Thixotropic Loop Research Articles

Effect of the interfacial interaction on the phase structure and rheological behavior of polypropylene/ethylene– octene copolymer/BaSO4 ternary composites

AbstractIn polypropylene (PP)/ethylene–octene copolymer (POE)/BaSO4 ternary composites, two different kinds of phase structures are assumed:(1) POE and BaSO4 filler are separately dispersed in the PP matrix and (2) POE‐encapsulated filler particles (core–shell structure) are dispersed in the matrix. This depends on the interfacial interaction of the composites. For the design of composites with different interfacial interactions, three routes for the preparation of BaSO4 master batches were developed. First, a mixture of BaSO4, POE, and maleic anhydride (MAH) in a certain ratio was extruded in the presence of dicumyl peroxide and then pelletized. In extrusion, MAH‐functionalized POE was in situ formed to enhance the interfacial interaction between POE and BaSO4. Second, a mixture of POE and BaSO4 was directly extruded and then pelletized. Third, after BaSO4 was treated with an organic titanate coupling agent, the treated BaSO4 and POE were blended in extrusion and then pelletized. Scanning electron microscopy observations showed that the core–shell structure in which POE encapsulates BaSO4 particles is formed through route 1, whereas POE and BaSO4 are separately dispersed into the PP matrix through routes 2 and 3. The rheological behavior of PP/POE/BaSO4 ternary composites was studied with a controlled stress rheometer. The results showed that the interfacial interaction in composites with core–shell morphology is the strongest. Interparticle interactions give rise to the formation of interparticle networks; the stronger the network is, the larger the shear yield stress is and the smaller the thixotropic loop area is. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1804–1812, 2002

EFFECT OF PROCESSING CONDITIONS ON THE VISCOELASTIC BEHAVIOR OF NU-TRIMX: AN OAT-BASED ?-GLUCAN-RICH HYDROCOLLOIDAL EXTRACTIVE

This paper examines the efsects of processing conditions and starting materials on the viscoelastic behavior of Nu-TrimX (a new fi-glucan-rich hydrocolloid product) suspensions (5 % by weight in deionized water). In general, Nu-TrimX suspensions exhibit clockwise thixotropic loops with shearthinning behavior over the experimental shear rate range (0-250 s-') at 25C. The rheological responses of the various Nu-TrimX suspensions were modeled using a power law constitutive equation. The suspensions behave as pseudoplastic materials with power law exponents ranging from 0.42 to 0.71.

The use of calcined clay as part replacement of titanium dioxide in latex paint formulations

Titanium dioxide, TiO2, being a prime pigment, has been used extensively in the paint industry. The growing demand and the increasing cost of TiO2 has driven the attention of paint technologists to search for other alternatives. There has been continuous effort to replace part of the TiO2 with a fine particle size extenders in paint formulations. A detailed study has been undertaken to replace TiO2 pigment by various percentages of calcined clays in two latex paint formulations. Properties such as thixotropy, hiding power, light fastness, film brightness, scrub resistance, and weather resistance have been determined to establish the optimum use of calcined clays in the paint formulation. Thixotropy, one of the important properties of latex paints, has been studied with the help of thixotropic loop area method. The viscosity data has also been analyzed by Casson's equation. It has been found that the calcined clays can replace up to 20% TiO2 in paint formulations without having any adverse effect on their properties. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1029 -1036, 2000

Understanding thixotropic and antithixotropic behavior of viscoelastic micellar solutions and liquid crystalline dispersions. I. The model

A simple model consisting of the Upper Convected Maxwell constitutive equation and a kinetic equation for destruction and construction of structure, first proposed by Fredrickson in 1970, is used here to reproduce the complex rheological behavior of viscoelastic systems that also exhibit thixotropy and rheopexy under shear flow. The model requires five parameters that have physical significance and that can be estimated from rheological measurements. Several steady and unsteady flow situations were analyzed with the model. The model predicts creep behavior, stress relaxation and the presence of thixotropic loops when the sample is subjected to transient stress cycles. Such behavior has been observed with surfactant-based solutions and dispersions. The role of the characteristic time for structure built up, λ, in the extent and shape of the thixotropic loops is demonstrated.

