Lubricant

This article is about industrial lubricants. For other uses, seeLubricant (disambiguation).

Alubricant(sometimes shortened tolube) is a substance that helps to reducefrictionbetween 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 aslubricity.

In addition to industrial applications, lubricants are used for many other purposes. Other uses include cooking (oilsandfatsin use infrying pansand baking to prevent food sticking), to reduce rusting and friction inmachinery,through the use ofMotor oilandgrease,bioapplications on humans (e.g., lubricants forartificial joints), 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 soapshave 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 theRoman era,lubricants were based onolive oilandrapeseed oil,as well as animal fats. The growth of lubrication accelerated in theIndustrial Revolutionwith 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 ofvacuum distillationof petroleum, as described by theVacuum Oil Company.This technology allowed the purification of very nonvolatile substances, which are common in many lubricants.

Properties

A good lubricant generally possesses the following characteristics:

• A high boiling point and low freezing point (in order to stayliquidwithin a wide range of temperature)
• A highviscosity index
• Thermal stability
• Hydraulic stability
• Demulsibility
• Corrosion prevention
• A high resistance tooxidation
• Pour Point (the minimum temperature at which oil will flow under prescribed test conditions)

Formulation

Typically lubricants contain 90% base oil (most oftenpetroleumfractions, calledmineral oils) and less than 10%additives.Vegetable oilsor synthetic liquids such as hydrogenatedpolyolefins,esters,silicones,fluorocarbonsand many others are sometimes used as base oils. Additives deliver reduced friction and wear, increasedviscosity,improved viscosity index, resistance tocorrosionandoxidation,aging or contamination, etc.

Non-liquid lubricants include powders (drygraphite,PTFE,molybdenum disulphide,tungsten disulphide,etc.), PTFE tape used in plumbing, air cushion and others.Dry lubricantssuch as graphite, molybdenum disulphide and tungsten disulphide also offer lubrication at temperatures (up to 350 °C) higher than liquid and oil-based lubricants are able to operate. Limited interest has been shown in low friction properties ofcompacted oxide glaze layersformed at several hundred degrees Celsius in metallic sliding systems, however, practical use is still many years away due to their physically unstable nature.

Additives

Main article:Oil additive

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 pointdepressants are compounds that prevent crystallization of waxes. Long chainalkylbenzenesadhere to small crystallites of wax, preventing crystal growth.
• Anti-foaming agentsare typicallysiliconecompounds which increasesurface tensionin order to discourage foam formation.
• Viscosity index improvers(VIIs) are compounds that allow lubricants to remain viscous at higher temperatures. Typical VIIs arepolyacrylatesandbutadiene.
• Antioxidantssuppress 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 metalscatalyzethe oxidation process, dithiophosphates are more useful. In the latter application the additives are calledmetal deactivators.
• Detergentsensure the cleanliness of engine components by preventing the formation of deposits on contact surfaces at high temperatures.
• Corrosion inhibitors(rust inhibitors) are usually alkaline materials, such as alkylsulfonate salts, that absorb acids that would corrode metal parts.
• Anti-wearadditives form protective 'tribofilms' on metal parts, suppressingwear.They come in two classes depending on the strength with which they bind to the surface. Popular examples includephosphate estersandzinc dithiophosphates.
• Extreme pressure(anti-scuffing) additives form protective films on sliding metal parts. These agents are often sulfur compounds, such as dithiophosphates.
• Friction modifiersreduce 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 vehiclesbut other industrial, marine, and metal working applications are also big consumers of lubricants. Although air and other gas-based lubricants are known (e.g., influid bearings), liquid lubricants dominate the market, followed by solid lubricants.

Lubricants are generally composed of a majority of baseoilplus 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 fromcrude oil.TheAmerican Petroleum Institute(API) designates several types of lubricant base oil:

• Group I – Saturates < 90% and/orsulfur> 0.03%, andSociety of Automotive Engineers(SAE)viscosity index(VI) of 80 to 120

Manufactured by solvent extraction, solvent or catalytic dewaxing, and hydro-finishing processes. Common Group I base oil are 150SN (solvent neutral), 500SN, and 150BS (brightstock)

• Group II – Saturates > 90% and sulfur < 0.03%, and SAE viscosity index of 80 to 120

Manufactured by hydrocracking and solvent or catalytic dewaxing processes. Group II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has water-white color.

• Group III – Saturates > 90%, sulfur < 0.03%, and SAE viscosity index over 120

Manufactured by special processes such as isohydromerization. Can be manufactured from base oil or slax wax from dewaxing process.

