Читать книгу The Rheology Handbook - Thomas Mezger - Страница 63

Polymer solutions, polymer melts, organic binders (e. g. methylcellulose), many coatings (e. g. primers, without inorganic gellants), glues, many shampoos

The terms shear-thinning and pseudoplastic are identical in their meaning (see also Chapter 14.3: 1925, with the concepts of Eugen C. Bingham [3.4], Wolfgang Ostwald jun. [3.2] [3.3] – the latter used the German term “strukturviskos” – and others on this subject [3.5]). However, pseudoplastic contains the word plastic, a behavior which cannot be exactly determined in a scientific sense since it is the result of inhomogeneous deformation and flow behavior (see also Chapter 3.3.4.2c and Figure 2.9: no. 4). This is the reason why the use of the term “pseudoplastic” is diminishing more and more in current literature.

Note: Apparent shear viscosity

Often the term “apparent viscosity” is used with different meanings about the measured viscosity values. Therefore, actually this term should be avoided as it is superfluous [3.33] [3.78].

Meaning 1: Related to rheology it means that a sample does not show ideal-viscous flow behavior. If the ratio of shear stress to shear rate varies with the shear load, the corresponding values are often called the “apparent shear viscosity” at the corresponding shear rate, to illustrate that these kinds of values are different from constant viscosity values of ideal-viscous fluids (according to ASTM D4092). Each one of these viscosity values obtained represents a single point of the viscosity function only. Therefore, these viscosity values can only be evaluated in the appropriate form if information is also given about the shear conditions. Examples for accurate specifications are as follows:

η( γ ̇ = 100 s-1) = 345 mPas, or η(τ = 500 Pa) = 12.5 Pas

Meaning 2: Related to rheometry, apparent viscosity means that the viscosity values depend on the measuring geometry used. This may be the case if tests with a rotational rheometer are performed by use of a too large or a too small measuring gap. By the way, this term is sometimes selected too when testing with capillary viscometers (and melt index testers) if too small or too large capillary diameters are selected (according to standards).