US3518194A - Lubricating composition - Google Patents

Description

United States Patent Office 3,518,194 Patented June 30, 1970 3,518,194 LUBRICATING COMPOSITION Joseph J. McGrath, Monroeville, and Harold 0. Strange,

Pittsburgh, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware N Drawing, Filed Feb. 29, 1968, Ser. No. 709,205 Int. Cl. Clllm 1/38, 3/32 US. Cl. 252-475 8 Claims ABSTRACT OF THE DISCLOSURE A lubricating oil composition having improved nonsludging properties is obtained by incorporating a phenothiazine carboxylic acid ester in the composition. Examples of the esters include n-butyl phenothiazine-l-carboxylate; isooctyl phenothiazine-2-carboxylate; dodecyl phenothiazine la-carboxylate; tridecyl phenothiazine-2-carboxylate; hexadecyl phenothiazine-4-carboxylate; di-isooctyl phenothiazine-Z,8-dicarboxylate.

This invention relates to a lubricating oil composition and more particularly to a lubricating oil composition which is highly resistant to oxidational changes at an elevated temperature for prolonged periods of time.

The current trend in designing more efficient and more economical engines has accentuated the need for lubricants which will effectively lubricate such engines under severe operating conditions over extended periods of time. Improved lubricants are required for automotive and aircraft engines as well as for combustion turbine engines and for steam turbines, including those for turbine reduction gears. Regardless of the use to which the lubricant is subjected, the lubricant must be highly resistant to oxidative deterioration. Products which are formed upon oxidation of an oil are acidic in nature and thus exert a corrosive effect on metal surfaces with which the oil comes in contact. Oxidation of the oil is further undesirable in that upon oxidation its viscosity is changed. Still further, oxidation of the oil gives rise to the formation of gum, varnish, sludge and other deleterious products which decrease the useful life of the oil.

For many years, highly refined paraffinic oils have been used as base oils in forming lubricating compositions. By the term highly refined parafiiinic oil we means a petroleum lubricating oil which has been refined by one of the more drastic refining methods known in the art, for example, by conventional solvent extraction and aluminum chloride refining adapted to remove all or substantially all of the unsaturated and aromatic constituents of the oil. Aluminum chloride refined or solvent extracted parafiinic Oils, such as the Pennsylvania oils, have provided excellent base oils for many lubricating compositions. Likewise, drastically refined Mid-Continent and Gulf Coastal oils have been widely used as base oils in forming lubricants.

In addition to these refining methods, lubricating oils of high quality have been obtained by hydrogenating vari ous charge stocks derived from Pennsylvania, Mid-Continent, West Coast, Middle-East crudes, etc. It has been known, for example, that improved lubricating properties can be obtained when the lubricating oil stocks are treated with hydrogen. Treating some lubricating oil stocks with hydrogen, for example, has resulted in obtaining stocks for making excellent multigrade lubricants, i.e., lubricants suitable for use under a wide range of temperature. Regardless of the treatment to which the various charge stocks are subjected, the mineral lubricating oil per se,

which is obtained is not completely satisfactory under the severe conditions encountered in some of the current engines.

Some synthetic lubricating oils have been produced which have high viscosity indices, low evaporation properties, high flash and fire points, and are substantially resistant to oxidational changes at temperatures up to about 300 F. The synthetic lubricating oils are of various types including polyalkylene oxides, silicones, esters of phosphoric and silicic acids, highly fiuorinated hydrocarbons, polyaryl ethers, aliphatic esters, etc. Among the polyalkylene oxides, silicones, silicates, fiuorinated hydrocarbons and polyaryl ethers which have been used may be mentioned polyisopropylene oxide, polyisopropylene oxide diether, methylphenyl silicone polymers, tetraisoctyl silicate, perfluorohydrocarbons, m bis(m-phenoxyphe noxy)benzene and rn bis(m-phenoxyphenoxy)phenyl ether. Certain organic esters including monoesters of monobasic acids, diesters of dibasic acids, triesters of tribasic acids, diesters prepared from glycols, triesters prepared from polyhydric alcohols and mixtures thereof have a majority of the properties required of aircraft lubricants and have become commercially important. Specific examples of the ester lubricants" include di-(3,5,5-trimethylhexyl) glutarate, di-(2-ethylhexyl)adipate, diisooctyl adipate, diisodecyl adipate, di-(2-ethylhexyl)pimelate, dihexyl azelate, diisooctyl azelate, di-(Z-ethylhexyl)azelate, di-(Z-ethylhexyl)sebacate and dipropylene glycol dipelargonate. While the ester lubricants in general have been satisfactory at temperatures up to about 300 F., these lubricants have not been completely satisfactory when used for prolonged periods of time under oxidizing conditions at temperatures above about 300 F.

The use of phenothiazine in mineral and synthetic lubricating oils has met with some success in reducing the oxidation of the oils. However, the presence of phenothiazine in some oils, particularly the synthetic ester lubricants, as been found to promote the deposition of sludge in internal combustion engines operating under oxidizing conditions at temperatures above about 300 F. for prolonged periods of time. The sludge thus formed is referred to as phenothiazine sludge or phenothiazine dirtiness.

Attempts have been made to overcome phenothiazine sludge formed at elevated temperatures by incorporating various detergents in the lubricating oil in combination with the phenothiazine. However, such combinations have not been completely satisfactory.

Alkyl derivatives of phenothiazine have also been used in lubricating oils in an attempt to avoid phenothiazine dirtiness but these derivatives also have certain disadvantages. The lower alkyl derivatives of phenothiazine, for example, alkyl phenothiazines having less than 4 carbon atoms in the alkyl radical are insoluble in mineral base lubricants in substantial concentrations and the use of conventional solutizers have failed to increase their solubility to a useful level. Alkyl phenothiazines having at least 4 carbon atoms, preferably 6 to 12 carbon atoms, in the alkyl radical are sufiiciently soluble in mineral and synthetic oils to give improved non-sludging properties at normal and moderately elevated temperatures. However, the higher alkyl phenothiazines are not sufiiciently stable at elevated temperatures to prevent sludging for prolonged periods of time. Such higher alkyl phenothiazines appear to decompose at elevated temperatures, thus reducing their effectiveness. The higher alkyl phenothiazines have a further disadvantage in that the amount of the additive required to give non-sludging properties for even a short period of time increases as the number of carbon atoms in the alkyl group increases. Thus, un-

desirably large amounts of the alkyl phenothiazine are required to give even temporary improvement.

We have found that a lubricating composition having improved thermal and oxidative stability for prolonged periods of time and which has non-sludging properties when used to lubricate bearings "t temperatures above aboct 300 F. under oxidizing conditions can be obtained by incorporating in the composition in oxidation-inhibiting proportions a phenothiazne carboxylic acid ester. Thus, the improved lubricating composition of our invention comprises a lubricating oil normally tending to undergo oxidational changes at an elevated temperature above about 300 F. and a small amount, sufficient to substantially retard such oxidational changes, of a phenothiazine carboxylic acid ester. The lubricating composition thus obtained not only has improved oxidation resistance but also has non-sludging properties.

The lubricating oil base stock used in the composition of the invention may be a mineral oil or a synthetic oil. Inasmuch as the synthetic oils are normally more resistant to oxidational changes than the mineral oils, the synthetic oils form a preferred class of lubricating bases. By the term synthetic oil we mean a composition having a majority of the properties of a hydrocarbon oil of lubricating grade including polyalkylene oxides, silicones, esters of phosphoric and silicic acids, highly fluorinated hydrocarbons, polyaryl ethers, aliphatic esters, etc. Probably those synthetic oils which have been more widely used commercially are the silicones and the ester lubricants. The ester lubricants are particularly susceptible to improvement in accordance with the present invention. In general, the lubricating oil content of the composition of the invention comprises about 90 to about 99.9 percent by weight of the total composition. The particular lubricating oil as well as the exact amount of such oil employed depends upon the characteristics desired in the final composition.

The phenothiazine carboxylic acid ester which we employ is represented by the formula:

where Y is selected from the group consisting of hydrogen and carboalkoxy (COOR) radicals and R is an alkyl radical containing from 4 to 16 carbon atoms, preferably 6 to 12 carbon atoms. The position of the carboalkoxy radical or radicals on the phenothiazine ring depends to some extent upon the method used in preparing the compound. We have obtained good results with compounds having the carboalkoxy radical in the 1 and 2 positions. For this reason, we do not wish to limit the invention with respect to the position of the carboalkoxy radical or radicals of the phenothiazine ring.

The R groups in the radicals of the structural formula shown hereinabove can be. the same or diiferent but those compounds in which the R groups are the same are preferred from the standpoint of ease of preparation. The R groups may be straight chain or branched in structure. The branched groups are usually preferred in that branched alkyl groups tend to improve oil solubility of the compounds.

Examples of R groups containing from 4 to 16 carbon atoms include n-butyl, tertiary butyl, n-pentyl, isopentyl, n-hexyl, n-heptyl, n-octyl, isooctyl, 2-ethylhexyl, nonyl, decyl, dodecyl, tridecyl, tetradecyl, hexadecyl and the like. Specific examples of some of the phenothiazine carboxylic acid esters are:

n-butyl phenothiazine-l-carboxylate tertiary butyl phenothiazine-l-carboxylate n-pentyl phenothiazine-l-carboxylate isopentyl phenothiazine-l-carboxylate 4 n-hexyl phenothiazine-l-carboxylate n-heptyl phenothiazine-l-carboxylate n-octyl phenothiazine-l-carboxylate isooctyl phenothiazine-l-carboxylate Z-ethylhexyl phenothiazine-l-carboxylate nonyl phenothiazine-l-carboxylate decyl phenothiazine-l-carboxylate dodecyl phenothiazine-l-carboxylate tridecyl phenothiazine-l-carboxylate tetradecyl phenothiazine-l-carboxylate hexadecyl phenothiazine-l-carboxylate n-butyl phenothiazine-Z-carboxylate tertiary butyl phenothiazine-Z-carboxylate n-pentyl phenothiazine-2-carboxylate isopentyl phenothiazine-2-carboxylate n-hexyl 'phenothiazine-2-carboxylate n-heptyl phenothiazine-Z-carboxylate n-octyl phenothiazine-2-carboxylate isooctyl phenothiazine-2-carboxylate 2-ethylhexyl phenothiazine-2-carboxylate nonyl-phenothiazine-Z-carboxylate decyl phenothiazine-2-carboxylate dodecyl phenothiazine-Z-carboxylate tridecyl phenothiazine-Z-carboxylate tetradecyl phenothiazine-2-carboxylate hexadecyl phenothiazine-2-carboxylate isooctyl phenothiazine-3-carboxylate decyl phenothiazine-3-carboxylate dodecyl phenothiazine-3-carboxylate tetradecyl phenothiazine-4-carboxylate hexadecyl phenothiazine-4-carboxylate di-tertiary-butyl phenothiazine-2,8-dicarboxylate di-isooctyl phenothiazine-2,8-dicarboxylate di-dodecyl phenothiazine-2,8-dicarboxylate di-hexadecyl phenothiazine-2,8-dicarboxylate butyl dodecyl phenothiazine-2,8-dicarboxylate pentyl tridecyl phenothiazine-2,8-dicarboxylate octyl butyl phenothiazine-2,8-dicarboxylate The amount of the phenothiazine carboxylic acid ester employed depends upon the particular lubricating oil base used as well as the severity of the conditions to which it is subjected and also upon the particular phenothiazine carboxylic acid ester chosen. Normally, the amount of the phenothiazine carboxylic acid ester comprises about 0.1 to about 5 percent by weight based on the weight of the total composition. In most instances, good results are obtained with about 0.5 to about 2 percent by weight. In any event a small amount, suflicient to inhibit oxidational changes normally tending to occur, is used.

