Introduction
The Immobilization Cell is used with the Kinexus rotational rheometer to characterize the rheological properties of a paint or coating while it is dried on a substrate. Such a test is useful to determine the effects of
- Solids content
- Porosity of the coating substrate
- Thickness of the coating substrate
- Water retention additives
- Applied pressure drop
on the rheological properties of a material.
Figure 1 shows the Immobilization Cell system.
A measurement is made by placing the sample on a substrate positioned on a porous sintered disk and a vacuum applied below to initiate the dewatering process. An upper geometry (cone or plate) of up to 45 mm in diameter can be used, and real-time measurements made in rotation (viscometry) to characterize the sample’s immobilization kinetics, or oscillation to detect changes in the viscoelastic properties.
Measurement Parameters
In the following, the rheological properties of a wall paint were measured during drying. Table 1 details the measurement parameters.
Table 1: Measurement Parameters
Device | Kinexus ultra+ rotational rheometer |
Type of test | Oscillation, time sweep |
Geometry | PP40 (plate/plate, diameter: 40 mm) |
Gap at measurement start | 1 mm |
Normal force during measurement | 0.5 N |
Frequency | 1 Hz |
Shear strain | 0.5% |
Remarks on:
Selection of the shear strain: The shear strain of 0.5% was selected because it is in the linear viscoelastic region (Région Viscoélastique Linéaire (LVER)In the LVER, applied stresses are insufficient to cause structural breakdown (yielding) of the structure and hence important micro-structural properties are being measured.LVER) and so does not lead to a breakdown of the sample´s structure. This was determined by means of an amplitude sweep experiment (results not displayed). Of course, the sample changes during the measurement because it is drying, so that its LVER may also change. A look at the harmonic distortion curve revealed that the sample remained in the LVER during the entire measurement.
The normal force applied during the measurement: A gap of 1 mm was chosen for the test, but to make allowance for the shrinkage that was expected from the drying of the sample, a small normal force was applied to ensure contact was maintained between the upper plate and the sample with the change in gap during the test. This technique prevented the sample from being ejected as maintaining a normal force resulted in the reduction in gap size matching the sample’s shrinking.
LVER – Linear Viscoelastic Range
- The LVER is the amplitude range where strain and ContrainteLa Contrainte est définie par un niveau de force appliquée sur un échantillon d’une section bien définie. (Contrainte = force/surface). Les échantillons qui possèdent une section rectangulaire ou circulaire peuvent être comprimés ou étirés. Les matériaux élastiques comme les élastomères peuvent être étirés jusqu’à 5 à 10 fois leur longueur initiale.stress are proportional.
- In the LVER, the applied stresses (or strains) are insufficient to cause structural breakdown of the structure and hence microstructural properties are being measured.
Harmonic Distortion
Within the LVER, input oscillation frequency is the same as the output oscillation frequency. Beyond the LVER, we get harmonic distortion. The input oscillation breaks down to higher (i.e., harmonic) frequency responses. As the strain goes out of the LVER, the harmonic distortion increases. It can be easily displayed in the NETZSCH rSpace software.
Measurement Results
Figure 2 displays the complex stiffness and the gap measured during drying of the wall paint.
After a 1-minute equilibration in which an oscillation was applied without vacuum, the pump was switched on and dewatering of the paint began. This resulted in a three–decade increase in the complex modulus. Complex stiffness and gap during paint drying 2(stiffness) within 11 minutes, while the sample shrank by more than 10%. After this time, the complex modulus as well as the gap plateaued, indicating the end of the drying process.
A demonstration of setting up the Immobilization Cell can be seen in this video: How to use the immobilization cell