How can defoamer work in Paint&coating?

How bubbles are generated and how to stable bubbles in coatings?

During the coating production process, air will be drawn into the air to form bubbles. Certain raw materials in the coating, such as surfactants and dispersants, will stabilize the bubbles.
Bubbles can also be generated during the coating process, which mainly depends on the way the coating is applied. For example: curtain coating can continuously entrain air in the paint, airless spraying is also easy to entrain air, and it is easy to generate air bubbles when spraying under relatively low humidity or high temperature conditions.Stable bubble principle:

l  Marangoni effect (fluid backflow caused by surfactants, counteracting gravity)

• Electrostatic effect: The electrostatic repulsion of the surfactant thickens the liquid layer film of the bubbles, thereby stabilizing the bubbles.
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2. Principle of paint defoamer

Defoamers work during or after the formation of a thin layer of foam:
The uniformly dispersed defoamer penetrates into the foam elastic film and distributes in the film, and causes the thin layer to rupture by reducing the surface tension.
The uniformly dispersed defoamer penetrates into the thin layer of foam and forms a monomolecular film, which reduces its adhesion and is easy to rupture the thin layer.
There is a third mechanism for defoamers containing hydrophobic particles. These hydrophobic particles reach the surface of the thin layer and adsorb the surfactant at the top of the thin layer. The thin layer was broken due to the absence of the surfactant.

3. Selection and evaluation method of paint defoamer:

The defoamer must be able to spread rapidly on the surface of the foam and to penetrate quickly, causing the foam to burst rapidly. Defoaming agents are currently commonly used in silicones and polyacrylates.
Silicone-based defoamers are usually of the polysiloxane type. For example: polydimethylsiloxane containing acrylate functional groups and polyether-modified polydimethylsiloxane. Organic silicon has high temperature resistance, low temperature resistance, stable physical properties, chemical inertness, and very low surface tension. It is a commonly used defoamer.
Defoamers such as polyacrylic acid are used to defoam by changing the polarity and molecular weight of the polymer to cause selective incompatibility. The use of this type of defoamer needs to be evaluated for gloss.
In the future, when selecting a suitable defoamer, the process of foam generation in the system, the compatibility of the system and the concentration, temperature, and viscosity must be considered. Each of these factors has a direct impact on the choice of defoamer.
The evaluation of foam control agents mainly considers several aspects: spreading rate; compatibility with the system; stability of defoaming and cost performance. However, the above factors are often contradictory in a formula. For example, the defoaming stability of the defoamer with the best compatibility with the system is often the worst; the worst compatibility is often the most spreading rate. fast.

Due to the diversity of coating materials and construction methods, paint defoamers need to be evaluated based on actual conditions.

• Add the defoaming agent to be compared to the varnish in a certain ratio, put it in a glass bottle, shake it in a shaker for 5 minutes, and take out the observation at the same time to determine the defoaming ability of the defoamer based on the amount of foam. ; After standing for 10 minutes / 30 minutes, observe the foam height again and compare the defoaming speed;
• Use a scraper fineness meter to scrape the coating liquid to determine the compatibility of the defoamer with the system (with or without shrinkage);
• After the system foam is eliminated, observe the system's clarity, whether there is turbidity, delamination, oil slick, etc .;
• Storage stability: After standing for half a month, repeat steps 1, 2, and 3, and determine the long-term effectiveness of the defoamer.
• Determine the amount of addition.

https://www.linkedin.com/company/china-defoamer
https://invelydefoamer.blogspot.com/
Edited by Invelychem.