Rheology for material formulation

Using rheology for faster, smarter material formulation

Rheology is often perceived as a theoretical, highly mathematical discipline but in fact it has very practical relevance in defining the behaviour of a wide range of materials including cement, asphalt, cosmetics, foods, paints, inks, blood and much more. Rheology is all around us and we are constantly performing rheological testing every time we apply a force to one of these materials in order to deform it or make it flow at the desired rate, be it squeezing toothpaste, spreading cement, or chewing food.  The ratio of the applied force (stress) to the measured deformation (strain) or flow rate (strain rate) is what governs such behaviour and is related to the rheological material properties such as modulus (stress/strain) or viscosity (stress/strain rate), the knowledge of which can help predict how a volume of material will behave under a given force or deformation.

We qualitatively assess the modulus, elasticity or viscosity of a material through our senses when we physically touch a material but we also make qualitative rheological assessments by sight since, for example, we can often tell the differences in the rheological properties of two different Ketchup samples by the way they sit on the plate. Such assessments are subjective and in most cases cannot be quantified except by a comparative panel score which may grade different products by thickness, stringiness, tackiness and so on. While such feedback is important, especially for the food and personal care industries, it is somewhat impractical in the early stages of product development where there are often a large number of formulation candidates. Therefore, objective metrics that reliably correlate with key aspects of product performance are extremely valuable.

Such objective metrics can be obtained using rheometry, and in particular rotational rheometry, where it is possible to measure both viscosity and viscoelastic properties of complex materials under conditions which are relevant to the application or process of interest. Key to the successful use of rheology is identifying the relevant parameters that correlate directly with the aspect of performance that is of interest. Ensuring relevance requires careful consideration of the following issues:

• Which test should I apply?
• Which rheological parameters should I measure?
• What test conditions should be applied?
• How do I interpret the results generated?
• What does the data mean?

Such questions are not always easy to answer which is why Malvern gave this significant consideration when developing the Kinexus rheometer platform, where the aim was to make rheological testing readily accessible to the novice while still providing the advanced functionality and flexibility to the expert user. This was realized with the rSpace software which uses Malvern’s cornerstone Standard Operating Procedure (SOP) approach and built in intelligence to provide support through every aspect of the measurement process; through sample loading, measurement, analysis and result interpretation, As well as all the standard rheological  tests there are also a collection of dedicated rSolution tests for benchmarking performance and material properties in relation to texture, spreading behavior, yield stress, product delivery characteristics, formulation stability and even tack testing.

Some of these simple but application specific tests are discussed in a recent Malvern article in Household and Personal Care magazine which shows how different personal products can be compared and evaluated in terms of their textural properties and spreading behavior. We also have much more application specific information on our website in terms of whitepapers and application notes, many of which have an associated rSolution test in the rSpace software. We also have a team of application focused rheologists who are more than happy to help answer any of the questions above.

In summary rheology is both a practical and important formulation tool which can give important insight into product microstructure and performance. Knowing which test to apply and how to make relevant measurements is the key to success and something Malvern’s rheology specialists are able to help with.