The phenothiazine carboxylic acid esters can be prepared according to known chemical procedures. Neither the compounds per se nor their method of preparation constitutes any portion of the invention. The phenothiazine carboxylic acid esters, for example, can be prepared by esterification of the corresponding phenothiazine carboxylic acid or by transesterification of the methyl ester of the phenothiazine carboxylic acid. The phenothiazine carboxylic acid from which the esters are prepared can likewise be prepared by various procedures. Methods for preparing phenothiazine-l-carboxylic acid have been described by Gilman et al., J. Org. Chem., 23, 1476 (1958); Gilman et al., J. Am. Chem. Soc., 71, 1499 (1949); and Gilman et al., J. Am. Chem. Soc., 66, 626 (1944). According to the latter reference, phenothiazine-l-carboxylic acid is prepared by metalating phenothiazine in the 1-position with n-butyllithium which, upon carbonation, gives phenothiazine-l-carboxylic acid. R. Baltzly et al., J. Am. Chem. Soc., 68, 2673 (1946) and J. Michels et al., ibid., 72, 888 (1950) describe the reaction of 10- acetylphenothiazine with acetyl chloride and aluminum chloride in carbon disulfide to give 2,10-diacetylphenothiazine and/or 2,-8,10-triacetylphenothiazine depending upon the amount of acetyl chloride used. The subsequent reaction of these compounds with sodium hypochlorite results in the formation of phenothiazine-2-carboxylic acid and phenothiazine-2,-8-dicarboxylic acid. Burger and Clements, J. Org. Chem., 19, 1113 (1953), have disclosed a similar method for preparing phenothiazine-2-carboxylic acid starting with l-acetylphenothiazine and chloroacetyl chloride. The esters are prepared from these acids by reaction with the desired alcohols in the presence of a strong acid catalyst or more easily by transesterification of the methyl ester by heating the methyl ester with the desired alcohol in the presence of sodium metal catalyst. Massie et al., J. Org. Chem., 24, 251 (1959), have disclosed a mehod for preparing the methyl ester of phenothiazine-Z-carboxylic acid by reacting phenothiazine-2- carboxylic acid with dimethyl sulfate and potassium carbonate in the presence of potassium iodide using acetone as a solvent.

The following specific examples will serve to illustrate the preparation of phenothiazine carboxylic acid and the esters thereof.

EXAMPLE 1 Phenothiazine-Z-carboxylic acid Into a 12-1iter flask equipped with a stirrer and a reflux condenser are placed 223 grams of IO-acetylphenothiazine, 104.5 grams of chloroacetyl chloride and 400 ml. of carbon disulfide. To the mixture thus obtained are added, with stirring, 375 grams of anhydrous aluminum chloride in 40-gram portions over a 2-hour period. The mixture is then stirred for 4 /2 hours at room temperature and then for 2 hours at a reflux temperature. The mixture is then cooled to room temperature and the supernatant carbon disulfide is decanted. The viscous residue which remains is decomposed by pouring onto a mixture of 3000 grams of cracked ice and 100 ml. of concentrated hydrochloric acid. A tan precipitate comprising 2-chloroacetyl-lO-acetylphenothiazine is obtained which is filtered, washed with water and recrystallized from ethanol, M.P. 174-175 C.

A mixture of 206 grams of the 2chloroacetyl-10-acetylphenothiazine, prepared above, and 1300 ml. of pyridine is stirred for /2 hour at 90 C. and then cooled to room temperature. The dark solution thus obtained is extracted With 3 liters of ethyl ether yielding a gummy yellow solid. The yellow solid is then stirred for 2 hours at 50 C. with 2 liters of a 5% aqueous solution of sodium hydroxide. The mixture is then filtered through a bed of Celite filter aid and charcoal. The filtered solution is then acidified with concentrated hydrochloric acid precipitating a yellow solid comprising phenothiazine-Z-carboxylic acid. Upon recrystallization from ethanol, 127 grams (80% yield) of phenothiazine-2-carboxylic acid having a melting point of 275 280 C. are obtained.

