Patent Publication Number: US-4320018-A

Title: Synthetic aircraft turbine oil

Description:
The application is a continuation in part application of my copending application Ser. No. 910,716, filed May 30, 1978, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. 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 at 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 and 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 containing a phenyl naphthylamine, a dialkyldiphenylamine, a polyhydroxyanthraquinone, a hydrocarbyl phosphate ester and a dialkyldisulfide compound as hereinafter described. More specifically, the lubricating oil composition of the invention comprises a major portion of an aliphatic ester base oil formed from the reaction of 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 phenyl naphthylamine or an alkyl or alkaryl derivative of phenyl naphthylamine in which the alkyl radical contains from 4 to 12 carbon atoms, 
     (b) from about 0.3 to 5 percent by weight of a dialkyldiphenylamine in which the alkyl radical contains from 4 to 12 carbon atoms, 
     (c) from about 0.01 to 0.5 percent by weight of a polyhydroxyanthraquinone, 
     (d) from about 0.25 to 10 percent by weight of a hydrocarbyl phosphate ester in which said hydrocarbyl radical contains an aryl ring and has from about 6 to 18 carbon atoms, and 
     (e) from about 0.01 to 0.5 percent by weight of a dialkyldisulfide. 
     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. 
    
    
     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, dodecanoic, 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 a 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 fluid 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 may be H or is an alkyl radical containing from about 4 to 12 carbon atoms or an alkaryl radical containing from 7 to 12 carbon atoms. This radical can be straight or branched chain alkyl radical with the tertiary alkyl structure being preferred or it can be an alkylaryl radical. 
     Specific effective compounds of this class include α- or β-phenylnaphthylamine, N-(para-tertiary-octylphenyl)-α- or β-naphthylamine, N-(4-cumylphenyl)α- or β-naphthylamine and the corresponding paratertiary-dodecylphenyl and paratertiary-butylphenyl alpha-and beta-naphthylamines. The preferred naphthylamines are those in which R is H or 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 1,2,5,8 tetrahydroxyanthraquinone. The preferred concentration of this component is from about 0.05 to 0.15 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. These compounds are preferably in the lubricating oil composition in a concentration ranging from about 0.5 to 5 wt. %. 
     Another essential component of the lubricating oil composition is a dialkyldisulfide. 
     The alkyl group of said dialkyldisulfide may contain from 4 to 20, preferably from about 8 to 16 carbon atoms each and may be linear or branched. The two alkyl groups of the dialkyldisulfide may be the same or they may be different. 
     Thus, specific examples by way of example include di-n-butyldisulfide, di-secbutyldisulfide, di-t-butyldisulfide, di-n-pentyldisulfide, di-n-hexyldisulfide, di-2-ethylhexyldisulfide, di-t-octyldisulfide, didodecyldisulfide, t-butyl-n-hexyldisulfide, 2-ethylhexyl-n-dodecylsulfide, di-n-octadecyldisulfide, 2-methylpentyl-n-nonyldisulfide and the like. 
     This componet is present in the lubricating oil composition in an amount of from about 0.01 to about 0.5% preferably in a concentration range of from about 0.05 to about 0.1% by weight. 
     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 dialkyldisulfide compound 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-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 lubricants 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 A                                                     
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ROLLS ROYCE (RR 1001) OXIDATION TEST                                      
260° C./6 HRS.                                                     
Di-t-octyldisulfide                                                       
                BASE               BASE                                   
                FLUID+             FLUID+                                 
                0.1 WT. %          0.05 WT. %                             
         BASE   ADDI-     BASE     ADDI-                                  
         FLUID  TIVE - A  FLUID    TIVE - A                               
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% Viscosity                                                               
Change at 100° F.                                                  
           96.7     41.7      96.0   60.1                                 
Total Acid                                                                
Number Change                                                             
           4.09     3.19      4.90   4.37                                 
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     The data in the Table above show that the oxidative stability of the lubricating oil compositions of the present invention containing di-t-octyldisulfide is significantly improved in comparison to the Base Fluid. 
     In comparison to the Base Fluid, the di-t-octyldisulfide species showed a reduction in Viscosity Increase of 54.9% at 0.1 wt. % concentration and a Total Acid Number Change of 22.0%. At a concentration of 0.05 wt. %, the reduction in Viscosity Increase amounted to 37.4% and the Total Acid Number Change was 10.8%, in comparison to the Base Fluid. 
     The lubricating oil composition containing the di-t-octyldisulfide species was further evaluated at 0.05 wt. % concentration in the Pratt &amp; Whitney Aircraft Specification PWA 521C Oxidation Corrosion Test, 425° F./48 HRS., and the Navy MIL-L23699B Specification 400° F./72 HRS. Oxidation Corrosion Test and was found to satisfy completely these specification requirements. 
     Another series of tests were conducted with the additive Di-benzyldisulfide, a known oxidation inhibitor in mineral oils. The test results are shown in Table B. 
     
                       TABLE B                                                     
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ROLLS ROYCE (RR 1001) OXIDATION TEST                                      
260° C./6 HRS.                                                     
Di-benzyldisulfide                                                        
                        BASE FLUID+                                       
                        0.1 WT. %                                         
              BASE FLUID                                                  
                        ADDITIVE - B                                      
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% Viscosity Change                                                        
                96.7        82.7                                          
at 100° F.                                                         
Total Acid Number Change                                                  
                4.09        3.49                                          
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     The data in the Table B above show that the oxidative stability of the lubricating oil compositions containing di-benzyldisulfide is not significantly improved in comparison to the Base Fluid. 
     In comparison to the Base Fluid, the Di-benzyldisulfide species only showed a reduction in Viscosity Increase of 14.5% by 0.1 wt. % concentration and a Total Acid Number Change of only 17.0%. These results are to be contrasted with those in Table A, using the additive of the invention, wherein the reduction in Viscosity Increase amounted to 54.9% and a Total Acid Number Change of 22.0% at 0.1% concentration. 
     These data show one cannot predict that the effectiveness of an anti-oxidant in one type of fluid will be satisfactory in any other type of fluid. 
     Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the appended claims.