EP additive
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EP additives, or extreme pressure additives, are additives for lubricants with role to decrease wear of the parts of the gears exposed to very high pressures. They are also added to cutting fluids for machining of metal.
EP additives are usually used in applications such as gearboxes, while AW additives are used with lighter loads such as bushings.
EP additives typically contain organic sulfur, phosphorus or chlorine 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 additive and the surface is confined to this area.
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
- Polymer esters
- Organophosphates (phosphoric acid esters), eg. tricresyl phosphate, TCP)
- Zinc phosphates (zinc dialkyl dithio phosphate, ZDDP)
- Polysulfides
- Molybdenum compounds
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 steel parts. [1]
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 organophosphates, 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.
Polysulfides serve as carriers of inactive and active sulfur.
Molybdenum compounds decompose under high pressure to form an in-situ deposited layer of molybdenum disulfide. Molybdenum dithiocarbamates are used as additives for greases.
Sulfur and phosphorus containing EP additives can cause serious problems in gears with parts made of bronze, brass and other copper alloys.
See also:
