Base fluids generally cannot satisfy the requirements of high performance lubricants, they need additives, that are chemical compounds added to lubricating oils to improve certain of its properties to the finished oils. Usually, the amount of additive used varies from 5 to 20%.
In addition to their beneficial effects, additives can have detrimental side effects, especially if the dosage is excessive or if interactions with other additives or with surfaces, seals and paints occur. So additives should be carefully balanced.
.Additives can be classified in three groups:
1. Lubricant Performance Enhancement Additives. Their mission is to improve base oil properties, allowing lubricant to work at extreme conditions.
a. Viscosity index (VI) improvers, long chain, high molecular weight polymers, as polymethacrylates (PMAs), poly-ethylene propylenes (OCPs), poly-styrenes-co-butadienes hydrogenated (HSDs), poly-isopropenes hydrogenated (SIPs), poly-styrenes-co-maleic-anhydride esterificated (SPE), to thicken the lubricant at elevated temperatures.
b. Pour point depressants, certain high molecular weight polymers function by inhibiting the formation of a wax crystal structure that would prevent oil flow at low temperatures, usually alkylaromatic polymers and polymethacrylates are used. A lowering of the pour point by about 11º - 17º C can be achieved.
c. Seal swell agents, promote slight swelling of seal material to counteract shrinking action of some highly paraffinic and PAO base oils. They are mainly formulated by esters.
d. Tackiness agents, are used to increase adhesiveness of lubricants on metal surfaces reducing run-off. They are formulated by high molecular weight polymers, aluminium soaps of unsaturated fatty acids.
e. Emulsifiers and demulsifiers, have the mission to emulsify water to avoid phases separation, and demulsify to separate water contamination from the lubrication system. Special polyethylene glycols and other ethoxylated substances have proved high efficiency.
2. Lubricant Protection Additives. Their mission is to protect the base oil, increasing the lubricant life.
a. Anti-oxidants, when oil is heated in the presence of air oxidation occurs, as a result of it both the oil viscosity and the concentration of organic acids in the oil increase, and varnish and lacquer deposits may form on hot metal surfaces exposed to the oil.
Oxidation inhibitors based in Zinc Dithiophosphate (ZDTPs), phenols compounds, diphenylamine alkylate, molybdenum and dithiocarbamate organic compounds, even sulphur and nitrogen compounds can be used; they react with the initiators, peroxy radicals, and hydroperoxides to form inactive compounds, or decompose these materials to form less reactive compounds, so they increase the live of lubricants.
b. Metal passivators, build a passivating protective layers thus preventing the solubilization of metal ions that would work as pro-oxidants. Can be classified into three groups: film forming compounds, complex forming chelating agents and sulfur scavengers. The mostly used are benzotriazole and tolytriazole as well as their alkylated derivatives.
c. Foam inhibitors, ability of oils to resist foaming varies considerably on type of crude oil, type and degree of refining, and viscosity, additives are formed by long chain polymers or silicones which act to destabilise surface foam.
d. Dispersants, are chemical compounds that disperse or suspend in the oil potential sludge forming materials, joined with Detergents can delay the formation of deposits and reduce the rate at which they accumulate on metal surfaces.
Typical dispersants are based on long chain hydrocarbons as polymeric succimides, olefin/P2S5 reaction products, polyesters, and benzylamides, that are acidified and then neutralized with compound containing basic nitrogen.
3. Metal Surface Protection Additives. Their mission is to provide active protection to metal surfaces, increasing life of equipment.
a. Anti-wear (AW), reduce friction and wear under boundary lubrication conditions; these additives form layers on the metal surface by absorption or chemisorption. They are formed by phosphorus compounds, sulfur and phosphorus compounds, sulfur and nitrogen compounds, sulfur compounds, and chlorine compounds.
b. Extreme pressure (EP), reduce friction and prevent scuffing and scoring of components operating under boundary lubrication conditions. They react with the metal surfaces producing a thin protective film in the same way of AW additives. they are formed with the same type of compounds of AW, but are much more reactive, but also passive EPs as sulfonated, especially calcium and sodium salts, and solid lubricating compounds as molybdenum disulfide. Typically EP aditives increase wear effects due their high reactivity.
c. Corrosion and rust inhibitors, there are two types of corrosion, by organic acids that develop in the oil itself, and by contaminants that are picked up and carried by the oil. Inhibitors form a protective film that prevents the corrosive materials from reaching or attacking the metal. Highly alkaline materials, as benzotriazole, substituted azoles, sulphured olefines, zinc diethyldithiophosphate, zinc diethyldithiocarbamate, trialkyl phosphites, in the oil will help to neutralize strong acids as they are formed.
Rust inhibitors having a high polar attraction toward metal surfaces, typical materials used are amine succinates and alkaline earth sulfonates.
d. Detergents, prevent the build-up of deposits in hot running engines, joined with Dispersants. They are generally to be compound that chemically neutralize deposit precursors that form under high temperature conditions or as the result of burning fuels with high sulfur content or other materials that form acidic combustion by-products.
The main detergents are organic soaps and salts of alkaline earth metals such as barium, calcium, and magnesium. these materials are often referred to as metallo-organic compounds.
4. Other Additives. We can include other additives as Anti-microbial pesticide to protect lubricants from contamination, fouling or deterioration caused by bacteria, fungi, protozoa, algae, or slime; or Dyes to give a specific color type for marketing, identification or leak detection; they have no influence in lubricant performance.
