Note: Descriptions are shown in the official language in which they were submitted.
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FUNCTIONAL FLUIDS WITH SERVO VALVE EROSION RESISTANCE
FIELD OF THE INVENTION
This invention relates to phosphate ester fluids used in transmitting
power in aircraft hydraulic systems. More specifically it relates to enhancing
the
anti-erosion properties of such fluids.
BACKGROUND OF THE INVENTION
Functional fluids are used in a wide variety of industrial
applications. For example they are used as the power transmitting medium in
hydraulic systems, such as aircraft hydraulic systems.
Functional fluids intended for use in aircraft hydraulic systems
must meet stringent performance criteria such as thermal stability, fire
resistance, low susceptibility to viscosity changes over a wide range of
temperatures, good hydrolytic stability, elastomer compatibility and good
lubricity.
Organic phosphate ester fluids have been recognized as a preferred
fluid for use as a functional fluid such as in hydraulic fluids. Indeed, in
present
commercial aircraft hydraulic fluids phosphate esters are among the most
commonly used base stocks.
As with other functional fluids, organic phosphate ester based
fluids require the incorporation of various additives to enhance the
performance
of the fluid. For example, experience has shown that orifices in the servo
control
valves of aircraft hydraulic systems are subject to erosion which is
attributed to
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streaming current induced by fluid flow. Valve orifice erosion, if extensive,
can greatly
impair the functioning of the valve as a precise control mechanism. Therefore
various
additives have been used in functional fluids as erosion inhibitors.
Nonetheless, there
remains a need for increased choice of useful erosion inhibitors, especially
for
improved erosion inhibitors.
One object of the present invention is to provide phosphate ester based
aircraft hydraulic fluids with enhanced anti-erosion properties.
SUMMARY OF THE INVENTION
Briefly stated a phosphate ester functional fluid, especially a hydraulic
fluid, is enhanced by incorporating in the fluid an erosion inhibiting amount
of an
additive or mixture thereof having the formula
RSO3M
where M is preferably an alkali metal such as lithium, sodium, potassium,
cesium. it
could also be ammonium (NH4). The R group consists of a per-fluorinated
hydrocarbyl
group from 1 to 12 carbon atoms. Preferred are the Cl to C8 carbon atoms. The
said
hydrocarbyl group can be linear or branched. Examples are metal salts of
1,1,2,2,3,3,4,4,4-nonafluorobutane-l-sulonic acid. The preferred M is
potassium.
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In a further aspect of the present invention, there is provided a method for
enabling a hydraulic fluid to meet the BoeingTM test BMS 3-1 1L for erosion,
wherein the
hydraulic fluid comprises: a phosphate ester basestock wherein the basestock
comprises:
to 100 weight percent of trialkyl phosphates; 0 to 75 weight percent of
dialkyl aryl
phosphates; 0 to 30 weight percent of alkyl diaryl phosphates; and 0 to 20
weight percent
of alkylated triaryl phosphates; wherein the alkyl groups have 4 or 5 carbon
atoms; and 1
to 20 wt%, based on the total weight of the fluid, of additives selected from
one or more
antioxidants, acid scavengers, viscosity index improvers, rust inhibitors and
defoamers,
the method comprising the step of adding to the fluid from about 0.01 wt% to
0.5 wt% of
the basestock potassium perfluorobutane sulfonate, wherein said hydraulic
fluid meets the
Boeing test BMS 3-11L for erosion.
In a further aspect of the present invention, the viscosity index improver is
a polyalkylarylate or methacrylate viscosity index improver wherein the
repeating units
substantially comprise n-hexyl and isodecyl acrylate or methacrylate and
wherein at least
95 wt% of the viscosity index improver has a molecular weight between about
50,000 and
900,000 and is present in an amount in the range 3 to 10 wt% based on the
total hydraulic
fluid weight and the acid scavenger is of the formula
d
11 R,
C (CH2) x\ I ! O
0
p CH R2
<D Y
wherein R1 is H or a C, to C4 alkyl group, x is 1 or 2, y is an integer of 1
to 4, and R2 is a
C, to C4 alkyl group or a phenyl group and is present in an amount in the
range 1 to
10 wt% based on the total hydraulic fluid weight.
DETAILED DESCRIPTION OF THE INVENTION
The anti-erosion properties of phosphate ester based functional fluids,
especially aircraft hydraulic fluids, are enhanced by adding to the fluid an
effective
amount of a salt or mixture of salts represented by the formula
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RSO3 M
where M is an alkali metal such as lithium, sodium, potassium, cesium. M can
also be
calcium or barium. It could also be ammonium (NH4). The R group consists of a
perfluorinated hydrocarbyl group from 1 to 12 carbon atoms. Preferred are the
C l to
C8. We unexpectedly discovered that the low molecular weight metal salt of
perfluoroalkyl sulfonic acid are very soluble in the phosphate ester base
stock whereas
the potassium 3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluorooctane-l-sulfonate (Zonyl
FS-
62) was found essentially not soluble. This is illustrated in Examples 2 and 3
(Fluid E).
