Defoamer introduction and common products

1.Mechanism of bubble formation and stabilization

Bubbles (primarily referring to relatively small bubbles dispersed in a liquid, not including the bubbles children play with) are gas-liquid dispersions formed by gas dispersed in a liquid, with the gas serving as the dispersed phase and the liquid serving as the dispersion medium. So, why do bubbles in pure water disappear quickly, while bubbles in water with detergent can persist for extended periods?

1) Why do bubbles in pure water disappear quickly?

Water has a high surface tension of 72.8 mN/m. When pure water is applied to a glass plate using a wire rod or a film applicator to form a water film, the surface tension causes the film to rapidly contract toward the center, forming a small puddle or large droplet. Similarly, when an external force draws air into water to form a bubble, the surface molecules of the water, attracted by their internal molecules, create a “contraction force” that attempts to minimize the bubble’s surface area. This high tension causes the bubble film to rapidly thin and rupture, leading to its rapid disappearance.

2) Why are the bubbles stabilized after adding detergent?

Surfactants in detergents (such as sodium dodecylbenzenesulfonate) contain both hydrophilic and hydrophobic groups. They adsorb at the water-air interface, reducing the surface tension of water from 72.8 mN/m to 25-30 mN/m. This lower surface tension makes the liquid film more flexible and deformable, forming stable bubbles. The hydrophobic end of the surfactant faces the air, while the hydrophilic end faces the liquid film of the bubble membrane, forming a bilayer inside and outside the liquid film. This bilayer not only inhibits liquid drainage but also dynamically repairs the liquid film. Furthermore, the electrostatic repulsion of ionic surfactants increases the thickness of the liquid film, making it more stable.

Because water has a very low viscosity, even with the addition of surfactants, bubbles formed will quickly rise to the liquid surface. However, in water-based paints, due to the high viscosity of most coatings and the small bubble diameter, it is difficult for gas to rise to the liquid surface quickly, and it will remain stable in the paint for a long time.

2.Bubbles in paint

1) The source of bubbles

Bubbles are generated throughout the production and application of paint. Some bubbles originate from mechanical mixing, while others arise from chemical reactions:

Bubbles introduced by mechanical mixing include: grinding of color pastes (fine-particle pigments or fillers have a large specific surface area, and air adsorbed on the surface is released into the system during dispersion, forming dense microbubbles); paint mixing, pumping and canning; redispersion and dilution during application; mixing of two-component paints; and application (brush, roller, spray, curtain coating, etc.).
Bubbles generated by chemical reactions include: gases released during the curing of two-component waterborne polyurethanes and aspartame polyurea resins.

2) Reasons for stable foam

Surfactants: Emulsifiers are required to synthesize water-based resins or emulsify hydrophobic resins into aqueous dispersions. Wetting agents and dispersants are used to disperse pigments and fillers in water. Emulsifiers, wetting agents, and dispersants are all surfactants, and they tend to generate and stabilize bubbles in water-based systems.
Viscosity: Thickeners increase the viscosity of the coating, slowing the rise of bubbles while enhancing the elasticity of the bubble membrane, making it more difficult for tiny bubbles to aggregate and escape. According to Stokes’ law, the rate at which bubbles rise in a liquid is proportional to the square of the bubble radius and inversely proportional to the viscosity of the liquid.

3) Hazards of bubbles

During the pigment and filler grinding stage, if the grinding slurry contains a large number of bubbles, these bubbles act like airbags, reducing grinding efficiency, prolonging grinding time, and increasing energy consumption. Bubbles can also affect the detection of grind fineness and occupy a certain volume, reducing equipment utilization.
During the pumping and canning stage, bubbles can reduce the specific gravity of the paint, hindering paint quality control and canning.
During the application stage, although bubbles in the paint will burst as the coating film dries, these bubbles often leave defects on the coating surface, such as pinholes, pinholes, and craters. Such defects are often unacceptable.

3.Defoaming agent

1) Defoaming mechanism of defoaming agent

Defoamers need to strike a balance between compatibility and incompatibility in a given system. Good compatibility results in poor defoaming ability, while poor compatibility results in strong bubble breaking ability, but can easily cause coating defects.

Surface Tension Reduction: Defoamers (such as silicones) have a lower surface tension than the bubble film. When adsorbed on the bubble film surface, they locally reduce the surface tension, causing uneven force on the film, leading to thinning and rupture.
Defoamer Elasticity: Defoamer molecules insert into the bubble bilayer, disrupting the orderly arrangement of surfactants and weakening the film’s elasticity (such as polyether defoamers), making it unable to resist external disturbances and rupture.
Hydrophobic Particle Penetration: Hydrophobic solid particles (such as silica) adsorb surfactants, reducing the concentration of stabilizing components in the film and causing puncture.

