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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.