In order to understand how the defoamer plays a role in the foaming medium, and to use the defoamer more reasonably and effectively, we must be familiar with its mechanism of action and general properties.






When the foaming system is vigorously foaming, the foam is immediately eliminated after the defoamer is added. In fact, the function of defoamer is to offset the foam stabilization effect of the foam aid substance. The foam is originally extremely unstable, such as pure water under agitation, will also form a foam, but once the agitation is stopped, the foam will be eliminated immediately. If a foaming aid is added to the water, it is difficult to burst the foam because of the stabilizing effect of the foaming aid. So even a pure surfactant that foams will not form a stable foam because there is no foam helper. The defoaming mechanism of defoaming agent is introduced below.






In 1941, the concept of expansion coefficient S was proposed, that is, S=γ m-γ int-γ a -- formula (1).


In formula (1) :γm- surface tension N/m of the foaming medium; γint- Surface tension N/m between defoamer and foaming medium; γa - Surface tension N/m of defoamer.






If the expansion coefficient S is positive, the defoamer can be diffused on the surface of the bubble film; if it is negative, it is difficult to diffuse. In other words, the larger the S value, the easier the defoamer






It diffuses across the membrane. It can be seen from formula (1) that the smaller the surface tension of the defoamer, the larger the S value, the easier it is to diffuse on the surface of the foam film, and the better the defoamer effect. In 1948, someone proposed the concept of immersion coefficient E, that is, E=γm + γint-γa - formula (2).






In the same way, the positive or negative value of the immersion coefficient E is used to judge whether the defoamer can enter the surface of the bubble film. The smaller the surface tension of the defoamer, the better. Note that formula (1) and formula (2) are different, the sign before γint is different, formula (1) is negative, formula (2) is positive. Ross, an American colloidal chemist, tested and observed the foaming system with various surfactants to find the corresponding relationship between the solubility of the defoamer in the foaming solution and the defoamer effect. On the basis of previous experiments, he put forward a hypothesis: in solution, the solute in the dissolved state is a foam stabilizer; A solute in an insoluble state, when the expansion coefficient and immersion coefficient in equations (1) and (2) are both positive, is an antifoam agent. Solutes that do not dissolve can be aggregated into molecular clusters.






The molecular droplets of defoamer contact the vesicle, because S<0 E<0 s="">0 E>0 E>0 E>0 first immersed; If S>0, it expands on the membrane. Then the membrane thinned locally and broke. Causing bubbles to merge or burst. When the immersion coefficient S and the expansion coefficient E are both negative, the droplet will not be immersed nor expand. When the immersion coefficient E is positive and the expansion coefficient S is negative, the droplet immersion is prismatic and can be immersed but not expanded. Only when both are positive, it may be an antifoam agent.






S<0 e="">0 This hypothesis laid the foundation for the action mechanism of defoamer, and was soon spread and widely expanded. But Tsunetaka Sasaki, a Japanese expert, points out that the Ross hypothesis is not comprehensive enough. Because defoaming includes two types: bubble suppression and bubble breaking. When the droplets of defoamer push away the surface active fracture agent on the surface of the bubble film and replace it into an unstable film, they may not completely push away the bubble film and embed it as Ross said. Therefore, it is believed that there are a variety of defoaming mechanisms. Although most of the defoaming agents are insoluble, the relationship between the expansion coefficient of the soluble additives, the immersion coefficient and the defoaming effect has no defoaming effect. However, it is true that part of the defoaming is carried out in the dissolved state. In other words, the defoamer is not an absolutely insoluble substance.












Defoaming mechanism related to stable bubble factors




With the continuous understanding of bubble stability factor, people's understanding of defoaming mechanism is also deepening. The factors of bubble stability are varied, and the mechanism of determining defoaming is also varied.










1) It can reduce the local surface tension of the foam and cause the bubble to burst.






