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Rheological investigation of cosmetics and pharmaceuticals

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 behavior 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.

Investigating cosmetics and pharmaceuticals using rheological tests

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

Typically measured cosmetics and pharmaceuticals:

Hair gel

Besides hairspray, hair gel is certainly one of the most popular hair products on the market. Modern hair gel must 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.

Rheological tests on hair gel

Gel is an abbreviation of “gelatin“ and has the same Latin root as “jelly“ (thickened fruit or meat juice). It is frequently a finely dispersed system consisting of at least a solid and a liquid phase, forming a three-dimensional network. Its consistence is viscoelastic and can be tested with a rheometer.

Constantly checking a product’s viscosity and yield point guarantees a consistently high quality. Whereas the viscosity of a hair gel is a particularly important parameter during production and for product evaluation during the development, the yield point is particularly important for the customer's evaluation of the end product. The yield point corresponds directly with the elastic properties of the sample at rest. Gels with a high yield point or a strong gel character are perceived by the customer as having "more volume". A strong gel character gives the impression of a high-quality product to a majority of customers.

With a rotational rheometer the flow curves and viscosity curves can be measured, for example, in a controlled shear stress test in rotation. Based on this, the yield point yield point can be calculated, e.g. using the Herschel-Bulkley flow curve fitting model. Another method is to determine the yield point by an amplitude sweep performed in oscillation.

This test requires a rheometer equipped with a Peltier temperature control system.

Nail polish

Nail polish is a material that needs to satisfy various rheological demands. On the one hand it should be thick enough to stick to the brush, on the other hand it has to be thin enough when transferred from the brush to the nail. Furthermore, the material should flow enough in order to level out any brush marks after the application. Finally, it has to dry fast enough for the customers to go about their business.

Rheological tests on nail polish

A very important factor with regard to nail polish is its thixotropic recovery. Thixotropy is a special kind of time-dependent viscosity behavior. Under constant shear, the viscosity of a thixotropic material decreases. Once shearing stops, the material will recover more or less completely. This behavior is observed in the decomposition and recovery of the inner material structure, which can be measured with rotational or oscillatory tests by performing a three-interval thixotropy step test.

Depending on their formulations, different types of nail polish may have different rates of thixotropic recovery. Determining the thixotropic behavior will show, for example, how often the nail polish has to be shaken during the application.

This test requires a rheometer equipped with a Peltier temperature control system.

Skin cream

The feel and long-term stability of skin cream, both in cosmetic and pharmaceutical applications, are important points for the acceptance by the consumer. These properties are largely influenced by the ingredients but also by the manufacturing process. The replacement of ingredients with functional properties, such as emulsifiers or stabilizers, may be necessary since some of these are suspected to cause allergies 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.

Rheological tests on skin cream

Characterizing the physical properties of an emulsion with a rheometer is the first step in determining and evaluating how a change in the composition or manufacturing process may influence the properties of the final product. Additionally the mechanical stability of the final product can be tested in a relatively short time with an amplitude sweep, which is an oscillatory test performed at variable amplitudes while keeping the frequency at a constant value. Using the values of the storage modulus G' and loss modulus G'' in the range of very low strain or stress values (which is called the linear-viscoelastic or LVE range), this kind of testing provides information about the structure strength and the viscoelastic character of a cream at rest.

This test requires a rheometer equipped with a Peltier temperature control system.