Showing papers on "Rheometer published in 1991"

Wall slip of molten high density polyethylene. I. Sliding plate rheometer studies

Abstract: Experiments were performed in a sliding plate rheometer with a high density polyethylene to determine the conditions for the onset of slip and the relationship between slip velocity and shear stress. It was found that melt slip occurs at a critical shear stress of approximately 0.09 MPa in both steady and transient shear tests. The effect of the presence of a layer of fluorocarbon at the interface on both the slip velocity and the critical shear stress for the onset of slip, was also studied. Exponential shear was used to study the effect of shear history on slip. Both steady state and dynamic models for the slip velocity are proposed that are consistent with the experimental observations. Results of oscillatory shear experiments suggest that melt slip is a physicochemical process in which the polymer–wall interface undergoes continuous change during successive cycles.

Rheology of extremely shear thickening polymer dispersionsa) (passively viscosity switching fluids)

Abstract: The flow properties of concentrated polymer dispersions which exhibit extreme shear thickening have been investigated in steady and transient shearing flows using various types of rheometers. A phenomenological characterization based on few fluid parameters is proposed and pertinent test modes for their determination are discussed. The dispersions consist of monodisperse, electrostatically stabilized, solid spherical particles (diameter <0.5 μm) dispersed in glycols. They show a reversible steplike viscosity transition of nearly three powers of ten when an apparent critical shear rate γc is exceeded. This jump from a low to a high viscosity state shows typical features of a shear‐rate‐induced phase transition, including hysteresis and the existence of a metastable low viscosity state above γc. The viscosity step can be traced continuously by using stress controlled rheometers. Pronounced fluctuations of the rate of deformation with time are observed at the critical shear rate. The apparent critical shea...

The rheology of a concentrated colloidal suspension of hard spheres

Abstract: The rheology of a model hard-sphere suspension has been studied at high volume fraction. Particular emphasis was placed on observing the transition between liquid-like and solid-like behavior at the maximum packing volume fraction. Capillary viscometry has shown that the suspension viscosity at low concentration agrees well with theory and other experimental work on hard-sphere systems. At higher concentrations the rheological properties, measured using steady shear, oscillatory shear, and creep techniques, change rapidly from viscous Newtonian to shear-thinning viscoelastic. When the volume fraction is greater than the maximum packing volume fraction the behavior is like that of an elastic solid, and a yield stress can be measured using cone and plate instruments and the vane method. At high volume fractions the product of a characteristic shear rate (or Peclet number) and the low shear limiting viscosity is found to be almost independent of concentration. It is possible to superimpose all the steady shear data using a scaling based on the Cross equation.

Yield stress and wall effects in magnetic colloidal suspensions

Abstract: The static yield stress of magnetic colloidal suspensions is measured as a function of the applied magnetic field in a controlled stress rheometer. The experimental results are analysed with the help of the magnetization curve for different volume fractions. The experimental yield stress scales as the square of the magnetization but its magnitude is always lower than the one predicted on the basis of interparticle forces. The nature of the material used for the plates of the rheometer cell, as well as the surface roughness, is shown to have a large effect on the measured yield stress.

Rheology and Elastohydrodynamic Lubrication

Abstract: 1. Introduction to the book. 2. History of Rheology and Newton. 3. Different Viscometers. 4. Limits for Newtonian Behaviour. 5. Rheological Models for Non-Newtonian Fluids. 6. Rheometers for the Non-Newtonian Range. 7. Solidification Theory. 8. Lund High Pressure Chamber. 9. Lulea High Pressure Chamber. 10. Lubricant Compressibility. 11. Interferometry Film Thickness Measurement. 12. Film Thickness at Combined Rolling and Sliding. 13. Transverse Sliding. 14. The Blue Laser Technique. 15. The Jumping Ball Apparatus. 16. Computerized Jumping Ball Apparatus. 17. The Newtonian Elastohydrodynamic Problem. 18. Non-Newtonian Fluid Model. 19. Pitting and Micropitting. 20. Mixed Lubrication. 21. Lubricant Contamination. 22. Influence of Residual and Static Stresses. 23. Overview of the Different Parameters.

