Читать книгу The Rheology Handbook - Thomas Mezger - Страница 145

1 When shaking continuously, the ketchup is becoming thinner and thinner. During the subsequent period of rest, it is thickening more and more and after approximately 10 minutes it has returned to its original gel-like consistency.

2 The paraffin oil is solid when at rest, but immediately turns to a liquid state when shaken. It takes approximately eight hours until the oil has reached its initially solid state again.

The term thixotropy is a combination of Greek words and is not that easy to translate: Thixis means touching, in the sense of deranging when bringing in motion (e. g. when stirring or shaking), and trepein means turning, changing, transforming; it is a “property of some solidified colloids to temporarily turn into a liquid state under the influence of mechanical forces” [3.65] [3.33] [3.80]. As a rheological term, thixotropy might be translated as structural change or transition due to a mechanical load. Unfortunately, in many industrial laboratories due to lack of knowledge, the term thixotropy is given various meanings, and correspondingly, there are a lot of more or less useful test procedures. See also Chapters 11.2.3d (slump), 11.2.4a (inclined plate) and 11.2.11a/c1 (gelation test with Bingham Build-Up/Gallenkamp Torsion-Wire Viscometer), as well as Chapter 12.4.3 (guideline), and the Index.

In order to perform useful rheological tests, in minimum two intervals have to be considered: Thixotropic behavior means reduction of structural strength during a shear load phase and a more or less rapid but complete structural regeneration during the subsequent period of rest [3.66]. This cycle of decomposition and regeneration is a completely reversible process (see Figure 3.38).

Figure 3.38: Time-dependent viscosity function of a thixotropic material

(1) structural decomposition when applying a constantly high shear load

(2) structural regeneration when at rest

Figure 3.39: Time-dependent viscosity function of a rheopectic material:

(1) increasing structural strength when applying a constantly high shear load

(2) decreasing structural strength when at rest

Thixotropic behavior is defined as time-dependent behavior, and is correctly determined in a scientific sense only if

1 both decomposition and regeneration of the structure are taken into consideration,

2 testing is performed at a constant shear load in each one of the test intervals.