Lubricant additives: classification, properties and applications
Lubricant additives are individual chemical compounds or their compositions (packages) introduced into base lubricating materials to improve their physicochemical, performance and environmental properties. Without additives, modern engine, transmission, hydraulic and industrial oils cannot provide reliable protection of equipment over a wide range of temperatures, loads and environments. It is the additive package that turns a base oil into a finished commercial product with specified characteristics.
Role of Base Oil and Additives
The base oil accounts for 70–95% of the finished product volume and determines its viscosity-temperature properties, volatility and compatibility with seals. Additives, whose content in oil is typically from tenths of a percent to 5–6%, and in compositions up to 10–20%, purposefully modify certain characteristics. Additives are more effective in well-refined oils, but their solubility and stability may decrease, so formulations require a balance between base oil refinement depth and component compatibility.
Classification by Functional Action
By purpose, lubricant additives are divided into several main groups.
Antioxidant additives protect the oil from oxidation by atmospheric oxygen. These include alkylphenols, aromatic amines, and barium and zinc dialkyldithiophosphates. The mechanism is based on breaking the chain reactions of hydrocarbon oxidation by interacting with peroxide radicals and hydroperoxides.
Detergent-dispersant additives reduce the formation of carbonaceous deposits and keep oxidation products suspended. Metal salts (calcium, barium) are used — alkylsulfonates, alkylphenolates, alkylsalicylates — as well as ashless nitrogen-containing compounds: Mannich bases and succinimide derivatives. These additives are especially important for engines operating under high temperatures and cyclic loads.
Anti-wear and extreme-pressure additives improve lubricating properties by forming protective films on friction surfaces. Anti-wear (AW) additives work at moderate loads, extreme-pressure (EP) additives at high contact pressures and temperatures typical of hypoid gears and heavily loaded units.
Friction modifiers reduce the coefficient of friction and ensure smooth operation of units, which is especially important for automatic transmissions and energy-efficient oils.
Viscosity (thickening) additives improve the rheological properties of oils. These are oil-soluble polymers — polyisobutylene, polyalkylstyrene, polymethacrylates. At low temperatures, macromolecules take a compact globular form and interact weakly with the oil, reducing viscosity; at high temperatures they uncoil and the viscosity of the thickened oil increases. This enables all-season oils with a wide operating temperature range.
Pour point depressants lower the pour point of the oil. Polyethers of methacrylic acid and synthetic fatty alcohols adsorb on the surface of paraffin crystals and prevent their growth and interlocking, maintaining oil fluidity at low temperatures.
Corrosion inhibitors protect metal surfaces from attack by aggressive media. Salts of synthetic fatty acids, dicyclohexylamine and other compounds are used, forming protective adsorption layers.
Antifoam and demulsifying additives improve oil behaviour in circulation systems: the former prevent stable foam formation, the latter accelerate water separation from oil. These properties are critical for hydraulic systems and turbine oils.
Additive Packages and Compatibility
In practice, balanced additive packages are introduced into oil rather than individual components, providing several functions simultaneously. Component compatibility is a key factor: some additives can enhance each other's action (synergism), others can weaken it (antagonism). Therefore, formulations are developed with special studies of mutual influence. Additives must improve oil quality without degrading other properties or causing corrosion, deposits or incompatibility with elastomers.
Applications
Lubricant additives are used in all types of lubricating materials: engine, transmission, hydraulic, industrial, compressor, turbine and special oils. For engine oils, detergent-dispersant, antioxidant and anti-wear properties are critical; for transmission oils — extreme-pressure and viscosity properties; for hydraulic oils — viscosity stability, demulsibility and filterability; for industrial oils — protection against corrosion and wear at moderate loads.
Development Trends
Modern trends are associated with tightening environmental requirements, reducing sulfate ash, phosphorus and sulfur content in engine oils, the transition to biodegradable base oils and the development of oils with extended drain intervals. Interest is growing in nanoscale additives and hybrid packages that simultaneously provide protection against wear, friction and corrosion. Selecting the optimal additive package for specific equipment and operating conditions requires an individual approach that takes into account unit design, seal materials and operating modes.