What is an antifoaming agent?

A chemical additive that reduces and hinders foam formation.

Main components of antifoaming agents

• Carrier (75-85%): mineral oil, vegetable oil, silicone, polyether, water, etc.

• Hydrophobic particles (3-10%): such as silicon dioxide, wax, metallic soap, higher fatty alcohols, etc.

• Surfactant (3-7%)

• Other additives (0.5-3%)

Foam generation and antifoaming mechanism:

Foam is a heterogeneous dispersed system in which insoluble gases enter a liquid under external force, forming a large number of bubbles isolated from each other by the liquid. When foam is generated, due to the rapid increase in the contact surface area between the liquid and the gas, the free energy of the system also increases rapidly. The increased free energy of the foam system is the product of surface tension and the increased surface area. The ease of foam generation is directly related to the surface tension of the liquid system. The lower the surface tension, the less free energy is required for the system to form foam, and the easier it is to generate foam.

Foam is a thermodynamically unstable system. Its elimination must go through three processes: redistribution of bubbles, thinning of the film, and rupture of the film. For stable foam systems, it takes a long time to achieve natural defoaming through these three processes, so defoamers are needed to achieve rapid defoaming.

Defoaming includes both foam suppression and foam breaking. When a defoamer is added to the system, the defoamer must be able to distribute throughout the system without dissolving, and its surface tension must be lower than that of the system. For the defoamer to work, it must first penetrate the bubble film, and after the defoamer penetrates the film layer, it must be able to spread out quickly.

The defoamer used in water-based coatings always penetrates the foam system in the form of particles. When the system is about to generate foam for some reason, the defoamer particles in the system immediately destroy the elastic film of the bubbles and inhibit the generation of bubbles. If foam already exists, after the added defoamer contacts the foam, it captures the hydrophobic chain ends on the surface of the foam and quickly spreads to form a very thin double-layer film. Further diffusion and layered intrusion replace the original foam cell wall. Because liquids with low surface tension always flow towards liquids with high surface tension, the low surface tension of the defoamer itself can cause the cell wall of the foam containing the defoamer to gradually thin, and it is strongly pulled by the surrounding film layer with high surface tension, causing stress imbalance in the entire bubble, thus leading to the rupture of the bubble.

Characteristics of antifoaming agents:

1. Insoluble or slightly soluble in the foaming liquid;

2. Surface tension lower than the foaming liquid;

3. Has a certain degree of affinity with the foaming liquid;

4. Does not chemically react with the foaming liquid;

5. Low volatility, long duration of action;

Dosage and addition method of antifoaming agents:

Generally, high-viscosity latex paints, due to the difficulty of defoaming and many foam stabilization factors, require a slightly higher dosage, generally 0.3% to 1.0%. Low-viscosity latex paints or water-soluble coatings, especially water-soluble coatings, due to the presence of a certain amount of solubilizers, can appropriately reduce the dosage, generally 0.01% to 0.2%. For other water-based coatings or resins, it is generally around 0.1%. The dosage is not the higher the better; too much will cause shrinkage holes, oil spots, and other paint defects. Too much silicone-containing defoamer will also affect recoatability.

The optimal time to add the antifoaming agent is generally at the stage when the viscosity of the coating is relatively high, such as adding it during the grinding and mixing stages; this can ensure that the antifoaming agent has good dispersibility and avoid undesirable phenomena such as precipitation or fish eyes in the coating.

In addition, to ensure the defoaming effect, it is best to add it in two steps, once added to the grinding material and once added to the finished paint, with half in each stage.

After adding the antifoaming agent, the coating should be thoroughly stirred to ensure that the antifoaming agent is evenly distributed in the coating to achieve the best defoaming effect.