FUNCTIONAL SELECTION
OF SYNTHETIC LUBRICANTS R. G. LARSEN AND A. BOND1 Shell Development Company, Emeryville, Calif.
T h e basic principles are summarized which guide the selection of synthetic lubricants for specific application in lubricating practice. The physical and chemical properties of the commercial and potential lubricants characterizing many chemical classes have been assembled in a single table, permitting quick selection of suitable compound classes for a given application. The use of this information is illustrated by a few typical lubricating problems which cannot altogether satisfactotily be solved by the use of conventional petroleum fractions. Consideration has also been given to economic factors such as raw material cost, which may often be as important an element in lubricant selection as the technical requirements; sofar, these factors have retardedthe applicationof synthetics in uses where products based directly upon petroleum fractions are satisfactory.
N HIS presidential address before the thirty-sixth annual meeting of the Institute of Petroleum in London, Evans (7) visualized the future of synthetic oils in these words: “It may not always be possible, either through economic or ‘political reasons, to subsist entirely upon Nature’s gifts which, after all, are not equally distributed throughout the world. T o meet the impact of adverse conditions it is necessary t o prepare in advance, by research, synthetic alternatives.” Scarcity of petroleumderived lubricants has already given impetus t o the development of processes which transform lower olefins [ethylene, FischerTropsch (Synthol) gasoline, and cracked wax] into oils. The physical and chemical properties of such oils are similar t o those of the natural oils with which we are already familiar. Thus it might seem surprising that, aside from measures adopted in Germany during the war, synthetic oils have had their greatest development in the United States where the availability of high quality mineral oils appears assured for many years t o come. I n the United States, however, the impetus is not economic pressure but performance; rapid developments in engineering have led t o performance requirements which the ordinary mineral oils cannot meet. Such requirements are specialized cases as a rule, where the advantage to be realized by the use of a synthetic oil must be appreciable if it is to justify the cost, which is almost inevitably greater. Where petroleum fractions give satisfactory performance, synthetics have not yet found a place, Moreover, in some instances special fractionation of natural petroleum oil to a very narrow boiling range and compounding with minor amounts of chemicals may permit oils t o meet requirements that uncompounded petroleum products do not. Synthetics are often distinctive as compared to hydrocarbons in one respect, but fall short in another-for example, the fluorocarbons, which are stable t o heat and oxygen and nonflammable, are characterized by great changes of viscosity with temperature, a definite disadvantage for some applications. Likewise, the silicones are outstanding with respect t o viscosity-temperature characteristics and oxidation stability, but are poor lubricants and therefore are barred from many uses. Thus, a synthetic oil is usually chosen because its particular characteristics are prime requisites for t h e intended purpose. This paper outlines the properties possessed by various types of synthetic oils, t o assist in the selection of the right lubricant for a given use.
I
FUNCTlONAL PROPERTIES
The development of a new synthetic oil involves the preparation of liquids with unusual physical or chemical properties in a viscosity range suitable for hydrodynamic lubrication under various operating conditions. Those who are t o apply the new oils require a detailed analysis of the functions the oil must fill. Usually this is not simple, because a description of temperature and pressure cycles, catalytic environment, and other conditions which the oil will encounter is not available. If any oil is selected on an empirical basis, either many tests must be run or one takes the risk that the oil may be well chosen for one characteristic but deficient or even harmful in other respects. Thus the lubriration engineer fipds himself with a n added responsibility in this field. Of the many properties that characterize the response of liquids to environmental conditions, only a limited number are important for its performance as a lubricant. Optical and magnetic properties, for example, can safely be neglected; other properties surh a s thermal conductivity and expansion, which are of occasional importance in lubrication engineering, are so nearly alike for most viscous organic liquids that they need not be considered where t h e emphasis is on differentiation. EGen after screening out those properties which, according t o the present status of experience, are either unimportant to performance characteristics or essentially independent of chemical structure, many arbitrary decisions and compromises must be made in order t o obtain a workable tabulation of pertinent properties. For example, in a series of polymers from a given starting material the grading of low temperature viscosity, freezing (pour) point, and volatility refers only t o the light grades (SAE lo),because these data are of practical significance mostly for these lighter grades. The relative position of the heavier grades may be inferred from the data on the more fluid representatives of the group. The properties finally selected for tabulation in Table I a s being most pertinent t o lubricating oil applications are listed below. It k ass.umed that proper cognizance is taken of such properties as odor, toxicity, attack on paints, packing and insulating materials, and other secondary properties which may be involved in the application of the lubricant. Flow Properties Viscosity level
Viscosity index range Low temperature suitability 2421
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 42, NO. .12
Table 1.
Functional Prop-
High Temperature Characteristics
Fluid Hydrocarbons Ethylene polymer Pronvlene oolvmer i3;i&&e pb1y"mer Isoarnylene polymer Wax olefin polymer Alkyl aromatics Low viscosity index
Viscosity Index (6)
50-> 70 70-> 70 70
-
< - 100-40 90-120
High viscosity index 70
10,000
>10,00010,000-
10,00010,000
