Present Problems and Future Trends in Lubrication - ACS Publications

Present Problems and Future Trends in Lubrication W. A. ZISMAN Naval Research Laboratory, Washington

25, 0. C.

This report summarizes the principal problems in the field o f lubrication and their relation to recent advances. Wherever possible the probable future trends of research and development are indicated. The lubrication problems discussed include: wear preventive and corrosive properties of oiliness additives, shear strength and durability o f extreme pressure agents, internal cohesion and temperature coefficient of viscosity of organic liquids, improvements in the viscosity index scale, synthetic oils and their improvement, antioxidants for higher temperatures, the colloidal properties o f antirust additives, the relation of oil solubility and shear stability o f viscosity index improvers to the structure o f polymers, the nature o f polyvalent soaps and amine-acid complexes and their micelle forming proper-

ties in oils, the rust inhibition and work stability o f greases, new gelling agents and their improvement, high temperature greases and bearings, and the approach toward lifetime lubrication in electric motors. Until 1940 the principal advances in the field of lubrication were based on improvements in petroleum refining methods and on the essentially empirical development of a technology of addition agents for lubricants. Advances of the future several decades are likely to result from an increased understanding o f how these additives function, through the gains in performance and maintenance economics made possible b y tailor-made synthetic oils and new gelling agents, and through the development of new solid lubricants, improved nonmetallic bearing materials, and better designs in rolling contact bearings.

NGINEERS and scientists are now well aware oi the UP precedented needs which have developed since 1940 for greatly improved lubricants and decreased lubrication maintenance. A large and remarkable variety of machines instruments, and other devices have had to be operated a t new extremes of speed, temperature, and altitude. Seasonal or other climatic change-over of lubricants has become increasingly troublesome. Today, some of the required advances in lubrication have been made. Because of these advances and the current trends in research, a reasonable basis now exists for speculating about future trends.

Early experiments on the friction and \Tear of electrical contacts by Holm (73) and thc later comprehensive investigations of Born-den and Hughes (19), Bowden and Young ( 2 3 ) ) and iYhit,ehead (123) on the friction of degassed solids have proved that in the absence of metallic oxide coatings or of adsorbed films of either water or oxygen, the coefficient of friction ( p ) of most clcan dry metals is much above unity, and values u p to 100 h a w h e n reported. This has led to the conclusion that absolutely clean, gas-free, unoxidized metals mill always adhere immediately upon contact. Adhesion occurs even in the air with some clcan solids, as Beilby (13) arid Tomlinson (118) proved over two dccades ago. As long a s t.he oxides and the usual adsorbed films of oxygen and water are present on t,he metals, 1.1 rarely exceeds 1 to 1.5. Na,ny common metals have values for p of 0.5; sonic unusually hard metals have values as low as 0.3, and a few soft metals or alloys cont,aining lead and indium have values of p ranging from 0.3 to 0.1. The problem of alloying metals or of using surface films of metal is a n old one to physicists and metallurgists. Since it has been thoroughlj. investigated and reviewed by Bowden and his coworkcr’s (28),it will not be discussed here. An enormous amount of effort, has been spent looking for useful liquids which would be more effective than oxides and adsorbed gases in decreasing ,u and the rate of wear of smooth uneontaminated metals. Liquids having good hounclary lubricating properties were found by the dozen, but in most instances later research revealed that the principal cause was minor concentrations of’ act,ive impurities. The recognition of this fact led to increasing attention to pure, polar-type liquids as in the classic investigations of Hardy (72). Unfortunately, of the most effective liquid compounds found by hiin and other investigators, many werP chemically reactive with common metals; some were liquids having large temperature coeficients of viscosity; and ot’here were too unstable to atmospheric oxidation for wide use. O w l . 30 years ago emphasis increased on the use of minor concentrations of surface-active or chemically reactive compounds in mineral oils. Practical gains resulted from much semiempirica! work and a n imposing though a t times confusing array of oiliness agents and wear preventives, such as oleic or other fatty acids, ( I l S ) , was patented; but it proved difficult to learn precisely

E

Research on Friction and Wear Prevention There are two extreme conditions of lubrication-hvdi odynamic and boundary. In the hydrodynamic condition, no contact exists between the rubbing solids; the parameters of importance involve only properties of the bulk liquid such a s the viscosity, the density, the temperature coefficients of the viscosity and the density, the heat transfer coefficients, and those defining the geometry of the bearing system. In the boundary condition, contacts always exist between the rubbing solids, and the physical and chemical properties of the contacting surfaces are important. Hydrodynamic conditions of lubrication are usually desired in the operation of mechanisms because the coefficient of f i iction may be a few hundredths or less, and practically no wear occurs‘ boundary conditions of lubrications are avoided where possible. because of the resulting power consumption and the wearing, galling, or seizure of the rubbing solids. I n the past 15 years the work of a large number of investigators and laboratories has revealed much about the friction of metallic solids, the true area of contact, the generation of “hot spots” under certain conditions of sliding, the effect of oxides and adsorbed gases, end the pressure welding which may occur a t points of contact at lower temperatures. As the review by Bowden and Tabor in 1946 (21) and their valuable book in 1950 (32)are now well known, it is preferable t o survey here only very recent progress and to concentrate on some of the important unsolved problems of mcar prevention. 1406

INDUSTRIAL AND ENGINEERING CHEMISTRY

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how these chemical agents performed the job of reducing friction, wear, or seizing. T h a t solutions of amphipathic polar compounds in nonpolar liquids have better boundary lubricating properties than the solvent has been known since the pioneer researches of Langmuir (79, 80), Hardy (72), the later studies by Sameshima et al. (104), and Beeck, Givens, and Smith (11), as well as numerous others. In the early years of World War 11, valuable investigations by Tabor (116), Hughes and Whittingham (75),and Frewing (69,60) on the temperature transitions in "stick slip" behavior of dilute solutions of long-chain polar compounds revealed that the friction-reducing films could be desorbed by raising the temperature, t h a t these films were physically adsorbed, and that they could be important in boundary lubrication. Our own wartime research (14, 15, 28) on the adsorption of these compounds from various nonpolar liquids demonstrated t h a t when such adsorbed films are first formed, they are always monolayers bound t o the metal by electrical dipole-image forces. The relative adsorptivity of long-chain polar compounds was shown t o increase in the order: esters, alcohols, carboxylic acids, and primary amines. This is precisely the order to be expected from the well-known expression for the dipole mirror-image adsorption energy

