Showing papers on "Rheometer published in 1971"

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 development of the velocity field in polymer melts in a reservoir approaching a capillary die

Abstract: Color motion pictures have been made of the flow of low-density polyethylene, polystyrene, and isotactic polypropylene at 180°C in the reservoir approach to a capillary extrusion rheometer Detailed observations of the variation of flow patterns with extrusion rate were made At low flow rates, essentially radial flow into the capillary entrance was observed in all polymers With increasing flow rate, the included entrance angle α for the polyethylene and polystyrene decreased from 180°C and a “wine glass” structured velocity field was observed with stagnant circulating regions in the corners and the melt channeling in through the wine glass to the capillary entrance The angle α was related to entrance pressure drop Δpe and capillary wall shear stress σw data through the semilogarithmic equation where α is in degrees; Δpe/σw is interpreted as a Weissenberg number The breakdown of stable laminar flow of the melts in the reservoir and the distortion of extrudates was observed These phenomena seemed to be initiated by the formation of a spiralling motion in the reservoir

Characteristics of Thixotropic Behavior

Abstract: The shear stress‐shear rate behavior of a thixotropic hectorite clay‐water suspension was studied in a Couette viscometer under a wide variety of rest and shear rate histories. Shear stress decay and build‐up transients resulting from step changes in shear rate were measured. The transient stress data were used to construct equilibrium and constant structure flow curves. The constant structure curves could be described by the Power Law model with a constant exponent but varying coefficient. Normalized stress transients could be described by an exponential time series similar to those used in stress relaxation studies on viscoelastic polymers. This method was applicable to literature data and to data obtained for other fluids in our laboratory. The transient behavior of thixotropic fluids is heavily influenced by past history of deformation. Structural rebuilding during rest seemed to begin with the formation of a weak gel structure. Structural rebuilding was also observed under shear rate. In experiments ...

Measurement of the rheological properties of polymer melts with slit rheometer. I. Homopolymer systems

Abstract: Two slit dies have been designed, having aspect ratios of 10 and 20. Three melt pressure transducers were flush-mounted on the long side of the rectangular slot, along the longitudinal centerline of each die. The dies were then used to measure wall normal stresses along the longitudinal direction of polymer melts flowing through the thin slit. The polymeric materials investigated were high-density polyethylene, low-density polyethylene, polypropylene, and polystyrene. The measurement of wall normal stresses were used to determine the rheological properties of melts, namely, the melt viscosity from the slope of axial pressure profiles and the melt elasticity from exit pressures. The present study shows that the rheological properties determined from the slit rheometer are in good agreement with those from the capillary rheometer reported in the author's earlier papers. Therefore it may be concluded that a slit die also may be used as a means of characterizing polymeric materials by their viscous and elastic properties in the molten state.

Determination of normal-stress differences in steady shear flow

Abstract: Using the cone- and-plate apparatus ofAdams andLodge, values of pressure gradient,r-d¯p/dr in a range 300 to 3000 dyn/cm2, have been measured to an accuracy of 1% at shear rates near 10 sec−1 in liquids of viscosity less than 200 poise. Using the constancy of pressure gradient at a given shear rate as an indicator of liquid stability, it was found that a polyisobutene liquid containing 2% of ‘Oppanol B 200’ in ‘Oppanol B I’, when stored at rest at 25 °C, was stable during a certain 7-day period after dissolution and unstable during the subsequent 30 days.

Response behaviour of hot wires in shear flow

Abstract: The response characteristics of a hot wire operated at constant temperature and exposed to a mean-velocity gradient along its length are examined both analytically and experimentally. The shear sensitivity of local wire temperature distributions, as measured with an infrared microscope, are compared with predicted temperature distributions in order to select a convective heat transfer correlation which can be applied locally along a wire in shear flow. On the basis of this correlation, the steady-state and dynamic response behaviour of platinum and tungsten wires in shear flow are examined by means of computer-generated data. Response curves of general applicability are presented which can be used to correct local mean-velocity and turbulence intensity measurements whenever a mean-velocity gradient exists along a wire.

Some suggestions for new rheometer designs I. Theory

Abstract: Considerable interest has recently been shown in new rheometer designs which make use of a steady flow to determine the complex viscosity of an elasticoviscous liquid. Here, we develop the necessary mathematical background to a number of new rheometers of this sort. The suggested measurements involve the components of the force or the components of the couple acting on one of the instrument members. Since commercial rheometers are available which make such measurements, it is anticipated that these instruments will be adapted by some experimentalists to include the new geometries suggested in this paper.

Time-dependent polymer rheology under constant stress and under constant shear conditions.