. The said hydrocarbyl group can be linear or branched. A preferred example is
metal
salts of 1,1,2,2,3,3,4,4,4-nonafluorobutane- l-sulfonic acid. M is preferably
an alkali
metal and most preferably potassium.
The preferred examples of this invention are:
Potassium trifluoromethanesulfonate or potassium inflate;
Lithium trifluoromethanesulfonate or lithium iriflate
Potassium 1,1,2,2,2-pentafluoroethane-1-sulfonate-;
Potassium 1,1,2,2,3,3,3-heptafluoropropane-l.-sulfonate-
Potassium 1,1,2,2,3,3,4,4,4-nonafluorobutane-l-sulfonate-;
Potassium 1,1,2,2,3,3,4,4,5,5,5-undecafluoropentane-l-sulfonate-;
Potassium 1,1,2,2,3,,3,4,4,5,,5,6,6,6-tridecafluorohexane-l-sulfonate;
Potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7-pentadecafluoroheptane-l-sulfonate 1;
and
Potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-l-sulfonate.
The foregoing additives are readily prepared by neutralization of the
corresponding acid (i.e., a compound of the above formula except that M is
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H with an alkali metal hydroxide or quaternary ammonium hydroxide.
Additives of the foregoing formula are also commercially available compounds.
The anti-erosion additive is incorporated in the phosphate ester
basestock in an amount sufficient to enhance the anti-erosive properties of
the
fluid. Typically the addition comprises from about 0.01 wt% to about 0.5 wt%
based on the weight of the basestock.
Phosphate ester base stocks used in this invention refer to organo-
phosphate esters selected from trialkyl phosphate, dialkyl aryl phosphate,
alkyl
diaryl phosphate and triaryl phosphate that contain from 3 to 8, preferably
from
4 to 5 carbon atoms. Suitable phosphate esters useful in the present invention
include, for example, tri-n-butyl phosphate, tri-isobutyl phosphate, n-butyl
di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl
phosphate, isobutyl diphenyl phosphate, di-n-butyl phenyl phosphate, di-
isobutyl
phenyl phosphate, tri-n-pentyl phosphate, tri-isopentyl phosphate, triphenyl
phosphate, isopropylated triphenyl phosphates, and butylated triphenyl
phosphates. Preferably, the trialkyl phosphate esters are those of tri-n-butyl
phosphate and tri-isobutyl phosphate.
'Be amounts of each type of phosphate ester in the hydraulic fluid
can vary depending upon the type of phosphate ester involved. The amount of
trialkyl phosphate in the base stock fluid comprises from about 10 wt% to
about
100 wt% preferably from about 20 wt% to about 90 wt%. The amount of dialkyl
aryl phosphate in the base stock fluid is typically from 0 wt% to 75 wt%
prefer-
ably from 0 wt% to about 50 wt%. The amount of alkyl diaryl phosphate in the
base stock fluid is typically from 0 wt% to 30 wt%, preferably from 0 wt% to
wt%. The amount of alkylated triaryl phosphate in the base stock fluid is
typically from 0 wt% to 20 wt% and preferably from 0 wt% to 15 wt%.
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The hydraulic fluids of this invention contain from 1 w /o to 20 wt%
based on total weight composition of additives selected from one or more
antioxidants,
acid scavengers, VI improvers, rust inhibitors, defoamers. The use of those
conventional additives provides satisfactory hydrolytic, oxidative stability
and
viscometric properties of the hydraulic fluid compositions under normal and
severe
conditions found in aircraft hydraulic systems.
Antioxidants useful in hydraulic fluid compositions in this invention
include, for example, polyphenols, trialkylphenols and di (alkylphenyl)
amines,
examples of which include bis (3,5-di-tert-butyl-4-hydroxyphenyl) methane,
1,3,5-
trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxyphenyl) benzene, 2,6-di-tert-
butyl-4-
methylphenol, tetrakis (methylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate)
methane, and di (n-octylphenyl) amine. Typical amounts for each type of
antioxidants
can be from about 0.1 wt% to 2 wt%.
Acid scavengers useful in hydraulic fluid compositions of this invention to
neutralize phosphoric acid and dialkyl phosphoric acid produced from the
hydrolysis
and thermal degradation of the phosphate ester base stocks. Examples of acid
scavengers include epoxy compounds such as epoxycyclohexane carboxylates.
