STRUCTURAL GUIDES FOR SYNTHETIC LUBRICANT DEVELOPMENT C. M. MURPHY
AND
W. A. ZfSMAN
N a v a l Research Laboratory, Washington,
D. C.
General structural guides for‘the development of synthetic lubricant fluids are presented.
To attain small temperature coefficients of viscosity, flexible long-chain structures with small coefficients of expansion are required. Of the various hydrocarbons, the n-alkanes have the better viscosity indexes. The alkyl polyethers and methyl silicones have even better viscosity indexes than the n-alkanes. The perhalocarbons, because of their rigidity and high thermal expansion, have very poor viscosity indexes; this is a fundamental limitation. Branching, or other hindrance to close alignment, is,the most reliable method for obtaining freezing point depressions. Boundary lubricant and rust-inhibition properties are related to the ability of the molecules either to form condensed physically adsorbed films or to react chemically with the boundary material, It is not possible to obtain the optimum bulk and surface properties in the same molecule. The properties of hydrocarbons, ethers, pol yethers, thioethers, mono-, di-, tri-, and tetraesters, silicones, perhalocarbons, and amides are discussed in relation to these guides.
H E increasing demand for liquid lubricants having small temperature coefficients of viscosity and low freezing points has focused attention on their synthesis. Certain structural considerations and generalizations derived from the study of hydrocarbons ( I 1, 26, 29, 32-34) have been the principal guides in the synthesis of new fluids.
T
Increasing the chain length increases the viscosity, raises the freezing point, and improves the viscosity-tem erature characteristics as evidenced by higher viscosity in&xes and lower A.S.T.M. slopes. The addition of side chains increases the viscosity, lowers the freezing point, and has an adverse effect on the viscosity index and A.S.T.M. slope. The position of the branched chain exerts a variable influence on the viscosity and viscosity index. Branches near the middle of the chain are more effective in lowering the freezing point. The addition of cyclic groups causes larger increases in viscosity and has a more adverse effect on the viscosity index and A.S.T.M. slope than alkyl groups. Increasing the ratio of the cross section of the molecule to its length adversely effects the viscosity index and A.S.T.M. slope. These generalizations were extended during World War I1 to the aliphatic esters having structures analogous to the aliphatic hydrocarbons (3,6). The advent of other synthetic lubricant fluids such as the silicones, halocarbons, polyethers, and a variety of esters of carboxylic and phosphorus-containing acids has made it desirable to attempt an even more general correlation of physical properties with molecular structure. This paper is concerned with what might be called the physical chemical principles basic to or guiding the synthesis and development of “tailor-made” lubricants. Many attempts have been made to predict the viscositytemperature characteristics of liquids. Among tAe earlier equations is that of Andrade (2) which has undergone many modifications and expansions, frequently with the introduction of a third constant and/or a molecular volume term such as by Van Wijk and Seeder (67,38). The introduction of a third constant makes i t unwieldy to use and the molecular volume term restricts its use to pure compounds or k n o v , ~mixtures. The equaare the most general but are tions of Eyring and co-workers (368) unsatisfactory for use over wide ranges of temperature, and they
have not been useful in forecasting the behavior of new liquids or in guiding synthesis. As pointed out elsewhere (27), it is very suggestive to treat the linear viscosity-temperature region of the graph on the A.S.T.M. chart as a region of “more normal” or “more ideal” behavior of each fluid and the regions of convexity or concavity as regions of ‘hbnormal” behavior having their origin in the increasing prominence of some chemical or physical transformation peculiar to the liquids studied. Such an approach makes i t necessary to learn more about the other properties of the fluid a t extreme temperatures. The applicability of the A.S.T.M. equation over a wide range of temperature has been shown (a7) for a number of the nonhydrocarbon liquids discussed. Therefore, this relation log log ( p
+ c ) = A log T + B
(1)
is employed here. As the graph of Equation 1 is linear over a wide range of temperature, the slope, S, of the graph is a very convenient measure of the viscosity-temperature characteristics. When S is measured from the graph plotted on the A.S.T.M. viscosity-temperature chart, .D 341-39, it is found that constant A in Equation 1 is related to S by A = 5.005. Below 1.5 cs. where the A.S.T.M. chart has been altered by varying c arbitrarily, this relation is not exact. A is related t o m in the Walther (39) equation, ( p c ) =~K , ~by the fact that m = - A . Despite the many attempts to represent the viscosity-temperature characteristics of liquids by a single index or number ( 1 , 12-14, 19, 31, 40, 41), none has been satisfactory for research purposes. Ramser (30) has shown that the change of viscosity with temperature cannot be represented by a single index but requires two independent parameters. N o index is truly independent of viscosity, and it is not possible to convert one to another without taking the viscosity level into consideration. In this paper the A.S.T.M. slope determined over the temperature range 210’ to 100’ F.is used to compare the viscosity-temperature characteristics, as i t was shown (30) to be less sensitive to viscosity than the viscosity index. If the A.S.T.M. slope is compared graphically with the viscosity, the dependence an viscosity is cared for.
