Showing papers on "Rheometer published in 2000"

Polymer and Composite Rheology

Abstract: Introduction to polymer rheology instruments for shear rheology shear viscosity of melts of flexible chain polymers shear viscosity of polymer solutions normal stress differences in polymers during shear flow dynamic mechanical properties extensional viscosity rigid-rod and liquid-crystal polymer rheology yield stress, wall slip, particle migration, and other observations with multiphase systems solid-in-liquid suspensions short-fibre suspensions emulsions gas-containing melts and foams rheology of powders and granular materials chemorheology and gelation flow through porous media melt fracture.

Interfacial shear force microscopy

Abstract: We present an experimental investigation of lossy and reactive shear forces at the nanometer scale using quartz-crystal tuning-fork shear-force microscopy. We show that this technique allows us not only to quantitatively measure viscous friction and elastic shear stress with a combination of high spatial and force resolution (better than 10 nm, and less than 1 pN, respectively), but also to obtain such quantities with the tip positioned at any arbitrary distance away from direct electrical tunnel contact with the sample surface. We are proposing that, even under vacuum conditions, the measured viscous and elastic shear stress (i.e., velocity dependent) are directly attributable to a third body filling the tip-sample gap. A simple model is given that allows us to obtain its local viscosity and shear modulus as a function of the tip-sample distance, showing that tuning-fork shear-force microscopy can be applied to quantitative analysis in nanotribology.

High-pressure rheology of polystyrene melts plasticized with CO2: Experimental measurement and predictive scaling relationships

Abstract: A high-pressure extrusion slit die rheometer was constructed to measure the viscosity of polymer melts plasticized by liquid and supercritical CO2. A novel gas injection system was devised to accurately meter the follow of CO2 into the extruder barrel. Measurements of pressure drop, within the die, confirm the presence of a one-phase mixture and a fully developed flow during viscosity measurements. Experimental measurements of viscosity as a function of shear rate, pressure, temperature, and CO2 concentration were conducted for three commercial polystyrene melts. The CO2 was shown to be an effective plasticizer for polystyrene, lowering the viscosity of the polymer melt by as much as 80%, depending of the process conditions and CO2 concentration. Existing theories for viscoelastic scaling of polymer melts and the prediction of Tg depression by a diluent were used to develop a free volume model for predicting the effects of CO2 concentration and pressure on polymer melt rheology. The free volume model, dependent only on material parameters of the polymer melt and pure CO2, was shown to accurately collapse the experimental data onto a single master curve independent of pressure and CO2 concentration for each of the three polystyrene samples. This model constitutes a simple predictive set of equations to quantify the effects of gas-induced plasticization on molten polymer systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3168–3180, 2000

Magnetorheology of magnetic holes compared to magnetic particles

Abstract: We compare the rheological behavior in a shear flow of two types of suspension of magnetic particles in the presence of a magnetic field. The first suspension is made of silica particles in a ferrofluid and the second one is made of carbonyl iron particles in silicone oil. The permeability curves of these two suspensions have been measured for different volume fractions as a function of the magnetic field in order to characterize the magnetic interactions. We show for both cases the existence of two different yield stresses: one associated with the solid friction of the particles on the plates of the rheometer and the second one with the rupture of the aggregates. This second yield stress presents a maximum with the volume fraction for the suspension of magnetic holes but grows faster than linearly with the volume fraction for the suspension of carbonyl iron. These features are explained by theoretical models based, respectively, on the deformation of aggregates of macroscopic size and on ruptures between particles at the particle scale. When plotted as a function of the Mason number, the curves for different fields well collapse on the same one but the slope of this master curve on a log–log plot is quite different from the one of the Bingham law, especially for the suspension of magnetic holes. For both suspensions the exponent depends on the volume fraction.

Novel rheometer for refractory castables

Abstract: A newly developed rheometer bridges the gap between advances in castable technology and rheological characterization techniques.

On the shear banding flow of elongated micellar solutions

Abstract: Under steady shear flow, elongated micellar solutions show shear stress saturation above a critical shear rate due to the formation of shear bands that result in non-homogeneous flow. Long transients and oscillations accompany this stress plateau. When measurements are done with a controlled stress rheometer, frequently a metastable branch is observed. At higher shear rates, a second upturn is observed above a second critical shear rate, which indicates that homogeneous flow is recovered. Here, a model consisting of the codeformational Maxwell constitutive equation coupled to a kinetic equation to account for the breaking and reformation of the micelles is presented to reproduce the features described above in steady shear flow. The model also predicts a second metastable branch and long transients at higher shear rates and the existence of an inflexion point in stress-shear rate plots above which no shear banding behavior is detected.

