Rheology is a lore about the flow and deformation of substances. Regarding latex paints, this is the science of flowing behavior during storage and handling, both during and after application. Rheology in its scope of research covers everything related to the flow and properties of paints during mixing, pumping, application (brush, roller, spray, etc.), or mixing after adding colorants or diluting with water. It is an extensive range of knowledge concerning not only rheological additives (thickeners), but their interactions with other raw materials in the formulation.
Designing latex paint formulations is a task that is often difficult at the stage of selecting thickeners responsible for the entire range of rheological efficiency. The mere achievement of a resting viscosity, the so-called low-shear viscosity, is not difficult and usually comes down to a single thickener. But the market expects latex paint to have a number of properties, and rheological additives must satisfy these expectations, including easy application with a brush, roller, or spray (especially by people who are not professional painters). Additionally, the additives must ensure that there is no syneresis, no spattering, good leveling, etc.