Solutions of the constitutive equations for the flow of an electrorheological fluid in radial configurations

Abstract: Analyses are presented of the effect of radial electric field distribution on the performance on an ideal electrorheological fluid in annular film shear and flow rheometers. Influences of the assumed form of yield stress–electric field relationship are shown. For the parabolic case few problems are raised provided the film thickness to radius ratio is small, though some asymmetry is seen in the flow velocity profiles. When the prospect of a barrier or activation voltage threshold is introduced, significant plug flow formation can be encountered. For high field strengths, where the ER effect is most attractive, a relatively uniform shear rate profile is achieved in a Couette viscometer only at higher operating speeds. In both cases the field distribution is taken to be that of a perfect dielectric and the properties of the fluid as a Bingham plastic.

The influence of materials of construction on biconical rotor and capillary measurements of shear viscosity of rubber and its compounds and considerations of slippage

Abstract: An experimental study is presented of the flow of elastomers and their compounds in biconical and capillary geometries using rotors and capillaries produced from different materials including aluminum, brass, copper, steel, and stainless steel plus polytetrafluorethylene (PTFE). The rotational rheometer could be operated at various controlled pressure levels. A butadiene–styrene copolymer (SBR) was investigated as well as an SBR compound with 20 vol % carbon black and a second SBR compound with 7% zinc stearate added. It was found in the pressurized rheometer, when the pressure was greater than 0.2 MPa, that the data for SBR and SBR–carbon black was essentially the same for smooth and grooved rotors fabricated from different metals. However when the pressure was reduced the torques were substantially reduced and the data for the grooved rotors were higher than for the smooth rotors. Generally, the copper and brass gave rise to higher shear stresses than the steel or stainless steel. The PTFE yielded the l...

Effects of Shear on Miscible Polymer Blends: In-Situ Fluorescence Studies

Abstract: The effects of shear flow on the phase behavior of a miscible blend of polystyrene with poly(vinyl methyl ether) near the lower critical solution temperature have been investigated by fluorescence measurements. For this purpose approximately 1 % of the polystyrene chains in the blend were labeled with anthracene and the fluorescence quenching was used as an indicator of miscibility. The modification of a Rheometrics RMS-800 rheometer for in situ fluorescence measurements along with simultaneous shear and normal stress studies under well-controlled temperature and deformation conditions is described. Following equilibration in the miscible state approximately 25 K below the coexistence temperature, slow heating to temperatures in the two-phase region shows that shearing at a constant rate increases the coexistence temperature, The elevation above the quiescent spinodal temperature is related to shear rate as AT/T, = (0.015 f 0.002)y0.w(w and this relation is independent of composition in the range of 2040% (w/w) polystyrene, within the experimental error. Isothermal experiments in the two-phase region also show shear-induced mixing. The in situ fluorescence measurements provide a sensitive indicator of phase transitions that cannot be unambiguously identified by the measured shear stress and first normal stress difference alone.

The use of the vane to measure the shear modulus of linear elastic solids

Abstract: The vane rheometer may be used to obtain an estimate of the shear modulus G of a gelled suspension. Complex variable techniques are used to determine the torque required to turn a two-bladed vane (i.e. a single plate) of diameter 2a through an angle θ when immersed in a linear elastic material with shear modulus G and Poisson's ratio σ. The torque per unit length is πa2Gθ(1 + k−1), where k = 3 – 4σ. When σ = 1 2 (a reasonable approximation when studying weak gels) the shear stress τxy over the surface of the vane is zero, and there is no tendency for slip to occur at the walls of the vane. The more general case of a two-bladed vane in the form of an ellipse is studied, with either a no-slip boundary condition, or a stress-free (slip) boundary condition. This latter case is resolved by representing the complex deformation potentials as infinite series of exponentials. These series are truncated, and the coefficients of each term are determined numerically. Boundary element methods are then used to compute the torque required to turn finite vanes, of length l = la, in a fluid of viscosity μ, at low Reynolds numbers. There is a direct analogy between the governing equations for low Reynolds number flow and the equations governing linear elastic or viscoelastic deformation when σ = 1 2 , with G replaced by μ and θ by the vane's angular velocity Ω. A no-slip boundary condition is assumed over the surface of the vane. The torque is found to be M = 2πa3μΩ(l + 0.66) for the two-bladed vane, and M = 2πa3μΩ(1.5l + 1.20) for a four-bladed vane, when l ⩾ 3. Numerical results on vanes with 3, 6 and 8 blades indicate that the torque per unit length on an n-bladed vane of infinite length is 2πa2μΩ(2 - 2n−1). These results were tested experimentally, and good agreement was found between theory and experiment.

