An agent added to oils, fuels, and coolants to impart specific beneficial properties to the finished products. Additives create new fluid properties, enhance properties already present and reduce the rate at which undesirable changes take place in a fluid during service. Selected additives are:

Dispersants, purpose: keep sludge, carbon and other deposit precursors suspended
Detergents typical compounds: succinimides, neutral metallic sulfonates, phenates, polymeric detergents, amines
Basic Metal purpose:neutralize acids, prevent corrosion
Compounds typical compounds:overbased metallic sulfonates, phenates, carbonates
Oxidation Inhibitors purpose:prevent or control oxidation of oil by forming oxygen-inactive compounds and / or scavenging free oxygen
typical compounds:zinc dialkyldithiophosphates; aromatic amines, sulfurized products, hindered phenols
Extreme Pressure (EP) Antiwear Additives purpose:reduce wear, prevent scuffing and seizing by forming protective surface films
typical compounds:zinc dialkyldithiophosphates; tricresylphosphates, organic phosphates, chlorine and sulfur compounds
Viscosity Index Improvers purpose:reduce the rate of viscosity change with temperature
typical compounds:polyisobutylene, methacrylate polymers, olefin copolymers- may incorporate dispersants
Rust Inhibitors purpose:prevent the formation of rust on metal surfaces by formation of surface film or neutralization of acids
typical compounds:high base additives, sulfonates, phosphates, organic acids or esters, amines
Corrosion Inhibitors purpose:prevent corrosive attack on alloy bearings or other metallic surfaces
typical compounds:organically active sulfur, phosphorus, nitrogen compounds such as phosphites, metal salts of thiophosphoric acid, sulfurized waxes, terpenes
Antifoamants purpose:reduce foam with surface tension modifiers that break up large surface bubbles, or small entrained bubble reducers
typical compounds:silicones, organic polymers
Friction Modifiers purpose:reduce or modify friction
typical compounds:long chain polar compounds, (amides, phosphates, phosphites, acids, etc.), molybdenum disulfides
Metal Deactivators purpose:form surface films so metal surfaces do not catalyze oil oxidation
typical compounds:zinc dialkyldithiophosphates, metal phenates, organic nitrogen compounds
Pour Point Depressants purpose:lower “freezing” point of oils assuring free flow at low temperatures
typical compounds:low molecular weight methacrylate polymers
Emulsifiers purpose:permit water-in-oil or oil-in-water emulsions by changing interfacial tension
typical compounds:soaps of fatty acids, sulfonic or napthenic acids
Demulsifiers purpose:lower emulsion stability
typical compounds:heavy metal soaps, alkaline earth sulfonates
Tackiness Agents purpose:increase adhesiveness of lubricant on metal surfaces; reduce runoff
typical compounds:high molecular weight polymers, aluminum soaps of unsaturated fatty acids
Oiliness Agents purpose:lower friction under near-boundary conditions
typical compounds: high molecular weight polar compounds such as fatty oils, oxidized waxes or soaps
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