Abstract: The kinetic rate theory previously presented for describing non‐Newtonian phenomena has been further modified to predict the flow behavior of viscoelastic materials under constant stress conditions. The thixotropic shear stress or shear rate is predicted by the kinetic theory, and the experimental stress or shear rate is obtained by modifying the thixotropic value by a stress or shear rate retardation term. The retardation term stems from a Maxwellian approach for stress retardation. In order to test the validity of this approach, transient and steady‐state data have been obtained for two solutions of polymethylmethacrylate in diethyl‐phthalate. Both constant stress measurements and constant shear rate data have been taken over a broad range. In a systematic manner as suggested by the theory, the parameters were evaluated from constant stress data, and were in turn used to predict constant shear rate data as well as the constant stress measurements. It should be emphasized that the parameters were not obtained from the best empirical fit to the data but were evaluated in a manner suggested by the theory. The agreement between theory and data was good enough to ascertain that the approach is adequate for correlating polymer rheology data. The overall average absolute mean deviation ranged from 4.2% for the steady‐state measurements to 11.2% for the constant stress transient measurements. It was further observed that stress overshoot at constant shear rate conditions normally occurred when the Deborah number was greater than unity. Gradual stress growth curves were observed when the number was less than unity.

A versatile surface rheometer

Abstract: In the study of the physical properties of hydrocolloid films at the aqueous-air interface and aqueousoil interface it is necessary to be able to look at the whole range of rheological properties from low viscosity liquid to viscoelastic solid. It is desirable to follow all these stages, in succession, sometimes, with one piece of apparatus.

Measurement of the rheological properties of polymer melts with slit rheometer. III. Two-phase systems—high-impact polystyrene and ABS resin

Abstract: Measurements were made of the rheological properties of commercially available rubber-reinforced two-phase polymer systems. The polymers chosen for study were two high-impact polystyrenes (Union Carbide Corp., TGDB9500 and TGDB2100) and an acrylonitrile–butadiene–styrene (ABS) resin (Monsanto Co., Lustran I240 Natural). For the study, a slit rheometer was used, which was described in part I of this series. The present paper, the third of this series, clearly demonstrates the usefulness of the slit rheometer for characterizing polymer systems of industrial importance by means of their viscous and elastic properties in the molten state.

Normal stress and shear stress in a viscoelastic liquid under steady shear flow: Effect of molecular weight heterogeneity

Abstract: Under steady shear flow, the normal stress and the shear stress in both dilute and concentrated solutions of monodisperse poly-α-methylstyrenes and their blends were measured. It was confirmed that the molecular theories of Rouse and Zimm extended to concentrated solutions can explain the relation between the zero-shear normal stress coefficient and the zero-shear steady-flow viscosity for both monodisperse and polydisperse systems. Shear-rate dependence of steady-flow viscosity can be understood fairly well by the molecular entanglement concept proposed by Graessley so long as the polymer is monodisperse or the amount of the higher molecular weight component is high. However, zero-shear viscosity of blended systems cannot be explained quantitatively by the theory of Graessley. The shear-rate dependence of steady-state compliance of blended systems was also observed, and it can well be explained by the theory of Tanaka, Yamamoto, and Takano which interpreted the shear rate-dependent steady-state compliance in terms of the relaxation time spectrum and its variation with shear rate.

Some suggestions for new rheometer designs II. Interpretation of experimental results

Abstract: We give further consideration to rheometers which make use of an `almost rigid-body' steady flow to determine the complex viscosity of an elastico-viscous liquid. A new force-measuring system, which is now commercially available, is described for the first time. In the present work, this is used in conjunction with the eccentric-cylinder geometry suggested by Abbott and Walters. An existing formula is shown to be inadequate to describe the experimental results due to the large end effects which are present. The formula is modified by recourse to a `lubrication approximation' and the revised equation is shown to lead to excellent agreement between theory and experiment in the case of viscous and elastico-viscous liquids. In all rheometers of the type under consideration, one of the instrument members is driven mechanically while the other is driven by viscous forces. The very small `lag' which is present due to the friction in the bearing is shown to have an important effect on the interpretation of the experimental results. Finally, the conclusion that end effects have to be taken into account in some rheometer designs motivates a reconsideration of the theory for some of the geometries suggested in I.

Direct Measurement of Shear Stress in a Flume

Abstract: The wall shear stress is an important parameter in many boundary layer type flow situations. The need for a more accurate versatile hydraulic skin friction balance than the Preston tube technique and other similarity techniques. This presentation gives the design of a floating element type balance which is believed to be sturdier and simpler. Some skin friction measurements, made by the balance, are compared with the shear stress values determined by the Preston tube techniques.

