Svnthetic Lubricant Fluids from Branched-Chain Diesters J
PHYSICAL AND CHEMICAL PROPERTIES OF PURE DIESTERS E. M. BRIED, H. F. KIDDER, C. M. RIURPHY,
AND
W. A. ZISXIAK
Naval Research Laboratory, Washington, D . C .
D
This paper reports the development of diesters most suitesters presumably being 2URIKG World War 11, designers, manufacable for use as synthetic lubricating fluids. The principal ethylhexyl. Although glycturers, and users of fire conrequisites were to secure chemically stable and noncorerol esters and monoesters rosive fluids having freezing and pour points below -40" F., have been used for lubricants trol equipment, depth bomb mechanisms, fuse mechaadequately low evaporation rates, low viscosities at or lubricant additives, the nisms, aeronautical com-40" F., and low viscosity-temperature slopes (or high use of diesters for such purpasses, gyroscopes, clocks, gun viscosity indices). From structural analogies and known poses is relatively new. il rules relating molecular structure of hydrocarbons to their number of undisclosed esters sights, bomb sights, recorders, physical properties, it was concluded that long-chain and and diesters were tested durand aerial photographic and essentially aliphatic diester molecules were needed having ing the war (1O), and it was other special aviation and ordnance equipment were conone or more short-chain alkyl branches suitably posireported that a commercial cerned with a variety of new tioned. Thirty-four diesters were prepared, purified, and grade of di-(2-ethylhesy!) lubrication problefis. The studied in this research program. The following propersebacate was an effective antities were examined and related when possible with diftack additive for use in the most common difficulty was ' ferences in molecular structure: viscosity-temperature bepetroleum hydraulic oils and that of obtaining sufficiently havior, volatility, flammability, specific gravity, thermal for the preparation of polyeffective and chemically expansion, solubility for water, and hydrolytic stability. mer-thickeneddampingfluids. stable lubricank with pour points ranging from -40" to From a study of the properties of these fluids, a smaller Some aromatic and aligroup was selected for development into lubricants of phatic diesters of dicarbos-80" F. Especially troubleylic acids were commercially immediate interest. some was the problem of satisfying the need for nonvolatile available a t the start of fluids having viscosities a t this investigation. Because looo F. of not over 10 t o 15 centistokes. Usually the smallest of the uncertainty in regard to purity and the difficulties involved in purification, it was decided to synthesize and purify all of the possible temperature coefficient of viscosity was desired. The best obtainable rust inhibition and good hydrolytic and oxidadiesters of interest in this investigation. tion stabilities were required to care for service use and for GENERAL STRUCTURAL CON SIDERATION S eventual long-time storage conditions. Other properties desired were nontoxicity, resistance t o mold growth, and compatibility An important advantage of diesters as compared to monowith petroleum lubricants t o care for accidental contamination. esters is that compounds of higher molecular weight can be preThe evaporation of the volatile, less viscous fraction of petropared without resorting t o the use of alcohols or acids of high leum oils used in low viscosity instrument oils has resulted in molecular weight. I n addition, a wide variety in molecular conconsiderably increased viscosities and pour points, and sometimes figuration is possible because of the availability in such compounds in practically dry bearings. Often the evaporation and reconof two reactive groups. A diverse group of diesters can be densation of the lubricant on optical parts of instruments was prepared with the many commercially available branchedsufficient to obscure vision. chain aliphatic alcohols. As in the development of vacuum The required combination of properties make it difficult to pump oils, the desired low vapor pressure can be assured by preproduce the required lubricants from petroleum oil fractions. paring compounds of sufficiently high molecular weight. I t was therefore considered advisable to develop a nonhydrocarIn order to keep the viscosities of the fluids sufficiently low at bon homologous group of synthetic organic chemicals for these -40" F., it is essential that the branches from the main diester applications. chain should be as short as possible, consistent with the need for obtaining a low freezing point. This can be obtained readily by The use of diesters as plasticizers is the earliest large scale using (a) branched alcohols or alcohol-ethers reacted with application known to this laboratory, the dimethyl, diethyl, distraight-chain dicarboxylic acids, ( b ) branched-chain monocarbutyl, dioctyl, diphenyl, and dibenzyl phthalates and sebacates boxylic acids and straight-chain dihydric alcohols such as the being among the commonly used plasticizers. More recently polymethylene glycols or the polyethylene