Oxidation Characteristics of Lubricating Oils Relation between Stability and Chemical Composition G . H. VON FUCHS AND H. DIAMOND Shell Oil Company, Incorporated, Wood River, 111. Analysis of rate curves obtained by measurement of oxygen absorption has elucidated the effect of basic composition, methods of processing, and addition agents on the stability of oils. Oxidation characteristics are strongly influenced by interaction phenomena between different types of components. Saturated hydrocarbons in an oil have a tendency to rapid autocatalytic oxidation whereas aromatic compounds act as antioxidants by a mechanism of autoretardation. The balance between these opposing effects is strongly dependent on the reaction temperature. Under given HE principal cause for the deterioration of lubricating oils while in service is generally understood to be a process of oxidation. Not only is part of the lubricant destroyed and eliminated as volatile oxidation products, but also other substances are formed which remain ip the oil with deleterious consequences. For example, sludge accumulates in the crankcase of an engine and clogs oil lines, corrosive acids are formed, and other surface-active compounds are produced which, in the case of turbine oils, cause emulsi.6cation (21). The problem of oxidation stability has been aggravated by the recent trend of the automotive industry toward more drastic operating conditions-namely, higher engine temperatures and smaller clearances between piston skirts and cylinder walls. Deposition of lacquer on pistons and ring sticking in certain engines are partly attributable to this cause. Demands upon the lubricant have been further increased by longer use of an oil fill and reduction in quantity of make-up oil which need be added between drains to replace that consumed. The introduction of solvent extraction as a refining process has improved the viscosity index and color of oils and reduced their tendency to precipitate sludge, but it has also been found that an excessively refined oil containing virtually no aromatics (i. e., a white oil) is prone to oxidize rapidly and to form soluble oxidation products. Considerable concern has therefore arisen over the possibility of overextraction in current practice, and it has been realized that less thorough refining to some intermediate aromaticity might yield a product of greater stability. Several investigators have shown in a general way that aromatic components in an oil lower its rate of oxidation (3, 4, 6, 8, 19, 24, 26). In a recent study of some factors controlling oxidation (16),a lubricating oil was separated into
conditions of temperature and metal catalysis, maximum stability is observed at an intermediate (optimum) aromaticity. The phenomenon of autoretardation is differentiated from conventional inhibitor action. Reactions involved in the formation of saponifiable material (free, combined, and potential acids) may account for most of the oxygen absorbed by an oil. Results of the laboratory oxidation experiments have shown good correlation with oil deterioration and lacquer deposition in small-scale engine tests. fractions which differed from one another and from the original stock, both in aromaticity and stability. Earlier workers (la, 13, 14) classified various oils’according to the form of their oxidation-rate curves but laid emphasis on the mathematical equations for the curves rather than on the nature of the components responsible for the observed phenomena. An attempt has therefore been made systematically to correlate the oxidation characteristics of lubricating oils with their basic composition, methods of processing, certain addition agents, etc., and in some measure to derive an explanation which is in agreement with the chemical behavior of typical compounds present in oils (23).
Experimental Method For purposes of fundamental investigation, the oxygenabsorption measurement was chosen since it can be performed with precision and is probably the best single criterion of total deterioration. Measurements of this type are well adapted to kinetic interpretation, as the primary process i n the oxidation of an oil is one involving oxygen regardless of the products which are eventually formed. On the other hand, sludge formation, for example, though a major problem in lubrication, does not indicate other phenomena which occur. Thus, a white oil may be oxidized until it contains large quantities of acids but without the formation of any insoluble products. Similarly, changes in viscosity may be due t o many causes. Furthermore, as discussed below, oxygenabsorption measurements properly applied and interpreted can furnish a means for evaluating stability which is more suitable than the customary routine tests from the viewpoint of actual service performance. 927
928
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 8
FIGURE 1. SCHEMATIC DIAGRAM OF
FOR hIEAsURIKG RATEO F OXIDATIOX
SYSTEM
A.
Glass cylinder Glass piston Iron core D. Solenoid E . Motor drive F. Valve system G. Reaction yessel H . Oil sample 1. Oil b a t h J. Thermostst K . Condenser L. Cold t r a p .>I. K10 a n d COS absorber K. Flowmeter 0. McLeod gage P. OzsupPlY Q . Vacuum R. Full-range manometer S. Vents T. Control manometer G. G a s buret V . Leveling bulb TI'. Lift motor S. C h a r t ; horizontal arrow indicates v o l u m ~ , vertical arrow indicates time Y . .iutomatic b u r e t , controller, and recorder
B. C.
o @
o
Q
I -
T h e apparatus a n d technique adopted were essentially those described by Dornte (f$), with some modifications and refinements. T h e oil t o b e studied was oxidized by pure oxygen under carefully controlled conditions, a n d t h e volume of gas consumed by t h e reaction measured at appropriate intervals. Oxygen absorption n-as plotted against time on a linear scale, and conclusions drawn both from t h e position a n d form of t h e r a t e curves were obtained. T h e work described was concerned only with phenomena occurring up t o a total oxygen absorption of approximately 2000 cc. (normal temperature and pressure) of oxygen per 100 grams of oil; reaction was usually n o t carried further, since beyond this point t h e oil would have been too far deteriorated for any practical considerations.
DESIGSOF APPARATUS.Oxygen was circulated in a closed system through a small (18-gram) sample contained in a glass absorption cell (reaction vessel) immersed in a bath a t constant elevated temperature. The cell was equipped with a preheating coil and fritted-glass filter which dispersed the gas into fine bubbles and thus ensured intimate contact with the liquid. The flo~ rat,e employed was well in excess of that necessary t o keep the oil saturated with oxygen, so t h a t reaction velocity was insensitive t o minor variations in flow above that rate ( 2 1 ) . I n early experiments the circulating pump consisted of a sylphon bellows, expanded and contracted by motor drive and connected t o a pair of mercury-filled traps which acted as check valves, to provide unidirectional flow. CIRCULATING PUMP. Subsequently, an improved pump was employed, consisting of a glass cylinder and closely fitting piston of glass tubing sealed at both ends with an iron core in the center. Reciprocating motion was transmitted t o the piston by magnetic attraction from a motor-driven solenoid surrounding the cylinder; the valves were glass balls resting on ground-glass seats ( 2 ) . Thus the circulated oxygen at no time came in contact with any metal, except that specially inserted in the oil in studies of catalysis, and the whole circulating system could be dismantled and cleaned before an experiment whenever it became contaminatcd with condensed oil vapors or fog from previous
runs. This arrangement facilitated the obtaining of accurate and reproducible results. Furthermore, the introduction of the all-glass valve system eliminated complications from a source irrelevant t o service conditions: there is evidence that certain active reaction products, possibl: organic peroxides, are 'removed by passage th
