To obtain excellent dispersion results, the millbase must exhibit certain rheological properties. Unfortunately, the flow behaviour of a millbase may not be expressed by one single parameter, such as the apparent viscosity.
Viscosity is the measure of the internal friction of a fluid, and is a product specific Constant Value which is defined as the quotient of shear stress (τ) and shear rate (D). Only Newtonian fluids retain a constant viscosity and are independent of variations in shear rate (that is i.e. water, mineral oil, etc.). All other substances which have a viscosity which is dependent on shear rate are classified as Non‒Newtonian and are more commonly found than Newtonian liquids. Millbase formulations are complex rheological systems, for whose characterisation information concerning apparent viscosity, plastic behaviour, yield stress, thixotropy, rheopexy, dilantancy is needed.