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">

Description

BACKGROUND OF THE INVENTIONThis 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 fro...

AI Technical Summary

Benefits of technology

Among the several objects 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 of 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.

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 100%, preferably between about 20% and about 99%, by weight of a trialkyl phosphate wherein the alkyl substituents are substantially isoalkyl C.sub.4 or C.sub.5, between about 0% and about 70% by weight of a dialkyl aryl phosphate wherein the alkyl substituents are substantially isoalkyl C.sub.4 or C.sub.5, and between about 0% and about 25% by weight of an alkyl diaryl phosphate wherein the alkyl substituent is substantially isoalkyl C.sub.4 or C.sub.5. The composition further comprises 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; 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.degree. F., at least about 9.0 centistokes at about 100.degree. F., and less than about 4200 centistokes at about -65.degree. F.; and an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxygen.

where R.sup.1 is hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl or alkoxyalkenyl, and R.sup.2 is alkyl, alkenyl or an aliphatic carboxylate. The invention is further directed to a fluid composition suitable for use as an aircraft hydraulic fluid comprising a fire resistant phosphate ester base stock. The base stock comprises between about 10% and about 100%, preferably 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 a triaryl phosphate. The alkyl substituents of the trialkyl phosphate and the dialkyl aryl 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 and the dialkyl aryl phosphate are isoalkyl groups. The aryl substituents of the dialkyl aryl phosphate esters and the triaryl phosphate esters are typically phenyl, but may also be an alkyl-substituted phenyl (alkylphenyl) wherein the alkyl substituent is C.sub.1 to C.sub.9, preferably C.sub.3 to C.sub.4. Nonlimiting examples of the alkyl-substituted phenyl substituents include tolyl (also known as methylphenyl), ethylphenyl, isopropylphenyl, isobutylphenyl, tert-butylphenyl, and the like. The fluid composition further comprises 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, 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 of at least about 3.0 centistokes at about 210.degree. F., at least about 9.0 centistokes at about 100.degree. F., and less than about 4200 centistokes at -65.degree. F.; and an antioxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxygen.

Problems solved by technology

In use, aircraft hydraulic fluids commonly become contaminated with moisture.

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.

Erosion problems may also be expected to increase with bulk fluid 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.

However, it was not designed for extended operations at temperatures in the range of 275.degree. F.

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