Coatings are materials in liquid form whose rheological and flow characteristics directly affect their behavior during preparation, processing and application. The rheological properties of a paint, that is, its flow pattern under the influence of external forces, are usually determined by the interaction between particles.
In the rheology of coatings, there are several basic flow types. Plastic fluid is a flow characteristic. Under the action of external force, only when the shear stress exceeds a certain limit can the internal structure be destroyed and flow can occur. The minimum shear stress required to start flow is called the yield point, the viscosity after flow is called plastic viscosity, and the viscosity before flow is the structural viscosity of the suspension system.
As the shear force increases, the internal structure of pseudoplastic fluid is continuously destroyed, resulting in a gradual decrease in viscosity. Plastic thixotropic fluids have a yield point. After the flow begins, the viscosity decreases with the destruction of the internal structure, which is called apparent viscosity. Different from pseudoplastic fluids, under the action of shear stress, the destruction and reconstruction of the internal structure of the fluid takes a certain amount of time to reach a new equilibrium. When the shear stress is removed, the reconstruction of the internal structure is also a function of time, causing the thixotropic fluid to produce a change curve during the shear process, and the area of ââthe closed loop can represent the size of its thixotropy.
The viscosity of dilatant fluids increases with increasing shear stress, a phenomenon that can be explained by geometric effects and particle agglomeration. When at rest, the suspended particles are staggered with each other. The external force destroys this state, causing the edges and corners of the particles to touch each other, increasing the gap and expanding the volume, thereby increasing friction and causing an increase in viscosity.
The rheological properties of coatings are affected by many factors, such as the volume of solid matter, temperature, type of pigment, type and amount of adhesive, degree of agglomeration of particles, etc. Coatings with high solids content generally have higher viscosities and yield points. Different types of pigments and solvents will also affect the flow characteristics of coatings. For example, casein-based coatings show different rheological types at different solid contents, and the size and content of particles in china clay coatings will also affect their rheological properties. They range from dilatant to thixotropic.
For the study of coatings, it is crucial to understand their rheological properties. By understanding the internal structure and inter-particle interactions of coatings, we can better regulate its flow behavior, optimize the preparation process, and ensure better performance and stability during application. The research and exploration of coating rheology in the coatings industry will provide an important reference for the development of coatings technology and bring more innovation and application potential.
