miércoles, 1 de octubre de 2014

Engine Lubricant oils. Current challenges.






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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