Mechanics of the “falling plate” extensional rheometer

Abstract: The low-rate falling-plate extensional rheometer experiments by Sridhar and coworkers on a mobile polymer solution were simulated using a free-surface finite-element code which accounts for gravity and surface tension. There is a reverse flow near the plates which is caused by the interaction between gravitational and surface tension forces, causing a delay in the development of the uniform cylindrical column and a difference between the local and imposed extension rates during the early stages of the experiment.

Dilatancy of concentrated suspensions with Newtonian matrices

Abstract: Suspensions filled close to their maximum packing fraction present special challenges in their processing and in their rheological characterization. In this report, the literature in the area of dilatancy of concentrated suspensions is reviewed. Furthermore, the shear viscosity of a Newtonian polymeric liquid filled with 60 vol. percent of ammonium sulfate has been investigated. Both capillary and parallel disk torsional flows, were employed, spanning three decades in shear stress. Upon correction for slip, the suspension exhibited shear thinning at low shear stresses and shear thickening at higher shear stresses. Above a critical wall shear stress, the shear viscosity of the suspension increased unboundedly and the flow became pluglike with apparent slip at the wall. These findings have important ramifications in the processing of composites from such concentrated suspensions.

Development of an in-line rheometer suitable for reactive extrusion processes

Abstract: This paper presents the development of a novel in-line extrusion rheometer based on the flow of polymer through a wedge (vertically tapered slit). This rheometer is suitable for measuring changes in rheological properties on-line during reactive extrusion, because it can be used to estimate the viscosity for a range of shear rates without the need to change the polymer flow rate (i.e., extruder throughput). Equations have been developed to estimate the parameters of the power-law equation, used to describe the viscosity-shear rate relationship, from measurements of pressure drops along the wedge. An experimental in-line wedge rheometer has been built and used to measure the viscosity for a series of polypropylenes prepared via reactive extrusion. Viscosity measurements from the experimental in-line wedge rheometer are compared with measurements from a capillary rheometer. Good agreement is found between the capillary and wedge rheometer measurements.

Step planar extension of polymer melts using a lubricated channel

Abstract: A technique to do step planar extension on polymer melts has been developed using a rectangular channel with lubricated walls and the linear motor of the Rheometrics System Four rheometer. Using this method we probe the stress relaxation of two polymer melts, a linear low density polyethylene (LLDPE) and a highly branched low density polyethylene (IUPAC X), and compare the step planar extensional data to step shear data. Since a step planar deformation is theoretically equivalent to a step shear in a rotating frame of reference, we expect that the nonlinear modulus for step planar extension should be equivalent to that for step shear. Although we find the time dependence of the stress relaxation modulus to be the same in both shear and planar extension, the strain dependence is surprisingly different for the two experiments.

Determination of polymer melt viscosity by squeezing flow with constant plate velocity

Abstract: A universal rheometer which measures squeezing flow properties of liquids at constant plate velocity has been built. The smallest plate velocity is 0.5 μm s −1 and plate distance is determined with a precision of approximately 0.2 μm, allowing accurate squeezing flow measurements to be performed at plate distances down to approximately 20 μm. The squeezing flow properties of three different polymer melts with viscosities ranging from 127 to 28 000 Pa s were measured at 25°C. Normal stress effects are found to contribute significantly to the squeezing force for polymer melts with typical melt viscosities of about 10 4 Pa s except at very small plate distances. A simple correction for normal stress effects gives promising results but the corrected Newtonian melt viscosities are slightly lower than the steady shear viscosities measured with cone-and-plate on a rotational instrument. The corrected melt viscosity of silicone elastomer SE-30 determined by squeezing flow in the non-Newtonian region is also somewhat lower than the dynamic viscosity η* obtained from oscillatory measurements, in disagreement with the Cox-Merz Rule. The effects of plate velocity, excess material and non-parallel plates were studied experimentally.

