Showing papers on "Rheology published in 1971"

Rheology of concentrated suspensions

Abstract: The dependence of the viscosities of highly concentrated suspensions on solids concentrations and particle size distributions is investigated by using an orifice viscometer. Based on the extensive amount of data on pertinent systems, an empirical equation which correlates the relative viscosities of suspensions (or relative moduli of filled polymeric materials) as a function of solids concentrations and particle size distributions is proposed. The equation has a constant which characterizes size distributions of spherical particles and can be determined experimentally without measuring viscosities. For uniform-size spherical particles, it reduces to the well-known Einstein equation at dilute solids concentrations.

Rheological properties of human erythrocytes and their influence upon the "anomalous" viscosity of blood.

Abstract: I I I . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 55 Auxil iary Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Prepa ra t ion of E r y t h r o c y t e s . . . . . . . . . . . . . . . . . . . . . 157 Control of H e m a t o c r i t . . . . . . . . . . . . . . . . . . . . . . . . 158 Suspending Media . . . . . . . . . . . . . . . . . . . . . . . . . . 158

Polymer Melt Flow Instabilities in Extrusion: Investigation of the Mechanism and Material and Geometric Variables

Abstract: A comprehensive study of the effect of material, geometric, and operating variables on the onset of melt flow instabilities in extrusion was made. Primary objectives were to compare such melt instabilities for a variety of materials and to correlate instability criteria with independent rheological parameters. Seven polymer melts were studied: two polyethylenes, polypropylene, polystyrene, two polybutadienes, and an SBR copolymer. Two distinct extrusion apparatuses were used: a die fed by a screw extruder system and a die fed by an Instron rheometer. The variables studied were temperature, entry cone angle, length/diameter ratio of the die, and flow rate. Independent rheological measurements were performed. These consisted of the evaluation of normal stresses on the Weissenberg rheogoniometer, entrance pressure drop measurements on the Instron rheometer, and viscosity measurements on both instruments and the screw extruder. In a companion study, flow visualization studies were made in the die entry region of the Instron rheometer, for most of the above materials and also a solution of polyisobutylene and glycerine. Distinctively different patterns were observed for the various materials, ranging from severe channeling with corner eddies to nonchanneling flow. Interrelationships between the onset of instabilities with other measurements were noted. In comparing a variety of materials, a decrease in the critical shear rate is generally associated with an increased tendency of the material to channel, an increased normal stress/shear stress ratio and an increased entrance pressure drop/shear stress ratio. It is argued that the instability phenomena are strongly associated with the elastic properties of the materials.

Rheological properties of molten polymers. II. Two‐phase systems

Abstract: A study is carried out for characterizing two-phase systems of molten polymers by their viscous and elastic properties. The two-phase systems chosen for study are blends of polystyrene and polypropylene, and blends of polystyrene and high-density polyethylene. For the study, measurements of wall normal stresses are made by use of a capillary melt rheometer described in part I of this series. The concept of the “exit pressure” is used to determine the elastic properties of the two-phase polymer systems. The present study shows anomalous viscous and elastic properties of two-phase systems, which are difficult to predict from knowing the viscous and elastic properties of their individual components. A detailed discussion is given on the state of dispersion of two incompatible polymer systems in the molten state, by presenting pictures of the microstructure of the extrudate samples. The state of dispersion appears to vary depending on the blending ratio, extrusion temperature, melt viscosities of individual components, and blending method.

The rheology of filled polymers

Abstract: The flow properties of polymer melts containing fillers of various shapes and sizes have been examined. If there is no failure of either the filler or polymer in the solid state, then the modulus enhancement for randomly distributed filler is equal to the melt viscosity enhancement under medium shear stress conditions (104 Nm−2) in simple shear flow or in oscillatory shear flow. Submicron-size fillers, in particular, can form weak structures in the melt that greatly increase the low shear rate viscosity without changing the modulus of the solid proportionately. The highly pseudo-plastic nature of polymer melts at shear stresses of 106 Nm−2 means that, even without orientation of filler particles toward the flow direction, the viscosity enhancement is less than at lower shear stresses.