Boron nitride as a processing aid for the extrusion of polyolefins and fluoropolymers

Abstract: The influence of a new processing additive (fine particles of boron nitride) on the processability of polyolefins and fluoropolymers in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer, and an extruder. Two metallocene polyethylenes and several Teflon® fluoropolymers were tested using these two pieces of equipment. The additive had a significant effect on the extrudate appearance of polyethylene and fluoropolymer particularly in the crosshead dies. It was found to eliminate surface melt fracture and to postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.005% to 0.5%). To explain the possible mechanism for the effect of the additive on the processability of the resins, rheological measurements using both parallel-plate and sliding-plate rheometers were carried out. The rheology of the resins did not seem to change significantly with the addition of boron nitride except for the low-shear-rate (low-frequency) range, where the behavior of the filled resin was found to be similar to that of a crosslinked polymer or a phase-separated entangled blend. Practical wire coating and tubing extrusion studies for these resins were also carried out.

Yield stress of structured fluids measured by squeeze flow

Abstract: Various structured fluids were placed between the parallel circular plates of a squeeze-flow rheometer and squeezed by a force F until the fluid thickness h was stationary. Fluid thickness down to a few microns could be measured. Most fluids showed two kinds of dependence of f on h according to an experimentally-determined thickness h*. If h > h* then F varied in proportion to h−1 as predicted by Scott (1931) for a fluid with a shear yield stress τ0. The magnitude of τ0 from squeeze-flow data in this region was compared with the yield stress measured by the vane method. For some fluids τ0 measured by squeeze flow was less than the vane yield stress, suggesting that the yield stress of fluid in contact with the plates was less than the bulk yield stress. If h < h* then F varied approximately as h−5/2 and the squeeze-flow data in this region analysed with Scott's relationship gave a yield stress which increased as the fluid thickness decreased. This previously unreported effect may result from unconnected regions of large yield stress in the fluid of size similar to h* which are not sensed by the vane and which become effective in squeeze flow only when h < h*.

Dynamics of weakly strain-hardening fluids in filament stretching devices

Abstract: We investigate the transient viscoelastic response of weakly strain-hardening fluids to imposed elongational deformation in filament-stretching devices. We combine time-dependent finite-element simulations with quantitative experimental measurements on a rheologically well-characterized test fluid to investigate how well the device reproduces the ideal transient uniaxial extensional viscosity that is predicted theoretically. A concentrated polymer solution containing 5.0 wt% monodisperse polystyrene is used as the test fluid and the experiments are conducted using the filament-stretching rheometer, developed by Spiegelberg et al. The axisymmetric numerical simulations incorporate the effects of viscoelasticity, surface tension, fluid inertia and a deformable free surface. Single and multi-mode versions of the Giesekus constitutive equation are used to model the rheology of the shear-thinning test fluid. Excellent agreement between the measured transient Trouton ratio and the numerical predictions over a range of deformation rates is reported. The numerical simulations also reveal some important aspects of the fluid kinematics exhibited by weakly strain-hardening fluids during stretching—including a rapid necking of the filament diameter near the axial mid-plane of the fluid column, and an associated elastic recoil phenomenon near the rigid end-plates. This necking instability of a viscoelastic filament can be understood through a generalized Considere criterion, as recently documented by Hassager et al. As a consequence of this necking, spatial and temporal homogeneity in the extensional deformation of the filament is never achieved, even at large Hencky strains. This is in sharp contrast to the numerical and experimental studies for strongly strain-hardening dilute polymer solutions that have been reported to-date. Nonetheless, the present computational rheology study shows that filament stretching devices can still be used to accurately extract the transient extensional viscosity function for weakly strain-hardening fluids, provided that the evolution history of the tensile force at the end-plate and the filament radius at the mid-plane are carefully measured and that the experimental data are correctly processed.

