Showing papers by "Pierre J. Carreau published in 1999"

Non-linear viscoelastic behavior of fumed silica suspensions

Abstract: Suspensions of fumed silica exhibit a wide range of rheological properties depending on the nature and magnitude of the interparticle forces. In a non-polar fluid, the particles interact through hydrogen bonding and can form a three-dimensional network. The microstructure formation is responsible for the non-linear viscoelastic behavior of fumed silica suspensions, even at very small strain. These non-linear rheological properties have been studied in small amplitude oscillatory experiments as a function of particle size, surface treatment of particles, suspending medium polarity and solids concentration. The non-linear viscoelastic behavior is characterized by a non-sinusoidal waveform of the signal response. For suspensions in a non-polar fluid, both the elastic and the loss moduli are shown to be sensitive to the strain amplitude: the elastic modulus is decreasing with increasing strain whereas the loss moduli is initially increasing with strain. We have chosen to examine the dissipated energy which is clearly related to the breakup of the suspension structure. A comparison of model predictions and the experimental data shows the limitations of these models, recently proposed in the literature to describe the behavior of colloidal suspensions.

Rheological modeling of concentrated colloidal suspensions

Abstract: The use of concentrated colloidal suspensions is common in several industries such as paints, foodstuffs and pulp and paper. These suspensions are generally composed of strongly interactive particles. If the attractive forces dominate the repulsion and Brownian forces, the particles aggregate to form a three-dimensional network yielding a gel structure. Under flow, the micro-structure of suspensions can be drastically modified and the rheological properties are then governed by structure breakdown and build-up. In this work, we propose a structural network model based on a modified upper convected Jeffreys model with a single relaxation time and a kinetic equation to describe the flow-induced micro-structure evolution. Three distinct kinetic equations are tested for this purpose. The proposed model describes yield and thixotropic phenomena, nonlinear viscoelastic behavior and output signal distortions observed for relatively small strain amplitude during oscillatory measurements, and overshoots observed in stress growth experiments. A comparison of model predictions and experimental data for fumed silica and coating colors is also presented. However, different model parameters must be used to correctly predict the different flow properties indicating that a more versatile or generalized kinetic equation must be proposed.

Morphological evolution of immiscible polymer blends in simple shear and elongational flows

Abstract: The evolution of the morphology of the dispersed phase of immiscible polymer blends is studied in both shear and extensional flows. In the simple shear flow, predictions of the Lee and Park [H.M. Lee, O.O. Park, Rheology and dynamics of immiscible polymer blends, J. Rheol. 38 (1994) 1405–1425] and of the modified Lee and Park as well as the modified Grmela and Ait-Kadi [M. Grmela, A. Ait-Kadi, Comments on the Doi–Ohta theory of blends, J. Non-Newtonian Fluid Mech. 55 (1994) 191–195] models ([C. Lacroix, M. Grmela, P.J. Carreau, Relationships between rheology and morphology for immiscible molten blends of polypropylene and ethylene copolymers under shear flow, J. Rheol. 42 (1998) 41–62] are compared to stress growth data. The size of the dispersed phase is strongly affected by moderately finite strain imposed during the stress growth experiments. The morphological changes after stress relaxation are well predicted by the models for a polypropylene (PP), ethylene vinylacetate (EVA) and ethylene methylacrylate (EMA) blend [C. Lacroix, M. Grmela, P.J. Carreau, Relationships between rheology and morphology for immiscible molten blends of polypropylene and ethylene copolymers under shear flow, J. Rheol. 42 (1998) 41–62]. The prediction is less satisfactory for a polystyrene (PS)/polyethylene (PE) blend. The morphological evolution for the elongation flows of the PP/EVA/EMA blends has also been investigated. The morphologies of samples extracted before and after extrusion through an hyperbolic shaped (nozzle) die show that the elongational flow induces fibrillar structures. The extensional viscosity data, needed to solve the interface governing equations of the Lee and Park model, have been obtained from entrance pressure drop measurements. These equations are shown to qualitatively describe the transition from a spherical to a fibrillar morphology.

Rheological properties of concentrated suspensions

Abstract: Publisher Summary This chapter discusses various aspects related to the rheological behavior of concentrated suspensions. Concentrated colloidal suspensions are characterized by yield stresses, shear thinning and thixotropic behavior, and for small amplitude shear flow, the elastic moduli could be significantly larger than the loss moduli. The rheological behavior of a concentrated suspension is the result of various contributions that are very difficult to measure and accurately describe. When dealing with industrial suspensions, other complications may arise from uncontrollable parameters, such as purity and technical specifications of the various components. The continuum concept is no longer valid for highly concentrated suspensions. Hence, in modeling the rheological properties of such materials, the assumption of affine deformation embedded needs to be relaxed. This can be done empirically using the Gordon- Schowalter derivative with a slip parameter instead of the usual upper convected derivative. A better and probably more successful route is to use computer (direct) simulations of the dynamics of particles injected in a suspending fluid. Such (2-D) computer simulations are carried out for a few hundred long rigid fibers and the qualitative results are shown.