US3679587A - Functional fluid compositions containing perfluoro surfactants - Google Patents

Abstract

PHOSPHATE ESTER BASED FUNCTIONAL FLUID COMPOSITIONS CONTAINING SMALL AMOUNTS OF PERFLUORINATED ANIONIC SURFACTANTS ARE NOVEL COMPOSITIONS OF MATTER WHICH WHEN EMPLOYED AS HYDRAULIC FLUIDS IN HYDRAULIC SYSTEMS SHOW INCREASED ABILITY TO INHIBIT EROSION OF MATERIAL AND DAMAGE TO MEMBERS IN SUCH HYDRAULIC SYSTEMS.

Description

United States Patent 3,679,587 FUNCTIONAL FLUID COMPOSITIONS CONTAIN- ING PERFLUORO SURFACTANTS Terrill D. Smith, Edmond, Okla., assignor to Monsanto Company, St. Louis, M0. N0 Drawing. Filed Mar. 10, 1970, Ser. No. 18,291 Int. Cl. C091: 3/00 US. Cl. 252-75 20 Claims ABSTRACT OF THE DISCLOSURE Phosphate ester based functional fluid compositions containing small amounts of perfluorinated anionic surfactants are novel compositions of matter which when employed as hydraulic fluids in hydraulic systems show increased ability to inhibit erosion of material and damage to members in such hydraulic systems.

This invention relates to functional fluid compositions having the ability to inhibit and control damage to mechanical members of hydraulic systems in contact with said fluid and more particularly to compositions comprising certain phosphate ester functional fluids containing an anionic perfluorinated surfactant in amounts suflicient to inhibit and control damage to mechanical members in a hydraulic system.

Many ditferent types of materials are utilized as functional fluids and functional fluids are used in many different types of applications. Thus such fluids have been used as electronic coolants, atomic reactor coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids and as filter medium for air conditioning systems. In many of these uses there have been reports of damage to the fluid during use and to mechanical members, especially metallic members, in contact with the fluid as evidenced by a a loss of weight of such member. Thus damage has been reported in aircraft hydraulic systems, gas turbine bearings, jet turbine control systems, steam turbine bearings and steam turbine control systems.

One particularly undesirable condition which exists during the use of a functional fluid and which can cause damage is the erosion of materials. One possibility for such erosion of material is cavitation which can be described as a phenomenon resulting from movement of a fluid at a given pressure to a lower pressure, the lower pressure being obtained by an increase in fluid velocity at a given point or zone in the pressure system. The reduction in pressure is of such a magnitude as to cause cavitation severe enough to damage mechanical members and/ or the fluid.

While there are many undesirable results caused by damage, one important aspect of the problem of damage is the effect on hydraulic systems and fluids experiencing such damage. For example, the structural mechanical parts in a hydraulic system, such as pumps and valves, exhibit a marked decrease in strength, and the geometry of the parts is altered. These changes in the case of pumps cause a decrease in the efliciency of operation and in the case of valves can cause faulty operation, excessive leakage or even hazardous conditions. This damage necessitates premature overhaul of mechanical parts which is both costly and time consuming. Furthermore, as damage occurs the metal from metallic mechanical parts in contact with the functional fluid contaminates the functional fluids requiring premature draining of the fluids from the system, filter clogging and replacement, and can cause a change in physical and chemical properties of the fluids. Metal contaminants can also reduce the oxidative stability of a fluid thereby causing further corrosion. In addition 3,679,587 Patented July 25, 1972 ice 7 water in such amounts to functional fluids will inhibit and control damage to valves and other mechanical members of a hydraulic system, various undesirable side effects are noted.

Thus, for example, large amounts of water present in the system can lead to the hydrolysis of the phosphate esters giving rise to amounts of the free acidic groups in the fluid which causes corrosion and other undesirable effects. The increase in the total acidity of the phosphate ester based functional fluid thus necessitates frequent draining of the hydraulic fluids after relatively short periods of operation.

It is therefore an object of this invention to provide phosphate ester based hydraulic fluids which are capable of inhibiting or preventing the damage to mechanical members by cavitation or erosion of material and which can contain small amounts of water or essentially no water in order to prevent the undesirable side effects such as hydrolysis of the ester and increases in the acid number of such fluids.

Further objects will be apparent from the following description of the invention.