Time dependent flow in equimolar micellar solutions: transient behaviour of the shear stress and first normal stress difference in shear induced structures coupled with flow instabilities

Abstract: Recently we studied time dependent structural changes that are coupled with flow instabilities (Fischer 1998; Wheeler 1998; Fischer 2000). Within a stability analysis, a classification scheme for the feedback circuit of coupled shear-induced structure and flow instabilities was derived by Schmitt et al. (1995) and applied to our samples. Here, inhomogeneous flow layers of different concentration and viscosity are generated by shear-induced diffusion (spinodal demixing) and, as consequence, one no longer observes a homogeneous solution but a type of shear banding that is seen here for the first time. In this paper we present the behaviour of the first normal stress difference observed in the critical shear-rate regime where transient shear-induced structure is coupled with flow instability. Similar to the oscillations of the shear stresses (strain-controlled rheometer) one observes oscillations in the first normal stress difference. This behaviour indicates that elastic structures are built up and destroyed while the shear-induced structures occur and that the induced phase is more elastic than the initial one. Oscillations of shear stress and first normal stress difference are in phase and indicate that both phenomena are caused by the same mechanism.

Shear-induced deformations of polyamide microcapsules

Abstract: The application of microcapsules for technical, cosmetic and pharmaceutical purposes has attracted increased interest in recent years. The design of new capsule types requires a profound knowledge of their mechanical properties. Rheological studies provide interesting information on intrinsic membrane properties and this information can be used to avoid premature release of encapsulated compounds due to the action of external mechanical forces (stirring, swallowing, spreading). In this publication we report a systematic study of polyamide microcapsules. These particles were synthesized by reacting 4-aminomethyl-1,8-diaminooctane and sebacoyl dichloride at the interface between silicone oil and water. Two different experiments were performed to get information on the mechanical properties of the capsule walls. First of all, we used an optical rheometer (rheoscope) to observe the capsule deformation and orientation in shear flow. The polymerization kinetics, relaxation properties, the regime of linear-viscoelastic behavior and the shear modulus of the flat membranes were independently measured in an interfacial rheometer. Both experiments gave complementary results. It turned out that the two-dimensional elongational modulus was about 3–4 times larger than the shear modulus. This result is in fairly good agreement with a theoretical model recently proposed by Barthes-Biesel. Due to the simple synthesis and well-defined structure, polyamide microcapsules can also serve as simple model systems to understand the complicated flow properties of red blood cells.

Workability of Self-Compacting Concrete

Abstract: The slump test is widely used to evaluate the workability of concrete. However, it has serious drawbacks, especially for self-compacting concrete (SCC). Other flow characteristics such as viscosity or filling capacity or time of flow through an orifice are needed to characterize flow in SCC. The goals of this study were multiple: 1) to test flow characteristics of SCC using various devices: two concrete rheometers, several standard tests, and the widely used V-flow and U-flow tests; 2) to determine the correlation between the various tests and especially between the two rheometers; and 3) to attempt to determine the rheological characteristics of SCC. Thirteen mixes were prepared with varying dosages of viscosity modifying admixture (VMA) and high-range water-reducing admixture (HRWR) to achieve a wide range of flow behavior. It was found that the plastic viscosities measured with the two rheometers were correlated at 84%, and that a SCC mixture is not defined by its high slump and slump spread alone.

Using filament stretching rheometry to predict strand formation and “processability” in adhesives and other non-Newtonian fluids

Abstract: The spinning of polymeric fibers, the processing of numerous foodstuffs and the peel and tack characteristics of adhesives are all associated with the formation, stability and, ultimately, the longevity of thin fluid `strands'. This tendency to form strands is usually described in terms of the tackiness of the fluid or by heuristic concepts such as `stringiness' (Lakrout et al. J Adhesion 1999). The dynamics of such processes are complicated due to spatially and temporally non-homogeneous growth of extensional stresses, the action of capillary forces and the evaporation of volatile solvents. We describe the development and application of a simple instrument referred to as a microfilament rheometer (MFR) that can be used to readily differentiate between the dynamical response of different pressure-sensitive adhesive fluid formulations. The device relies on a quantitative observation of the rate of extensional thinning or `necking' of a thin viscous or viscoelastic fluid filament in which the solvent is free to evaporate across the free surface. This high-resolution measurement of the radial profile provides a direct indication of the ultimate time to break up of the fluid filament. This critical time is a sensitive function of the rheological properties of the fluid and the mass transfer characteristics of the solvent, and can be conveniently reported in terms of a new dimensionless quantity we refer to as a processability parameter P. We demonstrate the usefulness of this technique by presenting our results in the form of a case study in which we measure the visco-elasto-capillary thinning of slender liquid filaments for a number of different commercial polymer/solvent formulations and relate this to the reported processing performance of the materials. We also compare the MFR observations with the prediction of a simple 1D theory derived from the governing equations that model the capillary thinning of an adhesive filament.

