US 3564044 A
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United States Patent Oflice 3,564,044 LIQUID ESTERS OF NEOALKYLPOLYOLS AND MIXTURES OF GEM AND STRAIGHT CHAIN OR ALKANOIC NEO ACIDS Tai S. Chao, Homewood, and William D. Hoffman, Park Forest, 11]., and Manley Kjonaas, Hammond, lnd., assignors to Sinclair Research, Inc., New York, N.Y., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 557,898, June 16, 1966. This application Oct. 5, 1967, Ser. No. 673,020
Int. Cl. C07c 69/32 US. Cl. 260-488 10 Claims ABSTRACT OF THE DISCLOSURE Neoalkylpolyol esters of mixtures of gem acids and straight or branched chain neo fatty acids are prepared. These esters possess improved oxidation resistance and good low temperature properties and are suitable for use as base fluids or blending stock for high temperature lubricants.
This application is a continuation-in-part of application Ser. No. 557,898 filed June 16, 1966, now U. S. Pat. No. 3,441,600.
This invention is concerned with neoalkylpolyol esters of mixtures of gem and straight or branched chain alkanoic neo acids. These esters are suitable, for instance, as base fluids or blending stock in high temperature lubricants.
The physical state of lubricants, i.e., whether they are fluids or solids under standard conditions, along with their oxidative and thermal stability is extremely important for high temperature lubricants such as those used in jet aircraft, since they must be able to flow and pour at subzero temperatures and at the same time function at high operating temperatures.
Lubricants composed in whole or in part of synthetic components have been developed in an effort to obtain superior lubricating compositions. In general, these lubricating compositions are characterized by higher viscosity indices, lower pour point and greater heat stability than mineral oils of corresponding viscosity. Such properties are of special value in lubricating engines which are subjected to high temperatures, such as combustion turbine engines.
Mineral oil lubricants, for example, event those containing added VI improvers, pour point depressors, or other additives are undesirable for use in such engines because of the relatively high volatility, low flash point, and poor thermal and oxidative stability of the mineral oil which also has a tendency to leave deposits which accumulate and interfere with the operation of the engine.
Certain neoalkylpolyol esters of straight chain fatty acids, e.g., pentaerythritol tetracaproate, which are known as Type II lubricants, have been employed in jet engines but do not possess the thermal and oxidative stability required of the more advanced engine models. On the other hand, esters derived from neoalkylpolyols and branchedchain neo fatty acids, e.g., pentaerythritol tetrapivalate, although having good oxidative stability, are solids at room temperatures and thus have poor handling, starting and low temperature properties. Polyphenyl ethers, e.g.,
3,564,044 Patented Feb. 16, 1971 m,m,-bisphenoxyphenoxybenzene, have pour points above 35-40" F. and do not flow and pour at sub-zero temperatures, as required to achieve their functions, especially during the critical start up time for jet engines.
To improve thermal and oxidative resistance of jet lubricants, compounds comprising aromatic, pseudo aromatic and other inherently stable groups have been suggested as base fluids. Such materials are polyphenyl ethers, pyrazines, trizines and phosphonitrile chloride derivatives. All of the above bear high costs which limit their use, and they are inferior in low temperature properties. This precludes their use in jet planes and in industrial gas turbine units, which are often exposed to sub-zero temperatures.
Various liquid esters such as di-Z-ethylhexyl sebacate, etc., have been used commercially in or as lubricants. In particular, esters of alkyl neo fatty acids and neoalkylpolyols of 3-5 hydroxyl groups such as trimethylolpropane and pentaerythritol have generally been good base fluids for high temperature synthetic lubricants. The presence of alkyl groups and the absence of I-I-atoms on the a-carbon atom impart many unique properties to alkyl neo fatty acid esters such as increased thermal, oxidative and hydrolytic stability. This increased stability rendders them highly desirable for use as base fluid for jet engine lubricants, functional fluids, greases and as plasticizers.
However, the presence of the alkyl groups on the a-carbon atom also imparts some severe disadvantages. The first disadvantage is due to the steric hindrance exerted by the alkyl groups on esterification, with the consequent need for more drastic conditions which increase processing costs and result in inferior products. The second and much more damaging effect is that, despite having greater thermal stability than straight chain acid esters, they also have higher melting points.
