PHYSICAL, CHEMICAL, A N D M E C H A N I C A L C H A R A C T E R I S T I C S OF A P O L Y P H E N Y L ETHER D O N N E L L R . W I L S O N , ' W I L L I A M A . M A R S H A L L S 2R O L A N D E . D O L L E s A N D R O B E R T J. B E N Z I N G Air Force Materials Laboratory, Research and Technology Division, WTight-PatteTSOn Air Force Base, Ohio
The formulation, properties, characteristics, and pump loop behavior are reviewed for m-bis(m-phenoxyphenoxy)benzene, one of the more promising polyphenyl ethers for high temperature hydraulic fluid applications. Data are given for kinematic viscosity, thermal conductivity, specific heat, density, vapor pressure, pour point, flash point, fire point, bulk modulus, evaporation weight loss, stability (hydrolytic, thermal, radiation, and oxidative), four-ball wear and transition temperature, effect of tricresyl phosphate as an antiwear additive, rolling contact fatigue, and pump loop behavior to 800' F. Excellent fluid performance was obtained in two closed pump loops at 400", 550", and 700" F.; however, at 800" F. gel tended to form and small increases in viscosity and neutralization number occurred.
HE past two decades have seen rapid advances in the requireTments for high temperature hydraulic fluids. Where systems operated a t 165" F. fluid temperatures 20 years ago, designs now exist for 700" F. bulk temperatures with hot spots well into the 800" F. range. Many fluid types are under consideration for the various applications. The wide variety of environments encountered by these fluids dictate a rather broad study of the physical and chemical properties of each fluid as well as its dynamic mechanical behavior in system mock-ups. The purpose here is to present the results for one of these fluids, a polyphenyl ether. The polyphenyl ethers today constitute a large class of fluids in an advanced stage of development. Preparation of the first members of the class was reported by Ullmann and Sponagel (1906). A copper-catalyzed coupling reaction, which now bears Ullmann's name, was used. Very little additional development work was done until 1955, when the U S . Air Force became interested in the polyphenyl ethers as potential radiation-resistant engine oils. The polyphenyl ethers as a class have many properties which make them of interest as high temperature hydraulic fluids: good tolerance to thermal and oxidative environments and high stability in radiation environments. In fact, these materials are classed among the most radiation-resistant and most thermally and oxidatively stable fluids currently available. Their high pour point [+40" F. for m-bis(m-phenoxyphenoxy)benzene] limits their operation to relatively high ambient temperatures unless external heating is provided. This is perhaps their biggest disadvantage. They are radiation-resistant with tolerances up to 10'0 ergs per gram C. They show promise as hydraulic fluids from room temperature to about 700" F. with short exposure to 800" F. Their lubricity is only fair, but antiwear materials, such as tricresyl phosphate, can be added. Although they do not meet all future high temperature requirements, they warrant consideration for many applications. Both Monsanto Chemical Co. and Shell Development Co. carried out much of the early Air Force contract work with the
' Present address, Midwest Research Institute, Kansas City, Mo. Deceased.
polyphenyl ethers. The Republic Aviation Co. (1963) also investigated one member of the polyphenyl ether class as a potential fluid for a 1000" F. system. These programs led to the decision to evaluate one ether, rn-bis(rn-phenoxyphenoxy)benzene, in an extensive hydraulic effort. The results of much of this work are presented here. rn-Bis(m-phenoxyphenoxy) benzene is an unsubstituted polyphenyl ether, primarily the meta isomer. I t is a five-ring member of the polyphenyl ether class with four ether linkages. I t is known by several designations: The Air Force designations are MLO-7463, MLO-59-692, ELO-62-29, ELO-64-68, and MLO-63-29; the Air Force designation of the material with 5y0 by weight of tricresyl phosphate is MLO-63-15; the Monsanto Chemical Co. designation of the base stock is MCS-192; and the chemical nomenclature abbreviation for the base material is mmm-5P4E. Most polyphenyl ethers can be made by coupling an alkali phenate with an aromatic halogen compound in the coppercatalyzed Ullmann ether synthesis (Ullmann and Sponagel, 1906). Both copper and copper salts have been used successfully. Methods of preparing the m-bis(rn-phenoxyphenoxy)benzene from m-phenoxyphenol and rn-dibromobenzene are discussed by Gunderson and Hart (Mahoney and Barnum, 1962) and Mahoney et al. (1959). Properties and Characteristics
Several of the more common properties of rn-bis(rn-phenoxyphenoxy)benzene are listed in Table I . Bulk Modulus. The isothermal secant and tangent bulk modulus values have been determined for this fluid to 700" F. and 10,000 p.s.i.g. with the apparatus shown schematically in Figure 1. The apparatus and experimental procedure are described by Hopkins et al. (1963, 1964). The isothermal secant and tangent bulk modulus values are plotted as functions of pressure and temperature in Figures 2 and 3. T h e secant bulk modulus is an average value determined over the pressure range from zero to any value selected on Figure 2. The tangent bulk modulus (Figure 3) applies to a particular pressure and was computed from measurements of the secant bulk modulus. The method of computation as well as data for other types of fluids is given by Hopkins et al. (1964). The bulk modulus values for mmm-5P4E are considerably higher VOL. 6
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Table 1.
