The Rheology Handbook. Thomas Mezger

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.

      3.4.2.1.3Examples of thixotropic materials

      Almost all dispersions (suspensions, emulsions, foams) such as pastes and creams; gels, ketchup, coatings, paints, printing inks, sealants, drilling fluids, plaster, dispersions (e. g. in geo-technics), soap, sols

      3.4.2.1.4b) Non-thixotropic behavior

      3.4.2.1.5Experiment 3.5: Stirring yogurt

      After stirring, yogurt remains considerably thinner compared to the initial state, even when waiting a long period of time.

      Each material displaying a certain degree of structural decomposition in the shear phase with time, of course is showing time-dependent behavior. However, such a material is not thixotropic if the initial structural strength does not completely return finally, even after an “infinitely” long period of time at rest. In this case a permanently remaining structural change has taken place. If structural regeneration does not occur completely (i. e. to 100 %), it is sometimes referred to as incomplete or “false thixotropy”. In this case, instead of thixotropy it is better to speak of partial regeneration, e. g. expressed in a percentage compared to the initial viscosity value.

      Example of a meaningful specification, related to the viscosity value: After a high-shear phase, during a rest period of t = 120 s, structural regeneration has taken place up to 70 % compared to the initial structural strength-at-rest before shearing.

      3.4.2.1.6c) Rheopectic behavior

      Rheopectic behavior means an increase in structural strength when performing a high-shear process which is followed by a more or less rapid but complete decomposition of the increased structural strength during a subsequent period of rest. This cycle of generation and re-composition, of increase and decrease in structural strength is a completely reversible process (see Figure 3.39). Rheopexy is sometimes called “anti-thixotropy” or “negative thixotropy” [3.9] which may lead to confusion. Rheopectic behavior is defined as a time-dependent behavior (like thixotropic behavior). Testing of rheopectic behavior is similar to the thixotropy test, with appropriate modifications (see Chapter 3.4.2.2).

      Materials showing rheopectic behavior tend to inhomogeneous flow. Wall-slip effects and phase separation should always be taken into account. In industrial practice, rheopectic behavior is much less common than thixotropic behavior.

      3.4.2.1.7Examples of rheopectic materials

      When working with dispersions showing a high concentration of solid matter or gel-like particles such as ceramic casting slips, latex dispersions and plastisol pastes, rheopectic behavior should always be taken into account.

      Note: Non-thixotropic behavior: Shear-induced and permanently remaining increase in structural strength

      3.4.2.1.8Example: Testing a dispersion under the following conditions

      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.