Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/778560
variety of lubrication challenges. For a mineral oil, the goal of the refining process is to optimize the resulting properties to produce a superior lubricant. For synthetically generated oils, the objective of the various formulations is to create a lubricant with properties that may not be achievable in a mineral oil. Whether mineral-based or synthetic-based, each base oil is designed to have a specific application. Some of the most important base oil prop - erties include the viscosity limitations and viscosity index, pour point, volatility, oxidation and thermal stability, aniline point (a measure of the base oil's solvency toward other mate - rials including additives), and hydrolytic stability (the lubricant's resistance to chemical decomposition in the presence of water). Base Oil Groups The 20th century saw a number of improvements in the refining process used for mineral oils along with the introduction of a variety of synthetics. By the early 1990s, the American Petroleum Institute (API) had categorized all base oils into five groups, with the first three groups dedicated to mineral oils and the remaining two groups predominantly synthetic base oils. Groups I, II and III are all mineral oils with an increasing severity of the refining process. Group I base oils are created using the solvent-extraction or solvent-refining technology. This technology, which has been employed since the early days of mineral oil refining, aims to extract the undesirable components within the oil such as ring structures and aromatics. Group II base oils are produced using hydrogen gas in a process called hydroge- nation or hydrotreating. The goal of this process is the same as for solvent-refining, but it is more effective in converting unde- sirable components like aromatics into desirable hydrocarbon structures. Group III base oils are made in much the same way as Group II mineral oils, except the hydrogenation process is coupled with high temperatures and high pressures. As a result, nearly all undesirable components within the oil are converted into desirable hydrocarbon structures. When comparing properties among the mineral base oil groups, you typically will see greater benefits with those that are more highly refined, including those with enhanced oxidation stability, thermal stability, viscosity index, pour point and higher operating temperatures. Of course, as the oil becomes more refined, some key weaknesses also occur, which can affect additive solubility and biodegradability. Group IV is dedicated to a single type of synthetic called polyalphaolefin (PAO). It is the most widely used synthetic base oil. PAOs are synthetically generated hydro- carbons with an olefinic tail formed through a polymerization process involving ethylene gas. The result is a structure that looks very much like the purest form of the mineral oils described in Group III. The advantages of PAOs over mineral oil include a higher viscosity index, excellent low- and high-temperature performance, superior oxidation stability, and lower volatility. However, these synthetic lubri- cants can also have deficiencies when it Cost L VH M M M VH H M Seal compatibility AA BA A BA BA AA AA AA Compatible with mineral oil - BA BA A A BA BA E Corrosion stability E AA A BA BA BA A E Oxidation stability BA AA AA AA A E E AA Viscosity range AA BA A BA BA A A A Flash point M E A AA A E E AA Pour point A AA M L L L VL L Temperature range BA A A AA A E E AA MINERAL OIL PHOSPHATE ESTER POLYGLYCOL POLYOLESTER DIESTER PERFLUOROPOLYETHER SILICONE POLYALPHAOLEFIN E EXCEPTIONAL AA ABOVE AVERAGE OR VERY GOOD A AVERAGE OR GOOD BA BELOW AVERAGE OR LESS THAN IDEAL VL VERY LOW L LOW M MEDIUM H HIGH VH VERY HIGH NA NOT AVAILABLE KEY Comparison of essential properties for base oils