Defoaming agent
Foam is generated when air enters a liquid containing surfactant under the action of external force, and the surfactant will be oriented and arranged at the air-liquid interface. The hydrophobic group faces the air in the bubbles, and the hydrophilic group faces the water, creating a large number of bubbles. There are several reasons why foam occurs in paint.
1: During the production of paint, air will be entrained into the paint due to mechanical stirring;
2: Air brought in during the coating process, such as brush coating, roller coating, high-pressure airless spraying, etc.
3: When mixing two-component paint before construction, stir the mixed air.
4: The object to be painted has many pores, and air bubbles are formed due to the penetration of the paint, such as when painting on porous wood and cement walls.
5: Bubbles generated by chemical reactions, such as polyisocyanate in two-component PU coatings reacting with trace amounts of water to produce carbon dioxide.

Bubbles are inherently unstable, and their destruction requires three processes: redistribution of bubbles, thinning of the film thickness, and rupture of the film. However, for a relatively stable foam system, it will take a long time to achieve natural defoaming through these three processes. Defoaming agents are mostly used in all production. It is generally believed that defoaming agents reduce the local surface tension of the foam liquid film and defoam. Because of the low surface tension of the defoaming agent itself, it always penetrates into the foam system in the form of particles to locally reduce the surface tension of the bubble film. Since liquids with low surface tension always flow toward liquids with high surface tension, the membrane wall gradually becomes thinner and is strongly pulled by the surrounding membrane layers with high surface tension, resulting in stress imbalance, which eventually causes the bubbles to burst.

Currently, there are three general categories of defoaming agents: mineral oil defoaming agents, silicone defoaming agents, and silicone-free polymer defoaming agents.
Defoaming agent selection
There are many factors that affect foaming and foam stabilization in coating formulations when selecting defoaming agents.
1: Surface tension The surface tension of the paint has a great influence on the defoaming agent. The surface tension of the defoaming agent must be lower than the surface tension of the paint, otherwise it will not have defoaming and foam suppression effects. The surface tension of the coating is a variable factor, so when selecting a defoamer, the surface tension must be constant, and then the surface tension factor must be taken into account.
2: Influence of other additives. Most of the surfactants used in coatings tend to be functionally incompatible with defoaming agents. In particular, emulsifiers, wetting and dispersing agents, leveling agents, thickeners, etc. will affect the effectiveness of defoaming agents. Therefore, when using various additives together, you must pay attention to the relationship between different additives and choose the best balance point.
3: Baking temperature When the paint enters high-temperature baking at room temperature, the viscosity will drop instantly and the bubbles can move to the surface. However, due to the volatilization of the solvent, the solidification of the paint, and the increase in surface viscosity, the bubble film will become more stable, leaving Shrinkage holes and pinholes will occur on the surface, so the baking temperature, curing speed, and solvent evaporation rate also have an impact on the effectiveness of the defoaming agent.
4: The solid content, viscosity, and elasticity of the coating are very difficult to defoam in coatings with high solid content, thick coatings, high viscosity, and high elasticity. In these coatings, it is difficult for the defoaming agent to diffuse, and the microbubbles become large slowly, and the foam moves toward the surface. There are many unfavorable defoaming factors such as reduced surface migration ability and high viscoelasticity of the foam film. The foam in these coatings is quite difficult to eliminate, so it is best to use a defoaming agent and a defoaming agent together.
5: Coating method and construction temperature There are many coating methods for coating construction, such as brush coating, roller coating, shower coating, scraping coating, high-pressure airless spraying, screen printing, etc. The degree of bubbling of the paint varies depending on the coating method used. Brush and roller coating produce more foam than spraying and scraping. The most foamy ink is screen printing, and it is difficult to eliminate. There are more bubbles when the temperature is high than when the temperature is low, but the foam is easier to eliminate when the temperature is high than when the temperature is low.

Defoaming agent use
1: Stir thoroughly before use and mix evenly.
2: Add defoaming agent while the paint is stirring.
3: Before use, it generally does not need to be diluted with water and can be added directly. If some varieties need to be diluted, they need to be mixed and used.
4: The dosage should be appropriate. If the dosage is too small, it will cause shrinkage, edge shrinkage, poor brushability and poor recoatability, etc. If the dosage is too small, the defoaming effect will be poor. The sweet spot should be determined.
5: It is best to add it in two steps, that is, before grinding the pigment slurry and after adding the emulsion to mix the paint. Generally, the amount added each time is half of the total amount, and it can also be adjusted according to the foam situation. In the pigment slurry grinding stage, a defoaming agent with good foam-suppressing effect can be used, and in the paint mixing stage, a defoaming agent with good foam-breaking effect can be used. Experiments show that it is best to add silicone defoaming agent during the grinding stage of pigment slurry, and add as little water as possible, so that it can be fully dispersed and the dosage can be reduced to a minimum. It is advisable to add non-silicone defoamer during the paint mixing stage.
6: It takes at least 24 hours after the defoaming agent is added to achieve the balance between defoaming performance and edge shrinkage and cavity shrinkage. Therefore, testing the coating performance must be carried out after 24 hours.