+
2415
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2416
HYDROCARBONS
From the generalizations ( 6 ) as to the variations in viscositytemperature characteristics of hydrocarbons n ith structure i t is concluded that the n-alkanes should have the better viscometric characteristics. T o verify this the A.S.T.M. slope, S, has been plotted against the viscosity in centistokes a t 100' F. The slopes of the hydrocarbons shown in Figure 1 were obtained from the literature or where necessary by plotting on an A.S.T.M
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Vol. 42. No. 1 2
cross section to greatest linear dimension. Secondly, they have the least hindrance to free rotation about all the carbon-carbon bonds; hence, they are the most flexible and can coil and uncoil in the liquid state under the influence of thermal agitation and molecular impacts. Other viscometric and Surface chemical evidence for the relation of flexibility to viscosity-temkerature characteristics has been given (16, 27). The freezing point of an alkane can be lowered by the addition of hydrocarbon side chains or branch groups which create steric hindrance to the parallel alignment of the main alkane chains of adjacent molecules. If the branch is large, i t causes a large increase in the viscosity; hence, i t is usually desirable to have one or more small branches comprising preferably only a few carbon atoms each, and rarely over 5 or 6. If many such small branches are required to attain a low freezing or pour point, i t is important that they not be placed so as sterically to hinder freedom of rotation and thus sacrifice the flexibility of the main alkane chain. When one such branch is used, the freezing pdint lowering has been shown (11, 33) to be the greatest when the branch occurs in the middle of the main chain. Th%freezing point can also be lowered b y using mixtures of isomers or several members of a homologous series. ETHERS
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chart the viscosity data given in the literature (11, 16, ,%'0-14, 16, 18, S1-45). It is seen that a smooth curve can be drawn through the points for the n-alkanes; with a few exceptions the points for the isoalkanes and aryl or cyclosubstituted alkanes are above this curve. The great majority of the points below the n-alkane curve are from the data of Zorn (35) and Land? et al. (10-%4),the latter as given b y Evans (15) in kinematic units. The viscosities of three of the Landa compounds-2,l l-dimethyldodecane, Zmethylheptadecane, and 2-methyltricosane-are lees than those of the n-alkane having the same long chain, which is absurd; so the purity of their compounds is doubtful. The data by Schiessler et al. on hydrocarbons for A.P.I. Project 42 are more reliable. The graphical points of the branched and cyclic hydrocarbons are above the n-alkane line a t distances roughly in accord with the generalizations on branching and cyclization previously given ( 6 ) . I n Figure 2 the A.S.T.M. slope has been plotted against Z , the number of atoms in the principal chain of the alkane. The graph is a straight line for values of 2 between 10 and 20. The graph becomes more convex for the higher values of Z , above 20. Unfortunately, data on higher alkanes, where 2 is greater than 28, are not available. If S for the isoalkanes is plotted against Z, the graphical points are above the graph of Figure 2 , for the large majority of compounds of reliable purity. The shorter the branches the nearer the graphical points approach the nalkane line. However, if there are many branches seriously to restrict the flexibility of the principal chain, the slope is greatly increased and the graphical point moves up away from-the nalkane line. Hence, n-alkanes have lower slopes than do the isoalkanes of the same 2. Isoalkanes having only one or two comparatively short side-chains will have slopes within roughly 570 of the slope of that n-alkane. This limiting or bounding relation of the slopes of the n-alkane to those of other hydrocarbons is also evidenced in Figure 1. The smaller slopes of the n-alkanes are due to their geometry and flexibility. First, they have the smallest ratios of molecular
The di-n-alkyl ethers because of their structural similarity to the n-alkanes would be expected to differ only slightly in viscometric properties, the differences decreasing as the alkyl groups increase in length. For purposes of this discussion, the hydrocarbon analogous to the ether is considered formed by replac-ing the ether oxygen with a methylene group. The viscosities and slopes of some di-n-alkyl ethers are given in Table I, where they are compared with the analogous alkanes. It is seen that the ethers are less viscous and have larger slopes than the analogous hydrocarbons. This is not unreasonable, a s the addition of an ether oxygen causes an increase in chain length comparable to that of a methylene group, but its cross-sectional area is less. As the ether oxygen is smaller in diameter and has no sidr chains attached, there is even more freedom of rotation about the bond between the ether oxygen and the adjacent carbon atoms than about the aliphatic carbon-carbon bonds.
Table
I.
Viscometric Characteristics of Di-n-alkyl n- AI kanes
Identification Di-n-butyl ether n-Nonane Di-n-amyl ether n-Undecane Di-n-heptyl ether n-Pentadecane Di-n-octyl ether Di-n-octyl thioether n-Heptadecane Di-n-decyl ether Di-n-decyl thioether n-Heneicosane Di-n-dodecvl ether n-Pentacosine a Extrapolated.
