Showing papers on "Rheometer published in 1994"

A new elongational rheometer for polymer melts and other highly viscoelastic liquids

Abstract: In polymer melt elongational rheometry only by the rotary clamp technique large elongations can be obtained homogeneously. However, as described in this paper, there still remain disadvantages that led to the development of a new rheometer with the following main features: The dimensions of the required sample are small (60 × 7 × 2 nun3), the sample is supported by a cushion of inert gas and, after having reached the test temperature of up to well above 300°C, it can be extended by a new type of clamps that make use of metal conveyor belts. The resulting tensile force is measured with a resolution of better than 100 mgf (0.001 N). The strain rate range is 0.001-1 s−1, and the maximum Hencky strain is 7, corresponding to a maximum stretch ratio of 1100. Within the sample, the temperature variation in time and space is less than 0.1°C. For the evaluation and documentation of the test performance, a video camera records the top and side views of the sample that carries a marking powder to permit the evaluation of the true strain rate. The operation of the instrument is easy, and so is the sample preparation, but care must be taken concerning the necessary isotropy and ‘internal homogeneity’. Examples of test results are given for several polymer melts at various temperatures: (1) Polystyrene up to a total Hencky strain larger than 7 at 170°C, (2) several types of polyethylene (LDPE, LLDPE, HDPE) at 150°C, (3) poly(amide) at 250°C, and (4) poly(ethersulfone) at 350°C. The wide applicability of the new rheometer is demonstrated by adding results obtained from samples of bread dough. The surface tension has no influence on the results if an error of 3% can be tolerated. From the results it follows that by means of the newly developed rheometer many problems in polymer melt elongation have been solved.

Binder characterization and evaluation. volume 3: physical characterization

Abstract: The research results in this report concern the characterization of physical properties of asphalt binders and the development of test procedures for aging and adhesion. Asphalt binders were shown to be linear viscoelastic materials at expected pavement strains. Test methods and models describing rheological behavior were developed with the assumption of linear behavior. A mathematical model that contains parameters that can be used to relate physical and chemical parameters was developed. The test methods and models allow the stiffness of asphalt binders to be characterized as a function of loading time and temperature with the dynamic shear rheometer at intermediate to upper pavement surface temperatures and with a newly developed bending beam rheometer at low temperatures, where thermal cracking is important. To characterize fracture properties, the project developed a direct tension test, in which failure strain, stress, and energy are determined as functions of loading time and temperature. Limited work on fatigue properties at low temperatures successfully related them to tensile properties. A new phenomenon, low-temperature physical hardening, was identified during the project. This phenomenon results in increased stiffness of the binder when it is held at a constant temperature at room temperature or below. The blister test for quantitative measurement of adhesion between asphalt binder and aggregate surfaces was found to be impractical. The investigation of various factors, including film thickness, aging temperature, the effect of oxygen versus air, and the effect of mineral surfaces, led to a new aging procedure for simulating in-service oxidative hardening of asphalt binders, which was adopted as part of the new Strategic Highway Research program (SHRP) binder specification.

Shear‐induced particle migration in Couette and parallel‐plate viscometers: NMR imaging and stress measurements

Abstract: Couette and parallel plate viscometers are two commonly used flow geometries to characterize shear viscosity of concentrated suspensions. In Couette flow, it is well documented that prolonged shearing causes a decrease in the apparent viscosity of concentrated suspensions due to shear‐induced particle migration from the annulus region to the stagnant region under the bob. In this study, the technique of nuclear magnetic resonance imaging (NMRI) was used to measure the evolution of suspension concentration profiles in Couette and parallel‐plate flow devices upon shearing. Neutrally buoyant suspensions of nearly monodisperse, non‐Brownian spherical particles at a volume fraction of 0.5 in a Newtonian fluid were used. The same flow cells and suspensions were also used in a rheometer to measure the changes in shear stress under identical experimental conditions such that a direct comparison can be made between the stress and concentration data. For Couette flow, the NMRI data correlated very well with the str...

Rheological consequences of microstructural transitions in colloidal crystals

Abstract: The flow properties of a suspension of 146 nm diameter charge, stabilized particles at a volume fraction of 0.33 are detailed, and links are made with small‐angle neutron scatteringstructure determinations. At rest the suspension is ordered, having hexagonally close‐packed planes lying parallel to the rheometer wall that show weak registration in the shear gradient direction. The suspension shows a distinct dynamic yield stress and two regions in which the stress is a decreasing function of shear rate. Between these regions, the suspension sustains steady‐state deformation with a time‐averaged microstructure consisting of a strained crystal in which the hexagonally close‐packed layers remain intact. In the second stress minimum, the suspension microstructure is polycrystalline, while at higher stresses a sliding‐layer microstructure is observed. Metastable viscosities(shear rates) occur in constant‐stress experiments in the regions near the local stress maxima. Neither shear melting nor shear thickening was observed at shear rates up to 1.1×104 s−1.