The uniaxial elongational rheometer RME - six years of experience

Abstract: For the increasing number of RME users, a compilation of possible measuring errors is made. A detailed description of sample preparation is given, starting from granular as well as powdery polymers. The preparation of smooth surface sample for optical rheometry is also addressed. A large part of the paper deals with the evaluation of true strain rates by particle tracking. A discussion of the errors arising from using different strain rate measures is made. Data on the elongation of LDPE at ɛ˙=0.01 s−1 is given to show the possibility of increasing the measuring sensitivity by repetition and averaging. The surface tension of the LDPE melt is measured as an example. The influences of sample thickness and the diameter of the distance pins on the measured viscosity are shown. The influences are small, and an upper limit of 1.5 mm for the sample thickness is recommended. The possibility of performing stress relaxation and recovery measurements after cessation of steady elongational flow is mentioned but not supported by our own measurements. The same holds for optical rheometry.

Establishing Linear Viscoelastic Conditions for Asphalt Binders

Abstract: The linear viscoelastic regime is defined in terms of the constitutive relationship between the stress and the strain. The set of equations that define the fundamental linear viscoelastic material properties in the time and frequency domains and their relationship to one another is based on the validity of the linearity principle. A material must obey two simultaneous conditions to be linear viscoelastic: the homogeneity (also called proportionality) condition and the superposition principle. On the basis of these considerations a testing procedure was developed to check linear viscoelastic conditions for tests performed on asphalt binders with the dynamic shear rheometer (DSR), the bending beam rheometer (BBR), and direct tension (DT). The testing procedure for the DSR requires performing strain sweeps and multiwave single-point tests. For the BBR, tests performed using different constant loads are required. In addition, the recovery part of the specification test is recorded. For the DT, tests performed at different strain rates and relaxation tests performed at different strain levels are required. When applied to asphalt binder data, the testing procedure found no departure from viscoelastic conditions for the DSR and BBR test data. However, the DT procedure indicated a departure from linear viscoelastic conditions.

The effect of boron nitride on the rheology and processing of polyolefins

Abstract: The influence of a new processing additive (fine particles of boron nitride) on the rheology and processability of polyolefins is studied. The equipment used includes an Instron capillary rheometer equipped with capillary and special annular dies (Nokia Maillefer wire coating crosshead) and two rheometers, namely a parallel-plate and a sliding-plate rheometer. Several types of boron nitride powders, varying in average particle size and distribution and in morphology are tested at various concentration levels. The additive with the smallest average particle size and free of agglomeration was found to have the greatest influence on the processability (melt fracture performance) of the polyolefins tested. Specifically, it was found that boron nitride not only eliminates surface melt fracture but also postpones the critical shear rate for the onset of gross melt fracture to significantly higher values, depending on the additive concentration, surface energy, and morphology. A flow visualization technique was used to visualize the polymer flow at the entrance of a transparent capillary die in order to determine the mechanism by which boron nitride eliminates gross melt fracture.

Estudo da viscosidade aparente das polpas de manga (Keitt) e abacaxi (Pérola)

Abstract: In this work, it was determined, experimentally, the rheological behavior of mango (Keitt) and pineapple (Perola) pulps, whole and centrifuged, in a Haake Rotovisco RV-20 rheometer with parallel plates PQ45, at 30° C. On the rheograms it was adjusted the Mizrahi-Berk (M-B) model, using the shear rate and shear stress. The results were presented as a ratio between the apparent viscosities (whole/centrifuged) with shear rate. This results showed the relative effect of suspended solids on the rheological behavior of each pulp.

Linear viscoelastic behavior of asphalt and asphalt based mastic

Abstract: The rheological properties of an asphalt mastic and its matrix are investigated. For the purpose of comparison a sample of thermal aged asphalt matrix is also considered. Dynamic and creep shear measurements are reported. The reduced shear rate concept proposed by Ohl and Gleissle is used to correlate mechanical properties of the three materials at the same temperature. We found that the concept gives only qualitative trends. A similar conclusion is found concerning the applicability of the time-temperature superposition principle for each sample. Our experimental results show also that the increase in viscosity due to thermal treatment or to the inclusion of solid particles is not uniform with temperature. The differences in the increase of the Vogel temperature from the asphalt to the mastic, or to the thermally aged asphalt, relate to the different mechanisms involved. Sedimentation of steel spheres in asphalt and mastic, is studied next. The Newtonian wall correction factor for the Stokes drag law holds for the three samples. Despite the similar behavior observed in conventional shear tests, Stokes' law gives the correct trend for the two asphalts although it overestimates the experimental settling velocity by a factor of approximately two in the case of the mastic.

