Lubricating oils and greases consist of oils and additives. Base oils, by themselves, may not exhibit the oxidative, antiwear, antirust and many other properties desired of a good lubricant. Additives make up for these deficiencies. The amount and type of additives vary depending on the lubricant's intended function. They can also improve or impart desired performance characteristics, as well as reduce the rate of lubricant degradation over time. A properly formulated lubricant, consisting of good quality base oil and additives, protects equipment and keeps it operating trouble-free.
Additives are grouped by function, some of which include:
- Preserve the lubricant (oxidation inhibitors, metal deactivators).
- Alter the lubricant's physical properties (viscosity index improvers, pour point depressants, thickeners).
- Impart performance properties to the lubricant (antiwear agents, extreme pressure agents, rust and corrosion inhibitors, foam inhibitors friction modifiers, emulsifiers and demulsifiers, detergents and dispersants).
The cast of characters
Lubricants oxidize when exposed to air, water, high temperatures and metal surfaces. Oxidation is a chain reaction that thickens the oil and produces sludge, varnish and corrosive chemicals. Oxidation inhibitors and antioxidants retard oxidation by stopping the chain reaction and decomposing the peroxide radicals that support oxidation. Three principal types of antioxidant compounds are hindered phenols, amines and sulfur compounds. They break the oxidation chain reaction at different temperatures, so some synergy may be obtained by using them in combination. Antioxidants are used in most industrial lubricants and motor oils exposed to air, metal surfaces and high temperatures.
Viscosity index improvers are long-chain polymers that alter the degree to which the oil's viscosity changes with temperature. At high temperatures, oils tend to thin out, and they thicken at low temperatures. Oils with a high viscosity index resist thinning and thickening better than those with a low viscosity index. Polymers that improve the viscosity index are like coils that uncoil as the temperature increases, and provide thickening to minimize the drop in the viscosity. The viscosity index improvers find use in motor oils and some hydraulic oils that need to be very fluid for cold temperature startup, while maintaining the desired viscosity at the equipment's elevated operating temperature.
The lubricant's pour point is the lowest temperature at which it flows, a figure that's a function of the amount of wax in the lubricant blend. It is desirable for the lubricant to flow, even at low temperatures, if it is to provide good lubrication during equipment startup. If it gels or stops flowing, it's possible to have catastrophic mechanical failure. Pour point depressants reduce the pour point, typically 15 to 30 ºF, by retarding wax crystal growth. The most widely used pour point depressants are polymethacrylates.
Antiwear additives reduce the wear that results from metallic parts rubbing or sliding against each other. At higher loading, friction and wear can increase if the oil film is not strong enough to keep the moving parts separated. Antiwear compounds are believed to react with the metal surface chemically to produce a coating that wears faster than the metal surface, thereby preventing metal wear. Zinc dithiophosphates are the effective antiwear additives used in hydraulic oils and engine oils.
At very high loads, such as with loaded gears, metal-to-metal contact can cause welding and result in scuffing and adhesive wear. Extreme pressure additives react with metal surfaces under high temperature conditions to form a protective surface film. Generally, the high temperature occurs as a result of high loads on metal surfaces rubbing against each other. Both antiwear and extreme pressure additives typically contain sulfur and phosphorous. Extreme pressure additives are generally used in gear oils, transmission oils and greases.
Rust inhibitors are compounds that prevent rusting of ferrous metals. Rusting occurs when iron is exposed to oxygen and water. The iron oxide formed by rusting is a mechanically weak material that is easily removed to expose new metal surfaces for further rusting. Rust inhibitors have polar components that attach to the metal surface and form a dense hydrophobic, monolayer of chemisorbed surfactant molecules that protects the metal from contact with oxygen and water. Corrosion inhibitors, on the other hand, prevent corrosion of nonferrous metals either by forming a protective coating on the metal surface or by reducing the amount of corrosive acids in contact with it. Rust and corrosion inhibitors sometimes affect the performance of extreme pressure and antiwear additives adversely because they are competing for the same metal surface. Phosphate esters, metal sulfonates, and fatty amines are good corrosion inhibitors. Rust inhibitors are generally used in most industrial oils, engine oils and greases.
Lubricant foaming can result from agitation, air entrainment and contamination. Foam decreases the oil's lubrication efficiency and may cause mechanical failure. Antifoam additives reduce the oil's tendency to foam. These additives don't reduce the amount of foaming directly. Instead, they reduce the surface tension of the air bubbles, increasing the rate at which the foam bubbles collapse. Most common antifoam agents are made from silicones, polydimethylsiloxanes and fluorosilicones. They find application in hydraulic oils, turbine oils and engine oils.
The performance of a circulating oil generally degrades as it retains water and forms an emulsion. Demulsifiers break up the emulsion and facilitate the formation of separate oil and water layers. Therefore, they find application in hydraulic oils where the oil and water have time to separate in the reservoir. The water is then drained from the system to maintain effective lubrication. Sometimes lubricants must retain water instead of forming separate oil and water layers. Emulsifiers reduce the surface tension between the oil and water interface, thereby promoting good mixing and the formation of a stable emulsion. Emulsions are stable if they do not easily separate into two layers. Emulsifiers find application in some fire-resistant hydraulic oils, soluble oils and metal working fluids.