Sulfur-Based Extreme Pressure (EP) Additives: Mechanism, Chemistry, and Performance
Abstract
Sulfur-based additives are among the most important classes of extreme pressure (EP) and antiwear (AW) agents used in industrial lubricants. This paper reviews the lubrication regimes relevant to sulfur carriers, their chemical structures and synthesis routes, and the working mechanisms responsible for EP performance. It further distinguishes between active and inactive sulfurized products, discusses the influence of carrier structure on additive efficiency, and highlights the role of polarity in determining performance. Simplified figures and tables are included to aid understanding of sulfurization routes and performance trends.
1. Introduction
Extreme Pressure (EP) and Antiwear (AW) additives are required to protect machine components under high load conditions. Sulfur-based additives are widely used because of their ability to form protective films on metal surfaces, reducing wear and preventing seizure. These additives function primarily in the boundary and mixed-film lubrication regimes where base oil viscosity alone is insufficient to maintain surface separation.
2. Chemistry of Sulfur-Based EP Additives
Sulfur carriers are synthesized through different routes that determine their activity, stability, and byproducts. The primary reaction routes include:
– Black sulfurization
– High-pressure H₂S sulfurization
– Mercaptan route
– Sulfur-chlorination route
3. Mechanism of Action
Sulfur-based EP additives function through adsorption on metallic surfaces, chemical reaction with the surface, or a combination of both. These processes lead to the formation of protective iron sulfide films that prevent metal-to-metal contact under high load conditions.
4. Active vs Inactive Sulfurized Products
Sulfurized olefins may contain active sulfur, which readily reacts with metal surfaces, or inactive sulfur, which is more stable. Although active sulfur can enhance reactivity, excessive activity can cause corrosion of yellow metals. Studies show that the total content of active sulfur has only a minor effect on EP performance, highlighting the greater role of chemical structure and carrier polarity.
5. Effect of Chemical Structure
The chemical backbone of the sulfurized additive influences both EP performance and stability. For example, esters and triglycerides as carriers provide better polarity and EP performance compared to olefin-based structures.
6. Summary
Sulfur-based EP additives are essential for lubricant formulations operating under high load conditions. They function primarily in boundary and mixed lubrication regimes through adsorption and chemical reaction mechanisms. The method of sulfurization influences color, stability, and byproduct formation. Active sulfur content is not the dominant factor for EP performance; instead, the polarity and chemical structure of the carrier play the most critical role. Part 2 of this study will examine issues of sulfur-induced wear, synergy with other additives, and corrosion challenges.