When the air enters the liquid containing surfactant, under the action of external force, the surfactant will be aligned at the gas-liquid interface. The hydrophobic group is toward the air of bubble, and the hydrophilic group is towards the water, producing a large number of bubbles. There are several reasons for foaming in paint.           

1: The air will be entrained into the coating due to mechanical agitation during coating production;         
2: Air brought in during coating process, such as brush coating, roller coating, high pressure airless spraying, etc.          
3: Two component coating, when mixing before construction, mix the mixed air.           
4: There are many pores in the coating, which are driven out to form air bubbles due to the penetration of paint into the air, such as when painting on porous wood and cement walls.           
5: The bubbles produced by chemical reaction, such as polyisocyanate in two-component PU coating, react with micro amount of water to produce carbon dioxide.           
The essence of foam is unstable, and its breakup has to go through three processes, namely, the redistribution of bubbles, the thinning of membrane thickness and the rupture of membranes. But a relatively stable foam system has to go through these three processes to achieve natural defoaming for a long time. Most of the production uses defoamer. It is generally believed that defoaming agent reduces the local surface tension of foam liquid film and defoaming. Because the low surface tension of the defoamer itself always penetrates into the foam system in the form of particulates, resulting in partial decrease of the surface tension of the film. Because the liquid with low surface tension always flows to the liquid with high surface tension, the membrane wall becomes thinner gradually, which is strongly drawn by the membrane layer with high surface tension around, resulting in the imbalance of stress, and finally leading to the bubble rupture.            At present, there are three kinds of general defoamers: mineral oil defoamer, organosilicon defoamer, polymer defoamer without organic silicon.