Continuous on-line measurement of fluid or slurry rheology

Abstract: A rheometer or viscometer for the continuous measurement of the rheological properties, such as viscosity and yield stress, of non-Newtonian fluids or slurries, the rheometer having a vertically disposed inverted U-tube conduit (3), the internal diameter and flow rate of which are chosen such that laminar rather than turbulent flow prevails in the U-tube and such that the fluid or slurry passing therethrough remains essentially homogeneous to provide accurate rheological measurements. The differential pressure across the U-tube (3) is measured by means of a transducer (4), with the flow rate being controlled by means of a flow meter (5) and a control valve (6), each under the operative control of a microprocessor (7). For a constant flow rate the differential pressure across the U-tube (3) is a measure of the fluid or slurry rheology or viscosity. Use of the rheometer on-line provides the basis for more efficient usage of chemical reagents and modifiers normally used for adjustment or modification of slurry rheology in slurry transport systems.

Measurement of very low strains using a stress rheometer: A new rotation sensing transducer (RST)

Abstract: The problems posed by a controlled stress rheometer are studied in the low strain and slow strain rate domain, characteristic of the Newtonian plateau of polymers or of the yield stress vicinity for fluids with a yield stress. A drift in the rheometer strain measuring system was observed which attenuates and may even reverse the sign of the strain measured in comparison with that actually occurring. This drift may give the impression that a yield stress value is observed by inducing a constant strain in creep and hence a shear rate equal to zero. Investigations have shown that the major perturbations are introduced by the rheometer motor and drift depends on the torque level, time, and thermal conditions inside the tube where the motor and transducers are located. A new and more precise automatic measuring system, referred to as RST, has been developed. This may easily replace the existing rheometer strain measuring system. The lower limit of the shear rate that can be measured under steady conditions is reduced from about 10−4 to 10−8 s−1 and rapid transient measurements are possible.

Comments on the accuracy of zero shear intrinsic viscosity of high molecular weight polyacrylamide

Abstract: The intrinsic viscosity of a polymer is traditionally measured with a capillary tube viscometer where the shear rate range is moderately high. Such method is valid when the polymers are non-ionic and have low to moderate molecular weight. The viscosity-shear rate curves obtained for dilute aqueous solutions of two high molecular weight polyacrylamides using two rotational viscometers indicate a strong shear-dependent viscosity in the medium to high shear rate regions. The zero shear intrinsic viscosity of the polymers determined by extrapolation from the high shear rate region to the zero shear condition may result in large errors. Its implication in predicting the molecular weight of polymers using the Mark-Houwink-Sakurada equation is discussed. A rheological equation for intrinsic viscosity as a function of shear rate is proposed.

A device to measure the dynamic shear properties of small samples

Abstract: An attachment for a standard torsional rheometer based on the sliding plate geometry is described which can measure the dynamic shear rheological properties of small samples with total volume on the order of 5–25 μl. The overall error is found to be 1.86% in measuring the dynamic viscosity and 4.04% in the storage modulus when compared to data collected with standard geometries for three fluids: a Brookfield oil, a high density polyethylene melt, and a paint base. It is found that there is an optimum gap between the sliding plates and surface tension can influence the results particularly for the lower viscosity Brookfield oil. This attachment is most useful for fluids which can degrade with repeated moulding, as must be done when preparing samples for the standard attachments, and may be used to determine various shear properties of suspensions including: large amplitude true dynamic shear properties, yield stress, and time‐dependent viscosity.

A multidirectional shear rheometer for polymer melts

Abstract: A parallel plate shear rheometer for polymer melts was built for superposed shear tests in two orthogonal directions. To examine its performance, a standard test was developed, including a change of the shear direction by angles between 0° and 180°. The results were analyzed to obtain preliminary information on the multidirectional shear behavior of a low-density polyethylene melt. The measured orthogonal components of the shear force were added and then split again into components of the original and of the changed shear direction. It was found, within the range of error, that these components depend only on time and on the shear rate, but not on the angle by which the shear direction is changed. The total shear rate was 0.56 s−1.