EXAMPLE 2 Methyl phenothiazine-2-carboxylate Into a flask equipped with a stirrer and a reflux condenser are placed 24.3 grams of phenothiazine-Z-carboxylic acid (Example 1), 25.2 grams of dimethyl sulfate, 13.8 grams of anhydrous potassium carbonate, about 1 gram of potassium iodide and 200 ml. of acetone. The mixture is stirred for 24 hours at a refluxing temperature. After cooling to room temperature, the solution is poured into 500 ml. of water whereupon a yellow solid comprising methyl phenothiazine-2-carboxylate is obtained. The yellow solid after recrystallization from ethanol-petroleum ether has a melting point of 160l65 C.

EXAMPLE 3 Isooctyl phenothiazine-Z-carboxylate A mixture of 13.4 grams of methyl phenothiazine-2- carboxylate (Example 2) and 0.2 gram of sodium metal dissolved in 26 grams of isooctyl alcohol is stirred at 120130 C. Methanol slowly distills from the mixture. After about 2 hours, the evolution of methanol ceases. The mixture is then cooled to room temperature and poured into hexane, whereupon a solid precipitates. The precipitate is a yellow solid melting at l06l08 C. and comprises the isooctyl ester of phenothiazine-Learboxylic acid.

EXAMPLE 4 Tridecyl phenothiazine-Z-carboxylate Phenothiazine-2carboxylic acid (Example 1) upon direct esterification with tridecyl alcohol in the presence of concentrated sulfuric acid produces tridecyl phenothiazine-2-carboxylate. The yield of product is low as compared to the yield obtained by transesterification.

EXAMPLE 5 Phenothazine-l-carboxylic acid Into a flask equipped with a stirrer and reflux condenser is placed a solution consisting of 100 grams of phenothiazine in 600 ml. of tetrahydrofuran. The solution is stirred at 010 C. while introducing a stream of dry nitrogen and a solution of grams of n-butyllithium in 500 ml. of ether. The solution of n-butyllithium is added at a rate to maintain the temperature below 10 C. After addition of the n-butyllithium is complete, the mixture is stirred for 4 hours at 10 C. and then for 8 hours at a reflux temperature. The mixture is then cooled to room temperature and poured slowly onto 1 kilogram of Dry Ice and 500 ml. of ether. After warming to room temperature, one liter of distilled water is added and, upon standing overnight, a yellow precipitate comprising phenothiazine-l-carboxylic acid, melting at l50-160 C. is formed.

EXAMPLE 6 Tridecyl phenothiazine-l-carboxylate A mixture consisting of 6.4 grams of phenothiazine-lcarboxylic acid (Example 5), 5.3 grams of tridecyl alcohol, 1 gram of concentrated sulfuric acid and ml. of toluene is refluxed for 12 hours after which time the solid phenothiazine-l-carboxylic acid is dissolved. The dark mixture thus formed is shaken with 200 ml. of a 5% aqueous sodium bicarbonate solution. The organic phase is then dried over anhydrous sodium sulfate and filtered. The volume of filtrate is reduced to about 30 ml. under vacuum and then diluted with 100 ml. of petroleum ether. Upon refrigeration of the ether solution, a yellow crystalline solid comprising tridecyl phenothiazine-l-carboxylate is obtained.

It will be understood that the foregoing examples are illustrative only and that other esters of phenothiazine carboxylic acid can be similarly prepared. The examples are not to be construed as limiting the scope of the present invention in any way.

The lubricating composition of the invention can contain minor amouts of other addition agents normally added to a lubricating oil for a specific purpose, if desired, such as a pour point depressant, a viscosity index improver, an oiliness and extreme pressure agent, a corrosion inhibitor, a sludge inhibitor, 21 dispersant, a detergent, a thickener and the like. Also, if desired, the lubricating composition can contain a foam inhibitor such as an organo-silicon oxide condensation product, an organosilicol condensation product and the like.

