I. Types and Characteristics of Common Silicone Defoamers

1. Polydimethylsiloxane (dimethyl silicone oil)
   • Characteristics: Low surface energy, low surface tension, low solubility in water and general oils and high activity. The main chain is a siloxane bond. It is a non-polar molecule with little affinity for water and general oils. It has low volatility, chemical inertness, stability and low toxicity. However, pure polydimethylsiloxane is difficult to be used as a defoamer without dispersion treatment. It has a high interfacial tension with water and a low spreading coefficient, and is not easy to be dispersed on the foaming medium.
   • Application method: A composite can be made by mixing hydrophobically treated SiO₂ aerosol into dimethyl silicone oil and treating it at a certain temperature and time.
2. Mechanically emulsified silicone defoamer
   • Composition and preparation: It is made by mechanically emulsifying silicone grease, emulsifier, waterproofing agent, thickening agent and an appropriate amount of water.
   • Characteristics: Low surface tension, high surface activity, strong defoaming power, small dosage, low cost. It is immiscible with water and most organic substances. It can defoam most bubble media. It has good thermal stability (can be used in a wide temperature range of 5℃ - 150℃). It has good chemical stability (can be used in acid, alkali and salt solutions without damaging product quality). It has physiological inertness (LD250g/Kg for mice and can be used in the food and pharmaceutical industries). It has both foam suppression and foam breaking functions for all bubble systems and is a broad-spectrum defoamer.
   • Application fields: Widely used for defoaming in production processes such as detergents, papermaking, pulp, sugar making, electroplating, fertilizers, additives, and wastewater treatment. In the petroleum industry, it is used for desulfurization of natural gas, accelerating gas-liquid separation, and controlling or suppressing bubbles in devices such as ethylene glycol drying, aromatic hydrocarbon extraction, asphalt processing, and lubricating oil dewaxing. In the textile industry, it is used for defoaming in processes such as dyeing, refining, and sizing. In the chemical industry, it is used for defoaming in processes such as synthetic resins, latex, coatings, and inks. In the food industry, it is used for defoaming in various concentration, fermentation, and distillation processes. Silicone grease can be applied on the pot wall, at the outlet or on a metal mesh for defoaming. Silicone grease can also be formulated into a solution for defoaming in oil-phase systems, or silicone grease can be combined with low-viscosity silicone oil to form a water emulsion for defoaming in various water-phase systems. In medicine, it is used to remove flatulence from organs or stomach organs before surgery, X-ray and gastroscopy for patients.

II. Classification and Examples of Silicone Defoamers
Silicone defoamers can be roughly divided into two categories:

1. Eliminating generated bubbles
   • Example: Ethanol, etc., can quickly eliminate generated bubbles.
2. Inhibiting bubble formation
   • Example: Emulsified silicone oil, etc., can inhibit the formation of bubbles at the initial stage of foam generation.

III. Silicone Defoamers Permitted for Use in China
The defoamers permitted for use in China include emulsified silicone oil, high-carbon alcohol fatty acid ester complex, polyoxyethylene polyoxypropylene pentaerythritol ether, polyoxyethylene polyoxypropylene amine ether, polyoxypropylene glycerol ether and polyoxypropylene, etc. These defoamers play an important role in different industries and fields. The specific application depends on their characteristics and the applicable foam system.