Shear History Research Articles

Rheological Behavior and Structural Interpretation of Waxy Crude Oil Gels

A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.

Morphology development of a PS/PMMA polymer blend during flow in dies

The morphology development in dilute and semi-concentrated blends (2 and 15 wt% disperse phase) of viscoelastic polymers is studied during flow in dies. In the entrance region the droplets deform into fibrils. In the die itself some of the fibrils can break up depending on their shear history and hence on their radial position. The morphology at the exit of the die is investigated by quenching the extrudate and visualizing the structure via scanning electron microscopy (SEM). For fibrils moving along the die axis, the theory of Tomotika for break-up of a fibril in a quiescent matrix describes the observations satisfactorily. Fibrils flowing off center undergo a shearing flow in the die, which could have an effect on the growth of the Rayleigh disturbances that cause break-up. It is observed that during flow break-up still occurs via Rayleigh instabilities. As a first approximation the theory of Tomotika also predicts the break-up of fibrils flowing off center, if the viscosity at the relevant shear rate is used.

The effect of block copolymer architecture on the coalescence and interfacial elasticity in compatibilized polymer blends

The effect of block copolymer architecture on the suppression of droplet coalescence and on the interfacial elasticity was studied in immiscible blends of polydimethylsiloxane (PDMS) and polyisoprene (PI) with a droplet-matrix morphology. The PDMS-PI diblock copolymers used in this study to compatibilize the blends differ in molecular weight and degree of asymmetry of the blocks. The general Palierne model with an interfacial shear modulus was used to analyze the dynamic measurements performed after different shear histories. It was shown that the coalescence suppression is more effective when the amount of compatibilizer increases and when the overall molecular weight of the block copolymer increases. When comparing the coalescence behavior of a blend and the inverse blend, it was shown that coalescence is suppressed more when the longest block of the block copolymer is located in the matrix. The interfacial relaxation time increases with molecular weight of the blocks for symmetric block copolymers. Asymmetry of the blocks also causes it to increase. A scaling relation is proposed for the interfacial relaxation time of PI/PDMS blends. This master curve is extended for varying viscosity ratios of the blend and with data of a polydimethylsiloxane (PDMS)/polyisobutylene (PIB) blend and a polymethylmethacrylate (PMMA)/polystyrene (PS) blend.

Localized Material and Processing Aspects Related to Enhanced Recycled Material Utilization Through Vibration-Assisted Injection Molding

With society focusing more and more on environmental issues, the recycling of materials of all types has become an important concern. As a result of previous processing and usage, the properties of recycled materials are often significantly inferior when compared to their unrecycled counterparts. Recycled polymeric materials, for example, have lowered molecular weights caused by previous thermal and/or shear histories. Thus, the scope of application for recycled materials has traditionally been very limited. With the aid of a new processing concept known as vibration-assisted injection molding (VAIM), the mechanical properties of products containing recycled polymeric materials could be dramatically improved. The current paper presents the results of an initial experimental investigation into the application of VAIM for recycled polystyrene (Recycled-PS) products. A number of recycled/new material blends were studied, and in all cases significant product mechanical property enhancements were realized through the use of VAIM. It was concluded that VAIM molded products containing as much as 50% low-grade recycled material could be made stronger than products made using traditional molding with 100% new polymer. Detailed results are presented in the paper, along with some insight into the likely fundamental mechanism underlying the utility of the VAIM technique.

The strength of paraffin gels formed under static and flow conditions

When the temperature of a solution of paraffins in oil is dropped below the ‘cloud point’ temperature, the high molecular weight n -paraffin molecules precipitate out of solution, crystallize and form a gel. The gelation and concomitant deposition of these paraffin gels in crude oil pipelines poses a major transportation problem by reducing the flow efficiency. Implementation of mechanical methods of remediation of paraffin deposition requires knowledge of the gel strength. Rheometric studies were performed on a model system to study the yield strength of paraffin–oil gels formed under various shear and thermal histories. It was observed that when the gel was formed under quiescent (shut-in) conditions, the yield stress of the gel decreased with an increasing cooling rate. However, when a shear stress was exerted on the gel during cooling (as would be experienced in a flow line), the trend of the yield stress vs. cooling rate curve was strongly influenced by the magnitude of this shear stress. Additionally, experimentation over a range of applied shear stresses revealed that the yield stress of the gel reaches a maximum at a moderate value of the applied shear. These rheometric results are explained with the help of 3-D polarized light microscopy observations of the paraffin crystal structure formed under various shear and cooling conditions using static and flow cell systems. The effects of crystal size on the gel properties are enunciated.

ELIMINATION OF REVERSAL FLOW IN BACK CLEARANCE GAPS OF A MINIATURE PEDIATRIC CENTRIFUGALBLOOD PUMP BY CFD

The configuration of a centrifugal pump has been widely used as a ventricular assist device (VAD) for adults and recently for infants and small children with failing ventricles. For such a centrifugal pump, the back clearance gap together with the central core and outer annular clearance gap jointly form a washout flow path. However, within the washout path, the flow demonstrates a complex 3-dimensional structure, whcih potentially causes cellular trauma and/or thrombosis and also affects hydrodynamic performance. The objective of this study was to eliminate the reversal flow in the back clearance gaps of a miniature pediatric centrifugal blood pump by CFD analyses and design optimization. A commercial CFD code, Star-CD, was utilized to simulate the 3-D blood flow within a miniature centrifugal pump. An implicit method based on multiple reference frames was used for the coupling between the rotational and stationary elements. A power law function was integrated into the code to evaluate hemolysis with the shear history being recorded by the Lagrangian particle tracking method. By CFD analyses and optimization design, it was found that secondary blades, located along the lower and side surfaces of the rotor, can generate antegrade blood flow in the clearance gaps. Through optimizing the secondary blades number and their geometry by CFD, we were able to achieve net antegrade flow with minimal zones of retrograde flow.

Three-Dimensional Structure of the Zone-Drawn Film of the Nylon-6/Layered Silicate Nanocomposites

Semicrystalline polymer/layered silicate nanocomposites exhibit a unique three-dimensional structure associated with the high-aspect ratio constituents (layered silicates and polymer crystallites), which is sensitive to shear and deformation history. Beginning from quenched biaxially extruded films of nylon-6/ montmorillonite nanocomposites, the impact of elevated temperature uniaxial drawing on the orientation of the layered silicate and crystallites, crystalline polymorphism, and layered silicate buckling was studied using X-ray and transmission electron microcopy. The quenched biaxially protruded films exhibited a uniplanar crystalline texture with the b-axis of the dominate γ-form oriented toward the normal to the film surface and the surface of the layered silicate oriented parallel to the film surface. Small-angle scattering features suggest that the lamellar of the γ-form is within the proximity of the layered silicate and has a fringed micelle structure. Upon uniaxial drawing, α-crystallites developed with a lamellar regularity of ∼15 nm along the draw direction, comparable to previous studies on pure nylon-6 fibers. The newly formed α-form exhibited a uniaxial texture with the b-axis parallel to the draw direction. This suggests that the α-form crystallized during the drawing of the film and does not have an orientation correlation with the layered silicate. Finally, the layered silicate within the zone-drawn film buckled perpendicular to the draw direction, analogous to failure of a uniaxially strained sheet of paper. The failure mode appears to occur for a collection of parallel aluminosilicate layers (2−4) and not individually. Recognition of the structural changes of the layered silicate and crystalline regions in response to uniaxial deformation of planar oriented films is critical to ascertaining the local transport properties of shaped layered silicate nanocomposite parts.

