Thixotropic fluids, properties

Bentonite, dispersed in water at the proper concentration, becomes a thixotropic fluid. When mixed with a chemical grout, bentonite adds this property to those which the grout already has. In contrast to a grout-cement... 

One further feature must be mentioned about pharmaceutical suspensions, namely, their desirable rheolt ical properties (7). In practice, a Bingham plastic" behavior is most used a minimum shear stress yield stress) is needed for the suspension to begin to flow. For tower stresses—and, of course, when the system is left undisturbed—the viscosity is so high that the particles will likely remain homogeneously dispersed. According to Falkiewicz (7). thixotropy is another flow characteristic that can be useful, since in thixotropic fluids a finite lime is needed to rebuild the structure after, for instance, shaking it for administration. For this reason, most formulations contain thixotropic flow regulators intended to confer optima viscous flow propertie.s to the suspensions. The reader is referred to Chapter 5 of this book for details. 

The rheological characterisation of non-Newtonian fluids is widely acknowledged to be far from straightforward. In some non-Newtonian systems, such as concentrated suspensions, rheological measurements may be complicated by non-linear, dispersive, dissipative and thixotropic mechanical properties and the rheometrical challenges posed by these features may be compoimded by an apparent yield stress. 

A closely related phenomenon is thixotropy. This is the property of some polymer solutions to show a time dependent reduction in viscosity when sheared. The longer the fluid is subjected to the shear stress, the lower its viscosity becomes. So a thixotropic fluid is one which takes a finite time to attain its equilibrium viscosity in shear after being subjected to a step change in shear rate. In... 

Time-Dependent Fluid Properties. This category includes thixotropic and rheopectic fluids. When a shear stress is applied to a thixotropic fluid, the viscosity decreases gradually, and when the stress is removed, a gradual recovery is observed. The opposite behavior is observed with rheopectic fluids a gradual increase of viscosity during shear followed by recovery on the removal of shear-inducing stress. 

Although there are materials which possess rheological properties that depend on the duration of shear (e.g., in thixotropic fluids the dynamic viscosity decreases, and in rheopectic fluids the dynamic viscosity increases with the time for which shearing forces are appli ), we will consider only those materials which do not change their properties with time. 

Clays and other fillers impart stiffness to adhesive films. Clays reduce the penetration of adhesives into porous substrates. Large-particle clays are better able to control penetration and also impart more rapid setting speed. A highly plastic clay, bentonite, makes adhesives thixotropic—fluid under high shear, thicker upon application. A study has been made of the effect of various fillers on the adhesive properties of polyvinyl acetate. 

A wide variety of nonnewtonian fluids are encountered industrially. They may exhibit Bingham-plastic, pseudoplastic, or dilatant behavior and may or may not be thixotropic. For design of equipment to handle or process nonnewtonian fluids, the properties must usually be measured experimentally, since no generahzed relationships exist to pi e-dicl the properties or behavior of the fluids. Details of handling nonnewtonian fluids are described completely by Skelland (Non-Newtonian Flow and Heat Transfer, Wiley, New York, 1967). The generalized shear-stress rate-of-strain relationship for nonnewtonian fluids is given as... 

Predictions on the effectiveness of a fluid loss additive formulation can be made on a laboratory scale by characterizing the properties of the filter-cake formed by appropriate experiments. Most of the fluids containing fluid loss additives are thixotropic. Therefore the apparent viscosity will change when a shear stress in a vertical direction is applied, as is very normal in a circulating drilling fluid. For this reason, the results from static filtering experiments are expected to be different in comparison with dynamic experiments. 

Control of Thixotropy. Often the adhesive application will require that the product be fluid for mixing and application, but it must not flow or sag once applied. For example, ASTM C920 defines a nonsag sealant as one that permits application in joints on vertical surfaces without sagging or slumping. This property is called thixotropy. Thixotropic materials, such as tomato catsup, toothpaste, etc., undergo a decrease in viscosity when subject to shearing. 

To date, relatively little is known about ways to eontrol the viscosity of cement pastes. In fact, to talk about viscosity is an approximation of reality ealeium phosphate pastes are generally non-Newtonian fluids and as a result, the viseosity is a function of shear forces (40). Furthermore, eements have transient properties meaning that the viscosity of a cement paste is a function of shear and time." Generally, ealeium phosphate pastes are thixotropic (shear-thinning)." " Both an increase in LSR and an increase in partiele size decrease paste viscosity."" "" Additives are also known to affect viscosity. For example, eitrate ions or poly(acrylic acid) decrease particle interaetions and hence deerease viscosity and cohesion."" ... 

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