THE CHEMISTRY OF LUBRICATING-OIL ADDITIVES
0
ARNOLD MILLER A m o u r Research Foundation, Chicago, Illinois
AT
Synthetic lubricants, such as the esters of dibasic acids THE turn of the century, the sole functions of a lubricant were to reduce friction and wear of moving or the silicones, should be described separately. parts and to act as a mild heat-transfer medium. BearANTIOXIDANTS ing loads were low and static; comparatively low operating speeds were the order of the day. Bearing metals, The use of new bearing alloys which exhibit catalytic such as tin-base babbitt or cast iron, used under the effects a t the higher operating temperatures in presentmild conditions then prevalent were generally highly day engines has made the conventional oils of the resistant to corrosion. At that time, refined petroleum 1930's obsolete with respect to oxidation stability. oils and waxes and simple calcium-and sodium-base Oxidation products may be polymeric sludges which greases were more than adequate for the lubrication raise the viscosity of the lubricant so that bearings are fouled, or they may be acids and acid precursors which requirements of existing machines. Mechanical improvements and advancements lead- contribute to corrosion of bearing surfaces. Attempts to study oxidation of lubricants in engines ing to the development of the internal combustion engine and all of its ramifications increased the severity of alone are not successful because of the large number of operating conditions in terms of high bearing loads, unknown or nucontrolled contributing factors. The greater speeds, and elevated temperatures. Speed be- mechanisms by which pure hydrocarbons are oxidized came the watchword. Metallurgists developed bear- has been studied during the past 20 years as an adjunct ing alloys of much greater load-bearing characteristics, to engine testing. On thebasis of work to &te ( I , $ , 5), but, in many cases, these materials were sensitive to the it is believed that the oxidation of hydrocarbons inravages of corrosion. Under these severe conditions, volves the formation of organic hydroperoxides by a it soon became evident that the demands placed upon free-radical chain reaction. The initiation reaction is effective lubricants were not being met by even the most thought to be the formation of a free radical through highly relined petroleum oil. It became necessary to the removal of a hydrogen atom from the hydrocarbon develop chemical additives which, when blended in the molecule. proper concentrations with petroleum stocks, improved RH-R'+H. Initiation the existing properties and/or added new important characteristics to the lubricant blends so that they This initiation reaction may be activated by metal ions, might function efficiently in modern high-speed engines. heat, or light. The free radical reacts with oxygen to form the perThe transition of a lubricant to a complex multifunctional material may be illustrated by a consideration of oxide radical, which in turn forms a hydroperoxide by the various types and functions of the additives now removing an atom of hydrogen from another hydrocarbon molecule. used in lubricant blends (see the table). The present discussion will be limited to agents which R' + O n ROO' peroxide Propagation can be added to petroleum-base liquid lubricants.
-
ROO' Lubricating41 Additivee A rlriitiiia
Antioxidant Lubricitv additive Extreme pressure agent Pour-point depressant Viscosity index improver Rugt inhibitor Deter~ent Dispersant Foam inhibitor
7ins
Prevent oil decomposition Reduce friction and wear a t moderate temperatures Reduce friction and wear a t high temperatures Decrease low-tem~eratmeuse limit Lower the viscosity-temperature coefficient Protect metal surfaces against corrosion Prevent formation of sludges and varnishes Keep insoluble decomposition products in suspension Break foams
+ R'H
+
ROOH R" hydroperoxide
+
The continuous repetition of the above two reactions propagates a chain reaction whose rate is dependent noon the stahilitv of the resultant hvdrooeroxide. he various hidroperoxides participate in or initiate a number of simultaneous reactions. The decomposition of the hydroperoxide molecule proceeds by breaking the 0--0 to produce two free radicals. This introduces branching in the reaction sequence.
" .
