US3296136A - Lubricant compositions of improved oxidation resistance - Google Patents

Description

United States Patent 3,296,136 LUBRICANT COMPOSITIONS OF IMPROVED OXIDATION RESISTANCE Daniel B. Eiciremeyer, Park Forest, and Tai S. Chao, Homewood, 11]., assignors to Sinclair Research, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Nov. 13, 1963, Ser. No. 323,275 6 Claims. (Cl. 252-475) This invention relates to ester-based lubricant compositions containing a novel combination of additive agents. More particularly the present invention relates to esterbased lubricant compositions which exhibit increased oxidation resistance.

Organic compounds, such as lubricating oils, undergo oxidation upon exposure to air. This process is accentuated by elevated temperatures such as occur in engines and other operating machinery. When such organic compositions are used as motor or machinery lubricants, their stability is still further drastically reduced due to their contact with metal surfaces which give up metallic particles into the lubricant. Such abraded or dissolved metals or metal salts appear to act as oxidation catalysts in the lubricant causing the formation of primary oxidation products which in turn cause further degradation of the organic compounds present in the composition. Problems of this nature are encountered in mineral oils but appear to be particularly troublesome in synthetic oleaginous fluids exemplified by esters.

The development of modern high speed jet turbine aircraft engines necessitates the search for lubricants which are resistant to high temperature oxidative degradation. Jet turbines are actuated by the energy of a burning fuel and are used to drive compressors which provide large amounts of air for the burning of the fuel. The combustion of the fuel provides the energy for driving the compressors with the remainder of the useful energy going into the propulsion of the aircraft. Jet turbine bearings are lubricated by pumping a lubricant to the bearings from a reservoir in a closed system. The design of more powerful jet turbines has led to an increase in the lubricant reservoir temperature and to greater difficulty in maintaining lubricant stability under the more severe conditions. For example, a lubricant which will operate satisfactorily at a reservoir temperature of 250 F. may sludge badly, build up viscosity, develop high acidity and corrode metals at a reservoir temperature of 440 F.

Thus jet turbine service requires a lubricant of superior thermal and oxidation stability as well as satisfactory physical characteristics in terms of viscosity, flash point, volatility, and load carrying properties. To arrive at such a lubricant it has been the general practice to add an antioxidant and other additives, such as foam inhibitors, anti-wear agents, etc. to a base fluid of suitable properties.

Numerous oxidation and corrosion inhibitors have been found for use in lubricating compositions and many combinations thereof also have been tested. For instance, antioxidants such as phenothiazine; phenyl-a-naphthyL amine; S-ethyl-lO, 10-dipheuylphenazasiline; etc. are well known in the art. The known inhibitors, however, are not adequate to prvoide the superior oxidation resistance now needed. Moreover, the eifectiveness of these known inhibitors is greatly impaired in the presence of metal.

It is the object of this invention to provide an ester abse synthetic lubricant having superior resistance toward oxidative and thermal degradation. It is a further object to protect and improve this resistance against the detrimental effects of metals with the use of suitable additives.

3,296,136 Patented Jan. 3, 1967 The present invention provides an improved ester based lubricant composition containing small amounts eflfective to retard oxidation, of each of a defined sulfur and nitrogen-containing organic compound and an arcmatic amine. The sulfur and nitrogen-containing organic compounds of my compositions are soluble in the base ester oil to the extent present and have the formula:

wherein R is hydrogen, an aromatic radical of say up to 18 carbon atoms or an alkyl, including cycloalkyl, radical of, for instance, up to about 18 carbon atoms; X is an amino or azo radical, that is, NR or -N=N; m is 0 to 1; n is 1 to 2; and p is 1 to 2. In any given compound the R radicals may be the same or difierently selected from the group of radicals given; however, at least one of the R groups is other than hydrogen and when m is O the R attached to the C(S) group is other than hydrogen. Exemplary of the R aromatic radicals are pyridyl, phenyl and naphthyl groups, which groups may be substituted with non-deleterious substituents such as one or more lower alkyl radicals. Also two R groups, say on opposite sides of the C(S)- radical, may be linked together, especially when alkyl groups are involved. R may also be a mixed radical, for instance of the aromatic alkyl type as in the case of benzyl.

