Compositions – Heat-exchange – low-freezing or pour point – or high boiling... – Organic components
Reissue Patent
1997-11-07
2001-03-20
Skane, Christine (Department: 1751)
Compositions
Heat-exchange, low-freezing or pour point, or high boiling...
Organic components
C252S075000, C252S077000, C252S078100, C508S433000, C508S282000
Reissue Patent
active
RE037101
ABSTRACT:
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® LD4. This composition contains 30 to 35% by weight dibutyl 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. Pat. No. 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 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.
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 10% and about 100%, preferably 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 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. In a preferred embodiment, the base stock of the composition comprises between about 50% and about 85% 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. In addition to the fire resistant base stock, 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 of at least about 3.0 centistokes (cst) 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 anti-oxidant in an amount effective to inhibit oxidation of fluid composition components in the presence of oxygen.
Preferably, as previously indicated, the alkyl substituents of the trialkyl phosphate, dialkyl aryl phosphate, and the alkyl diaryl phosphate contain between 4 and 8 carbon atoms, more preferably between 4 and 5 carbon atoms. 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. Most preferably, therefore, the alkyl substituents are isoalkyl C
4
and C
5
grou
Howrey Simon Arnold & White , LLP
Skane Christine
Solutia Inc.
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