Lubricant Base Stock

Group I

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 viscosity index (HVI) stocks, although they also contain a lot of wax. For certain applications, naphthenic crudes are preferred because they yield high-quality medium viscosity index (MVI) and low viscosity index (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 low viscosity index (LVI) stocks. For high viscosity index (HVI) and low viscosity index (LVI) stocks, some form of solvent extraction is necessary to remove colored, unstable and low viscosity index 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 -20ºC range. At the refiner’s option, the oils may be “finished” with hydrogen to remove sulfur, nitrogen and color bodies.
 

Group II & III

Hydro-Refined and Hydro-Isomerized Mineral Oils
An alternative refining process, which substitutes deep hydrogen treatment for solvent extraction, can yield viscosity indexes (VI) of over 100. An additional advantage of this approach is that such processes can increase the yield of high viscosity index (HVI) components from most crude. Instead of unwanted low viscosity index (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.
 

Group IV

Polyalphaolefins (PAO)
Originally, one of the most widely used synthetic base stocks in the market place, 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 -40ºC. 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.
 

Group V

Dibasic Acid Esters
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 -65ºC.

Advantages or 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 oxidation stability. In addition, the use of chemically resistant seals is recommended.

Polyol Esters
Like diesters, polyol esters 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.

Polyol esters contain no sulfur, phosphorus or wax. Pour points range from -30 to -70ºC and viscosity indexes from 120 to 160. These 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.