ALIPHATIC ESTERS Properties and Lubricant Applications GEORGE COHEN, C. M. MURPHY, J. G. O'REAR, HAROLD R A W E R , AND W. A. ZISMAN -Vaval Research Laboratory, Washington, D . C .
I
N 1942 this laboratory undertook a basic investigation of the problems involved in low temperature instrument lubrication. The results obtained using aliphatic diesters as lubricants have been reported (1-5, 6, 7,16, 25). It was found that a number of aliphatic branched-chain diesters possessed the requisite properties, for not only the lubrication of instruments but also many other military applications in aircraft and ordnance equipment. During the corresponding period, a German research group under the leadership of Zorn (58)investigated independently the use of esters as special-purpose lubricants and concluded that several branched-chain aliphatic esters were outstanding. Their work was later reviewed by Tingle (56) and by Horne (18). Since World War 11, a variety of special lubrication problems have arisen which increased interest in synthetic lubricants. Aircraft gas turbines, for example, require oils capable of providing adequate lubrication from temperatures of -65" to about 275" F. with temperatures on shutdown as high as 500" F. for intervals of 1 or 2 hours. Military automotive equipment is required t o operate a t what m-ere formerly considered prohibitively low temperatures ( 1 9 ) . At high altitudes or in winter, machine guns and automatic cannon frequently could not be made to f i e because of congealed lubricants (56). Synthetic ester lubricants have received increasing application where petroleum lubricants cannot function satisfactorily. Therefore, this laboratory has endeavored, through its own research and through encouragement of industry, to extend the supplies of such lubricants by locating additional sources of native raw materials for their production and by developing new classes of esters ( 2 7 ) . This report summarizes the progress since earlier publications (3,6). Certain generalizations relating the structures of hydrocarbons to their viscometric properties were found valid for esters (6,6, 8 25, 32, 32). These researches resulted in structural guides which reduced substantially the number of compounds required for synthesis and study. It was found that the high viscosity indexes of long-chain molecules are due to their ability to coil and uncoil reversibly with variations in temperatures ( I d , 31, 3 2 ) . Such coiling is possible because of the freedom of rotation about the chemical bonds in the principal chain. Branching restricts the rotational freedom about these bonds and also increases the ratio of cross section to length of the molecule, both effects contributing to smaller viscosity indexes. Unbranched diesters have slightly lower viscosity indexes than the analogous hydrocarbons because the carbonyl oxygen behaves as a short branch chain. Unusual surface active properties-viz., ability t o adsorb and orient at the solid-liquid interface, forming good wear-preventive f i l m e w e r e attributed t o the diesters by Zorn (58). Brophy and Zisman (8) showed that the wear-preventive and rust-inhibiting properties of diesters were caused by the presence of impurities, as the pure compounds were no better in these respects than white mineral oils or pure hydrocarbons. HIGH TEMPERATURE OXIDATION l N H l B I T O R S FOR ESTERS
It has been demonstrated ( 2 , s )that diesters can be adequately protected against oxidation a t 100" C. with conventional petroleum-type antioxidants. Until recently (29), little attention had been paid to inhibitors for more elevated temperatures. As the
