Caractérisation rhéologique de bitumes 70/100 utilisé comme matrice de confinement de déchets radioactifs

National audienceL'objectif de cet article est une étude rhéologique approfondie du bitume pur 70/100. Le bitume étant constitué de deux composants majeurs, les asphaltènes dispersés dans une matrice maltène, l'évolution de la microstructure du bitume en fonction de la température rend l'applicabilité du Principe d'Equivalence Temps-température (PETT) incertaine. Deux formes de représentation du PETT seront présentées et l'applicabilité du principe discutée. Le profil d'évolution de la viscosité en fonction du taux de cisaillement et un test de créneaux de contraintes confirment la présence d'un seuil d'écoulement, inférieur à 1 Pa, qui semble thermodépendant en dessous de 50°C. En appliquant la règle de Cox-Merz, une courbe maîtresse de viscosité peut être obtenue sur une très large gamme de taux de cisaillement pour les températures supérieures à 50°C. Elle est modélisée avec précision par une loi de Carreau-Yasuda à seuil

Impact de l'irradiation γ sur les propriétés rhéologiques et chimiques du bitume

International audienceL'objectif de cet article consiste à évaluer et simuler les effets de l'irradiation sur les bitumes, en particulier en terme de propriétés physico-chimiques structurales et rhéologiques. Pour cela, des irradiations γ externes ont été effectuées à différentes doses d'irradiations, allant de 1 à 7 MGy. Une augmentation de la viscosité en cisaillement et du seuil d'écoulement avec la dose a été observée. De même, les modules élastiques et visqueux (G' et G'') augmentent avec la dose, avec une augmentation plus marquée pour G', ce qui traduit un renforcement du caractère élastique, induit par les réticulations résultant de l'irradiation. Par ailleurs, un plateau à basse fréquence apparaît sur G', traduisant ainsi un comportement pseudo-solide (G' ≈ G'') et conduisant à l'augmentation de la viscosité complexe. La réticulation du bitume, induisant son renforcement, a été confirmée par diverses techniques. L'augmentation de la température de transition vitreuse (Tg), est observée par DSC modulée. Une augmentation des fractions aromatiques, concomitante avec une décroissance des fractions aliphatiques, a été observée par chromatographie d'exclusion stérique (SEC), spectroscopie infrarouge (FT-IR) et diffraction de rayons X (DRX)

Évolution des propriétés rhéologiques des enrobés bitume, vers une loi vieillissement/viscosité

This work is a contribution to the understanding of rheological properties of bitumen and their evolution under gamma irradiation. The prediction of swelling ratio is necessary to evaluate the state of the containers, particularly during the reversibility phase of the storage. The objective of this work is thus to establish the rheological data its evolution under irradiation in order to predict the container swelling with time. After the rheological and thermal characterization of pure bitumen, a series of extrusion trials has been carried out. The state of dispersion essentially depends on the ratio N/Q (screw speed to feed rate). Extreme values of N/Q show the highest yield stress, indicating an improved dispersion state. The industrial bitumen compound exhibits the highest viscosity compared to model compounds, which limits bubble migration. Finally, the effect of gamma irradiation on pure bitumen and compounds behaviour has been studied. External and structural modifications have been evidenced. A theoretical model based on Krieger-Dougherty equation has been developed and shows a good agreement with experimental data.Cette thèse est un apport à la compréhension de l'évolution des propriétés rhéologiques des enrobés bitume soumis à une irradiation γ. La prédiction du gonflement suivant la composition radiologique d'un enrobé est nécessaire pour évaluer l'intégrité des colis, en particulier durant la phase de réversibilité du stockage (durée séculaire). Le but de cette thèse est donc d'acquérir les données expérimentales rhéologiques et leur évolution sous irradiation pour in fine prédire l'évolution dans le temps des colis de bitume en terme de gonflement. C'est la raison pour laquelle une stratégie a été mise en place. Après avoir caractérisé le comportement du bitume exempt de sels grâce aux mesures rhéologiques et thermiques, une série d'extrusion d'enrobés modèles a été préparée. En extrusion bivis, l'état de dispersion dépend fortement des conditions opératoires (vitesse N et débit Q) et principalement du rapport N/Q ; des valeurs extrêmes de N/Q montrent un seuil d'écoulement élevé qui est un indicateur de l'amélioration de l'état de dispersion. Par ailleurs, l'enrobé industriel montre la viscosité la plus importante par rapport aux enrobé modèles, ce qui limite la migration des bulles. Une grande partie de ce travail a été consacrée à l'effet de l'irradiation γ sur le comportement du bitume et des enrobés. Des modifications externes et des changements structuraux ont lieu sous irradiation. A la fin de ce manuscrit, un modèle basé sur l'expression de Krieger-Dougherty a été établi et qui montre une très bonne adaptation aux résultats expérimentaux

Influence of Extrusion Conditions on the Rheological Behavior of Nuclear Bituminized Waste Products

International audienceIn France, bitumen has been used since four decades as a container material for low/intermediate activity and long lifetime radionuclides. Industrial Bituminized Waste Products (BWP) are produced by continuous extrusion of 60 wt.% bitumen and 40 wt.% salts, in which a very small fraction is radioactive. These incorporated salts are, in majority, of two types: soluble or insoluble. During storage, the organic matrix subjected to irradiation produces gases, which can give rise to BWP swelling in the container and then to a possible overflow. The extent of gas production is directly linked to the degree of dispersion of the salts into the bitumen matrix. To quantify this dispersion, the effect of extrusion operating conditions, such as feed rate and screw speed, have been studied using a laboratory scale co-rotating twin screw extruder. BWP rheological characterization shows the development of a pseudo-solid plateau at low frequency and a change in the linear viscoelastic domain when screw speed and feed rate are varied, indicating a modification of the dispersion state. Samples characterization with environmental scanning electron microscopy (ESEM) confirms in the case of insoluble and soluble salts a change in the number of agglomerates, accompanied by an improvement of the dispersion at high screw speed but less remarkable with the feed rate. A Carreau-Yasuda law with a yield stress is proposed to describe the theological behaviour of these materials. The effects of volume fraction and maximum volume fraction on viscosity are described using a Krieger-Dougherty equation