It has now been found that damage, herein defined to include damage to a functional fluid and to mechanical members in contact with said fluid, can be effectively reduced or inhibited in the phosphate ester based functional fluid systems by the incorporation of small amounts of perfluorinated anionic surfactant into a functional fluid. The incorporation of these perfluorinated anionic surfactants into phosphate ester based functional fluids improves the ability of such fluids to inhibit damage without adversely affecting the other properties of such fluids such as viscosity, oxidative and thermal stability, corrosion resistance in the presence of metal parts and the lubricating qualities of the functional fluid.

It has unexpectedly been discovered that a decrease in the erosion effects of phosphate ester based hydraulic fluids can be greatly diminished if a small amount of a perfluorinated anionic surfactant is incorporated into such fluid formulations.

The functional fluid compositions of this invention comprise a major amount of (A) liquid phosphate esters and mixtures thereof wherein the phosphate esters are represented by the formula wherein each R is individually an alkyl or alkoxyalkyl group containing from 2 to 12 carbon atoms, phenyl groups and substituted phenyl groups containing up to 20 carbon atoms wherein the substituents are alkyl, phenyl, or phenyl alkyl and wherein the alkyl groups contain from 1 to 10 carbon atoms and (B) a damage inhibiting amount of a perfluorinated anionic surfactant.

By the term major amount as employed herein is meant at least 50+% by weight of the phosphate ester. Preferably the phosphate ester comprises from 65 to 99.999 percent by weight of the functional fluid compositions of this invention. The functional fluid compositions of this 3 invention can also contain minor amounts, i.e. up to 49% by weight, of other lubricants for example, polyesters, polyphenyl ethers, and the like.

The amount of the perfluorinated anionic surfactant employed in the functional fluid compositions of this invention is from as little as 0.001 to as much as 5.0 parts surfactant per 100 parts of the phosphate ester. Amounts greater than 5.0 parts can be employed if soluble in the fluid, however, no commensurate advantages are obtained thereby.

The perfluorinated anionic surfactants employed in the compositions of this invention are the alkali metal salts of perfluoroalkyl sulfonic acids and have the general formula (II) R SO M where M is an alkali metal, for example sodium, lithium, potassium, rubidium or cesium and R; is a C F or a cyclic C,,F group where n is an integer of from 1 to 18 and a is an integer from 4 to 18. These salts are, for example, potassium perfluoromethane sulfonate, potassium perfluoroethane sulfonate, sodium perfiuorobutane sulfonate, sodium perfluorocyclohexane sulfonate, potassium perfluorooctane sulfonate, cesium perfluorooctadecane sulfonate, potassium perfiuorocyclopentane sulfonate, potassium perfluoropentane sulfonate, and the like. It is preferred that the perfluorinated anionic surfactants contain at least 5 carbon atoms and especially preferred that they contain from 7 to 12 carbon atoms. Other perfluorinated anionic surfactants which can be employed in the composition of this invention are the alkali metal salts of perfluorinated alkyl disulfonic acids and the like. These disulfonic acid salts are, for example, dipotassium perfiuorocyclohexane disulfonic C F (SO K) dipotassium bis(perfluorocyclohexane sulfonate) and the like.

The perfluorinated alkyl groups represented by R, include perfluorinated alkyl groups and perfluorinated cycloalkyl groups, for example, perfiuoromethyl, perfluoroethyl, perfiuoropropyl, perfiuorobutyl, perfluoropentyl, perfluoroheptyl, perfiuorooctyl, perfluorodecyl, perfluorooctodecyl, etc. The cycloalkyl groups are, for example, perfluorocyclopentyl, perfiuorocyclohexyl, perfluorocycloheptyl, perfiuoro (ethylcyclohexyl perfiuoro (cyclohexylmethyl) perfiuoro (cyclohexylethyl) perfluoro (cyclohexylpropyl) perfluoro (methylcyclohexyl) perfluoro dimethylcyclohexyl) and the like. In order to have the necessary surfactant properties and solubilities necessary for the compositions of this invention it is preferred that the perfiuoroalkyl group contain at least 5 carbon atoms and even more preferred that it contain 7 or more carbon atoms.

