The Rheology Handbook. Thomas Mezger

for investigating thixotropic behavior

      There are several possibilities to characterize thixotropic behavior, and correspondingly, different results may occur finally [3.66].

      3.4.2.2.1a) Step test consisting of three intervals

      Preset

      1 With controlled shear rate (CSR): profile γ ̇ (t) with three intervals as a step function (see Figure 3.40)

      2 With controlled shear stress (CSS): profile τ(t) with three intervals as a step function (similar to Figure 3.40)

      Result: Time-dependent viscosity function η(t), see Figure 3.41

       Figure 3.40: Preset profile: step function consisting of three test intervals, each one at a constant shear rate, at (1) low-shear, (2) high-shear, and (3) again at low-shear conditions

       Figure 3.41: Time-dependent viscosity of a thixotropic material, (1) at low-shear conditions showing the “reference value of the viscosity-at-rest”, (2) structural decomposition, and (3) structural regeneration

      For measurements like this, the following three test intervals are preset.

      1) Reference interval (low-shear phase)

      Shear conditions “at rest”, i. e. low-shear conditions, are preset in the period between t0 and t1. The aim is here to achieve a fairly constant η-value in the whole first interval, in terms of the reference value of the viscosity-at-rest. This value is used later as the reference value to be compared to the viscosity values obtained in the third test interval, showing structural regeneration if it occurs.

      2) High-shear interval

      High-shear conditions are preset in the period between t1 and t2 in order to break the internal structure of the sample. This interval is used to simulate the high-shear conditions occurring during an application process, for example, painting and coating using brush, roller or blade, or when spraying.

      3) Regeneration interval (low-shear phase)

      Shear conditions “at rest” are preset again in the period between t2 and t3, at the same shear conditions like in the first interval to facilitate regeneration of the sample’s structure. This interval is used to simulate the low-shear conditions occurring directly after the coating process when the material is only slightly stressed by its own weight due to gravity.

      For practical users, the crucial factor to evaluate structural regeneration is the behavior in the time frame which is related to practice. This period of time has to be defined by the user before the test according to the requirements, usually after a number of experiments performed (e. g. by the application department). Example A: For a wet coating, a regeneration time of t = 60 or 120 s is desired in order to obtain good leveling behavior. Example B: For a drop of an adhesive or for a printing ink a time of only t = 1 or 2 s is desired to achieve dot sharpness in a relatively short time. If the desired η-value has not been reached within this “time related to practice” (or“practice-relevant time”), then the sample is not considered to be thixotropic, related to this application.

      3.4.2.2.2Example 1: Presetting shear rates for all intervals

      1st interval (for t = 60 s, with 5 measuring points): at γ ̇ = 0.1 s-1 = const

      2nd interval (for t = 30 s, with 5 points): at γ ̇ = 1000 s-1 = const

      3rd interval (for t = 180 s, with 50 points or more): at γ ̇ = 0.1 s-1 = const again

      Exactly the same shear profile has to be preset for each individual test if thixotropy values of different tests are to be compared, and that counts for all parameters: shear rates, number of measuring points, and duration of the test intervals.

      3.4.2.2.3Example 2: Presetting shear stresses for all intervals

      1st interval: at τ = 10 Pa; 2nd interval: at τ = 1000 Pa; 3rd interval: again at τ = 10 Pa

      Interval times and number of measuring points should be selected like in Example 1.

      Example 3: Presetting shear rate and shear stress combined in series

      1st interval with controlled shear stress (at a low stress value, conditions at rest)

      2nd interval with controlled shear rate (at a high shear rate, to simulate the application process)

      3rd interval with controlled shear stress (at a low stress again, to simulate the low weight force of the applied wet coating layer)

      Using this type of test, sometimes differences between samples may be observed which might hardly be observed when using another test type. Sometimes, the first interval is omitted here (as in Figure 3.38). On the other hand, as a disadvantage must be stated that pre-tests are required to find out a useful shear stress value to be selected for the first and third interval.

      Note 1: Optimizing the test conditions

      In order to get a useful “reference value of the viscosity-at-rest”, the viscosity values should be as constant as possible in the first interval. If this condition is not met, the following actions can be taken:

      1 If the η(t)-curve comes from above and shows constant viscosity values only after a certain period of time, the preset shear rate was too high for the sample to be still in a state of rest. Therefore, at these shear conditions a certain degree of structural decomposition is already taking place. Action: A lower shear rate should be selected.

      2 If the η(t)-curve comes from below and shows constant viscosity values only after a certain period of time, then transient behavior is measured. Transient shear viscosity η+ is a function of both shear rate and measuring time, i. e. η+ = η( γ ̇ , t). In this case, the period of time was too short for the sample to adapt evenly throughout the entire shear gap to the applied low-shear conditions. Action: The measuring point duration should be extended. As a rule of thumb: The measuring point duration should be at least as long as the value of the reciprocal shear rate (1/ γ ̇ ). After a first trial with this preset, the measuring point duration may have to be extended further until good test conditions are achieved. Sometimes, however, even shorter times as t = 1/ γ ̇ are sufficient (see also the Note in Chapter 3.3.1b and Figure 2.9, no. 5: transient behavior).

      Note 2: Which mode of testing is more useful – shear rate or shear stress control?

      If the instrument is able to control very rapid changes in shear rates, then controlled shear rate (CSR) tests are often preferable to controlled shear stress (CSS) tests. The reason is that the process of structural decomposition and regeneration is directly dependent on the degree of the shear rate or deformation, respectively. Shear stress in fact is causing this deformation, but it is the deformation itself leading to the change in the structural strength. This is correct since the visco­sity value is only changing if the acting shear force indeed produces a sufficiently high deformation. As a result of this interrelation, in most cases, measuring results obtained from CSR tests are more reproducible.

      Optional methods to analyze structural regeneration

      A number of options to analyze thixotropic behavior are given below, many users prefer evaluation according to method M4.

      M1) The thixotropy value as difference between the minimum and maximum values of viscosity

      The extent of thixotropic behavior is determined in terms of the viscosity change Δη, which is the difference between the maximum viscosity after structural regeneration and the minimum visco­sity after structural decomposition. With ηmin at the time point t2