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Jul 21, 2021
Whenever we agitate the liquid or be introduced into the gas, many bubbles will be generated inside the surface of the liquid, and some bubbles generated by the liquid will quickly break and disappear. Some liquids not only do not disappear, but also gather more and form bubbles. Foam is a dispersion system formed by a large number of bubbles dispersed in a liquid. The dispersed phase is gas and the continuous phase is liquid. Among them, the volume fraction of the foaming liquid is very small, and the foam occupies a large volume. The gas is separated by a continuous liquid film, forming bubbles of varying sizes, which accumulate into foam.
The foaming medium is like a surfactant. A layer of surfactant molecules arranged in an oriented arrangement is adsorbed on the surface of the bubble. When it reaches a certain concentration, the bubble wall forms a solid film. The surfactant is adsorbed on the gas-liquid interface, causing the surface tension of the liquid surface to decrease, thereby increasing the gas-liquid contact surface, so that the bubbles are not easy to merge. The relative density of the bubbles is much smaller than that of the liquid itself. When the rising bubbles penetrate the liquid surface, they adsorb a layer of surfactant molecules on the liquid surface. Therefore, the bubble film with surfactant adsorbed in the air is different from the bubble film in the solution. It contains two layers of surfactant molecules to form a bimolecular film. The adsorbed surfactant protects the liquid film. effect. The defoamer is to destroy and inhibit the formation of this film. The defoamer enters the bimolecular oriented film of the foam and destroys the mechanical balance of the oriented film to achieve the foam breaking effect.
The defoamer must be a substance insoluble in the foaming medium, and it can be dispersed into the foaming medium in the form of droplets, or droplets that envelop solid particles, or solid particles. The defoamer has a lower surface tension than the foaming medium, and can spontaneously enter the liquid film to burst the bubbles.
The defoamer is easy to spread on the surface of the solution and automatically spreads out on the surface of the foam. It will take away a layer of solution adjacent to the surface and make the liquid film thin locally to reach a critical thickness. The liquid film ruptures and the foam is destroyed. The faster the defoamer spreads on the surface of the solution, the thinner the liquid film becomes, the faster the foam destruction speed, and the stronger the defoaming effect. Therefore, the reason for defoaming is that it is easy to spread. The adsorbed defoamer molecules replace the foaming agent molecules, forming a weaker film. At the same time, part of the solution adjacent to the surface layer is taken away during the spreading process, so that the foam liquid film becomes thinner, which reduces the stability of the foam and makes it easy to destroy.
An excellent defoamer must take into account the effects of eliminating and suppressing foam at the same time, that is, it should not only destroy the foam quickly, but also prevent the formation of foam for a long time. The reason for this situation may be related to whether the critical micelle concentration of the foaming agent (surfactant) in the solution is exceeded. In the excess solution, the defoamer (generally an organic liquid) may be solubilized, so that it loses the effect of spreading on the surface, and the defoaming effect is greatly reduced. After a period of time, as the defoamer is gradually solubilized, the defoaming effect is correspondingly weakened.
(1) Wide range of applications: Due to the special chemical structure of silicone oil, it is neither compatible with water or substances containing polar groups, nor with hydrocarbons or organic substances containing hydrocarbon groups. Due to the insolubility of silicone oil to various substances, it has a wide range of applications. It can not only use water system to defoam, but also can be used in oil system.
(2) Low surface tension: The surface capacity of silicone oil is generally 20-21 dyne/cm, which is smaller than that of water (72 dyne/cm) and general foaming liquids, and has good defoaming performance.
(3) Good thermal stability: Taking the commonly used dimethyl silicone oil as an example, it can withstand 150°C for a long time and 300°C for a short time, and its Si-O bond will not decompose. This ensures that the silicone defoamer can be used in a wide temperature range.
(4) Good chemical stability: Since the Si-O bond is relatively stable, the chemical stability of silicone oil is very high, and it is difficult to chemically react with other substances. Therefore, as long as the formulation is reasonable, silicone defoamers are allowed to be used in systems containing acids, alkalis, and salts.
(5) Physiological inertia: Silicone oil has been proven to be non-toxic to humans and animals, and its half-lethal dose is greater than 34 g/kg. Therefore, silicone defoamers (with suitable non-toxic emulsifiers, etc.) can be safely used in the food, medical, pharmaceutical and cosmetic industries.
(6) Strong defoaming power: Silicone defoaming agent can not only effectively break the foam that has been generated, but also can significantly inhibit the foam and prevent the formation of foam. Its usage is very small, as long as it is added to one millionth (1ppm) of the weight of the foaming medium, it can produce a defoaming effect. The commonly used range is 1 to 100 ppm. Not only is the cost low, but also does not pollute the defoamed substance.