The preferred esters of phosphoric acid which can be employed as base stocks in the compositions of the instant invention are those wherein the groups represented by R are alkyl, alkoxyalkyl, phenyl or substituted phenyl. The preferred base stocks are hereinafter referred to generically as phosphates and include trialkyl phosphates, triphenyl and/or substituted phenyl phosphates and mixed phenyl and/or substituted phenyl phosphates. The alkyl groups preferred are those containing from 2 to 12 carbon atoms with the total number of carbon atoms in the trialkyl phosphates being from 12 to 36 carbon atoms. These alkyl groups are for example, ethyl, propyl, isopropyl, butyl, hexyl, 2-ethylhexyl, dodecyl, decyl, and the like. The substituted phenyl groups represented by the Rs in Formula I are those containing up to 16 carbon atoms and the alkyl groups contain 1 to carbon atoms provided that the total number of carbon atoms in all of the alkyl groups attached to any one phenyl group be at most 10 carbon atoms. The alkoxyalkyl groups are for example, those having the formula H n 2n x a 2a wherein n is an integer having a value of from 1 to 10, a

is an integer having a value of from 2 to 10, and x is an integer of 1 to 10, preferably x is 1. These alkoxylalkyl groups are for example methoxyethyl, ethoxyethylpropoxyethyl, ethoxypropyl, ethoxypentyl, propoxydecyl, nonyloxyethyl, octyloxybutyl and the like. The substituted phenyl groups represented by R include the alkyl-substituted phenyl groups and are for example, methylphenyl, ethylphenyl, dimethylphenyl, propylphenyl, nonylphenyl, decylphenyl, dipentylphenyl, butylhexylphenyl and the like. Also included within the definition of the substituted phenyl groups represented by R, are, for example the phenylalkyl phenyl groups (C H C H --C H where n is 1 to 8) containing up to 20 carbon atoms, i.e. cumylphenyl, phenylmethylphenyl (C H CH C H phenylethylphenyl (C H C H C H phenylpropylphenyl (C H C H C H phenyloctylphenyl (C6H5C8H16C6H4 v and the like; and phenyl-substituted phenyl groups such as 0-, mand p-phenylphenyl (C H -C H o-, mand p-methylphenylphenyl (CH C H --C H o-, mand p-dimethylphenylphenyl ((CH C H C H and the like. Typical examples of these phosphate esters are for example, dibutylphenyl phosphate, triphenyl phosphate, tricresyl phosphate, tributyl phosphate, tri-Z-ethylhexyl, trioctyl phosphate, and mixtures of the above phosphates such as mixture of tributyl phosphate and tricresyl phosphate, mixtures of isooctyl diphenyl phosphate and 2-ethy1- hexyl diphenyl phosphate, and mixtures of trialkyl phosphates and tricresyl phosphates and the like. The particularly preferred phosphate esters are those which remain liquid at temperatures of about 30 C.

The functional fluid composition of this invention can also contain polymeric viscosity index improvers such as poly(alkylmethacrylate), poly(alkylacrylates), polyoxyalkylenes, polyurethanes and the like.

The preferred polymeric viscosity index improvers which may be employed in the compositions of this invention are the polymers of alkyl esters of alpha-beta unsaturated monocarboxylic acids having the formula wherein R and R are each individually hydrogen or a C to C alkyl group, and R is a C to C alkyl group. Illustration of the alkyl groups represented by R R and R within their definitions as given above are for example methyl, ethyl, propyl, butyl, t-butyl, isopropyl, 2-ethylhexyl, hexyl, decyl, undecyl, dodecyl and the like. These polymers include, for example, poly(butyhnethacrylates), poly('hexylmethacrylates), poly(octylacrylates), poly- (dodecylacrylates) and polymers wherein the ester is a mixture of compounds obtained by esterifying the 04-8 unsaturated monocarboxylic acid with a mixture of monoalcohols containing from 1 to 12 carbon atoms.