In preparing the improved lubricant, the phenothiazine carboxylic acid ester and other additives, if employed, can be added to the lubricating oil base as such or in the form of an oil solution. The latter practice is desirable in order to facilitate compounding of the composition.

In order to demonstrate the effectiveness of the phenothiazine carboxylic acid esters of the type described hereinabove as antioxidants in lubricating oils, we have conducted comparative oxidation tests with representative lubricating oils alone and with the same lubricating oils blended with phenothiazine and with phenothiazine carboxylic acid esters. The first series of oxidation tests were cant at a temperature of 347 F. At the end of each test an observation was also made as to whether or not varnish and/ or sediment were present.

The results of comparative tests on a lubricating oil alone, and the same lubricating oil with phenothiazine and 'wtih phenothiazine carboxylic acid esters are shown in test, the viscosity of the oil containing phenothiazine and tridecyl phenothiazine-Z-carboxylate increased only 5.7 percent and 3.4 percent, respectively. At the conclusion of the test, the oil with the phenothiazine contained a considerable amount of black sludge. The oil which contained the phenothiazine carboxylic acid ester formed no insolubles. When the test was further extended to 72 hours using an air flow rate of 8 liters per hour, there were still no insolubles present in the oil containing the phenothiazine carboxylic acid ester.

In order to illustrate still further the improved results obtained with a composition of the invention containing a phenothiazine carboxylic acid ester as compared to a composition containing phenothiazine, we have conducted comparative tests using the MIL-L-7808 Oxidation-Cor- Table I. rosion Test. In brief, the MIL-L-7808 test is conducted TABLE I A B C D Composition, percent by weight:

Lubricating oil, diisooctyl azelate 100 99 99 99 Phenothiazine compound:

Phenothiazine Isoootyl phenothiazine-2-carboxylate Tridecyl phenothiazine-l-carboxylate Oxidation test (air flow: 1 l./hr.): Viscosity, cps. at 100 F.:

Initial 12. 66 13. 00 After 48-hour test. 22. 79 13. 43 Increase 10. 13 0. 43 Percent increase in viscosity 80 3. 3 Acid number (ASTM D 974):

Initial 0. 07 0 09 After 48-hour test. 28. 68 0 90 Increase 28. 61 0 81 Visual observation.

1 Clear yellow.

2 Dark red, considerable black sludge. 3 Dark red-brown, no insolubles.

4 Dark red-brown, no insolubles.

The data in the above Table I clearly show that while phenothiazine improves the oxidation stability of the lubricating oil, the phenothiazine causes sludge or sediment to form. The phenothiazine carboxylic acid ester, on the other hand, improves the oxidation stability of the oil without causing the formation of sludge or sediment.

In order to demonstrate the effectiveness of a phenothiazine carboxylic acid ester under more severe oxidizing conditions, the above oxidation test procedure was repeated using an air flow rate of 8 liters per hour instead of one liter per hour. Again, the synthetic lubricating oil alone was compared with the same lubricating oil blended with phenothiazine and with a phenothiazine carboxylic acid ester. The results of the comparative tests are set forth in Table II.

by bubbling air at a rate of about five liters per hour through approximately one hundred milliliters of test lubricant contained in a glass container for a period of seventy-two hours. The test lubricant is maintained at a temperature of 347 F. 175 0.). Five different metal specimens, i.e., magnesium, aluminum, copper, steel and silver, are immersed in the test lubricant for the duration of the test. The metal specimens are weighed before and after the test to determine whether there is a loss or gain in weight. The viscosity of the test lubricant and its acid number are determined at the beginning and the conclusion of each test. The percent change in viscosity and the increase in acid number of the lubricant during the test is indicative of the oxidation stability of the test lubricant at a temperature of 347 F. At the end of each test an TABLE II E F G Composition, percent by weight:

Lubricating oil, diisooctyl azelate 100 99 99 Phenothiazine compound:

Phenothiazine 1 Tridecyl phenothiazine-2-carboxylate 1 Oxidation test (air flow, 8 l./hr.): Viscosity, cps. at 100 F.:

Initial 12. 28 12. 12. 63 After 48-hour test- 83. 19 13. 27 13. 06 Increase 70. 91 0. 72 0. 43 Percent increase in viscos 577 5. 7 3. 4 Acid number (ASTM D 974): Initial 0. 01 0. 03 0. 01 After 48-hour test. 58. 82 0.75 1.75 Increase 53. 81 O. 72 1. 74 Visual observation 1 Clear yellow. 2 Dark oil with considerable black sludge. 3 Red-brown with no insolubles.

The above data show that whereas the viscosity of the observation is also made as to whether or not varnish uninhibited oil increased 577 percent during the 48-hour 75 and/or sediment are present.

9 The test data obtained with the MIL-L7808 Oxidation- Corrosion Test are shown in Table III.

St Visual observation 1 Moderate varnish and sediment. 2 Slight varnish, no sediment.

The data in Table III clearly show that whereas sediment formed in Composition H containing phenothiazine, no sediment formed in Composition I which contained tridecyl phenothiazine-Z-carboxylate. The data show further that the metal specimens immersed in Composition H had weight changes of about 0.2 to about 0.3 milligram per square centimeter whereas except for copper, there Was very little change in weight of the metals immersed in Composition 1.

In order to show the oxidation inhibiting effect obtained by adding a phenothiazine carboxylic acid ester to a mineral oil, comparative tests were made using the Turbine Oil Oxidation Test, ASTM D 943-54. The lubrieating oil base stock was a mineral oil having an API gravity of 32.4; a viscosity of 150 SUS at 100 F. and 43.8 SUS at 210 F.; a viscosity index of 112; a flash point (OC) of 430 F.; a fire point of 470 F.; and a pour point of F. The results of of the test are set forth in Table IV.

The above data show that tridecyl phenothiazine-2-carboxylate effectively inhibits a mineral oil against oxidation.

Mineral lubricating oil compositions of the present invention in addition to exhibiting improved resistance to oxidation also show improved detergent characterstics. For example, a mineral lubricating oil containing from 0.1 to 1 percent by weight of isooctyl phenothiazine-2-carboxylate was found by the Coker Detergency Test to be substantially more resistant to the formation of deposits than the mineral oil alone. This test has been found to be a valuable measuring rod in determining the detergent characteristics of an oil.

The Coker Detergency Test is a modification of the Pratt and Whitney Test as described in the United States Air Force Military Specification MILL7808A. In brief, the test comprises splashing the test oil against a heated aluminum panel for a given period of time and thereafter determining the amount of deposit formed on the panel. At the start of the test, 225 milliliters of test oil are placed in a stainless steel reservoir the oil being heated to a temperature of about 150 to 165 F. As the test progresses additional oil is added to maintain the initial volume of oil. During the test period, air is drawn over the oil at a rate of about 10 liters per hour. A cylindrical brush made of stainless steel wire is positioned over the oil reservoir so that a portion of the brush dips below the surface of the oil. The splasher brush rotates at a speed of about 1000 r.p.m. during the test period. By this means test oil is thrown from the oil reservoir to the underside of an aluminum panel positioned over the reservoir so that the oil after hitting the test panel can fall back into the reservoir. The test panel is maintained at a temperature of about 500 F. during the test period. The test may be conducted for various periods of time but for comparative purposes any series of tests are run for the same period of time. At the conclusion of the test, the air flow and heaters are turned off and the splasher is stopped. Excess oil is allowed to drain from the panel. The panel is then washed with petroleum ether to remove any free oil and thereafter allowed to dry. The dried panel is then weighed to determine the increase in weight. The increase is reported as a deposit weight in milligrams.

The results obtained in the Coker Detergency Test are shown in Table V for a mineral oil alone and for the same oil blended with various amounts of isooctyl phenothiazine- 2-carboxylate. The mineral oil employed was a highly re fined paraflinic oil having as typical characteristics an API gravity of 32.2; a viscosity of 152 SUS and 43.8 SUS at and 210 F., respectively; a viscosity index of a flash point of 430 F.; a fire point of 495 F.; a pour point of 10 F.; and a color ASTM Union of -10.

TABLE V L M N 0 Composition, percent by weight:

Mineral oil 100 99.9 90. 75 99 Isooctyl phenothiazine-Z-carboxylate 0.1 0.25 1 Inspection, Coker Detergency Test, 8 hours,

deposit weight, mg 6.7 6.8 4.2

The effectiveness of the phenothiazine carboxylic acid esters in compositions of the invention in reducing deposits is clear from the data in Table V. It will be noted that 100 mg. of deposit was formed on the aluminum panel with the mineral oil alone. When the oil contained only 0.1 percent by weight of isooctyl phenothiazine-2-carboxylate, the deposits were reduced to 6.7 mg. When the oil contained 1 percent by weight of the phenothiazine carboxylic acid ester, the deposits were reduced to 4.2 mg.

Specific examples of other compositions within the scope of the invention are set forth in Table VI.

TABLE VI Composition, percent by weight P Lubricating oil:

Diisooetyl azelate 99.9 98 Di(2-ethylhexyl)sebacate 99.5 97 Mineral oil 99 95 Phenothiazine compound:

Tertiary butyl phen0thiazine-1-carboxylate l'sooctyl phenothiazine-l-carboxylate 0.5 Hexadeeyl phenothiazine-4-earboxylate. Dodecyl phenothiazine-Zi-carboxylate.

Di-isooctyl phenothiazine-2,S-dicarboxylate 3.... Octyl butyl phenothiazine-Z,8-dicarboxylate 5 'boxylic acid ester represented by the formula changes, of a phenothiazine carboxylic acid ester represented by the formula H Y j where Y is selected from the group consisting of hydrogen and carboalkoxy radicals of the formula COOR and R is an alkyl radical containing from 4 to 16 carbon atoms.

2. The lubricating composition of claim 1 wherein the amount of the phenothiazine carboxylic acid ester is about 0.1 to about 5 percent by weight of the composition.

3. A lubricating composition comprising a major amount of a synthetic ester lubricating oil normally tending to undergo oxidational changes at an elevated tem-* perature and a minor amount, sufficient to substantially retard such oxidational changes, of a henothiazine car- COOR Where Y is selected from the group consisting of hydrogen and carboalkoxy radicals of the formula COOR and R is an alkyl radical containing from 6 to 12 carbon atoms.

4. A lubricating composition comprising a major amount of diisooctyl azelate and a minor amount, sufficient to impart improved oxidation stability to said diisooctyl azelate, of isooctyl phenothiazine-2-carboxylate.

5. A lubricating composition comprising a major amount of diisooctyl azelate and a minor amount, sufficient to impart improved oxidation stability to said diisooctyl azelate, of tridecyl phenothiazine-Z-carboxylate.

COOR

6. A lubricating composition comprising a major amount of diisooctyl azelate and a minor amount, sufficient to impart improved oxidation stability to said diisooctyl azelate, of tridecyl henothiazine-l-carboxylate.

7. A lubricating composition comprising a major amount of a mineral lubricating oil and a minor amount, suflicient to impart improved oxidation stability to said mineral lubricating oil, of tridecyl phenothiaZine-2-carboxylate.

8. A lubricating composition comprising a major amount of a mineral lubricating oil and a minor amount, sufi'icient to impart improved detergency to said mineral lubricating oil, isooctyl phenothiazine-Z-carboxylate.

References Cited UNITED STATES PATENTS 3/1952 Smith. 4/1962 Spacht.

OTHER REFERENCES Burger et al.: Some Derivatives of Phenothiazine, J. Org. Chem., 19, 1113 (1953).

Cole et al.: Antioxidant Mechanism Studies of the Phenothiazine Type, Pet. Reprints v. 1, N0. 4, September 1956.

US. Cl. X.R. 252402