(v0 = ,$), since it is greatest for molecules

possessing both a large value of @ and a small value of T . Approximate calculations of Uofrom molecular constants agreed with measurements of UOobtained in studies of wettability (24, 28). Unfortunately, it is more difficult to obtain T accurately than M because of the basic problem of locating within the molecule the electrical center of gravity of the dipole. It was also demonstrated (14, 28) that the total energy of adsorption (U) of unbranched, aliphatic, polar molecules included a significant contribution ( UN), which is roughly proportional to the number ( N ) of carbon a t o m in the chain, arising from the van der Waals cohesive forces between neighboring methylene groups of adjarent molecules. Such a monolayer in the solid or condensed state makes a contribution to U which may be as large or greater than that due to the dipole adsorption energy ( Uo). Finally, the general conclusion resulted that regardless of which metal is used a maximum temperature (T,) always exists above which these physically adsorbed films must desorb, and at T , (if not a t lower temperatures) the friction ( p ) must increase rapidly. Measurements on the change with temperature of the organophobic property of a series of these films demonstrated that T , varied between 40" and 100' C. for alcohols, between 70' and 130' C. for carboxylic acids, and between 100' and 150' C. for primary amines. These observations are roughly in agreement with Frewing's values of the stick-slip temperature transitions for steel. These conclusions about the physical adsorption mechanism and relative adsorptivity have since been verified using various other methods by Karle and Brockway (76),Greenhill (66), Rowden and Moore (go), Menter and Tabor (86),and Daniel (38). Although organic acids first become attached to the metal ?through dipole-image forces, chemical combination with the surface atoms of some metals occurs later. Dubrisay (42) and Prutton et at. (100) demonstrated that the fatty acids react -to form the salt when the metal is coated with the oxide, Tingle (117) found that traces of water should be present also. Many soaps and salts of other organic acids formed in situ have low values of 9, and some manifest greater durability than others when exposed to moderate unit loads, This and related data accumulated by many workers is the basis of numerous patents on wear preventives and oiliness agents. The greater resistance of soaps in situ than the corresponding physically adsorbed fatty acids against being desorbed or scuffed o f f by friction a t high .temperatures has been demonstrated with the stick-slip machine by Bowden, Gregory, and Tabor (18) and in later studies on the .formation of soaps following adsorption of the acids by Menter :July 1953

and Tabor (86) and by Daniel (38). Among other things, they proved that the stick-slip transition temperatures obtained with each soap corresponded well with its softening point. Comparably careful research has not been reported on adsorbed acids or salts other than the fatty acids and their soaps. A major problem is to learn with which combinations of organic acids and metals and under what reaction conditions there will result the most useful wear preventive coatings; also, which combinations will favor corrosion or erosion of the metal. Answers to these questions are necessary in order to understand and apply p r o p erly the great variety of known chemical wear preventive and oiliness additives. Wide use has been made of antiwear and antiseize coatings resulting from the reaction at the oil-metal interface of inorganic acids or of organic compounds containing active or labile chlorine, sulfur, or phosphorus. Since the pioneer investigation of Beeck, Givens, and Williams (22) on phosphors-containing compounds and their polishing action on steel during friction, relatively little fundamental work has been reported on such compounds. Research on sulfurized compounds by Campbell (32), Simard, Russell, and Nelson ( I l l ) , Hughes and Whittingham (75),Clark, Gallo, and Lincoln (SS), Gregory (68), Davey (39),and Greenhill (65) has shown that thin films of some metal sulfides formed in situ reduce friction and seizing. When such active-sulfur compounds are present simultaneously with fatty acids, an added and valuable reduction in the coefficient of friction occurs. This explains the early practice of using sulfurized fatty oils as extreme pressure agents and wear preventives. Organic compounds containing active chlorine have been shown by Gregory (68) t o act similarly t o produce thin films of metal chlorides, but it was found essential not to have water present. Finch (52) has outlined reasons for believing that extreme pressure agents must have the property of being more fusible than the underlying metal. Evidently, a better understanding is needed of the mechanism by which extreme pressure agents becomes active in the oil and of the physical properties essential in the protective coatings formed. Research has shown, therefore, t h a t the coefficient of friction ( p ) of rubbing metals in the atmosphere can be reduced to as low as 0.10 through the physical adsorption of surface-active agents or oil additives; but the beneficial effects will always disappear a t temperature of from 50' to 200' C. because of increased desorption and solubility with the rise in temperature. More permanent reduction in friction and values of as low as 0.05 to 0.10 can be obtained, but only through the use of pure reactive liquids or of solutions of additives which react chemically with the metal to form protective surface coatings. If extremely high unit loads are t o be supported without damage, surfacp reactions leading to metallic compounds such a s phosphides, sulfides, or chlorides are necessary. Precisely why these coatings have low shear strength, but yet are sufficiently durable under boundary conditions of lubrication is, in most instances, not understood. Molecular Structure Required in a Synthetic Oil When in recent years chemists turned to the synthesis of tailormade oils for aviation, the basic question t o be answered was how should a molecule of a liquid be constituted in order t h a t it have the extreme temperature properties desired? An answer can be given today in reasonably general and useful terms, provided that it is agreed to relegate to chemical additives the tasks of caring for any unusual demands for oxidation inhibition, detergency, rust inhibition, wear prevention, and load-carrying capacity. Nowadays, the lubricating oil base most needed in aviation and in many other fields of application is one which will best satisfy the joint requirements of low viscosity a t ordinary temperatures, as small as possible a temperature coefficient of viscosity, better resistance to atmospheric oxidation, and a wider liquidus range than is obtainable at present from high viscosity index mineral oils of the same viscosity.


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Early work on pure hydrocarbons by Rfikeska (89), Evans (&), Nissan, Clark, and Nash (97), the A.P.I. Project No. 42 team of Schiessler and coworkers (37, 105-107), and many others had

related some of the important bulk properties of pure hydrocarbons to molecular structure. During World War I1 and since, several of these relations have been broadened and others have been given new interpretations, so t h a t they can be used now to answer many of these questions and to correlate and predict the properties of a much greater variety of nonassociated liquids such as esters, ethers, polyalkylene oxides, silicones, and fluorocarbons (96).

Figure 1. Comparison of Triethylene Glycol Diethyl Ether and n-Tetradecane The resulting "structural guides" have been of inestimable value. The following condensed and up-to-date version is presented here in order t o relate them to other topics t o be discussed later. First, to obtain a n organic liquid with a small temperature coefficient of viscosity, the molecule should not contain chemical groups having strong associating tendencies, and it should have a linear or rodlike molecular structure possessing the greatest possible flexibility through free rotations about the chemical bonds. Secondly, if the viscosity (at 100' F. for example) is to be low for a high boiling liquid, the molecule should have no side chains. Thirdly, the most reliable and effective way t o synthesize into a liquid the property of having a very low freezing point is to a b tach a minimum number of short side chains to the principal chain of the molecule in such a way as to create the maximum possible hindrance t o close interlocking or alignment of neighboring molecules. (Branch chains near the center of the principal chain are more effective than those near the end of the chain in reducing the freezing point.) Fourthly, the most satisfactory way to obtain a low viscosity liquid of high boiling point is to introduce into the molecule one or more nonhydrocarbon, nonassociating groups such as the ester group. Fifthly, other things being equal, the greater the molar coefficient of thermal expansion of a liquid, the greater the temperature coefficient of viscosity. This is simply one way to allow for the effects of large differences in the internal cohesive forces in the liquids. These qualitative rules are part empirical and part theoretical, and it is important t o try to express them some day in more precise, unified, and more fundamental terms. Kearly every one of the interesting synthetic oils developed in recent years obeys fairly closely the structural guides just described. For example, the temperature coefficient of viscosity (or the ASTM viscosity-temperature slope) for liquids having the same viscosity at 100' F. increases in the order: open-chain dimethyl silicones, double alkyl-stoppered polymers of ethylene oxide, then the corresponding 1:1 copolymers of ethylene-propylene oxide, the corresponding polymers of propylene oxide, the nalkanes, and the perfluoroalkanes (96). The decreased freedom of rotation about the main chain in going from a polyethylene oxide to a n-alkane is evident a t once upon comparing the FisherHirschfelder ball models of triethylene glycol diethyl ether and n-tetradecane shown in Figure 1. A similar comparison of the ball models will reveal that there is a marked decrease in the free-

