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Types of Defoamers and Defoaming Principles for Water-based Coatings

Jul 28, 2021

Silicones The most commonly used is polydimethylsiloxane, also known as dimethyl silicone oil. It has low surface energy, low surface tension, low solubility in water and general oil, and high activity. Its main chain is a silicon-oxygen bond, which is a non-polar molecule. It has no affinity with polar solvent water, and has little affinity with general oils. It has low volatility and chemical inertness, relatively stable and low toxicity. Pure dimethicone

Alkane, it is difficult to be used as a defoaming agent without dispersion treatment. It may be due to its high interfacial tension with water and low spreading coefficient, which makes it difficult to disperse on the foaming medium. Therefore, silicone oil is mixed into SiO2 aerosol to form a composite, that is, the hydrophobically treated SiO2 aerosol is mixed into dimethyl silicone oil, and processed at a certain temperature and a certain time, and then it can be prepared. Silicone defoamer is made of silicone grease, emulsifier, water repellent, thickening agent, etc., mixed with appropriate amount of water and mechanically emulsified. It is characterized by low surface tension, high surface activity, strong defoaming power, low dosage and low cost.

Most bubble media can defoam. It has good thermal stability and can be used in a wide temperature range of 5℃-150℃; its chemical stability is good, and it is difficult to react with other substances. As long as it is properly configured, it can be used in acid, alkali, and salt solutions. , No damage to product quality; it also has a physiologically inert LD250g/Kg mouse, usually used in the food and pharmaceutical industries. It has the functions of suppressing and breaking bubbles for all bubble systems, and belongs to the category of broad-spectrum defoamers. It is widely used for defoaming in the production process of detergents, papermaking, pulp, sugar, electroplating, fertilizers, additives, wastewater treatment, etc. In the petroleum industry, it is widely used for the desulfurization of natural gas to accelerate the separation of oil and gas; it is also used for the drying of ethylene glycol, the extraction of aromatic hydrocarbons, the processing of asphalt, the dewaxing of lubricating oil and other devices to control or suppress air bubbles. . In the textile industry, it is used for defoaming in the process of dyeing, scouring, sizing, etc.; in the chemical industry, it is used for defoaming in the process of synthetic resin, latex, paint, ink, etc.; in the food industry, it is used For various concentration, fermentation,
Defoaming during distillation. Silicone grease can be applied to the pot wall, outlet or metal mesh for defoaming. The silicone grease is formulated into a solution, which can be used to defoam the oil phase system. Silicone grease and low-viscosity silicone oil are combined to form a water emulsion, which can be used for defoaming in a variety of water phase systems. In medicine, it is usually used to remove flatulence from organs or stomach organs before surgery, X-ray and gastroscopy.
 
 Defoamers can be roughly divided into two categories: one can eliminate the bubbles that have been generated, Polyether-modified silicone is a polyether-siloxane copolymer (silicon ether copolymer for short) made from polyether segments in the siloxane molecule. Polysiloxane defoamers have the characteristics of rapid defoaming, long time of foam suppression, safety and non-toxicity, but it is difficult to dissolve in water, high temperature resistance, poor resistance to strong alkalis, and polyether defoamers have good water solubility and resistance High temperature, strong alkali resistance, but its defoaming speed and foam suppression time are not ideal. The polyether chain is grafted on the polysiloxane chain by condensation technology to make it have the advantages of the second type of defoaming agent. Become a defoamer with excellent performance and broad application prospects. In the molecule of the silicone ether copolymer, the siloxane segment is a lipophilic group, and the polyether segment is a hydrophilic group. In the polyether segment, the polyethylene oxide chain saves energy to provide hydrophilicity and foaming, and the polypropylene oxide chain saves energy to provide hydrophobicity and permeability, which has a strong effect on reducing surface tension. The polyether end groups also have a strong influence on the properties of the siloxane ether copolymer. 

Polysiloxane antifoaming agent Polysiloxane antifoaming agent is usually made up of the two main components of polydimethylsiloxane and silicon dioxide. It is made of polydimethylsiloxane as the base material. Defoamer is an ideal defoamer in the defoaming system, because it is insoluble in water and more difficult to emulsify. Polydimethylsiloxane has lower surface properties than carbon chain hydrocarbons, so it is more commonly used in the textile industry. The surface active agent has lower surface tension. Simple polydimethylsiloxane has poor foam suppression performance and is slow. The defoaming effect needs to be strengthened by silica particles. The silica particles are carried by the silicone oil to the air-water interface of the foam and enter the bubble liquid film due to its hydrophobicity. The contact angle of the foam droplets with the surfactant is greater than 90°, which forces the foaming liquid to be expelled from the surface of the solid hydrophobic particles, causing local rapid drainage of the foam and rupture. In this way, due to the synergistic effect, the two compositions The substance produced a good defoaming effect. Due to its excellent defoaming performance and other advantages, 

polysiloxane defoamers have been widely used in many fields, especially in the dyeing and finishing of textiles. The development is particularly rapid. Research has found that polydimethylsiloxane The silicone glycol defoamer with polyethylene copolymer as the base material has an ideal defoaming effect in jet dyeing and other textile processing due to its special solubility. Types of defoaming agents and defoaming mechanism of defoaming agents There are many types of defoaming agents, including organosiloxane, polyether, silicon and ether grafts, containing amines, imines and amides, which have a faster defoaming speed , The foam suppression time is longer, the applicable medium range is wider, and even the harsh medium environment such as high temperature, strong acid and strong alkali characteristics. The defoaming mechanism of defoamers .

The electrolyte collapses the surfactant electric double layer and causes the bubble to burst. The surfactant electric double layer interacts with the foam to produce a stable foaming liquid. The addition of ordinary electrolyte can collapse the surfactant electric double layer and defoam effect.

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