To determine the rheological behavior of a material – for example wall paint – a quick and practical solution can be to carry out a single-point measurement with a viscometer. On the other hand, using a rheometer to determine a measuring curve has its advantages.
We often get asked by users – in particular users who have no experience with rheology – whether a simple and presumably quick single-point measurement of the dynamic viscosity η is sufficient to describe the rheological behavior of a material. Unfortunately, there is no quick and easy answer to this question.
Depending on the required level of quality and the material requirements a single-point measurement can be an adequate – or inadequate – way to describe the viscosity behavior of a substance, for example a wall paint. A measuring curve consisting of a number of measurement values (“measurement points”) gives us much more information.
Example: wall paint
Wall paints, to remain with this example, usually have a shear-dependent flow behavior. Depending on the shear load which is applied to the wall paint, e.g. by pumping, stirring, applying using a brush or roller, spraying, etc., this leads to a more or less significant change in the dynamic viscosity η.
As can be seen in Figure 1, the dynamic viscosity η decreases with increasing shear rate (shear load) . The wall paint therefore clearly shows shear-thinning flow and viscosity behavior. This is often a requirement when applying wall paint to the substrate, e.g. with a paint roller or brush, as it is often beneficial to have a low viscosity to be able to apply the paint without a great deal of effort. On the other hand, the flow behavior or the dynamic viscosity at low shear rates can be of interest in order to obtain information about the stability of the wall paint (“sedimentation behavior”). The higher the value of the dynamic viscosity at shear rates <1 s-1, the higher the stability because in this case there is a stronger internal network structure in the wall paint which counteracts the sedimentation process of the pigments and fillers it contains.
From the single-point measurement to a measuring curve
In contrast to a measuring curve which covers a large shear load range in just one measurement, a single-point measurement only determines the shear load at a single shear rate. This means the user must know in advance which shear load should be depicted by the single-point measurement.
Of course, it is possible to carry out several single-point measurements one after another in order to obtain a simple measuring curve. However, this procedure is very time-consuming because the viscometer must be reprogrammed for each measuring point and a new sample which has not been previously placed under shear load must be used every time. Using a rheometer has the advantage here that one measurement covers a wide shear load range, from very low to high shear rates.
See things change
Single-point measurements using a viscometer deliver only very limited information on the shear-load-dependent flow and viscosity behavior. Measuring curves from a rheometer quickly provide a lot of information about the rheological behavior over a large load range. Besides giving insight into the behavior at low shear loads, which can give information on the stability of the wall paint, the behavior at high shear rates can give information on the processing properties when the paint is applied to the wall, e.g. using a brush or roller.
Anton Paar’s MCR rheometer series offers a range of possibilities for quickly and easily determining measuring curves. The newest rheometers in this family will have a big effect on your laboratory work: MCR 72 and MCR 92 were developed to simplify your lab routines. See things change.