The polyalkylmethacrylates and acrylates suitable for the purpose of this invention are in general those resulting from the polymerization of alkylmethacrylates or alkylacrylates in which the alkyl groups have from 4 to 12 carbon atoms. The alkyl groups may be mixtures such as derived from a mixture of alcohols in which case there may be included some alkyl groups having as low as 2 carbon atoms and as high as about 18 carbon atoms. The number of carbon atoms in the alkyl group should preferably be such that the polymer is compatible with the particular fluid use. Usually it will be satisfactory for the alkyl group of the methacrylate polymer to be from about 4 to 10 carbon atoms. The alkyl group may be branched chain or isoalkyl, but it is preferably normal alkyl. The molecular weight of the polymerized alkylmethacrylate can be from 15,000 to about 40,000 or the viscosity index improver can be a mixture of one or more polymers having different average molecular weights. For example, a

mixture of a polymer having average molecular weight of from 2,000 to 12,000 with one having an average molecular weight of from 15,000 to 40,000.

The total amount of viscosity index improver employed in the compositions of the instant invention can range from about 2 to about 20 parts per 100 parts of the total composition. Where a mixture of two polymers having different average molecular weight is employed, the ratio of high molecular weight viscosity index improver (A) to the lower molecular weight viscosity index improver (B) can be from 1 to 20 to 20 to 1. It is particularly preferred that the ratio of high molecular weight to the low molecular weight polymer be from about 2.5 to 7.5 to 7.5 to 2.5. It is even more preferred that the ratio be from 6 to 4 to 4 to 6.

Water can be piesent in the compositions of this invention; however, water is not necessary to obtain the inhibition of erosion of material. As little as 0.1 part of water per 100 parts of the total composition can be employed and can range as high as 2.0 parts per 100 parts of the total composition.

It has been found when a mixture of (A) a high molecular weight polymer of an alkyl ester of an alphabeta unsaturated aliphatic monocarboxylic acid with (B) a low molecular weight polymer of an alpha-beta unsaturated aliphatic monocarboxylic acid is employed as the viscosity index improver in the compositions of this invention a greater improvement in the inhibition of material erosion is obtained. By the term high molecular weight polymer is meant a polymer having an average molecular weight of from 15,000 to about 40,000. By the term low molecular weight polymer is meant a polymer having an average molecular weight of from about 2,500 to about 12,000.

In the controls and examples the base fluids are designated A, B, C, D and E, respectively. The surfactant is a perfluorinated anionic surfactant which is the potassium salt of the perfiuoroalkane sulfonic acid C F SO K. These base fluids have the following formulations:

BASE FLUID A Percent Dibutylphenyl phosphate 95.10 Poly(butylmethacrylate), average MW about 29,000 3.40 Bis(1,2-phenylmercapto)ethane 0.50 Paraplex G-62, an epoxidized soybean oil 1.0

BASE FLUID B Percent Dibutylphenyl phosphate 91.01-91.11 Poly(n-hexylmethacry1ate), approximately 5,600

average MW 3.00 Poly(butylmethacrylate) approximately 29,000

average MW 2.10 Bis( 1,2pheny1mercapto )ethane 0.50 3,4 epoxycyclohexylmethyl 3',4' epoxycyclohexanecarboxylate 3.10 Water .19-.29

BASE FLUID C Percent Tributyl phosphate 79.88 Tricresyl phosphate 10.9 Poly(n-hexylmethacrylate), approximately 5,600

average MW 4.0 Poly (butylmethacrylate), approximately 29,000

average MW 3.0 Bis(1,2-phenylmercapto)ethane 0.5 3,4-epoxycyclohexylmethyl 3,4'-epoxycyclohexane carboxylate 1.6 Water .12

6 BASE FLUID D Percent Tricresyl phosphate 10.9 Tributyl phosphate 79.50 Poly(n-hexylmethacrylate), approximately 5,600

average MW 4.0 Poly(butylmethacrylate) approximately 29,000

average MW 3.0 Bis(1,2-phenylmercapto)ethane 0.5 3,4-epoxycyclohexylmethyl 3',4'-epoxycyclohexanecarboxylate 1.6 Water 0.5

BASE FLUID E Percent Dibutylphenyl phosphate 90.80 Poly(n-hexylmethacrylate), approximately 5,600

average MW 3.00 Poly(butylmethacrylate), approximately 29,000

average MW 2.10 Bis(1,2-phenyhnercapto)ethane 0.5 3,4 epoxycyclohexylmethyl 3',4' epoxycyclohexanecarboxylate 3.1 Water 0.5

The following examples serve to further illustrate the invention. In the examples all parts are by weight unless otherwise expressly set forth.

EXAMPLE In these examples the following procedure was em ployed to show the effectiveness of the compositions of this invention to prevent erosion of material and valve damage.

