Synthetic oils, greases and lubricants are formulated with chemically modified petroleum base oils or base fluids synthesized from other raw materials. The base fluid is combined with additives in order to produce a product with a controlled molecular structure and predictable properties. Synthetic fluids are more expensive than non-synthetics, but are engineered to provide superior performance characteristics including thermal-oxidative stability, higher viscosity indexes, higher flash points, better solvency, chemical inertness, and lower pour points. They are based on compounds that consist of polyalphaolefins (PAOs), alkylated naphthalenes (AN), esters, polyethers, polyglycols, silicones, fluorinated compounds, or mixtures of synthetic fluids and water.

Base Oils

Base oils are raw stock fluids, composed of either a refined petroleum fraction or synthetic material. They are the primary constituent which is blended with additives in order to produce oils, greases, and lubricants. They are classified into five categories per the standard published by the American Petroleum Institute (API 1509, Appendix E). Groups I, II, and III are used to denote various grades of petroleum or mineral oils whereas group IV and V are considered synthetic fluids. (Base and Process Oils Information)

Groups I, II, II+, and III

Base oils in Groups I, II, II+, and III are refined mixtures of naturally occurring hydrocarbons. They are formulated with additives in order to produce petroleum or semi-synthetic fluids.

Group IV

Group IV base oils, PAOs, are chemically engineered hydrocarbons derived from alphaolefins. They possess a lower pour point, greater thermal stability, a higher viscosity index, and a higher degree of chemical inertness when compared to mineral oils.

Group V

Group V base oils encompass all other base oils including alkylated naphthalene (AN), esters, polyether, polyglycol, silicone, halogenated hydrocarbons, and other synthetic fluids. As they are considerably more expensive than mineral oils and PAOs they are most commonly used as fluid additives or otherwise used for specialized industrial applications.

Alkylated napthalene (AN)

Alkylated naphthalene (AN) is produced by the alkylation of naphthalene with an alkylating agent and an acid catalyst. AN fluids exceed the performance of PAOs in oxidizing environments as well as provide improved hydrolytic stability. They also possess low volatility and good solubility characteristics. They may be used as an additive or as a stock fluid.

Esters

Esters, diesters, and polyolesters are organic compounds formed through a reaction with an acid and an alcohol which produces an ester and water. The chemical structure of the products used dictates the properties of the ester. Hydrolytic stability can be a concern for some esters as excess water can break down the ester, reversing the chemical equation.

Diester lubricants have improved fire resistance and oxidation stability. They also resist sludge and varnish formation due to their high solvency and are commonly used as a lubricant for reciprocating air compressors.

Polyolesters exhibit excellent high temperature properties and long-term hydrolytic stability. They are used in rotary screw air compressors that operate above 180° F as is common in mining, oil well drilling, multi-stage compression, and when operating in hot ambient temperatures. They resist oil breakdown and have improved physical properties when compared to PAOs and diesters.

Polyether

Polyether or ether-based fluids, such as phenyl ether polymer or polyphenyl ethers (PPEs), are radiation-resistant fluids that offer superior thermal stability, oxidation stability, and a very low vapor pressure.

PPEs are limited by a relatively high pour point, 40° F, and are commonly used in high vacuums, very high temperatures, or when a radiation-resistant fluid is required.

Polyvinyl ether (PVE) is a hydrofluorocarbon ether lubricant with excellent performance characteristics including superior lubricity and solubility with process fluids. PVE is exstensively useful in refrigerant system as it is miscible with halogenated (HFC) refrigerants.

Polyglycol 

Polyglycol, glycol, polyalkyene glycol (PAG), and water-glycol fluids are often used for anti-freeze, circulating coolant, hydraulic fluids, and high water content fluids (HWCF). They offer fire resistance and excellent low-temperature properties as they dissolve moisture, prevent freezing, and have a subzero pour point.

Polyglycol lubricants are also used for hydrocarbon gas compressors as they reduce hydrocarbon solubility and have favorable viscosity and temperature characteristics. When operating under high pressures there can be issues with the formation of a hydrocarbon liquid phase. This phase can cause bearing failures if it is not separated from the lubricant before entering the compressor.

Water-glycol solutions tend to have higher viscosity index values than other compositions. Zinc, cadmium, and magnesium react with water-glycol solutions and water-glycol fluids should not be used in coolant systems where these materials exist.

Silicone

Silicone-based fluids include fluorosilicones, alkylmethylsilicones, and other silicone based fluids. They contain long linear polymers that easily slide past one another. They are also compressible and exhibit a low surface tension which limits their load carrying capacity. As a lubricant they are primarily used for light loads including metal-to-plastic and plastic-to-plastic lubrication applications. Their thermal stability, chemical inertness, and oxidation resistance allow for an extended life, making them suitable for electrical, chemical, and food and beverage applications as well as for closed systems.

