Freshly applied wet paint films are susceptible to a variety of surface pathogens. For example, when some pollutants or condensed solvent vapor droplets drop on the surface of the liquid film, they will spread rapidly on the surface of the film, discharge a lot of liquid paint, and form surface depressions, commonly known as "shrinkage cavities". The substance is called a shrinkage cavity donor. Shrinkage cavity is the general term for various , which can be divided into flat type, crater type, point type, exposed bottom type and bubble type in terms of shape, as shown in Figure 4-29, usually in the form of a drop Or a small piece of impurity as the center, forming a ring-shaped edge around it. The process is shown in Figure 4-30.
The occurrence of this phenomenon is related to the low surface tension of the shrinkage cavity donor. If its surface tension is high, it is impossible to expand and form shrinkage cavities, which is only possible when the surface tension is low.
In a coating formulation, if the surface tension of each component does not match, shrinkage cavities may occur. During the coating process, due to the creation of a huge fresh surface, the components with low surface tension inside the coating will be adsorbed to the surface layer and drive part of the material to migrate to the surrounding area. The flow of material driven by this flow may form shrinkage cavities. This adsorption process is a time course. If the viscosity of the system is small, the system can level off quickly; if the viscosity of the system is high, the process of surface adsorption and material flow is very slow, and the possibility of shrinkage cavity formation is small. Only when the viscosity is moderately low will shrinkage cavity occur.
External physical disturbances also change the surface composition, resulting in uneven distribution of surface tension on the surface of the coating film, and unequal surface tension everywhere. The low surface tension part will migrate to the high surface tension part, and drive part of the paint to migrate together, resulting in local flow and the formation of shrinkage cavities. If the liquid film is thick enough, the liquid can replenish from the bottom into the depression and close the shrinkage cavity. However, if the liquid film is thin and there is no liquid to replenish, permanent shrinkage will be formed.