The fluids were prepared in the normal manner and the potassium perfluoroalkane sulfonate added in the amount indicated with stirring. The solution was then filtered and the test conducted.

The test procedure is as follows: A valve is placed in a hydraulic system with a fluid supply heated to F. and maintained at a supply pressure of 3000 p.s.i.g. The valve is opened to allow a constant initial flow as indicated with the downside pressure being 1500 p.s.i.g. The test was conducted for 118 hours with the fiow rate being monitored. An increase in flow rate indicates a lack of effectiveness of the fluid to inhibit erosion of material. Valve 1 employed in the test was a Bendix pressure reduction and relief valve of the spool in sleeve type. Valve 2 was a flat orifice valve consisting of two cylindrical blocks of cold roll 1018 steel (Rockwell C Scale Hardness=4). Each block has eccentric holes on one fiat surface. The holes are drilled and finish reamed and the fiat circular surface is lapped. The blocks are mounted together by a centering pin and held together by four threaded rods and end plates fitted with hydraulic line connecting ports which has zero clearance between the fiat surfaces. The cylindrical blocks are rotated so that the flow rate is approximately 250 cc. per minute and the system operated with a supply pressure of 3000 p.s.i.g. and a down stream pressure of 1500 p.s.i.g.

When other anionic perfiuorinated surfactants such as C F C F -SO K, C F SO K, C F --C F -SO K (perfluoroethylcyclohexyl) and the like are substituted for the surfactant employed in the examples, similar inhibition of erosion of material is obtained.

The results of the tests of various functional fluids containing the perfluorinated anionic surfactant are given in Table I.

TABLE I Amount Flow rate of sur- (eoJminute) Test Test Base factant, time Change, number fluid percent Valve Initial Final (hours) percent Remarks A 0. 1 1, 000 2, 150 115 115 '""{A 0. 0 2 243 400 115. s i-63 A 0. 02 1 1, 000 600 118 -40 N0 damage, slight 1 deposit on valve;

s a 2 .522 a n2 1 -----{B 0.0 2 so 73 117 +46 2 {B 0 02 1 1,000 810 118 -19 B 0 02 2 250 266 118 +6 2:3 --.T:. .-f:f?. -29? -icei 3 {C 0 02 1 1,000 500 137 --50 1% 1% i 1 000 335 ii 59 00mm ""{D 01 0 2 250 250 117 o 4 {D 0 02 1 1,000 820 3 18 D 0 02 2 255 +2 I E 0.0 1 1,000 700 119 (-30) Base toi 1,900 stab? rs. eerease 0 700 due to siltation. E 0 0 2 250 327 119 +31 E 0 02 1,000 720 118 28 No pitting or damage, 5 slight deposit.

The test method as employed to determine relative damage has been found to correlate quite well to actual test runs on simulated hydraulic system test stands, such as the F airey Test Stand. In addition, the hydraulic system test stands for determining damage have correlated quite well with the hydraulic system of commercial aircraft where damage levels have been determined. Functional fluid compositions of this invention with a viscosity index improver system comprising the mixture of high molecular weight and low molecular weight polymers with water concentrations sufficient to inhibit the control damage have been evaluated in actual hydraulic systems in test stands and commercial aircraft and have been found to effectively inhibit damage and are far superior to the neat fluids with additive amounts of water but employing only a single polymer as the viscosity index improver.

The data in the examples demonstrate the significant inhibition of damage obtained by the incorporation of the mixture of viscosity index improvers into a base stock. In addition, the physical properties and the performance characteristics such as lubricity, fire resistance, and viscosity were essentially unaffected by the additive, an important consideration since a base stock is selected for a given fluid system because of its physical properties or characteristics and deviations from these properties and characteristics can bring about inferior fluid performance.

As a result of the excellent inhibition and control of damage utilizing the functional fluid compositions within the scope of this invention, improved hydraulic pressure devices can be prepared in accordance with this invention which comprise in combination a fluid chamber and an actuating fluid composition in said chamber, said fluid comprising base stock compositions hereinbefore described. In such a system, the parts which are so lubricated include the frictional surfaces of the source of power, namely the pump, valves, operating pistons and cylinders, fluid motors, and in some cases, for machine tools, the ways, tables and slides. The hydraulic system may be of either the constant-volume or the variable volume type of system.

