Thixotropy measurement

Another method for estimating thixotropy involves the hysteresis of the thixotropic loop. The area of the thixotropic loop is calculated or measured, which works well with printing inks (3). In a variation of this method, the up curve on an undisturbed sample is deterrnined. The sample is then sheared at high shear (>2000 ) for 30—60 s, followed by deterrnination of the down curve (22). The data ate plotted as Casson-Asbeck plots, vs 7 / (14), as... 

The area of the hysteresis loop is a measure of the thixotropy of the tested system. These measurements are easy to carry out and give a quick overview of the thixotropic properties of... 

Figure 3 gives an example of a typical force profile. The force is increased continuously and reaches the point - at the end of the first part of the force profile - where the pectin preparations start to flow. The so-called yield point is reached. The further increase leads to the continuous destruction of the internal structure and the proceeding shear thinning. The applied stress in part 3 of the stress profile destroys the structure of the fruit preparations completely. Now the stress is reduced linearly, see part 4 and 5, down to zero stress. The resulting flow curves 2, 3 and 4 and the enclosed calculated area from the hysteresis loop give important evidence about the time-dependent decrease of viscosity and a relative measure of its thixotropy. 

Since the rheology of many systems depends largely on the temperature, accurate and reproducible measurements require very careful temperature control. A 1°C temperature drop, for instance, increases the apparent viscosity / of an offset printing ink by approximately 15%. To demonstrate the correlation between thixotropy and temperature, Figs. 56 and 57 show the flow curves at different temperatures for two offset printing inks [134], Both materials clearly lose thixotropy-indicated by the area under the thixotropic loop-as the temperature increases. This effect is much more pronounced in the first case (Fig. 56), while the second ink exhibits a very slow decrease thixotropic behavior (Fig. 57). 

The causes for thixotropic and rheopectic behavior are possibly very similar to those for pseudoplasticity and dilatancy, respectively. The proposed causes of pseudoplasticity, i.e., the alignment of asymmetrical molecules and particles or the breakdown of solvated masses, could not always be expected to be instantaneous with respect to time. Therefore it seems that pseudoplastic behavior may simply be that form of thixotropy which has too small a time element to be measurable on most instruments in current use. Exactly the same argument may be applied... 

Since fluid shear rates vary enormously across the radius of a capillary tube, this type of instrument is perhaps not well suited to the quantitative study of thixotropy. For this purpose, rotational instruments with a very small clearance between the cup and bob are usually excellent. They enable the determination of hysteresis loops on a shear-stress-shear-rate diagram, the shapes of which may be taken as quantitative measures of the degree of thixotropy (G3). Since the applicability of such loops to equipment design has not yet been shown, and since even their theoretical value is disputed by other rheologists (L4), they are not discussed here. These factors tend to indicate that the experimental study of flow of thixotropic materials in pipes might constitute the most direct approach to this problem, since theoretical work on thixotropy appears to be reasonably far from application. Preliminary estimates of the experimental approach may be taken from the one paper available on flow of thixotropic fluids in pipes (A4). In addition, a recent contribution by Schultz-Grunow (S6) has presented an empirical procedure for correlation of unsteady state flow phenomena in rotational viscometers which can perhaps be extended to this problem in pipe lines. 

At higher shear rates, Watanabe and Kotaka (1983) observed thixotropy, i.e. stress decay increasing as a function of shear rate, in PS-PB diblocks in dibutyl phthalate (DBP), which is a selective solvent for PS.The fact that the flow crossed over from plastic to viscous non-Newtonian on increasing the shear rate indicated the breakdown of the micellar lattice structure, rather than of the individual micelles. This was confirmed by parallel measurements on a cross-linked PB-PS system, where stress decay and recovery were also observed. Thus the... 

