US3093585A - Ester base lubricant compositions - Google Patents

Description

United States Patent 3,093,585 ESTER BASE LUBRHIANT COMPOSITIONS Hans Low and William W. Reynolds, East Alton, Ill., and

Donald W. Schmulling, St. Louis, Mo., assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed June 6, 1960, Ser. No. 33,952

Claims. (Cl. 25237.2)

This invention relates to a lubricant composition containing a superior antioxidant combination. More particularly, the present invention relates to an additive combinatrion for ester base lubricant compositions having excellent stabilizing eiiects at extremely high lubricating temperatures.

One of the greatest problems in lubricating at high temperatures is to develop a composition which will not easily oxidize to form acidic components which in turn cause harmful deposits or sludges in the composition. This problem is especially difiicult to overcome when dealing with high temperatures such as encountered in modern aircraft gas turbine engines and hydraulic systems Where hot areas in the range of from 350-750 F. are contacted by the lubricant.

It is an object of the present invention to provide improved ester compositions. It is a particular object of the invention to provide ester compositions having reduced tendencies to form acidic oxidation products. It is a particular object of the invention to provide ester lubricants especially useful as gas turbine lubricants and hydraulic fluids.

Now, in accordance with the present invention, a superior lubricant composition is provided comprising an estertype base fiuid and oxidation stabilizing amounts of both an amine of the group consisting of certain triazines and certain pyridylamines in combination with critically limited proportions of complexes of such classes of amines with copper salts of fatty acids. In one aspect of the invention provision is made for partially oxidizing the amines to be used in the formation of such copper salt complexes. The optimum mol ratio of the combination is from 2 to 75 mols of total amine for each mol of copper.

The copper complexes which are formed between the amine and the copper salt are believed to be of the square planar type. However, the precise structure of the complexes has not been absolutely determined. They appear to exist in a ratio of about 2 mols of the amine for each mol of the copper salt. Part of the unexpected character of the present invention lies in the fact that copper is known to be a serious accelerator of oxidation. Consequently the present invention is based upon the discovery that an optimum ratio exists wherein an excess of firee amine is present beyond the amount necessary for forming the complex for the copper salt and within which ratio a remarkable stabilization of the oxidation properties of the ester base lubricant is observed.

The copper salts to be employed in the preparation of the subject complexes are preferably those with the copper in the second oxidative state (copper II), while the fatty acid radical may be derived from any fatty acid having from 2-24 carbon atoms. These include the usual acids such as acetic, propionic, butyric, hexoic, octanoic, :dodecanoic, cap-ric, caprylic, caproic, stearic, oleic, linoleic, etc. It is preferred that the acid be saturated and have from 2 to 12 carbon atoms per molecule.

The amines which are utilized in their free form and also in complex with the copper salt are of two general categories. These may be referred to as pyridylamines and triazines. The pyridylamines are those having at least one pyridyl radical and preferably two and comprise unsubstituted pyridylamines as well as alkyl substituted, and those pyridylamines having assiibstituent groups amino radicals, aryl radicals, aminoaryls, aminoalkyls, and triazyl radicals. Table A, which follows, lists typical pyridylamines coming within the scope of this invention.

TABLE A Pyridylamines Monopyr-idylamines:

2-pyridylamine 3 pyridylamine Dipyridylaniines:

2,2'-dipyridylamine 2,3 -dipyridylamine 2,4'-dipvridylamine 3,3'dipyridylamine 4,4'-dipyridylamine Alkylpyridylamines:

2,2-di(4-ethylpyridyl) amine 4,4'-di( Z-tert-butylpyridyl) amine 2-pyridyl-2'- 4-isopropylpyridyl) amine Aminopyridyl amines 2-tolyl-2 4-a1ninopyridyl amine 2,2-'( 3 -N-ethylaminopyridyl) amine Arylpyridylamines:

(Z'pyridyl 2'-benzyl) amine (4-pyridyl) (2'-phenyl) amine (Z-pyridyl i 2'-naphthyl) amine (Z-pyridyl) (Z-triazyl) amine The triazines may be used as the sole amine in the composition or may be combined with the pyridylamines. Suitable types are those constituted by unsubstituted triamine, or more preferably the guanamines in addition to monoarninotriazines, alkyl triazines, aryltriazines or triazinylamine. Table 13 which follows lists typical species suitable for this purpose.

