Лако-красочные материалы — производство

1.3.1 Rheology and its role in coatings

Rheology is the science of deformation and flow behavior of materi­als under the influence of parameters such as shear rate and time. Liquids with viscosity not dependent on shear rate are termed New­tonian liquids while non-Newtonian liquids exhibit changes in viscosity depending on shear rate. Most paints exhibit non-Newto­nian flow, which is desirable. Important types of non-Newtonian flows are pseudoplastic flow behavior (shear thinning), thixotro­pic flow behavior (time-dependent shear thinning,) and dilatant flow behavior (shear thickening). Pseudoplastic or thixotropic flow is often desirable in coatings because:

• Viscosity is fairly high at low shear rates, which avoids sedimen­tation of pigments and provides good anti-sag properties once the paint is applied on vertical surfaces.

• Viscosity is low at high shear rates, which allows better handling and easy application of coatings.

The rheology of a coating system is mainly influenced by the binder (chemistry and MW), solvents (type, content and solvency of resin), pigmentation level and additives such as wetting and dispersing

agents that have an indirect effect on rheology. Therefore, it is important to control the rheology to achieve certain desired cha­racteristics. Additives used for modifying rheological properties of coatings are known as thickeners or rheology modifiers. Apart from storage, processing, and application performance, the role of thickeners is also important to adjust the desired rheological cha­racteristics of the mill base to ensure maximum use of the supplied energy during the dispersing stage. They are important additives for both solventbased coatings and aqueous coatings.