Frequently used single-point viscosity tests in the starch plant are orifice pipettes,56 orifice funnels,57 the Hot Scott viscometer, and various methods to determine alkaline fluidity.58 For absolute measurements of the rheological properties, rotating viscometers with coaxial cylinders are used.59 The paper industry uses mainly the Brookfield viscometer and the Hercules viscometer for determining shear-dependent viscosity, pseudoplasticity, and thixotropy. Oscillatory and capillary viscometers are used for more detailed viscosity characterization, such as yield value, elastic properties, and viscoelasticity.60... 

This section draws heavily from two good books Colloidal Dispersions by Russel, Seville, and Schowalter [31] and Colloidal Hydrodynamics by Van de Ven [32] and a review paper by Jeffiey and Acrivos [33]. Concentrated suspensions exhibit rheological behavior which are time dependent. Time dependent rheological behavior is called thixotropy. This is because a particular shear rate creates a dynamic structure that is different than the structure of a suspension at rest. If a particular shear rate is imposed for a long period of time, a steady state stress can be measured, as shown in Figure 12.10 [34]. The time constant for structure reorganization is several times the shear rate, y, in flow reversal experiments [34] and depends on the volume fraction of solids. The viscosities discussed in Sections 12.42.2 to 12.42.9 are always the steady shear viscosity and not the transient ones. 

Entanglement networks and weak gels may exhibit thixotropy, or time-dependent flow, in addition to exhibiting either pseudoplastic or plastic flow. Thixotropy, which is noted in a rheogram as a hysteresis loop, occurs because the gel requires a finite time to rebuild its original structure that breaks down during continuous shear measurements. The degree of thixotropy depends on gel type, sample history, and experimental conditions. 

For the evaluation of the rheology of the silica dispersions, different test methods were applied (a) a shear rate-controlled relaxation experiment at = 0.5 s (conditioning), 500 s (shear thinning), and 0.5 s (relaxation) to evaluate the apparent viscosity, the relaxation behavior, and thixotropy (b) shear yield-stress measurements using a vane technique introduced by Nguyen and Boger [5] (c) low deformation dynamic tests at a constant frequency of 1.6 s in a stress range of ca. 0.5 - 100 Pa. All samples contained 3 wt% of fumed silica. 

In the case of thixotropic materials, the yield point depends on the conditions of measurement and on the previous history. At standstill, it acquires high values, while on stirring it may be close to zero. In practice, it means that vibration during handling of green ware may cause a decrease in yield point and subsequent deformation. Thixotropy is due to the formation of gel-type structures which are dcstro cd by dynamic effects. 

Steady state shear stress a-shear rate y measurements This requires the use of a shear rate-controlled instrument, and the results obtained can be fitted to models to obtain the yield value Up and the viscosity as a function of shear rate. Time effects (thixotropy) can also be investigated. 

Arheopectic pigmented bleach (alkali metal hypochlorite) hard surface cleaner formulated with bentonite clay is disclosed in U.S. Patent 5,688,435. Examples of time-dependent shear effects determined from constant shear rate measurements at 1, 10, 50, and 100 sec-1 are provided in the patent and shown in Figure 4.2 and Figure 4.3. The viscosity data show evidence of shear thickening as a function of time at constant shear rates of 1 and 10 sec-1 and thixotropy occurs at 50 and 100 sec-1. The formulation is rheopectic at 10 sec-1. Dynamic mechanical data are also contained in the patent and the storage and loss modulus as a function of strain amplitude is shown in Figure 4.4, for one patent example. 

Viscosity measurements can be made using several different methods and attendant equipment. To help relate viscosities reported in different units, Table III reports comparative viscosities using different methods of measurement. The accuracy will lessen as non-Newtonian flow (thixotropy, pseudoplasticity, etc.) increases, but it serves as an excellent guide. 

For a variety of reasons, it is necessary to measure the thixotropic behavior of lipids. Thixotropy is defined as an isothermal, reversible food structure transformation, and is a feature common to many food systems (Figure 4.12). The measurement of thixotropy results in a lowering of the apparent viscosity when the experiment is ongoing, with sufficient time, the apparent viscosity will return to its original value. 

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