Shake half a bottle of pure water vingorously and leave it still.The bubbles involved in the water will be released very quickly and the bubbles are basically invisible on the surface of the water.
Add a few drops of detergent to half a bottle of pure water,shake it vigorously and leave it still.The water surface is almost full of bubbies and these bubbles can exist stably for a long time
I. Mechanism of bubble formation and stability
Bubbles (here mainly refer to relatively small bubbles scattered in the liquid phase, not discussing bubbles played by children) are gas-liquid dispersions formed by gas dispersion in liquid, in which the gas is the dispersion phase and the liquid is the dispersion medium. So, why can the bubbles in pure water disappear quickly, and why can the bubbles in the water with detergents stay stable for a long time?
1) Why can the bubbles in pure water disappear quickly? The surface tension of water is as high as 72.8mN/m. When the pure water is coated on the glass plate with a wire rod or a film preparer to make a water film, under the action of surface tension, the water film will quickly shrink to the middle, forming a small beach or a large droplet. Similarly, when an external force rolls the air into the water to form bubbles, the surface molecules of the water form a "contraction force" due to the internal molecular attraction, trying to minimize the surface area of the bubbles. This high tension causes the bubble liquid film to become thin and broken rapidly, resulting in the rapid disappearance of the bubbles. 2) Why can the bubbles stabilize after adding detergent? The surfactants in the detergent (such as sodium lauryl benzene sulfonate) contain hydrophilic and hydrophobic groups, which are adsorbed at the water-gas interface and reduce the surface tension of water from 72.8 mN/m to 25-30 mN/m. Lower surface tension makes the liquid film more prone to bending and deformation, forming stable bubbles. The hydrophobic end of the surfactant is directed towards the gas, and the hydrophilic end is directed towards the liquid membrane of the bubble film, forming a bimolecular layer inside and outside the liquid film. On the one hand, this bimolecular layer can inhibit the discharge of liquid, and on the other hand, it can dynamically repair the liquid film. In addition, the electrostatic repulsion effect of ionic surfactants can increase the thickness of the liquid film and make the liquid film more stable.
Due to the very low viscosity of water, even if surfactant is added, the bubbles formed will rise to the surface of the liquid very quickly. However, in water-based paints, because most paints have high viscosity and small bubble diameters, it is difficult for the gas to float to the liquid surface in a short time, and it will be stable in the paint for a long time.
II. Bubbles in the paint
1) The source of bubbles
Paints produce bubbles from production to construction. Some bubbles come from mechanical mixing, and some bubbles come from chemical reactions:
- Bubbles brought in by mechanical mixing: grinding of color paste (fine-grained pigments or fillers have a large specific surface area, and the air adsorbed on the surface is released into the system to form dense microbubbles when dispersed), paint mixing, pumping and canning, redispersing during construction, dilution and mixing, mixing of two-component paints, construction (brush coating, roller coating, spraying , spray, etc.).
- Bubbles produced by chemical reactions: such as the gas released when two-component water-based polyurethane is cured, and the gas produced when Tiandong polyurea resin is cured.
2) The reason for the stable bubble
- Surfactant: The synthesis of water-based resins or the emulsification of hydrophobic resins into water-based dispersions requires the use of emulsifiers, and the use of wetting agents and dispersants are required to disperse the filler into water. Emulsifiers, wetters and dispersants are all surfactants, and they tend to generate and stabilize bubbles in the water-based system.
- Viscosity: The use of thickener will increase the viscosity of the paint, slow down the rising speed of bubbles, and enhance the elasticity of the bubble film, making it more difficult for tiny bubbles to gather and escape. According to Stokes' law, the rate at which a bubble rises in a liquid is directly proportional to the square of the bubble radius and inversely proportional to the viscosity of the liquid.
3) The harm of bubbles
- In the grinding stage of the filler, if the grinding paste contains a large number of bubbles, these bubbles are equivalent to airbags, which will reduce the grinding efficiency, prolong the grinding time, and increase energy consumption. Bubbles will affect the detection of grinding fineness, and bubbles will also occupy a certain volume, reducing the utilization rate of equipment.