Rheological properties of molten polymers. I. Homopolymer systems

Abstract: Experimental work has been carried out to investigate the influence of molecular weight distribution and long chain branching on both viscous and elastic properties of molten polymers, using a capillary rheometer, as described in a recent paper by Han. The materials used for the study are three high-density polyethylene samples of widely different molecular weight distributions and a low-density polyethylene containing much long-chain branching. For the analysis of the experimental data, and to obtain the information on the melt elasticity, the concept of the exit pressure recently advanced by Han is used. The study shows that the sample containing long-chain branching is much more elastic than the samples containing little or no long-chain branching, and that the broader the molecular weight distribution of the material, the more elastic the material is. These findings are in conformity with those reported in the literature. Also studied were blends of two high-density polyethylenes having widely different molecular weight distributions. The results of the blends systems show a maximum in melt viscosity as well as in elasticity for a certain blending ratio. The results of the present study may be of considerable interest to those who are concerned with modifying the structure of polymer and also with determining optimum processing conditions.

Rheology and plasticization of polyvinyl chloride

Abstract: The behaviour of polyvinyl chloride in the range of temperatures from 77°C to 230°C is characterized by the persistence of a slight amount of crystallinity, which gives rise to very peculiar rheological properties. In particular flow takes place partially through particle slippage, which predominates, with respect to molecular deformation, the lower the temperature and the higher the molecular weight and the crystallinity. The flow behaviour may change appreciably with shearing time and with thermal history. Viscous heat generation must be taken into account, even at relatively low shears. The molecular weight dependence of viscosity and relaxation time is anomalous for both rigid and plasticized samples. For the latter, the composition dependence of the glass transition temperature appears to be in line with predictions based on free volume concepts.

The rheology of synovial fluid

Abstract: A review has been undertaken of the rheological properties of synovial fluid in which the viscosity of synovial fluid under single-shear and multi-shear rate conditions, and the visco-elastic behaviour in conditions of oscillatory shear rate have been described. Synovial fluid is a pseudo-plastic (shear-thinning) fluid, both in normal and pathological conditions. The fluid exhibits elasticity and a normal-force effect. The low friction in joints is associated with a full film of lubricant which separates the surfaces and high friction results from thin films of localised boundary friction due to asperity contacts. A series of experiments with varying types of cartilage and synovial fluid have suggested that the nature of the lubricating fluid and the articular surfaces are important in determining shear stress during reciprocating motion. Scanning electron microscopy has confirmed the suggestion that surface aggregation of hyaluronic acid-protein complex can give enhanced lubrication.

Flow and rubber elasticity of polymeric systems

Abstract: The rheological properties of linear high molecular weight compounds at temperatures above their glass transition region and melting point are considered. When the deformation rates at steady flow conditions are increased high-elastic deformations are first observed, then a viscosity anomaly develops. The interrelation of these phenomena depends greatly on molecular weight distribution. Simple shear deformation processes at constant deformation rates are compared with those for uniaxial extension. The change in relaxation spectrum under continuous deformation of polymers is discussed. Attention is drawn to the presentation of rheological dependences in the form of master curves and it is shown that under simple shear the relaxation spectra of many polymeric systems change similarly. The forced transition of polymers from the fluid to the high elastic state due to an increase in shear rate has beesi studied. The phenomenon is distinct for high molecular weight polymers with narrow molecular weight distribution. It is accompanied by transition of steady laminar flow to slippage of the polymer along the capillary wall alternating with relaxation and adhesion to the wall. Slippage accelerates the output of polymer from the capillary by decimal orders. The effect of temperature on the phenomenon is estimated. The purpose of this lecture is to review the latest experimental studies of the rheological properties of linear polymers at temperatures above their glass transition and melting temperatures, carried out at the Laboratory of Rheology of the Institute of Petrochemical Synthesis of the USSR Academy of Sciences in collaboration with a number of industrial laboratories. The most important feature of rheological behaviour of polymeric systems is due to the combination of fluidity and high elasticity, i.e. to the ability to accumulate simultaneously unlimited irrecoverable and large recoverable deformations. However, while numerous papers have been devoted to measuring the viscosity of polymeric systems, information on their high elasticity, based on direct measurements, is very limited. The reason for this is that high-elastic deformations are estimated after unloading the samples. from their elastic recovery in time. This is much more difficult than measuring viscosity or normal stresses closely associated with high elasticity. At low shear rates and stresses there is a simple relationship1'2 between the first differential of normal stresses (P1 1—P22), shear stresses (r), high elasticity moduli (Ge) and high-elastic deformations (Ye). However, at shear 423 F G. V. YINOGRADOV stresses and rates () differing substantially from zero theory gives no correct predictions of the type of this bond. It is therefore necessary to determine it experimentally at least, especially if it can describe the properties not only of polymers with different molecular weight distribution (MWD), but also of their solutions. Naturally, this requires extensive measurements of the high-elastic properties of polymers and their solutions, as well as systematization of published data. Until recently most of the information on the rheological characteristics of polymeric systems pertained to simple shear and it will therefore be interesting to touch briefly on the peculiarities of behaviour of polymers during uniaxial extension. When considering the high-elastic properties of polymeric systems in the fluid state one is naturally interested in the peculiarities of their behaviour on passing into the high elastic state under the action of increasing deformation rates: this pertains primarily to the change in fluidity of polymeric systems upon such forced transition to the high elastic state. DEVELOPMENT OF HIGH-ELASTiC AND IRRECOVERABLE DEFORMATIONS Linear polymers display high elasticity simultaneously with a viscosity anomaly, when a continuous space network of macromolecular entanglements appears in them. This corresponds to the molecular weight reaching its critical value (Mcr) and to sharp enhancement of the dependence of initial viscosity (, = on this quantity. It was shown in ref. 3 for a series of polydisperse polybutadienes, that with M —' Mcr, '—* 0. This immediately Data for stationary shear flow Viscosity

