Showing papers on "Rheometer published in 1992"

Rheological and small angle neutron scattering investigation of shear‐induced particle structures of concentrated polymer dispersions submitted to plane Poiseuille and Couette flowa)

Abstract: Shear‐induced particle structures of rheologically well‐characterized concentrated polymer dispersions were investigated by small angle neutron scattering (SANS) in a wide range of shear rates. The dispersions consist of electrostatically stabilized styrene–ethylacrylate–copolymer spheres in glycol or water. Their viscosity functions show pronounced shear thinning and strong shear thickening versus shear rate as measured by various rotational rheometers and by capillary rheometry. A quartz slit die, which could be tilted with regard to the neutron beam, enabled us to achieve wall shear rates as low as 10−5 s−1 and wall shear stresses up to 104 Pa. Part of the measurements were repeated using a Couette shear cell. Spheres of 320 nm mean diameter at a volume concentration of 58.7% in glycol show an amorphous structure at rest. In the range of strong shear thinning the halo intensity becomes anisotropic, the intensity in flow direction being increased, but no long range particle structure shows up. This result shows that drastic viscosity transitions may occur, whereas the structural changes detectable by SANS may remain weak. Nearly monosized 165 nm particles at 52.3% solid in glycol exhibit in the shear thinning regime distinct hexagonal maxima superimposed on the halo and also in a second and third ring indicating the formation of a long range particle structure. With increasing shear rate these maxima disappear and an anisotropy of the halo intensity shows up. An intensity increase in flow direction at small angles is observed in the shear thickening regime. The particle structures observed after stop of flow depend on the preceding shear rate. Similar structural changes are found at 43.4% solid for the 165 nm particles. The results obtained in Couette flow are in qualitative agreement with the slit data but show much sharper intensity maxima. The underlying type of superstructure is compared with nonequilibrium molecular dynamics (NEMD) simulations for a soft sphere model fluid. The intensity patterns extracted from the simulation bear a remarkable similarity to the directly measured SANS data.

Role of slip and fracture in the oscillating flow of HDPE in a capillary

Abstract: Certain polymers exhibit two distinct branches in their capillary flow curves (wall shear stress versus apparent wall shear rate). This gives rise to oscillatory flow in constant‐piston‐speed rheometers and to flow curve hysteresis in controlled‐pressure rheometers. These curious phenomena have attracted considerable interest over a period of many years, but their basic mechanisms are still the subject of debate. Building on previous work we have developed a model that predicts all the essential features of the curves of pressure and flow rate versus time in the oscillatory flow regime. Fluid compressibility and the second branch of the flow curve are necessary features of the model, but fluid elasticity is found not to be an essential element. While our macroscopic measurements do not prove it conclusively, our data lead us to believe that on the high‐flow‐rate branch of the flow curve there is slip along a cylindrical fracture surface near the wall. The jump to the high‐flow branch occurs when this fracture occurs, at an upper critical value of the shear stress, while the jump back to the low‐flow branch occurs when adhesion is established at the fracture surface at a lower critical shear stress.

Viscoelastic Effects in MIL-L-7808-Type Lubricant, Part I: Analytical Formulation

Abstract: Analytical formulations for the computation of lubricant film thickness and traction in a high-speed rolling-sliding contact are presented with the objective of investigating the viscoelastic response of the MIL-L-7808-type lubricant Two types of relations are used to model the viscous shear strain rate. In the Type I model, a hyperbolic sine relation is used to model the viscous effect which becomes significant when the shear stress reaches a critical value. The Type II model employs a limiting shear stress, which the lubricant can withstand, and an inverse hyperbolic tangent function is considered to model the viscous behavior. Both models are based on three fundamental properties: lubricant viscosity, shear modulus and a critical shear stress. While the viscosity relations may be obtained by direct measurements, estimates of shear modulus and critical shear stress may be derived by curve-fitting the model predictions to experimental traction data. Presented at the 45th Annual Meeting in Denver, Colorad...