The aromatic amine component of the invention is soluble in the ester fluid to the extent present and can be represented by the following general formula:

wherein Q is a monovalent hydrocarbon of 1 to 20 carbons, preferably 6 to 12 carbon atoms, whose adjacent carbon atoms are no closer than 1.40 A (i.e. a non-olefinic, non-acetylenic, monovalent hydrocarbon), and Q is an aromatic hydrocarbon radical of 6 to 12 or 16 carbon atoms. Thus Q can be an alkyl group, including cyclokalyl or an aromatic group. Preferably, both Q and Q are aromatic and often at least one is a fused ring aromatic, e.g. naphthyl. Q and Q can be substituted with non-interefering substituents such as alkyl groups and amine groups, preferably alkyl or aromatic amines, and Q and Q can be linked together by means of a non-interfering element such as carbon, sulfur and oxygen. lllustrative of suitable amines are phenothiazine, N-phenyltit-naphthyl amine; dim-naphthyl amine); N,N'-diphenyl para-phenylene diamine; N,N-dioctyl para-phenylene diamine, N,N-heptylphenyl-para-phenylene diamine; diphenyl amine, N-butyl phenyl-(u-decyl) amine, di-betanaphthyl amine, etc.

The lubricant composition of this invention includes as the major component a base oil which is an ester of lubricating viscosity which may be, for instance, a simple ester or compounds having multiple ester groupings such as complex esters, di or other polyesters, and polymer esters. These esters are usually made from monoand polyfunction-al aliphatic alcohols or alkanols, and aliphatic mono and poly carboxylic or alkanoic acids. Frequently, the alcohols and acids have about 4 to 12 carbon atoms. The reaction product of a mono-functional alcohol and a monocarboxylic acid is usually considered to be a simple ester. A diester is usually considered to be the reaction product of 1 mole of a dicarboxylic acid, say of 6 to 10 carbon atoms, with 2 moles of a monohydric alcohol or of 1 mole of a glycol, for instance, of 4 to 10 carbon atoms, with two moles of a monocarboxylic acid, e.g. of 4 to 10 carbon atoms. The diesters frequently contain from 20 to 40 carbon atoms.

A complex ester is usually considered to be of the type XYZYX in which X represents a monoalcohol residue. Y represents 'a dicarboxylic acid residue and Z represents a glycol residue and the linkages are ester linkages. Those esters, wherein X represents a monoacid residue, Y represents a glycol residue and Z represents a dibasic acid residue are also considered to be complex esters. The complex esters often have 30 to 50 carbon atoms.

Polymer esters or polyester bright stocks can be prepared by direct esterification of dicarboxylic acids with glycols in about equimolar quantities. The poly esterification reaction is usually continued until the product has a kinematic viscosity from about 15 to 200 centistokes at 210 F and preferably 40 to 130 centistokes at 210 F.

Although each of these products in itself is useful as a lubricant, they are particularly useful when added or blended with each other in synthetic lubricant compositions. These esters and blends have been found to be especially adaptable to the conditions to which turbine engines are exposed, since they can be formulated to give a desirable combination of high flash point, low pour point, and high viscosity at elevated temperatures, and need contain no additives which might leave a residue upon vol-atiliaztion. In addition, many complex esters have shown good stability to shear. Natural esters, such as castor oil may be employed and also be included in the blends, as may be small amounts of a foam inhibitor such as a methyl silicone polymer, or other additives or lubricant components to provide a particular characteristic, for instance, extreme pressure or load carrying agents, corrosion inhibitors, etc., can be added.

The monohydric alcohols employed in these esters us ually contain about 4 to 20 carbon atoms and are generally aliphatic. Preferably the alcohol contains up to about 12 carbon atoms. Useful alkanols include butyl, hexyl, methyl, iso-octyl and dodecyl alcohols, C oxo alcohols and octadecyl alcohols. C to C branched chain primary alcohols are frequently used to improve the low temperature viscosity of the finished lubricant composition. Alcohols such as n-decanol, 2-ethyl-hexanol, oxo alcohols, prepared by the reaction of carbon monoxide and hydrogen upon the olefins obtainable from petroleum products such as diisobutylene and C olefins, ether alcohols such as butyl carbitol, tripropylene glycol mono-isopropyl ether, dipropylene glycol monoisopropyl ether, and products such as Tergitol 3A3 which has the formula C13H27O(CH2CH2O)3H, are suitable alcohols for use to produce the desired lubricant. If the alcohol has no hydrogens on the beta carbon atoms, it is neo-structured; and esters of such alcohols are often preferred. In particular, the neo-C alcohol--2,2,4-trimethyl-pentanol-1gives lubricating diesters or complex esters suitable for blending with diesters to produce lubricants which meet stringent viscosity requirements. Iso-octanol and iso-decanol are alcohol mixtures made by the x0 process from C 43 copolymer heptenes. The cut which makes up iso-octanol usual-1y contains about 17% 3,4-dimethylhexanol; 29% 3,5-dimethylhexaaol; 25% 4,5-dimethylhexanol; 1.4% 5,5-dimethylhexanol; 16% of a mixture of 3-methylheptanol and S-ethylheptanol; 2.3% 4-ethylhexanol; 4.3% a-alkyl alkanols and 5% other materials.

