Flow behavior thixotropic

Solutions of methylceUuloses are pseudoplastic below the gel point and approach Newtonian flow behavior at low shear rates. Above the gel point, solutions are very thixotropic because of the formation of three-dimensional gel stmcture. Solutions are stable between pH 3 and 11 pH extremes wiU cause irreversible degradation. The high substitution levels of most methylceUuloses result in relatively good resistance to enzymatic degradation (16). 

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... 

Many food systems exhibit complex flow behavior and are thixotropic, that is their viscosity decreases as their molecules shear. This depends upon the spherical nature of the particles in the suspension. Initially they are randomly orientated but begin to line up in one direction as shear/stress is applied. Sonication results in reduced viscosity probably due to the particles ordering themselves uniformly in the path of the ultrasound. 

FIGURE 8.8 Flow behavior of a symmetrically thixotropic system. (Adapted from Harris, J., Rheology and Non-Newtonian Flow, Longman Group, Ltd., London, 1977.)... 

Some systems exhibit flow behavior opposite of thixotropic systems, that is, viscosity increases with increasing shear rate. Such fluids are referred to as dilatant or rheopectic. This type of behavior is not common for liquid products containing a low concentration of the dispersed phase. 

Paints are rarely Newtonian in their flow behavior. They are usually found to be thixotropic, pseudoplastic, and sometimes dilatant. Paints are thixotropic if there is initial resistance to viscosity loss by agitation, if there is a reduction in viscosity with continued agitation, and if there is a return to the original viscosity after the paint is no longer agitated. A pseudoplastic paint is similar to a thixotropic paint except there is no initial resistance to viscosity loss or no yield factor. A dilatant paint differs because the viscosity increases or shows resistance to flow as the agitation is increased. Mill bases are usually slightly dilatant and are intended to be that way. 

Equation 7.42 well describes the flow behavior of polymeric systems, and it was found useful for polymer blends. It should be stressed that Equations 7.41-7.43 described the flow behavior of fluids without yield stress or thixotropicity. 

Deviations from Newtonian flow can occur when shear stress does not increase in direct proportion to shear rate. Such deviation may be in the direction of thickening (called dilatent flow) and in the direction of thinning (called pseudo plastic). Related to non-Newtonian flow is the behavior of thixotropic liquids when subjected to shear, as explained above. Flow behavior can be represented by the following equation ... 

The addition of proteins can induce an aggregation of xanthan chains according to the ionic composition of the solvent (presence of cations such as calcium or magnesium) and to the pH. Furthermore, the flow behavior of xanthan-protein solutions exhibited some thixotropic phenomena... 

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 ... 

To date many paint manufacturers use one-point measurement for measurement of paint consistency. This can be carried out, for example, with a simple Brookfield viscometer using one spindle at a given rpm. This one-point measurement can be misleading [49]. To illustrate this point let us consider three systems, namely Newtonian, Bingham plastic and pseudoplastic with thixotropy as illustrated in Fig. 4.27. At a specific shear rate, all the three systems show the same apparent viscosity although their flow behavior (using the full shear-stress curves) is significantly different [49]. These systems will show entirely different behavior on application at the shear rate at which the apparent viscosity is the same. A study of the flow curves indicates that the Newtonian system will flow at extremely low shear rates, whereas the plastic and thixotropic systems will show reluctance to do so because of their yield values. This is clearly reflected in the final film properties. Once the yield value is overcome, the... 

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. 

Flow Curves. In this type of testing, shear stress or shear rate is changed as a function of time and resulting shear rate or shear stress is measured. This type of test is generally used to survey flow behavior over one to two orders of magnitude of shear rate range. Thixotropic behavior of pastes can be determined from the hysteresis in shear stress-shear rate plots. There are several phenomenological models available to curve-fit the experimental data. Flow curves for various thick-film pastes are shown in Fig. 8.81. 

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... 

Thixotropy and rheopexy Now let us consider what happens when time is considered. Some fluids will display a change in viscosity with time under conditions of constant shear rate. Thixotropic fluids show a decrease in viscosity with time when subjected to constant shearing, as indicated in Fig. 6. Rheopexy is just the opposite (Fig. 6). Rheopectic fluids are very rare and rheopexy would be quite detrimental for A and S Thixotropy is frequently observed in materials such as greases, building adhesives, paints and it is done purposely by the formulator so that the adhesive will not sag when applied gently on a vertical surface but by brushing or rolling it will become more fluid and spread easily. Both thixotropy and rheopexy may occur in combination with any of the previously discussed flow behavior, or only at certain shear rates. The time element is variable under constant shear, some fluids will reach their final viscosity in a few seconds, while others may take up to several days. 

Thermosetting adhesives are typically available in liquid, paste, and solid forms. The liquid form is generally one- or two-part system which may contain solid fillers and colorants. The paste form typically exhibits flow behavior different from those of the unfilled or highly filled liquids. Certain pastes are modified to be thixotropic, which are gel-like materials at rest but act as fluid when agitated. Possession of such solid-like yielding behavior allows an unstrained mass of adhesive not to sag or flow unless forced to do so by mechanical action, such as by spreading with a spatula. 

For example, Seddon et al have shown that alkylimidazolium tetrafluoroborate family (number of carbon atoms typically 12) are thixotropic fluids whose viscosity decreases when increasing the shear rate. Kulkami et al have reported that the flow behavior may change from Newtonian to non-Newtonian by inereasing the alkyl chain length on the eat-ion like alkyl-methylimidazolium-based ionie liquids. Non-Newtonian shear thinning behaviors have been then reported for different aprotic or protic ionic liquids. 

The Newtonian and non-Newtonian fluids discussed in section 8.2 are time independent that is, the viscosity remains constant as long as the shear rate does not change. However, some fluids exhibit time-dependent flow behavior and their viscosities change with the time of shearing. Two most important time-dependent fluids are thixotropic and rheopectic. At a fixed shear rate, the viscosity of a thixotropic fluid decreases with time, while the viscosity of a rheopectic fluid increases with time (Figure 8.7). 

The viscosity of polymer dispersions is usually dependent on the shear rate. A distinction is made between pseudoplastic behavior (viscosity decreases with increasing shear), possibly with a flow limit, thixotropic behavior (viscosity decreases with in-... 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

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