What Does a Rheometer Have to Do With Sun Protection?

When we think of summer the smell and the feeling of sunscreen on our skin immediately come to mind. But hardly anyone is aware of the science behind this much-used product, especially about its connection with rheology.

That’s why I would like to tell you about it. As a parent, for example, you naturally want only the best for your child. One of the many “fears” parents have is that their children might get sunburn. The solution is, of course, to use a sunscreen, preferably a high-quality product purchased from a local pharmacy.

Rheometers tell us a lot about sunscreen and other gels, lotions, and creams

The pharmaceutical industry produces a wide range of products for different applications and these have to meet certain requirements. Our sunscreen, for example, should protect the skin against sunburn under all circumstances. Therefore, its structural behavior has to be thoroughly tested under different conditions. This is exactly what rheometers do. They test the structural – or more precisely the rheological – behavior of all kinds of samples. The viscosity, sometimes also referred to as the sample’s thickness, is one of the most important rheological parameters. Another parameter which is important in this industry is the flow or yield point of a sample describing the amount of force that must be applied before the sunscreen starts to flow.

What does a rheometer do in general?

We already learned that rheometers are used for measuring the structural behavior of different materials. I will explain using the example of the RheolabQC, a rotational rheometer from Anton Paar. It consists of a motor that turns a measuring bob (usually a cylinder). This cylinder is immersed in a cup that contains the sample, e.g. our sunscreen. The cylinder turns at a certain speed and the resulting torque is measured. The higher the resulting torque at a given speed, the “thicker” the sample is, i.e. the higher the viscosity of the sample is. This is how we can measure the viscosity of a certain substance. Of course, the rheometer additionally heats or cools the sample and it can simulate different conditions affecting the sample. For example, when measuring at a low speed, this means that the sample is at rest. When measuring at a high speed this determines the viscosity of the sunscreen when it is squeezed out of its tube or rubbed on the skin. Rheometers are widely used in the pharmaceutical industry for determining the structural behavior of all kinds of pharmaceutical products.

How is a sunscreen typically tested on a rheometer?

Some widely used rheological tests in the pharmaceutical industry are:

  • Flow and viscosity curves
  • Flow or yield point determination
  • Measurement of the structural regeneration, also called “thixotropy“ testing
  • Determination of the temperature-dependent behavior
  • Temperature swing test

These measuring methods are useful for optimizing the manufacturing process (homogenizing, pumping, filling etc.), the long-term stability of the dispersion and also the subsequent end-use of all kinds of gels, creams and lotions.
A viscosity curve for our sunscreen means that we determine the viscosity under different shear conditions with one test. One condition would be at rest (at low shear rates for simulating the behavior in the tube), another would be at high shear rates (for simulating the behavior when shaking the tube or during stirring or pumping in the production process).
The flow or yield point determination is important for finding out how much force is needed to squeeze the sunscreen out of its tube. This can also be tested by the rheometer quickly, reliably and easily under different conditions. Our sunscreen should be easy to squeeze out of its tube before applying it, no matter whether it was taken straight out of the fridge or has been lying in the car on a warm summer’s day.

Fig. 1: Determining the yield point of sunscreen. The red circle shows how much force is needed to squeeze the sample out of its tube at a certain temperature.
Fig. 1: Determining the yield point of sunscreen. The red circle shows how much force is needed to squeeze the sample out of its tube at a certain temperature.

Special requirements in the pharmaceutical industry

We say that “seeing is believing” but for any measurements in the pharmaceutical industry we have to say that documentation is mandatory. This is why not only the correct rheological measurements but also the respective documentation has to be carried out with our rheometers. To help customers in the pharmaceutical industry to qualify their instruments, Anton Paar GmbH offers the Pharma Qualification Package containing all relevant documents for instrument qualification in pharmaceutical companies. This package, together with the powerful and easy-to-use PC software RheoCompass™ means users are now well prepared for successful rheological measurements.

  Learn more @ Anton Paar

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