Showing papers by "Pierre J. Carreau published in 2009"

Rheological properties and percolation in suspensions of multiwalled carbon nanotubes in polycarbonate

Abstract: This paper is concerned with several issues related to the rheological behavior of polycarbonate/multiwalled carbon nanotube nanocomposites. The composites were prepared by diluting a masterbatch of 15 wt.% nanotubes using melt-mixing method, and the dispersion was analyzed by SEM, TEM, and AFM techniques. To understand the percolated structure, the nanocomposites were characterized via a set of rheological, electrical, and thermal conductivity measurements. The rheological measurements revealed that the structure and properties were temperature dependent; the percolation threshold was significantly lower at higher temperature suggesting stronger nanotube interactions. The nanotube networks were also sensitive to the steady shear deformation particularly at high temperature. Following preshearing, the elastic modulus decreased markedly suggesting that the nanotubes became more rigid. These results were analyzed using simple models for suspensions of rod-like particles. Finally, the rheological, electrical, and thermal conductivity percolation thresholds were compared. As expected, the rheological threshold was smaller than the thermal and electrical threshold.

Modeling fiber interactions in semiconcentrated fiber suspensions

Abstract: A set of rheological equations is developed for semiconcentrated suspensions of rigid fibers in a Newtonian fluid taking into account hydrodynamic and fiber-fiber interactions. The force generated by the fiber interactions is modeled using a linear hydrodynamic friction coefficient proportional to the relative velocity at the contact point, and weighted by the probability for contacts to occur. The equation of evolution of the second-order orientation tensor, containing advection and diffusion terms due to fiber interactions, is derived to predict fiber orientation under flow. The well known fourth-order orientation tensor, related to the hydrodynamic contribution, and a newly proposed fourth-order interaction tensor are used to evaluate the total stress in the composite. A linear and a quadratic closure approximation are proposed to describe the fourth-order interaction tensor. Results are presented using the quadratic form, which is found to be more accurate than the linear one. The model is shown to describe well simple shear data of suspensions of glass fibers in a Newtonian polybutene. Moreover, fiber orientation and the average number of contacts per fiber are predicted. The newly proposed interaction coefficient varies with fiber orientation, which appears to be realistic.

Rheological behavior of fiber-filled model suspensions: Effect of fiber flexibility

Abstract: The rheological behavior of model suspensions consisting of a Newtonian silicone oil and fibers of different flexibilities have been investigated in steady and transient shear flows. Various fiber suspensions have been prepared to examine the effect of flexibility parameters (stiffness and aspect ratio) as well as the role of interactions in the semi-dilute and semi-concentrated regimes. The viscous and elastic properties of the fiber suspensions are shown to be strongly enhanced by fiber flexibility. With increasing flexibility, the suspensions also exhibit enhanced shear-thinning and the shear rate for the onset of shear-thinning decreases. In start-up flow and especially at low shear rate, the suspensions show large stress overshoots, and under reversal flow delayed overshoots. The magnitude of the overshoot increases as fiber flexibility gets larger. This last effect has been attributed to less orientation in the flow direction during the forward flow for the more flexible fibers. Finally, the effects are more pronounced in the semi-concentrated regime, and this is attributed to enhanced fiber-fiber interactions.

Structural analysis of non-aqueous layered silicate suspensions subjected to shear flow

Abstract: Non-aqueous layered silicate suspensions exhibit a complex rheological behavior due to a multiple length scale structure, which is sensitive to flow and flow history. In the present work, the nature of flow-induced non-equilibrium and metastable structures in non-aqueous layered silicate suspensions based on natural and organo-modified sodium montmorillonites was examined using rheometry and confocal laser scanning microscopy (CLSM). The scaling behavior of their linear and non-linear viscoelastic properties was investigated. Based on fractal scaling theories, the scaling laws of the solid-like properties were ascribed to the presence of space-filling percolating networks consisting of clusters with a mass-fractal dimensionality, Df∼2. CLSM allowed us to detect the formation of aggregates under flow and to characterize their microscopic length scale. The shear-rate dependency of the microstructure characteristic length scale was attributed to a reversible shear-induced aggregation process. Upon cessation of flow, the observed thixotropic behavior of these suspensions was inferred from the CLSM observations to stem from local rearrangements at the nano-scale.

Investigation of the rheological properties of short glass fiber-filled polypropylene in extensional flow

Abstract: The behavior of short glass fiber–polypropylene suspensions in extensional flow was investigated using three different commercial instruments: the SER wind-up drums geometry (Extensional Rheology System) with a strain-controlled rotational rheometer, a Meissner-type rheometer (RME), and the Rheotens. Results from uniaxial tensile testing have been compared with data previously obtained using a planar slit die with a hyperbolic entrance. The effect of three initial fiber orientations was investigated: planar random, fully aligned in the stretching flow direction and perpendicular to it. The elongational viscosity increased with fiber content and was larger for fibers initially oriented in the stretching direction. The behavior at low elongational rates showed differences among the various experimental setups, which are partly explained by preshearing history and nonhomogenous strain rates. However, at moderate and high rates, the results are comparable, and the behavior is strain thinning. Finally, a new constitutive equation for fibers suspended into a fluid obeying the Carreau model is used to predict the elongational viscosity, and the predictions are in good agreement with the experimental data.

