When the solid and liquid contact, the original solid gas interface disappears and forms a new solid-liquid interface. This phenomenon is called wetting. When wetting the pigment filler with liquid and making it permeate into the polymer and aggregate of the pigment filler, if a small amount of surfactant is added to the liquid, wetting and permeation will be easier. Surfactants that wet or accelerate wetting of pigment fillers are called wetting agents. The liquid in water-borne coatings is water.
The surface properties of pigments and fillers, the types and dosage of wetting dispersants and the efficiency of dispersion equipment are the main factors affecting dispersion.
The stability or sedimentation of pigment and filler suspension dispersion system depends on the force, repulsion and attraction between particles. Repulsion is the main factor of stability, attraction is the main factor of agglomeration.
Foam is produced when air enters a liquid containing surfactant under external force, the surfactant is arranged in the gas-liquid interface. The hydrophobic group faces the air of the bubble, and the hydrophilic group faces the water, producing a large number of bubbles. The causes of foam in paint are as follows:
1: air will be grought into the paint due to mechanical stirring in paint production;
2: Air brought into coating process, such as brush coating, roll coating, high pressure airless spraying, etc;
3: two-component paint, air brought in by mixing or stirring before construction;
4: too many pores in the coating, the air is driven out to form air bubbles due to the infiltration of the paint, such as painting on porous wood and cement walls.
5: Bubbles produced by chemical reactions, such as polyisocyanate in two-component PU coatings, react with microscale water to produce carbon dioxide.
The foam is unstable in nature, and its breakage takes three processes: the redistribution of bubbles, the thinning of film thickness and the rupture of film. But a relatively stable foam system, through these three processes to achieve natural defoaming takes a long time, all production mostly use defoamer. It is generally believed that defoamer reduces the local surface tension of foam liquid film and defoames. Because of the low surface tension of the defoamer itself, the surface tension of the foam film is reduced locally by infiltrating into the foam system in the form of particles. Because the liquid with low surface tension always flows to the liquid with high surface tension, the membrane wall gradually becomes thinner and is strongly towed by the membrane layer with large surface tension around it, resulting in the imbalance of stress and finally the bubble rupture.
At present, there are three kinds of defoamers: mineral oil defoamer, silicone defoamer and silicone free polymer defoamer.