The main function of the coating is decoration and protection, if there are flow and leveling defects, not only affect the appearance, but also damage the protective function. If the formation of shrinkage holes causes insufficient thickness of the paint film, and the formation of pinholes will lead to the discontinuity of the paint film, these will reduce the protective performance of the paint film. In the process of coating construction and film formation, some physical and chemical changes will occur, and these changes and the properties of the coating itself will significantly affect the flow and leveling of the coating.
After the coating is applied, a new interface will appear, which is generally the liquid/solid interface between the paint and the substrate and the liquid/gas interface between the paint and the air. If the interfacial tension of the liquid/solid interface between the coating and the substrate is higher than the critical surface tension of the substrate, the coating will not be able to spread on the substrate, and leveling defects such as fisheyes and shrinkage porosity will naturally occur.
The evaporation of the solvent during the drying process causes a difference in temperature, density and surface tension between the surface and the inside of the film, which in turn leads to turbulent movements inside the film, forming the so-called Benard vortex. The Benard vortex results in the production of orange peel; In systems containing more than one pigment, Benard vortices are also likely to cause floating and blooming if there is some difference in the movement of the pigment particles, and vertical application can cause filaments.
During the drying process of the paint film, some insoluble colloids are sometimes produced, and the generation of insoluble colloids will lead to the formation of surface tension gradients, which often lead to the formation of shrinkage porosity in the paint film. For example, in a cross-linked curing system, where the formulation contains more than one resin, insoluble colloids may form as the solvent evaporates during the drying of the paint film. In addition, in formulations containing surfactants, if the surfactant is incompatible with the system, or if the solvent volatilizes during the drying process, its concentration changes and the solubility changes, forming incompatible droplets, and a surface tension difference can also be formed. All of these can lead to craters.
In the process of coating construction and film formation, if there are external pollutants, it may also lead to leveling defects such as shrinkage porosity and fisheye. These contaminants are usually oil, dust, paint mist, water vapor, etc. from the air, construction tools and substrates.
The properties of the coating itself, such as application viscosity and drying time, can also have a significant impact on the final leveling of the paint film. Too high application viscosity and too short a drying time often result in poorly leveled surfaces.
Therefore, it is necessary to add leveling agent to help the coating obtain a good leveling by adjusting some changes in the coating in the process of construction and film formation and the properties of the coating.