2) Types and properties of defoamers

Mineral oil defoamers: Typically composed of 75-90% mineral oil, 1-10% hydrophobic particles (such as fumed silica, polyurea), and 5-15% other ingredients (emulsifiers, preservatives, thickeners, etc.). Mineral oil defoamers are inexpensive and typically used in matte or semi-gloss architectural paints. Their defoaming efficiency is lower in systems with high resin content.
Emulsion defoamers: These are stable emulsions formed by dispersing hydrophobic active ingredients (such as silicones, mineral oils, or polyethers) in water using emulsification technology. They are specifically designed to eliminate or suppress foam in water-based coating systems. Many emulsion defoamers also incorporate hydrophobic particles to enhance defoaming efficiency.
Silicone defoamers: These use polydimethylsiloxane as the primary active ingredient. A balance between compatibility and defoaming efficiency is achieved by adjusting the degree of polymerization and organic modifier groups of the polydimethylsiloxane. This type of defoamer has exceptionally low surface tension and high defoaming efficiency, but its compatibility with certain resin systems can lead to coating defects. To prevent these defects, some automotive coatings prohibit the addition of any silicone components. Compared to mineral oil-based defoamers, silicone defoamers have less impact on paint film gloss and pigment color development.
Polymer-based defoamers: These defoamers are composed of polymers and hydrophobic particles. They are environmentally friendly and highly effective, and can be used in some food contact applications. There are many types of polymer-based defoamers, including polyether defoamers, polyacrylate defoamers, polyurethane defoamers, and specially modified polymer defoamers.

3) Common defoamer brands and manufacturers in the market

Mineral oil defoamer

BYK: BYK-1630, BYK-035, BYK-037, BYK-038, BYK-039. BYK-039 is silicone-free, while the other four contain silicone.
BASF: Foamaster® MO 2111 / 2133 / 2134 / 2141 / 2150 / 2157 / 2160, Foamaster® MO NDW / NXZ. NDW and NXZ have good compatibility in architectural coatings and a lower risk of fisheye formation.
Münzing: AGITAN® 230 / 282 / 5091. All are silicone-free.

Emulsion Defoamers

Evonik: TEGO® Foamex 2 (15% active, silicone-free), TEGO® Foamex 10 (15% active) / 11 (15% active) / 20 (20% active) / 24 (20% active) / 800 (20% active) / 822 (20% active) / 823 (20% active) / 825 (20% active) / 835 (50% active) / 842 (60% active) / 845 (20% active) / 1488 (20% active) / 8030 (20% active) / 8880 (20% active). Except for TEGO® Foamex 2, which does not contain silicone, all other grades contain silicone.
BYK: BYK-023 (18.5% solids), BYK-044 (57% solids), BYK-1610 (17% solids), BYK-1611 (17% solids), BYK-1615 (12.5% solids), BYK-1723 (18.5% solids). These grades all contain silicone as their primary active ingredient. BYK-1640, 1641, and 1642 (all 62% solids) are hyperbranched polymer defoamers containing polyamide particles and are silicone-free.
BASF: FoamStar® ED 2522 (20% solids), 2523 (27% solids), and 2528 (28% solids). All three products contain silicone.
Mingling: AGITAN® 100 / 105 / 107 (23% solids, silicone-free), AGITAN® 108 (29% solids, contains a small amount of silicone), AGITAN® 109 (50% solids), AGITAN® 150 (23% solids, modified silicone), AGITAN® 158 (25% solids, modified silicone), AGITAN® 159 (33% solids, modified silicone).
AFCONA: AFCONA 2524 (20% solids) / 2590 (35% solids) / 2592 (30% solids), all silicone emulsions.

Silicone defoaming agent

Evonik: TEGO® Airex 901W / 902W, TEGO® Foamex 16 / 26 / 32 / 840 / 843 / 844 / 852 / 883 / 3062 / 8050 / 8420, all 100% solids.
BYK: BYK-018, BYK-021, BYK-022, BYK-024, BYK-028, BYK-094, BYK-1730, all 100% solids, VOC-free.
BASF: FoamStar® SI 2210 / 2240 / 2250 / 2293 / 2299, all 100% solids.
Mingling: AGITAN® 731 / 760N / 761 / 765 / 766, all with 100% solids content.
AFCONA: AFCONA 2503 / 2505 / 2507 / 2508, all with 100% solids content.

Polymer Defoamers

Evonik: TEGO® Foamex 18 (polymer + hydrophobic particles) / 830 (polymer + silica) / 832 (polymer + hydrophobic particles) / 8820 (polymer + hydrophobic particles) / 8850 (polymer + hydrophobic particles).
BYK: BYK-011 (polyolefin solution + hydrophobic particles), BYK-012 (polyether + hydrophobic particles), BYK-015 (polyether + hydrophobic particles), BYK-016 (foam-breaking polymer + hydrophobic particles), BYK-1710 (foam-breaking polymer + hydrophobic particles), BYK-1711 (polyolefin solution + hydrophobic particles). All of these products do not contain silicone.
BASF: FoamStar® ST 2400 / 2412 (star polymers containing mineral oil), FoamStar® ST 2434 / 2438 (based on modified silicone and hyperbranched star polymers).

In addition to the manufacturers listed above, companies such as Solvay, Ashland, and Uniq Chem also offer a wide variety of defoamers.

Poor compatibility between defoamers and the paint system can easily lead to problems such as oil separation, gloss loss, and cratering. Many manufacturers offer different grades of the same defoamer. This is achieved primarily by fine-tuning the molecular structure to achieve controlled compatibility between different defoamers in different systems, achieving optimal defoaming effectiveness while minimizing the risk of side effects. Furthermore, many defoamers must be added to the paint system under moderate to high shear forces, such as during the grinding or paint mixdown stages. The active ingredient in the defoamer is inherently hydrophobic. If it is unevenly dispersed, the localized surface tension can be too low, easily causing film defects. Some defoamers can also reduce defoaming efficiency due to overdispersion.