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Shrinkage is the most common paint film defect in coatings, which refers to the phenomenon of shrinkage in the local coating area, and then the exposed or partially exposed back coating surface, forming pits and pockmarks, including shrinkage, pumping, as well as pockmarks, pockmarks, fish eyes, pits, etc., collectively referred to as shrinkage.

The main reason for shrinkage is the difference in surface tension gradient on the coated surface. For example, when a surface is contaminated, substances with low surface tension from the pollution source will cause the surface tension in the contaminated area to be lower than that in the surrounding area.

We know that the flow of liquid is from areas with low surface tension to areas with high surface tension, which creates this concave cavity and forms this shrinkage cavity.

There are many reasons for shrinkage, which can be roughly classified into the following categories.

The first one is that the coating composition contains shrinkage factors, because our coating itself is not well done, which means that the formation of shrinkage comes from the interior of the coating composition. The main reasons are as follows. The first one is that the resin contains a small amount of polymer molecules, and if the resin is not properly prepared, it will produce gel particles. After the insoluble particles in this solvent precipitate out, their surface tension is lower than that of the resin solution, and the coating is prone to shrinkage after film formation.

The second issue is the uneven evaporation of solvents, where some solvents evaporate too quickly or excessively during the evaporation process. It will disrupt the dissolution equilibrium between the solvent and the mixed resin, causing the resin with poor solubility to precipitate out. This type of shrinkage usually occurs during the drying process of the coating,

The third one is that the coating contains these active substances with low surface tension, such as adding poorly compatible or excessive defoamers. There are also other silicone fluorine additives, and the shrinkage caused by this reason will immediately appear as long as spraying is applied. The fourth is external pollution. It refers to the external pollution after the coating film is formed, such as the pollutants in the low surface slag floating in the air, such as some silicon particles, grease, and particles falling onto the newly coated film, which will cause shrinkage of the film.

In addition, some facilities such as painting, spray painting rooms, drying ovens, air compressors, paint storage rooms, etc., where the air is not clean, light mist particles or other airborne particles can also cause shrinkage when they fall onto wet film surfaces that have not been dried.

The fifth issue is that the surface tension of the substrate is low, and the surface tension of the coating is higher than that of the object being coated. For example, the surface tension of the material itself is very low, such as PE or PP plastic. In addition, there may be some coating on the surface of the substrate, such as extrusion molding materials that have not been cleaned with release agents or are coated with low surface tension substances. For example, galvanized nickel plated iron plates and so on. There are also coatings containing silicon or fluorine, etc. Another issue is that the substrate treatment is not qualified and the pollution is not completely cleaned up. In such cases, after coating, shrinkage will immediately occur, and in severe cases, it may even fail to form a film.

Even if the film is barely formed, its adhesion is not good.

The sixth issue is the imbalance of dynamic surface tension, especially when using high shear rate coating. Due to the significant increase in the bottom surface area of the coating in an instant, the surfactants in the coating cannot be rearranged on the newly added surface, resulting in an imbalance of surface tension in the coating. This imbalance of surface tension. If the coating falls on the substrate, it will cause shrinkage holes on the entire board, which is particularly evident in high-speed printing lines such as wallpaper ink. Adding additives that reduce dynamic surface tension, substrate wetting agents, such as alkynediol based substrate wetting agents, can effectively solve this problem.

There are many reasons for the occurrence of shrinkage, and there are also various solutions to the problem. The key to preventing shrinkage lies in prevention. Don’t solve problems again. If too strong defoamers are used in water-based coatings, it is particularly prone to shrinkage.

It must be thoroughly dispersed and stirred, or sandblasted, or diluted with defoamers before adding them to the system to avoid local concentration and shrinkage, or choose defoamers with good compatibility.

In addition, it is necessary to reduce external pollution, such as dust and oil removal treatment, and low surface tension leveling agents can also be added, such as fluorinated organic silicon leveling agents and substrate wetting agents, to achieve the goal of eliminating shrinkage. Usually, it is mainly composed of organic silicon or organic fluorine compounds. The shrinkage caused by dynamic surface tension imbalance can be improved by adding alkynediol based wetting agents to reduce dynamic surface tension.