Development of the Concept Dean and Davis Work

Many methods have been developed to represent changes in base stock viscosity with temperature. A useful method has to be able to express these changes for base stocks of all viscosities of interest, the meaning of the scale has to be readily apparent, the measurement must be relatively simple and cheap, and it must be widely accepted by base stock manufacturers and formulators and customers. The method that has emerged and lasted (too long for some ) is the viscosity index (VI) procedure. By this method the viscosity-temperature relationship of any base stock can be expressed by a single number, originally on a 0 to 100 

FIGURE 3.1 Viscosities (cSt) at 40°C and 100°C of n-paraffins of chain length Cn to C44. Sources J. Denis, The Relationship Between Structure and Rheological Properties of Hydrocarbons and Oxygenated Compounds Used as Base Stocks, Journal of Synthetic Lubricants 1(1—3) 201—238 (1984) J. Denis and G. Parc, Rheological Limits of Mineral and Synthetic Hydrocarbon Base Stocks, Journal of the Institute of Petroleum 56(556) 70-83 (1973) and Properties of Hydrocarbons of High Molecular Weight, Research Project 42, 1940-1966, American Petroleum Institute, New York. With permission. 

In 1929 E. W. Dean and G. H. B. Davis,4 of Standard Oil, used this representation for viscosities to develop the VI concept for lubricant feedstocks, intermediates, and base stocks. In modified form, this method for describing the viscosity-temperature relationship for base stocks and lubricants has become one of the bedrocks of the industry. The VI of a base stock now immediately brings to mind, rightly or wrongly, other features of interest to lubricants professionals, for example, oxidation stability and chemical composition (e.g., content of isoparaffins and cycloparaffins), and more recently, volatility. It is, however, best to regard this method as a means to express the rheological properties (the science of flow) of a base stock and use other more specific tests to measure other properties. 

As developed by Dean and Davis, this empirical method expresses viscosity variation with temperature numerically, initially on a simple scale of 0 to 100, based on two sets of reference distillate fractions. These oils were from two crudes whose distillates had not been refined in any manner (i.e., they had not been dewaxed or solvent refined). The viscosity changes with temperature of the 0 reference oil fractions were large, while those of the fractions from the 100 reference were small. These assignments of 0 and 100 were of course arbitrary and reflected experience at that time. It was assumed in developing the method that all distillation fractions from each of these reference crudes had the same VI (and that approximately agreed with the current knowledge) and that this was true for all other crudes and their lubricant fractions. A further assumption was that the Vis of all oils would fall between 0 and 100. 

To develop this method, Dean and Davis5 measured the viscosities (in SUS) at two temperatures (100°F and 210°F) for sets of distillation fractions from the reference crudes. The samples were, as mentioned, from two extremes of temperature dependence and were  

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