Temp., O F.
Visc., Cs. 356
100 200 210 300 400 500 550
... 13.1 ... 2.1 1.3 1.2
... 0.7 ...
600 ...
700 750 800 850 900
Properties of m-Bir(m-phenoxyphen0xy)benzene Thermal Conductivity, Density, Vapor Pressure, B.t.u. FtdHr. Specific Heat, G./Ml. Mm. Hg Sq. Ft. F. B.t.u./Lb./" F. 1.19 0.0845 0.349 1.15 0,0809 0.393
...
1.11 1.07 1.03 1.oo 0.98 0.94
... ... ...
0.1 0.7 3.2 6.8 12 39
...
75
... ...
150 330 700
...
0 .Oi90 0.0782 0.0780
... ... ... ... ... ...
...
ASTM pour point Cleveland open cup flash point Cleveland open cup fire point Isoteniscope initial decomposition temp. Min. autogenous ignition temp. Molecular weight
0:437 0.481 0.525
... ...
...
Evaporation in 6.5 Hr., % wt. Loss ,..
... 5.7 27.6 85.5
... ... ... ... ...
$40' F. 650' F. 695 O F. 870 F. 1050" F. 446.5
600,000 P
ul
BLOWDOWN
3
500,000
0 2
400,000 J Y
2
300,000
z a
THERMOCOUPLE
$ 200,000 J
a W
I
6
'" Schematic diagram of bulk modulus apparatus
than those determined for silicones, mineral oils, and esters (Hopkins et al., 1964). A hydraulic system containing this polyphenyl ether could be expected to be relatively stiff. Stability. Thermal stability experiments were conducted with mmm-5P4E in the stainless steel pressure cylinder a t the Petroleum Refining Laboratory, Pennsylvania State University (Table 11). T h e data indicate that thermal stability a t 850' F. is good, but that the fluid will degrade a t 900' F. and above. This observation is supported by the isoteniscope decomposition temperature of 870' F. for mmm-5P4E reported by Mahoney et al. (1959). ' The results of micro-oxidation (no metals present) and oxidation-corrosion (metals present) tests a t 500°, 550', and 600' F. are reported in Table 111. I n these tests, 1 liter per hour of dry air was bubbled through 20 ml. of heated test fluid for 24 hours. A 12-inch bulb-type reflux condenser was used. I n the corrosion test, five metal washers were mounted on the air entry tube and immersed in the test fluid. Basically, the experimental equipment and procedure used are described by Christian (1956). T h e neutralization number before and after each test was less than 0.1 mg. of KOH per gram of fluid. I n all tests (with and without metals), the average 100' F. viscosity increase after tests a t 500' F. was 4.070, at 550' F. was lO.O%, and a t 600" F. was 56.0%. The amount of fluid lost during all the experiments remained a t about 0.4% and was not appre82
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
700'F
0
tpoo
5,000
10,ooo
P R E S S U R E , PSlG
LPRESSURE VESSEL
Figure 1 .