Further, the successful use as lubricants of esters of alkyl neo fatty acids and neoalkyl polyols has been extremely limited, since most of them have been known to exist as solids at room temperature, this being especially true for the plural esters such as those wherein 3 or more of the hydroxyl groups of the alcohol are esterified.
It has now been found, in accordance with the present invention, that base fluids or blending stocks for high temperature lubricants, having improved oxidation resistance and good low temperature properties can be prepared by reacting neoalkylpolyols such as pentaerythritol, with mixtures of straight chain or alkanoic neo acids of 4 to 10 carbon atoms and preferably 5 to 8 car-bon atoms and certain well chosen gem alkanoic acids having the general structural formula:
wherein R R and R are C -C alkyl groups, n is 1 to 3 and the total number of carbon atoms is not more than 9. \Suitable gem acids include, for example, 3,3-dimethy1- pentanoic, 4,4-dimethylpentanoic, 3,3-dimethylbutanoic, 4,4 dimethylhexanoic, 5,5 dimethylhexanoic, 3,3,4-tr1- methylhex-anoic and 4,4,S-trimethylhexanoic acids.
The straight chain or alkanoic neo acids are usually employed in minor amounts in mixture with the gem acid, but may be employed in amounts of from 5 to 50% by weight and preferably at least 10% The gem acids can be prepared by known processes for preparing aliphatic acids. For example, 3,3-dimethylpentanoic acid can be prepared from 3,3-dimethylpentanol by oxidation With KMnO While 4,4-dimethylpentanoic acid can be prepared by treatment of 3,3-dimethyll-chloro-butane with NaCN, followed by acid hyrolysis.
It may be seen that these gem acids differ from neo acids, such as neopentanoic or pivalic, neoheptanoic, neodecanoic, acids, etc., in that the neo acids have no -CH groups between the branching and the COOH group.
The gem acids of this invention also differ from other branched-chain acids in that they contain at least one gem group:
and contain no tertiary H on the a-C atom. By tertiary H on the oc-C atom, it is meant the H shown in the formula:
The R is an alkyl group.
can be carried out in the same manner as the esterification of straight chain acids alone. No drastic conditions, such as the use of relatively high percentages of H 50 or the conversion to acid chloride are needed. In fact, for the preparation of esters for use in synthetic lubricants, the use of H 50 p-toluene sulfonic acid and other S-containing catalysts should be avoided. Even a trace of sulfur will result in deterioration of the quality of the finished product.
As may be seen, improved low temperature properties were obtained by incorporating a minor amount of a straight chain fatty acid, such as n-valeric acid, during the esterification of pentaerythritol with 3,3-dimethylpentanoic acid. By this means esters of lower pour points, lower viscosity at low temperatures and excellent low temperature storage stability were obtained. Table I shows the properties of four such esters. It can be seen that a pour point of F. was obtained with the incorporation of 18.5 mole percent (15.1 wt. percent of n-valeric acid in the acid mixture and one of F. was obtained with 28.0 mole percent (24.2 wt. percent) of n-valeric acid. The viscosity at 0 F. was reduced to 17,229 and 8,997 cs. respectively. Further reduction of KV at 0 F. to 3,851 cs. was observed when 48 mole percent (42 wt. percent) of n-valeric acid was used. The mixed esters so prepared remained as clear liquids after seeding (with the tetraester) and storing in refrigerator (20 F.) and in low temperature baths (at 0, -30, --40, and 50 F.) for over hours.
TABLE I.PROPERTIES OF PENTAERYTHRITOL ESTERS OF MIXTURES OF 3,3-DIMETHYLPENTANOIC ND VALE RIC ACIDS Pentaerythritol 3,3-Me2-pentanoic acid n-Valeric acid Mole percent n-valeric acid in acid mixture .I- Mole percent of n-valcric group in ester, by N MR.
