Analysis of the Function Principle of Defoamer

1. The generation and stabilization of bubbles in coatings:
During the production of coatings, air will be involved to form bubbles. Some raw materials such as surfactants and dispersants will stabilize the bubbles.
Bubbles also occur in the process of coating construction, which mainly depends on the way of coating construction. For example, the curtain coating can be continuously drawn into the air in the paint, and the airless spraying is also easy to get into the air. It is easy to generate bubbles when spraying under relatively low humidity or high temperature conditions.

Bubble stabilization principle:
Marangoni effect (liquid reflux caused by surfactants, counteracting gravity)
Electrostatic effect: The electrostatic repulsion of surfactants makes the liquid film of bubbles thicker, thus stabilizing bubbles.

2. The Action Principle of Defoamer
Defoamer plays a role in the formation or after the formation of thin foam.
Dispersing uniform defoamer permeates into the foam elastic film and distributes in the membrane, resulting in thin layer rupture through the decrease of surface tension.
Dispersing uniform defoamer penetrates into the foam thin layer and forms a monolayer, which reduces adhesion and is prone to lamina rupture.
There is a third mechanism for defoamers containing hydrophobic particles. These hydrophobic particles reach the surface of the thin layer and adsorb surfactants at the top of the thin layer. The thin layer ruptures due to the absence of surfactant.

3. Selection and evaluation of defoamers:
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.
Silicone defoamers are usually polysiloxane type. For example, polydimethylsiloxane containing acrylate functional group and polyether modified polydimethylsiloxane, etc. Silicone is a common defoamer with high temperature resistance, low temperature resistance, stable physical properties, chemical inertia and low surface tension.
Defoamers such as polyacrylic acid defoamers defoam selectively by changing the polarity and molecular weight of polymers resulting in incompatibility. The use of such defoamers needs to be evaluated for their 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 these factors will have a direct impact on the choice of defoamers.
Evaluation of foam control agents mainly consider several aspects: spreading speed, compatibility with the system, defoaming stability and cost performance. But these factors are often contradictory in a formula. For example, the defoamer with the best compatibility with the system has the worst defoaming stability, and the defoamer with the worst compatibility has the fastest spreading rate.
Due to the diversity of raw materials and construction methods, defoamer needs 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）. Use scraper fineness meter to scrape paint solution to determine the compatibility between defoamer and system (shrinkage or not);
（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 1, 2, 3 steps of experiment and determine the long-term efficacy of defoamer.
（5）Determine the dosage.
-- Reprinted from Wechat Paint Ring