Effect of PVDF characteristics on extruded film morphology and porous membranes feasibility by stretching

Abstract: Three different polyvinylidene fluoride (PVDF) resins were selected to develop porous membranes through melt extrusion and stretching. The effect of the polymer rheology on chain elongation in the melt state was studied. The possibility of generating a row-nucleated lamellar crystallization for precursor films was investigated. The arrangement and orientation of the crystalline phase were examined by wide angle X-ray diffraction (WAXD) and Fourier Transform Infrared Spectroscopy (FTIR). The extrusion conditions and the blend compositions were adjusted to obtain uniform precursor films with appropriate morphology. Annealing, cold and hot stretching were consequently employed to generate and enlarge the pores. It was found that a proper crystalline structure of the precursor films was strongly dependent on molecular weight of PVDF and process conditions. Blending of two PVDF resins having low and high molecular weights improved the water vapor permeability of the obtained membranes. The tensile response was monitored during the stretching process for membrane development and the results revealed a distinct behavior for the membranes having low or high permeability. The membranes with low permeability did not show any significant strain hardening during stretching whereas for highly permeable membranes, a noticeable strain hardening behavior was observed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1219–1229, 2009

Anisotropy of nonaqueous layered silicate suspensions subjected to shear flow

Abstract: Nonaqueous layered silicate suspensions have a complex rheological behavior due to the presence of a microstructure on multiple length scales, which is sensitive to flow and flow history. In the present work, the flow-induced orientation and anisotropy of the nonequilibrium metastable structures in nonaqueous layered silicate suspensions has been studied using a combination of light scattering, scattering dichroism, and advanced rheometric measurements, including two dimensional small amplitude oscillatory shear (2D-SAOS) flow experiments. The nature of the structures during flow was mainly studied by means of small angle light scattering patterns. Linear dichroism measurements in the vorticity and velocity gradient directions were used to assess the microstructural anisotropy. The changes observed in the vorticity plane developed in the same range of shear rate as the shear-thinning behavior of the suspensions. Scattering dichroism was used to demonstrate that the flow-induced anisotropy was locked in up...

Effect of Machine Direction Orientation Conditions On Properties of Hdpe Films

Abstract: In this study, the effects of machine direction orientation (MDO) conditions on crystalline orientation, morphology, and mechanical performance of a high-density polyethylene film were investigated. The MDO variables studied were feed speed, stretching temperature, draw ratio (DR) as well as annealing temperature. The type of crystals (spherulites or fibrils) and crystallinity of the films were considered using differential scanning calorimetry. The MDO process increased the film crystallinity by 18% and melting temperature (Tm ) by 5°C. The crystalline phase orientation was measured using Fourier transform infrared and wide angle X-ray diffraction. Stretching noticeably increased the crystalline alignment. The Young’s modulus decreased with increasing extrusion temperature and feed speed and enhanced with rising annealing temperature. However, up to DR = 6, the Young’s modulus decreased with draw ratio whereas it increased beyond DR = 6. Upon drawing, the tear resistance along the machine direction decre...

Heat transfer to newtonian and non‐newtonian liquids in a screw agitator and draft coil system

Abstract: Heat transfer has been investigated for several Newtonian and non-Newtonian liquids mixed in a flat bottomed vessel equipped with a screw agitator and a coil acting simultaneously as draft tube and heat exchanger. Heat transfer rates from coil to liquid were determined for different coil designs, rheological properties and operating conditions in the heating and cooling modes. A circulation Reynolds number is defined, the characteristic length being the effective height of the heat exchanger (the coil) and the characteristic velocity being the circulation velocity of the fluid which was determined experimentally. Using this Reynolds number, it was possible to establish a single correlation for the Nusselt number as a function of Reynolds and Prandtl numbers. Geometric ratios do not appear in this correlation which could adequately represent more than 200 experimental data. A similar single correlation could not be obtained when using the conventional mixing Reynolds number. This novel system is shown to be very efficient for handling rheologically complex fluids. On a etudie le transfert de chaleur pour plusieurs liquides newtoniens et non-newtoniens agites dans un systeme consistant en un recipient a fond plat muni d'un agitateur a vis et d'un serpentin servant a la fois de tube de circulation et d'echangeur de chaleur. On a determine les taux de transfert thermique entre serpentin et liquide pour une gamme de geometries du serpentin, de proprietes rheologiques et de conditions d'operation en chauffage et en refroidissement du liquide. Un nombre de Reynolds pour la circulation du fluide a ete defini dans lequel la longueur caracteristique est la hauteur effective de l'echangeur (le serpentin) et la vitesse caracteristique est la vitesse de recirculation du fluide determinee experimentalement. Il a ete possible d'obtenir une seule equation donnant le nombre de Nusselt en fonction du nombre de Prandtl et de ce nouveau nombre de Reynolds. Cette correlation qui represente de facon tres satisfaisante plus de 200 resultats experimentaux ne contient pas de rapport geometrique. Par contre, il n'a pas ete possible d'etablir une relation unique de ce type en utilisant le nombre de Reynolds de melange conventionnel. On montre que ce nouveau systeme est tres efficace pour traiter les fluides de rheologie complexe.