operating temperatures of military and industrial equipment are increasing, it was considered desirable to search for antioxidants suitable for diesterv a t temperatures above 150' C. Since the effectiveness of antioxidants may vary, owing to the presence of impurities in the additive or in the reference liquid, only compounde of high purity were used. The reference liquid, bis(2-ethylhexyl) sebacate, was selected because it is typical of the higher-boiling aliphatic diesters suitable for military lubricant applications. -411 the comparative data on antioxidants were obtained in an aeration-type oxidation apparatus using metal catalysts ( 2 , 3 ,SO). Among the criteria used to denote the efficacy of the inhibitors were the extent of the induction period, the change in viscosity, color, acid number, metal catalyst a-eight and appearance, and the presence of sludge and lacquer. The results on the oxidation of the uninhibited diester, and the antioxidant action of various inhibitor classes, are summarized in Table I. With no antioxidant present (Table 1,A) deleterious changes occurred in the bulk liquid and corrosion of the metal catalysts was observed a t 100" C. As would be expected, elevation of the test temperature resulted in a more rapid increase in acid numbers, viscosity changes, corrosion, and sludge and lacquer formation. \Then exposed to 163" C the oil gelled before the expiration of the 168hour run. Phenothiazine and many of its derivatives are particularly effective in diester liquids (69, SO). Results obtained with t x o phenothiazine derivatives are compared with those of the parent compound in Table 1,A. I t will be noted that phenothiazine provides satisfactory inhibition to the bulk oil a t all temperatures up to 175' C., although lacquer formation is noticeable a t 163" C. Both 3-fluorophenothiazine and the somewhat light-sensitive 3,7-difluorophenothiazine are about as effective a8 the parent compound. Phenyl-1-naphthylamine is the most effective of the amine class of antioxidants with an upper temperature limit of 163" C. Higher concentrations than those employed with phenothiazine are required t o effect comparable inhibition. The amines of lower molecular weight provide unsatisfactory inhibition at 150" C. An upper effective temperature limit of 125' C. is usually found for phenolic inhibitors. The exceptions are lauryl gallate and the conidendrins, which provide fairly effective inhibition a t 150" and 163' C., respectively. Alpha-conidendrin, whose chemical synonym is 1,2,3,4tetrahydro-6-hydroxy-4-(4-hydroxy3 - methoxyphenyl) - 3 (hydroxymethyl) - 7 methoxy - 2 naphthoic acid -01- lactone, is somewhat more soluble in the reference fluid than the beta form, although no apparent difference in their antioxidant action was noted through 163' C. Of the ether-type compounds investigated (Table 1,B) the selenides containing primary alkyl linkages are noticeably superior in antioxidant action t o those containing secondary linkages. I n the concentrations employed, n-alkyl selenides are effective at 163" C. I n contrast, the analogous sulfur compound, dihexadecyl thioether, has an upper effective temperature limit of 125' C. Of the phosphorus-containing compounds investigated, the phosphites inhibit adequately a t 100' and 125" C. The relatively low boiling points of these compounds may account, a t least in part, for their inability to inhibit oxidation above 125" C. Because of their hydrolytic instability, they should not
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1766
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August 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE I. OXIDATION INHIBITION
1761
IMPARTED TO BIS(2-ETHYLHEXYL)SEBACATE
(168-hour aeration test)
Antioxidant None
T!mp., C. A. 100 125 150 163
72 12 12 12
125 150 163 175 163 150 175
14 375 1000 Gelled
.. ..
.. .. ..
>168 > 168 > 168 > 168 >168 > 168 > 168
1.1 4.3 23.4 46.8 20.3 8.5 42.5
0.02 0.09 0.47 0.94 0.46 0.20 1.0
100 150 100 150 125 150 125 150 125 150 125 150 163 163 175
>168 > 168 > 168 36 >168 144 >168 >168 > 168 >168 > 168 >168 156 > 168 36
4.3 23.4 4.3 46.8 4.3 11.7 4.3 11.7 4.3 11.7 1.1 11.7 23.4 46.8 46.8
0.07 0.36 0.09 0.49 0.08 0.22 0.09 0.26 0.21 0.58 0.02 0.26 0.51 1.00 1.00
258 168 > 168 > 168
163 125 163 125 163 125 125 163 150 163
> 168
125 125
48
125 123 123
163 150 123 163 163 125 163 125 163 125 125 163 163 125
>168 >168 > 168 > 168 > 168 > 168 > 168 > 168 > 168 48
48 > 168 > 168 > 168
Concentration of Inhibitor Molality x 103 wt. %
v
Xeutralization No. acid
acid
0.40
2.9
0.54
0.08
