Lubricant


A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.
In addition to industrial applications, lubricants are used for many other purposes. Other uses include cooking, to reduce rusting and friction in machinery, through the use of motor oil and grease, bioapplications on humans, ultrasound examination, medical examination, and sexual intercourse. It is mainly used to reduce friction and to contribute to a better, more efficient functioning of a mechanism.

History

Lubricants have been in some use for thousands of years. Calcium soaps have been identified on the axles of chariots dated to 1400 BC. Building stones were slid on oil-impregnated lumber in the time of the pyramids. In the Roman era, lubricants were based on olive oil and rapeseed oil, as well as animal fats. The growth of lubrication accelerated in the Industrial Revolution with the accompanying use of metal-based machinery. Relying initially on natural oils, needs for such machinery shifted toward petroleum-based materials early in the 1900s. A breakthrough came with the development of vacuum distillation of petroleum, as described by the Vacuum Oil Company. This technology allowed the purification of very non-volatile substances, which are common in many lubricants.

Properties

A good lubricant generally possesses the following characteristics:
Typically lubricants contain 90% base oil and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefins, esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc.
Non-liquid lubricants include powders, PTFE tape used in plumbing, air cushion and others. Dry lubricants such as graphite, molybdenum disulphide and tungsten disulphide also offer lubrication at temperatures higher than liquid and oil-based lubricants are able to operate. Limited interest has been shown in low friction properties of compacted oxide glaze layers formed at several hundred degrees Celsius in metallic sliding systems; however, practical use is still many years away due to their physically unstable nature.

Additives

A large number of additives are used to impart performance characteristics to the lubricants. Modern automotive lubricants contain as many as ten additives, comprising up to 20% of the lubricant, the main families of additives are:
  • Pour point depressants are compounds that prevent crystallization of waxes. Long chain alkylbenzenes adhere to small crystallites of wax, preventing crystal growth.
  • Anti-foaming agents are typically silicone compounds which increase surface tension in order to discourage foam formation.
  • Viscosity index improvers are compounds that allow lubricants to remain viscous at higher temperatures. Typical VIIs are polyacrylates and butadiene.
  • Antioxidants suppress the rate of oxidative degradation of the hydrocarbon molecules within the lubricant. At low temperatures, free radical inhibitors such as hindered phenols are used, e.g. butylated hydroxytoluene. At temperatures >90 °C, where the metals catalyze the oxidation process, dithiophosphates are more useful. In the latter application the additives are called metal deactivators.
  • Detergents ensure the cleanliness of engine components by preventing the formation of deposits on contact surfaces at high temperatures.
  • Corrosion inhibitors are usually alkaline materials, such as alkylsulfonate salts, that absorb acids that would corrode metal parts.
  • Anti-wear additives form protective 'tribofilms' on metal parts, suppressing wear. They come in two classes depending on the strength with which they bind to the surface. Popular examples include phosphate esters and zinc dithiophosphates.
  • Extreme pressure additives form protective films on sliding metal parts. These agents are often sulfur compounds, such as dithiophosphates.
  • Friction modifiers reduce friction and wear, particularly in the boundary lubrication regime where surfaces come into direct contact.
In 1999, an estimated 37,300,000 tons of lubricants were consumed worldwide. Automotive applications dominate, including electric vehicles but other industrial, marine, and metal working applications are also big consumers of lubricants. Although air and other gas-based lubricants are known, liquid lubricants dominate the market, followed by solid lubricants.
Lubricants are generally composed of a majority of base oil plus a variety of additives to impart desirable characteristics. Although generally lubricants are based on one type of base oil, mixtures of the base oils also are used to meet performance requirements.

Mineral oil

The term "mineral oil" is used to refer to lubricating base oils derived from crude oil. The American Petroleum Institute designates several types of lubricant base oil:
  • Group I – Saturates < 90% and/or sulfur > 0.03%, and Society of Automotive Engineers viscosity index of 80 to 120
  • Group II – Saturates > 90% and sulfur < 0.03%, and SAE viscosity index of 80 to 120
  • Group III – Saturates > 90%, sulfur < 0.03%, and SAE viscosity index over 120
  • Group IV – Polyalphaolefins
  • Group V – All others not included above, such as naphthenics, polyalkylene glycols, and polyesters.
The lubricant industry commonly extends this group terminology to include:
  • Group I+ with a viscosity index of 103–108
  • Group II+ with a viscosity index of 113–119
  • Group III+ with a viscosity index of at least 140
Can also be classified into three categories depending on the prevailing compositions:
  • Paraffinic
  • Naphthenic
  • Aromatic

    Synthetic oils

Petroleum-derived lubricant can also be produced using synthetic hydrocarbons, "synthetic oils".
These include:
PTFE: polytetrafluoroethylene is typically used as a coating layer on, for example, cooking utensils to provide a non-stick surface. Its usable temperature range up to 350 °C and chemical inertness make it a useful additive in special greases, where it can function both as a thickener and a lubricant. Under extreme pressures, PTFE powder or solids is of little value as it is soft and flows away from the area of contact. Ceramic or metal or alloy lubricants must be used then.
Inorganic solids: Graphite, hexagonal boron nitride, molybdenum disulfide and tungsten disulfide are examples of solid lubricants. Some retain their lubricity to very high temperatures. The use of some such materials is sometimes restricted by their poor resistance to oxidation (e.g., molybdenum disulfide degrades above 350 °C in air, but 1100 °C in reducing environments.
Metal/alloy: Metal alloys, composites and pure metals can be used as grease additives or the sole constituents of sliding surfaces and bearings. Cadmium and gold are used for plating surfaces which gives them good corrosion resistance and sliding properties, Lead, tin, zinc alloys and various bronze alloys are used as sliding bearings, or their powder can be used to lubricate sliding surfaces alone.

Aqueous lubrication

Aqueous lubrication is of interest in a number of technological applications. Strongly hydrated brush polymers such as PEG can serve as lubricants at liquid solid interfaces. By continuous rapid exchange of bound water with other free water molecules, these polymer films keep the surfaces separated while maintaining a high fluidity at the brush–brush interface at high compressions, thus leading to a very low coefficient of friction.

Biolubricant

Biolubricants are derived from vegetable oils and other renewable sources. They usually are triglyceride esters. For lubricant base oil use, the vegetable derived materials are preferred. Common ones include high oleic canola oil, castor oil, palm oil, sunflower seed oil and rapeseed oil from vegetable, and tall oil from tree sources. Many vegetable oils are often hydrolyzed to yield the acids which are subsequently combined selectively to form specialist synthetic esters. Other naturally derived lubricants include lanolin.
Whale oil was a historically important lubricant, with some uses up to the latter part of the 20th century as a friction modifier additive for automatic transmission fluid.
In 2008, the biolubricant market was around 1% of UK lubricant sales in a total lubricant market of 840,000 tonnes/year.
, researchers at Australia's CSIRO have been studying safflower oil as an engine lubricant, finding superior performance and lower emissions than petroleum-based lubricants in applications such as engine-driven lawn mowers, chainsaws and other agricultural equipment. Grain-growers trialling the product have welcomed the innovation, with one describing it as needing very little refining, biodegradable, a bioenergy and biofuel. The scientists have reengineered the plant using gene silencing, creating a variety that produces up to 93% of oil, the highest currently available from any plant. Researchers at Montana State University’s Advanced Fuel Centre in the US studying the oil’s performance in a large diesel engine, comparing it with conventional oil, have described the results as a "game-changer".