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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 and ηmax at the time point t3 the following holds (see Figure 3.41):

Δη = ηmax – ηmin

Note 3: Alternative analysis methods (to method M1) from industrial practice

Some users in industrial laboratories evaluate thixotropic behavior by the following simple methods to carry out quality control of coatings.

Preset: Test, consisting of two measuring intervals: at first high-shear (HS), and then low-shear (LS) to enable structural regeneration of the sample. Analysis:

N3-a) The thixotropy index :

TI = (ηL – ηH) / tR

with ηH in mPas at the end of the HS interval; and ηL in mPas at the regeneration time tR after the beginning of the LS interval (Examples: tR = 60 s; or 30 or 90 or 300 s)

Calculation: Viscosity change in a previously defined time in the LS interval, in the form of the slope (Δη/Δt) of the η(t)-curve, with the unit (mPas/s). A straight line is therefore adapted between the last measuring point of the HS interval and the measuring point at time point tR in the subsequent LS interval. Evaluation: The faster the regeneration and the higher the corresponding

η-value obtained, the higher is the TI value. Example: with ηH = 100 mPas and ηL = 1000 mPas after tR = 60 s, then: TI = (900 mPas/60 s) = 15 mPas/s.

N3-b) The structure recovery index :

SRI = lg ηL – lg ηH

with ηH, ηL and tR as above. Calculation: Viscosity difference in the form of logarithmic viscosity values. Evaluation: The stronger the obtained regeneration, the higher the SRI value. Example: with ηH = 100 mPas and ηL = 1000 mPas (e. g. after tR = 30 s), then: SRI = (lg ηL – lg ηH) = 3 – 2 = 1. Usually here, specifications are given without any unit.

N3-c) The viscosity ratio during structural regeneration: VR(SR) = η1 (τ1) / η2 (τ2)

VR is a measure of the speed of structural recovery in the third test interval at a constantly low shear rate, with the viscosity values at the two time points t1 and later, at t2. As long as VR < 1, there is still a tendency for a continued structural recovery, and for VR = 1, the latter is finished.