A rheological model for lithium lubricating grease

The rheological properties of a lithium lubricating grease were examined under various settings of shear rate, time and temperature with an emphasis on the thixotropy of grease. A HAAKE RV 20 rheometer was employed in the experiments, utilizing a cone and plate configuration. Precautions were taken when scheming and carrying out the test in order to eliminate the effects of slip at the wall, fracture and other forms of disturbance. Flow curves and thixotropic hysteresis loops were obtained. Based on the measurement results, a new rheological model involving a structural parameter has been proposed. The characteristics of the proposed model and its implication for lubrication analysis were discussed.

Network development during shear-thickening in semidilute solutions of gently solubilized starches

The shear-thickening behavior of semidilute waxy maize (WM) starch solutions (90/10 DMSO/water) exhibit much of the behavior of classical nonlinear viscoelasticity. Small-amplitude oscillatory shear experiments were used to investigate the network structure formed during the shear-thickening regime. The solution viscoelastic properties of WM starch prior to the shear-thickening region could be described by the GLV model. The macromolecules behave as random coils with a longest relaxation time of 0.58 ± 0.03 s. For WM starch the observed shear-thickening region creates a stable, highly entangled network which resembles the behavior found in solutions in the concentrated regime. The longest relaxation time for the entangled solution is estimated to be 1.4 s. Semidilute solutions of normal maize (NM) starch in 90/10 DMSO/water do not exhibit a shear-thickening regime. The oscillatory shear data obtained before and after a thixotropic loop experiment were identical. The semidilute solution conformational dynamics of NM starch indicate some degree of rigidity in one of the components that constitute NM starch which might be associated with the helical structure obtained by amylose in DMSO. © 1998 John Wiley & Sons, Inc. 1 This article is a US Government work, and, as such, is in the public domain in the United States of America. J Polym Sci B: Polym Phys 36: 2085–2093, 1998

Solution Viscoelastic Properties of OATRIM‐10 and Cooked Oat Bran

ABSTRACTThe solution rheological behaviors of OATRIM‐10 and cooked oat bran were investigated. The rheological properties of the materials were investigated using both thixotropic loop and small‐amplitude oscillatory shear experiments. The cooked oat bran exhibited shear‐thinning behavior during a thixotropic loop experiment over a shear rate range of 0–250/sec. The shear‐thinning behavior was reproduced during the measurement of a second thixotropic loop. In contrast, OATRIM‐10 exhibited an unexpected region of shear‐thickening behavior at 20–80/sec. The shear‐thickening and subsequent shear‐thinning regions for OATRIM‐10 could be described by a transient network model indicating that the shear‐thickening behavior is caused by a shear‐induced entangled network that is partially disentangled at higher shear rates. Subsequent thixotropic loop experiments displayed the shear‐thickening region for OATRIM‐10, indicating that the network structure can be reformed during the imposition of a shear field. Small‐amplitude oscillatory shear data for cooked oat bran can be described reasonably well using a generalized linear viscoelastic (GLV) model. The oscillatory shear data obtained for OATRIM‐10 could not be described by the GLV model. OATRIM‐10 exhibited a distinctive plateau centered at 10/sec, and the low frequency response of storage modulus G′ decreased with a much larger slope in frequency than was predicted by the GLV model.

Rheological and mechanical properties of poly(butylene terephthalate)-modified epoxy resins

Poly(butylene terephthalate) (PBT) was used as modifier for epoxy resin. With the incorporation of 7.5 wt% PBT, the resin became a gel as shown by rheological measurements in steady shear, thixotropic loop and dynamic shear mode. The gel was very stable on storage. An abrupt change of rheological properties occurred at the dissolution temperature of the PBT spherulites. The PBT modifier did not impair mechanical properties of the cured resins, yet a moderate improvement in toughness was achieved.