Kinematic viscosity, 0. s.:
Pour point, Cloud point,
Flash point, F. Fire point, F... Melting point, F
Saponification Number 422.7 426.1 .I 4221 Low temperature storage test appearance after 60 hrs.:
20 F Solid Clear liquid--- Clc 1\ Too viscous to measure. b supercooled.
After seeding and cooling in refrigerator. Without seeding a sample stayed liquid for 2 years at room temperature.
The neopentylpolyols suitable for the purpose of this invention are organic compounds often having up to about 12 carbon atoms with 2-6 hydroxy groups or more and preferably not more than 7 carbon atoms with 24 hydroxy groups and containing at least one neopentyl group and at least two hydroxy groups attached to the carbon atoms next to the quaternary C atom. This structure is as follows:
I C HOCH2(IJCH2OII I Examples of such compounds include, for example, neopentyl glycol, 1,1,l-trimethylolethane, 1,1,l-trimethylolpropane (TMP), 1,1,1-trimethylolbutane, pentaerythritol (PE), dipentaerythritol, etc.
Esterification of the mixed gem alkanoic acids and straight chain or alkanoic neo acids with neopentylpolyols Although liquid esters can be prepared from neoalkylpolyols with mixtures of neo and straight chain acids, our studies indicated that a much higher percentage of the straight chain acid had to be used, with a corresponding 0 sacrifice in oxidation resistance. Thus, as may be seen in Table II, pentaerythritol esters of mixtures of neoheptanoic and n-valeric acids containing less than 50 mol percent n-valeric groups (as determined by NMR) are invariably solids. Even those containing more than 50 mole percent n-valeric groups showed poor low temperature stability. This is because the ester product is usually a mixture containing all possible single esters such as tetra-neo-C tetra-n-C tri-neo-C -mono-n-C di-neo-C di-n-C etc. Tetra-neo-C having a M.P. of 189l92 F. will crystallize out of this mixture upon standing, especially at low temperatures. Esters of mixtures of gem-C and n-C acids, on the other hand, did not show this tendency of crystallization. It is believed that this is due to the low melting point (-82 F.) of pure PE tetra-gem-O, and its strong tendency to remain as a super-cooled liquid.
TABLE II.PROPERTIES OF MIXED PENTAERYTHRITOL ESTERS OF NEOHEPTANOIO AND N-VALERIC ACIDS Sample No. A B O D E Mole percent neo-0 Calculated 70 Found (NMR) 100 73.9 60 8 Kinematic viscosity, cs.:
0F 8.171 6.469 5.712 5.245 4.744. 100 "F Solid Solid 42. 22--.- 35. 89 29 0 F 40 F Pour point, F Cloud point, Flash point, F 440 455 Fire point, F 500 520 6-0. 520. Melting point, F 189-192 135-144" 86-96 Low temp. stability, appearance after 60 hrs.:
F Solid So1id Solid Liquid and Clear crystals. liquid do Do. Do. Do. Do. Do.
n Too viscous to measure.
The excellent low temperature properties of mixed esters of gem and straight chain acids of this invention were also found with the esters of mixtures of gem and alkanoic neo acids. A pentaerythritol ester, prepared from 0.25 mole of pentaerythritol (PE), 0.75 mole of gem-C acid and 1.0 mole of neo-C group was found by NMR to contain 74.2 mole percent gem-C and 25.8 mole percent neo-C groups. It had a KV/210 F. of 11.91 cs., KV/ 100 F. of 181.0, KV/0 F. of 88,620 cs., pour point of -20 F., flash point of 485 F. In low temperature storage tests it stayed a liquid at 20 and 0 F. for more than 60 hours. These results are summarized in Table III.
TABLE III Properties of mixed pentaerythritol ester of 3,3-dimethylpentanoic acid (gem-7 acid) and neoheptanoic acid (neo-7 acid) Identity PE-gem-C -neo-C Kinematic viscosity, cs.:
210 F 11.91. 100 F 181.0. 0 F 8 8620. -40 F Pour point, F. 20. Cloud point, F. None. Flash point, F. 485. Fire point, F. 560. Melting point, F Mole percent gem-C 74.2. Low temp. stability, appearance after 60 hrs.:
20 F. Liquid. 0 F. Liquid. F. Glassy solid. F. e Glassy solid. F. Glassy solid.
The oxidation resistance of the esters of this invention was determined by two standard tests which have been correlated; well with gas turbine engine performance. The Oxygen Absorption Test measures the rate of O absorption in the presence of a standard inhibitor (1% by wt. of N-phenyl-l-naphthylamine), as well as the increase in viscosity and acid number after the absorption of 2500 ml. of oxygen and is a test to determine the oxidation resistance of lubricants. In principle it resembles the Dornte Oxygen Absorption Test (see WADC Tec. Report 59- 191, part III, pp. 30-32) and measures the rate of oxygen consumption by the lubricant at a given temperature.
