ABSTRACT
The dynamics of paint settling and sagging involve low shear rates. In contrast, the application of paint often requires a high shear rate. In solventbased and solventless coatings, the desire is to increase low-shear viscosity while not increasing high-shear viscosity. This property of shear thinning is referred to as pseudoplastic (shear dependent). Thixotropy is also very important because it is not only shear thinning but also time dependent; that is, viscosity decreases with time when sheared at a constant shear rate and viscosity increases with time after shear has stopped. There is a very notable
desire for thixotropy in trade sales or architectural coatings that are brushapplied. These paints are designed to maintain uniformity, minimizing the need for stirring, providing smooth application, quick leveling (eliminating brush marks), and fast recovery preventing runs and sag. Rheology additives prevent settling and sag by setting up a loosely held network structure that breaks down with shear and rebuilds as shear is reduced. The rheology additives vary widely in their effect on settling, sagging, spraying, brushing, leveling, rolling, misting, and so forth. They also vary in their effectiveness in different solvents, resins, and pigments. Each coating requires its own unique flow based on its application. Consequently, .the right rheology additive, or combination of rheology additives, must be selected from the many that are available on the market.