Influence of thermomechanical history on chemical and rheological behavior of bitumen

International audienceIt is well-known that asphaltene content plays an important role in determining the high viscosity of bitumen. This paper presents an experimental study of the specific effects of extrusion operating conditions on the physical and chemical properties of bitumen. Five bitumen samples were prepared by twin screw extrusion with different operating conditions (feed rate Q and screw speed N). Physical properties were studied by rheological measurements. Viscosity values were measured by steady state flow tests. Chemical changes in the bitumen structure were followed in the infrared region with attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy by measuring the evolution of the bands areas at 1700 cm-1 (C=O), 1030 cm-1 (S=O), and 1600 cm-1 (aromatic C=C) and the bands between 900 and 730 cm-1 attributed to aromatic C-H. An increase in feed rate Q induces a decrease in the Newtonian viscosity, as a result of a decrease in the asphaltene volume fraction. The characterization by ATR confirms that the decrease in feed rate entails the creation of C=O functional groups and the increase in sulfoxide (S=O) functional groups and C=C bonds, accompanied by a decrease in the C-H aromatic bonds. These results indicate a structure that is more oxidized and more aggregated at low feed rate, certainly as the result of an increase in the residence time into the extruder. The increase in screw speed also induces decreases in the viscosity and the volume fraction of asphaltenes, until a point after which the situation reverses. This change may be explained by the appearance of new peaks between 1200 and 1050 cm-1, attributed to C=S bonds, and between 640 and 540 cm-1, for S-S bonds. A competition between shear rate and residence time takes place. The thermomechanical history has, thus, a great influence on the chemical and rheological behavior of pure bitumen, and the chemical changes observed show that the asphaltene volume fraction is not the unique parameter that explains the variations in viscosity

Correlation between thermal and rheological studies to characterize the behavior of bitumen

International audienceThe objective of this work is a comprehensive thermo-rheological study of pure bitumen. The bitumen is a complex material consisting of asphaltenes dispersed in a maltene matrix. As a consequence, its flow behavior is characterized by the presence of a yield stress, which depends on temperature below 50°C. Applying the Cox-Merz rule, a master curve of viscosity can be obtained over a wide range of shear rates for temperatures above 50°C. It can be accurately modeled by a Carreau-Yasuda law with a yield stress. This specific rheological behavior can be explained by the changes induced by the temperature on the microstructure, evidenced by modulated differential scanning calorimetry measurements

Effect of γ irradiation on nuclear bituminized waste products (BWP): X-ray microtomography and rheological characterization

International audienceIn the last 50 years bitumen has been widely used in France as a matrix for embedding low- and intermediate-level radioactive waste. This organic matrix is subjected to α, β and γ radiation generating radiolysis gases, which consist mainly of hydrogen but also some carbon oxide and lighter hydrocarbons. The gases form bubbles whose growth entails swelling of the bitumen if the radiolytic process of gas generation is more important than the removal process. To assess the influence of γ radiation on the swelling and rheological behavior of the bitumen matrix, external γ irradiation experiments were performed on pure bitumen (PB) and on a model synthesized bituminized waste product (BWP: 60. wt% bitumen + 40. wt% salts). Greater swelling was observed for the BWP samples than for PB. X-ray microtomography shows that the hydrogen bubbles are more numerous in the case of BWP, indicating that the salts may be considered as preferential nucleation sites. Moreover, the rheological characterization of the samples shows that the Newtonian viscosity is higher for the BWP model sample, and that the irradiation enhances this phenomenon. In the case of accelerated irradiation (dose rate exceeding the actual dose rate), swelling increases with the irradiation dose since the gas production rate is higher than the removal process until a plateau is reached at approximately 4-5. MGy, indicating an equilibrium between gas production and release. Finally, the impact of the dose rate on the number, size and shape of bubbles and on macroscopic swelling is discussed on the basis of X-ray microtomography results

Rheological, structural and chemical evolution of bitumen under gamma irradiation

International audienceBitumen derived from crude oil by fractional distillation has been used in the nuclear industry as a radioactive waste encapsulation matrix. When subjected to α, β and γ self-irradiation, this organic matrix undergoes radiolysis, generating hydrogen bubbles and modifying the physical and chemical properties of the material. In this paper, the effects of irradiation on bitumen materials, especially in terms of its physical, chemical, structural and rheological properties, were characterized at radiation doses ranging from 1 to 7 MGy. An increase in the shear viscosity and melt yield stress was observed with increasing doses. Similarly, the elastic and viscous moduli (G′ and G″) increase with the dose, with a more pronounced increase for G′ that reflects enhanced elasticity arising from radiation-induced cross-linking. In addition, a low-frequency plateau is observed for G′, reflecting pseudo-solid behavior and leading to an increase of the complex viscosity. This behavior is due to increased interactions between asphaltene particles, and to aromatization of the bitumen by γ-radiations. Cross-linking of bitumen enhances its strength, as confirmed by various techniques (modulated DSC, DTA/TGA, SEC, FTIR and XRD)