The fumed silica is a high purity, fine, amorphous silica nano powder. The specific surface area is 150~380m2/g, the primary particle size is between 7~40 nm, the aggregate particle size is about 200-500 nm, the agglomerate particle size is about 5-20 microns, and the SiO2 content is not less than 99.8%. It is a multifunctional additive. In addition to being widely used in silicone rubber, paints, inks, paints, plastics, etc., it also acts as an important component in high-efficiency silicone oil defoamers, and plays a role of dispersion, adsorption, thickening and anti-settling. The fumed silica generally has two types of hydrophilic and hydrophobic products, wherein the hydrophobic product is obtained by surface chemical treatment using a hydrophilic product. As an important auxiliary in the industry, defoamer is used in a small amount but is indispensable. It is widely used in textile, paper, petroleum, food, pharmaceutical and other fields. Defoamers are targeted and specific, because the foaming mechanism and process conditions of each foaming system are different, and different foaming systems require different defoaming agents. It can be broadly classified into non-silicon based and silicon based defoamers. Non-silicone such as higher alcohols, acids, hydrocarbons, polyethers, esters, phosphates, etc., which are large in quantity and small in applicability; silicon-based defoamers are silicone oil/white carbon black silicone paste and modified silicone oil as defoaming agent. The main ingredient is the most widely used and most effective defoamer. Compared with other kinds of defoamers, there are several outstanding features: 1 low solubility, solubility parameters are very different from other organic substances, and will not occur with water, polar group-containing compounds and hydrocarbon compounds with different SP values. Association. It can be used in both water and oil systems, and has a defoaming effect on most foaming systems. 2 low surface tension, surface tension of 20 ~ 21mN / m, much lower than the surface tension of water (76mN / m) and general foaming liquid, which is the application of silicone oil defoamer and defoaming ability The main reason is that the general dosage is only 10~200ul/L of foaming liquid, which will not pollute the system and affect the processing performance of the product. 3 chemical stability, heat resistance, can adapt to the defoaming of weak acids, alkalis, salts and high temperature systems. 4 physiologically inert, can be used in food, cosmetics, pharmaceutical industry. There are many models of 5 varieties, which can be selected for a variety of foaming systems. In addition to disallowing systems containing dispersants (SiO2) such as some oily systems, fumed silica is an important component of silicon-based defoamers, and its effect is enormous.
1 The role of fumed silica in defoamers 1.1 Dispersion
  An important indicator of the quality of defoamers is its dispersion. A substance having a small solubility and a large dispersibility can be an active ingredient of an antifoaming agent. Dimethicone is the main component of the defoamer. It has low solubility and low dispersibility. It must use white carbon black, emulsifier and other dispersing additives to improve its dispersion. The fumed silica is divided into a hydrophilic type and a hydrophobic type, and the hydrophilic type contains a silicon hydroxy bond, which can form a hydrogen bond with water molecules, and can be used in an aqueous system to promote dispersion and stabilization of the defoaming agent. Hydrophobic fumed silica is obtained by surface chemical treatment of hydrophilic silica, which reduces the Si-OH number of silica to a suitable level, and can also be applied to water systems to reduce white carbon in water. The degree of association of the system to achieve the best defoaming effect. The application of fumed silica in defoamer is to first make it and dimethyl silicone oil into a white carbon black paste, which can improve its wettability with dimethicone and 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 hydrophilic white carbon black is used, since the inorganic white carbon black and the silicone oil have low compatibility, in order to achieve the purpose of mixing, the silicone oil and the white carbon black are mixed in a kneader and passed at 150 to 180 ° C. The N2 is stirred for 2 to 7 hours to improve the wettability of the silica and the silicone oil. If using hydrophobic white carbon, due to the table
The surface has been treated with organic matter, and its affinity with silicone oil is greatly improved. It can be stirred at 150 ° C for 2 to 5 hours with N2. The silicon paste made of fumed silica is transparent or translucent, and the ratio is generally silicone oil: white carbon black = (90 ~ 95): (10 ~ 5), dimethyl silicone oil is generally selected 25 ° C, 500 ~ 1000 cp For silicone oil, white carbon black can be used with a specific surface area of 200 m 2 /g (BET method) or more. As the active ingredient of silicone oil defoamer, silicone grease can be used directly or diluted with solvent, or self-emulsified with emulsifier or polyether modified silicone oil to achieve defoaming. Due to the presence of SiO2 particles, the silicone oil can be rapidly 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 defoamers. 1.2 adsorption
  An important role of fumed silica in defoamers is adsorption. Due to its small particle size, large specific surface area, high surface energy and three-dimensional network structure, white carbon black can form a huge force for adsorbing bubbles. Together with silicone oil, it constitutes the force to attack bubbles, that is, white carbon black absorbs and impacts. The bubbles cause weak points, and the bubbles are broken under the action of the low surface tension of the silicone oil to cause foaming. Therefore, the fumed silica is equivalent to a "needle tip" in the antifoaming agent, and the amount is small but the effect is enormous. It and silicone oil act as active ingredients of high-efficiency defoamer, with complex and efficient defoaming effect.
Due to its special three-dimensional network structure, fumed silica is the strongest of all white carbon blacks. Therefore, under the same conditions, defoaming of defoamer containing fumed silica The effect is the best. Therefore, in order to obtain a high performance, composite defoamer, it is recommended to use fumed silica. 1.3 thickening and anti-settling effect
  An important indicator of emulsion type silicone oil defoamers is defoaming effect and storage stability, especially for silicone-containing cream emulsions. There are many reasons that affect the stability of the emulsion, such as the particle size of the emulsion, the viscosity of the emulsion, and the like. The emulsion particle size is the most important control index of the emulsion type defoamer, and generally requires a particle size of less than 10 um. 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, and the silicone oil emulsion particles containing the white carbon black form a very small and a large number of basic defoaming units in the foaming liquid, so that the defoaming agent is optimal. Defoaming effect. At the same time, due to the small particle size, large specific surface area and stable hydrogen-bonded three-dimensional network structure, the white carbon black particles can be uniformly and stably distributed throughout the system, so that the system is stable in storage. Therefore, the fumed silica is a very good thickening and anti-settling agent in the antifoaming agent, and a model having a specific surface area of 200 to 300 m 2 /g is generally selected.
   In summary, the fumed silica black grease acts as a defoaming active in the silicone oil defoaming agent and plays an important role in high-efficiency compounding and synergistic effect, which greatly improves the defoaming efficiency of the silicone oil. 2 Application of fumed silica in defoamer
  Silicone oil defoamer can be divided into six types according to its product form: silicone oil type, silicone paste type, solution type, emulsion type, modified silicone oil type and solid type. The composition and application are as follows: