Chemically active additives such as dispersants, detergents, antiwear and extreme pressure (EP) agents, oxidation inhibitors, and rust and corrosion inhibitors, can interact chemically with metals to form a protective film, and with oxidation and degradation products to make them innocuous.
Chemically inert additives improve physical lubricant properties critical to effective performance. They include emulsifiers, demulsifiers, pour-point depressants, foam inhibitors, and viscosity improvers.
A balance of additives and their characteristics is critical to any performance package’s effectiveness.
Dispersants
These additives are used to suspend oil-insoluble resinous oxidation products and particulate contaminants in the bulk oil. This minimises sludge formation, particulate-related wear, viscosity increase, and oxidation-related deposit formation.
Dispersants are primarily used in gasoline engine and heavy-duty diesel engine oils, this accounts for 75-80% of total use. They are also used in natural gas engine oils, aviation piston engine oils, automatic transmission fluids and some types of gear lubricants.
Some dispersants can function as viscosity improvers as well, and are appropriately called dispersant viscosity improvers.
Detergents
Detergents perform similar functions to dispersants, keeping oil-insoluble by-products of combustion and oil oxidation in suspension. They help prevent contaminant build up, and also form a protective film on metal to keep equipment surfaces clean.
Additionally, overbased detergents neutralize acidic combustion and oxidation products, and hence control rust, corrosion, and resinous build-up in the engine. Some detergents can also act as oxidation inhibitors.
Detergents are primarily used in crankcase lubricants. Gasoline and diesel engine oils account for over 75% of total detergent use. Detergent levels in engine lubricant formulations are fairly high: marine diesel engine lubricants contain the highest concentrations as they use high-sulphur fuel which produces very acidic combustion products.
Detergents have additional use in automatic and manual transmission fluids as well as tractor hydraulic fluids. However their use in these applications is different - to modify the frictional properties of the fluid.
Antiwear and Extreme-Pressure Agents
Wear occurs in any equipment that has moving parts. There are three types of wear: surface-to-surface contact (frictional wear), surface contact with foreign matter (abrasive wear) and erosion by corrosive materials (corrosive wear). Methods of reducing wear vary:
- Abrasive wear is reduced through installing an efficient filter to remove debris.
- Corrosive wear is controlled with additives to neutralize whatever attacks the metal surfaces.
- Frictional wear is minimised through antiwear additives and Extreme Pressure (EP) agents.
Frictional wear is a major challenge to overcome. Under normal working conditions, the two metal surfaces are effectively separated by a lubricant film. This condition is called hydrodynamic, or thick-film, lubrication. As load increases, or speed decreases, metal-to-metal contact is more likely to occur. The temperature then rises in the contact zone due to frictional heat. Heat makes the lubricant lose viscosity, decreasing its film-forming ability, so it no longer minimises the frictional wear. This condition is called boundary lubrication and represents the typically adverse conditions modern lubricants have to be able to operate in.
Both EP and antiwear additives function by thermal decomposition, forming products that react with the metal surfaces to form a solid protective layer. This fills any unevenness in the surfaces, which reduces friction and prevents welding and surface wear. The only difference between the additives is that EP agents perform under more severe conditions.
Likely loads and temperatures must be considered before choosing antiwear or EP additives. Antiwear agents are commonly used in engine oils, automatic and manual transmission fluids, power steering fluids, and tractor hydraulic fluids.
EP agents are used in gear oils, other power-transmitting fluids, load bearing greases, and metalworking fluids. Extreme-pressure additives are usually supplemented with antiwear additives to make them effective across a wide range of conditions.
Many effective EP and antiwear additives are corrosive to metals. Therefore, lubricants are typically formulated to balance protection with corrosivity.
Antiseize additives are a separate class of antiwear additives that perform independently of temperature. They improve boundary lubrication by forming a protective film through deposition, and are generally used in greases, some industrial oils, and various break-in lubricants.
Friction Modifiers/Antisquawk Agents
Lubrication of moving surfaces is more efficient when also using friction modifiers. These additives prevent scoring, reduce wear and noise, and can also help to prevent micropitting in industrial gear lubricants.
Under heavy load conditions, the EP additive action replaces the friction modifier action and takes over the function of damage prevention. The process reverses again as the load eases off. Friction modifiers have a finite life, related to their oxidative and thermal stability.
Friction modifiers are commonly used in gasoline engine oils, automatic and manual transmission fluids, tractor hydraulic fluids, power steering fluids, shock absorber fluids, and metalworking fluids. In automatic transmission fluids and limited slip axle lubricants, friction modifiers control torque application through clutch and band engagements.
Antisquawk additives are functionally similar to friction modifiers. They are used to reduce mechanical noises such as squawk and chatter and are primarily used in automatic transmission and tractor hydraulic fluids, limited slip axle oils and industrial oils.
Oxidation Inhibitors
All modern lubricants, being hydrocarbon based, are susceptible to oxidation when exposed to oxygen in some manner. Mineral and synthetic base lubricants have stability thresholds, beyond which additional stabilizers or inhibitors are needed to slow oxidation. Any oxidation process produces acids that attack iron, copper or other metals present.
Heat doubles the rate of oxidation with every 10ºC rise. If not controlled, the lubricant decomposition will lead to oil thickening and the formation of sludge, varnish, resin, and corrosive acids.
Oxidation inhibitors break up the chain reaction of oxidation and slow down this vicious circle. They are used in almost all lubricants. Gasoline and diesel engine oils and automatic transmission fluids account for 60% of total use, as higher operating temperatures and greater air exposure demand higher levels of protection.
