Synthetic aircraft turbine oil
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention is concerned with a pentaerythritol ester base lubricating oil composition for a gas turbine engine. Gas turbine engines are operated under a wide range of temperature conditions. The lubricant must be fluid at extremely low temperatures and at the same time retain its lubricating properties in an engine which produces internal operating temperatures of 450°-550° F or above. The lubricant is subjected to severe oxidation stresses under the high running temperatures encountered in such engines. 
     Ester base lubricating oil compositions prepared from pentaerythritol and a mixture of fatty acids containing selected additive combinations are well known. These lubricants are functional over a wide temperature range and exhibit good thermal and oxidative stability. The search for a still more effective, long lived ester base lubricant composition, however, is a major goal of lubricant manufacturers. In addition, more advanced gas turbine engines currently being developed and tested will put higher stresses on the lubricant composition and are projected to require improved lubricant compositions. 
     SUMMARY OF THE INVENTION 
     The synthetic lubricating oil composition of the invention comprises a major portion of an aliphatic ester base oil containing an alkylphenyl- or alkarylphenyl- naphthylamine, a dialkyldiphenylamine, a polyhydroxy anthraquinone, a hydrocarbyl phosphate ester, 4,4&#39;-dithiomorpholine and 3-aminotriazole. More specifically, the lubricating oil composition of the invention comprises a major portion of an aliphatic ester base oil formed from the reaction of a pentaerythritol and an organic monocarboxylic acid having from about 2 to 18 carbon atoms per molecule containing: 
     A. FROM ABOUT 0.3 TO 5 PERCENT BY WEIGHT OF THE LUBRICATING OIL COMPOSITION OF ALKYL OR ALKARYL DERIVATIVES OF PHENYL -NAPHTHYLAMINES IN WHICH THE ALKYL RADICALS CONTAIN FROM 4 TO 12 CARBON ATOMS, 
     B. FROM ABOUT 0.3 TO 5 PERCENT BY WEIGHT OF A DIALKYLDIPHENYLAMINE IN WHICH THE ALKYL RADICALS CONTAIN FROM 4 TO 12 CARBON ATOMS, 
     C. FROM ABOUT 0.001 TO 1 PERCENT BY WEIGHT OF A POLYHYDROXYANTHRAQUINONE, 
     D. FROM ABOUT 0.25 TO 10 PERCENT BY WEIGHT OF A HYDROCARBYLPHOSPHATE ESTER IN WHICH SAID HYDROCARBYL RADICAL CONTAINS AN ARYL RING AND HAS FROM ABOUT 6 TO 18 CARBON ATOMS, 
     E. FROM ABOUT 0.005 TO 0.2 PERCENT BY WEIGHT OF 4,4&#39;-DITHIODIMORPHOLINE, AND 
     F. FROM ABOUT 10 TO ABOUT 100 PPM, BASED ON THE COMPOSITION, OF A METAL PASSIVATOR SUCH AS 3-AMINOTRIAZOLE. 
     The lubricating oil composition of the invention provides substantial improvements in oxidative stability, particularly excellent control of acidity and viscosity increase under severe oxidizing conditions, over prior art. 
     DETAILED DESCRIPTION 
     The base fluid component of the composition of the invention is an ester-base fluid prepared from pentaerythritol and a mixture of hydrocarbyl monocarboxylic acids. Polypentaerythritols, such as dipentaerythritol, tripentaerythritol and tetrapentaerythritol can also be employed in the reaction to prepare the base oil. 
     The hydrocarbon monocarboxylic acids which are used to form the ester-base fluid include the straight-chain and branched-chain aliphatic acids, as well as mixtures of these acids. The acids employed have from about 2 to 18 carbon atoms per molecule, and preferably from about 5 to 10 carbon atoms. Examples of suitable acids are acetic, propionic, butyric, valeric, isovaleric, caproic, decanoic, tertiarybutylacetic and 2-ethylhexanoic acid, including mixtures. 
     In general, the acids are reacted in proportions leading to a completely esterified pentaerythritol or polypentaerythritol with the preferred ester bases being the pentaerythritol tetraesters. Examples of such commercially available tetraesters include pentaerythritol tetracaproate, which is prepared from purified pentaerythritol and crude caproic acid containing other C 5-10  monobasic acids. Another suitable tetraester is prepared from a technical grade pentaerythritol and mixture of acids comprising 38 percent valeric, 13 percent 2-methyl pentanoic, 32 percent octanoic and 17 percent pelargonic acids, by weight. 
     The ester base comprises the major portion of the fully formulated synthetic ester base lubricating oil composition. In general, this ester base fluid is present in concentrations from about 90 to 98 percent of the composition, by weight. 
     The essential alkyl or alkaryl phenyl naphthylamine component of the invention is represented by the formula: ##STR1## in which R is an alkyl or alkaryl radical containing from about 4 to 12 carbon atoms. This radical can be a straight or branched chain alkyl radical with the tertiary alkyl structure being preferred or it can be an alkylaryl radical. The naphthylamine can be either an alpha or beta naphthylamine. Specific effective compounds of this class include N-(para-tertiary-octylphenyl)-alpha- or betanaphthylamine, N-(4 cumylphenyl) alpha- or beta-naphthylamine, N-(para-tertiary-octylphenyl) alpha- or beta-naphthylamine and the corresponding para-tertiary-dodecylphenyl and para-tertiarybutylphenyl alpha or beta-naphthylamines. The preferred naphthylamines are those in which R is a tertiary alkyl radical having from 6 to 10 carbon atoms therein. The preferred concentration of this component is from about 0.5 to 2.5 percent, by weight. 
     Another essential component of the lubricating oil composition of the invention is a dialkyldiphenylamine. 
     These compounds are represented by the formula: ##STR2## in which R is an alkyl radical having from about 4 to 12 carbon atoms. Suitable alkylamines include dioctyldiphenylamine, didecyldiphenylamine, didodecyldiphenylamine, dihexyldiphenylamine and similar compounds. Dioctyldiphenylamine is the preferred compound and the preferred concentration is from 0.5 to 2.0 percent, by weight. 
     The essential metal deactivator of the lubricating oil composition of the invention is a polyhydroxyanthraquinone. Suitable compounds in this class are the dihydroxyanthraquinones, such as 1,4-dihydroxyanthraquinone and 1,5-dihydroxyanthraquinone, and the higher polyhydroxyanthraquinones such as 1,2,4 trihydroxyanthraquinone and 1,2,5,8 tetrahydroxyanthraquinone. The preferred concentration of this component is from about 0.01 to 0.5 weight percent. 
     Another component of the lubricating oil composition of the invention is a hydrocarbyl phosphate ester, more specifically a trihydrocarbyl phosphate in which the hydrocarbyl radical is an aryl or alkaryl radical or mixture thereof containing from 6 to 18 carbon atoms and preferably from 6 to 12 carbon atoms. Effective specific compounds include tricresylphosphate, cresyl diphenylphosphate and triphenylphosphate. These compounds are preferably present in the lubricating oil composition in a concentration ranging from about 0.5 to 5 percent, by weight. 
     Another essential component of the lubricating oil composition is 4,4&#39;-dithiodimorpholine. This compound is available from Monsanto Company under the Trade Name as &#34;Sulfasan R&#34;. This component is present in the lubricating oil composition in an amount of from about 0.005 to about 0.2 percent, preferably in a concentration range of from about 0.05 to about 0.1 percent, by weight. 
     The final component of the lubricating oil composition is a minute amount, e.g., from about 25 to 100 parts per million, based on the lubricating oil composition, of a metal passivator such as 3 - aminotriazole or the like. 
     The novel lubricating oil compositions of the present invention exhibit improved oxidation stability, particularly excellent control of acidity and viscosity increase under severe oxidizing conditions. 
     The ester base oil employed in preparing the lubricating oil composition of the invention comprised pentaerythritol containing a minor amount of dipentaerythritol esterified with a mixture of fatty acids. It consisted of technical grade pentaerythritol ester made from a mixture of carboxylic acids consisting of (mole %): 
     
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        i - C.sub.5  8 ±3%                                             
        n - C.sub.5 23 ±5                                              
        n - C.sub.6 20 ±5                                              
        n - C.sub.7 27 ±5                                              
        n - C.sub.8  7 ±3                                              
        n - C.sub.9 16 ±3                                              
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     This ester base oil had the following properties: 
     
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Viscosity, cs at 210° F                                            
                        (5.01)                                            
Viscosity, cs at 100° F                                            
                        (25.6)                                            
Viscosity, cs at -40° F                                            
                        (7005)                                            
Viscosity Index         (140)                                             
Flash, ° F.      (515)                                             
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     The above ester oil was blended with all of the prescribed essential additives with the exception of the dithiodimorpholine and 3-aminotriozale to form a Base Fluid. Based on a fully formulated lubricant composition, the Base fluid consisted of about 95.4 weight percent of the ester base oil described above with 1.5 weight percent of t-octyl-phenyl-α-naphthylamine, 1.0 weight percent of dioctyl-diphenylamine, 2.0 weight percent of tricresylphosphate and 0.1 weight percent of quinizarin. 
     The oxidation-stability of the lubricant of the invention as compared to the Base Fluid was determined in the Rolls Royce (RR 1001) Oxidation Test. (D. Eng. R.D. 2497 Supplement Method No. 12). The results are set forth in the following Table. 
     
                                           TABLE                                   
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 ROLLS ROYCE (RR1001) OXIDATION TEST                                      
260° C./ 6 HRS.                                                    
                      BASE FLUID +                                        
                                BASE FLUID +                              
                                          BASE FLUID +                    
                      0.1% Additive &#34;A&#34;                                   
                                0.05% Additive &#34;A&#34;                        
                                          0.05% Additive &#34;A&#34;by weight+    
              BASE FLUID                                                  
                      by weight by weight 50 ppm Additive                 
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                                          &#34;B&#34;                             
% Viscosity Change at 100° F.                                      
              83.1    35.8      51.6      38.5                            
Total Acid Number Change                                                  
              4.92    2.35      2.65      2.44                            
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 Additive A - &#34;Sulfasan R&#34; 4,4&#39;-dithiodimorpholine                        
 Additive B - 3-aminotriazole as a metal passivator                       
 
    
     The data in the Table show that the oxidative stability of the lubricating oil compositions of the present invention are significantly improved in comparison to the Base Fluid. In comparison to the Base Fluid, the composition showed a reduction in Viscosity Increase of about 56 percent at 0.1 wt. % concentration, and of about 38 percent at 0.05 wt. % concentration in the presence of 50 ppm of 3-aminotriazole. 
     Similar good results were found with respect to the reduction in Total Acid Number Increase over the Base Blend, 52% at 0.1 wt. % concentration and 46% at 0.05 wt. % concentration with 3-aminotriazole, by weight. 
     The lubricating oil compositions were further evaluated in the Pratt and Whitney Aircraft Specification PWA-521B Oxidation-Corrosion Test 425° F/48 HRS. and the Navy MIL-L-23699B Specification 400° F/72 HRS. Oxidation-Corrosion Test. The compositions with 3-aminotriazole satisified the requirements these tests. 
     Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof and are indicated in the appended claims.