On the pressure dependency of the viscosity of molten polymers

Abstract: A slit viscometer to measure the viscosity of polymer melts under processing conditions is described. Along the slit a pressure drop is generated by applying a pressure at both the entrance and the exit. In this way the pressure in the center can be controlled independently of the shear rate. The pressure gradient in the slit is measured by means of three pressure transducers which are mounted in the region of fully developed flow. Results of pressure-dependent viscosity measurements on polystyrene, polyacrylonitrile-butadiene-styrene, and polypropylene are presented in a shear rate range of five decades. The flow curves obtained at different pressures and temperatures can be shifted onto a master curve. The shear thinning behavior of the three materials is adequately described with the generalized Cross-Carreau equation, while the zero shear viscosity is modeled with a generalized Arrhenius-W.L.F. relationship, incorporating a pressure dependency. Alternatively, it is possible to describe the zero shear viscosity in terms of the free volume fraction and the temperature.

Experimental tests of the scaling relation for textured materials in mixtures of two immiscible fluids

Abstract: To examine the scaling relation proposed for textured materials, viscoelastic properties of binary mixtures of immiscible Newtonian fluids were measured in steady shear flows and in the transient states after step changes of the shear rate. The sample was a mixture of silicone oil and resin oil, which have almost equal density and viscosity. It was observed that (i) the shear stress and the first normal stress difference, under the steady shear flows, are almost proportional to the shear rate, and that (ii) the plots of the scaled transient stresses after a step change of shear rate, against the scaled strain, compose a single curve. These results can be well explained by the scaling relation. However, the shape of the transient stress curves after step changes of shear rate do not agree with that predicted by the constitutive equation proposed by Doi and Ohta.

Rheological characterization of the time and strain dependence for polyisobutylene solutions

Abstract: The rheological response of polyisobutylene (PIB) solutions in Decalin and a related standard fluid S1 has been characterized in dynamic oscillatory flow, step strain, step-shear rate and steady shear using a Rheometrics RDSII rheometer. The time dependence represented as a discrete spectrum of relaxation times and the strain dependence characterized as an exponential damping function have been presented as a function of PIB concentration. The relaxation spectrum was calculated from the dynamic storage modulus and loss modulus. The damping function was determined from the non-linear relaxation modulus in a step-strain experiment. The Wagner integral viscoelastic model incorporated with the relaxation and the damping function has been used to predict the stress growth and the steady-shear behaviour, which were compared with the experimental data. A novel extensional rheotester was also used in this study to measure the stretching response of polymer solutions. The data gave a near single relaxation time for each solution, and this single relaxation time obtained from uniaxial extension was correlated to the relaxation spectrum obtained in simple shear.

Rheology of colloidal suspensions: Case of lubricating greases

Abstract: Lubricating greases are two‐phase colloidal suspensions consisting essentially of a mineral oil and a metal‐soap based thickener forming a three‐dimensional gelling network. This study examines the rheology of two greases, based on lithium soap and calcium complexes, under steady, transient and dynamic flow conditions, with homogeneous shear being applied in a cone‐plate geometry. Controlled speed and controlled torque rheometers were used in this study. Various precautions need to be taken in order to obtain reliable results. These include checking and eliminating slip at the walls and fracture, correct choice of apparatus, and suitable measurement procedures. It is shown that discrepancies may arise as a result of choosing the wrong test procedures. The lithium grease is characterized by a yield stress and shear‐thinning behavior, whereas the calcium complex has a minimum in the flow curve, for which controlled torque rheometer measurements are not the most suitable. Original test procedures are adopted...