TABLE B Triazines Guanarnines:

2,4-diamino-6benzyl-triazine 2,4-diamino-6-methyl triazine 2,4-diamino-6-propyl triazine 2,4-diamino-triazine 1-N-ethyl-2,4-diamino triazine l-N-phenyl-ZA-diamino triazine 2,3-di-(N-ethyl)-2,4-diamino triazine 1 35-tri (N-methyl -2,4-diaminotriazine 2,4-diamino 6-phenyl-triazine Monoamlno triazines:

2- (N-methylamino) -triazine Z-(N-phenylamino) -triazine 2-amino-tm'azine Alkyl triazines:

2,4-diethyltriazine 2,4,6-trimethyl triazine 2-methyl-4,-6-diethyl triazine 2-methyl-4-amino triazine The copper salt complexes of these amine types may be formed in situ or may be preformed before dispersal in the ester base lubricants. The complexes are immediately formed by combining solutions such as alcohol solutions of the amine and copper salt and evaporating to dryness. However, complexes appear to be just as efficiently" formed by combining the free amine and the copper salt in the ester lubricant.

While the structure of the complex has not been finalized, it is believed to be represented in a specific instance as according to the following diagram:

The formation of metal complex is an equilibrium reaction; therefore, an excess of free amine appears to serve two principal purposes. First, it minimizes the amount of free soluble copper ion which would act as a pro-oxidant and secondly it assures as large an amount of active inhibitor as possible by displacing the equilibrium toward the product, namely, the complex. If excessive amounts of the copper compound are present, then the pro-oxidant effect of copper becomes predominant and causes an acceleration rather than a retardation of oxidation. On the other hand, if excessive amounts of free amine are present (or not enough copper to form sufficient amounts of the complex, the active inhibitor) then the oxidation inhibition is minimized since the free amines do not appear to be very eifective inhibitors. Thus, it was found by means of comparative testing that the maximum oxidation inhibition was obtained when the mol ratio of total amine to copper was between about 2 and about 75, preferably be tween about and about 35.

The mixtures of free amines and copper complexes are found to be effective in aliphatic esters which are used primarily for synthetic hydraulic fluids and gas turbine lubricants as well as for other associated purposes. Broadly, the principal classes included within this broad class of aliphatic esters comprise esters formed between dicarboxylic acids and monohydric alcohols; esters formed between neopentyl glycols and monohydric-alcohols; esters of pentaerythritol or its dimers with monohydric alcohols; and the so-called complex esters which constitute various'combinations of dibasic acids, glycols and alcohols or dibasic acids, glycols and nonobasic acids. Table C outlines the various alternative structures contemplated as complex esters" as well as listing briefly typical starting materials for the formation of such esters.

TABLE C Complex esters (By ester-forming components):

Alcohol-(dibasic acid-glycol)-monobasic acid R OOCR COOR -OOCR See UJS. 2,575,195 Alcohol-dibasic acid-(glycol-dibasic acid) -alcohol 4 R OOCR COO(R -OOR COO) R See US. 2,703,811 Monobasic acid-glycol-(dibasic acid-glycol) monobasic acid R COOR -(OOCR COO--R -OOCR See US. 2,575,196 Monohydric alcohols:

Z-ethylbutyl alcohol 2-ethylhexyl alcohol Monobasic acids:

Caproic acids Pelargonic acid Capric acid Glycols:

Neopentyl glycol Ethylene glycol Propylene glycol Dibasic acids:

Sebacic acid Azelaic acid As diesters there may be used one or more of the compounds of the formula ROOC-Z-COOR wherein Z is an alkylene chain of four to eight carbon atoms and R is an alkyl group of four to fourteen carbon atoms, being the residue of an aliphatic monohydric alcohol from butyl to tetradecyl alcohols. Dicarboxylic acids which provide the above esters are adipic, pimelic, suberic, azelaic, and sebacic. The alcohols include butyl, isobutyl, or sec-butyl alcohols and the various amyl, hexyl, octyl, nonyl, decyl, undecyl, dodecyl, and tetradecyl alcohols. The alcohols with branched hydrocarbon chains provide diesters with particularly desirable properties. Typical of these are isobutyl, 2-ethylbutyl, 2-ethylhexyl, l-rnethylhexyl, l-methylheptyl, 1-methyl-4-ethyloctyl, 2,2, 4-trimethylhexyl, 2-isopropyl-3,3-dimethylbutyl, 1,4-dimethylbutyl, and 1-isobutyl-4-ethyloctyl alcohols.

