Efficient Rheology Control Additives
Rheology is defined as “the study of the change in form and the flow of matter embracing elasticity, viscosity and plasticity.” We concern ourselves with viscosity, further defined as “the internal friction of a fluid caused by molecular attraction, which makes it resist a tendency to flow.” Water is an invaluable solvent and vehicle but it is not without certain deficiencies. It is, above all, watery. The exercise of rheological control over water is a significant challenge for formulators. It is often too thin, it is generally runny and it is invariably unsupportive for insoluble particulates. Formulators are, therefore. forced to adjust water to suit their needs. Fortunately, this can be readily accomplished through the use of rheology modifiers. Specific control of water is enabled by the careful application of one or more of the rheology modifiers available for use in aqueous compositions. Familiarity with the rheological nuances of a particular modifier can at times make the difference between an exceptional formulation and a routine one. What follows is an overview of the hydrocolloids based additives most commonly used to control water. The intent is to make the formulator sufficiently familiar with the fundamental nature of each of these materials, so as to facilitate proper selection.
The terms used to characterize rheology are defined as follows:
Newtonian – The viscosity of such fluids will not change as the shear rate is varied. Water and thin motor oils show typical Newtonian behavior.
Non-Newtonian – The viscosity of such fluids will change as the shear rate is varied. There are several types of non-Newtonian flow behavior, characterized by the way a fluid’s viscosity changes in response to variations in shear rate. The most common types of nonNewtonian fluids you may encounter include:
• Pseudoplastic – This type of fluid will display a decreasing viscosity with an increasing shear rate. Probably the most common of the non-Newtonian fluids, pseudo-plastics include paints, emulsions and dispersions of many types. This type of flow behavior is
sometimes called “shear-thinning.” Moreover, they immediately recover their nonsheared viscosity once shear is removed.
• Dilatant – Increasing viscosity with an increase in shear rate characterizes the dilatant fluid. Although rarer than pseudoplasticity, dilatancy is frequently observed in fluids containing high levels of deflocculated solids such as clay slurries, candy compounds,
corn starch in water and sand/water mixtures. Dilatancy is also referred to as “shearthickening” flow behavior.
• Plastic – This type of fluid will behave as a solid under static conditions. A certain amount of force must be applied to the fluid before any flow is induced; this force is called the “yield value.” Tomato ketchup is a good example of this type of fluid; its yield
value will often make it refuse to pour from the bottle until the bottle is shaken or struck, allowing the ketchup to gush freely. Once the yield value is exceeded and flow begins, plastic fluids may display Newtonian, pseudoplastic or dilatant flow characteristics.
Yield Value – Yield value indicates the minimum force (the yield stress) that must be applied to a liquid to start disrupting the structure imparted by the rheology modifier, so that flow can occur. In practical terms, solids, oils and gases are trapped and segregated by this structure unless gravity or buoyancy can exert a force greater than the yield stress. Some fluids display a change in viscosity with time under conditions of constant shear rate. There are two categories to consider:
Thixotropy – Thixotropic fluids show a time-dependent response to shear. When subjected to a constant shear rate, they will decrease in viscosity over time. Often this is seen as a large initial viscosity loss, followed by gradual further loss. Once shear is removed,
thixotropic fluids recover their viscosity, but over a period of time, not instantaneously. These fluids are also considered to be pseudoplastic, but only in that they show decreasing viscosity in response to increasing shear rate. Thixotropy is frequently observed in materials such as greases, heavy printing inks and paints.
Rheopexy – This is essentially the opposite of thixotropic behavior, in that the fluid’s viscosity increases with time as it is sheared at a constant rate. Rheopectic fluids are rarely encountered. Both thixotropy and rheopexy may occur in combination with any of the previously discussed flow behaviors or only at certain shear rates. The time element is extremely variable; under
conditions of constant shear, some fluids will reach their final viscosity value in a few seconds, while others may take up to several days.
In addition, the term “synergism” is used to indicate that a combination of two rheology control additives provides a stronger rheological effect (e.g., viscosity or yield value) than would be anticipated by adding the individual contribution of each additive.