Rheology of Hydroxyapatite Dispersions

Hydroxyapatite (HA) is an interesting ceramic material for orthopedic applications, in particular for implant operations and bone regeneration, owing to its bioactivity and biocompatibility with the surrounding tissues. Even if different shaping processes can be used in order to obtain porous ceramic bodies with optimal final properties, the sponge impregnation method with aqueous HA dispersions is particularly suitable to achieve an appropriate macroporosity to bone regeneration. As for conventional slip or tape casting, the rheological behavior of HA suspensions must be properly fitted to process conditions and controlled through a satisfactory stabilization of the disperse phase, i.e., through an appropriate dispersant dosage and an accurate homogenization of dispersed powder. In the present work, HA powders with different crystallinity degree were used to prepare aqueous dispersions with different contents of solids and dispersant (ammonium polyacrylate). The specific surface area of HA powders is very high, and then special attention must be dedicated to the dispersant selection and the dispersion process since the rheology of HA suspensions is strongly influenced by the structural conditions of the disperse phase. Even small differences in dispersant concentration can lead to dramatic changes in the rheological properties also at relatively low values of solids volume concentration. Above a critical concentration, the viscosity drop associated with the apparently plastic behavior is confined within a very narrow stress range, a neat transition is observed in the linear viscoelastic properties, and the time‐dependent effects induced by the shear history become quite important and crucial for the experimental characterization.

CHanges in the Instantaneous Shear Modulus of Normally Consolidated Clay with Shear History

ABSTRACT Changes in the instantaneous shear modulus associated with the non-coaxiality of NC clay materials are presented. A series of hollow cylindrical shear tests was performed to measure the changes in the instantaneous shear modulus with shear history. After each normally consolidated clay specimen was compressively sheared until a prescribed shear level, it was continuously torsionally sheared under a constant volume to measure the instantaneous shear modulus. Coaxial constitutive relations, such as these of the conventional Cam-clay type of models, predict that the instantaneous shear modulus obtained, does not change regardless of the compressive shear history. On the contrary, it is known that non-coaxial constitutive relations theoretically bring about changes in the instantaneous shear modulus in relation to the compressive shear history. The test results supported the predictions made by the non-coaxial constitutive models. Finally, the proper formulation for the non-coaxiality between the stress and the incremental plastic strain is discussed.

Effect of shear history on the morphology and coarsening behaviour of polycarbonate/poly(styrene-co-acrylonitrile) blend

The morphology and coarsening behaviour of polycarbonate (PC) blend with styrene-co-acrylonitrile (SAN) random copolymer of 25wt% AN have been investigated as a function of shear history using transmission electron microscope (TEM) and time resolved light scattering techniques. Simple shear apparatus of two parallel plates geometry was used to generate different shear rate values depending on the different distances from the centre of the sample disk. The morphology of PC/SAN-25=70/30 blend showed that the dispersed phase of SAN was elongated and broken-up in the direction of flow with weaker contrast at high shear rate values. The shear rate was found to suppress the concentration fluctuations and enhance the miscibility of SAN (dispersed phase) in the PC matrix to a great extent. The average of the dispersed particle diameter was evaluated as a function of different shear memories at 240°C for different time intervals based on the Debye–Bueche theory. The obtained data were found to be shear memory dependent i.e. the average particle diameter decreases with increasing shear memory. This result indicated that the coarsening process is greatly suppressed by shear memory and the shear could produce a permanent morphological change (irreversible change) over the time scale of the measurement. This behaviour was attributed to the very high melt viscosities of the blend components, which in turn led to a very long relaxation time and consequently a high difficulty to erase the effect of shear at the experimental temperature. Furthermore, the coarsening process for all the measured samples followed the general power law, R¯3(t)=R¯3(0)+t, regardless of the shear memory of the blend. This behaviour implied that the shear could only retard the rate of domain growth without any effect on the coarsening mechanism.