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dom of rotation in going from a n-alkane t o a perfluoro-n-alkane. Marked flexibility in the open-chain dimethyl silicones has been firmly established by Fox et al. (57) and by Flory et al. (56) using independent methods. Flexibility is caused here by the presence of the many oxygen atoms in the siloxane chain, by the tetrahedral nature of the bond angles, and by the fact that the silicon atom is much larger than the carbon atom. Because of this difference in atomic dimensions there are no analogous dimethyl-substituted linear polymers among either the polyalkylene oxides or the polyethylenes. Such fundamental differences in the viscosity-temperature properties of many analogous families of liquids can be shown conveniently (96) in a plot of the ASTN viscosity-tempwature slope against either the 100' F. viscosity of the liquid (Figure 2 ) or against (Z) the total number of atoms in the principal chain of the molecule. The family of n-alkanes have the smallest ASTM slopes of all hydrocarbons of the same value of Z (96). The curve for the double, alkyl, chain-stoppered polypropylene oxides nearly coincides with that of the n-alkane$, while the curve for the double, alkyl, chain-stoppered 1:1 copolymers of ethylene and propylene oxide is below and nearly parallel to that for the nalkanes. The curve for the open-chain dimethyl silicones lies approximately parallel to but very much below the other curves. The need is widely recognized for a inore rational and more generally useful method of measuring and comparing the viscosity-temperature properties of liquids. Although the Hardiman and Nissan viscosity index scale (71) is being used more frequently than the Dean and Davis scale (do), it is not always as useful as the ASTM slope. For example, in our investigation ($4) of the viscometric properties of a large homologous series of very pure aliphatic monoesters, it was shown that the Dean and Davis viscosity index went through a maximum as the molecular weight increased, the Hardimaii and Kissan index increased rapidly to become practically constant about halfway up the series, and the ASTM slope decreased rapidly and then more C SO

o n ALKANES 0

0 80

A 0

Figure 2.

HIGHER MONOESTERS DIETHER OF POLYPROPYLENE OXIDE ETHER-ESTER OF POLYPROPYLENE OXIDE

Comparison of ASTM Slope vs. Viscosity for Analogous Liquids

slowly as if approaching an asymptotic minimum a t the end of the series (see Table I). It is difficult to believe that the molecular mechanism involved in the temperature decrease in viscosity is exhausted a t such low molecular weights as halfway through the series, Ramser (101) showed in 1949 that the change of viscosity of a liquid with temperature cannot be represented by a single index and that two parameters are required. I n Figure 2, in effect, two parameters are used-the ASTM slope and either Z or the 100' F. viscosity. Perhaps a more generally satisfactory system will be based some day upon the comparison of the properties of each liquid relative t o those of the n-alkane of the same reference viscosity. Hence, data are needed on the visco-


Vol. 45, No. 2

, Table 1.

ir

Some Viscometric Properties of Monoesters-

Compound Ethyl heptanoate n-Nonyl acetate Methyl laurate Ethyl myristate Ethyl stearate %-Amylstearate n-Decyl stearate Melissyl acetate n-Octadecyl stearate n-Deoyl melissate Melissyl laurate n-Octadecyl mellisate Melissyl melissate

Atoms in Viscosity CY. at Chain 2 210‘ F. 10 12 14 17 21

24 29 34 37 42 44 50 63

Long

Chain

Dean and Hardiman Davis and Slope V.I. (40) Nissan (71)

ASTM

0.589 0.641 1.07 1.45 2.17 2.64 3.83 6.45 6.13 8.91 9.69 13.1 18.9

metric properties of the pure molten n-alkanes, especially those containing over 36 carbon atoms per molecule. Synthetic Oils, Present and Future

h

Hydrocarbons having the proper configurations and molecular weights t o have low temperature coefficients of viscosity and low freezing points may exist in petroleum fractions, but no sufficiently cheap refining method has been developed by which they can be isolated. Also, the best of the hydrocarbons i n the low viscosity range will be a t a disadvantage in comparison with certain polyalkene oxides, esters, silicones, etc., with respect t o the joint properties of low temperature coefficient of viscosity, high boiling point, and low freezing point. Synthesis of high viscosity index hydrocarbons through the polymerization of olefins is a n obvious approach, especially since the structural guides make i t evident t h a t the ideal hydrocarbon structure is a polyethylene having a fairly low molecular weight and a small number of short aliphatic branches. However, the proper structure for a polyoleh has not been recognized until recently. Many attempts have been made t o prepare synthetic high viscosity index hydrocarbons from olefins since the pioneer effort of Sullivan and coworkers (114); the most interesting are the “polyethylene oils” developed in Germany during World War I1 (74, 94). None as good has been prepared since. Although reformed hydrocarbon oils are potentially the cheapest of the tailor-made liquid lubricants, it is difficult to conceive t h a t they will ever fully satisfy the growing market for synthetics; most probably they will be used in the less demanding applications or will be blended with other types of synthetics t o lower the cost and extend the supply. Since the earliest known lubricants were animal fats and vegetable oils consisting largely of glycerides or other esters of aliphatic carboxylic acids, the recent increase in the use of synthetic lubricating oils made from esters is of historical interest. Only in the past 10 years have we learned how to synthesize esters having the desired properties but free from the objectionable characteristics of the oils, fats, or other natural products from which they were derived (3, 24, Si,62, 74). Suitable aliphatic branchedchain esters including several dozen mono-, di-, tri-, and tetraesters are now available or will be. Fisher-Hirschfelder ball modds of bis(2-ethylhexyl) sebacate (Figure 3), bis(2-methylpentyl) azelate (Figure 4),bis(3-methylbutyl) adipate (Figure 5), and a polyethylene glycol diester of 2-ethylhexanoic acid (Figure 6) show clearly how these acids, alcohols, or glycols need t o be put together to obtain the structural characteristics desired in a liquid having a low freezing point, a high boiling point, and a small temperature coefficient of viscosity. Pure esters are not better boundary lubricants than aliphatic hydrocarbons or mineral oils, but many esters have the advantage t h a t products of their hydrolysis or oxidation are useful mildwear preventives and rust inhibitors (98). Their marked solvent Iuly 1953

properties make these esters behave like fairly good detergent oils. However, that same solvent ability leads to a swelling action on many types of rubber, plastics, and paints, and this has hindered the adoption of ester oils. Suitably resistant materials are available, but it is desirable t h a t the variety be widened. Aliphatic esters are extremely soluble in mineral oils, which makes them valuable coupling and blending materials for improving the properties of mineral oils.

