specific area or with a lower specific area at equivalent structure. This in situ characterization of dispersion mechanisms highlights that the effect of the carbon black characteristics on the two main dispersion mechanisms (rupture and erosion) is completely different

Influence of twin-screw processing conditions on structure and properties of polypropylene - Organoclay nanocomposites

International audienceThis study looks at the influence of extrusion parameters such as screw speed, feed rate and barrel temperature on the nanocomposite structure (size of agglomerates, level of intercalation and exfoliation) and its consequences on final mechanical properties. Nanocomposites of polypropylene, maleated polypropylene and organomodified montmorillonite, with respective mass fractions of 85/10/5, were prepared in a co-rotating twin-screw extruder using a masterbatch dilution method. The nanocomposites structure was quantified by scanning and transmission electron microscopy, X-ray diffraction and dynamic rheometry. Relationships between the microstructure at different levels (size and number of agglomerates, interlayer distance, melt yield stress to quantify the exfoliation level) and the processing conditions were established, revealing that specific mechanical energy received during extrusion was the key parameter controlling this microstructure. Mechanical properties in uniaxial tension (apparent Young's modulus) were measured and related to the microstructural parameters resulting from extrusion conditions

Characterization of dispersion mechanisms of agglomerated fillers in an elastomer matrix under shear by in-situ observations

International audienceThe mixing of carbon black or silica fillers and a polymer matrix in an internal mixer implies two processes: dispersion and distribution. To disperse implies to reduce the filler initial size (a hundred microns) down to the aggregate size (a few tens of nanometers). This size is necessary to ensure the reinforcement of the matrix. Although the mixing of a filler and a matrix is an usual operation, elementary mechanisms and key parameters responsible for the size reduction of the filler are not fully understood. The rheo-optical technique (counter-rotating shear cell coupled with an optical microscope) is an efficient technique to observe in-situ during shear the filler dispersion mechanisms. Kinetics and criteria of dispersion mechanisms such as erosion or rupture can be determined. The present work compares the shear-induced dispersion mechanisms and criteria of different grades of carbon black and silica in a common polymer matrix (styrene-butadiene rubber). The objective was to determine the role of the intrinsic parameters of the filler (specific area) on dispersion. This study shows differences in the dispersion mechanisms of silica and carbon black. The data will be discussed in terms of internal organisation of the fillers

Time-evolution of the structure of organoclay/ polypropylene nanocomposites and its influence on time/temperature superposition principle

International audienceWe investigate the rheological properties of a nanocomposite in small amplitude oscillatory shear. The evolution of the storage modulus with time shows that this material is not stable: the microstructure changes continuously with time, due to the disorientation of the clay platelets and the build-up of a 3D network. This evolution of the microstructure is evaluated by measuring the apparent yield stress. A two-step kinetics is evidenced, whose parameters directly depends on temperature. This time-evolution is generally assumed to violate the time-temperature superposition principle. However, we demonstrate that the time-temperature equivalence always exists if we consider samples having the same microstructure