A lubricant
is defined as a substance introduced between two surfaces in relative motion
with the aim of reducing the friction between them.
The basic
functions of a lubricant are friction reduction and wear prevention. Other
desirable features are contribution to the cooling, being anticorrosive, having
a cleaning action, preventing the
formation of deposits and facilitating the dispersion of pollutants.
Composition
The lubricant oils are formulated from a range
of basestocks and additives. The basestock provides the fluid layer that
separates the moving surfaces and eliminates heat and particles that cause the
wear, minimizing friction. Some properties of lubricating oils are improved by
additives, whose functions are described later.
Basestocks.
Classification
The basestocks are the major components of
lubricating oils for engines. They can be classified according to their main
components, its origin and its production process, among other classifications.
Depending on the major components, one can
speak of paraffinic base oils, naphthenic, and others, such as white oils,
electrical oils and process oils.
The paraffinic base oils are made from crude
oil with a high content of alkanes. They are characterized by good viscosity -
temperature relation, high viscosity index, good low temperature properties and
stability.
The naphthenic base oils are characterized by
containing no paraffins and high solvent power. The viscosity - temperature
relationship is worse than in the previous case and its viscosity is low -
medium.
The base oils can be classified according to
their origin, within two groups: mineral and synthetic base oils. Mineral base
oils are those obtained directly from the refinery, while the synthetic are
obtained after other processes. The oils can be classified according to their
production process:
• Mineral oils: produced by distillation and
refining using:
- Conventional technologies (e.g.
acid refining and solvent
extraction).
- Modern technologies such as hydrotreating and hydrocracking.
• Synthetic oils: produced by chemical
reaction:
- Synthetic hydrocarbons:
Polyalphaolefins, polyisobutenes dialkylbenzenes.
- Other synthetic fluids:
dicarboxylic esters, polyesters, neopentyl polyalkylene glycols, phosphate
esters, silicone oils, polyphenol ether, perfluoroalkyl ether
clorofluoroalquil ether, PAMA / PAO - cooligomers.
Mineral oils can be considered worse in
reference to properties like temperature stability, oxidation stability,
viscosity - temperature behavior, low temperature fluidity, high temperature
volatility, application temperature range, radiation stability and resistance
to ignition.
However, mineral oil may be superior with
regards to properties such as behavior towards hydrolysis, corrosion,
toxicology, compatibility with other materials, miscibility with mineral oils, compatibility
with seal materials, additive solubility, availability (in general or of
certain viscosity grades) and price.
Additives for engine
oils
The additives are substances capable of
improving the properties of the base oil. They are added from 5 to 20 % by
weight. They can be classified by their function. These include the anti-wear
additives, antioxidants, friction modifiers, demulsifiers, antifoaming agents,
detergents and dispersants. One additive usually have several functions.
• Antiwear additives: The aim of the antiwear
additives is to prevent wear of the parts that are in contact in the engine.
The best known are the ZDDP (zinc alkyl dithiophosphate) but following the
trend in the reduction of metal content in the engine lubricating oils anti-wear
additives based primarily on phosphorus and sulfur are used more and more.
• Antioxidant Additives: Antioxidants additives
are used to avoid the decomposition of oils by oxidation reactions. The
mechanisms to prevent degradation of the lubricant by oxidation are the removal
of free radicals and the decomposition of hydroperoxides. Some examples of
compounds that are usually used for this purpose are overbased phenates and
salicylates. The antiwear additives ZDDPs and P - S also have antioxidant
properties.
• Friction modifiers: The aim of the friction
modifiers is to reduce the friction and increase the fuel efficiency. Some of
the compounds that are commonly used for this purpose include carboxylic acids,
phosphoric, phosphonic acids and derivatives. Depending on the metal content,
we often speak of friction modifiers with metals (mainly molybdenum and
tungsten) and organic friction modifiers, which do not contain metals.
• Detergent and dispersant additives: The detergent
and dispersant additives are used to prevent carbon deposits on hot engine
parts and encourage the dispersal of pollutants to prevent the formation of
soot and sludge deposits. Compounds commonly used as detergent additives are
sulfonic acids, carboxylic acids and salicylic. With regards to dispersant
additives, succinimides and Mannich dispersants are generally used.
• Antifoaming additives: Anti-foaming additives
are used to avoid foaming, which may jeopardize the effective engine
lubrication. The effectiveness of the foams in the lubricant in wear prevention
is low Chemical compounds commonly used in the treatment of non-aqueous foams
are polydimethylsiloxanes. They are available in a wide range of molecular
weights. Furthermore, silicone-based fluids are characterized for having
limited solubility in the oil and less surface tension than oils from hydrocarbons.
Fluorosilicones show lower surface tension, but its use is limited by its high
price. Other commonly used antifoam compounds are aliphatic alkoxylated acids,
polyethers such as polyethylene glycol, branched polyvinyl ethers and poly
alcoxyamines.
New challenges of lubricating
oils for engines
Currently, manufacturers of lubricating oils for
engines are facing the following challenges:
- Reduction of metal content: Due
to environmental concerns, there is a trend in the reduction of metal
content. Particularly this reduction is considered in the sulfur,
phosphorus and total metal contents.
· Fuel
economy: fuel saving is one of the main research issues, because of the economic
and environmental consequences that it entails.
Due to this challenges, the research on this
area is full of opportunities to improve the properties of lubricant oil for
engines.
Author: Silvia Fernández Castejón. Chemical
Engineer. Complutense University of Madrid.
Bibliography:
Automotive Lubricants Reference Book. R.F. Haycock and
J.E. Hillier. Editorial SAE. 2nd
Edition.
Chemistry and Technology of Lubricants. R.M. Mortier,
M.F. Fox and S.T. Orszulik.
Editorial Springer. 3rd Edition.
Trends in lubricant oils for engines. British Petrol.
http://www.bp.com/genericarticle.do?categoryId=9013610&contentId=7021437
Trends in lubricant oils for engines. Shell.
http://www.shell.com/home/content/innovation/smarter_mobility/energy_efficienct_fuels_lubri
cants/advanced_lubricants/
Trends in lubricant oils for engines.. Shell. (2).
http://www.shell.com/home/content/lubes/media_centre/news_media_releases/2011/shell_conc
ept_lubricant.html
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