Rheology of chemically modified triglycerides

AbstractThe rheology of chemically modified plant oils was determined to aid in their processing for polymers and other applications. Epoxidized and acrylated triglycerides were derived from various plant oils and model triglycerides. The viscosity of the unmodified oils decreased as the level of oil unsaturation increased. However, the viscosity of the epoxidized oils increased slightly as the level of epoxidation increased. Furthermore, the viscosity of the acrylated oils increased exponentially as the level of acrylation increased because of increased polarity. In addition, the viscosity of the acrylated oils increased as the average distance of the acrylate groups from the fatty acid chain ends decreased. Chemically modified and unmodified oils did not exhibit any shear‐thinning behavior or any memory of shear history. The temperature dependence of the viscosity followed the Arrhenius law, and the activation energy decreased linearly with the level of acrylation. The addition of comonomers, such as styrene, reduced the viscosity of the acrylated oils exponentially as the comonomer concentration increased. However, the glass‐transition temperature and modulus of the triglyceride‐based polymers decreased as the styrene content decreased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 774–783, 2005

Online Optical Monitoring of Asphaltene Aggregation

An optical method is employed to investigate asphaltene aggregation kinetics in real time using a photometric dispersion analyzer (PDA). The size of the aggregates is obtained by coupling photographic technique and image analysis with the online output of the PDA. The aggregation and breakup of the flocs are studied in a stirred tank for various volume fractions of particle (φ), stirring speed, and solvent composition (i.e., ratio of toluene-to-heptane in the solution, T:H). Initially, the average aggregate size increases and eventually reaches a steady-state value that reflects a balance between aggregation and fragmentation. Increasing the stirring speed reduces the steady-state size. At a certain stirring speed, when either φ is increased or T:H is decreased, the steady-state floc size increases. The presence of maltenes in the bitumen greatly retards the aggregation kinetics of asphaltenes. A cyclic- and tapered-shear schedule decreases the average floc size. The steady-state size attained at a certain stirring speed depends on the shear history of the flocs.

Thixotropy and rheopexy of aggregated dispersions with wetting polymer

Different types of thixotropy of aggregated dispersions of magnetic metal, nonmagnetic iron oxide and carbon black particles in organic solvents with a polymer containing pendant wetting groups are studied. Thixotropic behavior depends on the type of aggregation which determines the type of structural recovery in and after shear. Metal particles form denser aggregates combined into flocs or networks recoverable in or after shear, which results in positive thixotropy with up-shear stress-vs-shear-rate curves running above down-shear curves. By contrast, iron oxide and carbon black particles form looser aggregates which re-flocculate slower in shear and do not re-flocculate at all at rest after shear, which results in rheopexy with recovery strongly accelerated by shear and negatively thixotropic loops. Viscoelastic moduli of the latter systems are determined by their shear history, e.g., G′ decreases with the preceding shear rate and does not recover. The effect of the wetting polymer on time-dependant phenomena is twofold: It suppresses thixotropy by consolidating aggregates, but slows down recovery, so that rheopexy is maximized at certain polymer-to-pigment ratios. Furthermore, two types of rheopexy are discriminated. Recovery rate of carbon black dispersions is controlled by the current shear rate only, resulting in thixotropic loops strongly dependent on sweep time. On the other hand, recovery rate of iron oxide dispersions has a long memory of shear history, which leads to thixotropic loops almost insensitive to sweep time. The difference is attributed to slow shear-induced dilation/shrinkage of aggregates built from elongated iron oxide particles, as opposed to stable aggregates of carbon black.

PEO-induced flocculation of fines: effects of PEO dissolution conditions and shear history

We have studied one of the retention aid systems used in the retention of fines in mechanical grade furnishes, the dual component system consisting of polyethylene oxide (PEO) and a cofactor. The efficiency of the retention aid was determined from fines flocculation experiments in a stirred beaker. It was found that the fines flocculation efficiency depended strongly on how the PEO solution, used in the experiments, was prepared. The way PEO was dissolved (stirring time and intensity during dissolution), stored (storage time and concentration), and injected in the flocculation vessel (shear conditions and dilution), all affected the efficiency to various degrees. The results are interpreted in terms of entanglements of PEO molecules. When PEO granules are dissolved in water, PEO is not instantaneously dissolved as single individual molecules, but initially PEO is present in the form of entanglements. These entanglements become smaller with time and shear. In general, entanglements are more efficient in flocculating fines than freely dissolved molecules. The implications for fines retention on a paper machine are discussed.

