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What Are Synthetic Lubricants

Brought to you by highly experienced Truck and Automotive Engineers and Lubrication Specialists. We have 25 years experience engineering and testing cars, trucks, engines, chassis components and systems, powertains and lubricants and we will put our extensive knowledge to use for you.

We are also longstanding Professional Grade members of the Society of Automotive Engineers (SAE). This prestigious position is ONLY granted to degreed engineers with documented work experience in the automotive engineering industry.

We have packed this website with useful information that will help you to make an informed decision regarding your choice of lubrication and filtration products for your vehicles and equipment. 

In this section I will cover what synthetic lubricants are and what they are made from for some of the most common types of synthetics. There are literally hundreds of types of specialized synthetic base stocks in existence and it is far beyond the scope of this book to cover them all. Highly specialized chemistry and scientific books exist on this topic for anyone interested in more in-depth research, but unless you have an extensive knowledge of chemistry they aren’t going to be very useful to you and the books very expensive are primarily only available in the research libraries of corporations involved in the synthetic lubricants engineering and manufacturing business. 

A synthetic lubricant is a product that is made from a chemical reaction (synthesis) of two or more simpler chemical compounds and also containing the necessary performance additives. The base stocks that form a synthetic lubricant are tailored through molecular restructuring in order to meet specific physical and chemical characteristics. 

Some of the most common synthetic lubricants are listed below  

1. Polyglycol fluids- Polyalkylene Glycol, Polyglycol Ethers, Polyalkalylene Glycol Ethers
2. Silicones
3. Esters: Diesters (Dibasic Acid Esters)
4. Esters: Polyolesters (Neopentyl Poly Esters)
5. Polymerized alpha olefin: Polyalphaolefin, Olefin Polymers, Olefin Oligomers-  synthetic hydrocarbons
6. Alkylated Aromatics- Dialkylbenzenes- a synthetic hydrocarbon
7. Phosphate Esters

There are many hundreds more types of synthetic lubricants and chemical variations of these synthetic lubricants. There is also no one specific synthetic lubricant that is superior in all respects, although a particular synthetic lubricant may possess certain specific advantages for a specific application. The synthetic lubricants listed in this book account for the majority of the volume of synthetic lubricant base stocks now used. 

Some of the common applications of each type of synthetic lubricant as well as the general process that the synthetic is manufactured where applicable are listed below: 

      Polyglycols 

These synthetic fluids were among the earliest used where extremes of temperature were encountered. The first use of polyglycols was for a water based hydraulic fluid for the U.S. Navy in 1943 for use in military aircraft so that fires would not result if bullets or shrapnel severs hydraulic lines. 

They have good lubricity, low sludge deposits, high natural viscosity indexes and good temperature stability.  Typical applications include automotive hydraulic brake systems (ethylene and polyethylene glycol), industrial gear oils, fire resistant fluids (by mixing the polyglycol with water), greases, metal working fluids and gas compressor oils.  Polyglycols were tested extensively for automobile engines but never developed into widespread use. Polyglycols are not compatible with petroleum oil. The chemical process used to manufacture polyglycols is beyond the scope of this book and is highly complex and one would need to be chemist or engineer to fully understand the process 

Silicones 

Silicones have high viscosity indexes and high thermal stability as well as excellent low temperature performance, which makes them good for use in certain greases, torsion dampers and in automotive brake hydraulic systems.  Silicone brake fluids have excellent temperature stability in newer vehicles, which have high performance brake systems but they are not nearly as water tolerant as Polyglycol brake fluids.  Water gets in brake systems over a period of time through the hydraulic lines, fittings and breather cap. As little as 2-3% water in a brake system is enough so that it can cause brake concerns. Silicone brake fluid and Polyglycol brake fluid are not compatible with each other and serious brake performance concerns can result if the two are mixed together. Additionally, the higher the temperature/performance rating of a brake fluid, regardless if it is Polyglycol or Silicone, the higher the affinity of the brake fluid to absorb water. 

Esters: Diesters (dibasic acid esters) 

During World War II a range of synthetic oils was developed. Among these, esters of long-chain alcohols and acids proved to be excellent for low temperature lubricants.  Following World War II, the further development of esters was closely linked to the aviation gas turbine. In the early 1960s, neopolyol esters were used in this application because of their low volatilities, high flash points and good thermal stabilities. 

Diesters are prepared by reacting a dibasic acid with an alcohol containing one reactive hydroxyl group. Note that the hydrolytic stability of diesters is not as good as mineral oils. Hydrolytic stability refers to how the lubricant reacts in the presence of water. Hydrolytic degradation can lead to acidic products, which, in turn, promote corrosion. Plus, hydrolysis can also materially change the chemical properties of the base fluid, making it unsuitable for the intended use. Systems that can contract high levels of moisture include systems that operate at low temperatures or that cycle between high and low temperatures and also certain fuels such as racing engines running alcohol, which has a cooling effect in the engine. Racing engines using ester based lubricants should have the lubricant changed regularly. 

Diesters have good lubricating properties, good thermal and shear stability, high viscosity indexes and have exceptional solvency and detergency. Diesters are superior fluids for aircraft engines and compressors, although mainly older jet aircraft. Diesters are also used as a base oil or part of a base oil for automotive engine oils and in some low temperature greases (note: modern military and commercial jet aircraft almost universally use lubricants formulated with polyol esters as the base fluid now). 

Diesters are incompatible with some sealing materials and can cause more seal swelling than mineral oils.  The scientific reason for this is as follows:  diesters have a low molecular weight that results in low viscosities. This combined with their high polarities makes them quite aggressive to elastomeric seals.  This can be reduced by using better elastomers or by carefully blending with PAO’s to nullify their swelling effects, since PAO base stocks are nonpolar.

Esters: Polyolesters (Neopentyl Poly Esters) 

Polyol esters are formed by reacting an alcohol with two or more reactive hydroxyl groups.  These fluids are used primarily for aircraft engines, high temperature gas turbines, hydraulic fluids and heat exchange fluids. Polyol esters are much more expensive than diesters. Lubricating greases with polyol esters as the base fluid are particularly suited to high temperature applications. Polyol esters have the same advantages/disadvantages as diesters. They are, however, much more stable and tend to be used instead of diesters where temperature stability is important. In general, a polyol ester is thought to be 40-50 deg. C. more thermally stable than a diester of the same viscosity.  Esters give much lower coefficients of friction than those of PAO and mineral oil.  By adding 5-10% of an ester to a PAO or mineral oil the oil’s coefficient of friction can be reduced markedly. 

Polymerized alpha olefin: Polyalphaolefin, Olefin Polymers, Olefin Oligomers- a synthetic hydrocarbon 

PAO’s are commonly used to designate olefin oligomers and olefin polymers. The term PAO was first used by Gulf Oil Company (later acquired by Chevron), but it has now become an accepted generic term for hydrocarbons manufactured by the catalytic oligomerization of linear alpha olefins having six or more carbon atoms.  PAO’s are gaining rapid acceptance as high-performance lubricants and functional fluids because they exhibit certain inherent and highly desirable characteristics.  These favorable properties include: 

A wide operational temperature range.

Good viscometrics (high viscosity index).

Thermal Stability.

Oxidative Stability.

Hydrolytic stability. *

Shear stability.

Low corrosivity.

Compatibility with mineral oils.

Compatibility with various materials of construction.

Low toxicity.

Manufacturing flexibility that allows “tailoring” products to specific end-use application requirements. 

* Of particular interest in relation to demonstrating superior hydrolytic stability of PAO fluids is a test that was conducted to find a replacement for a silicate ester based aircraft coolant/dielectric fluid used by the U.S. military in aircraft radar systems.  The test method required treating the fluids with 0.1% water and maintaining the fluid at 170 or 250 deg. F. for up to 250 hours.  Samples were withdrawn at 20- hour intervals, and the flash points were measured by the closed cup method.  A decrease in flash point was interpreted as being indicative of hydrolytic breakdown to form lower-molecular-weight products.  The PAO showed no decrease in flash point in any of the test conditions, while the silicate ester based fluid showed marked decreases.  The PAO fluid maintained started out with a flash point of 300 deg. F. and only dropped to 295 deg. F. at 80 hours into the test, while the silicate ester fluid, which started out with a flash point of 270 deg. F., ended up with a flash point of 220 deg. F. at only 55 hours into the test. 

PAO’s are used extensively as automotive lubricants (engine, gear, transmission, grease, hydraulic). PAO’s are also super premium oils for automotive applications operating in temperature extremes.  PAO’s are a synthetic hydrocarbon that is compatible with mineral oils. In industrial applications, they may be combined with organic esters to be used in high temperature gear and bearing oils, as well as gas turbines.  They are also used as a base fluid in some wide temperature range greases. 

The general manufacturing process used to form PAO’s is performed by combining a low molecular weight material, usually ethylene gas, into a specific olefin which is oligomerized into a lubricating oil material and then hydrogen stabilized.  There are a variety of basic building block molecules used to form the finished lubricant, which are dependent on the range of requirements of the specific lubricant. 

Seal compatibility is an important factor for any lubricant. Unlike mineral oils, PAO does not have a tendency to swell elastomeric materials.  Early commercial PAO products were not formulated properly to allow for this difference in behavior. Consequently, early PAO’s gained an undeserved reputation for leakage. Extensive tests have since shown that the addition of small quantities of an ester to the formulation easily alleviates this problem. 

Recent work has indicated that the proper choice of other performance additives may eliminate the need to employ esters, but this approach is not yet in practice for crankcase applications. In a test of a PAO vs. a mineral oil for seal compatibility, four seal materials were studied:  acrylate, silicone, nitrile and fluoroelastomer.  The seals were evaluated at the end of the test for changes in tensile strength, elongation, volume (seal swell), and hardness.  The PAO performance fell within the specification limits for all four elastomers.  The mineral oil failed with silicone. Similar tests have been carried out with fully formulated part- and full-synthetic PAO oils. In all cases the fluids met the specifications. 

Recent data shows that PAO-based fluids provide superior performance for the high-tech cars and trucks being built today. Today’s engines are smaller and more demanding and operate at higher RPM’s and under hood spaces is limited which causes increased operating temperatures.  Both the thermal conductivity and heat capacity of PAO fluids are about 10% higher than values for comparable mineral oils. The net result is that PAO-lubricated equipment tends to run cooler.

Alkylated Aromatics- Dialkylbenzenes- a synthetic hydrocarbon 

This synthetic hydrocarbon is compatible with mineral oils and is used as a base oil in many industrial applications such as engine, gear, hydraulic, air compressor and gas turbine fluids and in greases for sub-zero applications.  These fluids are somewhat toxic and have poor biodegradability 

This lubricant is formed by the alkylation of an aromatic compound, usually benzene. Alkylated aromatics have excellent low temperature fluidity. Their viscosity indexes are marginally higher than a high viscosity index mineral oil and they are oxidation resistant and stable at high temperatures and hydrolytically stable. 

Alkylated aromatics were developed for functional fluid use as early as the 1928-1936 time period but failed to gain any commercial prominence.  There was some development  and use of these fluids by the Germans from 1942-1945 due to petroleum oil supply interruptions cause by the war, but the war ended before they could get production volume increased to what they required. 

It wasn’t until the search for oil in Alaska and Canada in the 1960’s and the construction of the Alaska pipeline in the 1970’s that the good low temperature properties of alkylated aromatics became important. Conoco was the major company behind the introduction of alkylated aromatic base stock lubricants for service during this era.

Phosphate Esters 

The major feature of these fluids is their fire resistance. They find extensive use as hydraulic fluids in aircraft hydraulic systems, underground mining hydraulic systems, high temperature compressors and steam turbines, where fire resistance is critical. They have poor compatibility with some sealing materials and most paints. The manufacturing process of phosphate esters is extremely complex and will not be covered in this book.  Where safety is critical and there are high operating temperatures and/or pressures, phosphate esters are the lubricant of choice. 

Summary 

There is clearly no doubt that synthetic lubricants are superior to petroleum based oils. An excellent summary of in-depth studies that were conducted on the benefits of synthetic lubricants is presented in Appendix B of the Society for Automotive Engineers, Progress in Technology Series 22 and was conducted during the 1970’s and 1980’s.  The nine superior performance features of synthetic engine oils that were documented by extensive laboratory and field testing are listed below: 

Nine Superior Performance Features of Synthetic Engine Oils 

1. Engine Cleanliness.
2. Improved Fuel Economy (4.2% average increase)
3. Oil Economy (lower consumption)
4. Excellent Cold Starting and Low Temperature Fluidity
5. Outstanding Performance in Extended Oil Drain Field Service
6. High Temperature Oxidation Resistance
7. Outstanding Single and Double Length SAE-ASTM API “SE” and “SF” Performance Tests (note SE and SF specs were the latest at the time of the testing)
8. Excellent Wear Protection
9. Extended drain capability for heavy-duty diesel trucks and gasoline powered trucks.  Note: this particular test was based on truck fleet testing, however extended drain capability holds true for passenger cars as well.

These same superior performance features of synthetic engine oils hold true today just as they did when this extensive testing was conducted and has since been verified by many more studies and testing as well as countless millions of miles of field service in every possible type of vehicle and equipment application.

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