Viscosity, Cs.
100° F. 0.736 0.807 1.089 1.229 2.22 2.58 '3.06 3.84 3.59 5.26 6.67 5,900 8.18 IO. 4a
210e 2 '. 0.430 0.465
0,580
0.645 0.992 1,119 1.089 1,500 1.417 1.862 2.237 2.100 2.68 3.04
Ethers and A.S.T.11.
Slope, S 0.964 0.926 0,897
0,868
0.866 0.814 0.790 0.768 0.787 0.737 0.703 0 734 0 699
0.680
As the slopes of n-alkanes vary inversely as the viscosity a t a reference temperature, the ethers would be expected to have larger slopes than the analogous hydrocarbons, for they are less viscous. When the slopes of the ethers are plotted against the viscosity a t 100' F., the two ethers of lower molecular weight, di-n-butyl and di-n-amyl ether, are above the n-alkane line, while all the higher molecular weight ethers are below this line. The di-n-alkyl ethers have lower freezing and boiling points and higher densities than the analogous hydrocarbons. Even greater reduction in freezing points would be .expected of the
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December 1950
unsymmetrical ethers, though they would probably be slightly more viscous than their symmetrical isomers. A further reduction in freezing points (or pour points) can be obtained by suitr ably placed side chains, though this would be obtained a t the expense of the good viscometric properties. Evidently, such ethers offer an excellent synthetic approach on liquids of high viscosity index and low pour point. ~
Table
II.
Physical Properties of Long-chain Monoesters
Atoms Viscosin ity, A.S.T.M. M.P. or Chain, Cs. at Slope, Compound B.P., C. 2 100' F. S (Source) 10 1.15 0.920 Ethyl heptanoate" 67-70/8 mm. 1.42 0.969 12 n-Nonyl acetate) 0.862 12 1.57 n-Amyl caproateC .... 0.849 13 2.24 n-Decyl acetated 91.0-92.5/1.5 min. 0.826 14 2.50 Methyl lamatee . . ...... 0.815 15 2.53 n-He tyl heptanoate4 270-273 0.769 17 3.65 10-12 EthyP myristateC 0.795 17 3.44 102-104/1.5 mm. n-Decyl caproated 0.763 4.06 18 n-Amyl laurateb 21 0.718 6.40 Ethyl stearate' 0.740 21 7.61 n-Octadecyl acetateC 7.34 0.710 23 n-Decyl laurated 0.692 24 8.37 n-Amyl stearate* 0.650 29 13.9 n-Decyl stearated 31 17.5f 0 654 n-Octaderyl laurated 34 0.609 29f .\.ielissyl acetated 37 0.613 27f n-Octadecyl stearated 37 0.581 43f n-Amyl melissated 35f 38 _. 0.485 Meiissyl oaproated 42 0.560 42f n-Decy1 melissated 44 0.554 45f Mellinyl laurated 50 571 0.524 bIelissyl stasrated 0.519 651 50 n-Octadecyl .melissated 0.500 63 109f hlelliHyl inelissated a Wallace and Tiernan. b Carbide and Carbon Chemicals C o . Eastman Kodak ,Co. d Prepared by Division R , National Defense Research Council. e Chemiral Research Laboratory of Ariiiour & C o . / Eutrapolnted.
........... I......
