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This invention relates to improved synthetic phosphate ester functional
fluids particularly suitable for use as hydraulic fluids and the like.
More particularly, the invention relates to triaryl phosphate ester
functional fluid compositions derived from mixtures of
tertiary-butylphenyl/phenyl phosphates which exhibit a high viscosity
index and a low pour point, and also exhibit good hydrolytic, thermal and
oxidative stability, good wear properties, low shear stability, good
lubricity and fire-resistance.
The functional fluids of this invention are organic flame-retardant liquid
liquid compositions. Many different 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, heat
transfer fluids, heat pump fluids, refrigeration equipment fluids and as
filter mediums for air conditioning systems.
U.S. Pat. No. 3,992,309 to Dounchis teaches functional fluids with
exceptional thermal and oxidative stability which employ as a base stock
from 10% to 90% by weight of a "mixed t-butylphenyl/phenyl phosphate" and
90% to 10% by weight of organic esters of carboxylic acids with polyol
compounds such as pentaerythritol, trimethylolethane and
trimethylolpropane, neopentyl glycol esters and the like, the carboxylic
acids generally having from about 5 to 10 carbon atoms. Base stocks
employing the tertiary-butylphenyl/phenyl phosphate and polyol esters,
exhibit good extreme pressure and anti-wear properties. These are
desirable properties for a hydraulic fluid, particularly in combination
with the unusual degree of thermal stability, and the compatibility of
these materials when compounded with minor amounts of additives often used
for other functional fluids to improve the properties thereof. However,
the compositions of the '309 patent do not have both the high viscosity
index and the low pour point together with the otherwise desirable
properties.
U.S. Pat. No. 3,629,114 to Fairing discloses that the additive response of
a phosphate base stock will vary according to the type of phosphate base
stock. That is, the concentration of an ester compound will vary according
to whether the phosphate base stock is a trialkyl phosphate, a triaryl
phosphate, a mixed alkylaryl phosphate or a mixture of the above phosphate
base stocks. This lack of predictability makes it difficult to formulate a
phosphate base stock to meet specific requirements of functional fluids.
In general, a phosphate ester functional fluid is sufficiently nonflammable
to satisfy requirements for fire resistance. The viscosity characteristics
of a functional fluid must be such that it may be used over a wide
temperature range; that is, a low rate of change of viscosity with
temperature (the viscosity index). Further, its pour point should be low;
its volatility should be low at elevated temperatures of use; that is,
selective evaporation or volatilization of any important component should
not take place at high temperatures of use. It must possess sufficient
lubricity and mechanical stability to enable it to be used in hydraulic
systems of aircraft which are exceedingly severe on the fluid used. It
should be chemically stable to resist such chemical reactions as oxidation
and thermal degradation so rthat it will remain stable under conditions of
use against loss of desired characteristics, due to high and sudden
changes of pressure, temperature, high tensile stresses, and contact with
various metals.
The present invention is a shear-stable composition meeting the severe
requirements of a functional fluid comprising a mixture of a major amount
of a tertiary-butylphenyl/phenyl phosphate ester and a minor amount of a
polyol ester pour point depressant as a base stock, and from about 0.1% to
2% of a homopolymer polystyrene having a molecular weight of about 200,000
to about 300,000.
The tertiary-butylphenyl/phenyl phosphate ester is defined as a mixed
alkylated phenyl phosphate ester containing between about 15% to 60%,
preferably 30% to 50%, by weight mono-t-butylphenyl radicals or mixtures
thereof with di-t-butylphenyl radicals, the di-t-butylphenyl radicals
being present in an amount between about 1% to 10% by weight based on the
weight of the mono-t-butylphenyl radicals. A particularly desirable
phosphate ester is commercially available from FMC Corporation under the
designation of DURAD.RTM. 150-B phosphate hydraulic fluid.
The base stock of the present invention to comprise a major amount (greater
than 50% by weight) of a tertiary-butylphenyl/phenyl phosphate ester for
fire retardancy and a minor amount (less than 50% by weight) of a polyol
ester. Desirably, the base stock comprises between 60% to 90% by weight of
the phosphate ester and between 40% and 10% of a polyol ester preferably
about 70% to 80% by weight phosphate ester and about 20% to 30% by weight
of a polyol ester which functions as a pour point depressant.
The polyol ester incorporated into the base stock can be any polyol ester
suitable for use as a synthetic oil, such as esters of polyols and of
C.sub.4 to C.sub.12 straight or branched chained monocarboxylic acids.
