Eliminating Foam: How to choose an antifoam agent?

The occurrence of foam often causes inconvenience and problems in industrial production and daily life, so finding an effective method of defoaming has become the key.

The methods of defoaming can be divided into two categories: physical and chemical.

From a physical perspective, many methods can be used to eliminate foam. For example, baffles or filters can be used to mechanically prevent the formation and survival of foam; mechanical agitation can be used to break the stability of foam by external forces; electrostatic effects can change the charge distribution of foam and promote its rupture; freezing and heating can affect the physical properties of foam from the perspective of temperature changes, respectively, destroying the stability of the foam film; steam , radiation, high-speed centrifugation, pressurisation and depressurisation, high-frequency vibration, instantaneous discharge and ultrasound (sound wave liquid control) can all, to varying degrees, increase the gas permeability at both ends of the liquid film, accelerate the expulsion of the bubble film, and cause the foam’s stability coefficient to be lower than its attenuation coefficient, thereby gradually reducing the amount of foam. However, these physical methods are clearly limited by environmental factors, and it is difficult to achieve a high level of defoaming efficiency. However, they have the advantage of being environmentally friendly and have a high recycling rate, so they are still of value in some scenarios where environmental requirements are strict.

Chemical defoaming methods mainly include chemical reaction methods and defoamer addition methods. Chemical reaction methods involve adding specific reagents to the foam system to cause a chemical reaction with the foaming agent, generating substances that are insoluble in water, thereby reducing the concentration of surfactant in the liquid film and ultimately causing the foam to break. However, this method faces the dilemma of difficulty in determining the composition of the foaming agent, and the insoluble substances produced may damage the system equipment. In various industries today, the addition of defoamers is the most widely used means of defoaming. Its greatest advantage is its high defoaming efficiency and extreme ease of use, but the key is to find a suitable and highly effective defoamer product.

The working principle of defoamers can take on various forms. One way is to reduce the local surface tension of the foam. For example, spraying high-grade alcohols or vegetable oils into the foam, when they dissolve in the foam liquid, can significantly reduce the local surface tension. Since these substances have low solubility in water, the reduction in surface tension is limited to the local area of the foam, and the surface tension around it remains almost unchanged. The part of the surface tension that has been reduced is strongly pulled towards the surroundings and expands until the foam breaks. Take some reaction vessels in chemical production as an example. During the reaction, a large amount of foam is generated. If a moderate amount of high-grade alcohol defoamer is added, the stability of the foam can be effectively destroyed, so that the reaction can proceed smoothly. Second, after the defoamer is added to the foam system, it will spread to the gas-liquid interface, destroying the ability of the surfactant with foam stability to restore the elasticity of the film, thus causing the bubbles to break. Third, defoamers can promote the discharge of liquid films. The discharge rate of foam is closely related to foam stability. Substances that can accelerate foam discharge can also have a defoaming effect. Fourth, hydrophobic solid particles added to the surface of bubbles will attract the hydrophobic ends of surfactants, making the hydrophobic particles hydrophilic and enter the aqueous phase, thereby rupturing the bubbles. This principle can be used in some sewage treatment processes to eliminate foam by adding specific hydrophobic solid particles. Fifth, some low molecular substances that can be fully mixed with the solution, such as alcohols such as octanol, ethanol and propanol, can dissolve the surfactant on the surface of the bubble, reducing its effective concentration. Not only can they reduce the concentration of surfactant on the surface layer, they can also dissolve into the surfactant adsorption layer, weakening the stability of the foam. Sixth, for foaming liquids that rely on the interaction of the surfactant’s double electric layer in the foam to produce stability, adding a common electrolyte can destroy the double electric layer of the surfactant to achieve the purpose of defoaming.

Common defoamers can be divided into silicone (resin), surfactant, paraffin and mineral oil based on their composition. Silicone defoamers, also known as emulsifying defoamers, are used by emulsifying silicone resins in water with the help of emulsifiers (surfactants) and then adding them to the wastewater. Silica fine powder is another type of silicone defoamer with excellent defoaming properties. Surfactant defoamers are actually emulsifiers that use their dispersing effect to keep foaming substances in a stable emulsified state in water, thereby preventing foam from forming. Paraffin defoamers are made by emulsifying and dispersing paraffin or its derivatives with emulsifiers, and their use is similar to that of emulsifying defoamers with surfactants. Mineral oil is the main defoaming ingredient, and to enhance the effect, substances such as metal soaps, silicone oils, and silica are sometimes used in combination. In addition, various surfactants are added to make it easier for mineral oil to spread to the surface of the foaming liquid or to evenly disperse metal soaps in mineral oil.

