Industrial Lubrication Fundamentals: What’s In A Lubricant? (Synthetic Base Oils)
EP Editorial Staff | October 3, 2013
These man-made products are designed to work in the types of harsh conditions where animal/vegetable and mineral-based products typically can’t.
By Ken Bannister, Contributing Editor
In Element #4, we reviewed animal/vegetable and mineral base oils and their specific characteristics. Completing the trio of base stocks available to today’s lubricant blenders is a synthesized, man-made option commonly known as synthetic base oil. These “synthetics” are designed to perform in the harshest of climatic conditions (where their animal/vegetable and mineral-base-oil siblings are unsuited).
Most of today’s industrial oils use either a mineral or synthetic base oil. These base oils are categorized into five groups according to their refining or manufacturing process. Groups I, II and III represent conventional mineral-based lubricants, while groups IV and V are reserved for man- made synthetic base oils. (Refer to Table I above.)
- Group I reflects what is known as “conventional” base oils. They’re made from solvent refined crude stock and have a Viscosity Index between 80 and 120. Their sulfur content is above 0.03%, and their saturated hydrocarbon levels are less than 90%.
- Group II base oils are refined using a hydro-processing method known as “hydrotreating” that adds hydrogen to the base oil at temperatures above 600 F. This is done using a catalyst and applying moderate pressure over 500 psi to convert the base stock and reduce its sulfur content to less than 0.03% and increase its hydrocarbon saturation to levels of 90% and above.
- Group III base oils are known as “bright stock.” They’re primarily manufactured through a severe hydro-processing conversion method known as “hydrocracking” that employs a catalyst at a temperature above 650 F combined with pressures exceeding 1000 psi to take out undesirable elements like sulfur and nitrogen and replace them with hydrogen. The result is a more stable base oil with a Viscosity Index above 120 and a low pour point. In addition, remaining wax compounds are often removed to reduce the pour point further. Due to this more complex refining process, Group III base oils perform in a similar manner to Group IV pure synthetic base oils—and in most countries around the world, including North America, are allowed to be classified as a synthetic lubricant (even though they are hydrocarbon-based).
- Group IV base oils are reserved for Polyalphaolephin (PAO) synthetically manufactured base oils made up of very small synthesized hydrocarbon molecules.
- Group V base oils represent all other synthetic base oil configurations.
Specifics of synthetics
Synthetic lubricants owe their inception to the early jet engine: They were developed to cope with the extreme temperatures encountered when operating a jet aircraft. Using a polymerization process similar to that used in the plastic manufacturing industry, synthetic base oils are designed with specific and consistent molecular structures that result in highly stable base oils with a very high Viscosity Index (VI) rating.
Synthetic lubricants offer many advantages over mineral-based oils, the largest being the ability to operate reliably in both extremes of heat and cold at temperature ranges much wider than mineral oils. In addition to increased VI levels and improved thermal stability, synthetics also demonstrate improved oxidation stability (major reduction of sludge and acid buildup) and lower volatility, resulting in extended lubricant life and reduced oil consumption.
The disadvantages of synthetics are primarily associated with their cost—which, depending on the type, can range from as little as three times the cost of a mineral base oil to exponentially more. Certain synthetics can also cause seal swelling, and many are not compatible with any other base oil type.
Manufactured from chemically modified petroleum constituents or from a number of chemical bases and compounds, there are five common types of synthetic base oils. Their characteristics and costs compared to mineral-based products are summed up in Table II:
- Poly-alpha-olefin (PAO) is often described as a man-made mineral oil formulated by synthesis of ethylene gas molecules into a polymerized uniform structure similar to pure paraffin. With a pour point down to -90 F, a VI above 140 and good seal and mineral-oil compatibility, PAOs are widely used in automotive crankcase oils, industrial gear oils, compressor oils and turbine oils. Their negatives include poor additive solubility and biodegradability.
- Poly-alkylene-glycol (PAG) is an organic-chemical fluid with excellent lubricity (friction-reducing capabilities) and an inherent ability to volatilize (clean burn) any decomposed or oxidized products at high temperatures, thus leaving no sludge, acid buildup or damaging particles should oxidation take place. Polymers of alkylene oxides, PAGs are used primarily for industrial compressor oils, hydraulic oils (water glycol-type) and severe-duty gear oils. With a pour point of -60 F and a VI greater than 150, they have excellent biodegradability, but fall short in terms of mineral-oil and PAO compatibility.
- Di-basic acid ester (Di-Ester)originally saw use at the end of World War II for jet-engine lubrication (thanks to their high-shear VI stability of 150 and above). Formulated from a reaction between alcohol and acid-laden oxygen, it can suffer from poor hydrolytic stability (reacts to water presence) and poor seal compatibility. Today, Di-Ester is commonly used in high-temperature compressor oils.
- Polyol-Ester replaced Di-Ester as the preferred jet-aviation lubricant because of its increased thermal stability. With a low pour point of -95 F and a VI of 160, it is the preferred lubricant for gas turbines and two-cycle oil applications and is also used as a refrigerant oil. This type of synthetic is expensive (at up to 15 times the cost of mineral oil) and, like its Di-Ester cousin, suffers from poor hydrolytic stability, seal compatibility and corrosion stability.
- Silicone, one of the most expensive lubricants on the market, has a very high flash point and VI of over 250. These characteristics make silicone well suited for high-temperature applications—despite its poor lubricity. Typical applications are brake fluids and fire-resistant hydraulic oils.
More recent additions to the marketplace are semi-synthetic base oils blended with mineral base-oil products. These blends generally contain up to 20% pure synthetic product and are less expensive than pure synthetic oils. Because of the lack of standardization with them, their engineered value continues to be debated.
Changing base oil characteristics
All commercial oils are a unique proprietary blend of base oil and additives. Oil-company chemists and engineers choose the best base oil for the product’s intended purpose and use it as a canvas on which they color with additives to change and expand the base oil’s characteristics, delivering a new set of attributes for the finished product. In the next installment of this “ICML Domain of Knowledge” series (Element #6), we will investigate more base-oil characteristics and how typical additives affect them. Look for it in the November/December issue. LMT
Ken Bannister is a certified Maintenance and Lubrication Management Consultant with ENGTECH Industries, Inc., and author of the Machinery’s Handbook lubrication chapters, and the Lubrication for Industry text recognized as part of the ICML and ISO Domain of Knowledge. He teaches numerous preparatory training courses for ICML MLT/MLA and ISO LCAT certifications. Telephone: (519) 469-9173; or email: firstname.lastname@example.org.