THE FLOW PROPERTIES OF NATIVE, HEAT-MOISTURE TREATED, AND ANNEALED STARCHES FROM WHEAT, OAT, POTATO AND LENTIL

The effect of heat-moisture treatment (30% moisture, 100C, 16 h) and annealing (75 % moisture, 50C, 72 h) on the flow behavior of gelatinized starch pastes from wheat, oat, lentil and potato starches were studied at a concentration of 6% starch with a cone and plate viscometer (Wells Brookfield RVTDV II CP 200). The power law rheological model (σ=Kγn) was used to describe the flow behavior of the above starch pastes. All native starches exhibited a non-Newtonian shear thinning behavior. A thixotropic loop was evident only in oat starches and native potato starch. Among native starches, the magnitude of the shear thinning index (n) followed the order: oat > wheat > lentil > potato, while the corresponding order for the consistency index (K) was: potato > lentil > wheat > oat. Heat-moisture treatment decreased the K value of all starches. On annealing, K decreased in wheat and lentil starches, but increased in potato and oat starches. Heat-moisture treatment and annealing increased the n value of wheat, lentil and potato starches, but decreased that of oat starch. In all starches, the modification to the flow behavior was more marked on heat-moisture treatment than on annealing.

The Response of Viscoelastic Liquids to Complex Strain Histories: The Thixotropic Loop

The dynamics of viscoelastic liquids in a thixotropic shear loop is examined. Shear start‐up and thixotropic loop experiments with an aqueous poly(acrylamide) solution were conducted under a wide range of Deborah and Weissenberg numbers. The transient stresses generated by the liquid were measured and compared with predictions of the Wagner model. An efficient linear regression algorithm, utilizing a singular‐value decomposition routine, was developed and used to extract the linear parameters of the model from dynamic‐mechanical data. The nonlinear parameter was obtained from start‐up data by Wagner's method. The dynamic response of the liquid in a thixotropic loop is characterized by several distinct features, including stress overshoot and residual stress, which are well reproduced by the Wagner model. The general form of the dynamic response function is governed by the specific combination of the Deborah and Weissenberg numbers. Overall, the results indicate that viscoelastic effects may lead to distinct hysteresis in the response function in a thixotropic loop experiment, which may be erroneously interpreted as a rheological thixotropic effect.

Thixotropic properties of yellow iron oxide-alkyd paints: their dependence on pigment-resin-solvent interactions

The effect of pigment content and the nature of the solvent on the thixotropic properties of synthetic yellow iron oxide in alkyd paints have been studied by viscometry. The degree of thixotropy of a paint has been calculated from the area of the thixotropic loop. The thixotropic effect in these paints is found to appear at about 35% PVC content and increases with the level of pigmentation up to 55 to 60% PVC content. The degree of thixotropy of paints also depends on the nature of the solvents used in the formulation. Interactions between pigment and solvent and between alkyd and solvent help in building structure and consequently in developing thixotropy in yellow iron oxide-alkyd paints. The shear dependence has been analysed using the Power law and the Casson equation. These paints exhibit only pseudoplastic flow behaviour and their consistency index depends on the pigmentation level and the solubility parameter of the solvent. Sag resistance depends on the PVC content and the nature of the solvent, but the brushability property is independent of these parameters.

High‐Shear Viscometry with a Rotational Parallel‐Disk Device

A novel technique for measuring viscosity at high shear rates is described. The technique is an extension of a well‐established method, parallel‐disk viscometry, which has been limited traditionally to low shear rates ( 50,000 s−1) to be attained without disrupting the torsional flow field, and thus controlled measurements of viscosity could be made at those shear rates. The presence of flow disturbances such as shear heating, surface fracture, and radial migration, common impediments in high‐shear measurements, can be readily detected with a thixotropic loop program, and the effect of such disturbances can be separated from the actual rheological response of the fluid. Data are presented on some well‐defined fluids, both Newtonian and shear‐thinning, and the limitations of this technique are discussed.