glycols, or (c) using new plasticizers of interest here were made by reacting polyethylstraight-chain alcohols or alcohol-ethers reacted with bmnched ene glycol with 2-ethylhexanoic acid and 2-butoxyethanol with aliphatic dicarboxylic acids. A t present the relatively lower azelaic acid. Dibutyl phthalate vias investigated and recomcost and availability of the diesters made by methods n and b mended for use as a vacuum pump oil by Hickman and Sanford as compared t o c make them preferable. (IS,16). Later a variety of other diesters of phthalic acid were The low freezing points required can be obtained by three studied and applied by Hickman (14). I n the present high methods. The first is to cause branching of the molecule to give vacuum distillation practice, use is made of the dibutyl, diamyl, hindrance to the regular alignment and close packing of the: and dioctyl esters of either phthalic or sebacic acid, the dioctyl 484
April 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
major molecular chains. The second method is to apply the well known rule of alternation of the melting points of organic compounds of a homologous series and of the lower melting points of straight-chain compounds having an odd number of carbon atoms per molecule. The presence of the ester group itself should be advantageous in causing the diester compound to have a lower freezing point than the analogous hydrocarbon compound. The third method is t o employ a mixture of isomeric diesters. However, this method has t h e limitation of inherent nonreproducibility due to the difficulties in specifying and fixing the proportions of the various isomers present. The hydrolytic stability of a diester should be increased by the addition of one or more minor hydrocarbon chains branching from the major ester chain in such a way that each branch is close enough t o an ester group to create hindrance or blocking to the close approach of water or acid molecules. Hence, it is preferable that the branching of the major chain of the diester occurs on the beta carbon atom of the alcohol or of the acid. If the blocking effect is increased too much, however, there will be an added difficulty in obtaining good yields in the esterification reaction. Hence, long hydrocarbon chains branching from the beta or near-by carbon atoms are undesirable. Diesters made from alcohol-ethers instead of alcohols of the same molecular weight or those made by using polyethylene glycol reacted n-ith a branched-chain monocarboxylic acid will dissolve more water, and hence increased hydrolysis rates can be expected. The klscosity-temperature characteristics of only the lower molecular weight esters of the dibasic aliphatic acids have been investigated. Table I compares the kinematic viscosities of the diethyl esters of aliphatic dicarboxylic acids and of the normal paraffin hydrocarbons of the same chain length. The chain lengths of the diesters were considered to be the number of atoms in the longest chain; thus, one oxygen atom from each ester group is included in the calculated chain length. The kinematic viscosities of the diesters were calculated from the data of Dunstan, Hilditch, and Thole ( 7 ) , that of the normal hydrocarbons except tridecane and pentadecane were obtained from data given by Evans (9), and the viscosities of tri- and pentadecane were calculnted from data given by Doss (6). The viscosity of pentndecane is inconsistent with the other data. Table I reveals that diesters are more viscous than analogous paraffins of the same chain length but tend to approach them a t higher chain lengths. The esters of aliphatic alcohols and straight-chain dicarboxylic acids would be expected to have viscosity-temperature characteristics similar to those of the analogous paraffin hydrocarbons. Several investigators (4, 16, 18, PO, 21) have generalized on the effect of chain length, branching, cyAic groups, unsaturation, and functional substituents for hydrocarbons. These generalizations (summarized briefly below) were useful in predicting t’he \,iscosity and the viscosity-temperature characteristics of liquids from their structural configurations: 1. Increasing the chain length increases the viscosity and improves the viscosity-temperature characteristics as evidenced by high value of viscosity index and low value of the A.S.T.31. viscosity-temperature slope. 2. The addition of side chains increases the viscosity and decreases the viscosity-temperature slope. The amount is dependent upon the number and extent of the branches. 3. The position of the branched chain exerts a variable influence on the viscosity. 4. The addition of cyclic groups causes larger increases in viscosity and greater increases of viscosity-temperature slope than aliphatic chains. 5. Iiicreasing the ratio of the cross section of the molecule t o its length increases the viscosity-temperature slope.
These considerations led to the conclusion that only aliphatic diesters with short alkyl branches should be prepared. The results on the various new diesters were never found to be in disagreement with these generalizations.