Figure 3.

Ball Model of Bis(2-Ethylhexyl) Sebacate

Present requirements for military and industrial applications of ester lubricants are based mainly on the use of sebacates, adipates, and the diesters of various polyglycols; but increasing supplies a t low cost can be predicted soon for azelates, glutarates, pinates (92), and for certain monoesters and diesters prepared by reacting pelargonic acid with either glycols or branched alcohols (34). Many of these or equivalent esters can be made from acids, alcohols, and glycols derived from petrochemical sources and even from naval stores products. Applications of these esters as lubricant bases, either alone or blended with mineral lube oils, will become commonplace in the years to come.

Figure 4.

Ball Model of Bis(2-Methylpentyl) Azelate

Liquid polyalkylene oxides made from ethylene oxide and/or propylene oxide were originated by Fife, Toussaint, and Roberts (50, 108, 119) in the U.S.A. just before World War 11. Ball models of a polymer of propylene oxide ( a ) and of a 1:l copolymer of ethylene oxide and propylene oxide ( b ) are shown in Figure 7. Less well known is the subsequent development in Germany of liquid polyalkylene-oxide lubricants through the catalytic scission of tetrahydrofuran followed by either a polymerization reaction or copolymerization with ethylene oxide (74, 94). The first commercial American poducts (78)were chain-stoppered only at one end with a n alkyl ether group, but the change with temperature in the degree of association through the resulting terminal hydroxyl group caused a n increase in the temperature coefficient of viscosity a t subzero temperatures. When both ends of the chain were stoppered with either aliphatic acids or alkyl ethers (61, 90), this limitation was nearly eliminated. The ASTM slope is least for the diether as is shown in Figure 2. The volatility can be decreased most by using a n aliphatic arid as the chain stopper, and the resulting diester is in effect a close analog t o t h a t shown in Figure 6 and to the several other diesters made by us from polyethylene glycol during the World War I1 (3, 24). Since these liquids are good organic solvents for their oxidation products, they manifest appreciable self detergency; the use of detergent additives may increase this property further. Polyalkylene oxide lubricants have been used successfully in many


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high temperature systems (116). Because the oxidation products are either soluble in the liquid or else are volatile acids or aldehydes, the oxidized liquid remains remarkably free of sludge and insolubles while maintaining its lubricity. However, the same good solvent properties also cause these liquids to attack some types of plastics, rubbers, and paints. Many common materials are known that are adequately resistant. The many ether oxygen atoms in the polymer molecule caused these liquids t o have a higher solubility for water than petroleum or ester oils-for er-

Figure

5. Boll Model of Bis(3-Methylbutyl) Adipate

ample, some of the polymers of propylene oxide will dissolve several per cent of water or more a t ordinary temperatures. -4s the temperature increases, the solubility for water decreases, and this abnormality is interesting and deserves study. Rust prevention problems can bc more troublesome in some applications of these liquids t,han is usual with either petroleum or ester oils. Fortunately, we have shown t,hat the addition of polartype rust preventives to many types of polyalkylene oxide liquids will usually care for such difficulties (6. 7, 9). Because of their many desirable properties and the potentially large supplv from nat,ural gas or cracked petroleum fractions, one can safely predict that increasing volumes of the chain-stoppered polyalkylene oxides will be used in the futurc as lubricants as n-ell as in blends with mineral oils or aliphatic esters. Silicone liquids and their properties are now well known (55, 103). The uniquely low temperature coefficients of viscosity of the open-chain dimethyl silicones has not yet been explained fully. Our early demonstration of the great flexibility of these linear polymers (67) led us to identify that as the major rausc of the small temperature coefficient of viscosity (67, 96). This interpretation is theoretically sound, but nevertheless an adequate explanation should include any peculiarities of the cohesive forces between silicone molecules. Although the resistance of silicones t,o thermal oxidation is greater than that of analogous hydrocarbons, aliphatic esters, and polyalkylene oxides, the silicones have the unfortunate property of forming gels once thcy do oxidize (4,96). Theeffect of exposures a t temperatures froin 200" to 250' C. was shown by Murphy and coworkers (4, 96) to increase greatly the rate of chgnge of the viscosit?. x i t h time; it is less the greater trhe amount of phenyl substitution on the polysilosane chain. Elliott (44)has shown that the rate of oxidation can bri decreased considerably by adding iron, cobalt, nickel, or copper salts of various carboxylic acids; hut their practical appliration is much limited because of their low solubility in the silicones. Many advances in lubrication a t high temperatures could be made by using silicone liquids il inorc effective high tcrnpcrature antioxidants could be found. Boundary lubricating properties of silicones are much the same a s those of the pure aliphatic hydrocarbons when t,he surfaces lubricated are the usual combinations of one or more nonferrous bearing materials. But when both rubbing surfaces are steel, the resulting welding and tearing is so serious as t o limit t,heir application t o low unit loads (27, &5). Many suitable paire of bearing metals were found by us (56)which do not exhibit this difficulty, and so the equipment designer can often avoid trouble from this undesirable property. Yet the dimethyl silicones are surface active on water by virtue of the marked freedom of rota-

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tion about siloxane bonds and their ability to adsorb with all mct'hyl groups directed into tJhe air and oriented away from tlic water-air interface (67). However, these silicones are only weakly adsorbed on steel a t ordinary temperatures, for they manifest no ability to act a s hydrophobic film barriers to rusting. That' is why me found it necessary to care for the rust inhibitios of silicones by the addition of suitable polar-type additives (6, 7, I O ) . Since the surface oxide coating on most clean metals considcrably decreases the coefficient of friction ( I S , 19, 2Sj 73, 118, 223), thc preceding facts lead one to infer that the great susceptibility of steel sliding on steel to galling or welding when covered by silicones may be caused by an abnormally slow rate of renewal of the naturally occurring iron oxide as it is being rubbed off a t areas of contact. Further research on the boundary lubricating properties of silicones is needed. If this adverse property can be climinated without decreasing the resistance of silicones to oxidation and without increasing their remarkably low temperature coefficients of viscosity, it is certain there will result a rapid increase in the use of these materials in lubrication. Chlorinated hydrocarbons have rcceived early attention in lubrication; the most successful have been the ring chlorinated aromatics like benzene and biphenyl and the fully chlorinated olefins-ethylene, propylene, and butadiene. Such liquids are inherently limited for niauy lubrication applications by their very large temperature coefficients of viscosity. The causcs are the marked hindrances t,o free rotations in the molecule resulting from the very large radius of the chlorine atom, and the greatly increased van der Waals energy of cohesion between molecules of the liquid resulting from replacement of covalent hydrogen

Figure 6.