These compounds are prepared by reacting a polyol such as pentaerythritol,
dipentaerythritol, tripentaerythritol, trimethylol propane, trimethylol
ethane, trimethylol butane, neopentylglycol and the like with carboxylic
acids such as butyric acid, valeric acid, isovaleric acid, caproic acid,
hexanoic acid, caprylic acid, pelargonic acid, capric acid, lauric acid
etc. Particularly good results are obtained with mixed esters of mono- and
di-penta-erythritol with C.sub.5 -C.sub.10 straight chain carboxylic
acids. Exemplary synthetic fluid bases which are commerically available
include Hercolube J, Hercolube B, Hercolube A, Hercolube C, all marketed
by Hercules Incorp., Unilever 14,636, Unilever 14,735, marketed by
Unilever Corp., HATCOL 2377 and HATCOL 2925 marketed by Hatco Chemical
Corporation and Stauffer Base stocks 700, 704, 800, marketed by Stauffer
Chemical Company. Particularly desirable is an ester prepared from
commercial grade pentaerythritol and C.sub.5 to C.sub.10 short chain fatty
acids available from Hatco Chemical Corporation under the name of HATCOL
2970 polyol ester.
The present invention requires one to incorporate into the functional fluid
as a viscosity index improver from about 0.1 to about 2 parts of a
homopolymer polystyrene, per hundred parts of base stock, the homopolymer
polystyrene having a molecular weight between about 200,000 and 300,000.
Viscosity index improvers are generally long-chain, high molecular weight
polymers. The most common viscosity index improvers are methacrylate
polymers and copolymers, acrylate polymers, olefin polymers and
copolymers, and styrene-butyadiene copolymers. Modified polystyrenes
having a molecular weight between 10,000 and 150,000 are sometimes
employed as viscosity index improvers, such as the alkylated polystyrene
taught by U.S. Pat. No. 2,707,176, which employs a base solution of
triarylphosphate and a chlorinated biphenyl. However, the patent teaches
that the preferred molecular weight range is between 60,000 and 80,000 as
those prepared from a polystyrene having a molecular weight above 80,000
frequently become degraded during use, particularly where the functional
fluid is subjected to severe shearing stress. British Pat. No. 951,769
teaches a hydraulic fluid composition employing a triaryl phosphate, such
as tricresyl phosphate, and up to 15% by weight of polystyrene having a
molecular weight between 10,000 and 100,000. In view of these teachings of
the prior art it is suprising that a homopolymer of polystyrene having a
molecular weight between 200,000 and 300,000 would function as a viscosity
index improver in the base stock of the present invention. One
particularly desirable polystyrene has a molecular weight between 235,000
and 245,000 of with a glass transition temperature of 100.degree. C. and
is available under the name of STYRON 666D general purpose polystyrene
from Dow Chemical Company. To avoid affecting the pour point it is
desirable that the polystyrene be added in an amount from about 0.1 part
to 1 part per hundred parts of base stock.
As taught by the '309 patent to Dounchis other additives may be
incorporated into the base stock, such as very minor amounts, about 0.1%
to 5% by weight, of lubricant additives such as rust inhibitors, corrosion
inhibitors, anti-foam agents, anti-wear agents, cavitation inhibitors and
similar special purpose additives.
A preferred embodiment of the present invention is a hydraulic fluid
comprising a base stock consisting of 75 parts by weight DURAD.RTM. 150B
tertiary-butylphenyl/phenyl phosphate ester 25 parts by weight HATCOL 2970
polyol ester. The base stock is incorporated with 0.5% by weight
polystyrene with a molecular weight of from 235,000 to 245,000 and with
effective quantities of a copper passivator such as benzotriazole, a rust
inhibitor such as a mixed C.sub.8 to C.sub.12 mono and dialkyl acid
phosphate, an antioxidant such as
2,6-di-tertiary-butyl-dimethylamino-p-cresol, a silicone antifoam, and an
acid acceptor such as 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane
carboxylate.
The best mode of practising the present invention will be clear to one
skilled in the art from the following nonlimiting examples.
EXAMPLE 1
Preferred Composition
The base stock was prepared by mixing 75 parts by weight DURAD.RTM. 150B
tertiary-butylphenyl/phenyl phosphate with 25 parts by weight HATCOL 2970
polyol ester at room temperature. The 100 parts by weight of base stock
was heated to 121.degree. C. to speed the dissolution of 0.005 parts
benzotriazole (a copper passivator), 0.01 parts Ortholeum brand mixed
C.sub.8 -C.sub.12 alkyl acid orthophosphate (a rust inhibitor), 0.015
parts 2,6-di-t-butyldimethylamino-p-cresol (an anti-oxidant), 0.001 part
silicone antifoam and 1 part
3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate (an acid
acceptor). The temperature was reduced to about 100.degree. C. and 0.5
parts of finely ground STYRON.RTM. brand polystyrene resin was added
slowly with vigorous stirring. After all the resin was dissolved the
hydraulic fluid was cooled to room temperature with continued stirring.
The properties were determined by standard procedures and the results are
listed in Table I.
Table I demonstrates that the composition has excellent shear stability
with only a 9.06% drop in viscosity after being subjected to some shear
compared with the 15% allowable to pass the test. This is in contrast with
the teachings of the prior art that a viscosity improver based on a
polystyrene having a molecular weight of above 80,000 frequently becomes
degraded when subjected to severe shearing stress. The table further
demonstrates the excellent flame retardancy and stability properties
required for a hydraulic fluid.