485
A comparison of the boiling points of the diethyl esters of the dicarboxylic acids (3) with those of the analogous hydrocarbons reveals that the lower members of the diester homologous series have considerably higher boiling points, and therefore lower evaporation rates, than the hydrocarbons o f the same chain length. On going up this homologous series, the hydrocarbon portion of the molecule increases while the diester portion remains constant. Therefore, the influence of the latter on the boiling point will be less noticeable as the molecular weight increases. TABLE I. COVPARIsON AND
Diethyl Ester Oxalate Malonate Succinate Glutarate Adipate Pimelate Suberate .4zelate Sebacate
Viscosity a t 77a F., Centistokes 1.63 1.79 2.32 2.49
2.74 3.30
4.06 5:2S
OF \‘ISCOSITIES
O F ‘ILIPH.4TIC
SORMAL PAFAFFISS Chain Length, S o . Hydroof Atoms carbon 8 n-Octane 9 n-Sonanc! 10 n-Decane 11 n-Undecane 12 n-Dodecane 13 n-Tridecane 14 n-Tetradecane 15 n-Pentadecane 16 n-Hexadecane (cetane)
DIESTERS
Viscosity a t 77’ F., Centistokes 0.72 0.92 1.15
1.44 1.77 1.89 2.55 1.861 3.98
It is well known that the physical properties of the normal alkyl hydrocarbons differ from those of their branched-chain isomers. General rules have been given on the effect of branching on the boiling point. Briefly, they are as follows: (a) The boiling point of the branched-chain hydrocarbon will be lower than that of the normal hydrocarbon isomer. ( b ) The increase in boiling point by the addition of side chains will vary with the amount and extent of the branching and, to a lesser extent, will be influenced by the position of the side chain. From the foregoing considerations it is evident that the diesters of the normal alcohols will be less volatile than those of their branched-chain isomers. However, other properties are desired in an instrument lubricant; in particular, the freezing point and pour point of the oil must be considered. Since the diesters of the normal alcohols have not only higher boiling points than their branched-chain isomers but also higher freezing points, some compromise must be made to obtain fluids ITith low enough freezing points and volatilities. These analogies and structural conclusions were useful in outlining a program for the synthesis and development of a variety of diester lubricants. To obtain the smallest viscosity-temperature slope (or the maximum viscosity index), an entirely aliphatic diester was preferred. To obtain fluids with as low viscosity as possible consistent with the need for low freezing points and evaporation rates, branched-chain diesters viere preferred having a minimum number of branches of no more than several carbon atoms. Considerations of the present or probable future availability of starting materials and of the need for developing a group of fluids with a wide enough range of viscosities led to a definite synthetic and evaluation program. The preparation of the selected fluids, their purification, and their identification ivill be described elsewhere by other members of this laboratory ( 5 ) . FREEZING AXD POUR POIRTS
Table I1 lists the diesters synthesized and purified for this investigation. Some esters of phthalic acid are included for comparison. The freezing and pour points in Table I1 were taken from another investigation ( 5 ) . Liquids with extremely low freezing points can be prepared by using mixtures of the pure diesters or mixtures of the diester made XTith starting materials consisting of either isomeric mixed alcohols or mixed acids. Some simple two-component mixtures used by this laboratory during the war in connection with the development of instrument oils and greases for extremely low temperatures were prepared from the 2-ethylhexyl diesters of
INDUSTRIAL AND ENGINEERING CHEMISTRY
486
TABLE 11. FREEZING POINTS, VOLATILITIES, AND GRAVITIESOF DIESTERS hfol. Rt.
Identification
SPECIFIC
Freezing Volatility=, Sp. Cr., Point, O F , 74 T t . Loss di5
PHTHALlTEs
Diethyl phthalate Methylphthalylethyl glycolate Dibutyl phthalate E t h y l phthalyl ethyl glycolate Butvl phthalyl butyl glycolate Di-(2-ethylhexyl) phthalate
-2i < -31
2 26 26 .. 23 278.3 280.3 336.4 390.5
-3 1 468 < -31 t h yl ] Di-(undecyl) b Di-(tetradecyl) C Di-(heptadecy1)d
286.4 342.5 314 5 342 5 370.6 370. ,6 426 458.7 510.8 595.0 679.1
,
+ 2 8 t o 32 0 to - 6 / 14 +1 4 6 to9 -8
+
- 67
+8 6
. Froin I~olid energy d?terniinations it ~ o u l dbe predicted that tlie ester linkages are more thermally btable tliaii the C---C bond. I n :rgwrment with this conclusion, it \vas found that tile spontarirous ignition temperatures of the diesters (2) n-~i'eIiipher tli:itl thosr of the aliphatic hydrocarbon.: of the same chniii length. It ipossible that the diesters n.ill follo~\-the general rules eniinicwtcd by Egloff (6) relative to the oxidation of alltaries. The oxidation stabilities of ,several reprt.sentative prouph o f diesters were examined h y nieaiir of a dynaniic type of oxidatioii ~ 230 mill. test. The oxidation cell u x s :I cy1indric:il g l : veswl,
