Silicone grease

Silicone grease, sometimes called dielectric 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. Other thickeners may be used, including stearates and powdered polytetrafluorethylene (PTFE).[1] Greases formulated from silicone oils with silica thickener are sometimes referred to as silicone paste to distinguish them from silicone grease made with silicone oil and a soap thickener.

Use in industry

Silicone grease is commonly used for lubricating and preserving many types of rubber parts, such as O-rings, without swelling or softening the rubber, but is contraindicated for silicone rubber due to these factors. It functions well as a corrosion inhibitor and lubricant on non-metal-metal contact areas.

Silicone grease is soluble in organic solvents such as toluene, xylene, mineral spirits, and chlorinated hydrocarbons. It is insoluble in methanol, ethanol, and water.[2]

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.

Pure silicone grease is widely used by the plumbing industry in faucets and seals, as well as in dental equipment. This is due to it not being an ingestion hazard. Electrical utilities use silicone grease to lubricate separable elbows on lines that 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 this 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.[3][4] 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[5] 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.[6]

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.[7]

Consumer uses

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

Silicone greases are electrically insulating and are often applied to electrical connectors, particularly those containing rubber gaskets, as a means of sealing and protecting the connector. In this context they are often referred to as dielectric grease.[8][9]

A common use of this type is in the high-voltage connection associated with gasoline-engine spark plugs, where grease is applied to the rubber boot of the plug wire to help it slide onto the ceramic insulator of the plug, to seal the rubber boot, and to prevent the rubber's adhesion to the ceramic. Such greases are formulated to withstand the high temperature generally associated with the areas in which spark plugs are located, and can be applied to contacts 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 further advantage of sealing the contact area against electrolytes that might cause rapid deterioration of the metals by galvanic corrosion.[10]

Silicone grease can decompose to form an insulating layer at or next to switch contacts that experience arcing, and contamination can cause the contacts to prematurely fail.[11]

References
  1. Thorsten Bartels et al. "Lubricants and Lubrication" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Weinheim. doi:10.1002/14356007.a15_423.
  2. "Solubility of silicone fluids" (PDF). Retrieved March 6, 2019.
  3. 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.
  4. Lucian C. Pop and M. Saito (2015). "Serendipitous Reactions Involving a Silicone Grease". Coordination Chemistry Reviews. 314: 64–70. doi:10.1016/j.ccr.2015.07.005.
  5. Jamie S. Ritch and Tristram Chivers (2007). "Silicon Analogues of Crown Ethers and Cryptands: A New Chapter in Host–Guest Chemistry?". Angewandte Chemie International Edition. 46 (25): 4610–4613. doi:10.1002/anie.200701822. ISSN 1433-7851. PMID 17546579.
  6. Lucian C. Pop; et al. (2014). "Synthesis and structures of monomeric group 14 triols and their reactivity". Canadian Journal of Chemistry. 92 (6): 542–548. doi:10.1139/cjc-2013-0496.
  7. Fulmer, Gregory R.; Miller, Alexander J. M.; Sherden, Nathaniel H.; Gottlieb, Hugo E.; Nudelman, Abraham; Stoltz, Brian M.; Bercaw, John E.; Goldberg, Karen I. (10 May 2010). "NMR Chemical Shifts of Trace Impurities: Common Laboratory Solvents, Organics, and Gases in Deuterated Solvents Relevant to the Organometallic Chemist" (PDF). Organometallics. 29 (9): 2176–2179. doi:10.1021/om100106e.
  8. MotorBoating. February 2010. pp. 76–.
  9. EEE. Mactier Publishing Corporation. 1965.
  10. Tim Gilles (1 January 2015). Automotive Service: Inspection, Maintenance, Repair. Cengage Learning. pp. 765–. ISBN 978-1-305-44593-2.
  11. Dugger, M. T.; Groysman, D.; Celina, M. C.; Alam, T. M.; Argibay, N.; Nation, B. L.; Prasad, S. V. (2014). "Mechanically-induced degradation of metallic sliding electrical contacts in silicone fluid at room temperature". 2014 IEEE 60th Holm Conference on Electrical Contacts (Holm). pp. 1–6. doi:10.1109/HOLM.2014.7031029. ISBN 978-1-4799-6068-2.
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