• Group IV –Polyalphaolefins(PAO)
• Group V – All others not included above, such as naphthenics, polyalkylene glycols (PAG), andpolyesters.

The lubricant industry commonly extends this group terminology to include:

• Group I+ with aviscosity indexof 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 (derived ultimately from petroleum), "synthetic oils".

These include:

• Polyalpha-olefin(PAO)
• Syntheticesters
• Polyalkylene glycols(PAG)
• Phosphate esters
• Perfluoropolyether(PFPE)
• Alkylated naphthalenes(AN)
• Silicate esters
• Ionic fluids
• Multiply alkylated cyclopentanes(MAC)

Solid lubricants

Main article:Dry lubricant

PTFE:polytetrafluoroethylene(PTFE) 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 specialgreases,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,hexagonalboron nitride,molybdenum disulfideandtungsten disulfideare examples ofsolid 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.Cadmiumandgoldare used for plating surfaces which gives them good corrosion resistance and sliding properties,Lead,tin,zincalloys and variousbronzealloys 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 hydratedbrush polymerssuch 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

Biolubricantsare derived from vegetable oils and other renewable sources. They usually aretriglycerideesters (fats obtained from plants and animals). For lubricant base oil use, the vegetable derived materials are preferred. Common ones include high oleiccanola oil,castor oil,palm oil,sunflower seed oilandrapeseed oilfrom vegetable, andtall oilfrom 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 includelanolin(wool grease, a natural water repellent).

Whale oilwas a historically important lubricant, with some uses up to the latter part of the 20th century as a friction modifieradditiveforautomatic transmission fluid.

In 2008, the biolubricant market was around 1% of UK lubricant sales in a total lubricant market of 840,000 tonnes/year.

As of 2020,researchers at Australia'sCSIROhave been studyingsaffloweroil as an engine lubricant, finding superior performance and lower emissions thanpetroleum-based lubricants in applications such asengine-drivenlawn mowers,chainsawsand other agricultural equipment.Grain-growers trialling the product have welcomed the innovation, with one describing it as needing very little refining,biodegradable,abioenergyandbiofuel.The scientists have reengineered the plant usinggene silencing,creating a variety that produces up to 93% of oil, the highest currently available from any plant. Researchers atMontana State University’s Advanced Fuel Centre in the US studying the oil’s performance in a largediesel engine,comparing it with conventional oil, have described the results as a "game-changer".

Functions of lubricants

One of the largest applications for lubricants, in the form ofmotor oil,is protecting theinternal combustion enginesin motor vehicles and powered equipment.

Lubricant vs. anti-tack coating

Anti-tack or anti-stick coatingsare designed to reduce the adhesive condition (stickiness) of a given material. The rubber, hose, and wire and cable industries are the largest consumers of anti-tack products but virtually every industry uses some form of anti-sticking agent. Anti-sticking agents differ fromlubricantsin that they are designed to reduce the inherently adhesive qualities of a given compound while lubricants are designed to reduce friction between any two surfaces.

Keep moving parts apart

Lubricants are typically used to separatemoving partsin a system. This separation has the benefit of reducing friction, wear and surface fatigue, together with reduced heat generation, operating noise and vibrations. Lubricants achieve this in several ways. The most common is by forming a physical barrier i.e., a thin layer of lubricant separates the moving parts. This is analogous to hydroplaning, the loss of friction observed when a car tire is separated from the road surface by moving through standing water. This is termed hydrodynamic lubrication. In cases of high surface pressures or temperatures, the fluid film is much thinner and some of the forces are transmitted between the surfaces through the lubricant.

Reduce friction

Typically the lubricant-to-surfacefrictionis much less than surface-to-surface friction in a system without any lubrication. Thus use of a lubricant reduces the overall system friction. Reduced friction has the benefit of reducing heat generation and reduced formation of wear particles as well as improved efficiency. Lubricants may containpolaradditivesknown as friction modifiers that chemically bind to metal surfaces to reduce surface friction even when there is insufficient bulk lubricant present for hydrodynamic lubrication, e.g. protecting thevalve trainin a car engine at startup. The base oil itself might also be polar in nature and as a result inherently able to bind to metal surfaces, as withpolyolesteroils.