Cavitation in Viscoelastic Media

Abstract: A theoretical model for cavitation in viscoelastic media is developed which incorporates both interfacial and rheological arguments. The properties of interfacial and bulk cavities are correlated, and the influence of time‐dependent cavity vapor pressure Pν(t) or hydrostatic tension σ(t) are correlated with the extension ratio λ=r/r0 of the cavity surface in a stress relaxation model. Experimental data are analysed for cavitation by peeling, tensile loading of butt joints, and superheating of swollen elastomers. For each case an agreement between theory and experiments is obtained which identifies the specific mode of cavitation and indicates for all cases the presence of prenucleated cavities with radii r0⩾1.0 microns. The direct correlation of peeling stresses with viscoelastic restraints to interfacial cavitation provides a new type of experimental confirmation for the applicability of rate‐temperature superposition to peeling data. The implicit assumption that the interfacial failure stress depends u...

Flow of dilatant (shear‐thickening) fluids

Abstract: The conduit laminar flow of dilatant (shear-thickening) fluids was investigated. It was found that such flow agreed with the Metzner-Reed (friction factor-modified Reynolds number) correlation previously verified only for pseudoplastic fluids. The agreement was found to hold even for cases where the flow behavior index was 2.0 or greater, which caused the velocity term V in the modified Reynolds number Dn′ V2−n′/gc8n−1K′ to be raised to the zero or a negative power. It was also demonstrated that conduit laminar flow for dilatant (shear-thickening) fluids could be described solely with the Metzner-Reed correlation and rheological data taken with a small-scale laboratory viscometer. Studies of flow through fittings (90-deg. elbows, globe valves, and couplings) showed a definite effect for non-Newtonian fluids contrary to previous reports for pseudoplastics which indicated essentially Newtonian behavior.

A rheogram template for power law fluids: Technique for characterizing the rheological properties of emulsions and polymer solutions

Abstract: A simple “template” method is presented which extends the usefulness of the Brookfield Synchro-Lectric viscometer, with the use of a cylindrical spindle, to meaningful characterization of non-Newtonian fluids which follow the power law. Deviation from Newtonian behavior is characterized by a single constant called the shear thinning index (STI). By the method, a well-defined shear rate can be determined for each rpm setting and corresponding apparent viscosity values can be simply and rapidly obtained. The technique has wide application for the study and characterization of emulsions and polymer solutions. It has recently been employed for determination of agglomerate—creaming tendency in silicone emulsions and in the quality control of Polyox®, poly(ethylene oxide) resins.