1
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A variety of polyglycol ethers have been studied. These may be considered as condensation polymers of ethylene and isopropylene glycol. The polymers containing two unreacted terminal hydroxyl groups are associated liquids (27) and their Viscometric properties are abnormal, especially a t extreme temperature. Those which are chain-stoppered at one end with an alkyl group behave more normally. Data on slope os. viscosity of some of these liquids are graphed in Figure 3. It is seen that the polyisopropplene glycols have greater slopes than do the copolymt-rs containing both isopropylene and ethylene oxide units. This is in accord with the generalizations as to the effect of the number of branched chains on viscosity and slope or viscosity index. Both types of polymers have larger slopes than the comparable n-alkanes but have much lower pour points. The polyethers which are chain-stoppered a t both ends are also shown in Figure 3. Their graphical points fall below the n-alkane line. I n these condensation polymers of isopropyleoe glycol the effect of the ether linkages more than counteracts the adverse action of the methyl branches on the slope. The relative positions of the graphical point)s below the n-alkane line are presumably due to the size an$ shape of the alkyl group and to the type of linkage used to end-stopper the chain. The thioethers are more viscous than the analogous hydrocarbons and have smaller slopes. When the slopes are plotted against their 100' F. viscosities (Figure 3 ) it is seen that they are smaller than those of the alkanes of the same viscosity. Their boiling points are higher than the analogous n-alkanes and their fveczing pojiits are much lower. These changes in properties would be expected because of the larger diameter of the sulfur atom, resulting in an unsymmetrical long-chain molecule with a larger cross section. ESTERS
A comparison of the viscometric properties of esters with the analogous hydrocarbons arid ethers is informative. Some per-
2417
tinent properties of the aliphatic monoesters are given in Table 11. Many of the data shown there were obtained from a recent study of the physical properties of pure aliphatic monoesters (IO),in which the viscosities of the esters were determined from their melting points to 350' I?. The esters of lower molecular weight were obtained from commercial sources, while the higher molecular weight esters were obtained through Division B of the National Defense Research Council and are of exceptional purity. The 100" F. viscosities of the esters having melting points above this temperature were obtained by extrapolation on the A.S.T.M. viscosity-temperature chart. A comparison of the pairs of esters having the same number of atoms, 2, in the principal chain (2values of 12, 17, 21, 37, and 50, respectively) reveals that the esters of lower viscosity are the ones in which the ester group is nearer the center of the chain. Considering the carbonyl oxygen group as a branch on the main chain, this variation in viscosity is in accord with the rule applying ta branched-chain alkanes (I1,33). The viscosities and slopes of these monoesters are very nearly the same as those of the analogous alkanes-i.e., the one formed by replacing the ether oxygen with a methylene group. It was shown earlier that the ethers were less viscous than the analogous alkanes; therefore, it can be concluded that the carbonyl group is responsible for the somewhat higher viscosities of the esters.
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062
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ATOMS IN LONOLST
Figure
2.
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34
I 38
cnm
Slope
VI. N u m b e r of Atoms, in Longest Chain n-Alkanes and n - A l k y l Esters
0
n-Alkana
A n-Alkyl d m
The slope, 8,of each ester is plotted against the number of atoms in the longest chain, 2, in Figure 2, where they are compared t o the pointa of the analogous alkanes. The points for the esters either coincide with or are very close to the n-alkane curve. By using the points from the esters the curve can be extended to much longer chain lengths. This has not been shown on Figure 2 beyond Z = 40 to save space. There is no doubt that dS/dZ is not constant; its magnitude decreases more rapidly as 2 increases beyond 20 atoms per chain. When the slope of the alkyl monoesters is plotted against the 100' F. viscosity (Figure 4), the points fall very close to the n-alkane curve, the majority of them being slightly below the curve. As the monoesters have higher boiling points, wider liquidus ranges, and approximately the same viscosities and slopes as the analogous alkanes, they are interesting as ti class of synthetics having high viscosity indexes and low pour.points. The physical and chemical properties of a number of aliphatic diesters have been described (3, 6, 9, 17). The great majority of these diesters were branched-chain compounds, a5 the primary roncern was to obtain low slopes and freezing points. When the
INDUSTRIAL AND ENGINEERING CHEMISTRY
2418
slopes of these diesters arc plotted against the viscosity a t 100' F. (Figure 4)i t is found that all the points except three are above the n-alkane line. The distance above the alkane line is a function of the size (or length) and number of branches in the chain. This is in conformity with the rules for the effect of branching on the viiconietric properties of hydrocarbons. The three diesters having slopes smaller than the walkanes were the n-butyl and 3methylbutyl diesters of sebacic acid and the 3-methyl butyl diester of azelaic acid. The first compound is a straight-chain ~noleculeand the other two have only two short methyl branches on H long chain.