The properties of drilling muds at high pressures and high temperatures

Abstract: This paper describes an experimental study of the rheological properties of various aqueous bentonite suspensions which resemble those oil-well drilling fluids, or muds, which are encountered in practice. Data are presented for systems which are termed ‘concentrated’ muds, ‘thinned’ muds and ‘barite-loaded’ muds, under the conditions which commonly occur during oil-well drilling operations, namely, at high pressures and high temperatures. Concentrated muds comprise of suspensions containing 7—10% (by mass) clay particles. The thinned variety contains similar quantities of clay particles but relatively large amounts of electrolytes. The inclusion of barite particles in these suspensions is a routine commercial means of increasing the density of the media. The data have been obtained by the use of a rolling-ball type rheometer, for performing rheological measurements at pressures up to 1400 bar (1 bar » 105 Pa) and at temperatures up to 140 °C. The rheological response of these model drilling fluids is shown to approximate to those of Bingham fluids. The experimental data, which are reported in terms of two Bingham parameters (a yield stress and a plastic viscosity), show that the application of high pressure modifies the Bingham characteristic parameters in a way which is both temperature and mud composition dependent. The rheology of these muds is found to be shear-history dependent and the extent of this effect is described in terms of a ‘ten minute gel strength ’. This parameter provides a means of quantifying the thixotropic properties, or gel restructuring rate, of the muds. An induced volume change model, an application of the law of Corresponding States, has been introduced to describe the variations of the Bingham parameters (the yield stress and the plastic viscosity) of the concentrated muds as a function of pressure, temperature and clay content. The model is generally effective for the rationalization of the variation of the plastic viscosity. The model, however, presumes an equilibrium state for the system, and the rationalization of the yield stress data on this basis, particularly at low temperatures, is much less satisfactory. The data suggest that the characteristic gel restructuring times are long compared with the timescale of experiments and hence, complete gel formation may not have occurred during the timescale of our experiments, particularly at lower temperatures. Hence the full plastic yield strength potential was not achieved. At higher temperatures, it appears that the gel structure is more fully recovered within the experimental timescale and the inference is therefore that the gelation process is thermally activated. The data also suggested that thermally induced electrolyte dissolution may be responsible for certain features noted in the temperature dependence of the rheology. In addition, the densities and the sonic velocities of these muds at high pressures and high temperatures are described and discussed.

Turbulent pipe flow characteristics of low molecular weight polymer solutions

Abstract: Detailed mean velocity, normal Reynolds stress and pressure drop measurements were carried out with 0.4 to 0.6% by weight aqueous solutions of Tylose, a methylhydroxil cellulose (molecular weight 6000) from Hoechst after a selection process from a set of low molecular weight fluids. The viscosity measurements of the Tylose solutions showed shear-thinning behaviour, and the oscillatory and creep tests measured elastic components of the stress of the order of the minimal detectable values by the rheometer. These low molecular weight polymer solutions delayed transition from the laminar to the turbulent regime and showed drag reductions of half that reported to occur with other low elasticity shear-thinning high molecular aqueous polymer solutions. Near the wall the axial turbulent stress was higher than with water, whereas the two transverse components of turbulence were reduced. This near-wall behaviour is typical of drag reducing fluids based on high molecular weight polymers, but in the core of the pipe the three components of turbulence were higher than for the water flows, especially in the radial and tangential directions.

Method for Estimating the Chemical Crosslink Densities of Cured Natural Rubber and Styrene-Butadiene Rubber

Abstract: Chemical crosslink densities of gum and carbon black-filled natural rubber (NR) and styrene-butadiene rubber (SBR) were estimated by using a newly developed rheometer. The rheometer is the Rubber Process Analyzer (RPA 2000) which is designed specifically to measure dynamic properties such as shear storage modulus G′ and shear loss modulus G″ in cured and uncured rubber. It was found that the differences between the G′ values of dicumyl peroxide-cured NR and those of uncured samples yielded estimates of the crosslink densities which were nearly the same as the values inferred by chemical analysis. For TMTD-cured SBR, the same procedure yielded estimates of chemical crosslinks very close to those estimated by a tensile stress-strain method and by NMR. In addition, accelerated sulfur-cured natural rubber was also investigated. The agreement between the crosslink densities of these stocks determined from G′ values and from a solvent-swelling method was very good.

In-line measurement of rheological properties of polymer melts

Abstract: Shear viscosity (η), first normal stress difference (N1), and extensional viscosity (ηE) of polymer melts measured under processing conditions are important in process modeling, quality control, and process control. A slit rheometer that could simultaneously measure η, N1, and the planar extensional viscosity (ηp) was designed and tested by attaching it in-line to a laboratory model single-screw extruder. A tube (circular cross-section) rheometer to measure η and the uniaxial extensional viscosity (ηu) simultaneously was also designed and tested. Two commercial grades of LDPE (low density polyethylene) with melt index values of 6 and 12 were used as test materials for the study. Exit and hole pressure methods were used to estimate N1, and the entrance pressure drop method using the analyses of Cogswell, Binding, and Gibson (the last analysis used with the axisymmetric case only) was used to estimate ηE.

