Rheology of paint refers to the flow and deformation properties of paint under the action of external forces (such as gravity). In the process of production, storage, construction and film formation of paint, it may be affected by different types of forces, mainly including pure shear, tensile shear and simple shear, of which simple shear is the most important. When the coating is subjected to simple shear forces, there are velocity differences between different layers, and the viscosity (η) is defined as the ratio of the shear stress (τ) to the shear rate (γ), usually in PASCAL seconds (Pa·s), millipascal seconds (mPa·s), or centipascal seconds (cP). Viscosity is an important index of paint rheology, which is closely related to shear rate and shear stress.

The rheological properties of coatings are affected by film forming materials, solvents and fillers, and are significant at low shear rates. The aggregation of pigments, the colloidal properties of particles and a small amount of rheological additives 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 fluid dynamics of the paint, such as the mean particle size of the pigment particles, particle size distribution, shape, surface charge and adhesive properties.

Rheological properties of paint

The viscosity of the coating needs to be balanced in different application environments. When storing paint, it is hoped that the system has a high viscosity to prevent the precipitation of pigments and fillers. However, during construction, the lower viscosity helps to smooth the film, but the higher viscosity needs to be achieved within a certain period of time to avoid the film hanging and flowing problems. For powder coatings, sufficient leveling effect can be achieved only when the melt of the coating has a sufficiently low viscosity. In addition, viscosity also has an important effect on the dispersion of pigments in coatings.

In coating formulations, the combination of resin, pigment and solvent does not meet all requirements on its own, so formulations often require the addition of rheological agents to give the product the desired rheological properties. Rheological agents can control the flow characteristics of coatings, emulsions and pigment suspensions, and are suitable for water-based and solvent-based coating systems, as well as for situations with low shear viscosity (such as precipitation prevention, flow hanging and flow leveling) and high shear viscosity (such as construction and dispersion processes).

Rheological properties of paint

Rheology is one of the key properties of paint manufacturing and application, as the paint must undergo a rheological phase during its flow from the container to the surface of the coated object. The flow properties of coatings can be divided into Newtonian and non-Newtonian types, and the non-Newtonian fluids are further divided into shear rate dependent and time dependent types. The flow behavior of shear-rate-dependent fluids varies with the shear rate, including pseudoplasticity, dilatancy and plasticity. The time-dependent fluid is at a certain shear rate, its flow characteristics change with time, including thixotropy and shock setting, and most coatings belong to thixotropic fluids.

By understanding and mastering the rheology of coatings, coating manufacturers and users can better select, design and optimize coating formulations to meet the needs of various applications, ensuring stable and high-quality coating results under different conditions. The research and application of rheology has brought more possibilities to the coatings industry, and also provided more opportunities for innovation and development in all walks of life. By understanding rheology, we can better grasp coating technology and achieve better coating results.