Thixotropic flow

Usually the surfactant concentration in ointments and creams is significantly lower than in surfactant gels. Ointments are non-aqueous preparations, whereas creams result from ointments by adding water. The microstructure of both ointments and creams may consist of liquid crystals, as long as a liquid crystalline network or matrix is formed by amphiphilic molecules. In a liquid crystalline matrix, it is easier to deform the system by shear such formulations show plastic and thixotropic flow behavior on shear. In comparison to systems with a crystalline matrix which are usually destroyed irreversibly by shear, those with a liquid crystalline matrix exhibit a short regeneration time of... 

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. 

Suspending agent systems such as a pseudoplastic (sodium carboxymethylcellulose) in combination with a clay (hydrated colloidal magnesium aluminum silicate) or blends and coprecipitates of sodium carboxymethylcellulose and microcrystalline cellulose exhibit some thixotropic flow characteristics. Other pseudoplastics such as hydroxyethylcellulose or hydroxy-propyl methyl cellulose may be required to overcome possible in compatibilities with sodium carboxymethylcellulose. 

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. 

Thixotropic Alkyd Resins. Thixotropic flow is always associated with structural... 

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. 

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 behavior of W/O/W emulsion studied with a cone-and-plate viscometer has shown a negative thixotropic flow pattern, mostly under low shear rate. Upon raising the shear rate or the shear time an increase in the shear stress was observed, which induced phase inversion... 

The most common application of vegetable oil-modified polyamides is in the surface coatings and paints industry. The dimer acids of tall and soybean oils and amines are used to modify the flow behaviour of paints. This thixotropic flow prevents setting and sagging, enables easy application and improves surface appearance. Vegetable oil-based nylons are used as engineering polymers in the automotive and transport industry for fuel lines. Products are also used for the extruded and moulded components of fuel systems such as filler necks, gas tanks, reservoir modules, filters, fuel rails and vapour recovery systems. Nylon 11 is also used in power coatings to coat metals that must withstand abrasion, impact and corrosion. 

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. 

Suspensions containing silane Z-6076 and titanate LICA 12, on the other hand, showed large peld stresses, higher viscosities and highly thixotropic flow behavior, indicating that toe suspension is highly flocculated. The suspension prepared with silane Z-6020 is also not as well dispersed as toe suspension with the zircoaluminate but the relatively low peld stress, lower viscosity and absence of thixotropy indicates a much improved dispersion as compared with the silane 2r 6076 and titanate LICA 12. 

Rheology is the science of deformation and flow behavior of materials under the influence of parameters such as shear rate and time. Liquids with viscosity not dependent on shear rate are termed Newtonian liquids while non-Newtonian liquids exhibit changes in viscosity depending on shear rate. Most paints exhibit non-Newtonian flow, which is desirable. Important types of non-Newtonian flows are pseudoplastic flow behavior (shear thinning), thixotropic flow behavior (time-dependent shear thinning,) and dilatant flow behavior (shear thickening). Pseudoplastic or thixotropic flow is often desirable in coatings because ... 

Thick-fihn pastes often show thixotropic flow behavior. The shear history must be taken into account during paste viscosity measurements in order to explain the paste rheology... 

FIGURE 8.80 Thixotropic flow behavior of a thick-fUm paste illustrating the thixotropic loop. 

If the rate of structure buildup is slower than the rate of breakdown due to shear, thixotropic flow behavior is observed. Breakdown of the suspension structure can be reversible and the suspension structure is slowly recovered. The rate of structural breakdown and rebuilding is important to obtain optimum print quaUty during screen printing of thick film... 

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. 

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