If the viscosity of a substance can be maintained at different shear rates, we call it showing ideal viscosity or "Newtonian viscosity". Newtonian fluids generally only occur in low molecular weight liquids, such as water, solvents and mineral oil .

More complex systems often have shear-dependent flow characteristics. If the viscosity decreases with increasing shear rate, this flow characteristic is called shear dilution or pseudoplasticity. Most coatings and polymer solutions exhibit pseudoplastic properties.

A thixotropic fluid exhibits time-dependent viscosity reconstruction after applying a shear force. Once the application of shearing force is stopped, the viscosity will gradually recover. The degree of thixotropy is usually expressed in terms of the so-called hysteresis area. Thixotropic properties are advantageous for paints applied to vertical surfaces, because on vertical surfaces, the viscosity decreases under the shear of a brush or roller, leveling is achieved, and then gradually restored to prevent sagging. If the viscosity of the material increases with increasing shear rate, it is dilatant (shear thickening). For example, binder systems with higher solids or polymer concentrations can exhibit dilatant properties. In industrial practice, the dilatant flow characteristic is generally not desirable because it may cause problems in processes involving pumping or stirring. Throughout the life cycle of a paint or coating (such as production, mixing, filling, storage, coating, brushing, spraying), different shear rates and rheological requirements may be involved.

Viscosity—especially non-Newtonian viscosity characteristics—is an important material characteristic that makes up the properties of a fluid. Since paint is endowed with pseudoplasticity, it exhibits high stability during storage. He can also perform at low shear rates. Paints or coatings are usually applied under relatively high shear conditions (brushing, spraying). At this time the viscosity is more favorable, legend:

Generally, paints need to be leveled to a certain extent after application, so excessive recovery of viscosity is not desirable. However, if the recovery is too slow, it will cause drooping and dripping. And some systems may need to recover viscosity more quickly. In this case, a comprehensive understanding of the flow properties that different rheological additives can impart can help you choose the most suitable product for the desired application.