Extreme Pressure (EP) additives are a category of compounds used in lubricants, particularly in metalworking fluids and gear oils, to provide enhanced protection under high-pressure conditions. These additives are essential for reducing wear, friction, and scoring in metal-to-metal contact situations, where conventional lubricants may fail to provide adequate protection.

The primary function of EP additives is to form a protective film or boundary layer between metal surfaces under extreme pressure and temperature conditions. This film prevents direct contact between the metal surfaces, thereby reducing friction, wear, and heat generation. EP additives achieve this by chemically reacting with the metal surfaces to form a protective barrier, typically composed of sulfides, phosphates, or chlorides.

Common types of EP additives include:

Sulfur-Phosphorus Compounds: These additives, often referred to as sulfur-phosphorus EP additives, are the most widely used EP additives. They form a chemically active film on metal surfaces, providing excellent protection against welding, scuffing, and seizure under high-pressure conditions.

Chlorinated Compounds: Chlorinated EP additives are effective in reducing friction and wear in extreme pressure applications. They form a stable boundary film on metal surfaces, improving lubricity and preventing metal-to-metal contact.

Organic Compounds: Some EP additives are based on organic compounds, such as fatty acids or esters. These additives function by adsorbing onto metal surfaces, forming a protective layer that reduces friction and wear.

EP additives are commonly used in industrial applications such as metal cutting, forming, and machining, as well as in automotive gear oils, transmission fluids, and hydraulic fluids. However, it's important to note that while EP additives provide excellent protection under extreme pressure conditions, they may have limitations in certain applications. For example, sulfur-containing EP additives