The rheology of Coating refers to the flow and deformation properties of Coating under the action of external forces (such as gravity). During the production, storage, construction and film formation of Coating, it may be affected by different types of forces, mainly including pure shear, tensile shear and simple shear force, of which simple shear force is the most important. When Coating is subjected to simple shear force, there is a velocity difference between different layers, and viscosity (η) is defined as the ratio of shear stress (δ) to shear rate (γ), usually in Pascal seconds (Pa · s), milliPascal seconds (mPa · s) or centiPascal seconds (cP). Viscosity is an important indicator of Coating rheology and is closely related to shear rate and shear stress.
Coating rheology is affected by film formation substances, solvents and extenders, mainly at low shear rates. Aggregation of pigments, colloidal properties of particles, and a small amount of rheological aids can lead to significant changes in the viscosity of the system. At high shear rates, these structures will be destroyed, and the viscosity is mainly affected by the hydrodynamic factors of Coating, such as the average particle size of pigment particles, Particle size distribution, shape, surface charge, and Adhesive performance.
The viscosity of Coatings needs to be equilibrated in different application environments. When storing Coatings, it is desirable for the system to have a higher viscosity to prevent precipitation of pigments and extenders. However, during construction, lower viscosity contributes to the levelling of the film, but a higher viscosity needs to be achieved within a certain period of time to avoid sagging and flowing problems of the film. For powder coatings, a sufficient levelling effect can only be achieved when the melt of the Coating has a sufficiently low viscosity. In addition, viscosity also has an important impact on the dispersion of pigments in Coatings.
In Coating formulations, the combination of Resin, pigment and solvent alone does not meet all requirements, so formulations often require the addition of rheological agents to give the product the desired rheological performance. Rheological agents can control the flow characteristics of Coating, emulsion and pigment suspensions, and are suitable for water-based paints and solvent-based paints systems, and are suitable for low shear viscosity (such as preventing precipitation, sag and flow) and high shear viscosity (such as construction and dispersion processes, etc.).
Rheology is one of the key performances in the manufacture and application of Coatings, because the Coating must undergo a rheological phase during the flow of the Coating from the container to the coated surface. The flow properties of Coatings can be divided into Newtonian and non-Newtonian types, and non-Newtonian fluids are further divided into shear rate-dependent and time-dependent types. The flow behavior of shear rate-dependent fluids varies with shear rate, including pseudoplasticity, swelling and plasticity. Time-dependent fluids are at a certain shear rate, and their flow characteristics change with time, including thixotropic and shock, while most Coatings are thixotropic fluids.
By deeply understanding and mastering the rheology of Coatings, Coating manufacturers and applicators can better select, design and optimize Coating formulations to meet various application needs and ensure stable and high-mass painting effects under different conditions. The research and application of rheology brings more possibilities to the Coating industry, and also provides more opportunities for innovation and development in all walks of life. By understanding rheology, we can better master Coating technology and achieve better painting results.