Vol. 42, No. 12
terials, other possible uses, and properties such as hydrolytic, oxidation, and thermal stability will govern future efforts. I n general, the esters of secondary alcohols are nDt as stable as those of primary alcohols. This is in accord with well known organic principles. The secondary amyl esters seem particularly unstable, as such esters of adipic, azelaic, or sebacic acids when percolated through absorbants-e.g., fuller's earth, alumina, and silica gel-increased in acidity. When percolated through a column of warm silica gel there was also a copious evolution of amylenes, indicating that the esters had been dehydrated. HALOGENATED HYDRQCARBONS AND RELATED COMPOUNDS
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VISGOSITY. GENTISTOKES
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28
IOO'F
Slope vs. Viscosity of Ethers
n-Alkanes n-Alkyl erten n-Alkyl thioothen Polypropylene glycols chain4oppered at one end Poly(ethylene-propylene) glycol$ chain-stoppered at one end Polypropylene glycols chain-stoppered at both ends
Some physical properties of a variety of interesting esters are given in Table 111. The 100' F. viscosities and A.S.T.M. slopes of the three phthalate esters are nearly the same, indicating that in viscous flow they have nearly the s a n e configurations. Therefore, the normal graphical formula which shows the ortho ester as a U-shaped molecule is misleading. Fisher-HirschfelderTaylor atom models of these molecules reveal when arranged in the most linear form possible that the ortho isomer is the shortest and the meta is a t least as long as the para isomer. This agrees with the comparative viscometric properties given in Table 111. As would be expected, all three phthalate esters are more viscous and have larger slopes than the entirely aliphatic diesters of approximately the same chain length. The properties of two triesters, trimethylolethane tricaproate and tri( 2-ethylhexyl)tricarballylate, are given in Table 111. These triesters are more viscous and have larger slopes than diesters with the same 2,the tricarballylate ester being the more viscous. The tetraester, pentaerythritol tetracaproate, is much more viscous than the trimethyloletharle tricaproate, as would be expected. These two esters of caproic acid are similar to the caprylic esters described b y Zorn (43). Tri-(2-ethylhexyl)phosphate is given for comparison in Table 111. This ester is less viscous than the tricaproate of trimethylolethane with which i t is geometrically analogous. It also has a large slope considering its viscosity. Evidently these tri- and tetraesters have greater slopes and are moi-e viscous than the monoesters or diesters. The addition of the third and fourth ester groups introduced large branches on the principal chain, which caused the increased slope and larger viscosities observed. Evidently, a wide variety of esters can be prepared having low freezing points and volatilities and high viscosity indexes which are required for many lubricant applications. Clearly other considerations such as the availability and cost of starting ma-
As with the previously described classes of compounds, one would expect the perhalogenated n-alkanes to have the smallest temperature coefficients of viscosity of all the perhalocarbons This is found to be true, but the perhaloalkaoes are grossly iriferior to the comparable hydrocarbons in viscosity-temperature characteristics. The halogen atoms are very large as compared with hydrogen; therefore, they sterically hinder and restrict the rotation about carbon-carbon bonds. This effect increases in going from fluorine to iodine substituents. But the smallest halogen, fluorine, exerts such a powerful effect as an electron acceptor as to decrease the normal alkane carbon-carbon bond distance. These basic properties are in part responsible for the much larger A.S.T.M. slopes of the perhalogenated compounds than their n-alkane analogs ( 2 7 ) . One of the effects of elevating the temperature is to increase the distance between molecules (or the free volume), and that contributes to the decrease in viscosity with temperature. The temperature rate of change of the volume, V , occupied by R mole of the liquid is:
where M is the gram molecular weight and p the density. Available data show that bp/bT is three times larger for perfluoroheptane than for n-heptane (18). As the ratio of molecular weights is 358/100, a calculation with Equation 2 gives 2 to 1 as the ratio of a V / d T for the perfluorocarbon to that for the hydrocarbon. Coxrlparably large values ofbplb'l'have been reported ( 2 7 )for perfluorinated mineral oils and polychlorotrifluoroethylene. Clearly, this is another cause of the larger temperature coefficient of viscosity of perfluorocarbons than the analogous hydrocarbons. The larger value of bV/bT in perfluorocarbons is due to the same mechanism responsible for the anomalously low boiling points ( I @ , low surface tensions, and high cornpressibilities. Both are results of the weaker van der Waals forces of cohesion between the perfluoromethylene and perfluoromethyl groups than between the analogous methylene and methyl groups. Hence the large temperature coefficients of viscosity of thcx fluorocarbons are caused by two effects: inflexibility of the main carbon chain due to the steric hindrances of the large fluorine substituents and the rapid increase in the free volume with temperature due to the low van der Waals cohesive forces betweell molecules. These limitations are fundamental and limit the
Table
111.
Physical Properties of O t h r r Esters
Identification o-Di- (2-ethylhexyl) phthalate m-Di-(2-ethylhexy1)phthalate p-Di-(i-ethylhexyl) phthalate Trimethylolethane trioaproate Pentaerythritol tetraoaproate Tri-(Z-ethylhexyl) tricarhallylate
Tri-(2-ethylhexyl)phosphate a
Extrapolated.
210°
Viscosity, Ca.
F.