---------
Figure 2. Effect of pressure and temperature on the isothermal secant bulk modulus of rn-bis(rn-phenoxyphen0xy)benzene 600,000 m3 I 2
n
500,000
0
I y J
400,000
3
m
+ 300,000 z W (3
f 200,000 -I
2a
w I
s l-
100,000 0 1,000
5,000
l0,OOO
PRESSURE, PSlG
Figure 3. Effect of pressure and temperature on the isothermal tangent bulk modulus of rn-bis(rn-phenoxyphenoxy)benzene
ciably affected by increases in test temperature. The low fluid losses were undoubtedly due to the use of a reflux condenser. The aluminum, titanium, and 301 stainless steel specimens did not change weight during any of the corrosion tests, but the titanium and stainless steel specimens were some-
what tarnished. Generally, the silver specimens were lightly tarnished and lost some weight, the weight loss increasing slightly with temperature. The M-10 tool steel specimens were dark in color and gained a little weight a t 500' F. but a t 550' and 600' F. the weight change was nil. The fluid used in all tests became darkened and red-brown crystals were found in the bottom part of the reflux condenser after all 600' F. tests. The hydrolytic stability was determined a t 200' F. for 48 hours (Table IV). Only minor changes occurred in the oil viscosity, the weight of the copper specimens, and the neutralization numbers of the oil and aqueous layers. No insolubles were found in the oil layer. Foaming tendency was investigated (Table V). The collapse time is relatively long a t room temperature but rapid a t 200' F. Radiation resistance studies of several materials, including polyphenyl ethers, have been reported by Mahoney et al. (1959, 1962). The polyphenyl ethers are considerably more resistant to viscosity increases induced by radiation than are aliphatic esters or hydrocarbons. Radiation levels from a 3-m.e.v. Van de Graaff generator of 2.8 X loL1ergs per gram were sufficient to gel di-2-ethylhexyl sebacate, an aliphatic hydrocarbon, a methyl phenyl silicone, and a chlorophenyl silicone. I n contrast, polyphenyl ether viscosity increases were only moderate at 5.5 X 10" ergs per gram. The oxidation stability of irradiated fluids is also reported by Mahoney et al. (1959, 1962). Although the polyphenyl ethers have an initial rapid loss of oxidation stability, they are still less sensitive to radiation than a typical methyl phenyl silicone, phenyl phthalate, and ester-based oils. Meta-linked unsubstituted polyphenyl ethers, such as mmm-5P4E, have a very high initial oxidation stability. After irradiation to 2.8 x 10" ergs per gram, they will absorb oxygen more slowly than typical unirradiated inhibited lubricants. Four-Ball Evaluations. Two types of four-ball investigations were conducted: standard runs in which all conditions were held constant and transition temperature runs in which the temperature was increased a t 10' per minute from 100' to about 800' F. while all other conditions were held constant. The results of the standard experiments are listed in Table VI. Parameters studied were speed, fluid temperature, ball material, and load. I n addition, 5% by weight of tricresyl phosphate was added to the mmm-5P4E fluid used in two of the runs. Tentative conclusions may be made about the effect of each of the five variables:
d
'1
1. A speed increase from 600 to 1200 r.p.m. resulted in wear scar increases on the order of 130 to 145% with 52100 steel balls, 2. A temperature increase from 167' to 400' F. resulted in wear scar decreases to 65 to 90% of the 167' F. values. 3. Wear scars with M-10 tool steel were on the order of 150 to 175yo larger than those with 52100 steel. 4. Increases in load from 10 to 40 kg. increased the wear scar sizes about 140 to 160%. 5. Addition of 5% by weight of tricresyl phosphate reduced wear scar diameters about 45 to 55%. Transition temperature four-ball experiments were conducted to determine the temperature a t which boundary lubrication breaks down. Data from these tests, which are run under a special set of conditions, are useful in comparing fluid types but are not absolute values. A four-ball apparatus was used that had been modified (Hopkins and Wilson, 1964) to permit operation a t high temperatures and continuous recording of the torque transmitted from the spindle to the ball pot. Transition temperature data for several types of fluids, including VOL. 6 NO. 2
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Table 111.
