Stabilized phosphate ester-based functional fluid compositions

Abstract

A fluid composition suitable for use as an aircraft hydraulic fluid is disclosed. The fluid composition comprises a fire resistant phosphate ester base stock comprising between about 10% and about 100% by weight of a trialkyl phosphate, between about 0% and about 70% by weight of a dialkyl aryl phosphate, and from about 0% to about 25% by weight of an alkyl diaryl phosphate, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%. The alkyl substituents of the trialkyl phosphate, the dialkyl aryl phosphate, and the alkyl diaryl phosphate contain between 3 and 8 carbon atoms, preferably between 4 and 8 carbon atoms, more preferably between 4 and 5 carbon atoms, and are bonded to the phosphate moiety via a primary carbon. It is still further preferred that the alkyl substituents of the trialkyl phosphate, the dialkyl aryl phosphate, and the alkyl diaryl phosphate are isoalkyl groups. The fluid composition further comprises an acid scavenger, an anti-erosion additive, a viscosity index improver, and an antioxidant. A novel additive combination comprises a high molecular weight butyl/hexyl methacrylate viscos ity index improver, a perfluoroalkylsulfonate anti-erosion additive, a 3,4-epoxycyclohexanecarboxylate or a diepoxide acid scavenger, a di(alkylphenyl)amine, and a phenolic antioxidant comprising a mixture of a 2,4,6-trialkylphenol and a hindered polyphenol compound selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene and mixtures thereof. Preferably, the fluid composition further comprises a benzotriazole derivative as a copper corrosion inhibitor, and a 4,5-dihydroimidazole derivative, as an iron corrosion inhibitor and to enhance the stability of the fluid. <INS-S DATE="20010320" ID="INS-S-00001">The issued reissue patent is the reexamination certificate. <INS-E ID="INS-S-00001">

What is claimed is:  
     
       1. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock, the basic stock comprising between about 50% and about 85% by weight of a trialkyl phosphate in which the alkyl substituents are substantially isoalkyl C4 or C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 18% and about 35% by weight of a dialkyl aryl phosphate in which the alkyl substituents are as previously defined, and between about 0% and about 5% by weight of an alkyl diaryl phosphate in which the alkyl substituent is as previously defined, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) an acid scavenger in an amount effective to neutralize phosphoric acid and phosphoric acid partial esters formed in situ by hydrolysis of any of the phosphate esters of said base stock, wherein said acid scavenger comprises an epoxide compound;  
       (c) an anti-erosion additive in an amount effective to inhibit flow-induced electrochemical or zeta corrosion of the flow-metering edges of hydraulic servo valves in hydraulic systems, wherein said anti- erosion additive comprises an alkali metal salt of a perfluoroalkyl sulfonic acid ;  
       (d) a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity of at least about 3.0 centistokes at about 210° F., at least about 9.0 centistokes at about 100° F., and less than about 4200 centistokes at −65° F., wherein said viscosity index improver comprises a methacrylate ester polymer at least  95 %  by weight of the polymer having a molecular weight of between about  50 , 000  and  1 , 500 , 000   ; and  
       (e) an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxidizing agents, wherein said antioxidant is selected from the group consisting of at least one hindered phenol and a combination of at least one hindered phenol and at least one amine compound.  
     
     
       2. A fluid composition as set forth in claim  1  wherein said dialkyl aryl phosphate is a dialkyl phenyl phosphate. 
     
     
       3. A fluid composition as set forth in claim  1  wherein the viscosity index improver is present in an amount effective to cause the fluid composition to exhibit a viscosity between about 3 and about 5 centistokes at about 210° F. and between about 9 and about 15 centistokes at 100° F. 
     
     
       4. A fluid composition as set forth in claim  1  containing a viscosity index improver in a proportion of between about 3% and about 10% by weight of the fluid composition. 
     
     
       5. A fluid composition as set forth in claim  1   91 containing a 2,4,6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of the fluid composition, a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition, and a hindered polyphenol selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane and 1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene and mixtures thereof, in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition. 
     
     
       6. A fluid composition as set forth in claim  2  wherein phosphate esters containing an aryl substituent do not constitute more than about 25% by weight of the base stock. 
     
     
       7. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock comprising between about 10% and about 90% of a trialkyl phosphate wherein the alkyl substituents are substantially isoalkyl C 4  and C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are as previously defined, and between about 0% and about 25% by weight of an alkyl diaryl phosphate wherein the alkyl substituent is as previously defined, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) a viscosity index improver in a proportion of between about 3% and about 10% by weight of the fluid composition, the viscosity index improver comprising a methacrylate ester polymer, the repeating units of which substantially comprise butyl and hexyl methacrylate, at least 95% by weight of the polymer having a molecular weight of between about 50,000 and about 1,500,000;  
       (c) an anti-erosion agent in a proportion of between about 0.02% and about 0.08% by weight of the fluid composition, the anti-erosion agent comprising an alkali metal salt of a perfluoroalkyl sulfonic acid, the alkyl substituent of which is selected from the group consisting of hexyl, heptyl, octyl, nonyl, decyl, and mixtures thereof;  
       (d) an acid scavenger in a proportion of between about 1.5% and about 10% by weight of the fluid composition, the acid scavenger comprising an epoxide compound;  
       (e) a 2,4,6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of the fluid composition;  
       (f) a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition; and  
       (g) a hindered polyphenol selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl)-4 -hydroxyaryl)benzene and mixtures thereof in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition.  
     
     
       8. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock comprising between about 10% and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially isoalkyl C 4  or C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are as previously defined, and between about 0% and about 25% by weight of an alkyl diaryl phosphate wherein the alkyl substituent is as previously defined, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) a viscosity index improver in a proportion of between about 3% and about 10% by weight of the fluid composition, the viscosity index improver comprising a methacrylate ester polymer, the repeating units of which substantially comprise butyl and hexyl methacrylate, at least 95% by weight of the polymer having a molecular weight of between about 50,000 and about 1,500,000;  
       (c) an anti-erosion agent in a proportion of between about 0.02% and about 0.08% by weight of the fluid composition, the anti-erosion agent comprising an alkali metal salt of a perfluoroalkylsulfonic acid, the alkyl substituent of which is selected from the group consisting of hexyl, heptyl, octyl, nonyl, decyl, and mixtures thereof;  
       (d) an acid scavenger in a proportion of between about 1.5% and about 10% by weight of the composition, the acid scavenger selected from the group consisting of a derivative of a 3,4-epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                    
       wherein R 3  is an organic group containing 1 to 10 carbon atoms, from 0 to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R 4  through R 9  are independently selected from among hydrogen and aliphatic groups containing 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound; 
       (e) 2,4,6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of the composition;  
       (f) a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition; and  
       (g) a hindered polyphenol selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4 -hydroxyaryl)benzene, and mixtures thereof in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition.  
     
     
       9. A fluid composition as set forth in claim  8  further comprising benzotriazole or a benzotriazole derivative in a proportion of between about 0,005   0 . 005 % and about 0.09% as a copper corrosion inhibitor. 
     
     
       10. A fluid composition as set forth in claim  8  further comprising between about 0.0035% and about 0.010% by weight of an alkali metal arenate. 
     
     
       11. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock comprising between about 35% and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially isoalkyl C 4  and C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 35% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are as previously defined, and between about 0% and about 20% by weight of a triaryl phosphate, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) a viscosity index improver in a proportion of between about 3% and about 10% by weight of the fluid composition, the viscosity index improver comprising a methacrylate ester polymer, the repeating units of which substantially comprise butyl and hexyl methacrylate, at least 95% by weight of the polymer having a molecular weight of between about 50,000 and about 1,500,000;  
       (c) an anti-erosion agent in a proportion of between about 0.02% and about 0.08% by weight of the fluid composition, the anti-erosion agent comprising an alkali metal salt of a perfluoroalkyl sulfonic acid, the alkyl substituent of which is selected from the group consisting of hexyl, heptyl, octyl, nonyl, decyl, and mixtures thereof;  
       (d) an acid scavenger in a proportion of between about 1.5% and about 10% by weight of the fluid composition, the acid scavenger selected from the group consisting of a derivative of a 3,4-epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                    
       wherein R 3  is an organic group containing 1 to 10 carbon atoms, from 0 to 6 oxygen atoms, and from 0 to 6 nitrogen atoms, and R 4  through R 9  are independently selected from among hydrogen and aliphatic groups containing 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexanecarboxylate and the diepoxide compound; 
       (e) a 2,4,6-trialkylphenol in a proportion or between about 0.1% and about 1.0% by weight of the fluid composition;  
       (f) a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition; and  
       (g) a hindered polyphenol selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl) methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene and mixtures thereof in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition.  
     
     
       12. A fluid composition as set forth in claim  11  wherein the base stock comprises between about 80% and about 90% by weight of the trialkyl phosphate and between about 10% and about 208 by weight of a tri(alkylaryl)phosphate. 
     
     
       13. A fluid composition as set forth in claim  12  wherein the tri(alkylaryl) phosphate is selected from the group consisting of tri(isopropylphenyl)phosphate, tri(isobutylphenyl)phosphate, and tri(tert-butylphenyl)phosphate. 
     
     
       14. A fluid composition as set forth in claim  8  wherein the base stock comprises between about 10% and about 72% by weight of the trialkyl phosphate, between about 18% and about 70% by weight of the dialkyl aryl phosphate, and between about 0% and about 25% by weight of the alkyl diaryl phosphate. 
     
     
       15. A fluid composition as set forth in claim  14  wherein the base stock comprises between about 10% and about 25% by weight of the trialkyl phosphate, between about 45% and about 70% by weight of the dialkyl aryl phosphate, and between about 5% and about 25% by weight of the alkyl diaryl phosphate. 
     
     
       16. A fluid composition as set forth in claim  14  wherein the base stock comprises between about 50% and about 72% by weight of the trialkyl phosphate, between about 18% and about 35% by weight of the dialkyl aryl phosphate, and between about 0% and about 10% by weight of the alkyl diaryl phosphate. 
     
     
       17. A fluid composition as set forth in claim  8  wherein the base stock contains between about 0% and about 5% by weight of the alkyl diaryl phosphate. 
     
     
       18. A composition as set forth in claim  8  wherein said alkyl substituents are substantially isobutyl or isopentyl. 
     
     
       19. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock, the base stock comprising between about 10 and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially isobutyl or isopentyl and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are as previously defined, and between about 0% and about 25% by weight of an alkyl diaryl phosphate wherein the alkyl substituent is as previously defined, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) an acid scavenger in an amount effective to neutralize phosphoric acid partial esters formed in situ by hydrolysis of any of the phosphate esters of the base stock, wherein said acid scavenger comprises an epoxide compound;  
       (c) an anti-erosion additive in an amount effective to inhibit flow-induced electrochemical or zeta corrosion of the flow-metering edges of hydraulic servo valves in hydraulic systems, wherein said anti- erosion additive comprises an alkali metal salt of a perfluoroalkyl sulfonic acid ;  
       (d) a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity of at least about 3.0 centistokes at about 210° F., at least about 9.0 centistokes at about 100° F., and less than about 4200 centistokes at −65° F., wherein said viscosity index improver comprises a methacrylate ester polymer at least  95 %  by weight of the polymer having a molecular weight of between about  50 , 000  and  1 , 500 , 000   ; and  
       (e) an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxidizing agents, wherein said antioxidant is selected from the group consisting of at least one hindered phenol and a combination of at least one hindered phenol and at least one amine compound.  
     
     
       20. A fluid composition as set forth in claim  19  wherein the trialkyl phosphate is triisobutyl phosphate and the dialkyl aryl phosphate is diisobutyl phenyl phosphate. 
     
     
       21. A fluid composition as set forth in claim  19  further comprising benzotriazole or a benzotriazole derivative in a proportion of between about 0.005% and about 0.09% by weight as a copper corrosion inhibitor. 
     
     
       22. A fluid composition as set forth in claim  19  further comprising a 4,5-dihydroimidazole compound corresponding to the formula                    
       where R 1  is selected from the group consisting of hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl, and R 2  is selected from the group consisting of alkyl, alkenyl and aliphatic carboxylate. 
     
     
       23. A fluid composition as set forth in claim  22  wherein the 4,5-dihydroimidazole is selected from the group consisting of 2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and the condensation product of a C 14  to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       24. A fluid composition as set forth in claim  23  wherein the 4,5-dihydroimidazole compound is the condensation product of a C 16  to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       25. A fluid composition as set forth in claim  22   19 wherein the hindered phenol antioxidant comprises a mixture of a hindered phenol and a hindered polyphenol. 
     
     
       26. A fluid composition as set forth in claim  25  wherein the hindered phenol  polyphenol comprises a hindered polyphenol  compound selected from the group consisting of bis (3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trialkyl-2,4,6-tris(3,5 -di-tert-butyl-4-hydroxyaryl) benzene, and mixtures thereof. 
     
     
       27. A fluid composition as set forth in claim  25  wherein the antioxidant further comprises an amine compound. 
     
     
       28. A fluid composition as set forth in claim  27   25 wherein the antioxidant amine compound is a diarylamine. 
     
     
       29. A fluid composition as set forth in claim  28  wherein the diarylamine is di(p-octylphenyl)amine. 
     
     
       30. A fluid composition as set forth in claim  28  further comprising up to about 0.7% by weight of 2,6-di-tert-butyl-p-cresol. 
     
     
       31. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock, the base stock comprising between about 10% and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially isoalkyl C 4  or C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are as previously defined, and between about 0% and about 25% by weight of an alkyl diaryl phosphate wherein the alkyl substituent is as previously defined;  
       (b) an acid scavenger in an amount effective to neutralize phosphoric acid and phosphoric acid partial esters formed in situ by hydrolysis of any of the phosphate esters of the base stock, wherein said acid scavenger comprises an epoxide compound;  
       (c) an anti-erosion additive in an amount effective to inhibit flow-induced electrochemical or zeta corrosion of the flow-metering edges of hydraulic servo valves in hydraulic systems, wherein said anti- erosion additive comprises an alkali metal salt of a perfluoroalkyl sulfonic acid ;  
       (d) a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity of at least about 3.0 centistokes at about 210° F., at least about 9.0 centistokes at about 100° F., and less than about 4200 centistokes at −65° F., wherein said viscosity index improver comprises a methacrylate ester polymer at least  95 %  by weight of the polymer having a molecular weight of between about  50 , 000  and  1 , 500 , 000   ;  
       (e) an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxidizing agents, wherein said antioxidant is selected from the group consisting of at least one hindered phenol and a combination of at least one hindered phenol and at least one amine compound; and  
       (f) a 4,5-dihydroimidazole compound in an amount effective to increase the stability of the composition by at least 25% at 300° F. as measured by epoxide depletion, the 4,5-dihydroimidazole compound corresponding to the formula                    
       wherein R 1  is selected from the group consisting of hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl, and R 2  is selected from the group consisting of alkyl, alkenyl and aliphatic carboxylate. 
     
     
       32. A fluid composition as set forth in claim  31  wherein R 1  is hydrogen or lower alkyl and R 2  is a fatty acid residue. 
     
     
       33. A fluid composition as set forth in claim  31  wherein R 1  is hydroxyalkyl and R 2  is alkenyl. 
     
     
       34. A fluid composition as set forth in claim  31  wherein the 4,5-dihydroimidazole is selected from the group consisting of 2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and the condensation product of a C 14  to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       35. A fluid composition as set forth in claim  34  wherein the 4,5-dihydroimidazole compound is the condensation product of a C 16  to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       36. A composition as set forth in claim  35  wherein said alkyl substituents of said trialkyl phosphate and said dialkyl aryl phosphate are substantially isobutyl or isopentyl. 
     
     
       37. A fluid composition as set forth in claim  31  wherein the hindered phenol antioxidant comprises a mixture of a hindered phenol and a hindered polyphenol. 
     
     
       38. A fluid composition as set forth in claim  37  wherein the hindered phenol  polyphenol comprises a hindered polyphenol  compound selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene, and mixtures thereof. 
     
     
       39. A fluid composition as set forth in claim  37  wherein the antioxidant further comprises an amine compound. 
     
     
       40. A fluid composition as set forth in claim  39   31 further comprisingwherein the antioxidant amine compound is a diarylamine antioxidant . 
     
     
       41. A composition as set forth in claim  40  wherein said diarylamine comprises di(p-octylphenyl)amine. 
     
     
       42. A composition as set forth in claim  40  further comprising up to about 1.0% by weight of 2,6-di-tertiary-butyl p-cresol. 
     
     
       43. A composition as set forth in claim  31  wherein said alkyl substituents of said trialkyl phosphate and said dialkyl aryl phosphate are substantially isobutyl or isopentyl. 
     
     
       44. A fluid composition as set forth in claim  1  wherein the viscosity index improver is a methacrylate ester polymer, the repeating units of which substantially comprise butyl and hexyl methacrylate, at least 95% by weight of the polymer having a molecular weight of between about 50,000 and about 1,500,000. 
     
     
       45. A fluid composition as set forth in claim  19  further comprising an anti-foam agent. 
     
     
       46. A fluid composition as set forth in claim  45  wherein the anti-foam agent is a polyalkylsiloxane. 
     
     
       47. A fluid composition as set forth in claim  46  wherein the polyalkylsiloxane is polymethylsiloxane. 
     
     
       48. A fluid composition as set forth in claim  45  wherein the anti-foam agent is present in a concentration of from about 0.0001% to about 0.001% by weight of the fluid composition. 
     
     
       49. A fluid composition as set forth in claim  48  wherein the anti-foam agent is present in a concentration of about 0.0005% by weight of the fluid composition. 
     
     
       50. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock, the base stock comprising between about 10% and about 100% by weight of a trialkyl phosphate in which the alkyl substituents are substantially isoalkyl C 4  or C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 70% by weight of a dialkyl aryl phosphate in which the alkyl substituents are as previously defined, and between about 0% and about 25% by weight of an alkyl diaryl phosphate in which the alkyl substituent is as previously defined, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) an acid scavenger in an amount effective to neutralize phosphoric acid partial esters formed in situ by hydrolysis of any of the phosphate esters of the base stock, wherein said acid scavenger comprises an epoxide compound;  
       (c) an anti-erosion additive in an amount effective to inhibit flow-induced electrochemical or zeta corrosion of the flow-metering edges of hydraulic servo valves in hydraulic systems, wherein said anti- erosion additive comprises an alkali metal salt of a perfluoroalkyl sulfonic acid ;  
       (d) a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity of at least about 3.0 centistokes at about 210° F., at least about 9.0 centistokes at about 100° F., and less than about 4200 centistokes at −65° F., wherein said viscosity index improver comprises a methacrylate ester polymer at least  95 %  by weight of the polymer having a molecular weight of between about  50 , 000  and  1 , 500 , 000   ; and  
       (e) an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxidizing agents, wherein said antioxidant is selected from the group consisting of at least one hindered phenol and a combination of at least one hindered phenol and at least one amine compound.  
     
     
       51. A fluid composition as set forth in claim  50  further comprising benzotriazole or a benzotriazole derivative in a proportion of between about 0.005% and about 0.09% by weight as a copper corrosion inhibitor. 
     
     
       52. A fluid composition as set forth in claim  50  or  51  further comprising a 4,5-dihydroimidazole compound corresponding to the formula                    
       wherein R 1  is selected from the group consisting of hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl, and R 2  is selected from the group consisting of alkyl, alkenyl and aliphatic carboxylate. 
     
     
       53. A fluid composition as set forth in claim  52  wherein the 4,5-dihydroimidazole compound is present in a proportion of between about 0.01% and about 0.1% by weight of the fluid composition. 
     
     
       54. A fluid composition as set forth in claim  52  wherein the 4,5-dihydroimidazole is selected from the group consisting of 2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and the condensation product of a C 14  to C 15  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       55. A fluid composition as set forth in claim  54  wherein the 4,5-dihydroimidazole compound is the condensation product of a C16 to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       56. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock, said base stock comprising between about 10% and about 100% by weight of a trialkyl phosphate in which the alkyl substituents are substantially isoalkyl C 4  or C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 70% by weight of a dialkyl aryl phosphate in which the alkyl substituents are as previously defined, and between about 0% and about 25% by weight of an alkyl diaryl phosphate in which the alkyl substituent is as previously defined, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) an acid scavenger in an amount effective to neutralize phosphoric acid and phosphoric acid partial esters formed in situ by hydrolysis of any of the phosphate esters of the base stock, wherein said acid scavenger comprises an epoxide compound;  
       (c) an anti-erosion additive in an amount effective to inhibit flow-induced electrochemical or zeta corrosion of the flow-metering edges of hydraulic servo valves in hydraulic systems, wherein said anti- erosion additive comprises an alkali metal salt of a perfluoroalkyl sulfonic acid ;  
       (d) a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity of at least about 3.0 centistokes at about 210° F., at least about 9.0 centistokes at about 100° F., and less than about 4200 centistokes at −65° F., wherein said viscosity index improver comprises a methacrylate ester polymer at least  95 %  by weight of the polymer having a molecular weight of between about  50 , 000  and  1 , 500 , 000   ; and  
       (e) an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxidizing agents, wherein said antioxidant is selected from the group consisting of at least one hindered phenol and a combination of at least one hindered phenol and at least one amine compound.  
     
     
       57. A fluid composition as set forth in claim  56  wherein the phosphate esters containing an aryl substituent do not constitute more than about 25% by weight of the base stock. 
     
     
       58. A fluid composition as set forth in claim  56  wherein the viscosity index improver is present in a proportion of between about 3% and about 10% by weight of the fluid composition. 
     
     
       59. A fluid composition as set forth in claim  56 wherein the viscosity index improver is a methacrylate ester polymer, the repeating units of which substantially comprise butyl and hexyl methacrylate, at least 95% by weight of the polymer having a molecular weight of between about 50,000 and about 1,500,000wherein the hindered phenol antioxidant comprises a mixture of a hindered phenol and a hindered polyphenol. 
     
     
       60. A fluid composition as set forth in claim  56   98 wherein the antioxidant comprises a 2,4,6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of the composition, a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition, and a hindered polyphenol selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane and 1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene, and mixtures thereof, in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition. 
     
     
       61. A fluid composition as set forth in claim  60  wherein the 2,4,6-trialkylphenol is 2,6-di-tert-butyl-p-cresol and the hindered polyphenol is selected from the group consisting of bis (3,5-di-tert-butyl-4-hydroxyphenyl)methane, 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4 -hydroxyphenyl)benzene, and mixtures thereof. 
     
     
       62. A fluid composition as set forth in claim  56  further comprising benzotriazole or a benzotriazole derivative in a proportion of between about 0.005% and about 0.09% by weight as a copper corrosion inhibitor. 
     
     
       63. A fluid composition as set forth in claim  56  or  62  further comprising a 4,5-dihydroimidazole compound corresponding to the formula                    
       where R 1  is selected from the group consisting of hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl, and R 2  is selected from the group consisting of alkyl, alkenyl and aliphatic carboxylate. 
     
     
       64. A fluid composition as set forth in claim  63  wherein the 4,5-dihydroimidazole compound is present in a proportion of between about 0.01% and about 0.1% by weight of the fluid composition. 
     
     
       65. A fluid composition as set forth in claim  63  wherein the 4,5-dihydroimidazole is selected from the group consisting of 2-(8-heptadecenyl)-4,5 -dihydro-1H-imidazole-1-ethanol and the condensation product of a C 14  to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       66. A fluid composition as set forth in claim  65  wherein the 4,5-dihydroimidazole compound is the condensation product of a C 6  to C 8  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       67. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock, the base stock comprising between about 10% and about 100% by weight of a trialkyl phosphate in which the alkyl substituents are substantially isoalkyl C 4  or C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 35% by weight of a dialkyl aryl phosphate in which the alkyl substituents are as previously defined, and between about 0% and about 20% by weight of a triaryl phosphate, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) an acid scavenger in an amount effective to neutralize phosphoric acid partial esters formed in situ by hydrolysis of any of the phosphate esters of the base stock, wherein said acid scavenger comprises an epoxide compound;  
       (c) an anti-erosion additive in an amount effective to inhibit flow-induced electrochemical or zeta corrosion of the flow-metering edges of hydraulic servo valves in hydraulic systems, wherein said anti- erosion additive comprises an alkali metal salt of a perfluoroalkyl sulfonic acid ;  
       (d) a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity of at least about 3.0 centistokes at about 210° F., at least about 9.0 centistokes at about 100° F., and less than about 4200 centistokes at −65° F., wherein said viscosity index improver comprises a methacrylate ester polymer at least  95 %  by weight of the polymer having a molecular weight of between about  50 , 000  and  1 , 500 , 000   ; and  
       (e) an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxidizing agents, wherein said antioxidant is selected from the group consisting of at least one hindered phenol and a combination of at least one hindered phenol and at least one amine compound.  
     
     
       68. A fluid composition as set forth in claim  67  wherein the phosphate esters containing an aryl substituent do not constitute more than about 25% by weight of the base stock. 
     
     
       69. A fluid composition as set forth in claim  67  wherein the viscosity index improver is present in a proportion of between about 3% and about 10% by weight of the fluid composition. 
     
     
       70. A fluid composition as set forth in claim  67 wherein the viscosity index improver is a methacrylate ester polymer, the repeating units of which substantially comprise butyl and hexyl methacrylate, at least 95% by weight of the polymer having a molecular weight of between about 50,000 and about 1,500,000wherein the hindered phenol antioxidant comprises a mixture of a hindered phenol and a hindered polyphenol. 
     
     
       71. A fluid composition as set forth in claim  67   99 wherein the antioxidant comprises a 2,4,6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of the composition, a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition, and a hindered polyphenol selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane and 1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene, and mixtures thereof, in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition. 
     
     
       72. A fluid composition as set forth in claim  71  wherein the 2,4,6-trialkylphenol is 2,6-di-tert-butyl-p-cresol and the hindered polyphenol is selected from the group consisting of bis(3,5-di-tert-butyl-4-hydroxyphenyl)methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenyl)benzene, and mixtures thereof. 
     
     
       73. A fluid composition as set forth in claim  67  further comprising benzotriazole or a benzotriazole derivative in a proportion of between about 0.005% and about 0.09% by weight as a copper corrosion inhibitor. 
     
     
       74. A fluid composition as set forth in claim  67  or  73  further comprising a 4,5-dihydroimidazole compound corresponding to the formula                    
       where R 1  is selected from the group consisting of hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl, and R 2  is selected from the group consisting of alkyl, alkenyl and aliphatic carboxylate. 
     
     
       75. A fluid composition as set forth in claim  74  wherein the 4,5-dihydroimidazole compound is present in a proportion of between about 0.01% and about 0.1% by weight of the fluid composition. 
     
     
       76. A fluid composition as set forth in claim  74  wherein the 4,5-dihydroimidazole is selected from the group consisting of 2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and the condensation product of a C 14  to C 15  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       77. A fluid composition as set forth in claim  16  wherein the 4,5-dihydroimidazole compound is the condensation product of a C 16  to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       78. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: 
       (a) a fire resistant phosphate ester base stock comprising between about 35% and about 99% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially isoalkyl C4 or C 5  and are bonded to the phosphate moiety via a primary carbon atom, between about 0% and about 35% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are as previously defined, and between about 0% and about 20% by weight of a triaryl phosphate, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%;  
       (b) a viscosity index improver in a proportion of between about 3% and about 10% by weight of the fluid composition, the viscosity index improver comprising a methacrylate ester polymer, the repeating units of which substantially comprise butyl and hexyl methacrylate, at least 95% by weight of the polymer having a molecular weight of between about 50,000 and about 1,500,000;  
       (c) an anti-erosion agent in a proportion of between about 0.02% and about 0.08% by weight of the fluid composition, the anti-erosion agent comprising an alkali metal salt of a perfluoroalkyl sulfonic acid, the alkyl substituent of which is selected from the group consisting of hexyl, heptyl, octyl, nonyl, decyl, and mixtures there of;  
       (d) an acid scavenger in a proportion of between about 1.5% and about 10% by weight of the fluid composition, the acid scavenger selected from the group consisting of a derivative of a 3,4-epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                    
       wherein R 3  is an organic group containing 1 to 10 carbon atoms, from 0 to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R 4  through R 9  are independently selected from among hydrogen and aliphatic groups containing 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound; 
       (e) 2,4,6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of the fluid composition;  
       (f) a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition; and  
       (g) a hindered polyphenol selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene and mixtures thereof in a proportion of between about 0.3% and about 1.0% by weight of the fluid composition.  
     
     
       79. A fluid composition as set forth in claim  78  wherein the base stock comprises between about 80% and about 99% by weight of the trialkyl phosphate and between about 1.0% and about 20% by weight of the triaryl phosphate wherein the aryl substituents are alkyl-substituted phenyl. 
     
     
       80. A fluid composition as set forth in claim  79  wherein the alkyl substituent of the alkyl-substituted phenyl is C 2  to C 9 . 
     
     
       81. A fluid composition as set forth in claim  80  wherein the alkyl substituent of the alkyl-substituted phenyl is C 3  to C 4 . 
     
     
       82. A fluid composition as set forth in claim  81  wherein the alkyl-substituted phenyl is selected from the group consisting of isopropylphenyl, isobutylphenyl, and tert-butylphenyl. 
     
     
       83. A fluid composition as set forth in claim  79  wherein the triaryl phosphate is a tri(alkylaryl) phosphate selected from the group consisting of tri(isopropylphenyl)phosphate, tri(isobutylphenyl) phosphate, and tri(tert-butylphenyl) phosphate. 
     
     
       84. A fluid composition as set forth in claim  83  wherein the tri(alkylaryl) phosphate is tri(tert-butylphenyl) phosphate. 
     
     
       85. A fluid composition as set forth in claim  78  further comprising benzotriazole or a benzotriazole derivative in a proportion of between about 0.005% and about 0.09% by weight as a copper corrosion inhibitor. 
     
     
       86. A fluid composition as set forth in claim  78  or  85  further comprising a 4,5-dihydroimidazole compound corresponding to the formula                    
       where R 1  is selected from the group consisting of hydrogen, alkyl alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl, and R 2  is selected from the group consisting of alkyl, alkenyl and aliphatic carboxylate. 
     
     
       87. A fluid composition as set forth in claim  86  wherein the 4,5-dihydroimidazole compound is present in a proportion of between about 0.01% and about 0.1% by weight of the fluid composition. 
     
     
       88. A fluid composition as set forth in claim  86  wherein the 4,5-dihydroimidazole is selected from the group consisting of 2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and the condensation product of a C 14  to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       89. A fluid composition as set forth in claim  88  wherein the 4,5-dihydroimidazole compound is the condensation product of a C 16  to C 18  fatty acid and 4,5-dihydro-1H-imidazole. 
     
     
       90. A fluid composition as set forth in claim  1  wherein the hindered phenol antioxidant comprises a mixture of a hindered phenol and a hindered polyphenol.  
     
     
       91. A fluid composition as set forth in claim  90  wherein the amine antioxidant is a diaryl amine.  
     
     
       92. A fluid composition as a set forth in claim  1  wherein said epoxide acid scavenger is selected from the group consisting of a derivative of a  3 , 4 - epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                      
       
         wherein R 
         3  
         is an organic group containing  1  to  10  carbon atoms, from  0  to  6  oxygen atoms and from  0  to  6  nitrogen atoms, and R 
         4  
         through R 
         9  
         are independently selected from among hydrogen and aliphatic groups containing  1  to  5  carbon atoms, and mixtures of the  3 , 4 - 
         epoxycyclohexane carboxylate and the diepoxide compound.  
       
     
     
       93. A fluid composition as set forth in claim  7  wherein said epoxide acid scavenger is selected from the group consisting of a derivative of a  3 , 4 - epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                      
         wherein R   3    is an organic group containing  1  to  10  carbon atoms, from  0  to  6  oxygen atoms and from  0  to  6  nitrogen atoms, and R   4    through R   9    are independently selected from among hydrogen and aliphatic groups containing  1  to  5  carbon atoms, and mixtures of the  3 , 4   - epoxycyclohexane carboxylate and the diepoxide compound.    
     
     
       94. A fluid composition as set forth in claim  50  wherein the hindered phenol antioxidant comprises a mixture of a hindered phenol and a hindered polyphenol.  
     
     
       95. A fluid composition as set forth in claim  94  wherein the amine antioxidant is a diaryl amine.  
     
     
       96. A fluid composition as set forth in claim  95  containing a  2 , 4 , 6 - trialkylphenol in a proportion of between about  0 . 1  and about  1 . 0   %  by weight of the fluid composition, a di ( alkylphenyl ) amine in a proportion of between about  0 . 3   %  and about  1 . 0   %  by weight of the fluid composition, and a hindered polyphenol selected from the group consisting of bis (   3 , 5   - dialkyl -   4   - hydroxyaryl ) methane and  1 , 3 , 5   - trialkyl -   2 , 4 , 6   - tris (   3 , 5   - di - tert - butyl -   4   - hydroxyaryl ) benzene and mixtures thereof, in a proportion of between about  0 . 3   %  and about  1 . 0   %  by weight of the fluid composition.    
     
     
       97. A fluid composition as a set forth in claim  50  wherein said epoxide acid scavenger is selected from the group consisting of a derivative of a  3 , 4 - epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                      
         wherein R   3    is an organic group containing  1  to  10  carbon atoms, from  0  to  6  oxygen atoms and from  0  to  6  nitrogen atoms, and R   4    through R   9    are independently selected from among hydrogen and aliphatic groups containing  1  to  5  carbon atoms, and mixtures of the  3 , 4   - epoxycyclohexane carboxylate and the diepoxide compound.    
     
     
       98. A fluid composition as set forth in claim  59  wherein the amine antioxidant is a diaryl amine.  
     
     
       99. A fluid composition as set forth in claim  70  wherein the amine antioxidant is a diaryl amine.  
     
     
       100. A fluid composition as set forth in claim  19  wherein said epoxide acid scavenger is selected from the group consisting of a derivative of a  3 , 4 - epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                      
         wherein R   3    is an organic group containing  1  to  10  carbon atoms, from  0  to  6  oxygen atoms and from  0  to  6  nitrogen atoms, and R   4    through R   9    are independently selected from among hydrogen and aliphatic groups containing  1  to  5  carbon atoms, and mixtures of the  3 , 4   - epoxycyclohexane carboxylate and the diepoxide compound.    
     
     
       101. A fluid composition as set forth in claim  31  wherein said epoxide acid scavenger is selected from the group consisting of a derivative of a  3 , 4 - epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                      
         wherein R   3    is an organic group containing  1  to  10  carbon atoms, from  0  to  6  oxygen atoms and from  0  to  6  nitrogen atoms, and R   4    through R   9    are independently selected from among hydrogen and aliphatic groups containing  1  to  5  carbon atoms, and mixtures of the  3 , 4   - epoxycyclohexane carboxylate and the diepoxide compound.    
     
     
       102. A fluid composition as set forth in claim  56  wherein said epoxide acid scavenger is selected from the group consisting of a derivative of a  3 , 4 - epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                      
         wherein R   3    is an organic group containing  1  to  10  carbon atoms, from  0  to  6  oxygen atoms and from  0  to  6  nitrogen atoms, and R   4    through R   9    are independently selected from among hydrogen and aliphatic groups containing  1  to  5  carbon atoms, and mixtures of the  3 , 4   - epoxycyclohexane carboxylate and the diepoxide compound.    
     
     
       103. A fluid composition as set forth in claim  67  wherein said epoxide acid scavenger is selected from the group consisting of a derivative of a  3 , 4 - epoxy cyclohexane carboxylate and a diepoxide compound corresponding to the formula                      
         wherein R   3    is an organic group containing  1  to  10  carbon atoms, from  0  to  6  oxygen atoms and from  0  to  6  nitrogen atoms, and R   4    through R   9    are independently selected from among hydrogen and aliphatic groups containing  1  to  5  carbon atoms, and mixtures of the  3 , 4   - epoxycyclohexane carboxylate and the diepoxide compound.    
     
     
       104. A fluid composition as set forth in claim  7  wherein the base stock comprises between about  20 %  and about  90   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       105. A fluid composition as set forth in claim  104  wherein the base stock comprises between about  50 %  and about  90   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       106. A fluid composition as set forth in claim  8  wherein the base stock comprises between about  20 %  and about  90   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       107. A fluid composition as set forth in claim  106  wherein the base stock comprises between about  50 %  and about  90   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       108. A fluid composition as set forth in claim  19  wherein the base stock comprises between about  20 %  and about  90   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       109. A fluid composition as set forth in claim  108  wherein the base stock comprises between about  50 %  and about  90   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       110. A fluid composition as set forth in claim  31  wherein the base stock comprises between about  20 %  and about  90   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       111. A fluid composition as set forth in claim  110  wherein the base stock comprises between about  50 %  and about  90   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       112. A fluid composition as set forth in claim  50  wherein the base stock comprises between about  20 %  and about  99   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       113. A fluid composition as set forth in claim  112  wherein the base stock comprises between about  50 %  and about  99   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       114. A fluid composition as set forth in claim  56  wherein the base stock comprises between about  20 %  and about  99   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       115. A fluid composition as set forth in claim  114  wherein the base stock comprises between about  50 %  and about  99   %  by weight of a trialkyl phosphate, between about  0   %  and about  70   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  25   %  by weight of an alkyl diaryl phosphate.    
     
     
       116. A fluid composition as set forth in claim  67  wherein the base stock comprises between about  35 %  and about  99   %  by weight of a trialkyl phosphate, between about  0   %  and about  35   %  by weight of a dialkyl aryl phosphate and between about  0   %  and about  20   %  by weight of an triaryl phosphate.    
     
     
       117. A fluid composition as set forth in claim  116  wherein the base stock comprises between about  80 %  and about  99   %  by weight of a trialkyl phosphate, and between about  1   %  and about  20   %  by weight of an triaryl phosphate.    
     
     
       118. A fluid composition as set forth in any of claims  1 ,  19 ,  31 ,  50 ,  56  or  67  wherein in the anti- erosion additive comprising an alkali metal salt of a perfluoroalkyl sulfonic acid the alkyl substituent comprises from  5  to  12  carbon atoms.