Читать книгу The Rheology Handbook - Thomas Mezger - Страница 150

1st interval: for t = 1 min at γ ̇ = 100 s-1; result: viscosity increase η = 0.1 to 1 Pas

2nd interval: for t = 3 min at γ ̇ = 0.1 s-1; result: reaching η = 10 Pas after a short time, remaining on this high value afterwards

In this case, on the one hand an increase in viscosity in the high-shear interval can be observed. But on the other hand, there is no decrease of the viscosity value in the subsequently following low-shear interval. Therefore, this is not rheopectic behavior, since here, a permanently remaining shear-induced structural change has taken place. Of course, when testing these kinds of materials, loss of solvent or drying effects must be excluded. More about shear-induced effects: See Chapter 9.2.2 and Figure 9.23.

Note 1: Trials to describe thixotropic behavior with mathematical methods

Using mathematical models, several trials have been made to describe thixotropic and rheopectic behavior [3.68] [3.80]. In most cases, however, these kinds of estimates must fail, since above all in the interval of structural recovery, usually not all structural units do behave homogeneously, related to the total volume of the sample. Sometimes, these components, being typically in a size range of micrometers (e. g. 1 µm to 100 µm), such as particles, aggregates and agglomerates, may show – also at low shear rates – effects like shear-banding [3.26]. See also Note 2 below.

Note 2: Recovery time with thixotropic behavior and relaxation time

Relaxation time and retardation time, respectively, is a useful tool to characterize time-dependent behavior during a deformation and re-deformation process of viscoelastic liquids such as polymers (with a typical molecule size in the range of nanometers; e. g., mostly between 10 nm to 100 nm, and in maximum 1000 nm). This kind of behavior depends above all on the average molar mass and molar mass distribution of the polymer (see Chapters 7.3.3.3 and 6.3.4.4). In this case, however, chemical-physical interactions typically play a minor role – if at all. In principle, the recovery time of a thixotropic sample (with most “players” in the µm range) should not be compared with the relaxation process of polymer molecules (i. e., here showing clearly smaller “players” in the nm range), since in the former usually interaction forces are playing the dominant role. Please be aware: Within the nano-world there are typically existing clearly other rules compared to the micro-world or macro-world. See also Note 1 above.