Effect of shear memory on the coarsening behaviour of polystyrene and poly(vinyl methyl ether) blend

The effect of shear memory on the coarsening behaviour of polystyrene/poly(vinyl methyl ether) (PS/PVME) blend which shows a typical lower critical solution temperature (LCST)-type phase diagram has been thoroughly investigated for the near critical composition (PS/PVME=30/70) using a time-resolved light scattering technique. The measurements were carried out at 135 °C (20 °C above the quiescent cloud point) at two different directions, parallel and normal to the direction of flow. Different shear memories were generated in the melt using a simple shear apparatus of parallel plate geometry. The coarsening process was influenced to a great extent by the shear history of the blend over the time scale of the measurement. The average domain size of the dispersed particles obtained from the analysis of the light scattering data on the basis of Deby Bueche theory was found to be shear memory dependent. The coarsening process was elevated and suppressed at low and high shear memory, respectively. This behaviour was attributed to the shift of the cloud point observed under same values of shear rates. In addition, the coarsening behaviour of this blend was found to be flow direction independent due to the very high viscosity ratio of the blend, which led to in turn rather circular domains of PS in PVME matrix without any elongation or orientation in the direction of flow. Furthermore, the coarsening process for all the measured samples was followed the general power low, R̄3(t)=R̄3(0)+t, regardless the shear history and the flow direction of the blends. This result indicated that; the shear could only retard or elevate the rate of domain growth without any effect on the coarsening mechanism.

Extending the use of zinc‐containing biocides in PVC

AbstractWhile zinc is a crucial element in the formulation of many PVC stabilizers, high levels negatively affect heat and shear stabilities of PVC formulations. Thus, the use of zinc‐containing biocides in PVC has been limited to plastisol formulations, which are not subjected to the heat and shear histories of calendered and extruded formulations. Patented technology has been developed that overcomes many of the stability issues raised by the addition of zinc‐containing biocides to PVC. Specifically, the use of hydrotalcites in combination with zinc pyrithione improves sensitivity to heat and shear as well as retention of the antimicrobial activity of the zinc additive. J. Vinyl Addit. Technol. 10:95–98, 2004. © 2004 Society of Plastics Engineers.

Deformation conditions, kinematics, and displacement history of shallow crustal ductile shearing in the Norumbega fault system in the Northern Appalachians, eastern Maine

The Norumbega fault system in the Northern Appalachians in eastern Maine experienced complex post-Acadian ductile and brittle deformation from middle through late Paleozoic times. Well-preserved epizonal ductile shear zones in Fredericton belt metasedimentary rocks and granitic batholiths that intrude them provide valuable information on the nature, geometry, and evolution of orogen-parallel strike-slip Norumbega faulting. Metasedimentary rocks were ductilely sheared into phyllonite schistose mylonite, whereas granite into mylonite within the ductile shear zones. Ductile shearing took place at conditions of the lower greenschist facies with peak temperatures on the order of 300–350° based on comparison of plastic quartz and brittle feldspar microstructures, confirming a shallow crustal environment during faulting. Ductile shear strain was partitioned into two major shear zones in easternmost Maine—the Waite and Kellyland zones—but these zones converge toward the southwest. Megascopic, mesoscopic, and microscopic kinematic indicators confirm that fault motion in both zones was dominantly dextral strike-slip. Detailed mapping, especially in the plutonic rocks, reveals a complex ductile deformation history in the area where the Waite and Kellyland zones converge. Shear strain is broadly distributed in the rocks between Kellyland and Waite zones, and increases toward their junction. Multiple dextral high-strain zones oblique to both zones resemble megascopic synthetic c′ shear bands. Together with the Kellyland and Waite master shear zones, these define a megascopic S–C′ structure system produced in a regional-scale dextral strike-slip shear duplex that developed in the transition zone between the deeper (south-central Maine) and shallower (eastern Maine) segments of the Norumbega fault system. Granite plutons caught within the strike-slip shear duplex were intensely sheared and progressively smeared into long and narrow slivers identified by this study. The western lobe of the Deblois pluton and the Lucerne pluton have been recognized as the sources, respectively of the Third Lake Ridge and Morrison Ridge granite slivers. Restoration of both granite slivers to their presumed original positions yields approximately 25 km of dextral strike-slip displacement along only the Kellyland and synthetic ductile shear zones.

