1.1 Dispersion: An important indicator to measure the quality of a defoamer is its dispersion effect. Only substances with low solubility and high dispersibility can become the active ingredients of defoamer. As the main component of defoamer, simethicone has low solubility and low dispersibility. It must use silica, emulsifier and other dispersing aids to improve its dispersing effect. Fumed silica is divided into hydrophilic type and hydrophobic type. The hydrophilic type contains silicon-hydroxyl bonds and can form hydrogen bonds with water molecules. It can promote the dispersion and stability of defoamer when used in water systems. Hydrophobic fumed silica is obtained by surface chemical treatment of hydrophilic silica to reduce the Si-OH number of silica to an appropriate level. It can also be used in water systems to reduce the amount of silica in water. The degree of association of the system can achieve the best defoaming effect. The application of fumed silica in defoamer is to first make it and dimethyl silicone oil into silica paste, which can improve the wettability of it and dimethyl silicone oil and the dispersibility in silicone oil. The mixing method of the silicone paste has a certain influence on the stability and defoaming performance of the silicone paste. When using hydrophilic silica, due to the low compatibility of inorganic silica and silicone oil, in order to achieve the purpose of mixing, the silicone oil and silica are placed in a kneader and mixed at 150~180℃. Stir with N2 for 2 to 7 hours to improve the wettability of silica and silicone oil. If the hydrophobic silica is used, its affinity with silicone oil is greatly improved because the surface has been treated with organic matter, and it can be stirred at 150°C with N2 for 2 to 5 hours. The silicone paste made of fumed silica is transparent or translucent, and the ratio is generally silicone oil: silica = (90-95): (10-5), and dimethyl silicone oil is generally selected at 25°C, 500-1000cp Silicone oil and silica can be selected with a specific surface area of 200m2/g (BET method) or more. As the active ingredient of silicone oil defoamer, silicone ointment can be used directly or diluted with solvent, or self-emulsified with emulsifier or polyether modified silicone oil to achieve the purpose of defoaming. Due to the existence of SiO2 particles, the silicone oil can quickly spread in the foaming liquid, which improves the dispersion efficiency of the silicone oil, thereby improving the defoaming effect. Therefore, fumed silica is an important dispersant in defoamer.

1.2 Adsorption: An important role of fumed silica in defoamer is adsorption. Due to its small particle size, large specific surface area, high surface energy, and three-dimensional network structure, silica can form a huge force for adsorbing bubbles, which together with silicone oil constitutes a force to attack bubbles, that is, silica adsorbs and impacts The air bubbles create weak points, and under the action of the low surface tension of the silicone oil, the bubbles are ruptured and the bubbles are broken. Therefore, fumed silica is equivalent to the "needle tip" in the defoaming agent, and the dosage is small but the effect is huge. It and silicone oil are the active ingredients of high-efficiency defoamer, and have a composite, efficient and synergistic defoaming effect.

1.3 Thickening and anti-settling effect The important indicators of emulsion-type silicone oil defoamers are the defoaming effect and storage stability, especially the silicone-containing cream emulsion. There are many reasons that affect the stability of the emulsion, such as the particle size of the emulsion and the viscosity of the emulsion. The particle size of the emulsion is the most important control index for the emulsion type defoamer, and the particle size is generally required to be less than 10um. The particle size of the emulsion depends on the choice of emulsifier, the emulsification process and the size of the particles. The minimum particle size of fumed silica can reach about 5um. The silicone oil emulsion particles containing this silica form a very small particle and a large number of basic defoaming units in the foaming liquid, so that the defoaming agent can achieve the best Defoaming effect. At the same time, due to the small particle size, large specific surface area and stable hydrogen bond three-dimensional network structure, the silica particles can be uniformly and stably distributed in the entire system, making the system stable in storage. Therefore, as a very good thickening and anti-settling agent in defoamer, fumed silica should generally be selected with a specific surface area of 200-300m2/g. To sum up, the fumed silica silicone ointment plays a very important and efficient composite and synergistic role as a defoaming active in the silicone oil defoamer, which greatly improves the defoaming efficiency of the silicone oil.

2 Silicone oil defoamer can be divided into six categories according to its product form: silicone oil type, silicone paste type, solution type, emulsion type, modified silicone oil type and solid type.