Shear fracture of polystyrene melts and solutions

Abstract: We find that symptoms of polymer melt fracture, such as a time-dependent decrease in apparent sample modulus and apparent slip, can be induced by oscillatory torsional shearing flow of polystyrene melts and solutions, even when the polymer molecular weight is below the entanglement threshold, and thre strain amplidute is as low as 3% Visualization of samples during and after fracture show crack and bubble formation, as well as delamination of the polymer from the rheometer tools For polystyrene melts, the critical stress for fracture is τ* ≈ 01–10 MPa, depending on polymer molecular weight and temperature, and for solutions it is as low as 5 × 103 Pa Since “constitutive instabilities” require the viscoelastic properties to be highly nonlinear, our observations of melt fracture in unentangled polymers at shearing strains well within the linear viscoelastic range rule out this mechanism for some of our experiments, and show that melt fracture is not always caused by constitutive instabilities

Nuclear magnetic resonance imaging rheometer

Abstract: An apparatus and method for obtaining rheological information about a fluid using nuclear magnetic resonance is disclosed herein. A fluid flowing through a tube is subjected to nuclear magnetic resonance imaging signals to obtain the velocity profile of the fluid. The pressure gradient between two points along the tube is also obtained. The shear rate is then determined from the velocity profile, and the shear stress is determined from the pressure gradient. From a single velocity profile, data is obtained over shear rates ranging from zero at the center of the tube to the maximum shear rate at the tube wall. Alternatively, the velocity spectrum can be obtained and used in the same manner. The shear stress versus shear rate curve can thereby be obtained from a single nuclear magnetic resonance image taken at a specific value of the pressure gradient.

Single‐point correction for parallel disks rheometry

Abstract: The nonhomogeneous nature of the flow in the parallel disks rheometer necessitates the differentiation of the measured quantities (torque and normal force) with respect to the shear rate at the disk rim. Commercial instruments do not calculate true material functions online, rather they report apparent Newtonian values, i.e., ones obtained assuming the material functions are constants. In this work, we present a single‐point correction technique to obtain approximate values for material functions without numerical differentiation. The advantage of the single‐point correction method is that it gives more accurate results than the apparent Newtonian values and it takes less time than numerical differentiation. This can, therefore, be useful in quality control laboratories and for process‐line measurements where reasonably accurate data are needed in a short time. A single‐point correction is applied to the parallel disks device for the shear viscosity and a new correction method for the normal stress coefficients is also developed. The accuracy of these approximate methods is tested with experimental results for a polymer melt and a polymer solution. The correction for both shear viscosity η and normal stress coefficient Ψ≡Ψ1−Ψ2 avoids the numerical differentiation of the data and can be easily implemented in software that provide online material functions.

Shear Strength of Pressure Sensitive Adhesives and its Correlation to Mechanical Properties

Abstract: Correlations between shear resistance and the mechanical properties of pressure sensitive adhesives are studied by measuring the deformation behaviour in the static and the dynamic shear test and d...

Modelling the rheological behaviour of fibre suspensions in viscoelastic media

Abstract: A rheological model for fibre suspensions in viscoelastic media is developed. The model is based on two second order tensors: one for the viscoelastic matrix and the other for the fibre orientation. Several specific features of fibre-polymeric systems are included in the model. Model predictions in stress growth and steady shear flows are illustrated in the case of an FENE-P type polymeric matrix and the results obtained are in qualitatively good agreement with experimental observations on short fibre thermoplastic composites. Predictions in stress growth shear flows show that the short time behaviour is completely dominated by the behaviour of the viscoelastic matrix. Secondary transitions for both the viscosity and the first normal stress coefficient were observed at low shear rates before the steady state is reached. Steady shear predictions show that the presence of fibres has an important effect on the first normal stress coefficient mainly in the low shear rate region. In the high shear rate region, the behaviour is completely dominated by that of the viscoelastic matrix. The presence of fibres was found to increase the steady shear viscosity over the whole range of shear rates. However, the unbounded values of viscosity in the low shear rate region were not predicted. Several other characteristics of fibre suspensions in polymeric materials are presented.

Steady viscometric properties and characterization of dilute drag‐reducing polymer solutions

Abstract: Carefully prepared aqueous solutions of drag‐reducing polymers have been characterized by means of steady viscometric and molecular weight measurements. Solutions of anionic polyacrylamide (Cyanatrol‐750), 0.5–50 ppm, and nonionic poly (ethylene oxide) (Polyox WSR‐303), 100–1000 ppm, were investigated. Steady shear viscosity measurements, with a Rheometrics Stress Rheometer and a Contraves LS30 rheometer, reveal substantial shear thinning for Cyanatrol solutions above 2 ppm and Polyox solutions above 500 ppm, for the shear‐rate range investigated. Zero‐shear viscosity was accurately determined for all solutions. Intrinsic viscosity,hydrodynamic radius of gyration, and overlap concentration were also estimated. The Carreau and Carreau–Yasuda models successfully fit all shear viscosity data, and allow estimates of elasticity. Reduced variables were used for generalizing the shear viscosity data. Aging tests show that both polymers are stable. The molecular weight and polydispersity of Cyanatrol were measured using the technique of band sedimentation/low angle laser light scattering. The drag‐reducing capability of these polymers has been confirmed by pipe flow experiments.