EXAMPLE 2
A series of base stocks were prepared according to Example 1, but employing
varying proportions of tertiary-butylphenyl/phenyl phosphate and various
polyol esters as indicated in Table II. The additives were also
incorporated according to the method of Example 1. Some runs incorporated
a commercial viscosity index improver, "R&H", PA 4385 methacrylate polymer
as a control. In other runs the high molecular weight polystyrene "Poly S"
of the present invention was incorporated. As seen from Table II, it is
significant that the high molecular weight polystyrene was effective in
producing a viscosity index of over 100 at a use rate of less than 1 part
per hundreds parts of base stock. At that use rate the additive had
little, if any, adverse effect on the pour point. The commercial Rohm &
Haas viscosity index improver, "R&H", required use rates as high as 3 or 4
parts per hundred parts of base stock. Such a high use rate is undesirable
as it can adversely affect the flame-retardant properties of the
functional fluid composition as well as the thermal and oxidative
stability of the base stock. Table II also indicates the high molecular
weight polystyrene to be effective at use rates as low as 0.1 part per
hundred parts of base stock. Runs 1, 2, 11, 12, 16 and 18 correspond to
compositions taught by the '309 patent to Dounchis.
TABLE I
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TEST RESULTS OF EXAMPLE I
Test Results
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Kinematic Viscosity
37.0 .times. 10.sup.-6 m.sup.2 s (cts)
37.8.degree. C. (100.degree. F.)
Kinematic Viscosity
5.68 .times. 10.sup.-6 m.sup.2 s (cts)
100.degree. C. (212.degree. F.)
Viscosity Index 101
Pour Point -31.1.degree. C. (-24.degree. F.)
Total Acid No. 0.05 mg KOH/g
Specific Gravity 1.127 at 20.degree. C.
Flash Point, PMCC 280.degree. C. (536.degree. F.)
Autogenous Ignition
438.degree. C. (820.degree. F.)
Moisture Content 0.04 wt. %
Shell 4-Ball Wear Test
0.60 mm Scar Dia.
(ASTM D-2266)
1200 RPM/75.degree. C./40 kg/1 hr.
Sonic Shear Stability
-9.06%
(ASTM-D2603)
Rust Test, Part A Pass
(ASTM-D-665) Part B
Pass
Hydrolytic Stability: ASTM-2619
Cu, wt. loss 0.025 mg/cm.sup.2
Acid No. Increase, NIL mg KOH/g
Org. layer,
Total Water Layer Acidity
1.12 mg KOH
Thermal and Oxidative Stability: (FTMS 5308.6)
72 hrs. at 175.degree. C.
Metals Change, mg/cm.sup.2
w/air purge
Copper -0.007 mg
Steel NIL
Magnesium -0.007
Aluminum - 0.022
Silver -0.015
Total Acid No. After Test
0.05 mg KOH/g
Viscosity at 100.degree. F.,
36.76 cts
After Test
% Viscosity Change -0.65%
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Note: English units are enclosed in parentheses
TABLE II
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COMPARISON OF EFFICACY OF POLYSTYRENE AND A
COMMERCIAL VISCOSITY INDEX IMPROVER
BASE STOCK
PARTS Parts Viscosity
Pour
PO.sub.4 Polyol
Polyol Ester
Index Improver
Point
RUN Ester
Ester
Tradename
Poly S
R & H
.degree.C.
V.I.
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1 85 15 HATCOL 2970
0 0 -30
56
2 50 50 HERCOLUBE C
0 0 -43
78
3 75 25 HERCOLUBE C
0 3 -34
104
4 75 25 HERCOLUBE C
0 4 -36
102
5 85 15 EMERY 2935
0 3 -30
106
6 80 20 EMERY 2935
0 3 -31
106
7 70 30 EMERY 2935
0 3 -39
117
8 75 25 HERCOLUBE C
1 0 ND 129
9 75 25 HERCOLUBE C
0.5 0 -31
108
10 75 25 HERCOLUBE C
0.5 0 ND 101
11 85 15 HATCOL 2377
0 0 -26
48
12 85 15 HATCOL 2925
0 0 -32
51
13 85 15 HATCOL 2925
0.5 0 -31
100
14 85 15 HATCOL 2970
0.5 0 -29
99
15 85 15 HATCOL 2925
0.5 0 -30
96
16 75 25 HERCOLUBE J
0 0 ND 64
17 75 25 HERCOLUBE J
0.5 0 ND 106
18 75 25 HERCOLUBE F
0 0 ND 82
19 75 25 HERCOLUBE F
0.1 0 ND 89
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Key
ND = not determined
POLY S = homopolymer polystyrene (Inventive Examples)
R & H = Rohm & Haas PA 4835 polymethacrylate viscosity index improver
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