4.32 4.72 4.61 2.92
4.71 4.20 2.27
looo F. 32O F. 30.1 400' 34.1 435a 33.2 460" 6Za 11.5 23.4 165" 24.5 211 38' 8.21
A.S.T.M. Slope, S 0.849 0.834 0,836 0.764 0.716 0.788 0.79i
December 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY
application of perfluorocarbons in lubrication t o mechanisms operating over a limited temperature range. Nevertheless, i t is of interest to indicate some promising possibilities for obtaining perfluorocarbon structures with lower temperature coefficients of viscosity and freezing points. Polychlorotrifluoroethylene has a lower freezing point and a wider liquidus range than its perfluoro analog. The hindrances to close alignment of neighboring molecular chains caused by the large chlorine atom projecting from each monomeric unit is a t least in part responsible for the lower freezing. point of the former. The introduction of trifluoromethyl side chains should be even more effective in lowering the freezing point. Such a compound would result on the polymerization of perffuoropropylene. Similarly, liquid polymers of the perfluorobutenes should have low freezing points. Polymers of perfluoro-lbutene would be preferred over those of 2-butene, as the ethyl side chains would have a more adverse effect on the viscosity index. Obvious variations are possible by the partial substitution of chlorine for fluorine. Though the introduction of side chains will depress the freezing point below t h a t of the parent chain compound, it will have an adverse effect on the A.S.T.M. slope. An even more promising approach is through the formation of perfluorinated alkyl diethers or polyethers with trifluoromethyl side chains. Such compounds can be considered as homologs of the polyalkylene ethers. The advantage of such ethers is t h a t the periodic interruption of the aliphatic carbon chain by oxygen atoms will act to prevent the building up of stresses between adjacent close-packed -CF*-groups and so decrease steric hindrances to free rotaticn about the carbon-carbon bonds. Such structures should have lower A.S.T.M. slopes and freezing points than the analogous perfluorocarbons. If even lower freezing points are required, the introduction of short perfluoroalkyl side chains should have the desired effect without too adversely affecting the A.S.T.M. slope. The polyperfluoro ethers should be stable to oxidation, because the monoether, perfluorodibutyl ether, is reported to be remarkably stable. An analogous approach is to interrupt the main carbon chain with other atoms like sulfur having a covalence of two. A third approach is through the formation of mixed perfluorotertiary amines. The A.S.T.M. slope of perfluorodibutylmethylamine should be lower than t h a t of perfluorotributylamine, as the former has a much smaller branch chain. The dissymmetry of the tertiary amine structure and the action of the perfluoromethyl side chain would be expected t o result in a compound having a low freezing point. Perfluoro mixed tertiary amines with longer carbon chains such as the dilauryl methyl or ethylamine would also be of interest as fluids having higher viscosities. Inasmuch as perfluorotributylamine is reported t o be very stable, the unsymmetrical tertiary amines should also be stable to oxidation. SILICONES AND AMIDES
Although the silicones have been frequently discussed, their unusual viscometric properties have not been satisfactorily explained. On the A.S.T.M. viscosity-temperature chart the methyl silicones are straight lines over large temperature ranges (27). The slopes are so much smaller than those of the nalkanes of the same 100’ F. viscosity t h a t they cannot be shown in Figure 3. Surface pressure studies a t the authors’ laboratory (16)have shown t h a t the methyl silicones are readily coiled and uncoiled reversibly while adsorbed as oriented monolayers a t the air-water interface. This is good evidence t h a t the molecule is very flexible and that there is much freedom of rotation about the silicon-oxygen bonds. The greater flexibility of the methyl silicones compared to t h a t of other alkyl- or aryl-substituted silicones is, no doubt, one of the causes for the lower A.S.T.M. slopes of the former. The silicon atom, being much larger in diameter than the carbon atom, can accommodate methyl side chains with little loss of flexibility about the silicon-oxygen
2419
bond. This is not possible with the smaller carbon atom, as the geometrical analogs of the methyl-substituted polyethers or polymethylenes cannot be made, owing to the crowding of the methyl side chains. Consideration has been given to amides and especially to diamides as possible lubricant fluids, but several serious difficulties were encountered. The high melting points of amides made from primary amines are well known. This difficulty i s caused by hydrogen bonding between the polar groups of adjacent molecules, and it is well exemplified by N,N’-di-(2-ethyIhexy1)sebacamide and N,N’-di-(2-ethylhexyl)adipamide with. melting points of 87’ and 64’ C., respectively (25). When hydrogen bonding is eliminated by making monoamides or diamides from sebondary aliphatic amines, the melting points a r e lowered, but those studied still had much higher melting points than aliphatic esters having the same values of Z. Finally;, t h e expense of preparing unsymmetrical, secondary aliphatic amines did not justify the development of such liquid amides in the absence of evidence of a proportional gain in properties. LUBRICATING PROPERTIES OF SYNTHETIC QILS
When “thick film” or hydrodynamic conditions of lubrication prevail, the rheological and heat transfer properties of the 5uid are dominant. B u t in boundary friction and lubrication as well as rust inhibition, i t is the surface chemical rather than bulk properties of liquids which are important. One of the‘ most puazling questions in guiding the eynthesis of lubricants has been the molecular structural requirements for obtaining such properties in adequate measure. A brief comparison of the results of recent research on the boundary lubricating properties of synthetic ester oils not only has current interest but exemplifies the general solution t o the problem.
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Figure 4.