The pumps may be of various types, including centrifugal pumps, jet pumps, turbine vane, liquid piston gas compressors, piston-type pump, more particularly the variable-stroke piston pump, the variable-discharge or variable displacement piston pump, radial-piston pump or axial-piston pump in which a pivoted cylinder block is adjusted at various angles with the piston assembly, for example, the Vickers Axial-Piston Pump, or in which the mechanism which drives the pistons is set at an angle adjustable with the cylinder block; gear-type pump, which may be spur, helical or herringbone gears, variations of internal gears, or a screw pump; or vane pumps. The valves may be stop valves, reversing valves, pilot valves, throttling valves, sequence valves, relief valves, servo valves, non-return valves, poppet valves or unloading valves. Fluid motors are usually constantor variabledischarge piston pumps caused to rotate by the pressure of the hydraulic fluid of the system with the power supplied by the pump power source. Such a hydraulic motor may be used in connection with a variable-discharge pump to form a variable-speed transmission. It is, therefore, especially important that the frictional parts of the fluid system which are lubricated by the functional fluid be protected from damage. Thus, damage brings about seizure of frictional parts, excessive wear and premature replacement of parts.

The fluid compositions of this invention when utilized as a functional fluid can also contain dyes, pour point depressants, metal deactivators, acid scavengers, antioxidants, defoamers in concentration sufficient to impart antifoam properties, such as from about 10 to about 50,000 parts per million, lubricity agents and the like.

The componsitions of this invention can also contain up to 2 percent by weight of oxidation and corrosion inhibitors and acid scavengers such as compounds containing one or more epoxide groups in a molecule. Such epoxide containing compound containing one epoxy group are, for example, phenylglycidyl ether, glycidylcyclohexyl ether, glycidylalkylcyclohexyl ethers, butadiene monoxide, styrene oxide, 1,2-epoxydodecane, alkylglycidyl ether, 1,2-epoxyoctyloxypropane and the like. Epoxide compounds containing more than one epoxide group per molecule are, for example,

epoxidized soybean oil,

glyceryl,

tris acetoxyepoxy) stearate,

epoxidized linseed oil,

di 2-methyl-4,S-epoxycyclohexylmethyl) adipate,

4,S-epoxycyclohexylmethyl(3,4-epoxycyclohexylmethyl) carboxylate,

3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate,

3,4-epoxy-l-methylcyclohexylmethyl 3,4-epoxy-1- methylcyclohexanecarboxylate,

6-methyl-3,4-epoxycyclohexylmethyl 6-methyl-3,4-

epoxycyclohexanecarboxylate,

3,4-epoxy-Z-methylcyclohexylmethyl 3,4-epoxy-2- methylcyclohexanecarboxylate,

vinylcyclohexene dioxide,

butadiene dioxide,

1,4-bis(2,3-epoxypropoxy)benzene,

1,3-bis(2,3-epoxypropoxy)benzene,

1,3-bis 2,3 -epoxybutoxy) benzene,

4,4'-bis 2, 3-epoxypropoxy) diphenyldimethylmethane,

1,3-bis(2-hydroxy-3,4-epoxybutoxy)benzene,

the polymer prepared by reacting sorbitol with epichlorohydrin,

poly(allyl-2,3-epoxypropyl ether),

poly(2,3-epoxypropyl crotonate), and the like.

While this invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and that it can be variously practiced within the scope of the following claims.

What is claimed is:

1. A hydraulic fluid composition comprising (I) a major amount of at least about 65 percent by weight of the composition of a phosphate ester and mixtures thereof having the formula wherein each R is individually an alkyl or alkoxyalkyl group containing from 2 to 12 carbon atoms, a phenyl group or a substituted phenyl group containing up to 20 carbon atoms wherein the substituents are phenyl groups, phenyl alkyl groups or alkyl groups wherein the alkyl groups contain from 1 to 10 carbon atoms, and

(II) from about 0.001 to about percent by weight of phosphate ester of a perfluorinated anionic surfactant selected from the group consisting of alkali metal salts of perfluoroalkyl sulfonic acid and perfluoroalkyl disulfonic acid wherein the alkyl is from 1 to about 18 carbon atoms.