Ester Lubes bis Z-ethylhexyl) sebacate bis (isononyl) sebacate bis Z-ethylhexyl) adipate bis( 1-methylheptyl)azelate bis (2,2,4-trimethylhexyl) pimelate bis( 1,4-diethylhexyl) adipate bis( 1,4-dimethyloctyl) adipate bis 2-ethylbutyl) suberate 2-ethylhexyl) (isononyl) sebacate l-methylhexyl) (2-ethylbutyl) sebacate The aliphatic polyesters which may be utilized as the lubricating fluids are preferably tetraesters, such as the pentaerythritol esters or the dimers or trimers of the same. Preferably, each radical contains from 4 to 18 carbon atoms each and preferably from 6 to 14 carbon atoms each. Typical species include pentaerythritol tetracaproate, dipentaerythritol hexavalerate, pentaerythritol tetraheptoate, pentaerythiritol dicaproate divalerate, pentaerythritol tricaproate heptoate and mixtures thereof. It will be noted that the esters may be either mixed esters or mixtures of homo esters, the best rheological properties being obtained with mixed esters and with mixtures of esters. For use at elevated temperatures the dipentaerythritol esters are preferred due to their greater molecular weight and consequently higher boiling points.

The course of oxidation as indicated by the rate of oxygen absorption in the composition of this invention indicates that the amine in complex form with the copper salt is oxidized prior to its assumingits intended function of acting as an oxidation inhibitor. The oxidation of the amine may take place prior to or subsequent to incorporation of the amine in the ester base lubricant. Consequently, the invention will be understood to include the use of not only the amines per sebut also their oxidation products as' well as the copper complexes of such oxidized products. The exact nature of the oxidation product has not been elucidated; however, the fraction of the oxidized product found to be most active in the prevention of oxidation of the ester base lubricant has a nitrogen-to-copper molar ratio in the order of about 20-30, while the less effective fractions of the oxidized product have nitrogen-to-copper molar ratios in the order of 36.

Still further and unexpected reductions in oxidation of the ester base lubricant are obtained by the additional presence of certain phenolic compounds, particularly alkylene bisphenols. Such compounds are preferably those typified by the following structure:

METHYLENE BISPHENOLS The phenolic compounds may be modified by the presence of alkyl radicals on one or both of the phenolic nuclei. Typical compounds are as follows:

4,4-methylenebis(2,6-ditertiary-butylphenol) 4,4-methylenebis (2- 2*ethylhexyl) phenol) 2,2'-methylenebis(4-isopropylphenol) The data contained in the working examples will illustrate the benefits obtained by the use of such bisphenols in addition to the combination of the copper complex and free amines as described hereinbefore. The following examples illustrate the use of this invention.

EXAMPLE I Preparation of a copper complex betw en copper octoate and a tria zine.An alcoholic solution of copper (II) octoate was mixed with an alcoholic solution of 2,4-diamino 6-phenyl l,3,5-triazine(benzoguanamine). The green precipitate has a melting point of 2l0-214 C. and an elemental analysis showing that it contains 55.4 percent by weight of carbon, 7.42 percent hydrogen, 13.3 percent nitrogen and 14.0 percent copper. This agrees with the postulated formula However, the experimentally determined molecular weight was 945, almost twice as much as the empirical formula and indicates the likelihood of dimerization. This is more plausible since it enables the copper (II) to form a square planar complex.

EXAMPLE II Preparation of a copper salt compl x with a dipyridylamine.-Copper (II) acetate and 2,2dipyridylamine were mixed in an alcoholic solution and evaporated to dryness. Analysis of the product gave an empirical formula of (C H N -(Ou)(C H O Accordingly, one molecule of copper (II) acetate forms a planar complex with two molecules of the bidentate ligand dipyridylamine. The complex contains 17.5 percent copper and 11 percent nitrogen.