Transfer heat

Both gas and liquid lubricants can transfer heat. However, liquid lubricants are much more effective on account of their highspecific heat capacity.Typically the liquid lubricant is constantly circulated to and from a cooler part of the system, although lubricants may be used to warm as well as to cool when a regulated temperature is required. This circulating flow also determines the amount of heat that is carried away in any given unit of time. High flow systems can carry away a lot of heat and have the additional benefit of reducing the thermal stress on the lubricant. Thus lower cost liquid lubricants may be used. The primary drawback is that high flows typically require larger sumps and bigger cooling units. A secondary drawback is that a high flow system that relies on the flow rate to protect the lubricant from thermal stress is susceptible to catastrophic failure during sudden system shut downs. An automotive oil-cooledturbochargeris a typical example. Turbochargers get red hot during operation and the oil that is cooling them only survives as its residence time in the system is very short (i.e. high flow rate). If the system is shut down suddenly (pulling into a service area after a high-speed drive and stopping the engine) the oil that is in the turbo charger immediately oxidizes and will clog the oil ways with deposits. Over time these deposits can completely block the oil ways, reducing the cooling with the result that the turbo charger experiences total failure, typically with seizedbearings.Non-flowing lubricants such as greases and pastes are not effective at heat transfer although they do contribute by reducing the generation of heat in the first place.

Carry away contaminants and debris

Lubricant circulation systems have the benefit of carrying away internally generated debris and external contaminants that get introduced into the system to a filter where they can be removed. Lubricants for machines that regularly generate debris or contaminants such as automotive engines typically contain detergent and dispersant additives to assist in debris and contaminant transport to the filter and removal. Over time the filter will get clogged and require cleaning or replacement, hence the recommendation to change a car's oil filter at the same time as changing the oil. In closed systems such as gear boxes the filter may be supplemented by a magnet to attract any iron fines that get created.

It is apparent that in a circulatory system the oil will only be as clean as the filter can make it, thus it is unfortunate that there are no industry standards by which consumers can readily assess the filtering ability of various automotive filters. Poor automotive filters significantly reduce the life of the machine (engine) as well as make the system inefficient.

Transmit power

Main article:Hydraulics

Lubricants known ashydraulic fluidare used as theworking fluidin hydrostatic power transmission. Hydraulic fluids comprise a large portion of all lubricants produced in the world. Theautomatic transmission'storque converteris another important application for power transmission with lubricants.

Protect against wear

Lubricants prevent wear by reducing friction between two parts. Lubricants may also contain anti-wear or extreme pressure additives to boost their performance against wear and fatigue.

Prevent corrosion and rusting

Many lubricants are formulated with additives that form chemical bonds with surfaces or that exclude moisture, to prevent corrosion and rust. It reduces corrosion between two metallic surfaces and avoids contact between these surfaces to avoid immersed corrosion.

Seal for gases

Lubricants will occupy the clearance between moving parts through the capillary force, thus sealing the clearance. This effect can be used to seal pistons and shafts.

Fluid types

• Automotive
  • Motor oils
    • Petrol (Gasolines) engine oils
    • Diesel engineoils
  • Automatic transmissionfluid
  • Gearboxfluids
  • Brake fluids
  • Hydraulic fluids
  • Air conditioning compressoroils
• Tractor (one lubricant for all systems)
  • Universal Tractor Transmission Oil – UTTO
  • Super Tractor Oil Universal – STOU – includes engine
• Other motors
  • 2-stroke engine oils
• Industrial
  • Hydraulic oils
  • Air compressor oils
  • Food Grade lubricants
  • Gas Compressoroils
  • Gear oils
  • Bearing and circulating system oils
  • Refrigerator compressor oils
  • Steam and gas turbine oils
• Aviation
  • Gas turbineengine oils
  • Piston engineoils
• Marine
  • Crosshead cylinder oils
  • CrossheadCrankcaseoils
  • Trunk piston engine oils
  • Stern tube lubricants

"Glaze" formation (high-temperature wear)

A further phenomenon that has undergone investigation in relation to high-temperature wear prevention and lubrication is that of acompacted oxide layer glazeformation. Such glazes are generated by sintering a compacted oxide layer. Such glazes are crystalline, in contrast to the amorphous glazes seen in pottery. The required high temperatures arise from metallic surfaces sliding against each other (or a metallic surface against a ceramic surface). Due to the elimination of metallic contact and adhesion by the generation of oxide, friction and wear is reduced. Effectively, such a surface is self-lubricating.

As the "glaze" is already an oxide, it can survive to very high temperatures in air or oxidising environments. However, it is disadvantaged by it being necessary for the base metal (or ceramic) having to undergo some wear first to generate sufficient oxide debris.

Disposal and environmental impact

It is estimated that about 50% of all lubricants are released into the environment.Common disposal methods includerecycling,burning,landfilland discharge into water, though typically disposal in landfill and discharge into water are strictly regulated in most countries, as even small amount of lubricant can contaminate a large amount of water. Most regulations permit a threshold level of lubricant that may be present in waste streams and companies spend hundreds of millions of dollars annually in treating their waste waters to get to acceptable levels.