Electrorheology of a nematic poly(n‐hexyl isocyanate) solution

Abstract: The shear flow behavior of a unidomain nematic solution of poly(n‐hexyl isocyanate) (PHIC) subjected to an electric field was studied experimentally in a parallel plate rotational rheometer. At a shear rate of 0.4 s‐1, the Miesowicz viscosity ηc (with the director oriented along the velocity gradient) was found to be 35 times greater than the steady shear viscosity in the absence of an electric field. The mechanism for this electrorheological (ER) effect is the orientation of the permanent dipole moment of the PHIC molecules; the ER effect is an order of magnitude larger than that for low molecular weight liquid crystals. Director tumbling is postulated to occur at a shear rate of 0.4 s−1 for the PHIC solution if the applied dc electric field is lower than approximately 0.4 MV/m. At higher electric fields, flow alignment is regained. In this case, the transient stress undershoot is suppressed. Steady state viscosity vs electric field data were fit to Carlsson and Skarp’s two‐dimensional approximation of t...

Rheology of short glass fiber reinforced polypropylene

Abstract: An anisotropic rheological constitutive equation for short fiber reinforced polypropylene is presented. The bulk stress is the sum of a contribution of the Newtonian suspending fluid and of the fiber contribution. This contribution depends on the orientation which is described by an orientation tensor. The rate of change in orientation is calculated using semidilute suspension theories. After simplifications the model contains only one adjustable parameter and the transient relative viscosity depends only on the strain and not on the shear rate or time separately. Transient and steady‐shear viscosities were determined for two reinforced polypropylenes using a constant stress and a rotational parallel plate rheometer. An overshoot in the viscosity versus time curve was observed. It is attributed to changes in fiber orientation, and it depends on the fiber content but not on the shear rate. The model predictions for 20 and 30 wt. % glass fiber reinforced polypropylene are fairly good.

Determination of the wall slip velocity in the flow of a SBR compound

Abstract: Rubber compounds are known to exhibit slip at the wall in particular flow conditions. The slip velocity is usually determined by using the classical Mooney method. The rheological behavior of a styrene butadiene rubber (SBR) compound was studied with three different rheometers. Biconical rotational, capillary and slit die rheometers were used to define the true viscous behavior of the compound and the slip velocity. It was shown that it was impossible to apply the Mooney method to our experimental data. New characterizations were thus developed for both capillary and slit die experiments. They were based on the dependency of the slip velocity on the local flow gap. Contrarily to the Mooney method, they provided physically acceptable results and led to a power-law relationship between wall slip, wall shear stress and local geometry of the flow.

Response of Electrorheological Fluid-Filled Laminate Composites to Forced Vibration:

Abstract: Laminate composite beams consisting of metal or polystyrene outer strips and an electrorheological (ER) fluid (corn starch in corn oil) filler were constructed and tested using the standard Oberst test for viscoelastic materials. Application of an electric field of 2 kV/mm increased both the frequency of the various resonance modes and the loss factor associated with each. The apparent elastic modulus and loss factor of the composite beam and those of the ER fluid alone were calculated employing standard ASTM equa tions. Both modulus and loss factors of the composite beams decreased with resonance mode or frequency. The calculated shear modulus of the ER fluid increased with fre quency and the loss factor decreased, their magnitudes being in reasonable accord with those measured directly on the fluid employing a rheometer. However, the calculated mag nitude of the increase (GPa) of the apparent modulus of the metal/ER fluid composite beams with electric field and the fact that it decreases with frequency ...

Design and development of flow based electro-rheological devices

Abstract: The foundations for the design of electro-rheological (ER) actuating devices that use Poiseuille flow are laid. Systems of measurement to assess ER fluids are described including a high pressure flow apparatus. This was used to examine alternative valve geometries and fluids. A method is shown for the reduction of basic flow data to excess shear stress and shear rate. Standard actuator arrangements are reviewed before the design of a flying control surface servomechanism is discussed. The evaluation of this validates the methods and indicates that the forces developed are within striking distance of those required for operational systems.

The quasi‐periodic nature of a polyurethane melt in oscillatory shear

Abstract: Large amplitude oscillatory shear experiments were carried out on a polyurethane melt using a sliding plate rheometer incorporating a shear stress transducer. Three types of nonlinear behavior are described: periodic, quasi‐periodic, and chaotic, and common methods for characterizing these types of behavior are given. Spectral analysis is shown as the best way to distinguish between each type of behavior. Attractor reconstruction from chaos theory is introduced as a way to understand the dynamics of a system from only one dependent variable. At a low strain amplitude and frequency the behavior of the polyurethane melt is periodic but at higher amplitudes the response becomes quasi‐periodic.

Non-Newtonian rheology of leukemic blood and plasma: are n and k parameters of power law model diagnostic?