- In the pumping canning stage, bubbles will cause the specific gravity of the paint to become smaller, which is not conducive to paint quality control and canning.
- In the construction stage, although the bubbles in the paint will burst with the drying of the coating film, after the bubble rupture, some defects are often left on the surface of the coating film, such as shrinking holes, pinholes, volcanic craters, etc. Such defects are often unacceptable.
III. Defoamer
1) The bubble-breaking mechanism of defoamer
The defoamer needs to achieve a balance between compatibility and incompatibility in a given system. When the compatibility is good, the defoaming ability is poor. When the compatibility is poor, the bubble-breaking ability is strong, but it is easy to cause coating defects.
- The surface tension of reducing surface tension defoamers (such as silicone) is lower than that of the bubble liquid film. After its adsorption on the surface of the bubble film, the surface tension is locally reduced, resulting in uneven force on the liquid film, resulting in the thinning and rupture of the liquid film.
- The elastic defoamer molecules that destroy the liquid membrane are inserted into the bubble bimolecular membrane, which interferes with the orderly arrangement of surfactants and weakens the elasticity of the liquid membrane (such as polyether defoamers), making them unable to resist external disturbances and rupture.
- Hydrophobic particles puncture hydrophobic solid particles (such as silicon dioxide) adsorb surfactants, reduce the concentration of stable components in the liquid film, and puncture the liquid film.
2) Types and properties of defoamers
- Mineral oil defoamer: usually composed of 75-90% mineral oil, 1-10% hydrophobic particles (such as fumous silica, polyurea), 5-15% other ingredients (emulsifiers, preservatives, thickeners, etc.). Mineral oil defoamers are cheap and are usually used in matte or semi-glosse architectural coatings. The defoaming efficiency is low in systems with high resin content.
- Emulsion defoamer: a stable emulsion formed by dispersing hydrophobic active ingredients (such as silicone, mineral oil or polyethers) in the aqueous phase through emulsification technology, which is specially designed to eliminate or inhibit foam in the water-based coating system. Many emulsion defoamers will also be matched with some hydrophobic particles to improve the efficiency of defoaming.
- Silicone defoamer: with polydimethylsiloxane as the main active ingredient, the balance of compatibility and defoaming efficiency is achieved by adjusting the polymerization degree and organic modification group of polydimethylsiloxane. This kind of defoamer has a particularly low surface tension and high defoaming efficiency, but it has poor compatibility in some resin systems, which may lead to coating defects. In order to avoid coating defects in some automotive coatings, it is forbidden to add any silicone components to the coating formula. Compared with mineral oil defoamers, silicone defoamers have little impact on the gloss of the paint film and the color transplibility of pigments.
- Polymer defoamer: This kind of defoamer is composed of some polymers and hydrophobic particles, which is environmentally friendly and has high defoaming efficiency. It can be used in some food-contact industries. There are many types of such defoamer polymers, including polyether defoamers, polyacrylate defoamers, polyurethane defoamers, special modified polymer defoamers, etc.
3) Common brands of defoamers in the market and their manufacturers
Mineral oil defoamer
- BYK Chemical (BYK): BYK-1630, BYK-035, BYK-037, BYK-038, BYK-039, of which BYK-039 does not contain silicone, and the other four contain silicone.
- BASF: Foamaster® MO 2111 / 2133 / 2134 / 2141 / 2150 / 2157 / 2160, Foamaster® MO NDW / NXZ, NDW It has good compatibility with NXZ in architectural coatings, and the risk of fisheye production is low.
- Münzing: AGITAN® 230 / 282 / 5091, without silicone.
