Defoamer must be able to spread quickly on the foam surface and permeate quickly, so that the foam will burst quickly. At present, the commonly used defoamers are organosilicon and polyacrylate.           
Organosilicon defoamers are usually polysiloxane type. For example, polydimethylsiloxane containing acrylate functional group and polydimethylsiloxane modified by polyether, etc. Organosilicon is a common defoamer because of its high temperature resistance, low temperature resistance, stable physical properties, chemical inertness and low surface tension.           
Defoamers such as polyacrylic acid are defoaming by changing the polarity and molecular weight of the polymer, resulting in selective incompatibility. The use of such defoamers requires evaluation of the effect on gloss.           
The future selection of suitable defoamer must take into account the foam generation process, compatibility and concentration, temperature and viscosity of the system. Each of the above factors will have a direct impact on the selection of defoamers.           
Evaluation of foam control agents mainly consider several aspects: spreading speed, compatibility with the system, defoaming stability and cost performance. However, the above factors are often contradictory in a formula. For example, the defoamer with the best compatibility with the system has the worst defoaming stability; the defoamer with the worst compatibility has the fastest spreading rate.           
Due to the diversity of raw materials and construction methods, defoamers need to be evaluated according to the actual situation.           
1. A certain proportion of the defoamer that needs to be added is added to the varnish, put into the glass bottle, vibrate for 5 minutes in the oscillating instrument, at the same time take out the observation, determine the defoaming ability of the defoamer initially according to the number of bubbles; after placing the film for 10 minutes for /30 minutes, observe the height of the foam again and compare the defoaming speed;           
2. The compatibility between defoamer and system (with or without shrinkage cavity) is determined by scraping paint solution with scraper fineness meter;           
3, after the system bubble is eliminated, the clarity of the observation system, whether there is turbidity, layering, oil slick and so on;           
4. Storage stability: after half a month, repeat step 1, 2 and 3 experiments, and determine the long-term effect of defoamer           
5. Determine the amount added.