Briefly, a Weighed sample of the lubricant is placed in a Pyrex oxidation cell which is heated electrically in an aluminum block. Oxygen gas is bubbled through the lubricant at a specified rate. The exit gases go through a series of absorbants and a tube furnace so that all C0, C0 H 0 and organic materials are removed, leaving only the unconsumed oxygen which is circulated again through the hot lubricant. As the lubricant is oxidized and oxygen is usedup, the pressure in the closed system drops. This pressure drop which is directly proportional to the volume of O consumed is constantly monitored and recorded. A curve is obtained which indicates the volume of oxygen consumed vs. time. A lubricant of good oxidation resistance usually shows a very low rate of oxygen consumption at the beginning. As oxidation proceeds the inhibitors will gradually be used up and the pro-oxidation species (such as peroxy radicals) will accumulate. At one point the rate of oxidation will increase drastically and the curve makes a sharp bend. The time required for this to happen is generally called induction time (T When a specified volume (V of 0 has been consumed, the unit is shut down automatically. The time for this to happen is called total time (T The used oil is then analyzed for acid number and increased viscosity (A(KV/ or A(KV/2l0) for solid esters) and, sometimes, pentane insolubles. Therefore, this test also furnishes information on the extent of oxidative degradation (as indicated by increases in acid number and viscosity) upon the consumption of a definite amount of oxygen by the lubricant. This last-named property is often called oxygen tolerance. A lubricant of good oxygen tolerance and low rate of oxidation (high T and T,) generally will show less oxidative degradation in a bearing rig and in service. The amount of stain or deposit on the wall of the oxidation cell can often be used to predict the cleanliness of the lubricant in rig tests, although this relationship is not as reliable as the relationship mentioned above.
Table IV shows the results of the Oxygen Absorption Tests for esters for this invention (samples 1 and 2) and compares them with results obtained from tests of other neoalkylpolyl esters. As may be seen the pentaerythritol ester of the mixed C gem and neo acids (sample 2) gave significantly higher induction time (T and total times (T than the other esters while the other ester of this invention, the pentaerythritol ester of a mixture of 81.5% by weight C gem acid and 18.5% normal C acid (sample 1) was inferior in this respect only to samples 3 and 5.
TABLE IV.-02 ABSORPTION TEST RESULTS OF NEOIENTYLPOLYOL ESTERS Sample Ti, T v,, v,, A(KV/210), A(KV/100), Number Ester min. min. m]. mi. percent percent A (A.N.)
1 PE gem-CT-n-C (containing 81.5% gem-C 709 759 815 14.12 2-... PE ge1n-C -neo-C1 951 993 1,470 11.9 3 PE tetra-neo CL 723 308 410 5. 56 4.- PE tetra-nC 156 202 307 7. c3 5 PE tetra-gem-Cr... 733 772 1,360 9.39 6.. PE tetra (4, 4-dirnethyll1cxanoate) 271 303 524 10.61 7 PE tetra (5, S-dimcthylhexanoate) 332 368 1, 200 14. 8 TMP tri(4,4-dimethylhexanoate) 300 351 12.49 9 TMP tri(4,4, -trimethylhexanoate) 159 212 449 9.26
Another important test for oxidation resistance is the (a) a gem alkanoic acid having the structure:
Erdco High Temperature Bearing Head Test. This test R2 f r th 1' I has been used by yet engine manu acture s, syn e 1c RI C (CHZ)B COOH lubricant produces and govenment agencies as the final I screening tool of candidate lubricants before the engine test. For a Type 2 /2 lubricant, the temperatures used in the test are 50 F. higher than the Type 2 test. Table V shows the test results of three high temperature synthetic lubricants. Lubricant A is a Type 2 synthetic lubricant which passed the Type 2 bearing rig test and which comprises a mixture of straight chain carboxylic acid esters of monoand dipentaerythritol. It gave a \WADD rating of 73 and viscosity rise of 69% in the Type 2 test. When it was tested under 2 /2 conditions, the viscosity increase was so high (829%) that the test had to be terminated at 75 hrs. The WADD rating was 190.5. Using the same additives but replacing the base fluid with PE-neo-C -n-C (containing about 43.2% neo-C groups), lubricant B was obtained. The test was completed, but there was a high viscosity rise (250%) and a large amount of filter deposits. Lubricant C, which had the same additives as A and B but with PE-gem-C -n-C (containing 81.5% gem-C as base fluid, showed significant improvements in P&W rating, WADD rating, filter deposit, viscosity increase, as well as the final acid number. Since the three lubricants and the same additives, the difference is due to the difference in oxidation resistance of the base fluid. The base fluid for Lubricant C is superior to that of Lubricant B because the former contained a much smaller proportion of straight-chain groups which are more susceptible to attack by oxygen. Any reduction of the percent of straight-chain groups in the base fluid for Lubricant B will result in solids unsuitable for use in gas turbine engines.
TABLE V.-TYPE 2% ERDCO BEARING HEAD TEST RESULTS Conditions:
Duration. hrs.100 Test bearing temp. F.550 =1: 5 Oil in temp., 13- 150 :1: 10 Sump temp, F.490 :i: 5
1. A liquid ester formed from a neoalkylpolyol having up to about 12 carbon atoms with 2 to 6 hydroxy groups and a mixture of acids consisting essentially of:
wherein R R and R are C to C alkyl groups, n is 1 to 3 and the total number of carbon atoms is not more than 9; and
(b) an alkanoic acid selected from the group consisting of straight chain and alkanoic neo acids of 4 to 10 carbon atoms, and in amount of 5 to 50 percent of said mixture of acids.
2. The liquid ester of claim 1 in which the alkanoic acid selected from the group consisting of straight chain and alkanoic neo acids is of 5 to 8 carbon atoms and is in amount of 10 to 50 weight percent of said mixture of acids.
3. The liquid ester of claim 1 in which the neoalkylpolyol is a neopentylpolyol.
4. The liquid ester of claim 3 in which the neopentylpolyol is pentaerythritol.
5. The liquid ester of claim 4 in which the gem alkanoic acid is 3,3-dimethylpentanoic acid and the alkanoic acid (b) is n-valeric acid.
6. The liquid ester of claim 4 in which the gem alkanoic acid is 3,3-dimethylpentanoic acid and the alkanoic acid (b) is neoheptanoic acid.
7. The liquid ester of claim 2 wherein the neoalkylpolyol is pentaerythritol and the gem alkanoic acid is selected from the group consisting of 3,3-dimethylpentanoic, 4,4-dimethylpentanoic, 3,3-dimethylbutanoic, 4,4- dimethylhexanoic, 5,5-dimethylhexanoic, 3,3,4-trimethylpentanoic and 4,4,5-trimethylhexanoic acids.
8. The liquid ester of claim 7 wherein the straight chain and alkanoic neo acids are selected from the group consisting of n-valeric acid and neoheptanoic acids.
9. The liquid ester of claim 8 wherein the gem alkanoic acid is 3,3-dimethylpentanoic acid.
10. The liquid ester of claim 1 wherein the neoalkylpolyol has no more than 7 carbon atoms with 2 to 4 hydroxy groups and contains at least one neopentyl group and at least two hydroxy groups attached to the carbon atoms next to the quaternary carbon atom.
References Cited UNITED STATES PATENTS 2,798,083 7/1957 Bell et al. 260--410.6 2,991,297 7/1961 Cooley et al. 1111 2,991,297 7/1961 Cooley et al. 26041O.6 2,820,014 1/1958 Hartley et. al. 260-4106 LORRAINE A. WEINBERGER, Primary Examiner R. S. WEISSBERG, Assistant Examiner US. Cl. X.R. 260410.6