Different types of defoamers have their own advantages and disadvantages. Organic defoamers such as mineral oil, amides, lower alcohols, fatty acids and fatty acid esters, and phosphates belong to the first generation of defoamers. These defoamers were developed and applied relatively early, and have the advantages of being easy to obtain raw materials, environmentally friendly, and low in production costs. However, they have low defoaming efficiency, are highly specific, and have harsh usage conditions. Polyether defoamers are second-generation defoamers, and mainly include linear polyethers, polyethers with alcohols or amines as initiators, and terminal hydroxyl polyether derivatives. Their greatest advantage is their strong defoaming ability, and some also have excellent properties such as high temperature resistance and resistance to strong acids and alkalis. However, their application conditions are significantly limited by temperature, and their fields of application are relatively narrow. Their defoaming ability and bubble-breaking rate need to be improved. As a third-generation defoamer, silicone defoamer has strong defoaming properties, a fast bubble-breaking rate, low volatility, is non-toxic to the environment, physiologically inert, and has a wide range of applications. It has broad application prospects and huge market potential, but there is still room for improvement in its defoaming performance in some specific scenarios. Polyether-modified polysiloxane defoamer combines the advantages of polyether defoamer and silicone defoamer, and is the development trend of defoamers. Sometimes they can be reused based on their reverse solubility, but currently there are few types of this type of defoamer, and they are still in the research and development stage, so the production cost is relatively high.

When selecting a defoamer, a number of factors need to be considered. First, the defoamer should be insoluble or poorly soluble in the foaming liquid. Since the defoamer needs to concentrate and exert its effect on the foam film, it is important for the defoamer to be able to quickly reach this state, and for the antifoam agent to maintain it at all times. Therefore, only insoluble or poorly soluble defoamers can easily reach a supersaturated state in the foaming liquid, so that they can accumulate at the gas-liquid interface and concentrate on the bubble film to exert their effect at a lower concentration. For defoamers used in aqueous systems, the active ingredient molecules need to be strongly hydrophobic and weakly hydrophilic, and they work best when the HLB value is between 1.5 and 3. For example, in the production of water-based paints, if the defoamer is not properly soluble, it will not be able to effectively eliminate the foam, and may also affect the quality and performance of the paint. Secondly, the surface tension of the defoamer must be lower than that of the foaming liquid. Only when the intermolecular forces of the defoamer molecules are weak and the surface tension is lower than that of the foaming liquid, can the defoamer particles penetrate and expand on the foam film. It should be noted here that the surface tension of the foaming liquid is not the surface tension of the solution, but specifically the surface tension in the foamed state. Furthermore, the defoamer and the foaming liquid must have a certain affinity. Because the defoaming process is essentially a competition between the rate at which the foam collapses and the rate at which it is generated, the defoamer must be able to disperse quickly in the foaming liquid in order to have a rapid effect over a wide area. If the active ingredient of the defoamer is too similar to the foaming liquid, it will dissolve. If it is too dispersed, it will be difficult for it to take effect. The effect will only be good when the affinity is appropriate. In addition, the defoamer should not react chemically with the foaming liquid, otherwise it will lose its defoaming effect on the one hand, and harmful substances may be produced on the other, affecting microbial growth, etc. Finally, the defoamer should have low volatility and a long duration of action. When determining the system for using the defoamer, it is necessary to distinguish between a water-based system and an oil-based system. For example, in the fermentation industry, oil-based defoamers such as polyether-modified silicone oils or polyethers should generally be used, while water-based defoamers and silicone defoamers should be used in the water-based paint industry. At the same time, it is also necessary to compare the amount of defoamer added and refer to the price in order to obtain the most suitable and economical defoamer product.

The effectiveness of the use of defoamers is also affected by a variety of factors. The dispersibility of the defoamer in the solution has a significant effect on the defoaming performance. It should have an appropriate degree of dispersion, and too large or too small a particle size will affect the defoaming activity. In terms of compatibility in the foam system, when the surfactant is completely dissolved in the aqueous solution, it usually tends to stabilise the foam at the gas-liquid interface of the foam; while when the surfactant is insoluble or supersaturated, its particles will disperse in the solution and accumulate on the foam, thus defoaming. The ambient temperature of the foaming system should not be ignored. When the temperature of the foaming liquid is high, a special high-temperature defoamer must be used. Otherwise, not only will the defoaming effect of ordinary defoamers be greatly reduced, but demulsification may also occur. During packaging, storage and transportation, defoamers should be stored at 5-35°C. The shelf life is generally 6 months. They should be kept away from heat sources and exposed to sunlight. After use, they should be sealed to prevent deterioration. The addition ratio of the defoamer is also very important. The effect of adding the defoamer undiluted is different from that of adding it after dilution. Due to the low concentration of surfactant, the defoamer emulsion after dilution is extremely unstable, quickly separating into layers. It has poor defoaming performance and is not suitable for long-term storage. It is recommended to use the defoamer immediately after dilution. The addition ratio should be determined through on-site testing to ensure the desired effect, and it should not be over-added.