Rheology of complex formulated products

Most formulated products (foodstuffs, skincare and cosmetics, paints and varnishes, etc.) have a complex rheology, in the sense that they cannot be characterized by a single viscosity measurement. Indeed, the viscosity of these products is a function of the type and intensity of the mechanical stress applied, the time scale considered, the level of structuring or organization of the various constituent elements of the formulated product, and the balance of interaction forces between these various elements. Furthermore, formulated products must have properties and functions that are relevant to their use or application. For example, in the field of industrial paints, different formulations may have similar Störmer viscosities, but very different viscous behavior over a wide range of shear rates, which can condition the choice of a formulation in relation to the modes of application or use. What's more, these formulations need to possess well-balanced thixotropic properties to avoid drips or brush marks. These properties are often obtained through the presence of so-called rheology-modifying additives (polymers, particulate additives, surfactants, etc.). Controlling the action of these additives, as well as their mode and protocol of incorporation during a formulated product development process, is crucial to obtaining the ultimate characteristics of these products. There are strong and irreducible links between formulation, rheology and process engineering, which it is vital to understand and master.

In order to study these complex formulated products, we have developed what we call systemic rheology, which is proving to be an original methodological tool. Systemic rheology is not intended to replace traditional rheology, but rather to provide a more than complementary perspective, as three application examples will show.

Many complex formulated products take the form of colloidal suspensions. Rheo-optics is a branch of rheo-physics, which disregards standard laws of behavior and aims to predict rheological behavior on the basis of microscopic properties assessed in flow (a rapidly developing discipline). Rheo-optical techniques can be used to study rheological structures under controlled flow conditions.

Powdered products are also complex formulated products. They are increasingly present in many industrial application sectors. The implementation and use of these powdered products requires the use of indirect methods (angle of repose, flowability index, etc.) or direct methods (shear cells, etc.), which run up against difficulties of reproducibility and interpretation, linked to the topology of the stacks adopted during their conditioning