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.

Thixotropic Behaviour of Concentrated Fruit Pulps

Abstract: The thixotropic behaviour of peach pulp at soluble solids concentrations (SS) of 26°, 30° and 34.2° Brix was measured with a cone-and-plate rheometer. Time evolution of shear stress at a shear rate of γ = 1.0s-1 was acceptably represented by first order kinetics. Final, or equilibrium shear stress (τc) was found to increase linearly with SS. Considerable differences among the thixotropic parameters were found when comparing peach and plum pulp samples having practically the same SS, pectin and fiber levels. Scanning electron micrographs showed that differences in thixotropic behaviour could be attributed to differences in the microscopic structure of pulps.

A novel computerized viscometer/rheometer

Abstract: An innovative, Couette‐type viscometer/rheometer was developed, designed, and fabricated with the main objective being to measure viscosity and elastic properties of low‐viscous, non‐Newtonian, and visco‐elastic fluids, like dilute polymer solutions. The goal was to simplify and improve some existing drawbacks of commercial instruments employing several novel design solutions, particularly with regard to instrument precision and sensitivity. With a single pair of cylinders and a torsion bar stiffness of 0.116 Nm/rad, viscosities from 0.5 to 50 000 cP (centi‐Poise) in a shear‐rate range from 1 to 200 s−1, and oscillatory tests from 0.1 to 10 Hz, could be measured. The transducers’ electronic signals are handled by a software, developed in C language, and an IBM‐PC compatible computer with a data acquisition board. The innovative design solutions (use of a cruciform torsion bar, optoelectronics sensors, and a novel alignment procedure) have improved the critical instrument performance and reduced the number...

Nonlinear fluid behavior: from shear thinning to shear thickening

Abstract: A phenomenological model is introduced which describes both shear thinning and shear thickening behavior. Consequences of this model are presented for plane Couette (simple shear) flow. The non-Newtonian viscosity and normal pressure differences are discussed for a stationary situation. The dynamic behavior - stress growth and relaxation - is analyzed. A stress hysteresis is found in the shear thickening regime. The model is also applied to the vorticity free planar (biaxial) deformational and (uniaxial) elongational flow geometries.

A sliding plate normal thrust rheometer for molten plastics

Abstract: A sliding plate rheometer has been developed to measure the normal thrust of a molten plastic in large amplitude oscillatory shear. The normal thrust of a molten plastic in large amplitude oscillatory shear is a significant nonlinear effect that has previously been unobtainable. Normal thrust measurements may now be used to help understand and characterize nonlinear viscoelastic behavior inherent in most molten plastics. The new rhemoeter incorporates a piezoelectric pressure transducer in a very stiff plate that minimizes compliance. Normal thrust measurements with cone and plate rheometers are made by measuring the total force on the plate (or cone) and are subject to error due to edge effects. The new rheometer measures the local pressure in the sample and is therefore unaffected by sample size or edge effects. Normal thrust measurements in large amplitude oscillatory shear are reported for both molten Phillips TR480 high density polyethylene pipe resin which contains 2% by weight of carbon black filler and IUPAC LDPE X. Crosstalk due to shear stress on the active face of the pressure transducer causes signal error which for oscillatory shear is filtered out using a discrete Fourier transform.

The effect of suppression of offgassing on the rheometry of thermotropic liquid crystalline polymers

Abstract: Thermotropic liquid crystalline polymers commonly develop a bubble texture at time scales necessary to perform low‐rate rheological experiments. We have fabricated a capillary rheometer capable of high‐sensitivity (relatively low‐rate) measurements under a hydrostatic pressure adequate to suppress formation of visible bubbles. Vectra A (a copolyester of 73% HBA and 27% HNA) exhibited a transient reduction in viscosity at low rates that is independent of strain history and dependent only on time in the melt; no change in inherent viscosity was observed despite a threefold change in viscosity. The transient, which was not observed at high rates on the time scale of the experiment, may reflect changes in the defect structure. The flow curve for a given thermal history (time in the melt) exhibited a flattening at low to intermediate rates, suggestive of the flow curves reported for lyotropic systems. This result is qualitatively in agreement with rotational rheometer measurements known to be contaminated by o...