ROOH
-+
RO'
+ .OH
Branching
The resultant free radicals which came from the hydroperoxide decomposition in the branchmg step may participate in a complex sequence of reactions. A 308
VOLUME 33, NO. 7, JULY, 1956
large variety of end products results. A few of the primary steps are shown below. 'OH
+ RH
-
HOH water
+ free Rradical '
H
I
RCO'
RCO
-+
1
k
H
aldehyde
H
I
TCO'
+
+ R'H
k
-
RCHIOH
H. hydrogen atom
+
R"
alcohol
free radical
RCH,OR1
+ hydrogen H'
H I
RCO'
A
+ R'H
+
ketone
atom
ble products are formed which prevent further participation of that particular chain in the reaction sequence. The other type is characterized by the ability to passivate metal-bearing surfaces or metal ions so that the catalytic effect on the oxidation rate (3) no longer exists. The chain-terminating additives are usually organic aromatic amines, phenols, or sulfides. Catalyst poisons are usually organic sulfides, phosphites, or thiophosphates. The demands for even more oxidation-resistant lubricants for the future in newly designed high-speed engines spurs on further study of lubricant oxidation mechanisms. Even more stable antioxidants must be developed. At present, dialkyl selenides (4) and phenothiazines (5) appear to offer great promise as high-temperature oxidation inhibitors. LUBRICITY AND EXTREME-PRESSURE ADDITIVES
The various lubrication domains which are known to exist are represented in Figure 1. Under optimum operating conditions, journal bearings exhibit frictional k formaldehyde free radical characteristics which are determined by applications of the laws of hydrodynamics propounded by Reynolds The newly formed hydrocarhon free radicals may and Tower in 1885. The frictional drag is dependent then be oxidized to form more peroxide radicals; the on the geometry of the hearing, the applied load, the aldehydes, ketones, and alcohols formed may be further shaft velocity, and the viscous-flow properties of the oxidized, and secondary condensation reactions may produce high molecular-weight polymers. This pro- luhricant. Bearings are so designed that oil is drawn automatically into the bearings as the relative velocity duction of long chains of repeating, self-propagating of the shaft exceeds a given minimal value, if the hearreactions after the initial absorption of oxygen results ing load is kept comparatively low. In the hydrodyin autocatalytic oxidation a t an ever increasing rate. namic region, the two mating surfaces are separated As the supply of oxygen or reactive fragments is exby a wedge of compressed lubricant with a thickness -hausted, the termination processes begin to outweigh greater than the surface roughness of the bearings. the propagat,ion step, and the oxidation tends to stop. Termination may occur by a number of processes. The composition of the hearing materials does not enter into the friction calculations, and the problem of wear ' The recombination and disproportion of hydrocarhon induced by metal-to-metal contact is unimportant. radicals can lead to stable molecular end products, or The viscosity of the lubricant is the only important the reactions can form free radicals which are too stable physical property to be considered in this type of to participate in further chain transfer. Again it lubrication. In hydrodynamic systems both the fricshould be emphasized that the type of oxidation product and the exact nature of the individual secondary BEARING processes are determined by the strncture of the hydrocarbons and the temperature of the reaction system. R O I L FILM 2R' 2R00'
--
RR ROOR
+
01
Termination
Mixtures of hydrocarbons generally do not oxidize in a manner which would he predicted by adding the effectsexpected from the individual components singly. For example, substituted naphthalenes are relatively stable toward oxidation by themselves; when mixed with a highly refined paraffin-base oil they are oxidized preferentially so that the blend oxidizes rapidly. At present the occurrence of these induced oxidation effects makes it impossible to predict accurately the behavior of new hydrocarhon blends. Inhibitors have been developed to reduce markedly the over-all rate of oxidation. The inhibitors may act through two types of reaction. One inhibitor type functions as a chain-terminating agent by reacting with the free radicals formed in the propagation step. Sta-
t
-+I + JOURNAL
Z
QUASIHYDRODYNAMIC
5 '
-r lo-' -BOUNDARY LL
BEARING
BEARING
ggggFq
lo-" Z
JOEAL
Y
Y
HYDRODYNAMIC
-
kW 0
U
10.'