EXAMPLE III Oxidation inhibition of a mix d pentaerythritol ester.-- A pentaerythritol ester of a mixture of fatty acids averaging C C per ester linkage was modified with 0.025 mol of 2,2-dip-yridylamine (22DPA) and with other TAB LE 1 Oxidation Inhibitor, 0.025 M time, hours 0 22-DgA/Cu acetate comple N Cu, Fe, Ag, AL- 68 o Copper octoate None 16 Metal washers.

It will be seen by reference to the above table that the unmodified base fluid oxidized to the arbitrary limit set in about 14 hours (the arbitrary limit is that time required for the absorption of 1.0 millimole of oxygen per gram of base fluid). The table also shows that the use of a well-known antioxidant, phenothiazine, resulted in extending this oxidation time (OT) to 37 hours. A standard phenolic inhibitor was not as effective as the latter. It is significant to note that 2,2'-di-pyridylamine had essentially no oxidation inhibiting effect. Copper octoate also exhibits substantially no oxidation inhibiting activity when utilized in the absence of an amine of the present invention. However, it is significant to note that when a mixture of 2,2'-dipyridylamine and the copper acetate complex are mixed in the same base lubricant highly effective oxidation inhibition is obtained. Similar formulations wherein the metal catalysts (metal washers of copper, iron, silver and aluminum) are replaced by a soluble copper salt exhibit the same inhibitory effect in the several compositions tested.

An investigation of the effect of mol ratio of free amine to copper complex was made employing the same pentaerythritol ester containing 0.025 mol of 2,2-dipyridylamine to which was added varying proportions of copper octoate. Table 2 which follows shows the data obtained in this respect and indicates that the optimum oxidation inhibition is obtained with molar ratios of free amine to copper between about 2 and about 75. The data given in the middle column of this table reflects the total amine versus copper salt. Since the copper takes up two rnols of amine per mole of copper salt, the free amine is the net of the total amine minus 2.

The pentaerythritol ester base fluid was modified with 0.025 mol of benzoguanamine per liter of solution. The table which follows shows the effect upon oxidation inhibition of adding copper octoate to this composition. Copper octoate added in this manner forms a complex with a part of the amine in situ.

TABLE 3 Copper Molar ratio octoate in total OT, hours p.p.m. Cu+ Amine/Cu EXAMPLE V Oil properties of oxidized oil.'1"'able 4 which follows shows the benefits gained by the use of a combination of benzoguanamine (BG) with varying amounts of copper catalyst. It is noteworthy that within the claimed molar ratio the acid number of the oxidized oil is at a minimum, thus indicating that the peroxide production was also minimized by this combination.

TABLE 4 Amine 0.025 M Metal Molar ratio, OT, Acid N o.

amine/copper hours *Added as copper octoate.

EXAMPLE VI TABLE 5 (OT-14) N0. Additive, 0.025 M OT, 0T-14 l Combhours 1 None 14 2 Benzoguanamine 15 3 4,4-n1ethylenebis (2,4-ditert butylphenol) 27 Cu ootoate, 5O p.p.n1. Cu+ '16 (2)+(4) 62 +45 (3)+(4) 24 10 5 2)+ 3)+(4) 91 77 +61 EXAMPLE VII Preparation of oxidized 2,2-dipyridylamine and its copper complex.To 10 g. of 2,2-dipridylamine in a 100 ml. round-bottom flask was added 15 ml. glacial acetic acid. The solution was heated for 1 hour at 190 C. mantle temperature. After cooling to room temperature, 20 ml. of 30 percent hydrogen peroxide was added and the liquid temperature raised to C. for 1 hour and 80 C. for 2 hours. An additional 5 ml. of 30 percent hydroperoxide were added and the temperature raised to .C. for 1 hour. To half of this solution was added 3.4 g. of copper acetate monohydrate and the solution boiled to a semisolid. This was stored in a vacuum desiccator for 4 days with Drierite and sodium hydroxide pellets. 7

The above material was separated into 4 fractions by solvent extraction. The first portion was insoluble in ethanol, M.P. 400 C. Dilution of the alcohol solution with water gave a water insoluble second fraction, M.P. 132-135 C. The third portion was a precipitate obtained by adding ether to an ethanol solution of fraction 2, M.P. 18l-l92 C. The filtnate of fraction 3 gave fraction 4 on evaporation of solvent; it was a dark green viscous liquid and was ether, water and ethanol soluble.