Ball Model of Polyethylene Glycol Bis( 2-Ethylhexanoate)

by rhlorine. The uses of chlorinated oils have also l)ocn restricted greatly by t'heir toxicity and detrimental effects on many types of paints, rubbers. and plastics. Suitable viscosity index improvers are knom-n-for example, Ideland (89) used polystyrcne successfully with chlorinatcd hydrocarbons to develop high viscosity index! nonflammable. hydraulic fluids. Probably t,he chlorinated liquids will cont,iiiue to have very limited application in lubrication. Fully fluorinated hydrocarbons received much attelltion i n World F a r I1 and since tlien because of their high thermal stahility and remarkable resistance to oxidative and other chemical attack (67). The perfluoroalkanes have much less flesibility than the n-alkanes because fluorine atoms are larger than hydrogen atoms (96). Also, the uniqucly loiv van der Waals cohesive forces between such niolecules (96, 109) cause the liquids to have abnormally high molar thermal expansion coefficients. I-Ience, the perfluoroalkanes have much larger temperature coefficients of viscosity than the n-alkancs (96). The smaller liquidus rnngc of perfluorocarbons, -which also results from the low van der *Waals cohesive forces bet'ween molccules, ia still another limitation. As pointed out earlier, the most promising synthetic approaches by which perfluorinated liquids with smaller temperature coefficients of viscosity may be obtained is hy preparing polymers of perfluoroalkylene oxides (96). This should be investigated. Perfluorocarbons are also very bad solvents for common organic materials; this makes it difficult to improve them with addition agents or to blend thrm with petroleum or wit,h the othcr synthetic liquids. rlt present, thc relatively small volume of such liquids in use in lubrication is being applied mainly in valves and


Vol. 45, No. 7

in various special devices not exposed t80a wide range of temperature. Nevertheless, the extraordinary stability of perfluorocarbons will continue to make them a subject for much research. Antioxidants

a

.*

,.

dA

Antioxidants are the most commonly used lubricant additives. Until recently knowledge of how they function has been essentially empirical. However, significant progress has been made in the past 15 years as a by-product of basic research on chain reaction mechanisms, on the properties and reactions of peroxides and hydroperoxides, and on the nature and reactions of free radicals. It is now widely believed (196) that most organic oxidation reactions proceed by a chain mechanism with decomposition of hydroperoxides serving as the rate determining step. The most useful class of antioxidants functions by reacting with one of the products of the oxidation chain to terminate it. Most effective inhibition seems to occur when the chain is broken a t the rate determining step. The best inhibitors appear to have oxidation products which are also inhibitors-Le., they are selfregenerative. Another class of antioxidants are (in effect) metal deactivators. These coat certain metals like copper and iron, which accelerate many of the common oxidation reactions, and the resulting unreactive and firmly adhering films put a stop to participation in the reaction. The precise mode of operation of every type of metal deactivator is not known, although many are believed t o chelate with metal atoms of the surface. Competent and imaginative research is still needed in the study of oil antioxidants. New types of antioxidants are still being discovered, recent examples being Denison and Condit's work on the dialkyl selenides (41) and our own studies of phenothiazine and its derivatives (93, 94). Once a new and effective antioxidant has been found, it is sometimes possible t o give a plausible explanation of why i t acted so; but it is almost impossible to predict this property in a new type of compound from a knowledge of chemical constitution alone. Why one type of antioxidant is efficient in one kind of oil and inefficient in another is often perplexing. Most valuable of all would be an understanding of the precise cause of the upper temperature limit of operability of each type of antioxidant, and also what determines the location of the upper limit for antioxidants in general. Some results of our laboratory (34) exemplify this problem well. With a few exceptions, a variaty of phenolic antioxidants, which, a t 125' C. effectively inhibited the oxidation of the reference oil bis(2ethylhexyl) sebacate, failed to do so a t 150' C.; a variety of substituted aromatic amine antioxidants were effective a t 150' C,, but all failed a t 163' C.; the dialkyl selenides were effective a t 163' C., but failed a t 175' C.; and phenothiazine and several of its derivatives were effective inhibitors a t 175" C., but were unusable at 190' C. Yet in all these comparisons equimolal concentrations of antioxidants were compared, and the concentrations used were always high enough t o be in the range of efficient activity as antioxidants. These results indicate the problem, but they also indicate lively possibilities for the development of better high temperature antioxidants for lubricants.

Rust Inhibitors Rust inhibition in oils is caused by the physical adsorption on ferrous metals of certain types of high molecular weight, polarnonpolar molecules (6, 7 , 8). A water repellent film is formed whose pores become clogged quickly with molecularly dispersed water and the early products of reaction with the metal surface. This results in an essentially impermeable barrier to the progress of corrosion. Although physical adsorption is the initiating mechanism, surface chemical reaction of the polar compound with the ferrous metal to form a soap in situ may occur if the July 1953

rust inhibitor is an organic acid (8). The most durable inhibiting films are formed when the acid has the proper structure and a sufficiently large, nonpolar, hydrocarbon structure to form a closepacked and water-insoluble film. Unfortunately, such high molecular weight polar compounds are only slightly soluble in oils; the more effective the rust inhibitor, the less the solubility. A sufficiently high concentration of the additive must always be present in the oil so that any chance damage to the protecting monolayer may be healed immediately. Usually, the necessary concentration of the additive is above the solubility limit. Therefore, a considerable proportion of the additive must be retained in the oil in the undissolved or dispersed condition. Problems of colloid stability are evidently to be expected in attempting to obtain an optimum useful rustrinhibition in an oil. This is one of the principal causes of the difficulties encountered frequently with significant losses in the rust preventive properties of oils during storage.