Burning the lubricant as fuel, typically to generate electricity, is also governed by regulations mainly on account of the relatively high level of additives present. Burning generates both airborne pollutants and ash rich in toxic materials, mainly heavy metal compounds. Thus lubricant burning takes place in specialized facilities that have incorporated special scrubbers to remove airborne pollutants and have access to landfill sites with permits to handle the toxic ash.

Unfortunately, most lubricant that ends up directly in the environment is due to the general public discharging it onto the ground, into drains, and directly into landfills as trash. Other direct contamination sources include runoff from roadways, accidental spillages, natural or man-made disasters, and pipeline leakages.

Improvement in filtration technologies and processes has now made recycling a viable option (with the rising price of base stock andcrude oil). Typically various filtration systems remove particulates, additives, and oxidation products and recover the base oil. The oil may get refined during the process. This base oil is then treated much the same as virgin base oil however there is considerable reluctance to use recycled oils as they are generally considered inferior. Basestock fractionally vacuum distilled from used lubricants has superior properties to all-natural oils, but cost-effectiveness depends on many factors. Used lubricant may also be used as refinery feedstock to become part of crude oil. Again, there is considerable reluctance to this use as the additives, soot, and wear metals will seriously poison/deactivate the critical catalysts in the process. Cost prohibits carrying out both filtration (soot, additives removal) and re-refining (distilling,isomerization, hydrocrack, etc.) however the primary hindrance to recycling still remains the collection of fluids as refineries need continuous supply in amounts measured in cisterns, rail tanks.

Occasionally, unused lubricant requires disposal. The best course of action in such situations is to return it to the manufacturer where it can be processed as a part of fresh batches.

Environment:Lubricants both fresh and used can cause considerable damage to the environment mainly due to their high potential of serious water pollution. Further, the additives typically contained in lubricant can be toxic to flora and fauna. In used fluids, the oxidation products can be toxic as well. Lubricant persistence in the environment largely depends upon the base fluid, however if very toxic additives are used they may negatively affect the persistence.Lanolinlubricants are non-toxic making them the environmental alternative which is safe for both users and the environment.

Societies and industry bodies

• American Petroleum Institute(API)
• Society of Tribologists and Lubrication Engineers(STLE)
• National Lubricating Grease Institute(NLGI)
• Society of Automotive Engineers(SAE)
• Independent Lubricant Manufacturer Association(ILMA)
• European Automobile Manufacturers Association(ACEA)
• Japanese Automotive Standards Organization(JASO)
• Petroleum Packaging Council(PPC)

Major publications

• Peer reviewed
  • ASME Journal of Tribology
  • Tribology International
  • Tribology Transactions
  • Journal of Synthetic Lubricants
  • Tribology Letters
  • Lubrication Science
• Trade periodicals
  • Tribology and Lubrication Technology
  • Fuels & Lubes International
  • Oiltrends
  • Lubes n' Greases
  • Compoundings
  • Chemical Market Review
  • Machinery lubrication

See also

• Lubrication– The presence of a material to reduce friction between two surfaces.
• Motor oil– Lubricant used for lubrication of internal combustion engines
• Oil analysis– Laboratory analysis of an oil based lubricant's properties and contaminants
• Penetrating oil– Low-viscosity oil
• Tribology– Science and engineering of interacting surfaces in relative motion

References

Sources

• API 1509, Engine Oil Licensing and Certification System, 15th Edition, 2002. Appendix E, API Base Oil Interchangeability Guidelines for Passenger Car Motor Oils and Diesel Engine Oils (revised)
• Boughton and Horvath, 2003, Environmental Assessment of Used Oil Management Methods, Environmental Science and Technology, V38
• I.A. Inman.Compacted Oxide Layer Formation under Conditions of Limited Debris Retention at the Wear Interface during High Temperature Sliding Wear of Superalloys,Ph.D. Thesis (2003), Northumbria UniversityISBN1-58112-321-3
• Mercedes-Benz oil recommendations, extracted from factory manuals and personal research
• Measuring reserve alkalinity and evaluation of wear dependence
• Testing used oil quality, list of possible measurements
• Leslie R. Rudnick (2003).Lubricant Additives: Chemistry and Applications.CRC Press.ISBN978-0-8247-4740-4.

External links

• SAE-ISO-AGMA viscosity conversion chart
• Chart of API Gravity and Specific gravity