In the production of architectural coatings, due to the existence of additives such as emulsifier, dispersant and thickener in the formula, not only a large number of bubbles will be produced, but also can stabilize bubbles, resulting in a large number of bubbles in architectural paint. In the film forming process, if the bubble can not disappear, it will cause the film shrinkage, pinhole and other maladies. Therefore, adding antifoaming agent to eliminate these bubbles can achieve satisfactory results.
Causes and prevention of bubbles
A bubble is a dispersion system of gas in a liquid. In architectural coatings, The presence of surfactant can produce a large amount of bubbles and stabilize the bubble. These bubbles open towards the air interface - but are still surrounded by a surface film with surfactants, forming a bimolecular bubble sandwich. The main reasons for bubbles in the production of architectural coatings: (1) the pigment filler in the coating has a large specific surface area during the production when the pigment filler can adsorb air into the coating to produce bubbles; (2) Coating in production and application due to the addition of various surface active substances, such as emulsifier, dispersant wetting agent, adding agent, so in is easy to produce bubbles; (3) Due to mixing, dispersing, grinding construction, etc., it is easy to form bubbles through air intervention system; (4) Some side reactions produce CO2, resulting in bubbles.
Mechanism of antifoaming action
For architectural coatings, defoaming agent must be combined with a stable surfactant layer and infiltrate into the bubble membrane bilayer, after contact with bubbles
Fast spread, forming a very thin double film layer. With the further spread of defoaming agent invasion, replace the original foam membrane wall. Since the liquid with low surface tension always flows to the liquid with high surface tension, the surface tension of defoaming agent is lower than that of bubble liquid, so the molecule of defoaming agent adheres to the surface of bubble film to reduce the local surface tension on the surface of bubble film. However, the remaining part of the membrane still maintains a large surface tension. This difference in surface gravity on the bubble film makes the stronger tension pull the part with the weaker tension, so that the whole bubble will produce stress imbalance, resulting in bubble rupture.
Selection of defoamer and its application in architectural coatings
Defoaming agent is very targeted, often in a bubble system can defoaming and in another system is poor or ineffective, or even help bubbles. The selection of defoaming agent for waterborne architectural coating is a complex problem. There are many kinds of additives in the coating system. How to match them with defoaming agent to produce a good overall effect needs to be determined by test. How to choose defoaming agent, theoretically speaking, the main test of zhi surface tension and miscibility, in the selection should consider the influence of various factors: (1) due to the different varieties of coating, the main film, additives are different, the selection of defoaming agent different defoaming effect is different, need to be carefully analyzed. ② Several kinds of defoamer were screened to select the best defoamer.