What Is a Defoamer?

Substances that can reduce the surface tension of water, solutions, suspensions, etc., prevent the formation of foam, or reduce or eliminate the original foam. ^[1]

Defoamer is an additive to eliminate foam. In the production and application process of coatings, textiles, medicine, fermentation, papermaking, water treatment and petrochemical industries, a large amount of foam will be generated, which will affect product quality and production process. Based on the suppression and elimination of foam, a specific amount of defoamer is usually added to the product during production. ^[3]

The study of bubbles can be traced back to Plato's time, but for hundreds of years, people have not formed a unified understanding of the definition of bubbles. American colloid chemist L.I.Isipow and Dow Corning R.F. Smith define the foam from the density of the foam; Japan's Ito Koichi defines the foam from the perspective of the foam structure, but ignores the inter-bubble China s famous surface physicist Professor Zhao Guoxi defines foam as: foam is a dispersed system in which gas is dispersed in a liquid, gas is a dispersed phase (discontinuous phase), liquid is a dispersed medium (continuous phase), and liquid The bubbles in the liquid rise to the surface of the liquid, forming a small amount of liquid, which is formed by a liquid film to separate gas bubbles. At present, domestic and foreign scholars agree that foam itself is a thermodynamically unstable system. When gas enters a solution containing a surfactant, a stable foam system will be formed for a long time. ^[4]

There is no unified understanding on the action mechanism of defoamers. According to the defoamer mechanism proposed by previous people, there are roughly the following: ^[4]

The composition of the defoamer mainly includes active ingredients, emulsifiers, carriers, and emulsifiers. Among them, the active ingredient is the main core part, which plays the role of breaking bubbles and reducing surface tension. Emulsifiers disperse the active ingredients into small particles. , In order to better disperse into oil or water, and have a better defoaming effect; the carrier accounts for a larger proportion in the defoamer, and its surface tension is not high, which mainly acts as a support medium and suppresses foam 3. The defoaming effect is favorable, which can reduce the cost; the emulsifying aid is to make the emulsifying effect better. ^[3]

Defoamer Selection Considerations

The physical and chemical properties of an ideal defoamer must meet the requirements of the use system. In general, the following requirements must be considered when selecting a defoamer ^[7] :

(1) Strong defoaming ability, it can effectively eliminate foam when using very small amount;

(2) It has a lower surface tension than the defoamed system;

(3) After the defoamer is added, it does not affect the basic performance of the defoamed system;

(4) Insoluble in the defoamed system and not easily solubilized by the surfactant in the system;

(5) The surface tension should be well balanced;

(6) Does not react with the defoamed medium, nor is it degraded and degraded, and has good chemical stability;

(7) It has good diffusivity and permeability, has a positive diffusion coefficient in the foam medium, and has a rapid spreading ability on the foam surface;

(8) Good heat resistance, will not lose effectiveness at high temperature;

(9) Has good gas solubility and permeability;

(10) It has high physiological activity and safety in the system to be defoamed, and the defoamer itself is a non-toxic or low-toxic substance;

(11) Low chemical oxygen demand (COD), biochemical oxygen demand (BOD) and total oxygen demand (TOD);

(12) It has good storage stability;

(13) It has a long-term defoaming effect, and some defoamers can quickly defoam, but it will fail after a long time;

(14) Low cost;

(15) Does not increase the surface viscosity of the aqueous surfactant solution.

It is difficult for the defoamer to meet the above requirements at the same time, and it can only meet the requirements of the system to be defoamed in terms of main properties. Each defoamer can only be effective for a certain system or several systems, and the same foam system can also use a variety of different defoamers. Because of this, users should conduct tests to select the most effective and economical Defoamer. ^[7]

Antifoam turbidity problem

The main components of the defoaming agent are generally hydrophobic particles, silicone oil and emulsifier. The hydrophobic particles adsorb the silicone oil, so that the maximum effect of the silicone can be achieved with as little as possible. Silicone oil, as the main defoaming medium, has a small surface tension and is neither oleophilic nor hydrophilic. It is suspended in the system. When the defoamer is in the middle of the foam wall, the oil-water phase is discharged to produce a defoaming effect. Consumption, when the silicone oil outside the hydrophobic particles is completely consumed, the foam system becomes cloudy. Therefore, the amount of hydrophobic particles, silicone oil, emulsifier, and color used in the defoamer are different, which results in very different performances of the defoamer. When the defoaming agent has a good defoaming effect and a long foam suppression time, generally no turbidity appears in the system. ^[7]

Defoaming oil bleaching problem

Because the defoamer is not dissolved in the system, but is dispersed in the system, the uniformity of the dispersion of the defoamer in the system becomes very important. When the defoamer is uniformly dispersed in the system, it has little effect on the transparency of the system, and it takes longer to agglomerate into larger particles, which can remain in the system for a relatively long time. When many small particles are agglomerated, on the one hand, it will affect the transparency of the system, and on the other hand, it will shorten the time for the defoamer to agglomerate into large particles. This will cause turbidity after the defoamer is added to the system, and the next day Drifting oil. The methods that can be adopted to avoid oil bleaching are: moving the defoamer addition order forward; diluting before adding to the system, the diluent can be water or a surfactant in the system. ^[7]

Defoaming time

The properties of the silicone oil in the defoamer determine the defoaming time of the defoamer. The content of the silicone oil determines the consumption cycle of the defoamer in use. If the amount of silicone oil is too small, the defoaming performance of the defoamer will not reach the required value. Too much addition will affect the performance of the defoamer and reduce the defoaming performance of the defoamer. The particle size of the defoamer determines the filter resistance of the defoamer. Too large a particle size may cause the defoamer to be easy. It is filtered to produce bleaching oil, which has an effect on foam suppression. Stirring time is also an important indicator of the antifoaming ability of antifoaming agents. Insufficient mixing may produce turbidity, oil bleaching, weakening of antifoaming ability, and shortening the foaming suppression time. ^[7]

Defoamer failure

Acid-base stability. Silicone oil can destroy the surface tension of the liquid, and it has the effect of eliminating foam. If the defoamer has poor acid and alkali resistance, it will cause the silicone oil to decompose, which will cause the defoaming capacity to decrease or even fail. Adding silicate to the system will generally inhibit its decomposition; Defoamer solubility. Certain chemical components dissolve the silicone oil into the system, so that the defoaming agent no longer has a defoaming effect, but exists as a surfactant in the system, and the foam of the system is higher than when no defoaming agent is added. ^[7]