The Role of Rheology in the Pipelining of Mineral Slurries

Abstract: This paper combines new experimental evidence and pipeline flow models to challenge widely held views in the literature. These include the existence of a yield stress and its determination, the existence of shear-thinning at high shear rates, and the continuum approximation which is routinely applied to these slurries. The implications of these critical points for laminar, laminar/turbulent transition and turbulent flow are investigated. It is shown that interpretation of rheological data at low and high shear rates is crucial for flow modelling in all three types of flow. It is concluded that the yield pseudoplastic model should be used for mineral slurries and low shear rate phenomena may be ignored. A high shear rate asymptote was not found or deemed necessary. The yield stress is fundamentally important for the laminar/turbulent transition and results in the critical velocity becoming independent of pipe diameter. Accurate rheology is important in smooth turbulent flow. Rough turbulent flow r...

Dynamic Rheological Properties of Highly Concentrated Protein-Stabilized Emulsions

Abstract: We have measured the dynamic rheological properties of highly concentrated emulsions of soya oil in water, stabilized with sodium caseinate, as a function of oil phase volume fraction (φ ≥ 0.70). Experiments were carried out in oscillatory mode on a controlled stress rheometer, as a function of applied stress and frequency. At low stresses these emulsions behave as a gel whose elastic properties, as a function of φ, follow the empirical model of Princen (J. Colloid Interface Sci. 1986, 112, 427) and are controlled by the Laplace pressure of the oil droplets. At higher stresses, the frequency spectra show a Maxwellian behavior with a single relaxation time. The frequency sweeps can be superimposed at each φ value onto a single master curve scaled against their Maxwellian relaxation time. Strain sweeps, derived from the frequency spectra at these higher stresses are also superimposable using as scaling factor the ratio, reference stress to the applied stress. A frequency-dependent yield stress is also obser...

Morphological and Rheological Responses to Shear Start-up and Flow Reversal of Thermotropic Liquid-Crystalline Polymers

Abstract: Novel rheo-optical methods have been used to directly observe morphology evolution, during shear start-up and reversal flow, in semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs). Using a specially designed microrheometer allowing for simultaneous transient optical and mechanical observations, we observed band formation upon reversal of flow direction. It was seen that this band formation causes asymmetric light diffraction in HV small-angle light scattering mode, indicating a tilted arrangement of the regularly spaced bands. Also conducted were shear start-up and flow reversal experiments using a cone-and-plate rheometer under the same thermal and deformation histories as those in rheo-optical experiments for polymers of differing spacer lengths at equal temperature difference below the nematic-to-isotropic transition temperature (TNI). It was observed that, during both shear start- up and flow reversal, the first normal stress difference N1 + (A o , t) exhibits a large overshoot followed by an oscillatory decay, while shear stress U + (A o , t) exhibits a large overshoot followed by a monotonic decay. It was found that the higher the applied shear rate, the larger the overshoot of N1 + (A o , t) and U + (A o , t), and the longer the persistence of oscillations in N1 + (A o , t). Similarity was found between the ratio N1 + (A o , t)/ U + (A o , t) and flow birefringence ¢n + (A o , t) during shear start-up and flow reversal of the TLCPs investigated in this study. Further, we found that the ratio U + (t,A o )/U scales with A o t but the ratio N1 + (A o , t )/ N1 does not, where U denotes shear stress at steady state and N1 denotes first normal stress difference at steady state.

Concentration dependence of the linear viscoelastic properties of particle suspensions

Abstract: The response under small amplitude oscillatory deformations of a suspension of non-Brownian spheres dispersed in a viscoelastic fluid is investigated. The correspondence principle of linear viscoelasticity is used to derive a simple constitutive model from a model for a suspension in a Newtonian liquid. The theory predicts that for a specific particulate system the concentration dependence of the viscoelastic properties should collapse to a single master curve when the values are normalized with those of the carrier fluid alone. Measurements with the micro-Fourier rheometer using oscillatory squeeze flow are carried out on two suspensions of 60 and 80 μm sized particles dispersed in polymeric fluid and in silicon oil, and the master curve is verified.