Halogenated hydrocarbons

Fluids based on halogenated (fluorinated and/or chlorinated) hydrocarbons include chlorofluorcarbons (CFC), halogenated fluorocarbons (HFC), halogenated chlorofluorocarbon (HCFC), and perfluorocarbon (PFC) fluids. They have excellent chemical inertness and solvent resistance as is required for strong oxidizing and corrosive environments while their use is somewhat limited by high cost.

Limitations

Synthetic fluids have superior performance characteristics compared to mineral oils, but come at a higher cost. Synthetic fluids also have material compatibility issues and can decompose in certain industrial environments, crack or deteriorate plastic components, and can come out of suspension in the presence of leaded fuels used in aviation applications. Switching from conventional oil to synthetic oil can cause problems stemming from miscibility between the two fluids. Degradation to seal material can also pose a problem due to the high solvency nature of synthetic fluids. Care should be taken to ensure synthetic fluids are suitable for a given application.

Semi-synthetics

Semi-synthetic fluids, also referred to as synthetic blends, are generally defined as formulations that contain no more than 30% synthetic fluids. The distinction between synthetics and semi-synthetics varies between industries. In some cases products formulated with group III base oils, hydrocracked mineral oils, are described as synthetic fluids. The cost and heat transfer performance of semi-synthetic fluids falls between those of synthetic and petroleum based fluids.

Additives

Additives are chemical compounds that improve performance characteristics when present in small concentrations. Common additives used in synthetic fluids include corrosion inhibitors, detergents, extreme pressure (EP) additives, micro dispersions, low-foaming additives, and viscosity index (VI) improvers.

• Corrosion inhibitors are used to eliminate or control the rate of corrosion. They often have penetrating or water displacement characteristics. They encompass anti-oxidants, rust preventatives, as well as other corrosion inhibitors.
• Detergents are surface active agents that control chemical breakdown of the fluid by neutralizing impurities that can lead to sludge deposits.
• Extreme pressure (EP) additives include chemically active agents (sulfur, phosphorous, chlorinated compounds) that are reactive and form a film, preventing seizure, sticking, or surface adhesion in high pressure applications.
• Micro-dispersions are oils or lubricating fluids that contain a dispersion of solid lubricant particles, such as polytetrafluoroethylene (PTFE), graphite, molybdenum disulfide, or boron nitride (BN) in a mineral, petroleum, or synthetic oil base. Solid lubricants protect metal surfaces experiencing higher loads and lower speeds.
• Low-foaming or non-foaming additives break out entrained air. Entrained air can cause pump damage due to cavitation. Foaming can also reduce the cooling ability and the bulk modulus (or stiffness) of the fluid.
• Viscosity index (VI) improvers are used to control the rate at which viscosity decreases with respect to temperature. VI improvers increase the operating temperature of the finished product by combatting the tendency of a fluid to thin out at high temperatures.

Applications

Synthetic oils, greases, and lubricants are used in a variety of industries and applications. Some products are designed for aerospace, automotive, marine, or military industries. Applications include:

• Bearings
• Combustion engines
• Processing equipment
• Compressors
• Piston pumps
• Gears
• Final drives
• Circulating coolants
• Flood or mist coolants
• Insulating oils
• Lapping compounds
• Lubricants
• Heat transfer fluids
• Thermal oils
• Grinding
• Metal working
• Metal forming
• Metal cutting
• Hydraulic fluids
• Transmission fluids
• Quenchants
• Vacuum fluids

Specifications

• Kinematic viscosity is the time required for a fixed amount of fluid to flow through a capillary tube under the force of gravity. Units of kinematic viscosity are stoke, centistoke (1/100 of a stroke), and Saybolt Universal Seconds (SUS).

• The viscosity index (VI) is an arbitrary value describing the change in viscosity at two temperature extremes: 210° F (98.9° C) and 100° F (37.8° C).

• Pour point is the lowest temperature at which a fluid flows. It is typically 15° F to 20° F below its operating temperature.

• Flash point is the lowest temperature at which a liquid produces enough sufficient vapors to form an ignitable mixture in air near the surface of the liquid.

• Autogenous ignition temperature (AIT) is the temperature at which ignition occurs spontaneously.

• Thermal conductivity is a measure of the ability to transfer heat.

• Dielectric strength is the maximum voltage field that a material can withstand before electrical breakdown occurs.

• Specific gravity is the density of a fluid normalized to the density of water. Density is the mass per unit volume of a material.

Standards

AGMA 99FTM17 - Synthetic oil for worm gear lubrication

ASTM D4304 - Mineral and synthetic lubricating oil used in steam or gas turbines

9986113 - Tackified, PTFE filled synthetic grease 

References

Synthetic Lubricants What Are They

Understanding the Difference in Base Oil Groups  

What Makes a Lubricant Great?

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