Thixotropy, viscosity measurements

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. 

Thixotropy may also be expressed as the thixotropic index (TI) which is the ratio of viscosities measured at speeds that have a ratio of 1 10 rpm, for example at 2 rpm... 

The term thixotropy has often been confused with the pseudoplastic state which is a reduction in viscosity as the rate of shear increases. Thixotropy is a reduction in viscosity with time. Almost all thixotropic hquids are also pseudoplastic, but not all pseudoplastic liquids are thixotropic. Both pseudoplastic and thixotropic behaviour are caused by structure which involves the interactions between particles in the liquid. The strength of these interactions, and the rate at which they form, controls the rheology of the system. Viscosity measurement is not an instantaneous process and the rate of formation of interactions relative to the time of measurement determines the rheology. If interactions are formed relatively fast an equilibrium will be set up between formation (constant) and destruction (rate dependent on shear) of interactions, and the system will appear to be pseudoplastic. If interactions are formed slowly the apparent viscosity will decrease, this is because the viscometer will record a value before equilibrium is reached. In a brushing paint, the removal of brush marks requires a slow rate of forming structure. If the paint is applied to a vertical surface a fast build-up of structure is required to prevent flow of the paint down the surface, a phenomenon termed sagging . 

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). 

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. 

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. 

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. 

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. 

Measurement of Flow Properties For the precise scientific measurement of viscosity and thixotropy in absolute units, see Rheology of Ceramic Systems . Here we shall restrict ourselves to a consideration of the torsion viscometer. 

The rheology, or flow properties, of adhesives is important not only to achieve adequate coverage and wetting, but also to develop the process parameters necessary to dispense the adhesive (see Chapter 4). Flow properties are determined by measuring viscosity and thixotropy. 

The thixotrqiic behavior of suspensions and emulsions is a rather difficult property to measure, and this is further cmnplicated by the contribution of viscoelastic behavior (see next section). The main reason for this difflculty lies in the nature of the phenomenon involved. Thixotropy arises because of the existence of interparticle forces that produce three-dimensional microsmictures called flocculates or aggregates (see Sec. III). Depending on the magnitude of these forces, these microstructures are more or less prone to destruction by shearing, in consequence, any manipulation of a thixotropic sample may induce structural breakdown, thereby changing, most often irreversibly, the viscosity and yield stress of the sample. 

Consistency n. The property of a material or composition which is evidenced by its resistance to flow, represented by an undefined composite of properties, each measurable from the complete, force-rate flow curve as plastic viscosity, yield value and thixotropy. The term is applied to a variety of materials. For Newtonian liquids, consistency is simply viscosity. While consistency is an accepted rheological term, it has qualitative... 

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