Rust and Corrosion Inhibitors
Rust and corrosion indicate metal damage from oxygen and acid attack. The speed of attack increases in the presence of water and polar impurities, and internal combustion engines contain plenty of these elements.
Rust and corrosion inhibitors provide a barrier between the metal surface and harmful elements. Some inhibitors neutralise acids, others form protective films. Basic detergents are excellent rust and corrosion inhibitors, because they protect in both ways.
For many applications, rust and corrosion inhibitor systems are required to protect surfaces both above and below the lubricant level. These additives have major uses in engine oils, gear oils, metalworking fluids, and greases.
Emulsifiers and Demulsifiers
Emulsifiers enable two immiscible fluids to form a mixture called an emulsion. They work by reducing the surface tension of water to facilitate thorough mixing. Water and oil mixtures are often used as lubricants because they are low cost, easy to dispose of, and have fire-retardant properties.
Emulsifiers are primarily used in metalworking, rock drill and hydraulic applications. Emulsions should be stable over long periods of time, must possess good lubricating properties, should not attack seals and metals, and should be easy to demulsify for disposal.
Demulsifiers are used in applications where water contamination of the lubricant is a problem. Certain lubricant formulations, in the presence of water, have an increased tendency to form emulsions – due to other additives, such as detergents, that act as surfactants. Automatic transmission fluids, hydraulic fluids, and industrial gear oils are examples. Demulsifiers are added to such formulations to enhance water separation and suppress foam formation.
Pour Point Depressants
The pour point is the lowest temperature at which a fuel or an oil will pour when cooled under defined conditions. In general, the pour point is indicative of the amount of wax in an oil. At low temperatures the wax tends to separate, trapping a substantial amount of oil, which inhibits oil flow and hinders lubrication.
Most of the wax is removed during base oil refining. However, some wax is desirable for achieving the right viscosity. So, pour point depressant additives are used to enable mineral oils to function efficiently at low temperatures, whilst retaining the benefits of the wax at higher temperatures.
Good additives can lower the pour point by as much as 40 °C. These are commonly used in applications requiring mineral oil, usually below 0°C. Pour point depressants function by altering the wax crystal size. This inhibits lateral crystal growth and keeps the bulk oil in a liquid state.
Pour point depressants are used in most lubricant types, for example crankcase engine oils, automatic and power transmission fluids, automotive gear oils, tractor fluids, hydraulic fluids, and circulating oils.
Foam Inhibitors
Most lubricant applications involve agitation, which encourages foam formation by trapping air in the lubricant. Excessive foaming results in ineffective lubrication and, over time, oxidation and possibly cavitation.
The lubricant viscosity and surface tension determine the foam stability. Low viscosity oils produce foams with large bubbles that tend to break quickly. High viscosity oils, on the other hand, generate stable foams containing fine bubbles that are difficult to break. Surface active materials, such as dispersants and detergents, further increase foaming tendency.
Foam inhibitors alter the surface tension of the oil and help to weaken the structure of air bubbles.
Viscosity Improvers
Mineral oil lubricants become less effective at high temperatures. The drop in viscosity with heat reduces their film-forming ability. Unbelievably, to try and counter this problem, seasonal oil changes used to be the norm for some applications! Fortunately, with the advent of viscosity improvers, this is no longer the case.
Viscosity improvers are polymers added to low viscosity oils to improve high temperature lubrication. They effectively thicken the oil as temperature increases. This means that the lubricating effect of mineral oils can be extended across a wider temperature range.
A balance between thickening efficiency and shear stability is important when selecting a polymer for use as a viscosity improver. Higher molecular weight polymers make better thickeners but tend to have less resistance to mechanical shear. Lower molecular weight polymers are more shear resistant, but do not improve viscosity as effectively at higher temperatures and therefore have to be used in larger quantities.
Polymer additives can also undergo thermal and oxidative degradation, unzipping back to smaller monomers, which reduces their effect. The highest possible degree of thermal and oxidative stability is desirable in addition to the features above.
Viscosity improvers are primarily used in multigrade engine oils, gear oils, automatic transmission fluids, power steering fluids, greases, and some hydraulic fluids.
Other Components
Lubricants also contain a number of other additives, including seal-swell agents, dyes, biocides, and couplers.
Seal-swell agents:
Seals are used within lubrication systems to:
- Isolate lubrication environments from harmful elements
- Help maintain hydraulic pressure
- Allow removal and replacement of malfunctioning parts without total dismantling
- Minimize contamination and the loss of lubricant
Certain base fluids and additives in lubricants can cause shrinkage, brittleness, and deterioration of the seals. This impairs the performance of the lubricating system.
Seal swell agents help maintain the integrity of the seals. These materials are commonly used in transmission and hydraulic fluids.
Dyes:
Used in some lubricants and fuels as a color code. This ensures the lubricant is used in the correct application, and acts as a leak detection aid. Automatic transmission fluids contain a red dye, whereas the two stroke cycle oils contain a blue or purple dye.
Biocides:
Mineral oil based lubricants tend to resist microbial attack because of high operating temperatures and biocidal action. However, high water content lubricants e.g. certain metalworking fluids and hydraulic fluids, are easily attacked by microbes. Control of bacterial growth is essential to minimize product deterioration and possible worker health hazards.
Couplers:
Additives used in water-based lubricants to help stabilize microemulsions.