Some people studied the defoaming process of polysiloxane in the oil system. They continuously took photos of the foam system at the speed of 1/1000 seconds, and the photos were enlarged 100 times. The schematization of the four photos taken in succession can be seen that the droplets of silicone oil defoaming agent arrived on the foam film, causing the surface of the foam film to burst and merge into a large bubble. Finally, the gas and liquid are rapidly separated, the large bubble is destroyed, and the bubble is broken at 4. This also objectively explains why a drop of defoamer goes down and a series of bubbles burst.






When the defoamer is attached to the bubble film, it is immersed in the bubble film liquid, which will significantly reduce the surface tension there. Because in the water system, the solubility of the active ingredient of the defoamer to water is small, the reduction of the surface tension is limited to the local part of the bubble film, and the surface tension around the bubble film hardly changes. The part of the surface tension is reduced, which is strongly pulled around and extended, and finally causes the bubble to burst.










2) The defoamer can destroy the elasticity of the film and cause the bubble to burst






Due to the adsorption of surfactant on the surface of the membrane, the surface tension is reduced. Therefore, under local pressure, the membrane becomes locally thin at the same time, and the surface tension will increase due to the thinning of the surfactant. It is because of the difference in surface tension between the new surface and the original surface that the elastic recovery force is generated when the membrane is thinned by the external impact, and the membrane is not broken and plays a role in stabilizing the bubble. If we manage to break this elasticity, we can break the stability of the foam.






Some experts believe that the role of defoamer is to destroy the elasticity of the bubble film. When the defoamer is added to the foam system, it diffuses to the gas-liquid interface, which makes it difficult for the surfactant with foam stabilization function to restore the elasticity of the membrane.






3) Defoamer can promote liquid film drainage resulting in bubble collapse






The substance (defoamer) that accelerates the liquid discharge of the bubble membrane can be added to defoamer. If the bubble film is thick, the process of drainage to 30~40nm is long, and the self-healing effect of the bubble film is strong, and the elasticity of the bubble film is good, so the foam life of the thick bubble film is long, and the liquid drainage rate of the bubble film can reflect the stability of the foam. The liquid film discharge rate is fast, so that the liquid film becomes thin, thin to a certain extent, the foam burst.






4) Mechanism of hydrophobic solid particle defoamer






In the foaming water system, the addition of hydrophobic silica (that is, hydrophobic white carbon black) particles alone has the effect of defoaming. The hydrophobic white carbon black particles attract the hydrophobic end of the surfactant on the bubble surface, and the hydrophobic particles become hydrophilic particles and enter the aqueous phase, completing the defoamization. The solid particles of hydrophobic silica are generally white carbon black particles treated with methyl silicone oil, silazane or DMC, which are locally immersed on the surface of the bubble, making the surface tension of the local bubble smaller, coupled with the effect of solid particles, like the tip of a needle inserted into the bubble, accelerating the collapse of the bubble. Therefore, the defoamer containing hydrophobic white carbon black generally has a better foaming effect.












Other mechanism of action of defoamer








① Impact caused by expansion










When the membrane is subjected to a certain degree of impact, it will burst. The impact of the added defoamer on the bubble film can also cause the bubble film to rupture.






② so that the foaming surfactant is solubilized










Some low molecular substances that can be fully mixed with the solution can increase the solubility of the foaming surfactant and reduce its effective concentration. It is often found that surfactants have lower surface activity in mixed solvents than in pure solvents. A low molecular substance, such as octanol, having this effect. It not only reduces the surfactant in the surface layer, but also dissolves into the surfactant adsorption layer, reducing its degree of adhesion. Through the above two effects, the foam stability is weakened. This is a mechanism of action of soluble defoamer ethanol, propanol, octanol, etc. In order to facilitate defoaming, appropriate alcohols are often added to the formula of silicone defoaming agent, which also plays a certain role in auxiliary defoaming.






③ Electrolyte disintegrates the surfactant double electric layer






For the foam stable foaming liquid produced by the mutual repulsion of the double electric layer of the surfactant, adding some ordinary electrolytes and adding some ordinary electrolytes can collapse the double electric layer of the surfactant and play a defoaming effect.