Abstract: When discussing the rheological properties of normal and leukemic blood it must be considered that blood is a suspension of cells in aqueous solution which is also known as plasma. Whole blood viscosity and plasma viscosity were determined by Rheometer LS30 which allows measuring whole blood and plasma viscosity in the middle and low shear rate ranges. The measurements of the viscosity showed that whole blood and plasma behave as non-Newtonian power law fluid. The values of n (non-Newtonian index) and k (consistency index) of power law fluid were calculated for both leukemic blood and plasma samples. The importance of this phenomenon for the micro-circulation is discussed.

Instrument effects on stress jump measurements

Abstract: Stress jumps occur in fluids that possess a viscous contribution to their total stress. On cessation of shear, such a fluid will show an instantaneous loss of stress, while some of the stress will remain and subsequently decay in some manner, either exponential or otherwise. The results in this paper show that filtering, present in most rheometers, will eliminate the stress jump and not allow its measurement. The torque head inertia for a spring-type torque measuring system is found to greatly influence the measurements and a stress jump is not seen. In fact, the material's true stress relaxation behaviour is far from that measured. The force rebalance torque measuring system is found to accurately measure stress jumps.

Adjustable gap rheometer

Abstract: An improved rheometer of the present invention includes an adjustable gap through which a flowing system is forced. The inner cavity dimension of the gap is variable so as to give a user of the rheometer an ability to vary the separation gap and the length over diameter ratio. This, in turn, allows the user to determine the shear rate and shear stress along the wall of the cavity for varying deformation rates. Thus, the shear viscosity of the flowing system can be characterized for non-Newtonian systems without varying the volumetric flow rate. One of the preferred embodiments of the adjustable gap rheometer is a slit rheometer in which the flowing system is forced through a slit having generally more length than width and more width than gap. The slit rheometer is made adjustable by holding the body of the rheometer stationary while moving an adjustable wall which forms one of the sides of the slit. Another preferred embodiment of the adjustable gap rheometer includes two concentric portions comprising a mandrel portion and a bushing portion. Axial movement of one of the portions relative to the other varies the separation between the components.

Flow Mechanics on Extrusion through a Square‐Entry Die

Abstract: The relationships between the constants of the Herschel-Bulkley (H-B) equation describing plastic flow and the Benbow equation for extrusion through a square-entry die were studied via computer simultions. These relationships are fundamental for characterizing the rheology of ceramic extrudates using the square-entry rheometer and are difficult to obtain from experiments. Results showed that the apparent shear rate should be used instead of extrudate velocity in the Benbow equation, and the Benbow equation was modified accordingly. The exponent in the H-B equation was exactly the same as the shear thinning exponent for the extrudate body in the modified Benbow equation. The die-entry velocity factor was a function not only of the shear rate factor but also of the shear thinning exponent in the H-B equation. The yield strength of the modified Benbow equation was zero when the yield strength in the H-B equation was zero.

The second normal stress difference for pure and highly filled viscoelastic fluids

Abstract: In the present work the measurement and description of the second normal stress difference in pure viscoelastic fluids and in suspensions of these fluids is discussed The various measurement methods implemented to date are described briefly Following this, the cone-and-plate distance method, which was introduced by Jackson and Kaye, is discussed The analysis method of this experimentally relative simply implemented technique is modified This is done by assuming that the ratio ψ* of the second normal stress difference to the first is independent of shear rate This permits the precalculation of the measured function with ψ* as a curve parameter The best possible fit of the measurement leads to the determination of ψ* This method is used to measure the normal stress ratio of pure polyisobutene and of a 345% suspension of the same fluid The result for the pure fluid matches literature values; ψ* of the suspension was found to have negative sign, as for the pure fluid, but to be of much greater magnitude

Nonlinear viscoelasticity of concentrated polystyrene solutions : sliding plate rheometer studies

Abstract: Because of the degree to which the linearity and polydispersity of polystyrene samples can be controlled in laboratory polymerizations, the rheological properties of melts and solutions of this polymer have been extensively studied However, in previous work the maximum strain or strain rate was limited to small values due to edge effects in the rotational rheometers used We used a sliding plate rheometer equipped with a shear stress transducer and a birefringence apparatus to measure simultaneously the shear stress and the third normal stress difference, N3 (≡σ11−σ33), during step strain, start‐up of steady shear, and exponential shear Depending on the strain history, the maximum strain achieved was between 20 and 80 The steady‐state shear stress in steady shear flow was found to be nearly independent of shear rate over a range of shear rates with a strong suggestion of a maximum, a phenomenon predicted by the Doi–Edwards theory The relaxation modulus for the shear stress was found to be superposable