Typical
amounts that can be used as acid scavenger can be from about I to about 10 wt%
based
on the total weight of hydraulic fluid. A preferred acid scavenger is
represented by the
following formula
0
Rr O5]
CH2~\ I
0 CH y R2
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where R, is H or a C, to C4 alkyl group, xis I or 2, y is an integer of 1 to
4, and R2 is a C, to C4
alkyl group or a phenyl group.
Suitable viscosity index improvers comprise poly alkylacrylates and
methacrylates
wherein at least 95 wt% of the viscosity index improver has a molecular weight
between 50,000
to 900,000.
EXAMPLE 1
This example illustrates the preparation of an additive of the present
invention. To 100 g of sulfonic acid in 750 ml methanol is added with stirring
enough
of the corresponding alkali metal hydroxide to neutralize the
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acid. The methanol is removed by flushing the solution with nitrogen at 40 T.
The product salt is then dried in a vacuum oven at 70-80 C for 24 hours.
EXAMPLE 2
This example describes the functional fluids containing an alkali
metal salt of a perfluoroalkyl sulfonate. The following functional fluids can
be
prepared by incorporating the particular salt into a tributyl phosphate,
triarylphosphate base oil containing conventional VI improver, epoxide acid
scavenger, antioxidant rust inhibitor and difoamer. FC-98 is currently used in
commercial aviation phosphate esters and has been used in Fluids D and F as a
comparison. FC-98 is a potassium perfluoroalkylsulfonate where R is a
cyclohexanic ring. Examples of salts are shown in Table 1.
TABLE 1
Fluid Additive Salt Concentration, wt%
1 Potassium 0.01
2 Lithium 0.5
3 Rubidium 0.01
4 Cesium 0.01
Potassium 0.5
6 Lithium 0.1
7 Quarternary 0.05
Ammonium
8 Ammonium 0.03
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TABLE 2
Fluid A Fluid B Fluid C Fluid D Fluid E Fluid F
Trialkyl Phosphate, wt% 70-75 70-75 70-75 70-75 70-75 70-75
Triaryl Phosphates, wt% 10-12 10-12 10-12 10-12 10-12 10-12
VI Improver, wt% 5-7 5-7 5-7 5-7 5-7 5-7
Additives, wt% 8 8 8 8 8 8
Anti-erosion, wt%
Lithium triflate 0.05 0 0 0 0 0
Potassium triflate 0 0.05 0 0 0 0
Potassium perfluorobutane 0 0 0.05 0 0 0
sulfonate
Potassium perfluorooctane 0 0 0 0.05 0 0.025
sulfonate (e.g., FC-98)
Potassium 3,3,4,4,5,5,6, 0 0 0 0 0.05 0
6,7,7,8,8,8-tridecafluoro
octane- l-sulfonate
EXAMPLE 3
This example illustrates the properties of the fluids from this
invention. The anti-erosion additive provides electrical conductivity which is
essential for the anti-erosion performance of the fluids. The example also
shows
an excellent solubility of the anti-erosion additive in the phosphate ester
fluid.
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TABLE 3
Fluid A Fluid B Fluid C Fluid D Fluid E*
Properties
Appearance after 1 week Clear Clear Clear Clear Clear
at -51C
Potassium, wppm 0 101 52 34 6
Lithium, wppm 21 0 0 0 0
Conductivity, S/cm 1.77 0.66 0.52 0.47 0.09
* 178 mg deposit collected on the filter after filtration of 500 g fluid due
to low
solubility.
EXAMPLE 4
This example shows the effectiveness of the compositions of this
invention to prevent erosion of the servo valve. The fluids used were those
described in Example 2. The erosion rig test was performed according to the
Boeing BMS 3-11L specifications. The tests were run at 225F and supplied
pressure was maintained at 3000 psi at the exception of Fluid D which has been
run at 275F. About 1000 ppm chlorine as 1,1,1-trichloroethane (methyl
chloroform) was added after about 200 run hours. About 200 ppm chloride was
used for Fluid D because it was run at higher temperature according to BMS 3-
11L specifications. The servo valve edge was analyzed by SEM to confirm the
presence of any servo valve edge electro-chemical erosion. The results of the
tests of the various functional fluids containing the perfluorinated
alkylsulfonic
metal salt of this invention are presented in Table 4.
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TABLE 4
Change
Initial Final in
Valve Leakage Leakage Run Leakage
(Different Rate, Rate, Time, Rate, Servo Valve
Valve) cc/min cc/min Hours cc/min Comments
Fluid A 5 & 7 225 740 119 621 No damage but
deposit on valve
Fluid B 2&4 245 265 650 20 No damage, no
deposit
Fluid C 6 & 8 300 325 650 25 No damage, no
deposit
Fluid D 6 & 8 290 275 524 15 No damage, no
deposit
Fluid F 1 &3 300 280 663 20 No damage, no
deposit
