What is the defoaming mechanism of defoamer in coatings?
As we all know, architectural coatings often produce a lot of foam during the production process. After the coating dries, these original foams will bring unbearable performance. Not only will the performance of the coating be greatly affected, the appearance will be damaged, and in severe cases, the coating will even fall off the wall. At this time, defoamers need to be added to eliminate the foam.
Generally speaking, water-based defoamers need to be added to architectural coatings. The following editor will introduce in detail how defoamers work effectively in architectural coatings.
For architectural coatings, defoamers must be combined with a stable surfactant layer and penetrate into the bubble film bimolecular membrane. After contacting the bubbles, they spread rapidly to form a very thin double membrane layer. As the defoamer further spreads and invades, it replaces the membrane wall of the original foam. Since liquids with low surface tension always flow toward liquids with high surface tension, the surface tension of the defoamer is lower than that of the bubble liquid. Therefore, the defoamer molecules attach to the surface of the bubble membrane to reduce the local surface tension of the bubble membrane, while the rest of the membrane surface still maintains a large surface tension. This difference in surface gravity on the bubble membrane causes the stronger tension to pull the weaker tension part, causing stress imbalance in the entire bubble, which leads to bubble rupture [2~5].
The important role of the defoamer is to eliminate bubbles as quickly as possible. The defoamer combines with the stable surfactant layer and penetrates into the bubble membrane bilayer. Its penetration ability can be expressed by the penetration coefficient E. After the defoamer penetrates, it can quickly spread out. Its spreading ability can be expressed by the spreading coefficient S. According to the formula proposed by ROSS:
Penetration coefficient E=γF+γDF-γD(1)
Spreading coefficient S=γF+γDF-γD(2)
Where: γF———Surface tension of foam medium; γD———Surface tension of defoamer; γDF———Interface tension between foam medium and defoamer.
When E>0, the defoamer penetrates into the foam film; when E<0, it cannot penetrate. When S>0, the defoamer spreads to the film surface; when S<0, it cannot spread. Therefore, when E>0 and S>0, the defoamer can produce a defoaming effect.
