Extreme pressure additive

EP additives ( extreme pressure ) additives are added to lubricants and improve their tribological properties, ie they prevent the welding of two rubbing against each other metallic materials.


Once extremely high pressures or loads between two materials rubbing against each other occur, the welding of the two materials can occur. In this case, EP additives are indispensable in lubricants. Under high pressures and loads, high temperatures are in the lubricant. This is from the EP additive sulfur ( sulfur carrier ), released a phosphoric acid derivative ( phosphorus-containing compounds ) or chlorinated hydrocarbons ( chlorinated paraffins). The liberated substance reacts under these conditions immediately with the metal surface to form metal sulfides, phosphates or chlorides. The compounds form on the metal surface layers, which are sheared under a high pressure, whereby the welding of the metal surfaces is prevented. At the same time, however, EP additives can increase the wear, as is lost due to the resulting metal -metal compounds from the surface of the materials.

Requirements and combination of additives

The following tribological demands on lubricants:

  • Reduction of friction
  • Reduction in wear
  • Protection against welding

Since the EP additives protect prior to a welding, but even increase wear, the additives in lubricants must be matched carefully to each other. In order to find the optimum between the additives, many trials and experience are necessary. Overdoses of certain additives may communicate its benefits in turn.

Used chemical compounds

Usually sulfur, phosphorus or chlorine-containing organic compounds are used as EP additives. The compounds have to be constructed so that in the lubricant only when needed (ie, at high pressures or loads), the substance is released, which forms on the metal surface a protective layer, such as sulphides, phosphates or chlorides.

Sulfur-containing compounds (sulfur carrier )

Sulfur-containing compounds columns at high pressure sulfur from that on the metal surface in a layer - for iron from the iron sulphide, iron ( II ) sulfide ( FeS ), iron ( II) - disulfide ( FeS2 ) and iron (III ) sulfide ( Fe2S3 ) - leads.

Sulfur carriers are classic EP additives and up to 40 wt -% sulfur. Mostly these are organic compounds with double bonds (olefins, esters, fatty acids or triglycerides), which are sulfur cured. A distinction is made inactive, and active sulfur carriers, depending on whether the sulfur is released at a low or high temperature. Sulfur carrier with polar groups act as well as a friction modifier.

In the district of Mannheim, Rhein Chemie Rheinau provides numerous products here, which differ in the chemical backbone and the sulfur content.

Phosphorus-containing compounds

These compounds cause high pressure to form metal phosphates on the metal surfaces.


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

Dimercaptothiadiazole derivatives

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

Chlorine-containing compounds ( chlorinated paraffins)

Chloroparaffins split off at high pressure and high temperature hydrogen chloride which reacts with the metallic material layer and a chloride, such as iron (II ) chloride ( FeCl2 ), and iron (III ) chloride ( FeCl3) 2 forms.

Chlorinated paraffins are sulfur-free EP additives, whose technology is obsolete and its use is declining. The mixtures used chlorinated hydrocarbons are highly effective, but health was precarious. Due to their extremely high stability of chlorinated paraffins cause high disposal costs.