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

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

Dispersion de charges agglomérées dans une matrice polymère sous l'action d'un champ de cisaillement

International audienceUne cellule contra-rotative transparente couplée à un microscope optique a été utilisée pour étudier les mécanismes de dispersion d'une charge agglomérée dans une matrice soumise à un cisaillement. Le comportement d'un noir de carbone et de deux silices de précipitation dans une matrice élastomère commune est comparé. Différents mécanismes de dispersion sont observés en fonction des caractéristiques de la charge. L'origine de ces différences de comportement mécanique est discutée

Ecoulement dans une géométrie plan-plan confinée: validité de la géométrie pour l'étude de fluides viscoélastiques et application à la dispersion du noir de carbone

Congrès Annuel de la société Européenne de Rhéologie (AERC 2010), Göteborg , Suède (07/04/2010)International audienceSi avoir un écoulement stable est une condition nécessaire pour faire des mesures de rhéologie en régime continu, il en est de même lors d'études rhéo-optiques où des observations d'évolution de structure ou de comportement de particules ou inclusions en suspension dans une matrice sont réalisées in-situ pendant le cisaillement. Dans cet article, nous montrons comment l'installation d'une bague de confinement autour d'une cellule de cisaillement plan-plan contra-rotative transparente permet de réduire le développement d'instabilités d'écoulement dans une matrice polymère viscoélastique pendant le cisaillement, sans modifier l'écoulement sur une zone relativement large de la cellule. Des mesures de périodes de rotation de billes de verre et de rhéologie en régime continu ont été effectuées pour montrer la validité de cette géométrie d'écoulement. Cette cellule a été utilisée pour déterminer les conditions de dispersion d'une charge de noir de carbone dans une matrice polymère sous l'action d'un champ de cisaillement. La détermination des conditions de rupture d'un noir de carbone en suspension dans une matrice polymère dans deux géométries d'écoulement différentes (avec et sans bague de confinement) permet de montrer que le critère de rupture s'exprime bien en terme de contrainte hydrodynamique. L'utilisation de la bague a permis d'atteindre des taux de cisaillement plus élevés dans des conditions stables d'écoulement

Numerical and experimental study of dispersive mixing of agglomerates

Best paper 2006 award in the Extrusion divisionInternational audienceThe degree of filler dispersion has a major influence on the physical properties of rubber compounds. Typical fillers, e.g. carbon black and silica, are difficult to disperse, particularly if they are fine and low structured. As a result, the quantity of undispersed fillers generally amounts for 1% to 10% of the compound. The elimination, or at least the reduction, of agglomerates will result in rubber parts (e.g. tires, seals, belts) with improved properties and higher reliability. Clearly, a better understanding of the physics of batch mixers would help improve their mixing performance. Due to the complexity of the real process, experiments on a representative device were held from which a model has been deduced. It appears to be a generalization of the law of Kao and Mason, but for high viscous matrices. The next step was to get a model available for statistically large number of pellets as can be found in any sample taken out of the mixer. A statistical approach is used where we define a model describing the evolution of mass density function of agglomerate sizes. Eventually, we implement this model within available numerical simulation tools to estimate dispersion in real mixers

Analysis of cellular responses of macrophages to zinc ions and zinc oxide nanoparticles: a combined targeted and proteomic approach

Two different zinc oxide nanoparticles, as well as zinc ions, are used to study the cellular responses of the RAW 264 macrophage cell line. A proteomic screen is used to provide a wide view of the molecular effects of zinc, and the most prominent results are cross-validated by targeted studies. Furthermore, the alteration of important macrophage functions (e.g. phagocytosis) by zinc is also investigated. The intracellular dissolution/uptake of zinc is also studied to further characterize zinc toxicity. Zinc oxide nanoparticles dissolve readily in the cells, leading to high intracellular zinc concentrations, mostly as protein-bound zinc. The proteomic screen reveals a rather weak response in the oxidative stress response pathway, but a strong response both in the central metabolism and in the proteasomal protein degradation pathway. Targeted experiments confirm that carbohydrate catabolism and proteasome are critical determinants of sensitivity to zinc, which also induces DNA damage. Conversely, glutathione levels and phagocytosis appear unaffected at moderately toxic zinc concentrations

Molecular responses of mouse macrophages to copper and copper oxide nanoparticles inferred from proteomic analyses

The molecular responses of macrophages to copper-based nanoparticles have been investigated via a combination of proteomic and biochemical approaches, using the RAW264.7 cell line as a model. Both metallic copper and copper oxide nanoparticles have been tested, with copper ion and zirconium oxide nanoparticles used as controls. Proteomic analysis highlighted changes in proteins implicated in oxidative stress responses (superoxide dismutases and peroxiredoxins), glutathione biosynthesis, the actomyosin cytoskeleton, and mitochondrial proteins (especially oxidative phosphorylation complex subunits). Validation studies employing functional analyses showed that the increases in glutathione biosynthesis and in mitochondrial complexes observed in the proteomic screen were critical to cell survival upon stress with copper-based nanoparticles; pharmacological inhibition of these two pathways enhanced cell vulnerability to copper-based nanoparticles, but not to copper ions. Furthermore, functional analyses using primary macrophages derived from bone marrow showed a decrease in reduced glutathione levels, a decrease in the mitochondrial transmembrane potential, and inhibition of phagocytosis and of lipopolysaccharide-induced nitric oxide production. However, only a fraction of these effects could be obtained with copper ions. In conclusion, this study showed that macrophage functions are significantly altered by copper-based nanoparticles. Also highlighted are the cellular pathways modulated by cells for survival and the exemplified cross-toxicities that can occur between copper-based nanoparticles and pharmacological agents

The opportunistic pathogen Pseudomonas aeruginosa activates the DNA double-strand break signaling and repair pathway in infected cells.

International audienceHighly hazardous DNA double-strand breaks can be induced in eukaryotic cells by a number of agents including pathogenic bacterial strains. We have investigated the genotoxic potential of Pseudomonas aeruginosa, an opportunistic pathogen causing devastating nosocomial infections in cystic fibrosis or immunocompromised patients. Our data revealed that infection of immune or epithelial cells by P. aeruginosa triggered DNA strand breaks and phosphorylation of histone H2AX (γH2AX), a marker of DNA double-strand breaks. Moreover, it induced formation of discrete nuclear repair foci similar to gamma-irradiation-induced foci, and containing γH2AX and 53BP1, an adaptor protein mediating the DNA-damage response pathway. Gene deletion, mutagenesis, and complementation in P. aeruginosa identified ExoS bacterial toxin as the major factor involved in γH2AX induction. Chemical inhibition of several kinases known to phosphorylate H2AX demonstrated that Ataxia Telangiectasia Mutated (ATM) was the principal kinase in P. aeruginosa-induced H2AX phosphorylation. Finally, infection led to ATM kinase activation by an auto-phosphorylation mechanism. Together, these data show for the first time that infection by P. aeruginosa activates the DNA double-strand break repair machinery of the host cells. This novel information sheds new light on the consequences of P. aeruginosa infection in mammalian cells. As pathogenic Escherichia coli or carcinogenic Helicobacter pylori can alter genome integrity through DNA double-strand breaks, leading to chromosomal instability and eventually cancer, our findings highlight possible new routes for further investigations of P. aeruginosa in cancer biology and they identify ATM as a potential target molecule for drug design

Enhanced susceptibility of T lymphocytes to oxidative stress in the absence of the cellular prion protein.: PrPC and oxidative stress in T lymphocytes

International audienceThe cellular prion glycoprotein (PrP(C)) is ubiquitously expressed but its physiologic functions remain enigmatic, particularly in the immune system. Here, we demonstrate in vitro and in vivo that PrP(C) is involved in T lymphocytes response to oxidative stress. By monitoring the intracellular level of reduced glutathione, we show that PrP(-/-) thymocytes display a higher susceptibility to H(2)O(2) exposure than PrP(+/+) cells. Furthermore, we find that in mice fed with a restricted diet, a regimen known to increase the intracellular level of ROS, PrP(-/-) thymocytes are more sensitive to oxidative stress. PrP(C) function appears to be specific for oxidative stress, since no significant differences are observed between PrP(-/-) and PrP(+/+) mice exposed to other kinds of stress. We also show a marked evolution of the redox status of T cells throughout differentiation in the thymus. Taken together, our results clearly ascribe to PrP(C) a protective function in thymocytes against oxidative stress