Molecular rheology of polymer systems

Abstract: Some general principles of constructing molecular theories of polymer systems are discussed. The theories based on statistical mechanics combined with simplified concepts in molecular structure comprise the following fundamental equations: equation of continuity in an N-dimensional configuration space, kinetic equations describing the motion of the structural units involved, dynamic equations, i.e. relations between the local tension and configuration characteristics, kinetic equations describing the rates of formation and dissociation of structural units in the system. Some simplified concepts in intraand inter-molecular interactions in macromolecular systems are given. Two examples of model systems are analysed systematically; a dilute solution of flexible chain-macromolecules, and an entangled network system typical for ëoncentrated polymer solutions and melts. INTRODUCTION The importance of molecular considerations in polymer rheology is rather obvious and hardly need be argued. The theory of mechanical (as well as any other) macroscopic characteristics of polymer systems formulated in terms of molecular structure leads to a deeper understanding of the mechanisms responsible for the observed phenomena, and enables one to predict the physical behaviour of various materials, and to design new materials with modified structure and properties. Molecular theories seem to make the best basis for physically reasonable assumptions in the derivation of constitutive equations from continuum considerations. The term 'molecular theory' is. however, far from being unequivocal. There are in the polymer literature many treatments claimed to be, and (even worse) considered by many readers as, 'molecular theories' while based on quite arbitrary or unfounded model assumptions. So, for example, in attempts to reproduce the empirical '3.4 power law' (shear viscosity proportional to the 3.4th power of the molecular weight of polymer) Bueche1 assumed some 'rigid rotations' of macromolecules in entangled systems; for the same purpose Graessley2 postulated a kind of 'slalom motion' of a macromolecule through entanglement loops of other chains, while Hayashi3 and Pokrovskii4 assumed special forms of frictional coefficients. None of these concepts was based on systematic molecular considerations or had a clear physical significance. Various criteria can be considered in the comparison of individual

Fluid-Dynamic Consideration of Bottom Materials

Abstract: Bottom materials often involve clay-water suspensions which evidence complex rheology. Studies of the non-Newtonian behavior of kaolin are reported herein. Concentric viscometer analysis at concentrations of 5, 10 and 20% by weight indicate Bingham-plastic-body behavior; Newtonian fluid behavior is evidenced at low concentration and alkalinity. The dependence of the fluid properties, yield stress, and plastic viscosity on pH and concentration is characterized; both these properties increase with increase in concentration, or decrease in pH (increase in acidity). Measurements of the drag of simple bodies in this mixture are reported. The drag coefficient of spheres and discs in the laminar boundary-layer region (Reynolds numbers below 2 x 10 5 ) indicate negligible non-Newtonian effect above a Reynolds number of 1,000. At lower Reynolds number, a yield stress effect is superimposed. The dependence of this effect on the Hedstrom number is indicated for values of this parameter up to 4,000. The occurrences with spheres and discs are found to be much the same.

Molecular weight distribution and rheological properties of polyisobutylene solutions

Abstract: The molecular weight distribution of a series of polyisobutylenes was determined using osmotic pressure measurements, gel permeation chromatography, and intrinsic viscosity. All of the polymers except for one, a blend of the highest and lowest molecular weight constituents, had similar moderate molecular weight distributions. The “extended chain length” method of calibrating the gel permeation chromatograph for polyisobutylenes was found to be effective. Steady state and transient shear stresses and normal stresses were measured on 5% decalin solutions of these polymers. The zero shear viscosity increased with the 3.3 power of molecular weight, and the zero shear normal stress coefficient (σ11 − σ22)/Γ2 varied with the 7.5 power. Relative elastic memory as measured by (σ11 − σ22)/σ12 or stress relaxation increased with increasing molecular weight (and at constant number- or weight-average molecular weight) with breadth of distribution. Stress overshoot also correlated with this tendency.

The influence of molecular weight on the melt rheology of polypropylene

Abstract: Melt viscosity and melt elasticity data were obtained over a broad range of temperatures and shear rates on a series of four polypropylenes of different molecular weight but approximately the same molecular weight distribution. The superposition technique was used with both temperature and molecular weight to shift flow curves for all four materials at three temperatures each along the shear rate axis to generate a master flow curve at a given temperature and molecular weight. For polypropylenes of this type, and molecular weight distribution shift, factors which can be used to extend the useful range of experimentally obtained flow data were determined. The dependency of apparent viscosity on weight average molecular weight at shear stresses as high as 106 dynes/cm2 is shown. The dependency of melt elasticity on molecular weight and temperature is discussed.

The rheological properties of annealed tantalum

Abstract: The influence of strain rate on the mechanical behavior of 99.9 pct pure, fully recrystallized tantalum is examined. Stress-strain curves in tension are obtained from strain rates of 10−5 to almost 104 sec−1. The rate dependence of the upper and lower yield stresses is closely predicted by the Johnston-Gilman model. However, the model does not adequately describe the postyield hardening behavior of the metal.

