FUNCTIONAL FLUID

Abstract: A functional fluid comprising a novel base stock composition comprising between about 50 % and about 70 % by weight of a trialkyl phosphate in which the alkyl substituents are C3 to C8 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 C3 to C8 and are bonded to the phosphate moiety via a primary carbon atom, and from 0 to about 5 % by weight of an alkyl diaryl phosphate. Preferably, the alkyl substituents are isobutyl or isopentyl. The fluid 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 viscosity index improver, a perfluoroalkylsulfonate anti-erosion additive, a 3,4-epoxycyclohexane carboxylate 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 composition selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxyaryl)benzene and mixtures thereof. Preferably, the composition further comprises a 4,5-dihydroimidazole derivative to enhance the stability of the fluid.

Description: FUNCTIONAL FLUID Background of the Invention This invention relates to phosphate ester functional fluids and more particularly to phosphate ester fluids of improved thermal, hydrolytic and oxidative stability useful as aircraft hydraulic fluids. Functional fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids, heat transfer fluids, heat pump fluids, refrigeration equipment fluids, and as a filter media for air-conditioning systems. Hydraulic fluids intended for use in the hydraulic system of aircraft for operating various mechanisms and aircraft control systems must meet stringent functional and use requirements. Among the most important requirements of an aircraft hydraulic fluid is that it be stable against oxidative and hydrolytic degradation at elevated temperatures. In use, aircraft hydraulic fluids commonly become contaminated with moisture. Water enters the hydraulic system with air bled from an engine compressor stage. During operations, the moisture level in Type IV aircraft hydraulic fluids normally ranges from about 0.2 to about 0.35% by weight. Water causes hydrolytic decomposition of phosphate esters to produce partial esters of phosphoric acid. Hydrolytic breakdown of the ester is accelerated if water content exceeds about 0.5% by weight. Conventionally, phosphate ester aircraft hydraulic fluids are formulated to contain an acid scavenger which neutralizes partial esters of phosphoric acid released by hydrolytic breakdown of the triester. Over time., however, the acid scavenger becomes depleted and organometallic compounds are formed by complex reactions involving the phosphate triester, phosphoric acid partial esters, and surfaces of the metal environment within which the hydraulic fluid is ordinarily contained. These organometallic compounds, of which iron phosphate is usually the most prominent by-product, are not soluble in the hydraulic fluid. Higher performance aircraft are .operated under conditions which expose hydraulic fluids to increasing temperatures. Current Grade A fluids operate at maximum temperatures in the range of 225 to 240°F. However, projected aircraft applications will expose aircraft hydraulic fluids to bulk fluid temperatures in the range of 275°F or higher. At such temperatures, the potential for oxidative and hydrolytic breakdown of phosphate esters is substantially increased. Degradation of phosphate ester hydraulic fluids is also accelerated where the fluids are exposed to compressed air. The rate of air oxidation of such fluids also increases with temperature. Thus, for application at 275°F or higher, a need exists for fluids of both enhanced thermal oxidative stability and enhanced thermal hydrolytic stability. Erosion problems may also be expected to increase with bulk fluid temperature. Erosion is a form of electrochemical corrosion, more precisely referred to as zeta corrosion, the rates of which are increased with temperature. The incidence of cavitation, which is one of the mechanical sources of erosion problems, is also likely to increase with temperature. As erosion progresses, the presence of metallic or other insoluble components may result in filter clogging and replacement, and can cause a change in the physical and chemical properties of the fluid, thereby requiring premature ■ draining of fluids from the system. Metal contaminants also reduce oxidative stability of the fluid, accelerating corrosion. In addition to any effects resulting from contamination by metal (or other) contaminants, the fluid may suffer deterioration in numerous other ways, including: a) viscosity change; b) increase in acid number; c) increased chemical reactivity; and d) discoloration. A hydraulic fluid useful in aircraft is available from applicants' assignee under the trademark Skydrol© LD-4. This composition contains 30 to 35% by weight d. jutyl phenyl phosphate, 50 to 60% by weight tributyl phosphate, 5 to 10% of viscosity index improvers, 0.13 to 1% of a diphenyldithioethane copper corrosion inhibitor, 0.005% to about 1% by weight, but preferably 0.0075% to 0.075% of a perfluoroalkylsulfonic acid salt antierosion agent, 4 to 8% by weight of an acid scavenger of the type described in U.S. Patent 3,723,320 and about 1% by weight of 2,6-di-tertiary-butyl-p-cresol as an antioxidant. This composition has proved highly satisfactory in high performance aircraft application. However, it was not designed for extended operations at temperatures in the range of 275°F. Summary of the Invention Among the several objectε of the present invention, therefore, may be noted the provision of an improved functional fluid useful as a hydraulic fluid in aircraft applications; the provision of such a fluid which exhibits improved hydrolytic stability, especially at elevated temperatures; the provision of such a fluid which exhibits improved oxidative stability at elevated temperatures; the provision o.f such a fluid which exhibits advantageous viscosity characteristics and especially viscosity stability under shear conditions; the provision of such a fluid of relatively low density; the provision of such a fluid which has not only high resistance to oxidation but also low toxicity; the provision of such a composition which has improved anti-erosion properties; and the provision of such a fluid composition which exhibits improved resistance to corrosion of metal components of an aircraft or other hydraulic fluid system. Briefly, therefore, the present invention is directed to a fluid composition suitable for use as an aircraft hydraulic fluid. The composition comprises a fire resistant phosphate ester base stock, the base stock comprising between about 50% and about 72% by weight of a trialkyl phosphate, between about 18% and about 35% by weight of a dialkyl aryl phosphate, and from 0 to about 5% by weight of an alkyl diaryl phosphate. The alkyl substituents of the trialkyl phosphate and the dialkyl aryl phosphate contain between 3 and 8 carbon atoms and are bonded to the phosphate moiety via a primary carbon atom. The composition further contains 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; 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; a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity index 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; and an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxygen. Preferably, the alkyl substituents of the trialkyl phosphate and dialkyl aryl phosphates are substantially C 4 or C 5 , most preferably isobutyl or isopentyl. The composition is further directed to a fluid composition suitable for use as an aircraft hydraulic fluid and containing a novel combination of additives. The composition comprises a fire resistant phosphate ester base stock comprising between about 10% and about 90% by weight of a trialkyl phosphate, between about 0 and about 70% by weight of a dialkyl aryl phosphate and from 0% to about 25% by weight of an alkyl diaryl phosphate. The alkyl substituents of the trialkyl phosphate and dialkyl aryl phosphate contain between 3 and 8 carbon atoms and are bonded to the phosphate moiety via a primary carbon atom. The composition further comprises a viscosity index improver in a proportion of between about 3% and about 10% by weight of the composition. The viscosity index improver comprises 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. The composition further comprises an anti-erosion agent in a proportion of between about 0.02% and about 0.08% by weight of the composition, the anti-erosion agent comprising an alkali metal salt of a perfluoroalkylsulfonic acid, the alkyl substituent of which is hexyl, heptyl, octyl, nonyl or decyl. The composition comprises an acid scavenger in a proportion of between about 1.5 and about 10% by weight of the composition, the acid scavenger comprising a derivative of 3,4-epoxycyclohexane carboxylate or a diepoxide compound of the type disclosed in U.S. patent 4,206,067. The composition further contains a 2,4, 6-trialkylphenol in a proportion of between about 0.1% and about 1% by weight, a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1% by weight, and a hindered polyphenol composition selected from the group consisting of bis(3 , 5-dialkyl-4-hydroxyaryl)methane, 1,3, 5-trimeth l-2,4, 6-tris(3, 5-di-t-butyl-4-hydroxyaryl) benzene and mixtures thereof in a proportion of between about 0.3% and about 1% by weight of the composition. The alkyl substituents of trialkyl phosphate and dialkyl aryl phosphate are preferably butyl or pentyl . The invention is further directed to a fluid composition suitable for use as an aircraft hydraulic fluid comprising a fire resistant organophosphate ester base stock. The base stock comprises between about 10% and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially isobutyl or isopentyl, between about 0 and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are substantially isobutyl or isopentyl and between about 0% and about 25% by weight of an alkyl diaryl phosphate. The composition further comprises 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; an anti-erosion additive in an amount effective to inhibit flow-induced electrochemical corrosion of the flow metering edges of hydraulic servo valves in hydraulic systems; a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity index 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 about -65°F; and an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxygen. The invention is further directed to a fluid composition suitable for use as an aircraft hydraulic fluid comprising a phosphate ester base stock. The base stock comprises between about 10% and about 90% by weight of trialkyl phosphate wherein the alkyl substituents are substantially butyl or pentyl, between about 0 and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are substantially butyl or pentyl, and between about 0% and about 25% by weight of an alkyl diaryl phosphate. The composition further comprises 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; 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; a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity index 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; an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxygen; and a 4, 5-dihydroimidazole compound in an amount effective to decrease by at least about 25% the rate of breakdown at 300°F of phosphate triesters in the composition to phosphoric acid partial esters, as measured by epoxide depletion. The 4, 5-dihydroimidazole compound corresponds to the formula where R 1 is hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl or alkoxyalkenyl, and R 2 is alkyl, alkenyl or an aliphatic carboxylate. Brief Description of the Drawings Figures 1 to 12 are plots of epoxide depletion versus time for hydraulic fluid formulations tested under varying conditions of temperatures, moisture content, and other parameters; and Figure 13 is a bar graph illustrating the superior anti-corrosion properties of the functional fluid of the invention. Description of the Preferred Embodiments In accordance with the present invention, it has been discovered that a hydraulic fluid of improved thermal, hydrolytic, and oxidative stability is provided by utilizing a phosphate ester base stock which contains a high concentration of alkyl ester moieties and contains relatively small proportions of phenyl or other aryl esters. The base stock comprises a mixture of trialkyl phosphate and dialkyl aryl phosphate, in each of which the alkyl substituent is C 3 to Cg, preferably C 4 or C 5 . The alkyl substituents are bonded to the phosphate moiety via a primary carbon. Optionally, the base stock further contains a small proportion of alkyl diaryl phosphate. Further advantages are realized if the alkyl substituents of the trialkyl and dialkyl aryl phosphate esters are primarily constituted of isobutyl or isopentyl, in preference to the normal isomers thereof. In this preferred instance also, attachment of the alkyl substituent to the phosphate should be via a primary carbon. In addition to the improved base stock, the composition of the invention preferably contains a combination of additives which further enhances the properties of the fluid as compared to fluids previously available in the art for use in the aircraft hydraulic systems. Moreover, it has been found that the additive combinations of this invention are effective in enhancing the properties of base stock compositions previously known in the art or otherwise differing from the preferred base stock of the functional fluids of this invention. But the most advantageous properties are realized using both the additive package and the base stock of the invention, especially where the alkyl substituents of the trialkyl phosphate and dialkyl aryl phosphate are isobutyl or isopentyl. The preferred base stock is characterized by a very low alkyl diaryl phosphate ester content, preferably not more than about 5% by weight, more preferably not more than about 2% by weight. It is further preferred that the sum of the proportions of esters containing an aryl substituent, i.e., dialkyl aryl, alkyl diaryl, and triaryl phosphates, does not constitute more than about 25% by weight of the base stock: More particularly, it is preferred that the base stock composition comprise between about 50 and about 72% by weight of a trialkyl phosphate where the alkyl substituent is substantially C 4 or C 5 , between about 18% and about 35% by weight of a dialkyl aryl phosphate in which the alkyl substituent is substantially C 4 or C 5 and from 0 to about 5% by weight of an alkyl diaryl phosphate. Preferably the aryl substituents are phenyl or alkyl-substituted phenyl such as, for example, tolyl, ethylphenyl or isopropylphenyl. As contrasted, for example, with Skydrol® LD-4 hydraulic fluid, which has a significantly higher diphenyl ester content, the base stock of the functional fluid of the invention exhibits significantly improved hydrolytic stability at temperatures substantially above 225°F using the same acid scavenger system as that incorporated in LD-4. Using the same anti-oxidant additive as LD-4, a composition comprising the base stock of this invention exhibits significantly enhanced thermal oxidative stability. As a result of the relatively low diphenyl ester content of the base stock, the functional fluid of the invention has relatively low density, which is advantageous in aircraft hydraulic fluid applications. In the preferred base stock of the invention, it is particularly preferred that the alkyl substituents be isobutyl or isopentyl, most preferably isobutyl. It has been found that a base stock composition comprising triisobutyl or triisopentyl phosphate and diisobutyl or diisopentyl phenyl phosphate affords multiple advantages as compared to same compositions in which the alkyl substituents are n-butyl and n-pentyl. Toxicity studies indicate that the isobutyl and isopentyl esters are of even lower toxicity than their n-butyl and n-pentyl counterparts. In particular, the isobutyl and isopentyl esters causes less dermal sensitization than the normal alkyl esters. Systemic toxicity is also lower. Table A compares the toxicity properties of butyl vs. isobutyl phosphate esters. Table A TBP TIBP Oral LD 50 1200 mg/kg >5000 mg/kg Dermal LD 50 >10,000 mg/kg >5000 mg/kg Eye Irritation mildly irritating practically non-irritating Skin Irritation severely irritating moderately irritating TBP TIBP Subchronic Bladder hyperplasia in d rats >1000 ppm none observed in ? rats >5000 ppm NOEL 200 ppm NOEL 5000 ppm Hen Neurotox not neurotoxic not neurotoxic tested at LD 50 = tested at LD 50 1500 mg/kg > 5000 mg/kg Genotoxicity Ames not yet tested CHO/HGPRT in vitro cytogenetics in vivo cytogenetics Significantly, in the context of the present invention, the isobutyl and isopentyl esters have further been found to exhibit hydrolytic stability superior to that of the corresponding normal esters at the high temperatures to which the hydraulic systems of high performance aircraft are exposed. Isobutyl and isopentyl esters also contribute markedly to seal integrity, the materials of which hydraulic system seals are commonly fabricated being found much less subject to swelling when in contact with the isoalkyl esters than in contact with the corresponding normal esters. Moreover, it has been found that the isobutyl and isopentyl esters are even lower density than the normal alkyl esters, which means that the weight of fluid in a given aircraft hydraulic system is lower, resulting in improved aircraft fuel efficiency. In addition to the improved base stock, the composition of the invention preferably contains a combination of additives which further enhances the properties of the fluid as compared with fluids previously available in the art for use in aircraft hydraulic systems. More particularly, the composition incorporates an acid scavenger in a proportion sufficient to neutralize phosphoric acid partial esters formed in situ by hydrolysis of components of the phosphate ester base stock under conditions of the service in which the hydraulic fluid composition is used. Preferably, the acid scavenger is a 3,4-epoxycyclohexane carboxylate composition of the type described in U.S. patent 3,723,320. Also useful are diepoxides such as those disclosed in U.S. patent 4,206,067 which contain two linked cyclohexane groups to each of which is fused an epoxide group. Such diepoxide compounds correspond 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^ are independently selected from among hydrogen and aliphatic groups containing 1 to 5 carbon atoms. Exemplary diepoxides include 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane, bis (3,4-epoxy-6-methylcyclohexylmethyl adipate) , 2-(3,4-epoxycyclohexyl)-5,5-spiro(3,4-epoxy)cyclohexane-m- dioxane. The concentration of the acid scavenger in the fluid composition is preferably between about 1.5% and about 10%, more preferably between about 2% and about 8% by weight, which is generally sufficient to maintain the hydraulic fluid in a serviceable condition for up to appropriately 3000 hours of aircraft operation. To limit the effect of temperature on viscosity, the composition further includes a polymeric viscosity index improver. Preferably, the viscosity index improver comprises a poly(alkyl methacrylate) ester of the type described in U.S. Patent 3,718,596. Generally, the viscosity index improver is of high molecular weight, having a number average molecular weight of between about 50,000 and about 100,000 and a weight average molecular weight of between about 200,000 and about 300,000. Preferably, the viscosity index improver of the invention has a relatively narrow range of molecular weight, approximately 95% by weight of the viscosity index improver component having a molecular weight of between about 50,000 and about 1,500,000. This result is achieved in part by utilization of predominantly butyl and hexyl methacrylate esters. The viscosity index improver is present in a proportion sufficient to impart a kinematic viscosity of: at least about 3.0, preferably between about 3 and about 5 centistokes at 210°F; at least about 9, preferably between about 9 and about 15 centistokes at 100°F; and not more than about 4200 centistokes at -65°F. Superior shear stability characteristics are also imparted by the viscosity index improver used in the composition. Preferably the fluid composition contains between about 3% and about 10% by weight of the viscosity index improver. A particularly preferred viscosity index improver is that sold under the trade designation PA6703 and/or PA6477 by Rohm & Haas. The viscosity index improver is conveniently provided in the form of a solution in a phosphate ester solvent, preferably a trialkyl phosphate ester such as tributyl or triisobutyl phosphate, or a combination of alkyl and phenyl derivatives. The proportions referred to above for the viscosity index improver are on a solids (methacrylate polymer) basis. The phosphate ester solvent becomes in effect part of the base stock, and the ranges of proportions of phosphate esters, as discussed above, reflect the phosphate ester added as a vehicle for the viscosity index improver. An anti-erosion agent is incorporated in an amount effective to inhibit flow-induced electrochemical corrosion, more precisely referred to as zeta corrosion. The anti-erosion additive is preferably an alkali metal salt, more preferably a potassium salt of a perfluoroalkylsulfonic acid. Such anti-erosion additives are more fully described in U.S. Patent 3,679,587. Typically, the alkyl component comprises hexyl, heptyl, octyl, nonyl, decyl, or mixtures thereof, with perfluorooctyl generally affording the best properties. It is particularly preferred that the anti-erosion agent predominantly comprises the potassium salt of peirfluorooctylsulfonic acid in a proportion of between about 250 and about 1000 most preferably at least about 500 ppm. In the operation of an aircraft hydraulic fluid system, the sulfonic acid moiety of the anti-erosion agent tends to lower the surface tension of the hydraulic fluid and thereby better cover the metal surfaces with which the hydraulic fluid normally comes in contact. The metering edges of servo valves are generally the most important metal parts which need protection from electrochemical corrosion. Positive ions in the fluid, including the alkali metal ion of the anti-erosion agent, are adsorbed onto the metal surface and neutralize the negative charges on the metal that are otherwise created by the rapid flow of the hydraulic fluid over the servo valve metering edges. Enhanced erosion resistance is provided in the composition of the invention, which preferably contains a perfluoroalkylsulfonic salt content about twice that of the prior art composition sold as LD4. Limiting the diaryl ester content of the base stock contributes to thermal, oxidative, and hydrolytic stability of the fluid. The composition of the invention also contains a combination of antioxidant additives, preferably including both a hindered phenol and a hindered polyphenol. Hydrolytic stability has been found to be improved by partially substituting the hindered polyphenol for the phenol, and it is thus preferred that the composition contain not more than about 1.0%, preferably not more than about 0.7% by weight of a phenol such as a 2,4 , 6-trialkylphenol . It is generally preferred that the composition contain between about 0.1% and about 0.7% of a 2, 4 , 6-trialkylphenol, preferably 2 , 6-di-tertiary-butyl-p-cresol ("Ionol"). The composition should further include between about 0.3% and about 1% of a hindered polyphenol composition, such as a bis(3 , 5-dialkyl-4-hydroxyaryl) methane, for example, the bis(3 , 5-di-tertiary butyl-4-hydroxy phenyl) methane sold under the trade designation Ethanox® 702 by the Ethyl Corp., a l,3,5-trialkyl-2,4,6-tris(3,5 dialkyl-4-hydroxyaryl) aromatic compound, for example, the 1, 3 , 5-trimethyl-2,4, 6-tris(3, 5-di-tertiarybutyl-4- hydroxyphenyl)benzene sold under the trade designation Ethanox® 330 by the Ethyl Corp., or mixtures thereof. The composition may also include an amine antioxidant, preferably a diarylamine such as, for example, phenyl-α-napthylamine or alkylphenyl-α-naphthylamine, or the reaction product of N-phenylbenzylamine with 2,4,4-trimethylpentene sold under the trade designation Irganox® L-57 by Ciba-Geigy; diphenylamine, ditolylamine, phenyl tolylamine, 4, 4 '-diaminodiphenylamine, di-p-methoxydiphenylamine, or 4-cyclohexylaminodiphenylamine; a carbazole compound such as N-methylcarbazole, N-ethylcarbazole, or 3-hydroxycarbazole; an aminophenol such a N-butylaminophenol, N-methyl-N-amylaminophenol, or N-isooctyl-p-amino-phenol; an aminodiphenylalkane such as aminodiphenylmethanes, ,4 '-diaminodiphenylmethane, etc., aminodiphenylethers ; aminodiphenyl thioethers; aryl substituted alkylenediamines such as 1,2-di-o-toluidoethane, 1,2-dianilinoethane, or 1, 2-dianilinopropane; aminobiphenyls, such as 5-hydroxy-2-aminobiphenyl, etc.; the reaction product of an aldehyde or ketone with an amine such as the reaction product of acetone and diphenylamine; the reaction product of a complex diarylamine and a ketone or aldehyde; a morpholine such as N-(p-hydroxyphenyl)morpholine, etc.; an amidine such aε N,N'-bis-(hydroxyphenyl)acetamidine or the like; an acridan such aε 9 , 9 '-dimethylacridan, a phenathiazine such as phenathiazine, 3, 7-dibutylphenathiazine or 6, 6-dioctylphenathiazine; a cyclohexylamine; or mixtures thereof. An alkyl substituted diphenylamine such as di(p-octylphenyl) amine is preferred. Certain amine components can also act as a lubricating additive. The amine antioxidant is also preferably present in a proportion of between about 0.3 and about 1% by weight. By maintaining the lonol content of the fluid composition below 1.0%, preferably below 0.7%, and more preferably below 0.5% by weight, toxicity of the composition is even lower than that of Skydrol® LD-4 hydraulic fluid. As a copper corrosion inhibitor, the composition of the invention preferably includes a benzotriazole derivative, such as that sold under the trade designation Petrolite 57068. This corrosion inhibitor is present in an amount sufficient to deactivate metal surfaces in contact with the fluid composition against the formation of metal oxides on the metal surfaces in contact with the fluid, thereby reducing rates of copper dissolution into the hydraulic fluid, and also reducing dissolution of perhaps parts fabricated from copper alloys. Advantageously, the composition contains between about 0.005% and about 0.09% by weight of the benzotriazole derivative, preferably between about 0.02 and about 0.07% by weight. Phosphate ester functional fluids are known to corrode iron alloys as well as copper alloys. Numerous iron corrosion inhibitors are available for use in functional fluids, but these are known in many instances to increase rateε of eroεion and thuε have a net deleterious effect on the performance properties of the hydraulic fluid. However, in accordance with the invention, it has been discovered that certain 4 , 5-dihydroimidazole compoundε are effective iron corrosion inhibitors ' , yet do not adversely affect the erosion properties of the fluid. Useful 4, 5-dihydroimidazole compounds include those which correspond to the structural formula where R 1 is hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl or alkoxyalkenyl, and R 2 is alkyl, alkenyl or an aliphatic carboxylate. Exemplary groups which may constitute R^ include hydrogen, methyl, ethyl, propyl, butyl, pentyl, octyl, vinyl, propenyl, octenyl, hexenyl, hydroxyethyl, hydroxyhexyl, methoxypropyl, propoxyethyl, butoxypropenyl, etc. Exemplary group, which may constitute R 2 include, octyl, dodecyl, hexadecyl, heptadecenyl, or a fatty acid substituent such as 8-carboxyoctyl, 12-carboxydodecyl, 16-carboxyhexadecenyl, or 18-carboxyoctadecyl . In a particularly effective embodiment, R 1 is hydrogen or lower alkyl and R 2 is a fatty acid residue containing at least about 9 carbon atoms, i.e., -Cg-COOH to -C 18 COOH, preferably C j g-C^g-COOH. In another preferred embodiment, R 1 iε a lower hydroxyalkyl and R 2 is a Cg-C 18 alkenyl. In the latter instance, however, the most satisfactory inhibition of Fe corrosion is realized only if the 4 , 5-dihydro-imidazole is used in combination with an amino acid derivative, more particularly an N-substituted amino acid in which the N-substituent contains both polar and oleophilic moieties, for example, an N-alkyl-N-oxo-alkenyl amino acid. It has further and unexpectedly been discovered that the presence of such a , 5-dihydroimidazole compound, typically in a proportion of between about 0.01% and about 0.1% by weight, not only inhibits iron corrosion but contributes markedly to the stability of the functional fluid as indicated by epoxide depletion. It has been found that the salutary effect of the .4 , 5-dihydroimidazole compound is enhanced if it iε uεed in combination with a phenolic antioxidant, especially a complex hindered polyphenol such as a bis (3 , 5-dialkyl-4-hydroxyaryl) methane or a 1, 3 , 5-trialkyl-2, 4, 6-tris(3, 5-t-butyl-4- hydroxyaryl) aromatic compound. Optimal effect on stability has been observed using a combination of the condensation product of 4 , 5-dihydro-lH-imidazole and 16 -C 18 fatty acid (sold under the trade designation Vanlube RI-G by the Vanderbilt Co.) with a hindered polyphenol and an alkyl substituted diarylamine such as di(p-octylphenyl)amine. Also effective as a 4 , 5-dihydroimidazole compound in such combination is 2-(8-heptadecenyl)-4 , 5-dihydro-lH-imidazole-l-ethanol (sold under the trade designation Amine-0 by Ciba-Geigy) to function as an iron corrosion inhibitor, the latter compound is preferably used in combination with an amino acid derivative such as, e.g., the N-methyl-N(l-oxo-9-octadecenyl)glycine sold under the trade designation Sarkoεyl®-0 by Ciba-Geigy. To function as an iron corroεion inhibitor, the latter compound should be used in combination with an amino acid derivative such as, e.g., the N-methyl-N(l-oxo-9-octadecenyl) glycine sold under the trade designation Sarkosyl®-0 by Ciba-Geigy. It has been found that a still further enhancement in high temperature stability is realized where the 4 , 5-dihydroimidazole compound is used in combination with a base stock in which the ester substituents are substantially isobutyl or isopentyl. Although they have not been found to produce the subεtantial advantageous effect on high temperature stability that iε afforded by the use of an a 4 , 5-dihydroimidazole compound, other iron corrosion inhibitors have been found effective in the functional fluid of the invention without adverse effect on erosion characteristics. Acceptable iron corrosion inhibitors include, for example, the product sold by Petrolite under the trade designation Petrolite P-31001. As necessary, the fluid composition may also contain an anti-foaming agent. Preferably, this is a silicone fluid, more preferably a polyalkylsiloxane, for example, the polymethylsiloxane sold under the trade designation DC 200 by Dow Corning. Preferably the anti-foam agent is included in a proportion sufficient to inhibit foam formation under the test conditions of ASTM method 892. Typically, the anti-foam content of the compoεition iε at leaεt about 0.0005% by weight, typically about 0.0001% to about 0.001% by weight. Preferably, the pH of the composition of the invention iε at least about 7.5, more preferably between about 7.5 and about 9.0. To impart a pH in this range and to enhance the acid scavenging capacity of the formulation, the composition may further include between about 0.0035 and about 0.10%, preferably between about 0.01% and about 0.1% by weight, most preferably between about 0.02% and about 0.07% of an alkali metal phenate or other arylate. Potassium phenate is preferred. In addition to neutralizing . acidic components of the composition, the alkali metal arylate serveε to pacify the metal εurfaceε when the compoεition haε been added to a hydraulic εyεtem, thereby reducing corroεion. Although optimal propertieε are realized in a compoεition of low alkyl diaryl phoεphate content and particularly in compoεitionε using the base stock of the invention as described above, the additive combination of the invention also affords beneficial results when used in combination with any of a variety of base stock compositions known to the art. The benefit of using esters whose alkyl substituents are predominantly comprised of isobutyl or isopentyl also extends beyond the preferred concentration ranges outlined above. Broadly, the additive combination can be used with an organophosphate ester base stock comprising between about 10% and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially butyl are pentyl, between about 0 and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl subεtituentε are substantially butyl or pentyl, and between about 0% and about 25% by weight of an alkyl diaryl phosphate. More preferably, the additive combination is used with a baεe stock compriεing between about 35% and about 90% by weight of a tributyl or tripentyl phosphate, between about 0% and about 35% by weight of a dibutyl aryl or dipentyl aryl phosphate, and between about 0% and about 20% by weight of a triaryl phoεphate. The additive combination is also effective in combination with other ranges of baεe εtock compoεitionε aε set forth below: Table 1 Weight % Baεe Base Base Baεe Eεter Stock I Stock II Stock III Stock IV Tri(C 4 /C 5 alkyl) 10-72% 10-25% 50-72% 80-90% Di(C4/C5 alkyl) Aryl 18-70% 45-70% 18-25% Alkyl diaryl 0-25% 5-25% 0-10% Triaryl 10-20% Aε discussed hereinabove, optimal properties are achieved by combining the preferred isobutyl and isopentyl ester base stock with the additive combination of the invention. However, εignificant benefitε in lower toxicity, lower density, hydrolytic stability, thermal stability, and seal integrity are afforded by the use of the isoalkyl esters with other additive combinations as well. Preferably, the isoalkyl ester base stock contains between 50 and about 72% by weight of a trialkyl phosphate wherein the alkyl substituents are subεtantially iεobutyl or iεopentyl, between about 18 and about 35% by weight of a dialkylaryl phosphate wherein the alkyl substituents are substantially isobutyl or isopentyl and between 0 and about 10% by weight, preferably between about 0 and 5% by weight, of an alkyl diaryl phosphate. However, the benefits of using the isoalkyl substituents are so εubεtantial that they are realized to a εignificant extent over a conεiderably broader range of compoεition. Generally, therefore, a base stock which utilizes isoalkyl esterε may comprise between about 10% and about 90% by weight of a triisobutyl or triisopentyl phosphate, between about 0 and about 70% by weight of a diisobutyl or diisopentyl aryl phoεphate and between about 0 and about 25% by weight of an alkyl diaryl phoεphate. Preferably, the alkyl substituent of the alkyl diaryl phosphate is also isobutyl or isopentyl, especially when the alkyl diaryl phosphate content exceeds about 5%. The aryl substituent of these esters is typically phenyl but may also be an alkylphenyl such as tolyl, ethylphenyl or isopropyl phenyl . The isoalkyl base stock should be combined with an acid scavenger in an amount effective to neutralize phosphoric acid partial esters formed in εitu by hydrolyεiε of any of the phosphate esters of the base stock. The acid scavengers described above are preferred but other acid scavengers known to the art may be used. The isoalkyl based functional fluids should also contain an antierosion additive in an amount effective to inhibit flow induced electrochemical corrosion of flow metering edges of hydraulic servo valves in hydraulic systems. These fluids should also contain a viscosity index improver in an amount effective to cause the fluid composition to exhibit the viscosity index stated above. The composition should further include an antioxidant in an amount effective to inhibit oxidation of the fluid composition components in the presence of oxidizing agents. Preferably, the anti-erosion agent, viscosity index improver, and antioxidant composition are as described above, but the benefits of the use of an isoalkyl base εtock are alεo realized with other additive combinationε known to the art. Methodε known to those εkilled in the art may be used for the preparation of the compositionε of the invention. For example, a base stock comprising the phosphate esters may be prepared by mixing in an agitated stainless steel vessel. Additives may then be blended into the base stock in the same vessel. As noted above, the viscosity index improver is preferably added in the form of a solution in a phosphate ester solvent. At temperatures above 200°F, the more preferred functional fluid compositions of the invention exhibit thermal, oxidative, and hydrolytic stability two to three times greater than that of Skydrol® LD-4 hydraulic fluid as measured by the depletion of epoxide acid scavenger as a function of time. Superior stability iε exhibited even in the presence of halogen-containing compounds such as trichloroethane. When a 4,5-dihydroimidazole compound is included, the extent of improvement is even greater. Aε a reεult of the relatively low phenyl eεter content, the composition of the invention has a denεity of leεε than one gram per cc, typically between about 0.98 and about 0.99 gramε per cc. This is a desirable feature from the standpoint of fuel burn (consumption) in aircraft. Shear stability of the fluid compoεition alεo compareε favorably with commercially available aircraft hydraulic fluidε. Thuε, for example, after 500 hour expoεure to an accelerated degradation teεt in a typical aircraft hydraulic pump εyεtem, the viεcoεity of the compoεition at -65° dropε only from 4000 to 2400. In part, thiε advantage iε believed to result from the narrower range of molecular weight of the viεcoεity index improver. Exposure to εhear conditions tends to degrade higher molecular weight viεcosity index improvers, so that compositions in which the molecular weight of the viscoεity index improver is distributed over a broad range tend to suffer a greater losε of effectiveneεε over time due to breakdown of the higher molecular weight species. In part due to the relatively low concentration of 2, 6-di-tertiary-butyl-p-cresol, the toxicity of the fluid compoεition in the invention is very low. Where an isoalkyl ester base εtock iε uεed, toxicity is even lower, The following examples illustrate the invention, Example 1 A hydraulic fluid having the composition set forth in Table 1 was prepared by mixing at ambient temperature in a 50 gallon stainless steel tank agitated with a 25 horsepower agitator having an anchor type impeller. The phosphate ester components were introduced into the tank first and, after a 30 minute period of initial mixing, the other additives were added in the sequence indicated in Table 2. Table 2 Basis: Basis: 100 Gram 80 Gallon Batch Batch Component Grams Grams / Pounds Tributyl Phosphate, Neat 49.0135 148,216.8 / 326.8 Dibutyl Phenyl Phosphate 26.34 79,652.2 / 175.6 DRUM Of Low Diphenyl 2(~220#) Content (Less Than 2% By Weight) Methacrylate Ester 16.56 50,077 / 110.4 Viscoεity Index 22684.9 Improver (PA6477, gSLDS 45.3% εolidε in 54.7% tributyl phoεphate) 3,4 Epoxycyclohexane 6.3 19,051 / 42 Carboxylate Potassium 05 151.2 / Perfluoroctylsulfonate (FC98) Benzotriazole type 05 151.2 / Copper Corrosion Inhibitor Table 2 Cont'd (P57068,Petrolite (50% Active) , EXI663 Iron Corrosion Inhibitor .05 151.2 / (90-31001,Petrolite (50% Active) Dye .001 3.024 / Potasεium Phenate .035 105.84 / Biε-(3, 5-Di-tertiary .90 2,722 / 6 Butyl-4-Hydroxyphenyl) Methane (Ethanox® 702) Di(p-octylphenyl) amine 0.45 1,361 / 3 2, 6-di-t-butyl-p-creεol 0.25 756 / 1.667 Antifoam (Dow-Corning) 0.0005 1.512 / Thiε compoεition had a denεity of 0.996 g/cc at a temperature of 25°C. Of the εource of dibutyl phenyl phoεphate, 77.135% by weight was dibutyl phenyl phosphate or butyl diphenyl phosphate, so that 20.3% by weight of the overall composition waε conεtituted of phosphate esterε containing a phenyl moiety. However, the butyl diphenyl phoεphate content waε leεε than 1% by weight. Triphenyl phoεphate content waε eεεentially nil. Example 2 A εecond aircraft hydraulic fluid compoεition was prepared in the manner generally described in Example 1. The composition of thiε fluid is set forth in Table 3, Table 3 Density of Basis: Components 100 Gram Batch Variableε Grams Tributyl Phosphate 50.5988 Dibutyl Phenyl Phosphate 24.0947 Of Low Diphenyl Content (Less Than 10 2% By Weight) Methacrylate Ester 22,684.9 Viεcoεity Index gSLDS Improver (PA6477, Total 43.8% εolids/56.2% 15 tributyl phosphate) 3,4 Epoxycyclohexane 6.3 19,051 42 Carboxylate Potassium 05 151.2 Perfluorooctylsulfonate 20 (FC98) Benzotriazole Type 05 151.2 Copper Corrosion Inhibitor (P57068,Petrolite; 25 50% Active) Iron Corrosion Inhibitor 05 151.2 (90-31001,Petrolite (50% Active) , EXI663 Dye 30 Potaεεium Phenate Bis-(3, 5-Ditertiary Butyl-4-Hydroxy Phenyl) Methane (Ethanox 702) Table 3 Cont'd Di(p-octylphenyl) amine .45 1,361 / 3 Dow Corning Anti-Foam .0005 1.512 / 2,6 Di-tertiary-Butyl- .25 756 / 1,667 P-Cresol This composition also exhibited a density of 0.996 g/cc at a temperature of 25°C. Of the source of dibutyl phenyl phosphate, 84.751% by weight was constituted of esters which contained no phenyl moiety. The overall composition contained 20.3% by weight of phosphate esters having a phenyl moiety, but less than 1% by weight butyl diphenyl phosphate and essentially no triphenyl phosphate, Set forth in Table 4 are a partial elemental analysiε and meaεured phyεical properties of the compoεitionε of Exampleε 1 and 2. These data establish that the fluid composition of Examples 1 and 2 meet or exceed the airframe manufacturers' specification, for properties needed to qualify a product for use as an aircraft hydraulic fluid. Table 4 Table 4 Cont'd AIT,F 850 920 FLASH PT. 350 360 FIRE PT. 360 390 CONDUCTIVITY .65 .55 OXIRANE NO. .39 .40 FOAM SEQ 1 170/65 180/20 2 30/10 40/44 3 80/35 140/56 PARTICLE COUNT 5-15 7247 3116 15-25 1444 513 25-50 460 180 50-100 75 53 >100 14 10 SILTING INDEX 1.18 1.05 Example 3 Tests were conducted comparing the thermal, oxidative and hydrolytic stability of the hydraulic fluid compositionε of Exampleε 1 and 2 with commercially available hydraulic fluids. In each of these testε, a 301 εtainless steel tube was filled to 80% capacity with the fluid to be tested. The temperature was maintained constant in each test. Comparative testε were run at 250°F and 275°F, and further tests of the compoεition of the invention were run at 300°F In all teεtε, five corrosion coupons were immersed in the fluid. In some of the testε, the head εpace in the tube waε filled with air, in otherε it waε filled with nitrogen. After each tube was filled with the appropriate teεt compoεition, it waε capped and heated to a predetermined teεt temperature and maintained at that temperature so that hydrolytic stability at εuch temperature could be determined. Each tube waε monitored over time and εampleε were taken to follow trends in the fluid's chemical composition, in particular the concentration of the acid scavenger (epoxide) present in the sample. When the epoxide is 100% depleted, the fluid is typically degraded to the point that itε usefulness as an aircraft hydraulic fluid has essentially been exhausted. As epoxide depletion approached 100%, test specimens were titrated for acidity. When the neutralization number of the fluid reached 1.5 or greater, the test was halted. Illustrated in Figs. 1 to 3 are epoxide depletion curves for the compositions of the invention as compared to previously available aircraft hydraulic fluids. In these curves, and in those relating to the further examples set forth below, the legends "W17" and "W17R" deεignate a compoεition of Table 1 or 2 above. "2495B1" referε εpecifically to the composition of Table 1, and "2495B2" to the composition of Table 2. "H4A" refers to commercial hydraulic fluid sold by Chevron under the trade designation "Hyjet IVA®." "Epox A" meanε that the test was run with air in the head space of the stainleεε steel tube, so that the test εpecimen was exposed to thermal, hydrolytic, and oxidative effects. "Epox T" means that the head εpace contained nitrogen, so that the test primarily measured thermal hydrolytic effects only. Example 4 Further thermal, hydrolytic, and oxidative stability teεts were conducted on the compoεitionε of Example 1 and 2. Theεe teεts were carried out generally in the manner described in Example 3, except that 0.5% moisture was incorporated in the test samples to determine the effect of moisture on thermal stability. Test temperatures were 250°F and 275°F. The results of these testε are plotted in Figs. 4 and 5. Example 5 Additional thermal, oxidative, and hydrolytic stability tests comparing the compositionε of the invention with thoεe previously available in the art were conducted in sealed pyrex tubes. In certain of the testε, corroεion couponε were immerεed in the liquid contained in the pyrex tube. Except for the use of pyrex rather than stainless steel tubes, the tests were conducted in essentially the manner described in Example 3. Both the compositions of the invention and comparative fluids were tested at 300°F in the preεence of 0.1 to 0.5% moisture with five corrosion coupons immersed in the test samples. The results of these teεtε are εet forth in Figε. 6 to 8. Additional teεts on the compositions of the invention were conducted at 375°F without moisture addition. The results of these tests are set forth in Fig. 9. Example 6 Further thermal, oxidative, and hydrolytic stability teεtε were conducted generally in the manner deεcribed in Example 3, except that trichloroethane waε added, in varying amounts, to the teεt specimens in order to determine the effect on stability. Test temperatureε were 275°F and 300°F. The results of the testε of this example are set forth in Figε. 10 and 11. Example 7 The oxidation and corroεion reεiεtance of the fluid compositionε of Exampleε 1 and 2 waε compared with that of previouεly available aircraft hydraulic fluidε by teεting in accordance with federal teεt method FTM5308.7 This test severely stresseε the fluid with regard to oxidation stability. In each test the fluid was charged to a glass tube and tested in accordance with FTM 5308.7. The fluid was heated to a fixed temperature of 350°F after which dried air was purged through the test fluid at a rate of 5 liters per hour. Samples were taken every 24 hours, or more frequently, and the test was halted when the neutralization number of the fluid reached 1.5 or greater. The results of the tests in this Example are illustrated in Fig 12. Example 8 Because erosion is a form of electrochemical corrosion, erosion characteristicε of a hydraulic fluid compoεition can be measured by wall currents obtained during flow of the fluid through small simulated orifices εimilar to thoεe in a test servo valve. Using a standard erosion test apparatus, testε were conducted comparing the eroεion propertieε of the compoεitionε of Exampleε 1 and 2 with aircraft hydraulic fluid compositions previously available to the art. In this test εyεtem, favorable eroεion propertieε were indicated by low wall currentε and the moεt favorable characteriεticε are indicated by a negative wall current. Set forth in Table 5 is a summary of the data obtained in testing the compositionε of the invention and thoεe previously available commercially. Further erosion testε were conducted on variouε functional fluid compoεitions afte.r storage in glass containers at contact with air at 225°F. Set forth in Table 6 are the resultε of theεe tests for samples stored for the indicated number of hours. In these tables, two meaεurementε are reported for conductivity of the specimen, one taken by applicant's aεεignee and the other by an outεide teεting laboratory. I w deεignateε wall current, i t deεignateε threshold current, and R v is the rate of erosion. R v is related to I w and i t by the function: • R v = 150I W - 18i t In Tableε 5 and 6, the term: "LD4" refers to the product sold under the trademark "Skydrol® LD-4" by Monsanto; "SKY500B" and "B4" refer to another functional fluid product available from Monsanto under the trade designation "Skydrol® 500B4"; "LD5" refers to the composition of the invention; "FC96" refers to an antierosion agent compriεing a potassium salt of perfluorohexylsulfonic acid; "Ca+2" referε to the preεence of Ca +2 di(perfluoromethylsulfonate) in a tested fluid; "AO" means that an antioxidant was preεent, typically a combination of lonol and a hindered polyphenol such as bis(3 , 5-di-t-butylhydroxyphenyl)methane; "XI" with reference to the antieroεion agent in LD-4 meanε that the antierosion- agent FC98 is present in the standard commercial concentration; "X2" and "X3" mean that the FC98 concentration has been doubled or tripled; "TBP" refers to tributyl phosphate; "DBPP" refers to dibutyl phenyl phosphate; "TEHP" referε to triethylhexyl phoεphate; "Si-HC" refers to a tetraalkyl silane composition; "HT" is used to deεignate Skydrol® HT, a functional fluid formulation that haε been sold by applicant's assignee; "TiBP" refers to triisobutyl phosphate; "FC98" refers to an antierosion agent comprising a potasεiu εalt of perfluorooctylsulfonic acid; "EXI 663" refers to a benzotriazole Cu corrosion inhibitor; 31001 refers to a Petrolite Fe corrosion inhibitor; HALS refers to a hindered amine light stabilizer; "H4A" refers to various εampleε of the functional fluid sold commercially by Chevron under the trade deεignation Hyjet IVA; "W6" , "W7", "W8," etc. refer to. the compositions of the invention; "ERT" means the εpecimen had been use in Erosion Resiεtance Teεtε; and "ECT" means the specimen had been used in Eroεion Control Tests. Table 5 EROSION TEST DATA SUMMARY 4>- 10 15 IΛI 10 15 I 10 15 20 10 15 20 O-J OO 10 15 20 Table 6 Erosion Test Data After Oven Heating 225 F, In Glasε; Air @ Start Only; Includes 1020 Steel and Cu Corr. Coupons 10 Example 9 The compositions of Examples 1 and 2 were compared with an available commercial hydraulic fluid in a storage test at 375°F in the presence of iron. After 21 hours storage at such conditions, analyses were made of the solids build-up in the fluid. More particularly, measurementε were made of the build-up of metal εolidε, other εolidε, and total εolidε. The reεultε of these teεts are illustrated in Fig. 13. Example 10 Aircraft hydraulic fluids of the invention were formulated, subεtantially in the manner deεcribed in Example 1, and εubjected to the Eroεion Reεiεtance Teεt of Boeing Material Specification for Fire Reεistant Hydraulic Fluid, BMS 3-11G (Rev. 7/17/86). Set forth in Tables 7, 7A, and 7B are the compositionε of the fluidε teεted. Set forth in Table 8 are the reεultε of the erosion tests. Set forth in Tables 9 and 9A is a comparison of the propertieε of the fluidε before and after εubjection to the eroεion teεtε. In theεe tableε, "HF 400," "HF-411," and "HF-460" refer to poly(butyl/hexyl methacrylate) viεcoεity index improvers. In each entry, the table stateε the butyl methacrylate polymer εolidε content, the balance being trialkyl phosphate solvent. "AEA" refers to an antieroεion agent, "PANA" designates phenyl-α-napthylamine; "APANA" designates an alkylphenyl-α-naphthylamine. "DODPA" refers to di(p-octylphenyl) amine; "P58526 Petrolite" is an iron corrosion inhibitor; "DC 200, 100 CST" is a Dow-Corning antifoam; "SARK 0" referε to the N-methyl-N-l-OXO-9-octadenyl) glycine εold under the trade deεignation "Sarkosyl-O" by Ciba-Geigy; "AMINE O" refers to the 2-(8-heptadecenyl)-4,5-dihydro-lH-imidazole-l-ethanol sold under the trade designation "Amino-O" by Ciba-Geigy; "90-31001" refers to Petrolite 31001; and "FH-132" refers to diphenyldithioethane. 10 15 •P- J 10 15 20 Table 7 Cont'd DC 200,100 CST .0005 .0005 0005 SARK 0 .004 AMINE 0 .004 5 FH132 25 J VARIABLES TBP,REDIST.. 5 TBP . 50.844 50.8935 DBPP,LOW DI-PHENYL,ROD/C2 30. 25. 25. DBPP,LOW DI-PHENYL,ROD/C4 HF400,43.6%S/7.5%FINAL . HF411,35.5%S/3.75%FINAL 10.42 10.275 10.275 . P , I 10 HF460,58.5%S/3.75%FINAL 6.41 6.41 6.41 MCS 1562 5.8 5.8 5.8 AEA,FC98 .05 .05 .05 P57068, PETROLITE .055 .1 .1 (50% ACTIVE) 15 DYE .001 .001 .001 KP .035 .035 .01 E702 .761 PANA .625 . . APANA .76 .9 20 DODPA . .625 .45 P58528,PETROLITE . .1 .1 (50% ACTIVE) Table 7A Cont'd DC 200, 100 CST .0005 SARK 0 004 AMINE O 004 5 (1)KP, SELFMADE KP 2% BDPP IN DBPP -P- 10 J ^1 15 20 RUN NUMBER RIG USED CASE DRAIN TEMPERATURE (°F) RESERVOIR TEMPERATURE Cl ADDED, PPM TOTAL RUN TIME, HR OPERATING PROBLEMS ^ 10 BOEING VALVE DATA SLIDE AND SLEEVE NO. PORT NUMBERS FLOW INCREASE, cc/min, erratic ACCEPTABLE? 15 EDGE APPEARANCE s light PUMP DATA MANUFACTURER Abex SERIAL NO. 491761 20 HRS AT START (§225F Table 8 Cont'd HRS TO FALURE 468 856 1426 no 476 1980 failure CAUSE OF FAILURE 0 ring Oring bearings — bearings bearings shaft seal SECOND PUMP (IF USED) MFR S/N HRS AT START $ HRS TO FAILURE RUN NUMBER RIG USED CASE DRAIN TEMPERATURE (°F) RESERVOIR TEMPERATURE Cl ADDED, PPM TOTAL RUN TIME, HR OPERATING PROBLEMS BOEING VALVE DATA SLIDE AND SLEEVE NO. PORT NUMBERS FLOW INCREASE, cc/min, 15 ACCEPTABLE? EDGE APPEARANCE PUMP DATA MANUFACTURER 20 SERIAL NO. HRS AT START HRS TO FAILURE CAUSE OF FAILURE SECOND PUMP (IF USED) MFR S/N HRS AT START •vΛ 10 HRS TO FAILURE 130 RUN NUMBER 5 RIG USED ASE DRAIN TEMPERATURE (° ESERVOIR TEMPERATURE 1 ADDED, PPM TOTAL RUN TIME, HR 10 OPERATING PROBLEMS BOEING VALVE DATA SLIDE AND SLEEVE NO. PORT NUMBERS 15 FLOW INCREASE, cc/min. ACCEPTABLE? EDGE APPEARANCE PUMP DATA 20 MANUFACTURER SERIAL NO. HRS AT START HRS TO FALURE 25 CAUSE OF FAILURE Table 8 Cont'd SECOND PUMP (IF USED) MFR Abex S/N ' 116815 HRS AT START 0 HRS TO FAILURE 117 10 15 20 10 15 en o 10 15 20 120 Table 9 Cont'd 10 15 20 (l)LESS EROSION THAN H4A AT 225 F FOR 600 HRS, Table 9 Cont'd -INCR. IN INT. LEAKAGE CC'S/MIN 5 0 HR-RUN END 200< <500 HRS <600 EROSION TYPE 10 ;VISUAL EROSIVE, PUMP - YES YES ,BECK — YES PUMP RIG C - C C -_ - B A C#l O&C LIFE - - - - _ _ _ 15 SPAN (3 350 F,HRS 120 NA ICAP DATA: W15/F W15/U W15/U W17/F Bl B2 W17/U W17/U W17/,U Na .94317 3.063 .606 .56 <.5 <.5 2.05 2.39 <.5 K 84.14 601.9 46.99 76.15 82.4 85.1 35 45.5 46.62 S 79.39 64.14 87.5 59.11 63.9 61.9 60.6 561.7 69.2 20 Cu <.5 1213 9.811 <.5 <.5 <.5 9.32 95.76 11.34 Table 9 Cont'd Fe <.5 43.53 293.3 <.5 <.5 <.5 8.24 60.89 50.4 Mn < .5 .435 1.775 <.5 <.5 < .5 <.5 <.5 < .5 Zn <.5 <.5 58.02 <.5 1.76 2.16 1.09 13.22 14.17 Al .94 2.475 27.2 1.59 < .5 < .5 <.5 <.5 <.5 Cd < .5 < .5 c.5 <.5 <.5 < .5 <.5 <.5 <.5 W15/F W15/U W17/F W17/U W17/U W17/U ^ FOAM 280/170 440/268 NA 210/93 70/25 55/18 60/20 ° TEST 240/130 10 (250/100)F (400/250)U Example 11 Formulations were prepared which substantially corresponded to the compositions of Example 1, except that the trialkyl phosphate and dialkyl aryl phosphate components were triisobutyl phosphate and diisobutyl phenyl phosphate, respectively, and the compositions varied with respect to the compound included as an iron corrosion inhibitor. Erosion valve leakage tests were run on these compositions in the manner described in Example 9, and epoxide depletion tests were conducted on these compositions generally in the manner described in Example 1. The results of these tests are set forth in Table 10. The table indicates that composition M-l used a "combination" of antioxidants . Initially, M-l contained lonol, Ethanox 702 and di(p-octylphenyl)amine (DODPA). After the erosion test had progressed for 25 hours, further amounts of Ethanox 702 and DODPA were added to the composition. At 153 hours, a phenolic antioxidant was added; at 267 hours, an amine antioxidant was added; and at 503 hours a mixture of Ethanox 703 and Ethanox 330 was added. Ethanox 703 is a trade designation for 2 , 6-di-t-butyl-α-dimethyl amino-o-cresol . The phenolic antioxidant added at 153 hours was a mixture of t-butyl phenol derivatives sold under the trade designation Iganox L-130 by Ciba-Geigy; and the amine antioxidant added at 267 hours was a reaction product of N-phenylbenzylamine and 2 ,4 , 4-trimethyl pentene, sold under the trade designation L-57 by Ciba-Geigy. Table 10 TES Erosion Test Iron Erosion Epoxide Additives Corrosion Valve Depletion Run Basestock Phenolics Amines Inhibitor Leakage @ 300°F M-l TIBP/DIBPP Continuation Combination None <100 cc >95% a M-2 TIBP/DIBPP E703/E330 DODPA None at the >200 cc 65% a 10 start. At 22 hrs. Petrolite 31001 added, M-3 TIBP/DIBPP Ionol/E702 DODPA Vanlube 100 cc 22% a 15 RI-G M-4 TIBP/DIBPP Ionol/E702 DODPA Vanlube 78.9 9-b RI-G M-5 TIBP/DIBPP Ionol/E702/E330 DODPA Vanlube 58 °.a RI-G 20 a Boeing BMS-3-11G Erosion Resistance Test ' b Boeing, BMS-3-11G, Erosion Control Test These data and those of Example 9 demonstrate that the iron corrosion resistance agents Petrolite 31001 and Vanlube RI-G are both satisfactory with respect to effect on erosion. Neither appears to significantly accelerate erosion, and the compositions containing these additives exhibit satisfactory antierosion properties. The combination of a triisobutyl phosphate/di- isobutyl phenyl phosphate base stock with the 4,5-dihydroimidazole derivative of Vanlube RI-G provides a remarkable and unexpectedly favorable effect on the stability of the composition at elevated temperature. This effect is not seen with iron corrosion inhibitors other than 4,5-dihydroimidazoles of the above described type.

Claims: WHAT IS CLAIMED IS: 1. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: a fire resistant phosphate ester base stock, said base stock comprising between about 50% and about 72% by weight of a trialkyl phosphate in which the alkyl substituents contains between 3 and 8 carbon atoms 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 contain between 3 and 8 carbon atoms and are bonded to the phosphate moiety via a primary carbon atom, and from 0 to about 5% by weight of an alkyl diaryl phosphate; 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 said base stock; 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; a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity index . 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; and an antioxidant in an. amount effective to inhibit oxidation of fluid composition components in the presence of oxidizing agents. 2. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: a fire resistant phosphate ester base stock, said base stock comprising between about 50% and about 72% by weight of a trialkyl phosphate in which the alkyl substituents are substantially C 4 or C5, between about 18% and about 35% by weight of a dialkyl aryl phosphate in which the alkyl substituents are substantially C4 or C 5 , and from 0 to about 5% by weight of an alkyl diaryl phosphate; 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 said base stock; 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; a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity inde 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; and an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxidizing agents. 3. A fluid composition as set forth in claim 2 wherein said dialkyl aryl phosphate is a dialkyl phenyl phoεphate. 4. A composition as set forth in claim 2 wherein said viscosity index improver is present in an amount effective to cause the fluid composition to exhibit a viscosity index between about 3 and about 5 centistokes at about 210°F and between about 9 and about 15 centistokes at 100°F. 5. A composition as set forth in claim 2 containing a viscosity index improver in a proportion of between about 3% and about 10% by weight of said composition, said viscosity index improver comprising a methacrylate ester polymer, the repeating units of which substantially comprise butyl and hexyl methacrylate, at leaεt 95% by weight of said polymer having a molecular weight of between about 50,000 and about 1,500,000. 6. A composition as set forth in claim 2 containing a 2, 4 , 6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of said composition, a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1% by weight of said composition, and a hindered polyphenol composition selected from the group consiεting of bis(3 , 5-dialkyl- -hydroxyaryl)methane and l,3,5-trialkyl-2,4,6-tris(3, 5-di-t-butyl-4-hydroxyaryl) benzene and mixtures thereof, in a proportion of between about 0.3% and about 1% by weight of said composition. 7. A composition as set forth in claim 2 wherein esterε containing an aryl substituent do. not constitute more than about 25% by weight of said base stock. 8. A composition as set forth in claim 1 wherein said alkyl substituents are substantially isobutyl or isopentyl. 9. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: a fire resistant phosphate ester base stock comprising between about 10% and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents contain between 3 ant 8 carbon atoms and are bonded to the phosphate moiety via a primary carbon, between about 0 and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents contain between 3 and 8 carbon atoms and are bonded to the phosphate moiety via a primary carbon atom, and between about 0% and about 25% by weight of an alkyl diaryl phosphate; a viscosity index improver in a proportion of between about 3% and about 10% by weight of said composition, said 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 said polymer having a molecular weight of between about 50,000 and about 1,500,000; an anti-erosion agent in a proportion of between about 0.02% and about 0.08% by weight of said composition, said anti-erosion agent comprising an alkali metal salt of a perfluoroalkylsulfonic acid, the alkyl substituent of which is hexyl, heptyl, octyl, nonyl or decyl or mixtures thereof; an acid scavenger in a proportion of between about 1.5% and about 10% by weight of said composition, said acid scavenger comprising an epoxide compound; a 2,4, 6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of said composition; a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1% by weight of said composition; and a hindered polyphenol composition selected from the group consisting of bis(3, 5-dialkyl-4-hydroxyaryl)methane, l,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4- hydroxyaryl)benzene and mixtures thereof in a proportion of between about 0.3% and about 1% by weight of said composition. 10. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: a fire resistant phosphate ester base stock comprising between about 10% and about 90% by weight of a trialkyl phosphate wherein the alkyl εubstituents are substantially butyl or pentyl, between about 0 and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are substantially butyl or pentyl, and between about 0% and about 25% by weight of an alkyl diaryl phosphate; a viscosity index improver in a proportion of between about 3% and about 10% by weight of said composition, said 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 said polymer having a molecular weight of between about 50,000 and about 1,500,000; an anti-erosion agent in a proportion of between about 0.02% and about 0.08% by weight of said composition, εaid anti-erosion agent comprising an alkali metal salt of a perfluoroalkylsulfonic acid, the alkyl substituent of which is hexyl, heptyl, octyl, nonyl or decyl or mixtures thereof; an acid scavenger in a proportion of between about 1.5% and about 10% by weight of said composition, said acid scavenger selected from the group consiεting of a derivative of a 3,4-epoxycyclohexane 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 said 3,4-epoxycyclohexane carboxylate and said diepoxide compound; a 2,4,6-trialkylphenol in a proportion of between about 0.1% and about 1.0% by weight of said composition; a di(alkylphenyl)amine in a proportion of between about 0.3% and about 1% by weight of said composition; and 0 a hindered polyphenol composition selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4- hydroxyaryl)benzene and mixtures thereof in a proportion of between about 0.3% and about 1% by weight of said 5 composition. 11. A composition as set forth in claim 10 further comprising benzotriazole or a benzotriazole derivative in a proportion of between about 0.005% and about 0.09 ppm as a copper corrosion inhibitor. 12. A composition as εet forth in claim 10 further comprising between about 0.0035% and about 0.10% by weight of an alkali metal arylate. 13. A fluid composition as set forth in claim 10 wherein said base stock comprises between about 35% and about 90% by weight of a trialkyl phosphate wherein the alkyl subεtituents are substantially butyl or pentyl and between about 0 and about 35% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are subεtantially butyl or pentyl, and between about 0 and about 20% of a triaryl phoεphate. 14. A fluid compoεition as set forth in claim 13 wherein said base stock compriseε between about 80% and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents are subεtantially butyl or pentyl ' and between about 10% and about 20% by weight of a tri(alkylaryl)phosphate. 15. A fluid composition as set forth in claim 14 wherein said tri(alkylaryl)phosphate comprises tri(isopropylphenyl)phosphate. 16. A fluid composition aε set forth in claim 10 wherein said base stock compriseε between about 10% and about 72% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially butyl or pentyl, between about 18% and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are substantially butyl or pentyl, and between about 0% and about 25% by weight of an alkyl diaryl phosphate. 17. A fluid composition as set forth in claim 16 wherein said base stock comprises between about 10% and about 25% by weight of a trialkyl phosphate wherein the alkyl substituents are εubstantially butyl or pentyl, between about 45% and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are subεtantially butyl or pentyl, and between about 5% and about 25% by weight of an alkyl diaryl phoεphate wherein the alkyl εubstituent iε εubεtantially butyl or pentyl. 18. A fluid compoεition as set forth in claim 16 wherein said base stock compriseε between about 50% and about 72% by weight of a trialkyl phosphate wherein the alkyl subεtituents are substantially butyl or pentyl, between about 18% and about 35% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are substantially butyl or pentyl, and between about 0 and about 10% by weight of an alkyl diaryl phosphate. 19. A fluid composition as set forth in claim 10 wherein said base εtock containε between about 0 and about 5% by weight of alkyl diaryl phoεphate. 20. A composition as set forth in claim 10 wherein said alkyl subεtituents are subεtantially iεobutyl or isopentyl. 21. A fluid composition suitable for use as an aircraft hydraulic fluid, comprising: a fire resistant phosphate eεter baεe stock, εaid base stock comprising between about 10% and about 90% by weight of a trialkyl phosphate wherein the alkyl substituents are substantially isobutyl or isopentyl, between about 0 and about 70% by weight of a dialkyl aryl phoεphate wherein the alkyl substituents are substantially isobutyl or isopentyl, and between about 0% and about 25% by weight of an alkyl diaryl phosphate; 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 said base εtock; an anti-erosion additive in an amount effective to inhibit flow-induced electrochemical corrosion of the flow-metering edges of hydraulic servo valves in hydraulic systems; a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viεcoεity index of at least about 3.0 centistokes at about 210°F, at least about 9.0 centistokeε at about 100°F, and less than about 4200 centistokes at -65°F; and an antioxidant in an amount effective to inhibit oxidation of fluid compoεition components in the presence of oxidizing agents. 22. A composition as set forth in claim 21 wherein said trialkyl phosphate is triisobutyl phosphate and said dialkyl aryl phosphate is diisobutyl phenyl phosphate. 23. A composition as set forth in claim 21 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. 24. A fluid composition as set forth in claim 21 further comprising an 4,5-dihydroimidazole compound corresponding to the formula where R-- is selected from the group consiεting of hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl, and R 2 iε εelected from the group consisting of alkyl, alkenyl and aliphatic carboxylate. 25. A composition as set forth in claim 24 wherein εaid 4,5 dihydroimidazole is selected from the group consiεting of 2-(8-heptadecenyl)-4,5-dihydro-lH- imidazole-1-ethanol and the condensation product of a C 14 to C 18 fatty acid and 4,5-dihydro-lH-imidazole. 26. A compoεition as set forth in claim 25 wherein said 4,5-dihydroimidazole compound is the condensation product of a C 16 to C 18 fatty acid with 4,5-dihydro-lH-imidazole. 27. A composition aε εet forth in claim 24 wherein said antioxidant comprises a hindered phenol. 28. A composition as set forth in claim 27 wherein said hindered phenol compriseε a hindered polyphenolic compound selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane 1,3,5-trialkyl-2,4,6-tris(3,5-ditertiarybuty1-4-hydroxy aryl)benzene and mixtures thereof. 29. A composition as set forth in claim 27 wherein said antioxidant further compriseε an amine compound. 30. A compoεition aε εet forth in claim 29 further compriεing a diarylamine antioxidant. 31. A compoεition as set forth in claim 30 wherein said diarylamine comprises di(p-octylphenyl)amine. 32. A composition aε εet forth in claim 30 further compriεing up to about 0.7% by weight of 2,6-di-tertiary-butyl p-creεol. 33. A fluid composition suitable for uεe aε an aircraft hydraulic fluid, comprising: 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 butyl or pentyl, between about 0 and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are subεtantially butyl or pentyl, and between about 0% and about 25% by weight of an alkyl diaryl phosphate; 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 said base stock; 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; a viscosity index improver in an amount effective to cause the fluid composition to exhibit a viscosity index 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 centistokeε at -65°F; an antioxidant in an amount effective to inhibit oxidation of fluid compoεition componentε in the preεence of oxidizing agents; and a 4,5-dihydroimidazole compound in an amount effective to increaεe the stability of the composition by at least 25% at 300°F aε measured by epoxide depletion, said 4,5-dihydroimidazole compound corresponding to the formula where R 1 is selected from the group consiεting of hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl and R 2 iε εelected from the group consiεting of alkyl, alkenyl and aliphatic carboxylate. 34. A compoεition as set forth in claim 33 wherein R-- is hydrogen or lower alkyl and R 2 is a fatty acid residue. 35. A compoεition as set forth in claim 33 wherein R- 1 - iε hydroxyalkyl and R 2 is alkenyl. 36. A composition as set forth in claim 33 wherein εaid 4,5-dihydroimidazole iε εelected from the group consiεting of 2-(8-heptadecenyl)-4,5-dihydro-lH- imidazole-1-ethanol and the condensation product of a C 14 to C j8 fatty acid and 4,5-dihydro-lH-imidazole. 37. A composition as set forth in claim 36 wherein said ,5-dihydroimidazole compound iε the condensation product of a C^g to C 18 fatty acid with 4,5-dihydro-lH-imidazole. 38. A compoεition as εet forth in claim 37 wherein εaid alkyl εubεtituentε of said trialkyl phosphate and said dialkyl aryl phosphate are substantially isobutyl or isopentyl. 39. A composition as εet forth in claim 33 wherein εaid antioxidant comprises a hindered phenol. 40. A composition as set forth in claim 39 wherein said hindered phenol comprises a hindered polyphenolic compound selected from the group consiεting of biε(3,5-diaIky1-4-hydroxyaryl)methane 1,3,5-trialkyl-2,4,6-triε(3,5-di-tertiary-butyl-4-hydroxy aryl)benzene and mixtureε thereof. 41. A composition as set forth in claim 39 wherein said antioxidant further compriseε an amine compound. 42. A compoεition aε εet forth in claim 41 further compriεing a diarylamine antioxidant. 43. A composition as set forth in claim 42 wherein said diarylamine compriseε di(p-octylphenyl)amine, 44. A composition as set forth in claim 42 further comprising up to about 1.0% by weight of 2,6-di-tertiary-butyl p-cresol. 45. A compoεition aε set forth in claim 33 wherein εaid alkyl substituentε of said trialkyl phosphate and said dialkyl aryl phosphate are substantially isobutyl or isopentyl.

Inventors: DEETMAN GERBRAND, DEETMAN, GERBRAND
Applicants: MONSANTO CO, MONSANTO COMPANY
Application year: 1993
Main classification: C10M105
IPC codes: C10M105, C10M129, C10M133, C10M135, C10M169, C10N10, C10N20, C10N30, C10N40