Silicone grease is a waterproof grease made by combining a silicone oil with a thickener. Most commonly, the silicone oil is polydimethylsiloxane (PDMS) and the thickener is amorphous fumed silica. Using this formulation, silicone grease is a translucent white viscous paste, with exact properties dependent on the type and proportion of the components. More specialized silicone greases are made from fluorinated silicones or, for low temperature applications, PDMS containing some phenyl substituents in place of methyl groups. For food applications, the thickener is calcium stearate. For applications involving highly reactive substances, powdered polytetrafluorethylene (PTFE) is the thickener.
Use in industry
Silicone grease is commonly used for lubricating and preserving rubber parts, such as O-rings. Additionally, silicone grease does not swell or soften the rubber, which can be a problem with hydrocarbon based greases. It functions well as a corrosion-inhibitor and lubricant for purposes that require a thicker lubricant.
Thermal grease often consists of a silicone grease base, along with added thermally conductive fillers. It is used for heat transfer abilities, rather than friction reduction.
Special versions of silicone grease are also used widely by the plumbing industry in faucets and seals, as well as dental equipment. These special versions are formulated using components not known to be an ingestion hazard. Electrical utilities use silicone grease to lubricate separable elbows on lines which must endure high temperatures. Silicone greases generally have an operating temperature range of approximately −40 to 200 °C (−40 to 392 °F) with some high-temperature versions extending that range slightly.
Use in the chemical laboratory
Silicone grease is widely used as a temporary sealant and a lubricant for interconnecting ground glass joints, as is typically used in laboratory glassware. Although silicones are normally assumed to be chemically inert, several historically significant compounds have resulted from unintended reactions with silicones. The first salts of crown ethers (OSi(CH3)2)n (n = 6, 7) were produced by reactions of organolithium and organopotassium compounds with silicone greases or the serendipitous reaction of stannanetriol with silicone grease to afford a cage-like compound having three Sn-O-Si-O-Sn linkages in the molecule.
Silicone grease is soluble in organic solvents, and lubrication of an apparatus with silicone grease may result in the reaction mixture being contaminated with the grease. The impurity may be carried through purification by chromatography in undesirable amounts. In NMR spectroscopy, the methyl groups in polydimethylsiloxane display 1H and 13C chemical shifts similar to trimethylsilane (TMS), the reference compound for those forms of NMR spectroscopy. As with TMS, the signal is a singlet. In 1H NMR, silicone grease appears at a singlet at δ = 0.07 ppm in CDCl3, 0.09 in CD3CN, 0.29 in C6D6, and -0.06 ppm in (CD3)2SO. In 13C NMR, it appears at δ = 1.19 ppm in CDCl3 and 1.38 ppm in C6D6. Tables of impurities commonly found in NMR spectroscopy have been prepared, and such tables include silicone grease.
Silicone-based lubricants are often used by consumers in applications where other common consumer lubricants, such as petroleum jelly, would damage certain products, such as latex rubber and gaskets on dry-suits. It can be used to lubricate fountain pen filling mechanisms and threads. It is used to seal and preserve O-rings in flashlights, plumbing, waterproof watches, and air rifles. Silicone grease is widely used to lubricate threads of water submersible flashlights used for diving and spearfishing. This grease improves water resistance of the flashlights and protects threads from wearing out. Silicone grease is used with waterproof devices as it has a very thick body and doesn't dissolve in water, as most spirits and other liquids would.
Various household uses include lubricating door hinges, shower heads, threads on bolts, garden hose threads or any thread or mechanism that can be lubricated.
As a sealant around electrical contacts
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Dielectric grease is electrically insulating and does not break down when high voltage is applied. It is often applied to electrical connectors, particularly those containing rubber gaskets, as a means of lubricating and sealing rubber portions of the connector without arcing.
A common use of dielectric grease is in high-voltage connections associated with gasoline engine spark plugs. The grease is applied to the rubber boot of the plug wire. This helps the rubber boot slide onto the ceramic insulator of the plug. The grease also acts to seal the rubber boot, while at the same time preventing the rubber from becoming stuck to the ceramic. Generally, spark plugs are located in areas of high temperature and the grease is formulated to withstand the temperature range expected. It can be applied to the actual contact as well, because the contact pressure is sufficient to penetrate the grease film. Doing so on such high pressure contact surfaces between different metals has the advantage of sealing the contact area against electrolytes that might cause rapid deterioration from galvanic corrosion.
Another common use of dielectric grease is on the rubber mating surfaces or gaskets of multi-pin electrical connectors used in automotive and marine engines. The grease again acts as a lubricant and a sealant on the nonconductive mating surfaces of the connector. It is not recommended to be applied to the actual electrical conductive contacts of the connector because it could interfere with the electrical signals passing through the connector in cases where the contact pressure is very low. Products designed as electronic connector lubricants, on the other hand, should be applied to such connector contacts and can dramatically extend their useful life. Polyphenyl ether, rather than silicone grease, is the active ingredient in some such connector lubricants.
Silicone grease should not be applied to (or next to) any switch contact that might experience arcing, as silicone can convert to silicon carbide under arcing conditions, and accumulation of the silicon carbide can cause the contacts to prematurely fail. (British Telecom had this problem in the 1970s when silicone Symel® sleeving was used in telephone exchanges. Vapor from the sleeving migrated to relay contacts and the resultant silicon carbide caused intermittent connection.)
- Thorsten Bartels et al. "Lubricants and Lubrication" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Weinheim. doi:10.1002/14356007.a15_423
- Haiduc, I., "Silicone Grease: A Serendipitous Reagent for the Synthesis of Exotic Molecular and Supramolecular Compounds", Organometallics 2004, volume 23, pp. 3-8. doi:10.1021/om034176w
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