Emulsion defoamer
- Winchuang: TEGO® Foamex 2 (15% active content, silicone-free), TEGO® Foamex 10 (15% active content) / 11 (15% active content) / 20 (20% active content) / 24 (20% active content) / 800 (20 % active content)/ 822 (20% active content) / 823 (20% active content) / 825 (20% active content) / 835 (50% active content) / 842 (60% active content) / 845 (20% active content) / 1488 (20% active content) / 8030 (20% active content)/8880 (20% active content), except for TEGO® Foamex 2 without silicone, the above products all other brands contain silicone.
- Bike Chemical: BYK-023 (18.5% solid content), BYK-044 (57% solid content), BYK-1610 (17% solid content), BYK-1611 (17% solid content), BYK-1615 (12.5% solid content), BYK-17 23 (18.5% solid content), the main active ingredients of the above brands are silicone. BYK-1640/1641/1642 (both 62% solid content), these three products are super branched polymer defoamers containing polyamide particles and silicone-free.
- BASF: FoamStar® ED 2522 (20% solid content) / 2523 (27% solid content) / 2528 (28% solid content), all 3 products contain silicone.
- Mingling: AGITAN® 100 / 105 / 107 (23% solid content, excluding silicone), AGITAN® 108 (29% solid content, containing a small amount of silicon), AGITAN® 109 (50% solid content), AGITAN® 150 ( 23% solid content, modified silicone), AGITAN® 158 (25% solid content, modified silicone), AGITAN® 159 (33% solid content, modified silicone).
- AFCONA: AFCONA 2524 (20% solid content) / 2590 (35% solid content) / 2592 (30% solid content), all are silicone emulsions.
Silicone defoamer
- Winchuang: TEGO® Airex 901W / 902W, TEGO® Foamex 16 / 26 / 32 / 840 / 843 / 844 / 852 / 883 / 3062 / 8050 / 8420, all of which are 100% solid content.
- Bike Chemical: BYK-018, BYK-021, BYK-022, BYK-024, BYK-028, BYK-094, BYK-1730, are all 100% solid content, VOC-free.
- BASF: FoamStar® SI 2210 / 2240 / 2250 / 2293 / 2299, all of which are 100% solid content.
- Mingling: AGITAN® 731 / 760N / 761 / 765 / 766, all of which are 100% solid content.
- AFCONA: AFCONA 2503 / 2505 / 2507 / 2508, all of which are 100% solid content.
Polymer defoamer
- Winchuang: TEGO® Foamex 18 (polymer + hydrophobic particles) / 830 (polymer + gas silicon) / 832 (polymer + hydrophobic particles) / 8820 (polymer + hydrophobic particles) / 8850 (polymer + hydrophobic particles).
- Bike Chemical: BYK-011 (polyolefin solution + hydrophobic particles), BYK-012 (polyether + hydrophobic particles), BYK-015 (polyether + hydrophobic particles), BYK-016 (bubble-breaking polymer + hydrophobic particles), BYK-1710 (bubble-breaking polymer + hydrophobic particles ), BYK-1711 (polyolefin solution + hydrophobic particles), none of the above products contains silicone.
- BASF: FoamStar® ST 2400 / 2412 (star polymer containing mineral oil), FoamStar® ST 2434 / 2438 (based on modified silicone and superbranched star polymer).
IN ADDITION TO SOME OF THE MANUFACTURERS LISTED ABOVE, SOLVAY, ASHLAND, UNIQ CHEM AND OTHER COMPANIES ALSO HAVE MANY TYPES OF DEFOAMERS.
When the compatibility between the defoamer and the system is poor, it is easy to have oil separation, gloss reduction, shrinkage and other defects. Many manufacturers have many different brands of the same type of defoamer, mainly by fine-tuning the molecular structure to achieve controlled compatibility of different defoamers in different systems, so as to obtain better defoaming effect and reduce the disadvantages. The risk of disease. In addition, many defoamers need to be added to the coating system under medium and high shear forces, such as the grinding stage or the paint preparation stage. The active ingredients of the defoamer itself are hydrophobic particles. If the dispersion is uneven and the local surface tension is too low, it is easy to cause coating defects. Some defoamers may be caused by excessive dispersion. The efficiency of foaming is reduced.