Application of defoamers in architectural coatings
In the production of architectural coatings, the presence of additives such as emulsifiers, dispersants and thickeners in the formula not only generates a large number of bubbles, but also stabilizes the bubbles, resulting in a large number of bubbles in the architectural coatings. During the film-forming process, if the bubbles cannot disappear, it will cause defects such as shrinkage holes and pinholes in the coating. Therefore, adding defoamers to eliminate these bubbles can achieve satisfactory results.
1. Causes and prevention of bubble generation
Bubbles are a dispersed system in which gas is dispersed in liquid [1]. In architectural coatings, surfactants can generate a large number of bubbles and stabilize them. These bubbles rise to the air interface, but they are still surrounded by a surface film containing surfactants, forming a bilayer, i.e., a bubble interlayer. The main reasons for the generation of bubbles in the production of architectural coatings are: ① The pigments and fillers in the coatings have a large specific surface area. The pigments and fillers added during the production of the coatings can absorb air into the coatings to generate bubbles; ② In the production and application of the coatings, various surfactants such as emulsifiers, dispersants, wetting agents, thickeners, etc. are added, so bubbles are easily generated during production and construction; ③ Due to stirring, dispersion, grinding, construction, etc., bubbles are easily formed through the introduction of air into the system; ④ Due to certain side reactions, CO2 is generated, which generates bubbles.
2. Defoaming mechanism
For architectural coatings, the defoamer must combine with a stable surfactant layer and penetrate into the bubble film bimolecular membrane. After contacting the bubble, it spreads rapidly to form a very thin bimolecular 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 to liquids with high surface tension, the surface tension of the defoamer is lower than the surface tension of the bubble liquid, so the defoamer molecules adhere to the surface of the bubble film to reduce the local surface tension of the bubble film surface, while the rest of the membrane surface still maintains a large surface tension. This difference in surface gravity on the bubble film causes the stronger tension to pull the weaker tension part, causing the entire bubble to produce stress imbalance, thereby causing the bubble to burst.
The important role of the defoamer is to eliminate bubbles as soon as possible. The defoamer combines with a stable surfactant layer and penetrates into the bubble film bimolecular membrane. Its penetration ability can be expressed by the penetration coefficient E. After the defoamer penetrates, it can spread quickly. 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, S>0, the defoamer can produce a defoaming effect.