The analysis of each fraction and effect on oxidation follow:

Fraction y Percent N Percent Cu N/Cu mol OT, hours ratio We claim as our invention:

1. A synthetic ester base lubricant composition comprising a major proportion of a pentaerythritol tetraester of a fatty acid and minor oxidation-inhibiting proportions each of benzoguanamine and a copper octoate complex with benzoguanamine, the mol ratio of total amine to copper being between about 5 and about 35.

2. A synthetic ester base lubricant composition comprising a major proportion of a pentaerythritol tetraester of a fatty acid and minor oxidation-inhibiting proportions each of 2,2-dipyridylamine and a copper diacet-ate complex with 2,2'-dipyridylamine, the mol ratio of total amine to copper being between about 5 and about 35.

3. A synthetic ester base lubricant composition comprising a major proportion of a pentaerythritol tetraester of a fatty acid and minor oxidation-inhibiting proportions each of an aryldiaminotriazine and a copper complex of a copper C 24 fatty acid salt with a triazine, the mol ratio of total amine to copper being between about 5 and about 35. Y

4. A synthetic ester base lubricant composition comprising .a major proportion of a pentaerythritol tetraester of a fatty acid and minor oxidation-inhibiting proportions each of 2,2'-dipyridylamine and a copper dioctoate complex with 2,2'-dipyridylamine, the mol ratio of total amine to copper being between about 5 and about 35.

5. A synthetic ester base lubricant composition comprising a major proportion of a pentaerythritol tetraes-ter of a fatty acid and minor oxidation-inhibiting proportions each of 2,2'-dipyridylamine and a copper C -24 fatty acid salt complex with 2,2'-dipyridylamine, the mol ratio of total amine to copper being between about 5 and about 35.

6. A synthetic ester base lubricant composition comprising a major proportion of a pen-taerythritol ester of an aliphatic monocarboxylic acid and minor oxidationinhibiting amounts each of diaminotriazine and a copper C 24 fatty acid salt complex with a pyridylamine, the mol ratio of total amine to copper being between about 2 and about 75.

7. A synthetic ester base lubricant composition comprising a major proportion of a pentaerythritol ester and minor oxidation-inhibiting amounts each of a triazine and a copper C 24 fatty :acid salt complex, with a pyridylamine, the mol ratio of total amine to copper being between about 2 and about 75. V

8. A synthetic ester base lubricant composition comprising a major proportion of a pentaerythritol ester and minor oxidation-inhibiting amounts each of a pyridylamine and a copper C 24 fatty acid salt complex with a pyridylamine, the mol ratio of 'total amine to copper being between about 2 and about 75.

9. A synthetic ester base lubricant composition comprising (a major proportion of a pentaerythritol ester of an aliphatic monocarboxylic acid and minor oxidationinhibiting amounts each of a dipyridylamine and a copper of the same group, the mol ratio of total amine to cop- 10 per being between about 2 and about 75.

1 19 References Cited in the file of this patent UNITED STATES PATENTS 2,961,406 McNeil Nov. 22, 1960 2,962,439 Lauer Nov. 29, 1960 2,976,238 Elliot et a1. Mar. 21, 1961 OTHER REFERENCES Atkins et al.: I. and E. Chem., vol. 39, No. 4, April 1957, pp. 491-493.

Barnes at 211.: Lubrication Engineering, August 1957, pp. 454-458.

Claims (1)

10. A SYNTHETIC ESTER BASE LUBRICANT COMPOSITION COMPRISING A MAJOR PROPORTION OF AN ALIPHATIC ESTER OF AN ALIPHATIC CARBOXYLIC ACID AND MINOR OXIDATION-INHIBITING AMOUNTS EACH OF A COPPER C2-24 FATTY ACID SALT COMPLEX OF AN AMINE OF THE GROUP CONSISTING OF PYRIDYLAMINES, AMINOTRIAZINES AND MIXTURES THEREOF AND A FREE AMINE OF THE SAME GROUP, THE MOL RATIO OF TOTAL AMINE TO COPPER BEING BETWEEN ABOUT 2 AND ABOUT 75.