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Ready Reference for Lubricant and Fuel Performance
Lubricant Basics — Base Stocks
The base stocks used to formulate lubricants are normally of mineral (petroleum) or synthetic origin, although vegetable oils may be used for specialized applications. Synthetics can be made from petroleum or vegetable oil feedstocks and are tailor made for the job they are expected to do.
Mineral Oils
Mineral stocks are refined by a number of processes of selection from the crude oil barrel. For this reason, the choice of crude is important. Most favored are paraffinic crudes, which give a good yield of high-VI (HVI) stocks, although they also contain a lot of wax. For certain applications, naphthenic crudes are preferred because they yield high-quality medium-VI (MVI) and low-VI (LVI) stocks with very little wax and naturally low pour points.
Distillation under atmospheric pressure removes the gasoline and distillate fuel components, leaving a "long residue" containing the lube oil and asphalt. Further distillation under vacuum yields "neutral distillates" overhead and an asphalt residue. Simple treatment with sulfuric acid, lime and clay turns the distillates into acceptable LVI stocks. For HVI and MVI stocks, some form of solvent extraction is necessary to remove colored, unstable and low-VI components. Finally, wax is removed by dissolving the oil in methylethyl ketone (MEK) and chilling and filtering to yield oils with pour points in the -10 to -20°C range. At the refiner's option, the oils may be "finished" with hydrogen to remove sulfur, nitrogen and color bodies.
The viscosity of the finished stocks is determined by the boiling range of their components. Most refiners settle for three or four stocks from which they blend their range of finished oils. For solvent extracted HVI oils, VI in the range of 90 to 100 is usual.
Hydro-Refined and Hydro-Isomerized Mineral Oils
An alternative refining process, which substitutes deep hydrogen treatment for solvent extraction, can yield VI of over 100. An additional advantage of this approach is that such processes can increase the yield of HVI components from almost any crude. Instead of unwanted LVI components being extracted, they are chemically changed into HVI materials, usually of lower molecular weight. This enables the blender to increase the output of light oils (for instance, SAE 5W-30) for which there is a growing market. In addition to the cracking of large molecules into smaller ones, hydro-isomerization reconstructs cracked waxes into branched paraffins. These structures offer excellent low temperature properties. This technology is growing globally to meet global standards for lubricants.
Synthetic Base Stocks
Synthetic processes enable molecules to be built from simpler substances to give the precise properties required. The main classes of synthetic material used to blend lubricants include:
Type Principal Applications
Olefin Oligomers (PAOs) Automotive and Industrial
Dibasic Acid Esters Aircraft and Automotive
Polyol Esters Aircraft and Automotive
Alkylated Aromatics Automotive and Industrial
Polyalkylene Glycols Industrial
Phosphate Esters Industrial
With the exception of polyglycol fluids, all have viscosities in the range of the lighter HVI Neutral mineral oils. Their viscosity indexes and flash points, however, are higher and their pour points are considerably lower. This makes them valuable blending components when compounding oils for extreme service at both high and low temperatures.
The main disadvantage of synthetics is that they are inherently more expensive than mineral oils, and are in limited supply. This limits their use to specialty oils and greases that command premium prices. Esters suffer the further disadvantage of greater seal-swelling tendencies than hydrocarbons; so, caution must be exercised in using them in applications where they may contact elastomers designed for use with mineral oils.
Polyalphaolefins are the most widely used synthetic lubricants in the U.S. and Europe. They are made by combining two or more decene molecules into an oligomer, or short-chain-length polymer.
PAOs are all-hydrocarbon structures, and they contain no sulfur, phosphorus or metals. Because they are wax-free, they have low pour points, usually below -40°C. Viscosity grades range from 2 to 100 cSt, and viscosity indexes for all but the lowest grades exceed 140.
PAOs have good thermal stability, but they require suitable antioxidant additives to resist oxidation. The fluids also have limited ability to dissolve some additives and tend to shrink seals. Both problems can be overcome by adding a small amount of ester.
Dibasic acid esters are synthesized by reacting an acid and an alcohol. Diesters have more varied structures than PAOs, but like PAOs, they contain no sulfur, phosphorus, metals or wax. Pour points range from -50 to -65°C.
Advantages of diesters include good thermal stability and excellent solvency. They are clean-running in that they tend to dissolve varnish and sludge rather than leave deposits. In fact, diesters can remove deposits formed by other lubricants.
Proper additive selection is critical to prevent hydrolysis and provide oxidative stability. In addition, chemically resistant seals are recommended.
Polyol esters, like diesters, are formed by the reaction of an acid and an alcohol. "Polyol" refers to a molecule with two alcohol functions in its structure; examples include trimethylolpropane (TMP), neopentylglycol (NPG), and pentaerythritol (PE).
Polyol esters contain no sulfur, phosphorus or wax. Pour points range from -30 to -70°C and viscosity indexes from 120 to 160. The fluids have excellent thermal stability and resist hydrolysis somewhat better than diesters. With the proper additives, polyol esters are more oxidatively stable than diesters and PAOs. Seal-swell behavior is similar to that of diesters.
Alkylated aromatics are formed by the reaction of olefins or alkyl halides with an aromatic material such as benzene. The fluids have good low-temperature properties and good additive solubility. Viscosity index is about 50 for fluids with linear molecules and zero or lower for fluids with branched side chains. Thermal stability is similar to that of PAO, and additives are required to provide oxidative stability.
Polyalkylene glycols (PAGs) are polymers of alkylene oxides. Lubricant performance and properties of a particular PAG depend on the monomers used to manufacture it, molecular weight, and the nature of the terminal groups. Thus, a wide range of properties is possible.
In general, PAGs have good high-temperature stability and high viscosity indexes, and they can be used over a wide temperature range. They exhibit low deposit formation and tend to solubilize their decomposition products. Like other synthetics, PAGs require additives to resist oxidation.
Phosphate esters are synthesized from phosphorus oxychloride and alcohols or phenols. They are used both as base oils and as antiwear additives in mineral and synthetic lubricants. Thermal stability is good, and pour point ranges from -25 to -5°C. However, viscosity index is extremely low, ranging from 0 to -30, which limits their high-temperature capabilities.