-
I
! VELOCITY
--+
I
_
J
[SHAFT VELOCITY] [VISCOSITY] [BEARINGLOAD]
310
tional drag and the wedge thickness increase as the viscosity of the lubricant is raised. It is therefore necessary to choose a lubricant blend whose viscositytemperature characteristics will meet service pour-point requirements and maximum allowable frictional drag, and yet also will maintain a suitable thickness of oil wedge. When the bearing load is increased, or the oil viscosity decreased, or the shaft velocity reduced, the oil-wedge thickness decreases continuously until more and more oil-wedge breakdown occurs. In the moderate breakdown region, the load is incompletely born by the partially shattered oil wedge. In this mixed, or quasihydrodynamic, lubrication region the frictional r e tarding forces increase many hundredfold and the laws of fluid mechanics are incapable of fully explaining the observed high frictional forces. New effects involving the summation of a myriad of microscopic surface interactions must be included. If the thickness of the oil wedge is allowed to fall much below 6000 A., all apparent hydrodynamic effects cease and the efficiency of the lubricant is defined by the laws of boundary friction. This oil thickness is less than the height of the asperities on the bearing surfaces. I n the boundary lubrications region the frictional retarding forces present are essentially independent of the shaft velocity, applied load, bearing geometry, and lubricant viscosity. The frictional forces are a manifestation of microscopic surface chemical phenomena in which there is interaction between the metals of the bearing faces and the components of the lubricant. The load is not uniformly distributed, but is borne a t the tips of the surface asperities. Under the conditions that result from concentrating the load a t the peaks of the surface asperities, plastic deformation of the peaks in contact occurs until the true area of contact is sufficiently great to support the load. Pressure welds are formed a t the tips of the surface asperities. These are sheared during the relative motion of the hearing surfaces. Frictional heating and galling result. The boundary friction forces stem from this shearing of the many microscopic surface welds. Lubricants applied in a thin film to a hearing system under boundary lubrication are believed to function by interaction with the metals in the mating surfaces. This changes the nature of the surface and decreases the coefficient of friction either by decreasing the number of microscopic surface welds or by lowering the shear strength of the welds (6). Since boundary friction is a surface phenomenon, the lubricating ability of a particular oil blend may be altered by the addition of small concentrations of surface-active agents. These radically change the nature of the mating surface by theformation of oriented monolayers adsorbed on it. The net effectiveness of the oil is determined by the type and nature of the reactions with the specific bearing metals, Studies (7) of additive systems have revealed that two adsorption mechanisms are possible. One involves the physical adsorption of the additive to form the monolayer, while the other involves the chemisorption of a
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reactive functional group with the surface atoms of the metal in the bearings. In the case of physical adsorption, the resultant friction obtained for a given additive blend through a range of temperature is relatively independent of the nature of the bearing metals. However, where chemisorption is involved, the reactivity of the bearing metals becomes extremely important. The effectiveness of a given lubricity additive may be shown to he dependent. on a number of variables (8). These include the adhesional force between the polar group and the adsorbent, the sum of the van der Waals interactions between the adjacent methylene groups of the surfactant which make up the cohesional force, the resistance of the monolayer t,oward mechanical compression, and the ability of the surfact,ant molecules to pack closely together. The same additive may function by either a chemisorption or a physical adsorption mechanism, depending 011 whether or not the bearing metals are of a type which can react, with it. Thus, oleic acid is physically adsorbed on gold-bearing surfaces, hut forms a soap during chemisorption on lead surfaces. "Extreme-pressure" (EP) agents are a misnamed class of materials which may be added to lubricating oils to lower the coefficient of boundary friction in bearing systems under certain conditions. High temperature, rather than the high loads ("extreme pressure"), present in the bearing system are necessary for the effective functioning of EP agents. These compounds thermally decompose a t the high temperatures occurring on the surfaces of heavily loaded bearings. This produces reactive fragments which form compounds with the hearing metals. These surface compounds have a lower shear strength than the corresponding base metal; the resulting coefficient of friction is lower. Compounds containing sulfur or chlorine (7) have been utilized as extreme pressure agents for many metal systems. Representative compounds are dibenzyldisulfide and chlorinated waxes. Phosphorus compounds (9) act as polishing agents in &eel systems by forming a low-melting eutectic with steel. This aids the wear-in process. I n the case of extremepressure additives, it is again important to study the agents in specific hearing-metal combinations, since the nature of the reaction products, and hence the coefficient of friction, is established by the pair of hearing metals which is being used. The high-temperature solubility of any potentially good new additive must be such that the surface-active agent will maintain a sufficiently high concentration at the liquid-solid interface. Finally, the resultant compound formed on the metal surface must be insoluble in the bulk solution. If it were soluble, its leaving the metal would produce detrimental corrosion. The development of additives capable of reducing friction between bearing surfaces a t high loads has made possible the widespread use of the hypoid gear in the automobile industry, extension of the lifetime of the cylinders of aircraft engines, and the effective use of special high-speed antifriction bearings.