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I2 16 20 V I S C O S I T Y , CENTISTOKES AT
Slope vs. Viscosity of Esten
n-Alkanes n-Alkyl esters 0 Alkyl diesters 0 Phthalate diesten oPhorphate esters Polye,ten. Trimethvlolelhane tricaproate pentaerythritol tetraca roate, and tri(2.‘ ethylhexyl) tricarbally()atc
%
During World War I1 the interest in synthetic ester lubricants was not confined t o this country (3,6, 9, 17), as the Germans, primarily under the direction of Zorn (66,.@), carried on much research in this field. From the results’of lubrication tests on these synthetic materials and extrapolations from the surface chemical properties of esters’ at the oil-mercury and air-water interfaces, Zorn concluded that esters (especially the aliphatic diesters) were remarkable lubricants. He attributed the lubricating ability of the diesters to their unusual adsorptive properties, and their mode of orientation when adsorbed a t the solidoil interface (36). The authors’ work was based on the ability
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INDUSTRIAL AND ENGINEERING CHEMISTRY
t o incorporate simultaneously in branched-chain aliphatic, diesters the three bulk properties (low viscosity-temperaturc coefficients, low freezing points. and low volatilities) desired in aviation and ordnance instrument oils. Earlier surface-chemiral research had shown that esters were only weakly adsorbed at the oil-water and metal-oil interfaces (42) while the diesters had practically no rust-inhibition properties (4, 5 ) . Thus, it is improbable that the boundary lubricating properties of diesters would be any better than the analogous pure hydrocarbons (or white mineral oils). -4large class of effective ant>iwearagents anti tust inhihitors physically adsorb as close-packed or condensed films. Brophy a n d Zisman ( 8 ) showed from surface-chemical considerations that t,he van der Waals energy of cohesion amountkd for a large proportion of the total energy of adsorption when such conditions preva,il. Structural requirements for an aliphatic polar molecule to have a very low freezing point :ire precisely those which make it unable to adsorb as it condensed or close-packed film. IIenoe, such ester liquids when pure cannot have good wear- or rust-preventive properties and are comparable in this respect to the analogous hydrocarbons. Experiments a t this laboratory on pure di-, tri-, and tetraesters using both the Rowden-I,eben stick-slip machine ;ind the Larson-Perry modification of t,he Boerla.ge &ball machine ( 7 . 8) verify the conclusion that. the diesters are not good antiwear agents. Other investigations (4-6’) showed that pure diesterx had little or no rustiiihihitive properties. In general, if any synthetic oil molecule is to possess boundary lubricating and rust-inhibitive properties, i t must contain one or more polar (or other reactive) groups which are not sterically hindered from adsorbing on or reacting with the metal boundaries. If the film so formed is to be physically adsorbed, it must have a low compressibility or be condensed to prevent wear and rusting, and the moleclilar configuration required is such as to oppose obt,aining t,he bulk properties of low viscosity-temperature c:oefficicnt and low freezing point. TJnder any conditions, the ivear-preventive nl)ility of the oil will disappear, owing to tliernid agitation ant1 dcwrption at temperatures of 100’ to 200” C, or less. If the protect,ive film is to be formed by a cheniic:al reaction a t the niet,al-oil boundary, as with soap fonnntion in situ or \vith sulfur-, chlorine-, or phosphoruscontaining extreme pressure addition agents, the bulk oil will be cBhemically less stable. Also, the presmce of the reactive groups in the oil niolecules will ‘invite association effects which will hinder obtaining the lowest possible temperature coefficient of viscosity. Therefore, it is better to synthesize lubricant fluids of high viscosity index and low pour point for the optimum bulk properties and tci rely upon addition agents for imparting the controlled dcgree of the surface activity needed for wear and rust prevention. .4s a n cis:tmple, the synthetic esters, like all other liquids wntainiirg c.:trt)on-c;trbon bonds, can deteriorate under certain conditions to form easily adsorbed or reacted acids. Although this property may seem advantageous in some special applications, it is not always reliable and often is insufficient. Hence, thp controlled use of efficient antiwear, antirust. and antioxidant additives is preferable. ,This is precisely what the lubricant producer has learned to do empirically. With minor exceptions based on compatibility and the specific effects of d e terioration products on, the stability of the base oil, the same technology can be applied to m y new synt.hetic lubricant fluid. LITERATURE CITED
(1) Am. Petroleum Inst., Standard 533-43; -4.S.T.M. Designation D 341-43. (2) Andrade, N,, Nuture, 125, 309, 580 (1930). (3) Atkins, D. C., Baker, H. R., Murphy, C. M., and Zisman, W. A . , IND. ENG.CHEM., 3 9 , 4 9 1 (1947). (4) Raker, H. R., Jones, D. T., and Ilisinan, W.A,, Ibid., 41, 137 (1 949).