Shear-induced fat particle structure variation and the stability of food emulsions: I. Effects of shear history, shear rate and temperature

The shear-induced fat particle structure variation and the stability of food emulsions were investigated by the back-light scattering technique. The effects of temperature, shear rate, and shear history on the fat particle structure and stability of food emulsions were studied. Increasing the shear time initially improves the fat particle structure; afterwards, the fat particle packing structure becomes less ordered. The effect of temperature on the fat particle structure is complicated. It was also found that the optimal temperature, shear rate and shear history exist in order to obtain a well-developed fat particle structure. A qualitative explanation of the experimental results is provided.

Effect of shear memory on the coarsening behaviour of polystyrene and poly(vinyl methyl ether) blend

The effect of shear memory on the coarsening behaviour of polystyrene/poly(vinyl methyl ether) (PS/PVME) blend which shows a typical lower critical solution temperature (LCST)-type phase diagram has been thoroughly investigated for the near critical composition (PS/PVME=30/70) using a time-resolved light scattering technique. The measurements were carried out at 135 °C (20 °C above the quiescent cloud point) at two different directions, parallel and normal to the direction of flow. Different shear memories were generated in the melt using a simple shear apparatus of parallel plate geometry. The coarsening process was influenced to a great extent by the shear history of the blend over the time scale of the measurement. The average domain size of the dispersed particles obtained from the analysis of the light scattering data on the basis of Deby Bueche theory was found to be shear memory dependent. The coarsening process was elevated and suppressed at low and high shear memory, respectively. This behaviour was attributed to the shift of the cloud point observed under same values of shear rates. In addition, the coarsening behaviour of this blend was found to be flow direction independent due to the very high viscosity ratio of the blend, which led to in turn rather circular domains of PS in PVME matrix without any elongation or orientation in the direction of flow. Furthermore, the coarsening process for all the measured samples was followed the general power low, R ̄ 3(t)= R ̄ 3(0)+t, regardless the shear history and the flow direction of the blends. This result indicated that; the shear could only retard or elevate the rate of domain growth without any effect on the coarsening mechanism.

Stress relaxation after steady shear flow in immiscible model polymer blends

Stress relaxation in immiscible blends is studied for a well defined shear history, i.e. after prolonged steady state shearing. Model systems are used that consist of quasi-Newtonian liquid polymers. Hence the relaxation is dominated by changes in the morphology of the interface. Both shear stress and the first normal stress are considered. The measurements cover the entire concentration range. For dilute blends the interfacial contribution to the stress relaxation compares well with model predictions. Deviations occur when the matrix phase is slightly elastic. In that case the similarity between the relaxation of shear and normal stresses is also lost. The latter is attributed to a wider drop size distribution. Increasing the concentration of the disperse phase results in a complex evolution of the characteristic relaxation times. The normal stresses relax systematically slower than the shear stresses and the concentration curve includes two maxima. Even for equiviscous components the concentration curves are not symmetrical. It is concluded that even a slight degree of elasticity in the matrix phase drastically affects the morphology and the interfacial relaxation of such blends.