Lower Newtonian viscosities of polystyrene

Abstract: A falling coaxial cylinder viscometer was used to measure the melt flow behaviour of a commercial polystyrene with Mw 260,000. The shear stress region extended down to 0.6 × 104 dynes/cm2 and shear rates were as low as 3 × 10−2 sec−1 at 186°C. The shear rate-shear stress plots were linear at low shear stresses with slopes (differential viscosities) of 3.3 × 105 poises at total shear less than 120 units and decreasing differential viscosity with higher total shear. The flow curves at relatively low total shear were initially dilatant and became pseudoplastic with increasing shear stress. The inflection point represents a Newtonian apparent viscosity, which agrees fairly well with literature values for polystyrenes of the same Mw. Newtonian apparent viscosity is characteristic of a point value of shear stress and shear rate and is not necessarily a plateau region. Observation of a Newtonian region with decreasing shear stress or shear rate does not prove that this flow regime persists unchanged to zero values of the experimental parameter. The existence and magnitude of the Newtonian apparent viscosity reflects shear history of the polymer as well as its constitution and molecular weight distribution.

Portable turbulent flow comparison rheometer

Abstract: The invention is a portable turbulent flow comparison rheometer for evaluation of fluids exhibiting drag-reducing properties. The device is capable of directly measuring and recording differences in flow rates produced by identical pressures between two fluids.

Determination of the Shear Rates of Non‐Newtonian Fluids from Rotational Viscometric Data. I. Concentric Cylinder Viscometer

Abstract: A method is proposed for determination of the shear rate of non‐Newtonian fluids sheared in a concentric cylinder viscometer. The mathematical expression of the shear rate is an exact solution. The method has been shown to be convenient. Furthermore, only one set of experimental data, torque versus angular velocity, is needed in the calculation. Three examples are given to illustrate the application of the proposed method.

Determination of the Shear Rates of Non‐Newtonian Fluids from Rotational Viscometric Data. II. Cone‐and‐Plate Viscometer

Abstract: A method for determination of the shear rate in a cone‐and‐plate viscometer is developed. The method to obtain the shear rate is shown to overcome the limitation of the existing method which requires a small angle between the cone and the plate as well as a linear velocity field with respect to the angle. Examples for non‐Newtonian fluids with and without prior knowledge of the rheological equation of state are presented to demonstrate the proposed method.

Crystallization of Cis-Polyisoprenes in a Capillary Rheometer. III. The Effects of Certain Additives

Abstract: Cis-polyisoprenes are readily crystallized under the pressure and orientation forces existing in a capillary rheometer. Oil-extension retards the rheometer crystallization process. However, natural rubber/oil blends containing up to 50 phr of oil are readily crystallized above room temperature. The retardation effect of oil-extension is not nearly as drastic as that produced by slight alterations in the stereoregularity of the polymer chains. ISAF carbon black significantly enhances the ease at which solidification of the melt can be effected in the capillary. Aging the black/rubber mix at room temperature enhances the ease of solidification. Shearing the aged black/rubber mix results in a retardation of the solidification process but subsequent aging of the sheared mix again enhances the ease of solidification in the capillary. Acetone extracted natural rubber crystallizes more easily in the capillary rheometer than the non-extracted material; a behavior in marked contrast to that observed under...

Viscous behavior of some liquid rubber resins

Abstract: The flow behavior of several low molecular weight polymers has been studied as a function of shear rate and temperature. These polymers, which had terminating hydroxyl or bromine groups, included homopolymers of polybutadiene and acrylonitrile–butadiene and styrene–butadiene copolymers. Viscosity was measured as a function of shear rate for the temperature range 25°–35°C, and the limiting zero shear viscosities were obtained for the range 25°–60°C. A cone plate viscometer was employed to measure the effect of shear rate on viscosity, and a Brookfield viscometer was used to verify the zero-shear rate values. A tendency of the fluid to flow out of the cone-plate gap was observed for some of the materials studied. From the viscosity data, characteristic times were estimated, and the data were compared with two constitutive equations. A modified Arrhenius equation was fitted to the zero-shear viscosity data. In the case of one material, it was possible to test the Nakajima relationship between viscosity and molecular weight distribution. The dependence of material parameters on temperature is discussed in detail.

A moderately high shear viscometer for measuring low specific viscosities

Abstract: A simple, moderately high shear capillary viscometer which can be used up to a shear rate of 150,0000 sec−1 is described. The use of a twin viscometer arrangement has eliminated the need of elaborate pressure control and adjustment units. Experimental results obtained by use of the viscometer to measure the specific viscosities of a charged colloid at two different shear rates are presented.

Analysis of dynamical behavior of viscoelastic fluids

Abstract: A theoretical study is undertaken on the dynamical behaviour of viscoelastic fluids in simple shearing flow. In the present study differential-type constitutive equations are chosen in attempting to describe the dynamical behaviour of polymer solutions experimentally observed byPhilippoff. The dynamical system studied here takes into account shear rate and normal stresses as the dependent variables with shear stress kept constant during the transient period. It is found that the 3-constantOldroyd model describes qualitatively the experimentally observed behaviour.