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Base
Oil Composition Table of Contents Physical & Chemical Properties Crude oils, from which lubricating mineral oils are obtained, contain thousands of compounds, some of which remain after the refining processes. Most commercial lubricants are mixtures where the composition is chosen for the proposed use. The major mineral oil components are paraffinic, naphthenic, and a smaller amount of aromatic compounds. The variations in composition directly affect lubricant performance. Paraffinic oils are straight chain or branched aliphatic hydrocarbons belonging to the series with the general formula CnH2n+2. Paraffin's are saturated with respect to hydrogen. A typical paraffinic oil molecule with 25 carbon and 52 hydrogen atoms has a molecular weight of 352. Very high molecular weight paraffins are solid waxes, also dissolved in small amounts of mineral oils. Naphthenic or alicyclic oils have the characteristics of naphthenes, which are saturated hydrocarbons of which the molecules contain at least one closed ring of carbon atoms. Base oils may contain a small amount of Aromatic hydrocarbons. Aromatics are unsaturated molecules identified by one or more benzene rings or by chemical behavior similar to benzene. The concentration of the three major types of compounds in a given base oil is determined by chemical analysis. The results are given as %CP for paraffinics, %CN for naphthenics and %CA for aromatics. For example, a commercial base oil is 68%CP, 32%CN, and less than 1%CA. The amount of each type of compound in a mineral oil is calculated from equations given in ASTM D3238-90 which require values of molecular weight, refractive index and density. Mineral base oils contain many minor components, such as sulfur, nitrogen, oxygen, metal compounds, and water. The type and quantity of these compounds depends upon the degree of refining. For example, medicinal white oil contains the least amount of minor components, and a "bright stock" contains the most. The sulfur content of a pure mineral oil, medicinal white oil, is less than 20 ppm, whereas the sulfur content of a refined base oil depends on the type of refining process, and may be 200 to 6,000 ppm, while bright stock may range from 5,000 to 10,000 ppm sulfur. These sulfur compounds are surface active and affect boundary lubrication. Medicinal white oil is an extremely poor boundary lubricant often used as a reference, primarily for research. However, medicinal white oil is used in food machinery lubricants where there may be a chance of it being ingested. A bright stock is known to have better, but limited, boundary lubrication properties. Some of the minor components also provide natural oxidation inhibition for the base oils. For more information call 1-888-HERGUTH (437-4884) |
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