Generally, the glycols contain from about 4 to 12 carbon atoms; however, if desired they could contain a greater number. Among the specific glycols which can be employed are 2-ethyl-l,3-hexanediol, 2-propyl-3,3-

heptanediol, 2 methyl 1,3 pentanediol, 2-butyl-1,3-

butanediol, 2,4 diphenyl 1,3 butanediol, and 2,4-dimesityl-1,3-butanediol. In addition to these glycols, ether glycols may be used, for instance, where the alkylene radical contains 2 to 4 carbon atoms such as diethylene glycol, dipropylene glycol and ether glycols up to 1000 to 2000 molecular weight. The most popular glycols for the manufacture of ester lubricants appear to be polypropylene glycols having a molecular weight of about 100-300 and 2-ethyl hexanediol. The 2,2-dimethyl glycols, such as neopentyl glycol have been shown to impart heat stability to the final blends. Minor amounts of ether glycols or other materials can be present as long as the desired properties of the product are not unduly deleteriously affected.

One group of useful monocarboxylic acids includes those of 8 to 18 or even 24 carbon atoms such as stearic, l-auric, etc. The carboxylic acids employed in making ester lubricants will often contain from about 4 to 12 carbon atoms. Patent No. 2,575,195, and include the aliphatic dibasic acids of branched or straightchain structures which are saturated or unsaturated. The preferred acids are the saturated aliphatic carboxylic acids containing not more than about 12 carbon atoms, and mixtures of these acids.

sebacic acids and isosebacic acid which is a mixture of a-ethyl suberic acid, oc,oz'-diethyl adipic acid and sebacic.

acid. This composite of acids is attractive from the viewpoint of economy and availability since it is made from petroleum hydrocarbons rather than the natural oils and fats which are used in the manufacture of many other dicarboxylic acids, which natural oils and fats are frequently in short supply. The preferred dibasic acids are sebacic and azelaic or mixtures thereof. Minor amounts of adipic used with a major amount of sebacic may also be used with advantage.

The ester base oils to which incorporation of the additive combination of the invention is particularly advantageous are the oils commonly referred to as neopentyl polyol polyesters, i.e. having more than one ester group. These are the esters of aliphatic carboxylic acids, gen

erally monoalkanoic acids, of about 4 to 12 carbon wherein n is 0 to l and R is a lower alkyl group, preferably of about 1 to 4 carbon atoms, which can be straight or branched chain, or a hydroxy methyl group. These esters can be made by reacting a mole of the alcohol with about 2 moles up to the stoichiometric equivalent of the carboxylic acid.

Illustrative of polyhydric alcohols free of beta hydrogen are neopentyl glycol, trimethylolethane, ,trimethylolpropane, pentaerythritol, dipentaerythritol, 2-butyl-2-ethyl-1, 3-propanediol, etc. Suitable aliphatic carboxylic acids with which the polyhydric alcohols free of beta hydrogen may be esterified are n-butyric acid, isobuty-ric acid, npentanoic acid, isopentanoic acid, n-heptanoic acid, isoheptanoic acid, n-octanoic acid, isooctanoic acid, pelargonic acid, n-decanoic acid, lauric acid, myristic acid, stearic acid, n-dodecanoic acid, valeric acid, n-hexabutyric acid, etc.

The additives of the invention are incorporated in the base oil in amounts sufiicient to retard oxidation of the oil, and the concentrations employed for optimum results may depend on the particular base oil and second additive component selected. Ordinarily about 0.05 to t 5%, preferably about 0.1 to 2%, by weight of each of the sulfur-nitrogen compound and the amine additive corn- Suitable acids are described in U.S.

wnaphthylamine, Test No. 8; base fluid A plus 0.1% thionicotinamide, Test No. 2; and base fluids A and B plus 1% or 0.5%, respectively, of N-phenyl-a-naphthylamine and a given amount of the sulfur-nitrogen additive of the 2. The lubricant composition of claim 1 wherein Q is an aromatic hydrocarbon radical and Q and Q each has 6 to 12 carbon atoms.

3. The lubricant composition of claim 1 wherein the present inventtion, the remaining tests through Test No. 5 sulfur-nitrogen compound is present in an amount of 21. In Test No. 22 the lubricant was the same as that of about 0.1 to 2% by weight and the aromatic amine is Test No. 3, except for the addition of 1% of 2,2'-dipyridylpresent in an amount of about 0.1 to 2% by weight. amine; and in Tests Nos. 23 and 24, the fluid of Test N0. T l ri an composition Of claim 1 wherein the 22 contained the stated amount of the identified additional ester base fluld s an ester of an elkanol havmg the SFN wmpounds. eral formula:

The times to the end of the inhibition period and to the j: end of the test are designated Ti and Tt, respectively, and HOCHz- CH2-OCH2 OH2OH the volume of oxygen absorbed is designated Vt. L 1!; J,

TABLE I Test No. Base blend Sulfur-Nitrogen (S-N) Additive Wt. percent Ti (min.) Tt (min) Vt (1111.)

