- EP additive
EP additives, or extreme pressure additives, are additives for
lubricants with a role to decrease wear of the parts of the gears exposed to very high pressures. They are also added to cutting fluids for machiningof metals.
EP additives are usually used in applications such as gearboxes, while
AW additives are used with lighter load applications such as hydraulic and automotive engines.
gear oils 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 oils. The sulfur or chlorine compounds contained in them can react with water and combustion byproducts, forming acids that facilitate corrosionof the engine parts and bearings. [ [http://www.pecuniary.com/faq/EP-additives.html pecuniary.com FAQ] ]
EP additives typically contain organic
sulfur, phosphorusor chlorinecompounds, including sulfur- phosphorusand sulfur-phosphorus- boroncompounds, which chemically react with the metal surface under high pressureconditions. 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 reactionbetween the additives and the surface is confined to this area.
The early EP additives were based on
leadsalts of fatty acids ("lead soaps"), "active sulfur" compounds (eg. thiols and elementary sulfur), and chlorinated compounds. During the 1950s the use of lead soaps was eliminated and replaced by zincand phosphorus compounds such as Zinc dithiophosphate. [ [http://www.noria.com/learning_center/category_article.asp?articleid=496 noria.com learning center] ]
Some of the EP additives are:
* Dark inactive sulphurized fat
* Dark active sulphurized fat
* Dark active sulphur hydrocarbon
* Short and medium chain chlorinated
alkanes (see chlorinated hydrocarbons and chlorinated paraffins)
Esters of chlorendic acid
Molybdenumcompounds Aliphatic chlorinated hydrocarbons ( 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 fluids, their role is largely confined to formulations for forming complex stainless steelparts. [http://www.umweltbundesamt.de/umweltvertraegliche-stoffe-e/pressure.htm]
The activity of halogenated hydrocarbons increases with decreasing stability of the
carbon- halogenbond. At local contact temperatures ranging between 305-330 °C, the additive thermally decomposes and the reactive halogen atoms form a surface layer of iron halogenideson 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 carbonare 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.
organophosphates, with or without zinc, have excellent high-pressure and antiwear properties, and provide corrosionprotection 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. Polysulfides serve as carriers of inactive and active sulfur. Molybdenumcompounds decompose under high pressure to form an in-situ deposited layer of molybdenum disulfide. Molybdenum dithiocarbamates are used as additives for greases.
Sulfur containing EP additives can cause
corrosionproblems in gears with parts made of bronze, brassand other copper alloys when high temperature environment is encountered.
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