VOLUME 33, NO. 7. JULY, 1956 POUR-POINT DEPRESSANTS
One of the requirements for modern lubricants is that oil blends be able t o pour and be pumped easily under severe cold environments, yet not be excessively volatile a t elevated temperatures. The lubricant must function effectively both in a cold engine and, after warm-up, in the hottest regions of the piston walls. I n most cases, straight petroleum-oil blends do not meet the strict viscosity requirements set up by present-day operating conditions. Polymeric materials have been developed which modify the flow properties of lubricants so that they can function adequately through a wide temperature range. The ponr point of an oil is arbitrarily defined as the lowest temperature a t which the oil will flow when chilled under specific conditions (10). Two separate and distinct causes for an oil's having an observed ponr point are known. One reason for an oil's no longer being able t o flow is the crystallization of the wax component in the oil blend t o form a rigid semisolid. I n wax-free oils, the increase in viscosity as temperature decreases may become so great that flow ceases. The wax pour point may be lowered by a chilling and filtering operation to remove the wax. The viscous pour point may be decreased by the use of low-viscosity stocks as components in the blend. However, dewaxing operations are expensive and time consuming, while suitable lolv-viscosity components for blends are not always readily available. Other means for solving this problem had t o be developed. Studies of the rheological properties of lubricants below the wax pour point have been most useful in elucidating some of the essential features of the mechanism of wax pour-point depression and necessary structural features of pour-point depressants (11, 12). The wax pour point may be lowered chemically by the introduction of small concentrat.ions of polymeric materials which adsorb on the surfaces of the growing wax crystals. These so alter the crystal habit that intercrystal growth is hindered. The concentration of wax crystals remains the same a t a given temperature, but the crystallites do not form a rigid interlocked matrix which would impede lubricant flow. Pour-point depressants are usually high molecular-weight polymers which have essentially the same solubility in the lubricant blend as that of the wax. Polyacylates, n-alkyl polymethacrylates, and alkyl-aryl condensation polymers are among the materials presently used as pourpoint depressants. Pour-point depressant,^ of the type used a t present have a number of drawbacks. The phenomenon of "pour-point reversion" occurs when the temperature cycle to which the blend is subjected is such that the wax phase is allowed to exist in crystalline form for extended periods of time. The pour point of the blend containing the depressant under these conditions tends to increase gradually; it tends t o approach the pour point of the straight base oils. There appears to be a slow interlocking of the crystallites in spite of the pres-
ence of the initially adsorbed depressant, so that a fairly rigid matrix is eventually formed. The present knowledge of pour-point depressant behavior is such that trial-and-error methods have to be used for determining optimum solubility characteristics. Additional investigation of these factors is needed. VISCOSITY-INDEX IMPROVERS
As previously discussed, the rate a t which viscosity changes with respect to temperature is important t o lubrication performance. In order to deal with viscosity-temperature relationships in a numerical manner, the petroleum industry adopted the concept of viscosity index ( V I ) as a relative measure of the dependence of lubricant viscosity on temperature (IS). The empirical VI scale is such that the greater the V I of a lubricant, the smaller the viscosity-temperature coefficient of the oil. It has been shown (14, 15) that series of high molecular-weight linear polymers, such as polybutenes or polymethacrylates, are effective in increasing the VI of some petroleum blends. The mechanism of VI improver action is not completely understood, but present concepts involve both the limited solubility of the polymer in the base oil and the additive effect of introducing a high-viscosity polymer (16, 17). At low temperatures, the polymeric additive is only slightly soluble and is solvated to a small extent; the bulk of the additive exists in either colloidal suspension or in a highly convoluted state. As temperature rises, more and more of the polymer goes into true solution, the polymer molecules become highly solvated and uncoiled, and the high viscosity of the polymer contributes t o a greater and greater extent toward the total viscosity of the lubricant blend. Thus, a t low temperatures, the viscosity of the lubricant is essentially that of the bulk oil carrier, while a t higher temperatures it is dependent on the concentration and composition of the given VI improver. Hence, the net result is to decrease the variation of the viscosity with temperature. I n many cases, V I improvers are subject either t o thermal or shear breakdown (18, 19, 20). This markedly diminishes the desired viscosity-temperature effects. The addition of VI improvers to a base oil increases the viscosity of the blend; thus a usable high VI blend which exhibits both low viscosity and a relatively low volatility cannot be prepared. With the present knowledge, proper solubility characteristics must be determined by trial and error. RUST INHIBITION
The extended storage of engines under adverse, humid conditions and the operation of equipment such as steam turbines under extremely severe environments require special protection of metal surfaces against rusting. A refined petroleum oil by itself is inadequate for this purpose. Water droplets easily displace the oil film a t the liquid-solid interface and initiate corrosion. It has been observed that alkaline earth salts of high molecular-weight petroleum sulfonic acids, certain
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naphthenates, function by peptizing sludge deposits and keeping them in colloidal suspension. The detergents, such as barium and magnesium phenolates, function either by controlling oxidation processes to prevent formation of insoluble products or by reacting with the oxidation products so that they do not tend to polymerize. Pro-oxidants are oxidation directors which, when acting along with a suitable antioxidant, reduce the over-all oxidation rate. ADDITIVE RESEARCH Fi-
2.
A. Mineral oil
Reeistanse of Oil Blend. Toward water Leashine
+ rust inhibitor;
B, pure mineral oil.