S-N Additive None 5 41 2, 500 Thionicotinamide 0. 1 10 80 2, 500 one 91 128 2,500 s-Di-n-butylthiourea 0. 1 108 153 2, 500 Ethylenethiourea 0. 1 439 1, 370 2thiobenzoy1amino-4-rnethylpyr1 0. 3 420 1, 240 N-phenyl-N-2-pyridylthiourea 0. 23 168 276 2, 500 None 163 2. 500 s-Dimethylthi0urea 1. 0 293 2, 500 s-Di-n-buty1thiourea 1. 0 382 2, 500 s-Diphenylthiourea 1. 0 492 2, 500 s-Diphenylthiourea 0. 1 202 2, 500 N-phenyl-N-2-pyridylthiourea 0. 2 406 2, 500 N -ph enyl-N-l-naphthylthiourea. 0. 5 365 2, 500 s-Dl-p tolythiourea 0. 5 399 2, 500 N-phenyl-N-laurylthiour 1. 0 316 2, 500 Diphenylthiocarbazone. 0. 2 252 2, 500 2-thiobenzoyl-aminopyridine. 0. 5 349 2. 500 N,N-di1auryl-dithiooxarn.ide 0. 5 327 2, 500 1,1-diphenyl-2-tbiourea 0.1 193 2,500 s-Dibenzylthiourea 0. 5 279 2, 500 m 353 2,500 2-thiobenzoylarnino--methylpyridine 0.15 1, 200 2, 500 Thiom'cotin'tmide 0. 5 466 1, 373

1 Test Conditions: 450 F., 1 cu. ft. O /hL, 75 g. sample.

2 Base Fluid A: An essentially complete ester of pentaerythritol and a mixture of alkanoic acids containing an average of seven carbons per mole and characterized by an acid number of 0.01, a sap. number of 400 and kinematic viscosities at 210 F. of 5.30 cs. and at 100 3 Base Fluid B: diisooctyl azelate; 80% complex ester from neopentyl glycol, azelaic acid and isooctyl alcohol, mole ratio approx. 112:2.

We claim:

1. A lubricant composition consisting essentially of a major amount of an ester base fluid of lubricating viscosity and small amounts of each of an ester base fluid-soluble compound having the following structural formula:

R is selected from the group consisting of hydrogen, alkyl radicals of up to 18 carbon atoms, phenyl, naphthyl References Cited by the Examiner and pyfi UNITED STATES PATENTS X is selected from the group consisting of NR1 a d 2,201,170 5/ 1940 Hanford 260-3 13 2,242,621 5/ 1941 Schulze et a1 44-74 0 to 13 2,302,552 4/1942 Johnson 25247 "1 and 2,322,184 6/1943 White 252-47 P 2,373,049 4/1945 Pedersen 252-47 with the proviso that at least one R 1s other than hydrogen 2 396 156 3/1946 Clarkson and when m is 0 the R attached to the CS- group i 2484257 10,1949 W t o 252 47 other than hydrogen, and an ester base fluid-soluble arO- n mafic amine having the general formula: 2,657,984 10/ 1953 Braithwarte 2S247 Q-N-H FOREIGN PATENTS wherein Q is a non-olefinic, non-acetylenic monovalent 550 340 7 19 5 d hydrocarbon radical of 1 to 20 carbon atoms and Q is an 601,174 7/1960 aromatic hydrocarbon radical of 6 to 16 carbon atoms, said amounts being effective to retard oxidation of said ester base fluid at temperatures in excess of 400 F., and said ester base fluid being an ester of an alkanol of 4 to 20 carbon atoms and an alkane carboxylic acid of 4 to 18 carbon atoms.

DANIEL E. WYMAN, Primary Examiner.

L. G. XIARHOS, Assistant Examiner.

Claims (2)

1. A LUBRICANT COMPOSITON CONSISTING ESSENTIALLY OF A MAJOR AMOUNT OF AN ESTER BASE FLUID OF LUBRICATING VISCOSITY AND SMALL AMOUNTS OF EACH OF AN ESTER BASE FLUID-SOLUBLE COMPOUND HAVING THE FOLLOWING STRUCTURAL FORMULA:

4. THE LUBRICANT COMPOSITON OF CLAIM 1 WHEREIN THE ESTER BASE FLUID IS AN ESTER OF AN ALKANOL HAVING THE GENERAL FORMULA: