Grease (lubricant)
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Grease is a lubricant of higher initial viscosity than oil, consisting originally of a calcium, sodium or lithium soap jelly emulsified with mineral oil.
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[edit] Properties
A true grease consists of an oil and/or other fluid lubricant that is mixed with another thickener substance, a soap, to form a solid. Greases are a type of shear-thinning or pseudo-plastic fluid, which means that the viscosity of the fluid is reduced under shear. After sufficient force to shear the grease has been applied, the viscosity drops and approaches that of the base lubricant, such as the mineral oil. This sudden drop in shear force means that grease is considered a plastic fluid, and the reduction of shear force with time makes it thixotropic. It is often applied using a grease gun, which applies the grease to the part being lubricated under pressure, forcing the solid grease into the spaces in the part.
Soaps are the most common emulsifying agent used, and the type of soap depends on the conditions in which the grease is to be used. Different soaps provide differing levels of temperature resistance (relating to both viscocity and volatility), water resistance, and chemical reactivity. Powdered solids may also be used, such as clay, which was used to emulsify early greases and is still used in some inexpensive, low performance greases.
[edit] Uses
Greases are employed where heavy pressures exist, where oil drip from the bearings is undesirable, and/or where the motions of the contacting surfaces are discontinuous so that it is difficult to maintain a separating lubricant film in the bearing. Grease-lubricated bearings have greater frictional characteristics at the beginning of operation. Under shear, the viscosity drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. Calcium- and sodium-based greases are the most commonly used; sodium-based greases have higher melting point than calcium-based greases but are not resistant to the action of water. Lithium-based grease has a drip temperature at 190 to 220 C (350° to 400°F) and it resists moisture, hence it is commonly used as lubricant in household products such as garage door openers.
Grease used for axles are composed of a compound of fatty oils to which, is added tar, graphite, or mica to increase the durability of the grease and give it a better surface.
[edit] Additives
Teflon is added to some greases to improve their lubricating properties. Gear greases consist of rosin oil, thickened with lime and mixed with mineral oil, with some percentage of water. Special-purpose greases contain glycerol and sorbitan esters. They are used, for example, in low-temperature conditions. Some greases are labeled "EP", which indicates "extreme pressure". Under high pressure or shock loading, normal grease can be compressed to the extent that the greased parts come into physical contact, causing friction and wear. EP grease contains solid lubricants, usually graphite and/or molybdenum, to provide protection under heavy loadings. The solid lubricants bond to the surface of the metal, and prevent metal-to-metal contact and the resulting friction and wear when the lubricant film gets too thin.
[edit] Other greases
Other types of lubricating material that are soft solids or high viscocity liquids at room temperature are often called grease, though they may not exhibit the shear-thinning properties typical of the oil/soap grease. Petroleum jellies, such as Vaseline, are also sometimes called greases, and are commonly used for lubricating food-handling equipment.
[edit] Silicone grease
Silicone grease is an amorphous fumed, silica-thickened, polysiloxane-based compound, which can be used to provide lubrication and corrosion resistance. Since it is not oil-based, it is often used where oil-based lubricants would attack rubber seals. Silicone greases also maintain stability under high temperatures, and are often used, in pure form or mixed with zinc oxide, to join heat sinks to computer CPUs.
[edit] Fluoroether-based grease
Fluoropolymers containing C-O-C (ether) bonds for flexibility are soft, often used as greases in demanding environments due to its inertness. Krytox by duPont is a prominent example.
[edit] Laboratory grease
- See also: Laboratory glassware
Apiezon, silicone-based, and fluoroether-based greases are all used commonly in laboratories for lubricating stopcocks and ground glass joints. The grease helps to prevent joints from "freezing", as well as ensuring high vacuum systems are properly sealed.
Apiezon or similar hydrocarbon based greases are the cheapest, and most suitable for high vacuum applications. However, they dissolve in many organic solvents. This quality makes clean-up with pentane or hexanes trivial, but also easily leads to contamination of reaction mixtures.
Silicone-based greases are cheaper than fluoroether-based greases. They are relatively inert and generally do not affect reactions, though reaction mixtures often get contaminated (detected through NMR near δ 0). Silicone-based greases are not easily removed with solvent, but they are removed efficiently by soaking in a base bath.
Fluoroether-based greases are inert to many substances including solvents, acids, bases, and oxidizers. They are, however, expensive, and are not easily cleaned away.
[edit] Water soluble grease analogs
In some cases, the lubrication and high viscosity of a grease are desired in situations where non-toxic, non-oil based materials are required. Carboxymethyl cellulose, or CMC, is one popular material used to create a water based analog of greases. CMC serves to both thicken the solution and add a lubricating effect, and often silicone based lubricants are added for additional lubrication. The most common form of this type of lubricant, used as a surgical and personal lubricant, is K-Y Jelly.
[edit] References
- This article incorporates text from the public domain 1911 edition of The Grocer's Encyclopedia.
[edit] External links
- U.S. Army Corps of Engineers grease definition and application guide (PDF file)
- Silicone grease (includes link to MSDS and application guide)