The status of the technology of lubricating-oil additives is a fleeting, changing one. As new high-speed mechanical devices are conceived and built, existing lubricant materials and formulations are rendered obsolete. New compositions must be developed and pressed into use. While the components and nature of lubricating oils may differ, the fundamental physicochemical principles underlying their action do not. The growing knowledge of these principles serves to guide further study in the field of additives for lubricants.
amines and amine salts, and other materials have rust preventative characteristics. However, empirical studies by themselves soon seemed to exhaust the sources of possible additives to meet newer, more severe conditions. Fundamental studies ($0,$1) on the mode of action of rust inhibitors have served to give information for the rational selection of new additive types. LITERATURE CITED There are a number of ways in which a rust inhibitor (1) ZUIDEMA, H. H., Chem. RPVS.,38, 197 (1946). may function. Amine inhibitors, for example, may (2) ROBERTSON, A., AND W. A. WATERS,Tmns. Fapaday Soe., neutralize corrosive acids produced during oxidation of 42, 201 (1946). the oil. Most inhibitors, however, are believed to func- (3) \VATSON,It. W., AND T. B. TOM,Ind. Eng. Chem., 41, 918 tion because of their surface activity. This latter type (19491. ~,~ DENISON, G. H., JR., AND P. C . CORIDIT, Ind. Eng. Chem., of additive is usually an asymmetric long-chain organic 41, 944 (1949). molecule which has a polar group a t one end. The MURPHY,C. M., JR., R. RAVNER,AND N. L. SMITH,2nd. polar group is selectively adsorbed to the metal, huildEng. Chem., 42, 2479 (1950). ing up oriented close-packed monolayers at the metalBOWDEN, F. P., AND D . TABOR,J. A p.p l . Phys., ~. 14, 80 (1943): oil interface. The resultant monolayer is hydrophobic Ibid., "The Friction and Lubrication of Solids," Oxford and resists the leaching effect of water droplets. FigUniversity Press, Oxford, 1950. ure 2 shows the result of a test to demonstrate the corBROPHY, J. E., AND W. A. ZISMAN,Ann. N. Y. Aead. Sci., rosion resistance of a given oil base with and without an 53. 8x6 - - - (lS.511. \-~--,. asymmetric polar additive. The strength of the polar (9) BEECK,O., J. W. GIVENS,AND A. WILLIAMS, PTOC.ROY. Soe. (London),A177, 90 (1940). bond between the metal and the additive, the compactness of the resultant film, the solubility of the additive (10) Am. Soe. Testing Materials, Designation D 97-47. (11) GAVLIN, G., E. A. SWIRE,A N D S. P. JONES,Ind. Eng. Chem., in both water and oil, and the temperature of the system 45, 2327 (1953). will determine how effective the rust inhibitor will be. (12) JONES,S. P., AND J. TYSON,J . Colloid Sci., 7 , 272 (1952). AND G. H. DEAN.Oil. Ca8. Since many of these factors depend on the composition (13) . . DAVIS. E. W.. L. C. LAPEQROUSI. J . , 30, 92 ('1932). and type of hulk lubricant used, it is important to deVANHORNE,W. L., Ind. Eng. Chem., 41, 952 (1949). velop rust inhibitors that are tailor-made for the par- (14) (15) THOMAS, R. M., J. C. ZIMMER, L. B. TURNER,R. ROSE, ticular oil blend in question. AND P. K. FROLICH, Znd. Eng. Chem., 32,299 (1940). ~
~
~
~
--.
OTHER ADDITIVES
Other material spurposely added to lubricating-oil blends include foam inhibitors, dispersants, detergents, and pro-oxidants. Oil blends containing high molecular-weight VI improvers foam to a marked degree when agitated. This makes necessary the use of foam inhibitors, such as silicones or selected esters. Dispersants and detergents are utilized to keep engines free of sludge deposits. The dispersants, such as metal
(16) EVANS,H. C., AND D. W. YOUNG, Ind. Eng. Chem., 39,1676 (1947). (17) ALPREY,T., A. BARTOVICS, AND H. MARK,.I. Am. Chem. Soc., 64, 1557 (1942). (18) . . Am. Soe. Testino Materials Sveeial Technical Publication. No. 111 (1950j. (19) BESTUL,A. B., J. Appl. Phys., 25, 1069 (1954). (20) KLAUS,E. E., AND M. R. FENSKE,Lubrication Eng., 1955, ini
(21) BAKER,H. R., AND W. A. ZISMAN, Ind. Eng. Chem., 40,2338 (1948). (22) WAI,TERS, E. L., AND R. G. LARSEN,Corr08ion, 6 , 9 2 (1950).