Rheological characteristics of pigment dispersions: Flow behaviour under steady shear flow

Abstract: The rheological characteristics under steady shear flow of dispersions of pigment in a mid-litho oil varnish have been examined. Using a Weissenberg rheogonio-meter the influence of pigment concentration and dispersion temperature on flow behaviour have been examined in some detail. Data have been examined in terms of the initial peak stresses and of the equilibrium stresses encountered. By taking the ratio of these two as a measure of thixotropy it would appear that thixotropy goes through a maximum with increasing pigment concentration. The results also indicate that particle/particle interaction controls flow up to the point at which a peak stress occurs. Thereafter the flocculation/deflocculation process predominates.

Poly(ethylene sulfide). IV. A rheological method for determination of “apparent molecular weight”

Abstract: By combining a number of theoretical relationships concerning the effect of molecular weight on melt rheology, an equation may be derived for conversion of melt indexer flow rate to “apparent molecular weight.” In spite of certain evident theoretical shortcomings of this derivation, the method was applied to the determination of the molecular weight of highly crystalline insoluble poly(ethylene sulfide), for which no other method appeared to be satisfactory. When applied to polymer specimens of presumably known molecular weight (e.g., certain ionically initiated specimens), reasonable agreement was found.

Rheological properties of blood from diabetic Wistar rats.

Abstract: The rheological properties of blood from rats made diabetic by subtotal pancreatectomy or alloxan injection were compared with those of blood from normal Wistar rats. Blood viscosity, plasma viscosity, hematocrit, erythrocyte sedimentation rate, plasma osmolality, and serum glucose were measured. The alloxan-treated rats had a significantly higher blood viscosity than the normal rats. The plasma viscosity remained unchanged.

Flow of non-Newtonian fluid through circular tubes

Abstract: According to Newton's law of viscosity τyχ = Dvyχ/dy. But experiments have shown that τyχ is indeed proportional to −dvx/dy for all gases and for homogeneous nonpolymeric liquids. There are however, a few industrially important materials, e.g. plastics, asphalts, crystalline materials that are not described by the equation given by Newton's law of viscosity and they are referred to as non-Newtonian fluids. The steady state rheological behaviour of most fluids can be expressed by the generalised form, τyχ = –η(dvyχ/dy) where η may be expressed as a function of eitherdvx/dy or τyχ (where η is independent of the rate of shear, the behaviour is Newtonian with η=μ). Numerous empirical equations or ‘models’ have been proposed to express the steady-state relation between τyχ anddvx/dy. The flow of Newtonian fluids through circular tubes have been discussed before by many. Here we shall discuss the case of two such models of non-Newtonian fluids through circular tubes. The flow of fluids in circular tubes is encountered frequently in Physics, Chemistry, Biology and Engineering.

Rheology of friction - reducing polymer solutions

Abstract: Stress decay characteristics of concentrated (0.14%-1.03%) polyethylene oxide solutions were measured. Apparent viscosity losses of up to 50% were recorded at 30c. Shear rates ranged up to 1370/sec. Limiting viscosities were found to be relatively independent of solution history. Disk flow was used to investigate the phenomenon of turbulent drag reduction and subsequent mechanical degradation of dilute aqueous solutions of polyethylene oxide. The relationship of drag reduction and degradation to the molecular weight, solution concentration and temperature was investigated. Reduced viscosity measurements(which are a measure of molecular weight of polymer)were carried out directly on the dilute solutions tested.

Rheological model for ionic polyacrylamide solutions over extremely wide shear rates

Abstract: Non-Newtonian shear viscosities were measured for 24 solutions of 3 different polyacrylamides. The polymer concentration varies from 50 to 2,500 ppm. Distilled water and 20,000-ppm NaCl solutions were used as solvents to afford comparison of the rheological properties between fresh and saline solutions. Two of the polymers are available commercially and have been used in the field. The third, however, is a new, developmental polymer and was provided especially for this study by the manufacturer. By using a Weissenberg rheogoniometer and various capillary tubes, the polymer solution viscosities were measured over 8 decades of shear rate, ranging from 10U-3D to 10U5D secU-1D. Thus, the results of this study are more extensive than any in the literature. It is shown that for all 3 polymers, the solution viscosity can be represented by a power-law type rheological model over the range of 10U-1D to 10U4D secU-1D with an error of less than 4%. Such viscometric relationships are essential and convenient for the description of the flow behavior of the polymer solutions in porous media when using reservoir simulation models. Beyond these shear rates, the viscosity-shear rate relations resemble parabolas approaching limiting viscosities. (13 refs.)