Stress Relief during Solid-State Transformations in Minerals

Abstract: This work estimates the power dissipated by solid-state phase changes in the transition layer of the mantle. Earlier studies have shown these transitions occur by nucleation and growth, and that dislocations are induced in the matrix around the grain during growth. The small-scale flow allowed by the dislocations dissipates power as a sample transforms. This dissipation is easily found in high-pressure experiments by studying the growth of an isolated grain in a rigid, perfectly plastic solid, and using the observed dislocation densities to infer the yield stress. Extrapolation of this result to the mantle raises a difficulty. If the deviatoric stress depends on the strain rate, the high shear stresses inferred around the grains in the experiments may not occur at the low dilatation rates typical of the transition layer. This difficulty is treated here by studying growth in two limits. Either the growth rate is determined by: (i) the kinetics of interface propagation, or (ii) the kinetics of matrix deformation. In case (i) it is argued that the deviatoric stress in the matrix can be taken as independent of strain rate, and the problem of extrapolation is trivial. It is shown that some form of layered convection is likely in case (i), because the yield stress inferred from experiments is such that if all the volume flow in the mantle were to cross the transition layer, the extra power loss would exceed the total viscous dissipation outside the transition layer. In case (ii), the difficulty of extrapolation is significant, and a test is given to identify this case in practice. Such ‘ slip-controlled ’ growth is shown to have a characteristic signature: if the pressure is fixed, the volume of an isolated spherical grain grows exponentially with time if the matrix is isotropic, and the deviatoric stress is a functional of the strain history. The small-scale flow can then be used as a rheometer.

Experimental problems and recent results in polymer melt rheometry

Abstract: Recent developments helped to solve some experimental problems in polymer melt rheometry: By a stability test one can find out whether under test conditions the individual polymer melt is sufficiently long stable. For melts with long relaxation times, an incomplete creep-test followed by recovery is proposed to determine the linear viscoelastic material functions. In a cone-and-plate rheometer, a very constant heating system and the separation of the plate into a central disk and an outer ring allow us to measure the two normal stress differences in viscometric flows also at higher temperatures. For a commercial LCP, a special sample preparation technique is reported. Melt elongation followed by recovery in combination with morphological studies are presented for a polymer blend (PMMA/PS). From the combination of recovery after melt extension and morphology the interface tension between the melts of PS and PMMA can be determined.

Rheology of foods

Abstract: Rheology and texture of foods tutorial review. Measurements of thixotropy in suspension of microspheres. Failure testing of gellan gels. Thermorheological studies of food polymer dispersions. Rheological characterization of melting of margarines and tablespreads. The effect of shear rate and strain on the pasting behavior of food starches. Comparison of small angle oscillatory and large deformation measurements to characterize the rheology of wheat flour doughs. Flow profiles in a tube rheometer. The use of ultrasonic testing for non-destructive evaluation of theological properties. Calibration of rheological techniques used for foods.

The Dispersion of Carbon Black in Rubber Part III. The Effect of Dispersion Quality on the Dynamic Mechanical Properties of Filled Natural Rubber

Abstract: The Part I of this series described a rapid method for determining the quality of carbon-black dispersion. The second paper (Part II) was concerned with the kinetics of the dispersion of carbon black into natural rubber (dispersion quality as a function of mixing time) in an internal mixer. In this paper we consider the effects of changes in dispersion quality on the dynamic mechanical properties of both unvulcanized and vulcanized natural rubber. The effects of changes in the degree of carbon-black dispersion were measured by using a new prototype moving-die rheometer (which is presently being developed at Monsanto Instruments & Equipment Research Laboratory). By using this prototype rheometer, G′ and G″ were measured as functions of shear-strain amplitude, temperature, and frequency. Increases in the degree of carbon-black dispersion in uncured natural rubber (starting from very poor dispersion quality) give decreases in the values of both G′ and G″. The decrease in dynamic moduli with increase...

Viscosity determination of stirred yoghurt

Abstract: Viscosity of commercial stirred yoghurt was evaluated with a Couette type, strain-controlled rheometer by using concentrical cylinder, cone and plate and parallel plate geometries. Yoghurt showed a yield stress at low shear rates and obeyed power law behaviour at shear rates >1 s -1 . With the concentrical cylinder system, a significantly lower yield stress was observed. Total time where a particular shear rate range was applied affected both yield stress and the coefficients of the power law equation