Figure 7. Ball Models of (a)an lsopropylene Oxide Polymer and (b) a 1 :1 Ethylene Oxide : lsopropylene Oxide Copolymer The most effective rust inhibitors are acids or acid-generating compounds like the polyvalent salts of organic acids, the products of the reaction between amines and acids, and some esters. Too little is known about the first two classes of materials (8). Polyvalent salts of the higher carboxylic acids are seldom welldefined compounds. Generally, they contain variable and complex combinations of acids and metal hydroxides whose structure may be polymeric in the anhydrous condition as McBain (85), Sheffer (110),Singleterry (8), and Alexander and Gray (64) have shown. The reaction of a tertiary amine and a higher carboxylic acid was only recently proved by Kaufman and Singleterry (77) to result in a hydrogen-bonded association complex consisting of from 2 to 4 moles of acid per mole of amine. No one yet knows the difference in association encountered in going from tertiary, to secondary, and to primary amines. Alkaline or alkaline earth salts of the petroleum sulfonic acids having molecular weights of 400 or more are preferred inhibitors. Precisely how they function as rust inhibitors is not known; it is not the same as the mechanism for carboxylates. Research on this question is needed, and in answering it meticulous work will be required because of the well-known difficulties in preparing organic sulfonates of sufficiently high purity for reliable surface chemical work. During World War I1 there developed a serious shortage of high molecular weight petroleum sulfonates, and it is essential that this should not happen again. Action to increase the supply is complicated by the fact that petroleum sulfonates are by-products of the white mineral oil industry and that it is only possible t o produce them cheaply when all the resulting white oil can also be marketed profitably. This dilemma can be solved by the large scale production of synthetic sulfates from reasonably well-defined, cheap, substituted aromatic hydrocarbons such as the alkyl-substituted naphthalenes. Rust inhibitors, which are as effective as the petroleum sulfonates and have the added advantage of behaving much more reproducibly, are being produced in this way.

INDUSTRIAL A N D ENGINEERING CHEMISTRY

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Viscosity Index Improvers

.

By 1940 the use of the polybutenes as viscosity index improvers (98, l a d ) had become well established in the petroleum industry. The wartime need for high viscosity index hydraulic and recoil oils made such polymer additives assume increasing importance. Esters of polymethylacrylic acid (121) were first introduced in these applications about 10 years ago. Their use in ester-base lubricants became standard practice because of their compatability and well-controlled properties (3, 63, 69). In the wartime Hydrolube development of our laboratory (6, 26, 125), water-soluble polyalkylene oxides were introduced as viscosity index improvers for water-base lubricants. Some of those developed since have differed mainly in modifications of the chain terminating groups of the polymcr. By adjusting the solubility of the polymer closer to the precipitation point, it has been possible since to obtain much more viscosity index improvement. Alkylated polystyrenes have also been used widely. It is generally recognized that viscosity index improvers have one fundamental limitation; because they increase the viscosity of the fluid base, they cannot be used to make a high viscosity index liquid with a low viscosity unless a liquid base of very high volatility can be tolerated. Linear polymers are the most effective viscosity index improvers, but not every linear polymer is a good improver in any specific liquid. A suitable linear polymer should be neither too soluble nor too insoluble in the oil. Evans and Young (47) have emphasized this in their study of the effect of varying the viscosityindex of an oil by adding a high boilingliquid todecreasethe solubility of the polymer. The importance of adjusting the solubility of the water-base hydraulic fluids in order to obtain optimum useful viscosity index improvement has been especially evident in Esposito’s (45)recent modification of the Hydrolube fluids. Unfortunately, it is still true that the proper solubility balance in a given combination of polymer and oil must be found by trial and error. In 1942, Alfrey, Bartovics, and Mark (1) first explained t h a t in a good solvent the polymer is highly solvated and is in a n extended configuration; hence, it has lost most of its ability t o extend its average configuration as temperature rises, and this means little or no viscosity index improvement. I n a poor solvent the polymer is solvated little, and hence it is curled up into a compact bundle; as temperature rises, solvation becomes more appreciable, the molecule uncoils on the average, the viscosity increases, and so the viscosity index is improved. Thus, the best viscosity index improver should be a highly flexible linear polymer which is as near the precipitation point as is practicable for the particular application. If this approach could be expressed in more precise and quantitative terms with reasonably accessible parameters, it would help in the perfection of present viscosity index improveis and in the discovery of improved materials. But there are other limitations to the use of polymers for viscosity index improvement. Linear polymers having average molecular weights around 5000 to 10,000 may be required in concentrations of about 10% to impart a certain viscosity index improvement in a given oil; if the average molecular weight is between 30,000 and 50,000 only about 1% of the same kind of polymer may be needed. Limitations to the use of either extreme value of the average molecular weight are imposed by the cost, when using the polymer of lower molecular weights, and by the appearance of the phenomenon of shear breakdown, when using the polvmers of higher molecular weight. The permanent decrease in viscosity and viscosity index of oils containing linear polymers was first treated successfully by Pohl a t our laboratory in 1943 in a report (99) which was circulated a t the time by N D R C and the Navy to contractors and t o industry. H e demonstrated theoretically that the effects being encountered widely were caused by the rupturing of covalent bonds in the principal chain of the polymer under the large shear stresses developed in 1412

gears and other high pressure hydraulic pumps and relief valves Typical viscosity index improvers like polybutene or the polymethacrylates, which had average molecular weights around 15,000 to 25,000, showed no significant amount of such polymer degeneration unless the velocity gradient (or rate of shear) was around 106 sec.-1 or more. Asimilar calculation was published a year later by Frenkel (68), and the phenomenon was discussed much later by Schnurmann (108). It is hoped this brief survey will make it evident that there is still room for research on the properties of viscosity index improvers. Detergency in Oils

At least several distinct mechanisms are believed to operate in modern detergent oils. Although such oils are widely used, the patents are still the main source of information concerning them, and never has t h a t literature been so empirical or so unconcerned with establishing scientific principles. Detergency can easily be confused with the ability of the oil to dissolve its own oxidation products. This solvent mechanism appears to be especially important in oils like the esters, ethers, the polyalkylene oxides, or their petroleum blends. Any additive t,hat changes the course of the oxidation reaction so t,hat the resulting products are more soluble or dispersible in the oil may impart significant det,ergency. Finally, from the chemical structures of the detergent additives reported in the literature many may be able t o form micelles in the oil in which some of the oxidized products are solubilized. Until recently no one had shown that soaps or other addit.ives did form micelles in oils in dilute solut,ion. However, this fact has been established through the postwar investigations of Singleterry and coworkers (8)and by van der Waarden (180). Singleterry’s work included the discovery of soluble fluorescent dyes suitable for use in measuring critical micelle concentrations of additives in nonaqueous systems, and a new method based on the polarization of fluorescent light for measuring the size of such micelles. With these tools to study well-defined polyvalent soaps it has been shown that critical micelle concentrations are often even lower in hydrocarbons than were found in mater systems by Corrin and Harkins (36) and later workers. Progress of research is now limited mainly by the time-consuming job of preparing pure soaps, sulfonates, and other micelle-forming compounds. Activities in a number of laboratories (2, 64, 84, 110) on the variable and associated strurture oi sQaps should prove helpful. Evidently more research is needed on detergency in oils and on the related dispersing and colloidal properties of soaps and sulfonates. lubricating Greases S o class of lubricants has undergone more radical and interesting changes than have greases in the past 15 years. To begin with, a large gain in the temperature range of greases was made possible by Earle’s work (43) on the lithium soaps as jelling agents. Later came a variety of new greases made from new types of soap complexes such as those reported by McLennen (85). Valuable knowledge about the colloidal structure of greases made from various gelling agents has resulted from the pioneer optical and electron microscope studies of Farringtori (48, 49), and the recent method developed by Browning (30) of observing the meshed structure of soap gelled greases by extracting all the oil with selective solvents. Koteworthy publications have resulted concerning the structures of greases made from calcium soaps (26), sodium soaps (17, 91), and lithium soaps (29). McBain and coworkers (83), the Volds (122), Puddington ( e l ) ,and Lawrence (81) have investigated the colloidal properties of various soap and oil systems, the phases formed and the phase transitions with temperature by using x-ray diffraction, differential thermal analysis, and other methods. These investigations need t o be continued and


