Rheology in Cosmetics

If you have never heard of the word rheology, it is the scientific field that encompasses the flow phenomena of matter (solids, liquids, and gases) and notably involves time-dependent behavior under the influence of stresses.

The rheological characteristics of cosmetics and pharmaceuticals are important for production, packaging, filling, and storage. Customers appreciate the sensation of a skin cream when applied; they want the toothpaste to remain on the brush after squeezing it out of the tube, and they want medicine in the optimal condition to take effect wherever it should.

As cosmetics and pharmaceuticals are developed for external application – for example in the form of lotions, gels, creams – or for oral application, for example as pills or syrups, they occur as liquids, semi-solids, and solids. All these forms can be investigated using a rheometer.

Rheological behaviour of cosmetics and pharmaceuticals

Rheology is widely used in the cosmetic and pharmaceutical industry to test a variety of products. The fields of application range from very common tests like measuring the stability of a cream at different temperatures to very challenging measurements of small sample quantities like antibodies and injectable hydrogels, which may be very expensive and time-consuming.

In this case the instrumentation required to test such kinds of materials must be extremely sensitive and precise to generate reproducible analytical results, for example at ambient and body temperatures.

Rheological tests are useful for:

• Controlling the quality of raw materials, final products, and manufacturing processes (mixing, pumping, packaging, and filling).
• Evaluating the effect of different parameters such as formulation, storage time, and temperature on the quality and acceptability of a final product.
• Testing the flow and deformation of biological fluids and cells.

Here are some examples of cosmetics and pharmaceuticals which typically undergo rheological testing:

Hair gel: Modern hair gel is expected to meet high requirements. On the one hand it has to be extremely strong, moisture-resistant, and should provide long-lasting hold; on the other hand it should not lead to sticky hands or sticky hair.

Nail polish: Nail polish is a material that needs to satisfy various rheological demands. It needs to be thick enough to stick to the brush, but must also be thin enough when transferred from the brush to the nail. Furthermore, the material should flow enough to level out any brush marks after the application. It also needs to dry quickly.

Skin cream: The feel and long-term stability of skin cream, both in cosmetic and pharmaceutical applications, are important when it comes to gaining favour with the consumer. This is determined by both ingredients and by the manufacturing process. The replacement of ingredients with functional properties, such as emulsifiers or stabilisers, may be necessary since regulatory changes do occur and are therefore no longer accepted by the consumer. However, the quality of the products needs to stay the same. Likewise, an ingredient exchange – to reduce production costs, for example – should not necessarily result in a change of the physical properties typically associated with a certain product.

For more information on how rheological testing to ensure product stability and success contact Botanichem.

[Information: https://wiki.anton-paar.com/en/basics-of-rheology]