Summary of the working principle of defoamers

1. Bubble Generation and Stabilization Principle in Coatings:
During the coating production process, air is entrained, forming bubbles. Certain raw materials in the coating, such as surfactants and dispersants, stabilize these bubbles.
Bubbles are also generated during coating application, depending primarily on the application method. For example, curtain coating can continuously entrain air into the coating, and airless spraying also easily incorporates air. Bubbles are more likely to form under relatively low humidity or high temperature conditions. The
stabilization principle is
the Marangoni effect (the liquid backflow phenomenon caused by surfactants counteracts the effect of gravity).

Electrostatic effect : The electrostatic repulsion of surfactants thickens the liquid film on the bubbles, thereby stabilizing them.

2. The Working Mechanism of Defoamers
: Defoamers work during or after the formation of the foam thin layer:
A uniformly dispersed defoamer penetrates into the elastic foam film and distributes within it, causing the thin layer to rupture due to a decrease in surface tension. A
uniformly dispersed defoamer penetrates the foam thin layer and forms a monomolecular film, reducing its adhesion and making it easier for the thin layer to rupture.
Defoamers containing hydrophobic particles have a third mechanism. 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 lack of surfactants.

3. Selection and evaluation methods for defoamers:

Defoamers must be able to spread rapidly on the foam surface and penetrate quickly, causing the foam to break down rapidly. Commonly used types of defoamers include silicone-based and polyacrylate-based agents.

Organosilicon defoamers are typically polysiloxanes. Examples include polydimethylsiloxanes containing acrylate functional groups and polyether-modified polydimethylsiloxanes. Organosilicones are commonly used defoamers due to their high and low temperature resistance, stable physical properties, chemical inertness, and very low surface tension.

Defoamers such as polyacrylic acid defoam by selectively incompatible polymers due to changes in their polarity and molecular weight. When using these defoamers, it is necessary to assess their impact on gloss.

In the future, the selection of a suitable defoamer must take into account the foam generation process in the system, the system’s compatibility and concentration, temperature, and viscosity. Each of these factors will have a direct impact on the selection of the defoamer.

The main aspects to consider when evaluating foam control agents are: spreading rate; compatibility with the system; defoaming stability; and cost-effectiveness. However, these factors are often contradictory in a formulation. For example, the defoamer with the best compatibility with the system often has the worst defoaming stability; and the one with the worst compatibility often has the fastest spreading rate.

Due to the diversity of coating raw materials and application methods, defoamers need to be evaluated based on the specific circumstances.

1. Add the defoamer to be compared to the varnish in a certain proportion, place it in a glass bottle, shake it in a shaker for 5 minutes, and take it out to observe. The amount of foam is used to preliminarily determine the defoaming ability of the defoamer . After standing for 10 minutes and 30 minutes, observe the height of the foam again and compare the defoaming speed.

2. Use a scraper fineness gauge to scrape the paint liquid to determine the compatibility of the defoamer with the system (whether there are pinholes);

3. After the foam in the system is eliminated, observe the clarity of the system and check for any turbidity, layering, floating oil, or other phenomena.

4. Storage stability: After half a month, repeat steps 1 , 2 , and 3 of the experiment to determine the long-lasting effect of the defoamer.

5. Determine the amount to add.