EP additive

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EP additives ( Extreme Pressure Additives ) are added to lubricants and improve their tribological properties, i. That is, they prevent the welding of two metallic materials rubbing against one another .

Mode of action

As soon as extremely high pressures or loads occur between two materials rubbing against one another, the two materials can weld. In this case, EP additives in lubricants are indispensable. High temperatures arise in the lubricant under high pressures or loads. Here, sulfur (sulfur carrier ), a phosphoric acid derivative (phosphorus-containing compounds) or chlorinated hydrocarbons (chlorinated paraffins) are released from the EP additive . Under these conditions, the released substance reacts immediately with the metal surface to form metal sulfides, phosphates or chlorides. The connections form layers on the metal surface that are sheared off under the high pressure, which prevents the metal surfaces from welding. At the same time, however, EP additives can increase wear , as the resulting metal compounds lose metal from the surface of the materials.

Requirements and combination of additives

The following tribological requirements are placed on lubricants:

  • Reduction of friction
  • Reduction of wear and tear
  • Protection against welding

Since the EP additives protect against welding, but also increase wear, the additives in lubricants must be carefully coordinated with one another. A lot of trials and experience are necessary to find the optimum between the additives. Overdosing certain additives can in turn reverse their benefits.

Chemical compounds used

As a rule, organic compounds containing sulfur , phosphorus or chlorine are used as EP additives. The connections must be structured in such a way that a substance is released in the lubricant only when required (i.e. at high pressures or loads), which forms a protective layer on the metal surface, for example made of sulfides , phosphates or chlorides .

Sulfur-containing compounds (sulfur carriers)

Sulfur-containing compounds split off sulfur at high pressure, which forms a layer on the metal surface - in the case of iron from the iron sulfides iron (II) sulfide (FeS), iron (II) disulfide (FeS 2 ) or iron (III) sulfide ( Fe 2 S 3 ) - leads.

Sulfur carriers are classic EP additives and can contain up to 40% by weight of sulfur. Most are organic compounds with double bonds ( olefins , esters , fatty acids or triglycerides ) that are sulfur-crosslinked. A distinction is made between inactive and active sulfur carriers, depending on whether sulfur is released at a low or high temperature. Sulfur carriers with polar groups also act as friction modifiers .

Compounds containing phosphorus

At high pressure, these compounds lead to the formation of metal phosphates on the metal surfaces.

Dialkyl dithiophosphates

Dialkyldithiophosphates are multifunctional additives. They act primarily as an AW additive , but also as an EP additive and as an antioxidant .

Dimercaptothiadiazole derivatives

Dimercaptothiadiazole - derivatives are classic non-ferrous metal inhibitors. But they also act as an EP additive.

Compounds containing chlorine (chlorinated paraffins)

Chlorinated paraffins split off hydrogen chloride at high pressure or high temperature , which reacts with the metallic material and forms a chloride layer, for example made of iron (II) chloride (FeCl 2 ) and iron (III) chloride (FeCl 3 ) 2 .

Chlorinated paraffins are sulfur-free EP additives whose technology is out of date and their use is in decline. The mixtures of chlorinated hydrocarbons used are highly effective, but very harmful to health. Due to their extremely high stability, chlorinated paraffins cause high disposal costs .

literature

  • Fritz Klocke, Wilfried König : Manufacturing process: forming. Volume 4 of the production process series. 5th edition, Springer, 2006, ISBN 3-540-23650-3
  • Werner Baumann, Bettina Herberg-Liedtke: Chemicals in metal processing: data and facts on environmental protection. Springer, 1995, ISBN 3-540-60094-9
  • Kurt Lange, Mathias Liewald (ed.): Umformtechnik. Handbook for Industry and Science, 2nd edition, Springer Verlag, Berlin / Heidelberg 1990, ISBN 978-3-662-10687-7 .
  • Hans-Jürgen Blanke, Wilfried J. Bartz, Uwe Jens Möller: Expert Practice Lexicon Tribology Plus. 2010 Terms for study and work, 2nd edition, Expert Verlag, Renningen 2000, ISBN 3-8169-0691-5 , p. 215.

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