Silicone oil generally refers to linear polysiloxane products that remain liquid at room temperature. They are generally colorless (or light yellow), odorless, non-toxic, non-volatile liquids, insoluble in water, methanol, ethylene glycol, and benzene. Methyl ether, carbon tetrachloride or kerosene are mutually soluble, slightly soluble in acetone, dioxane, ethanol and butanol.
       


        Classification of silicone oil






        Silicone oil is divided according to chemical structure, including methyl silicone oil, ethyl silicone oil, phenyl silicone oil, methyl hydrogen silicone oil, methyl phenyl silicone oil, methyl chlorophenyl silicone oil, methyl ethoxy silicone oil, methyl trifluoro Propyl silicone oil, methyl vinyl silicone oil, methyl hydroxy silicone oil, ethyl hydrogen silicone oil, hydrogen hydrogen silicone oil, cyanide-containing silicone oil, and the like.


        From the point of use, there are damping silicone oil, diffusion pump silicone oil, hydraulic oil, insulating oil, heat transfer oil, brake oil and the like.


        From the processing status can be divided into primary products and secondary products. The former refers to silicone oil products before processing, including hydroxy silicone oil, silicon functional silicone oil, carbon functional silicone oil and inactive modified silicone oil. Habitually, the first two categories are called linear silicone oils, the latter two types are called modified silicone oils; the latter are made from silicone oils, blended with thickeners, surfactants, solvents and additives, and processed by specific processes. Products such as greases, lotions and solutions, such as silicone grease, silicone paste, defoamer, release agent and release agent.


        Silicone oils are often not used alone, but are formulated to be effective by special formulation; not as a raw material, but as an auxiliary material to produce a certain effect. In general, there are three basic forms of silicone oil on the market: pure silicone oil, solution, and emulsion.










○ 100% pure silicone oil


        Pure silicone oil is very stable, heat resistant, antifreeze, good storage, high flash point and high ignition point, and has few storage and transportation problems.


○ Solution


        In fact, it is often necessary to dilute the silicone oil solution, and it is not convenient for the user to configure the solution by himself. In use, it is sometimes desirable to add active adjuvants (catalysts, crosslinkers, etc.) to the solution for storage and use. An aliphatic, aromatic or chlorinated hydrocarbon can be used as a solvent as needed. Other types of solvents are used under special circumstances. The polymer polysiloxane solution can only reach 35% concentration, otherwise the viscosity is too large.


○ lotion


        A large part of silicone oil or silicone oil-containing products are made into an emulsion form. Compared with solvent silicone oil, it has the advantages of being cheap, not easy to burn, and harmless to health. Different from the solvent type, all silicone emulsions are “oil-in-water” type (O/W type) heterogeneous systems. The external closed phase is the aqueous phase, and the silicone oil is dispersed in the water phase with very fine droplets. Dilute to the desired concentration with water for application. The stability of this emulsion can be achieved by minimizing the droplets and stabilizing effects of the emulsifier.






Silicone oil properties






         Silicone oil has many special properties:


○ Good heat resistance


         Since the main chain is composed of a -Si-O-Si- bond, the polysiloxane molecule has a structure similar to that of an inorganic polymer, and has a high bond energy, so that it has excellent heat resistance.


○ Good oxidation resistance and weather resistance


○ Good electrical insulation


        Silicone oil has good dielectric properties, and its electrical characteristics change little with changes in temperature and cycle number. The dielectric constant decreases with increasing temperature, but with little change. Silicone oil has a low power factor and increases with temperature, but there is no rule with frequency. The volume resistivity decreases as the temperature rises.


○ Good hydrophobicity


        Although the main chain of the silicone oil consists of the polar bond Si-O, the non-polar alkyl groups on the side chains are oriented outward, preventing water molecules from entering the interior and acting as a hydrophobic agent. The interfacial tension of silicone oil to water is about 42 dynes/cm. When diffused on the glass, the contact angle of about 103oC is formed due to the water repellency of the silicone oil, which is comparable to paraffin.


○ Small viscosity coefficient


        Silicone oils have low viscosity and small changes with temperature, which is related to the helical structure of silicone oil molecules. Silicone oil is a type of oil with the best viscosity-temperature characteristics of various liquid lubricants. This property of silicone oil makes sense for damping equipment.


○ High resistance to compression


        Since the silicone oil molecules have a large helical structure characteristic and a large inter-molecular distance, they have high compression resistance. By utilizing this property of silicone oil, it can be used as a liquid spring, and the volume can be greatly reduced compared with a mechanical spring.














○ Low surface tension


        Low surface tension is a characteristic of silicone oil. Low surface tension means high surface activity, therefore, silicone oil has excellent defoaming and antifoaming properties, isolation property with other substances, and lubricating properties.


○ Non-toxic, odorless and physiologically inert


        From a physiological point of view, siloxane polymers are one of the most inactive compounds known. Dimethicone is inert to organisms and has no rejection reaction with animal organisms. Therefore, they have been widely used in the departments of surgery and medicine, medicine, food and cosmetics.


○ Good lubricity


        Silicone oil has many excellent properties as a lubricant, such as high flash point, low freezing point, thermal stability, small viscosity change with temperature, no corrosion of metal, and no adverse effect on rubber, plastic, paint, organic paint film and low surface tension. Easy to spread on metal surfaces and other characteristics. In order to improve the lubricity of the steel of the silicone oil to the steel, a lubricity additive which is miscible with the silicone oil may be added. The introduction of a chlorophenyl group on the siloxane chain or the substitution of a dimethyl group with a trifluoropropyl group greatly improves the lubricating properties of the silicone oil.


○ Chemical properties


        Silicone oil is relatively inert because the Si-C bond is very stable. But strong oxidants are easy to work with, especially at high temperatures. Silicone oil reacts violently with chlorine gas, especially for methyl silicone oil. Sometimes there is an explosive reaction. Si-O bonds are easily broken by strong bases or acids. Concentrated sulfuric acid reacts very quickly at low temperatures, breaking the siloxane chain and attaching it to it. In this respect, a silicone oil having a high alkane group and a phenyl group is relatively stable, but concentrated sulfuric acid breaks the benzene-silicon bond of the phenyl group and releases benzene.






Silicone oil regeneration






        The chemical properties of silicone oils are relatively temperature-sensitive, but they also deteriorate after infiltration due to oxidation and foreign matter. Because silicone oils are expensive, they should be recycled and recycled separately.


        Waste silicone oils are usually regenerated in two ways. The waste silicone oil in the electrical equipment is mainly treated by adsorption refining, and the waste silicone oil can be contacted and refined with an adsorbent such as silica gel, activated alumina, activated clay and activated carbon. Purified regenerated silicone oil can be obtained by vacuum degassing (e.g., vacuum degassing at 95 ° C) or blowing nitrogen at 20-80 ° C after filtering off the spent adsorbent.


        In another method, the liquid waste silicone oil is treated with a solvent vapor, and the impurities in the waste silicone oil are dissolved and extracted by the dissolution at the time of solvent condensation, and another liquid phase which is immiscible with the silicone oil is formed. In this way, polychlorinated polycyclic aromatic hydrocarbons (PCBs) in the waste silicone oil can be dissolved and removed. The treated silicone oil also contains a trace amount of solvent which can be removed by vacuum drying. Commonly used are hydrocarbon solvents and chlorinated hydrocarbon solvents.