2. A composition of claim 1 wherein the surfactant is potassium perfluorooctane sulfonate.

3. A composition as claimed in claim 1 wherein the phosphate ester is dibutylphenyl phosphate.

4. A composition as claimed in claim 1 wherein the phosphate ester is tributyl phosphate.

5. A composition as claimed in claim 1 wherein the phosphate ester is a mixture of tributyl phosphate and tricresyl phosphate.

6. A composition as claimed in claim 1 wherein the phosphate ester is a mixture of a trialkyl phosphate and cresyldiphenyl phoosphate.

7. A composition as in claim 2 wherein the phosphate ester is tributyl phosphate.

8. A composition as in claim 2 wherein the phosphate ester is dibutylphenyl phosphate.

9. A composition as in claim 2 wherein the phosphate ester is a mixture of tributyl and tricresyl phosphate.

10. A composition as in claim 2 wherein the phosphate ester is a trialkyl phosphate and cresyldiphenyl phosphate.

11. A composition of claim 1 wherein the perfluorinated anionic surfactant has the structure wherein M is an alkali metal, m is an integer of 1 or 2, and R; is a C,,F or a cyclic C F group wherein n is an integer of from 1 to about 18, and a is an integer of from about 4 to 18.

12. A composition of claim 11 wherein R, is a perfluoroalkyl or perfiuorocycloalkyl group of 7 to 12 carbon atoms.

13. A composition of claim 11 wherein M is selected from the group consisting of sodium, potassium, lithium, rubidium, and cesium.

14. A hydraulic fluid composition comprising (I) a major amount of at least about 65 percent by weight of the composition of a phosphate ester and mixtures thereof having the formula wherein each R is individually an alkyl or alkoxyalkyl group containing from 2 to 12 carbon atoms, a phenyl group or a substituted phenyl group containing up to 20 carbon atoms wherein the substituents are phenyl groups, phenyl alkyl groups or alkyl groups wherein the alkyl groups contain from 1 to 10 carbon atoms, and

(II) from about 0.001 to about 5 percent by weight of phosphate ester of a perfluorinated anionic surfactant selected from the group consisting of alkali metal salts of perfluoroalkyl sulfonic acid and perfiuoroalkyl disulfonic acid wherein the alkyl is from 1 to about 18 carbon atoms, and

(III) from about 2 to 20 percent by weight of the composition of a viscosity index improver which is a polymer of an ester having the structure wherein R and R are each individually hydrogen or a C to C alkyl group, and 'R" is a C to C alkyl group.

15. A composition of claim 14 wherein the viscosity index improved is a mixture of (A) from 1 to 20 parts of a high molecular weight polymer having an average molecular weight of from 15,000 to 40,000, and

(B) from 1 to 20 parts of a low molecular weight polymer having an average molecular weight of from 2,000 to 12,000.

16. A composition of claim 15 wherein (A) and (B) are polyalkylmethacrylates wherein the alkyl groups are of 4 to 10 carbon atoms.

17. A composition of claim 16 wherein the phosphate ester is dibutylphenyl phosphate.

18. A composition of claim 17 containing up to about 2 percent by weight of 3,4-epoxycyclohexylmethyl-3,4'- epoxycyclohexanecarboxylate.

19. In a method of operating a hydraulic device wherein a displacing force is transmitted to a displaceable member by means of a hydraulic fluid, the improvement which comprises employing as said fluid a composition of claim 1.

20. In a method of operating a hydraulic device wherein a displacing force is transmitted to a displaceable member by means of a hydraulic fluid, the improvement which comprises employing as said fluid a composition of claim 2.

(References on following page) References Cited UNITED STATES PATENTS Brice et a1. 260-503 Rutkowski et a1. 260-513 Victor 252-389 Groslambert 252-78 XR Nychka 260-513 Langenfeld 252-78 Fielding 260-513 Vienna et a1. 252-389 XR 12 OTHER REFERENCES The Electrochemical Approach To Cavitation Damage and Its Prevention, Preiser et al., vol. 17, Corrosion (1961), pp. 535t, 536t, 540t, 541t.

Cavitational Erosion and Means for Its Prevention, Bogacher and Mints, U.S. Clearing House, pp. 99, 100, 101, 103, 107, and 112.

LEON D. ROSDOL, Primary Examiner H. A. PITLICK, Assistant Examiner US. Cl. X.R.