Thixotropic flow property

It is a hydrophilic white powder soluble in cold water forming a clear viscous solution. The solution of MC exhibit pseudoplastic non-thixotropic flow properties. 

The solntions of HPMC exhibit psendoplastic non-thixotropic flow properties. 

False body n. Thixotropic flow property of a suspension or dispersion. When a composition thins down on stirring and builds up on standing it is said to exhibit false body. The term false body is also used in practice for buttery materials, which are characterized by a relatively low viscosity and high yield value. 

Fermented milk products exhibit thixotropic rheological properties, i.e. the viscosity (resistance to flow) decreases as the rate of shear increases a typical relationship is shown in Figure 10.33. The rheological properties are major parameters of quality and are controlled by varying the total solids content of the milk, the heat treatment and homogenization of the milk and the use of hydrocolloids, e.g. gelatin or carageenan. 

Thixotropy is a rheological property that results in yield stress on standing. Thixotropic flow is defined as a reversible, time-dependent, isothermal gel-sol transition. Thixotropic systems exhibit easy flow at relatively high shear rates. However, when the shear stress is removed, the system is slowly reformed into a structured vehicle. The usual property of thixotropy results from the breakdown and buildup of floccules under stress. A small amount of particle settling takes place until the system develops a sufficiently high yield value. The primary advantage of thixotropic flow is that it confers pourability under shear stress and viscosity and sufficiently high yield stress when the shear stress is removed at rest. 

In the temperature range where the carrageenin is normally employed, viscosity characteristics become dependent on temperature, other solutes, and mode of observation. In a system like this it would be more appropriate to speak of consistency, fluidity, or some similar subjective term, and measuring instruments should be selected to reflect the property desired. The texture of a gel is a combination of strength and elasticity, both of which can be measured. The mouth feel of a paste is a combination of yield, viscosity, and melting temperature, the first two of which can be determined by measurements of thixotropic flow. Of particular significance is that measurements assuming Newtonian viscosity do not show the thixotropic characteristics that influence sensory experience. 

In many cases, highly filled paste adhesives are inherently thixotropic due to the inorganic fillers used such as alumina, sihca, other metal oxides, or metals. However, small amounts (1-5%) of thixotropic agents can be added to further control the flow properties. A popular and widely used thixotropic agent is fumed silica, also known as... 

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. 

Like almost all paints and lacquers, water-based dispersions also contain other components besides binder, pigment, and solvent. For example, the latex structure is favorably influenced by addition of poly(acrylamide). Added silicones reduce foaming. The flow properties are improved by hydroxyethyl cellulose or vegetable gums. The desired thixotropic poperties are produced by addition of bentonite, zirconium carbonate, or triethanolamine aluminate, etc. 

Rheopectic fluids. These fluids are quite rare in occurrence and exhibit a reversible increase in shear stress with time at a constant rate of shear. Examples are bentonite clay suspensions, certain sols, and gypsum suspensions. In design procedures for thixotropic and rheopectic fluids for steady flow in pipes, the limiting flow-property values at a constant rate of shear are sometimes used (S2, W3). 

Table 1 lists polymers used for skin care cosmetics. As can be seen from the table, the polymers used for skin care exhibit generally pseudoplastic or thixotropic flow and possess properties to form gels readily [3]. The roles these polymer gels play are extremely significant from the viewpoints of ease of use and functionality. Here, viscosity control, stabilization of emulsion, and maintenance of moisture are the functions of polymers in skin care cosmetics. 

Frequently, the flow properties of adhesives and sealants are measined with a Brookfield viscometer at two different spindle speeds at a rotation ratio of 10 1. The ratio of the low spindle speed viscosity to the high spindle speed viscosity is termed the Shear Thinning or Thixotropic Index and is a measine of shear sensitivity (pseudoplasticity) see Figure 3. 

Another method to measure flow properties after application involves the application of microdielectric measurements that have shown promise for characterizing fihn formation and cure (5,10-13). In this technique, the paint is applied to a thin, flat sensor connected to a frequency generator, an impedance analyzer and a computer. The sensor monitors the dielectric properties of the paint film at and near the interface between the sensor and the coating. The dielectric parameter used to estimate flow is called ion viscosity, but really is electrical resistivity rather than a trae viscosity. Ion viscosity is a measure of both the number and mobility of ions in a specimen. It is affected by changes in temperature, loss of solvent, crosslinking and other chemical reactions, formation of physical structure (such as by a thixotrope), ionic impurities, and ionic additives. The advantage of this technique is that measurements can be made in situ during application, flash, and the bake. 

The flow properties of a fluid vary depending on the method of application and material type. For materials that have Newtonian characteristics, viscosity is independent of shear rates. Non-Newtonian solutions, (suspensions), occur when the suspension causes either dilatency or thixotropy. A dilatant suspension is quite fluid until stressed, at which time the suspension becomes resistant to motion. Thixotropy is the opposite, in that the suspension is thick and gelatinous, but when stirred or mixed it flows more readily. While solder pastes typically exhibit similar (but not the same) thixotropic behavior, they only differ in the shear force necessary to provide flow characteristics. The difference among solder paste characteristics is due to the quantity and type of thixotropic agents utilized, which also affects paste rheology. 

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