Vol. 45, No. 1

*

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extended to a wider variety of oils, gelling agents, and conditions of production. New greases made from synthetic oils were introduced early in World War I1 commencing with the aliphatic diesters (69) and followed by the polyalkylene oxides (go), the silicones (YO,88),the silicone-diester blends (88),and most recently the perfluorocarbons. Many postwar investigations have been concerned with nonsoap gelling agents, and this trend has been accelerated by a threatening shortage in lithium soaps. Greases from colloidal silica were followed by others made from hydrophobic bentonite (53), copper phthalocyanine (64, 87), various kinds of carbon black, and hydrophobic silica; the most recent arrival is treated Attapulgite. New materials have brought new problems. Both silica and bentonite when uncoated were each too sensitive to the presence of water t o be useful in greases. T h a t difficulty was cured by coating each particle with a hydrophobic or waterproof coating. However, the resulting greases were deficient in rust-preventive properties. Hence, polar-type inhibitors were added, but there resulted B decrease in the effectiveness of the gelling agent and a loss in mechanical and heat stability. Also the hydrophobic coatings on these materials appear to oxidize around 250’ F. or more, and so the greases gradually become hard and caked. These difficulties still limit the fields of application of greases from coated particles. Other problems must be solved which have needed attention for several decades. These relate to improving the rust prevention of greases and to better methods of measuring t h a t property. Much more information is needed about the mechanical stability of greases despite recent significant research on the subject by Moore and Cravath (91) and Singleterry and Stone (112). Also, a more convenient method of measuring t h a t p r o p erty is desirable. Despite these difficulties, continued progress can be predicted in the improvement of properties of greases such as their work and storage stability, their resistance t o oxidation, and their temperature range of operation. Much longer storage and operating lives in grease-lubricated systems can be confidently predicted. Indicated improvements in greases, rolling contact bearings, and in the end-bell design of electric motors (96) will make it possible to approach much closer to the engineers’ goal of lifetime lubrication for equipment operating in the temperature range -65’ to 250’ F. At operating temperatures of 300’ F. relubrication intervals will probably be lengthened eventually to 5000 hours, while a t 400’ t o 450’ F. the relubrication interval will be at least 1000 hours. The increased cost of the new greases and bearings will be more than justified b y improved operation and the decreased cost of maintenance and repairs. General Trends

s(

Until 1940 the principal chemical advances in the field of lubrication were based on improvements in petroleum refining methods and on the essentially empirical development of a technology of addition agents for lubricants. Advances of the future several decades are likely t o result from a n increased understanding of how these additives function, through the gains in performance and maintenance economies made possible by tailor-made synthetic oils and new gelling agents, and through the development of new solid lubricants, improved nonmetallic bearing materials, and better designs in rolling contact bearings. Beyond a doubt, the great bulk of lubricants used in the next several decades will be manufactured from mineral oil because of availability and low cost, and because most needs in lubrication will be satisfied thereby. Mineral lubricating oils will continue t o be improved through research on additives. B u t there is no basis for predicting startling gains. Mineral oil greases will probably be improved considerably with respect t o storage stability and reproducibility. July 1953

Synthetic tailor-made oils will be used increasingly for the manufacture of lubricating oils and greases. The volume of such liquids used in the manufacture of blends of mineral and synthetic oils will grow rapidly t o large proportions as they are measured in the organic chemicals industry. Possibly within a decade, petrochemical suppliers will be producing a considerable proportion of the synthetic oils needed. Synthetic lubricants will be used mainly in equipment requiring long periods of operation without maintenance; in complex or delicate equipment whose relubrication is best handled during partial or complete disassembly; and in equipment required t o operate at.an extreme temperature or over a wide range of t,emperatures. Armed service cars, trucks, tractors. tanks, and gun carriers are likely t o be lubricated with oils and greases containing substantial proportions of synthetic liquids. But it will be a long time before any considerable use is made of synthetic oils a s crankcase lubricants in civilian automobiles. Greases used in such automobiles probably will contain increasing proportions of synthetic oil in the course of time in order t o obtain better allyear service and decrease relubrication intervals. Aqueous Iubricants will improve as the necessary technology of additives is developed, and eventually they probably will be used in large volume in hydraulic systems, shock absorbers, and buffer systems. It is very probable t h a t the excellent results with Hydrolubes already obtained by the Navy will lead to their much wider use in aircraft for the sake of nonflammability and decreased maintenance. Eventually aircraft and all their components will be lubricated by synthetics. Ordnance equipment such as rapid-fire guns, rifles, etc., will demand increasing volumes of synthetics. Electric motors (especially in the fractional horsepower range) will probably become entirely lubricated by oils and greases containing major proportions of synthetics. Such oils and greases will become commonplace in the shop and home for the lubrication of small mechanisms varying from fans and locks t o guns, fishing reels, and sewing machines. literature Cited (1) Alfrey, T., Bartovics, A., and Mark, H., J . Am. Chem. SOC.,64, 1557 (1942). (2) Arkin, L., and Singleterry, C. R., Ibid., 70, 3965 (1948); , J . Colloid Sci., 4, 537 (1949); J . Am. Chem. SOC.,73, 4574 (1951). (3) Atkins, D. C., Baker, H. R . , Murphy, C. M., and Zisman, W. A , , IND. ENG. CHEM.,39, 491 (1947). (4) Atkins, D. C., Murphy, C. M., and Saunders, C. E., Ibid., 39, 1395 (1947). (5) Baker, H. R., Jones, D. T., and Zisman, W. A., Ibid., 41, 137 (1949). ( 6 ) Baker, H. R., Spessard, D. R., Wolfe, J. K., and Zisman, W. *4., U. S. Patent 2,602,780 (July 8 , 1952). (7) Baker, H. R., and Zisman, W. A , , IND.ENG.CHEW,40, 2338 (1948). (8) Baker, H. R., and Zisman, W. A., Lubrication Eng., 7 , 117 (1951). (9) Baker, H. R . , and Zisman, W. A., U. S. Patent 2,434,978 (Jan. 27, 1948). (10) Baker, H. R., and Zisman, W.A., Ibid., 2,447,483 (-4ug.24, 1948). (11) Beeck, O., Givens, J. W., and Smith, A. E . , Proc. R o y Soc., A177, 90 (1940). (12) Beeck, O., Givens. J. W., and Williams, E. C., Ibid., A177, 103 (1940). (13) Beilby, G . T., “hggl.egatiOn and Flow in Solids,” New York, Macmillan Co., 1921. (14) Bigelow, W. C . , Glass, E., and Zisman, W. A, J. Colloid Sci., 2, 563 (1947). (15) Bigelow, W. C., Pickett, D. L., and Zisman, W. A., Ibid., 1, 513 (1946). (16) Birdsall, D. H., and Farrington, B. B., J . Phys. Chem., 52, 1415 (1948). (17) Bondi, A., et al., Proc. 111 World Petroleum Congress, 7 (1951). (18) Bowden, F. P., Gregory, J. N., and Tabor, D., Nature, 156, 97 (1 945). (19) Bowden, F. P., and Hughes, T. P., Proc. Rog. SOC.,A172, 263 (1939). (20) Bowden, F. P., and Moore, A. C . , Research, 2 , 585 (1949).


