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Extreme Pressure Antiwear Agent as the Main Classification of Oil Additive

2020-11-18 14:07:04  Knowledge

What is extreme pressure antiwear agent?

In boundary lubrication, when the metal surface is only under moderate load, if there is an additive that can be adsorbed on the metal surface or wears violently with the metal surface, this additive is called an anti-wear additive. When the metal surface is subjected to a high load, a large number of metal surfaces are in direct contact and a large amount of heat is generated, and the film formed by the anti-wear agent is also destroyed and no longer protects the metal surface. If there is an additive that can interact with the metal surface The chemical reaction generates a chemical reaction film, which acts as a lubrication to prevent metal surface scratches and even fusion welding. This most demanding boundary lubrication is usually called extreme pressure lubrication, and this additive is called extreme pressure additive.

Chlorine extreme pressure antiwear agent

Chlorine-containing extreme pressure anti-wear agent has low price and good performance. It has good compounding effect with phosphorus and sulfur-containing additives. It is one of the earliest extreme pressure anti-wear components used in the lubricant industry. Commonly used chlorine-containing extreme pressure antiwear agents are aliphatic chloride and aromatic chloride. The use effect of chlorine-containing extreme pressure antiwear agent mainly depends on its molecular structure and the chemical activity of chlorine atoms. When the chlorine atom is at the end of the aliphatic hydrocarbon, the activity is the highest, and the extreme pressure antiwear performance is the best; when the chlorine atom is in the middle of the carbon chain, the activity is second; when the chlorine atom is on the carbon ring, the activity is the worst, and the corresponding extreme pressure antiwear performance is The weakest. Therefore, aliphatic chlorides have poor stability, strong chemical activity, and good extreme pressure and anti-wear properties, but they are easy to cause metal corrosion, such as Chlorinated Paraffin; aromatic chlorides have good stability, low chemical activity, and extreme pressure resistance. The abrasiveness is poor, but the corrosiveness is less, such as Pen-tachlorobiphenyl (Pen-tachlorobiphenyl).

In recent years, as people’s awareness of environmental protection has been gradually strengthened and environmental protection regulations have become increasingly strict, the use of chlorine-containing additives has gradually decreased due to toxicity and corrosive problems. For example, chlorine-containing gear oils have been replaced by sulfur-phosphorus gear oils. 

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Sulfur-containing extreme pressure antiwear agent

Studies have shown that the extreme pressure and antiwear properties of sulfur-containing compounds are closely related to the C-S bond energy in their molecular structure. The smaller the C-S bond energy, the easier it is to break and form a protective film during the friction process, resulting in a better extreme pressure and anti-wear effect. The mechanism of action of the sulfur-containing extreme pressure antiwear agent is to first adsorb on the surface of the friction pair to reduce the friction between the friction pairs; as the load increases, the contact temperature between the friction pairs rises rapidly, and the sulfur-containing compound reacts with the metal to form sulfur The iron alcohol film has an anti-wear effect; with the gradual increase of the load, the CS bond begins to break, forming an extreme pressure chemical reaction film of FeS, which has the effect of anti-abrasion and anti-sintering.

Due to the high melting point of iron sulfide (1193℃-1199℃), it has good heat resistance effect, and sulfur-containing extreme pressure antiwear agent has very high sintering load. However, the iron sulfide film does not have the layered structure of graphite, molybdenum disulfide, ferric chloride, etc., and its anti-friction performance is poor. At the same time, although the iron sulfide film has high melting point and hardness, its brittleness is also high. In addition, considering the sulfur content The compound has strong corrosive wear, so the anti-wear performance of sulfur additives is poor.

The iron sulfide film formed by sulfur additives in the friction process has a high melting point and good extreme pressure performance. It is suitable for use under severe conditions such as high temperature and high load. It is a commonly used extreme pressure and antiwear additive in lubricating grease. At present, commonly used sulfur-containing extreme pressure antiwear agents mainly include sulfurized olefins, sulfurized oils (esters), polysulfide compounds, dibenzyl disulfide, and xanthogenic acid.

Phosphorus-containing extreme pressure antiwear agent

Phosphorus-containing additives have multiple functions such as extreme pressure, anti-wear, anti-friction, and anti-oxidation. They have good thermal stability, low corrosivity, and good compounding effects with other functional additives. They have been widely concerned by researchers and have developed rapidly in recent years. . There are also different opinions on the mechanism of action of phosphorus additives. Early scholars believed that phosphorus-containing compounds decomposed under the action of the instantaneous high temperature at the raised points of the friction surface, and formed iron phosphide with iron, and then formed the low-melting co-fusion gold with iron to flow to the recesses, smoothing the friction surface and preventing wear. Call this effect "chemical polishing". In recent years, some scholars believe that under boundary lubrication conditions, phosphide and iron do not produce iron phosphide, but a mixture of iron phosphite.

Phosphate is first adsorbed on the iron surface, and then the C-O bond breaks under boundary conditions to form iron phosphite: or iron phosphate organic film, which plays an anti-wear effect; under extreme pressure conditions, the organic iron phosphate film further reacts to form inorganic The iron phosphate reaction film prevents direct contact between metals, thereby protecting the metals and acting as extreme pressure. The variety of phosphorus-containing additives is more complex, not only in the type of compound, but also in the element composition. Some contain a single element of phosphorus, some contain sulfur and phosphorus, and some contain sulfur, phosphorus, nitrogen and other elements. Even if the element composition is the same, the compound structure can be different, and different phosphorus compounds are used for different purposes. Generally, phosphorus-containing additives can be roughly classified into phosphorus type, phosphorus nitrogen type, sulfur phosphorus type, sulfur phosphorus nitrogen type, phosphorus nitrogen boron type, sulfur phosphorus/sulfur phosphorus nitrogen boron type, etc. according to the active elements therein.

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