EP additive
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'''EP additives''', or '''extreme pressure additives''', are [[oil additive|additives]] for [[lubricant]]s with a role to decrease wear of the parts of the [[gear]]s exposed to very high pressures. They are also added to [[cutting fluid]]s for [[machining]] of [[metal]]s.
EP additives are usually used in applications such as [[transmission (mechanics)|gearboxes]], while [[AW additive]]s are used with lighter load applications such as hydraulic and automotive [[engine]]s.
EP [[gear oil]]s perform well over a range of temperatures, speeds and gear sizes to help prevent damage to the gears during starting and stopping of the engine. Unlike AW additives, EP additives are rarely used in [[motor oil]]s. The sulfur or chlorine compounds contained in them can react with water and combustion byproducts, forming acids that facilitate [[corrosion]] of the engine parts and bearings.<ref>[http://www.pecuniary.com/faq/EP-additives.html pecuniary.com FAQ]</ref>
EP additives typically contain organic [[sulfur]], [[phosphorus]] or [[chlorine]] compounds, including [[sulfur]]-[[phosphorus]] and sulfur-phosphorus-[[boron]] compounds, which chemically react with the metal surface under high [[pressure]] conditions. Under such conditions, small irregularities on the sliding surfaces cause localized flashes of high [[temperature]] (300-1000 °C), without significant increase of the average surface temperature. The [[chemical reaction]] between the [[Gasoline additive|additive]]s and the surface is confined to this area.
The early EP additives were based on [[lead]] salts of [[fatty acid]]s ("lead [[soap]]s"), "active sulfur" compounds (eg. [[thiol]]s and elementary sulfur), and chlorinated compounds. During the 1950s the use of lead soaps was eliminated and replaced by [[zinc]] and phosphorus compounds [[(ZDDP)]]. <ref>[http://www.noria.com/learning_center/category_article.asp?articleid=496 noria.com learning center]</ref>
Some of the EP additives are:
* Dark inactive sulphurized fat
* Dark active sulphurized fat
* Dark active sulphur hydrocarbon
* Short and medium chain chlorinated [[alkane]]s (see [[chlorinated hydrocarbon]]s and [[chlorinated paraffins]])
* [[Ester]]s of [[chlorendic acid]]
* Polymer [[ester]]s
* [[Polysulfide]]s
* [[Molybdenum]] compounds
[[Aliphatic]] [[chlorinated hydrocarbon]]s ([[chlorinated paraffins]]) are cheap and efficient, however they persist in environment and have strong tendency for [[bioaccumulation]]. Therefore they are being replaced with alternatives. In [[cutting fluid]]s, their role is largely confined to formulations for forming complex [[stainless steel]] parts. [http://www.umweltbundesamt.de/umweltvertraegliche-stoffe-e/pressure.htm]
The activity of halogenated hydrocarbons increases with decreasing stability of the [[carbon]]-[[halogen]] [[bond]]. At local contact temperatures ranging between 305-330 °C, the additive thermally decomposes and the reactive halogen atoms form a surface layer of [[iron]] [[halogenides]] on the part surface. Eventual failure of the contact point comes when the contact temperature exceeds the melting point of the iron halide layer. Under such conditions, small particles of [[carbon]] are generated as well. Some compounds used in lubricant additives are chloroalkanes, trichloromethyl phosphine acids, organic esters of a-acetoxy-b,b,b-trichloroethyl phosphonic acid, trichloromethyl esters of [[phosphoric acid]], trichloromethyl derivates of sulfur, trichloroacetoxy compounds, esters or amine salts of [[chlorendic acid]], 1,2,3,4,7,7-hexachloro-5-dimethylbicyclo[2.2.1]-2-heptene, etc.
Oil-soluble [[organophosphate]]s, with or without zinc, have excellent high-pressure and antiwear properties, and provide [[corrosion]] protection especially in presence of chlorinated hydrocarbons. ZDDP starts decomposing at 130-170 °C, while the activation temperature of TCP typically exceeds 200 °C. Their reaction products form a chemically bonded lubricating film on the surfaces.
[[Polysulfide]]s serve as carriers of inactive and active [[sulfur]]. <!-- Expand on its role. -->
[[Molybdenum]] compounds decompose under high pressure to form an in-situ deposited layer of [[molybdenum disulfide]]. Molybdenum [[dithiocarbamate]]s are used as additives for [[Petroleum|grease]]s.
Sulfur containing EP additives can cause [[corrosion]] problems in gears with parts made of [[bronze]], [[brass]] and other [[copper]] [[alloy]]s when high temperature environment is encountered.
==References==
<references />
==See also==
* [[Metalworking]]
[[Category:Lubricants]]
[[Category:Oil additives]]
[[de:EP-Additiv]]