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(21) Bowden, F. P., and Tabor, D., Ann. Repts. Chem. SOC.,42,20 (1945). (22) Bowden, F. P., and Tabor, D., “Friction and Lubrication of Solids,” New York, Oxford University Press, 1950. (23)Bowden, F.P., and Young, J. E., Nature, 164,1089(1949). (24) Bried, E. M., Kidder, H. F., Murphy, C. M., and Zisman, W. A., IND. END.CHEM.,39,484(1947). (25) Brophy, J. E., Fitzsimmons, V. G., O’Rear, J. G., Price, T. R., and Zisman, W. A., Ibid., 43,884(1951). (26) Brophy, J. E., Larson, J., Singleterry, C. R., and Zisman, W. A., Naval Research Laboratory, Washington 25,D. C., Eept. 3971 (May 28,1952). (27) Brophy, J. E., Militz, R. O., and Zisman, W.A., Trans. Am. SOC. Mech. Engis., 68,355 (1946). (28) Brophy, J. E., and Zisman, W. A., Ann. N . Y . Acad. Sci., 53, 836 (1951). (29) Brown, J. A , Hudson, C. N4,and Loring, L. D., Natl. Lub. Grease Inst., Annual Meeting, Chicago (October 1951) (30) Browning, G. V., Ibid., Chicago (October 1949). (31) Bruner, W.M., IND. ENG.CHEM.,41,2860(1949). (32) Campbell, W. E.,Trans. Am. SOC. Mech. Engrs., 61, 633 (1939). (33) Clark, G. L., Gallo, S.G., and Lincoln, B. H., J . Applzed Phys., 14,428 (1943). (34) Cohen, G., Murphy, C. M., O’Rear, J. G., Ravner, H., and Zisman, W. A., Naval Research Laboratory, Washington 25, D. C., Rept. 4066 (September 1952). (35) Callings, W.R., Chem. Eng. News, 23,1616 (1945). (36) Corrin, M. L., and Harkins, W. D., J . Am. Chem. SOC.,69,679 (1947). (37) Coaby, J. N., and Sutherland, L. H., Proc. Am. Petroleum Inst., Sec. 111,22,13 (1941). (38) Daniel, S.G., Trans. Faraday SOC., 47,1345(1951). (39) Davey, W., J . Inst. Petroleum, 31,73,154 (1945). and Davis, G. H. B., Chem. Met. Ena., 36, (40) Dean, E. W., . . 618 (1929). (41)Denison, G. H., and Condit, P. C., IND.EXG.CHEM.,41,944 (1949). (42) Dubrisay, R.,Compt. rend., 210,533 (1940). (43) Earle, C. E.,U. S. Patents 2,274,673-6(March 3,1942)and 2,293,052(Aug. 18,1942). (44) Elliott, J. R., Ibid., 2,445,587(July 20,1948). (45) Esposito, V.,Ibid., 2,588,970(March 11, 1952). (46) Evans, E.B., J . Inst. Petroleum, 24,321 (1938). (47) Evans, H. C., and Young, D. W., IND.ENG.CHEM.,39, 1676 (1947). (48) Farrington, B. B., and Birdsall, D. H., Inst. Spokesman X I , No. 1,4(1947). (49) Farrington, B. B., and Davis, W. N., IND.ENG. CHEM.,28, 414 (1936). (50) Fife, H.R., and Robert.s, F. H., U. S. Patent 2,448,664(September 7,1948). (51) Fife, H. R., and Toussaint, W. J.,Ibid.,2,457,139 (Dec.28,1948). (52)Finch, G. I., Proc. Roy. SOC.,B63,465 (1950). (53) Finlayson, C. M., and McCarthy, P. R., Inst. Spokesman, 14, No. 2, 13 (1950). (54) Fitzsimmons, V. G., Merker, R. L., and Singleterry, C. R., IND. ENG.CHEM.,44,556(1952). (55) Fitzsimmons, V. G., Pickett, D. L., Militz, R. O., and Zisman, W. A . , Trans. Am, SOC. Mech. Engrs., 68,361 (1946). ( 5 6 ) Flory, P. J., Mandelkern, L., Kinsinger, J. B., and Schultz, W. B.,J . Am. Chem. SOC.,74,3364 (1952). Taylor, P. W.,and Zisman, UT.A., IND.ENG. (57)Fox, H. W., CHEM., 39,1401 (1947). (58) Frenkel, J., Acta. Physicochim. U.R.S.S., 19,51 (1944). (59) Frewing, J. J.,Proc. R o y . Soc., A181,23 (1942). (60)Ibid., A182,270 (1944). (61) Gallay, W., and Puddington, I. E., Can. J . Research, 21B,202, 211,225(1943). (62)Glavis, F.J., and Stringer, H. R., A S T M Spec. Tech. Pub., 77 (1947). (63) Glavis, F. J., IND. ENG.CHEW,42,2441 (1950). and Alexander, -4.E., J . P h y s . Chem., 53,9,23 (64) Gray, V. R., (1949). (65) Greenhill, E. B., Council Sci. I n d . Research (Australia), A97, No. 36 (1944). (66) Greenhill, E. B., Trans. Faraday Soc., 45,625 (1949). (67) Grosse, A. V., and Cady, G. H., IND.ENG. CHEM.,39, 367 (1947). (68) Gregory, J. H., C m n c i l Sci. 2nd. Reseaich (Australia), A134, No. 49 (1945). (69) Hain, G. M., Jones, D. T., Merker, R. L., and Zisman. W. A., IND. ENQ.CHEM.,39,500(1947). (70) Hain, G. M., and Zisman, W. A,, U. S. Patent 2,448,667 (Sept. 7,1948). 1414

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Present Problems and Future Trends in Lubrication - ACS Publications

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