3iTT test
The following requirements apply to structural regeneration of coatings:
- not too fast in order to allow for good leveling,
- not too slow in order to prevent sagging and to ensure a sufficient wet-layer thickness.
Thus, a compromise must be found between these two requirements. This compromise is balanced time-dependent behavior. To investigate time-dependent behavior, it is recommended that a step test be carried out, in this case as a rotational test with three intervals. This measurement is usually performed as a time-dependent controlled-shear-rate test (Figure 1):
- Interval (1) Very low shear to simulate behavior at rest at a preset low shear rate,
- Interval (2) Strong shear to simulate structural breakdown of the sample during the coating process at a preset high shear rate, for example when applying paint with a brush or by spraying,
- Interval (3) Very low shear to simulate structural regeneration at rest after application using the same preset low shear rate as in the first interval. Usually, the result is presented as a time-dependent viscosity function (Figure 2):
- With viscosity values almost at rest,
- During structural breakdown under high shear conditions,
- During structural regeneration almost at rest.
Figure 1: Preset profile for a step test with three intervals used to evaluate the behavior of a brush coating: (1) constant low shear rate as in the state at rest, (2) constant high shear rate as applied in a coating process, and finally (3) the same shear rate as in the first interval during structural regeneration at rest after application.
Figure 2: Typical result of a step test with three intervals depicted as a time-dependent viscosity function: (1) at the beginning, high viscosity at rest, (2) decrease in viscosity caused by structural breakdown induced by high shear, (3) increase in viscosity by structural recovery at rest. The example shows a material with complete regeneration of viscosity.