Vol. 42, No. 12
( 5 ) Baker, H. R., and Zisman, W.d.,Ibid., 40,2338 (1948). (6) Bried, E. M., Kidder, H. F., Murphy, C. RI., and Zisman, W. A . , Ibid., 39, 484 (1947). (7) BroIihy, *J. E., Clinton, W. C., Thomas, T. M., and Zisman, (8)
19) (10)
(11)
FV. A . , “Boundary Lubricating Prop,erties of Synthetic Esters,” to be published as Naval Research Lah. report. Brophy, J. E., and Zisman, M’. A , , ”Surface Chernical Phenomena in Lubrication,” Confererlce on E’rlndamental Aspects of Lubrication. New York Acad. Sciences, March 3, 1950 (to be published as bound report of coiiference). Bruner, W. M . , IND. CNO.CHEX.,41,2860 (1949). Cohen, G., and Murphy, C . M . , “Viucomet,ric and Related Properties of Pure Alkyl Monocstws,” to be published as Naval Research Lab. r,eport. Cosby, ,J. N., and Sutherland, I,. H., P,oc. A4ni.Pctrolrum
111, 22, 1 3 (1941). (12) Davis, G. H. B., Lapeyrouse, W.. atld Dean, E. W., Oil Ce: &a J . , 30, No. 46, 92 (1932). (13) Dean, E. W., Bauer, A. D., and Herrlund, ,J. H., IND., ENQ. CHEM., 32, 102 (1940). (14) Dean, E. W., and Davis, G. H. B., Cliern. M e t . E r f g . , 36, 618 (1929). (15) Evans, E. B., J . Inst.’PrtroZeurrL, 24,321 (19:38). (16) lFox, H. W., Taylor, P. IV.,and Zisman, JT. .I.,IND.ENG. C H E M . , 39, 1401 (1947). (17) Glavis, F. .J., and Stringer, H. I t . , Symposium on Synthetic Lubricants, Atlantic City, N. .J., June 1947, Am. Soc. Testing Materials, S p e c . Tech. Pub. 77. (18) Grosse, A. V.,andCady, G. H., IND.ENG.CHEM.,39,367 (1947). (19) Hardiman, E. W., and Nissan, A. H., J . Inst. Prtro!eicm. 31, 255 (1945). (20) Landa, Y., Cech, J., and Silva, V., ColEection Czechosloi. C‘hrm. Communs., 5, 204 (1933). (21) Landa, S.,and Habada, M., Ibid., 8, 473 (1936). (22) Landa, S.,and Kejvan, A., Ibid., 3, 368 (1931). (23) Landa, S . , and Riedl, R., Ibid., 2, 520 (1930). (24) T,anda, S..and Silva, V., Ibid., 4,538 (1932). (25) MacGregor, J. ‘H., and;Ward, F., J . Soc. Chem. I d , , 66, :344 (1 947). (26) Mikeska, L. A., IND.ENG.CHEM.,28, 970 (1936). (27) iMurphy, C. M., Romans, J. n., and Zisman, W-. A , , ‘ / ‘ m a s . Am. Soc. Mech. Engrs., 71,561 (1949). (28) Natl. Bur. Standards, Circ. C461 (1947). (29) Nissan, A. H., Clark, L. V. W.,and Nash, A. W., .J. IrL8t. Petroleum, 26, 155 (1940). (30) Ramser, J. H., IND. ENG.CHEM.,41,2053 (1949). (31) Sanderson, R. T., Ibid., 41, 368 (1949). (32) Schiessler, R. W., Clark, D. G., Rowland, C. S.,Sloitt,nian, W. S., and Herr, C. H., Proc. A m . PetroZezim Ilast., 111. 24, 49 (1943). (33) Schiessler, R. W., Cosby, J. N., Clarke, D. G . , Rowland. C . S.. Sloatman, W. S., and Herr, C. H., Ibid., 111, 23, 15 (1942). (34) Schiessler, R. W., Herr, C. H., Rytina, A. W., Weisel, C. A . ,
Fisohl, R., McLaughlin, R. L., and Kuehner, H. H., “Synthesis and Properties of Hydrocarbons. of High llolecular Weight,” Am. Petroleum Inst. Meeting, St. Louis, Ma., June 2, 1947. (35) Tingle, E. D., J. Inst. Petroteurn, 34, 743 (1948). (36) Tobolsky, A., Powell, R. E., and Eyring, H., “Elastic-Viscous Properties of Matter in Frontiers in Chemistry,” Vol. I , p. 125, New York, Interscience Publishers, 1943. (37) Van Wijk, W. R., and Seeder, W. A.,Physica, 4, 1073 (1937). (38) (39) (40) (41) (42) (43)
Ibid., 6 , 129 (1939). Walther, C., ErdGZ u. Teer, 7, 382 (1931). Walther, C., PTOC. World Petroleum Cong. London, 2, 419 (1933). Wilcock, D. F., Mech. Eng., 66, 739 (1944). Zisman, W.A., J . Chem. Phys., 9, 5 3 4 , 7 2 9 , 7 8 9 (1941). Zorn, H., “Esters as Lubricants,” U. S. Air Force translation, Rept. F-TS-957-RE (1946-47).
RECEIVED June 23, 1950.