Thixotropy of a coal water mixture.

The shear stress-shear rate behaviour of a thixotropic coal water mixture (CWM) was studied in coaxial rotating cylinder rheometer under a wide variety of rest and shear rate history. The transient stress data were used to construct equilibrium and constant-structure flow curves. These transient stress data, i.e. the stress relaxation data and the stress recovery data, were also used to evaluate the breakdown and buildup processes of the internal structure. A phenomenological thixotropy model which has the structural stress as a thixotropic structural parameter was proposed. This model can predict the thixotropic hysteresis loop precisely.

A rheological model of a thixotropic viscoelastoplastic medium

A model is proposed which allows for the difference between instantaneous and slow deformations during loading and unloading (of a mixture based on mineral binding substances). At different constant deformation rates the model reflects the dependence of the relaxation phenomena on the velocity and describes the thixotropy loop.

Thixotropic changes occurring on reheating a coal tar pitch containing mesophase

A coal tar impregnation pitch has been heated to 420°C under shear conditions and cooled. Microstructural investigation of the material showed ellipsoidal mesophase particles produced by shear deformation. On reheating in quiescent conditions, these, particles were observed to revert to the spherical shape after a 3.5 hr treatment at 300°C. Material which experienced the same initial heating cycle but no shear was also reheated, with isothermal intervals, up to 300°C. Flow curves measured isothermally at temperatures between 250 and 300°C showed marked thixotropic behaviour. The thixotropic loop widened with increasing ageing time at each temperature, but the effects were reversible indicating a physical phenomenon. It is suggested that this behaviour is probably the result of a slow coagulation of the “solid-like” mesophase particles to form a weak structure which can be broken down at low rates of shear.

Rheological Factors for Bentonite Suspensions

The flow behavior of bentonite colloidal suspensions is given in terms of the stress‐strain relations with respect to variables of concentration, suspension medium, hydrogen‐ion concentration, temperature, and the successive application of shear stress. Repeated cycling (successive reruns of upcurve and downcurve) caused thixotropic hysteresis loops which decreased in size to a final constant and reproducible flow curve. A hysteresis loop did not appear for a mixture of less than 20 wt% bentonite and glycerin. The flow curves are discussed in terms of bondings in “cardhouse” and “blockhouse” models for dilute and concentrated suspensions respectively. Eyring's rate theory of viscosity is discussed briefly in terms of the well‐known generalized viscosity equationwhich was applied successfully to given flow curves. Physical and chemical characterizations are compared in terms of thixotropic flow models for aqueous, glycerol, and oil (Bentone grease) suspensions.

Rheological Factors for Attapulgite Suspended in Water

Stress‐strain curves and stress‐relaxation curves were determined at various temperatures (1° to 70°C) and concentrations (16 to 22 wt%) for attapulgite suspended in water. Stress‐relaxation experiments were conducted under different conditions on samples which were relaxed for various durations of time between successive cyclic deformations. The flow properties are given in terms of rheopexy and thixotropy. The flow curve (for shear rates 0 to 700 per second versus shear stress) shows, first, a rheopectic hysteresis loop which changes on repeated shearing to a reversible and reproducible non‐Newtonian flow curve. After resting the system for about 12 hours, the resulting flow curve shows a thixotropic loop. Viscosity as a function of pH shows an inversion at pH ⋍ 6.3. A model for explaining the observed facts is proposed. The script S type of looped‐over flow curves showing both rheopectic and thixotropic characteristics is explained in terms of breakage of “logjammed” and crystalline bundles. A plot of the yield‐point viscosities versus the reciprocal of temperature for the rheopectic and thixotropic curves shows activation energies of about 1 and 2 kcal respectively. An autographic rotational viscom‐eter was designed and utilized to obtain the data reported so that the objectionable effects of frictional heating in the sample were minimized. The calibration response of the viscometer to a glycerol standard shows good agreement with published viscosity data.