Factor 1: surface tension
The surface tension of defoamer must be lower than that of coating, otherwise it will not have defoaming and antifoaming effect. The surface tension of coating is a variable factor, so when choosing defoamer, the surface tension should be constant, and then the surface tension change factor should be taken into account.
Factor 2: influence of other additives
Most of the surfactants used in coatings are incompatible with defoamers. In particular, emulsifier, wetting and dispersing agent, leveling agent, thickener and so on will affect the effect of defoamer. Therefore, we must pay attention to the relationship between different auxiliaries and choose the best balance point.
Factor 3: baking temperature
When the coating enters the high temperature baking at normal temperature, the viscosity will decrease immediately, and the bubbles can move to the surface. However, due to the volatilization of the solvent, the curing of the coating and the increase of the surface viscosity, the foam will become more stable, and it will be trapped on the surface, resulting in shrinkage holes and pinholes. Therefore, the baking temperature, fixed speed and solvent evaporation rate will also affect the effect of the defoamer.
Factor 4: solid content, viscosity and elasticity of coating
Coatings with high solid content, high viscosity and high elasticity are very difficult to defroster. In these coatings, defoamer diffuses difficultly, the bubble speed is slow, the migration of foam to the surface decreases, and the viscoelasticity of foam is large. The foam in these paints is quite difficult to remove. It is best to choose defoamer and defoaming agent.
Factor 5: coating method and construction temperature
There are many coating construction methods, such as brush coating, roller coating, spray coating, scraping coating, spraying, screen printing coating, etc. The foaming degree of different coating methods is also different. Brush and roller coated foam is more than spray and scraping. When the temperature is higher than the temperature, foam will be more, but when the temperature is high, the foam will be eliminated.
Selection of defoamers for different applications
There are many kinds of defoamers with different characteristics, so there are different Defoamers in different formulations.
Flat coating
Formula features: low viscosity, low defoaming pressure
Suggestion: mineral oil or mineral oil modified polyether
Elastic coating
Formula features: high viscosity, high defoaming pressure
Formula suggestion: polyether modified organosilicon
Surface coating and laminating adhesive
Formula suggestion: polyether, which can effectively prevent surface defects and has little effect on gloss
Colorful water in water
Formula features: more surfactants, stable foam
Formula suggestion: use polyether or mineral oil to modify polyether
Precautions for use of defoamer
Before use, it must be fully stirred and evenly mixed;
Add when the coating is stirred;
Before use, generally do not add water to dilute;
If the dosage is too much, it will cause shrinkage, edge shrinkage and poor painting performance;
It is better to add it in two times, i.e. in the grinding stage and paint mixing stage respectively. Generally, each time is half of the total amount. In the grinding stage, use defoamer with good foam suppression effect and in the paint mixing stage, use defoamer with good foam breaking effect.
Source: coating technology and construction skills
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