Showing papers by "Pierre J. Carreau published in 1994"

Rheology of concentrated poly(ethylene oxide) solutions

Abstract: This paper reports on the rheological properties of a series of concentrated poly(ethylene oxide) solutions. The polymers were obtained from two sources and dissolved in two solvents. Data on viscosity, primary normal stress differences, and dynamic rigidity were obtained using two Weissenberg rheogoniometers, a Rheometrics stress rheometer and a Bohlin VOR instrument. All the viscosity and normal stress data could be correlated using two key parameters: the zero shear viscosity and a characteristic time constant. Master curves are obtained which are almost independent of concentration, solvent, and molecular weight. The Cox–Merz rule is found to be valid for all solutions; however, the analogy between the primary normal stress coefficient and the storage modulus is verified only at low shear rates or frequencies. In addition, a modified rheological model based on the conformation tensor is shown to well represent the shear data.

Rheological Properties of Blends: Facts and Challenges

Abstract: Three forces will continue to provide incentive for the industry to produce new polymer blends: the ability to tailor product properties to a particular performance cost balance; fast entry into new markets; and it is easier to develop products for small or niche markets than by synthesizing new polymers. The utilization of polymer blends in commerce represents some 16% of all plastics and the sector is growing at a rate of approximately 10% per year. We expect that polymer blends will grow at a much faster pace in the next decade. The most important class of blends from the commercial viewpoint are the so-called immiscible blends, where the presence of a distinct minor phase may be observed. The minor phase in an immiscible polymer blend is deformable and a wide range of sizes and shapes can thus be obtained for this dispersed phase during processing [1]. Both the size and the shape of the minor phase can influence a multiplicity of physical properties. At present the processing/ morphology/ property relationships in immiscible and partly miscible blends remain poorly understood and a given processing strategy may result in high value added blends. Rheological methods are powerful tools to elucidate such relationships. The morphology of blends is strongly dependent on the rheological properties of the components and rheological properties are needed to understand the processing of blends and changes occurring during processing. Conversely, the blends’ morphology affects the rheological properties.

Mixing in the transition flow regime with helical ribbon agitators

Abstract: The effects of the non-Newtonian properties on the effective deformation rate, mixing and circulation times and flow behaviour have been investigated in the transition flow regime of mixing systems. Based on the equivalent Couette flow, three models are proposed and are shown to predict similar and drastic increases of the effective deformation rate with the impeller rotational speed in the transition regime. The predictions are shown to fit very well data obtained for various non-Newtonian fluids mixed with helical ribbon agitators, and with literature data for anchor, blade turbine and flat disc agitators. The elasticity along with shear-thinning properties appear to have slight effects on the dimensionless mixing and circulation times in the transition regime, whereas their effects in the laminar regime are quite drastic, as reported by others. L'influence des proprietes non newtoniennes sur la vitesse de deformation effective, les temps de melanges et de circulation et sur les patrons d'ecoulement a ete etudiee pour le melange dans un reservoir en regime transitoire. Utilisant I'analogie avec l'ecoulement Couette, on a propose trois modeles, qui predisent de facon semblable des augmentations tres marquees de la vitesse de deformation effective en fonction de la vitesse de I'agitateur en regime transitoire. Les modeles decrivent tres bien les resultats obtenus pour plusieurs fluides non newtoniens melanges par des agitateurs a lames helicoidales et les resultats de la litterature pour des agitateurs de type ancre, turbine a lames et disque plat. Les proprietes elastiques ainsi que la rheofluidifiance ont peu d'effet sur les temps adimensionnels de melange et de circulation en regime transitoire alors que leurs effets en regime laminaire sont tres marques, tel que rapporte par d'autres auteurs.

Rheology of several hundred rigid bodies

Abstract: A novel nonequilibrium molecular dynamics, originating in mesoscopic theory of suspensions, is introduced to investigate the behavior of model polymeric fluids consisting of several hundred ellipsoids of revolution (spheroids) that interact via the Gay-Berne potential. This dynamics is used to generate new microstructural, thermodynamic and rheological data. The microcanonical equtions of motion for the translational and angular momenta as well as for mass-centers and orientational unit vectors are derived from a Hamiltonian. These expresssions are then augmented by SLLOD-like and Gaussian thermostat terms added consistently to equations for both the rotational and translational degrees of freedom; the role of Gaussian thermostat is to maintain constant kinetic temperature of the assembly of spheroids. The thermodynamic results are calculated along one isotherm (nondimensional temperature T maintained at unity). Rheology is investigated for two state points (namely for particle number density p equal to 0.25, 0.4 and T set to 1), that lie well inside the isotropic phase if no external flow is applied. A state point is defined by the fluid's temperature T,. and the concentration of particles per unit volume p. As indicated by snapshots of molecular configurations, at the intermediate shear rates (nondimensional shear rate approximately 1–2), ellipsoids become aligned to the direction of flow and the stress tensor begins to be nonsymmetric. At even higher shear rates, this configuration breaks down leading to the formation of a transitory isotropic-type fluid, and then to the build-up of a highly ordered structure exhibiting global orientation of particles in the direction of the vorticity axis. For ϱ = 0.4, the first (N1) and the second (N2) normal stress differences are positive and negative respectively, but at low densities (ϱ = 0.25), n1 becomes slightly negative. In addition to the stress tensor, we compute the conformation tensor, the order parameter and the components of the pair radial distribution function. At high shear rates the radial distribution functions become significantly anisotropic. Furthermore, we investigate the phenomenon of the stress overshoot at the inception of the simple shear flow from a molecular perspective, and study the evolution of the distribution of translational velocities as a function of the shear rate.

Extrudate roughness of high‐density polyethylene and linear low‐density polyethylene: Determination of key parameters using screening design

Abstract: A screening design is used to establish the contribution of various parameters to the roughness of cylindrical extrudates. A dimensionless response variable is proposed to characterize the extrudate roughness, and the effects of ten parameters on this roughness response were examined. Two polyethylenes (one high density and one linear low density) were investigated using a 45 mm single screw extruder. The results show that the main parameters affecting the extrudate roughness are, in order of importance, apparent shear stress at the die wall, die diameter, ratio of die length to diameter, and type of polymer. The other six parameters (the use of an additive, recycling, type of entrance adapter, die material, die temperature, melt temperature) were found to have a non-significant contribution to roughness.