Micro-oxidation and Oxidation-Corrosion Data for m-Bis(m-phenoxyphen0xy)benzenea
(24-hr. duration, 20-ml. sample, 1 liter air per hour, 12-inch bulb-type reflux condenser) Test TfmP.9 F. 500
100" F . Viscosity, Cs.
yo
W t . Change ( M g . / S q . Cm.) of Corrosion Specimens and Appearance after Testb Aluminum Titanium M-70 steel Type 301 SS'
% Fluid
Loss Silver RemarksC 0.4 No corrosion specimens used 0.4 No corrosion specimens used 0 -0.04/LT 0 . OO/NT 0 .OO/LT +O .04/DT O.OO/MT 0 -0.01 /LT 0 . OO/NT 0 . OO/LT +0.02/DT 0 . OO/MT 550 365 0.4 No corrosion specimens used 0.4 No corrosion specimens used 0.4 -0,04/LT O.OO/NT O.OO/LT O.OO/DT O.OO/MT 0.4 -0,06/LT O.OO/NT O.OO/LT O.OO/DT O.OO/MT 600 365 0.8 No corrosion specimens used Red-brown crystals at 0.4 No corrosion specimens used bottom part of re0.4 O.OO/d O,OO/NT O.OO/LT +O.Ol/DBT O.OO/MT flux condenser 0.4 -O.lO/NT O.OO/NT O.OO/LT +O.Ol/DBT O.OO/MT NT-no tarnish. DT-dark tarnish. a Tests conducted in accordance with procedure described in W A D C - T R - 5 5 - 4 4 9 , Part 3 (Christian, 1956). LT-light tarnish. DBT-dark blue tarnish. MT-moderate tarnish. c Fluid darkened during all tests. d Specimen had a thin brown coating that could not be wiped off, which may have compensated for any weight loss due to incipient corrosion. Coating was not present in duplicate test. Before
After
Change
365
378 378 379 383 399 410 399 399 618 602 507 550
3.6 3.6 3.8 4.9 9.3 12.3 9.3 9.3 69.3 64.9 38.9 50.7
1
Table IV.
Hydrolytic Stability of m-Bis(m-phenoxyphen0xy)benzene" Oil viscositv change at 210" F.. % -1.1
Neut. No. GhangePoil layer, mg.'KOH/gram Neut. No: change, aqueous layer, mg. KOH/gram Copper weight change, mg./sq. cm.-appearance Before brushing After brushing Insoluble material in oil layer
-0.02
+o. 01 - 0 . 0 1/2B, - 0.01/2B,
moderate tarnish moderate tarnish
0.00 Test conducted at 200" F . for 48 hr. per Federal Test Method Std. No. 791, Method 3457 (General Services Administration, 1955). a
Table V.
Foaming Tendency of m-Bis(m-phenoxyphenoxy)benzene" Foam Volume for 5-Min. Blow Period MI. F.
75 200 75
175 85 40 Foam Collapse Time
F.
Rolling Contact Fatigue. The rolling contact fatigue life of M-50 steel at 425' F. was studied with several fluids by Bamberger and Moore (1961). Five of the fluids, including a mixed isomer of bis(phenoxyphenoxy)benzene, were candidates for lubricants in high mach air-breathing propulsion systems. Tests were run in a General Electric rolling contact rig, described by Bamberger and Moore (1961), using the diester MIL-L-7808 as the reference fluid and the life (fatigue life that 90% of the specimens in a group will exceed) as the criterion (Table VIII). The stress pattern produced by the rolling contact rig is similar to that imposed on the inner race of a ball bearing. TWOof the candidate fluids, bis(phenoxyphenoxy)benzene (GTO-929) and methyl phenyl silicone (0-58-48), were appreciably superior to the diester MIL-L-7808 fluid. Two other fluids, GTO-948 and 0-60-7, which are both ester-based derived from trimethylolpropane, yielded results essentially the same as MIL-L-7808. The other candidate fluid, ester-based 0-58-18, was inferior to MIL-L-7808. A grade 1100 mineral oil, reference oil B, was also evaluated and had the shortest Blo life. Behavior in Pump loops at High Temperatures
Min.
26 0.67 7 a Test conducted per Federal Test Method Std. No. 791, Method 321 7 ( A S T M D 892).
One polyphenyl ether, mmm-5P4E, has been pumped through two hydraulic circuits fabricated for evaluation of the effects of long-term shear a t high temperatures on potentially useful hydraulic fluids and lubricants. Schematics of these circuits are shown in Figures 4 and 5 .
mmm-5P4E, are listed in Table V I I . The experimental procedure consisted of starting each run with the fluid temperature at 100' F. with a given speed and load. The fluid temperature was then increased at about 10' per minute, while the load and speed were held constant. The transition temperature was defined as that bulk fluid temperature a t which the torque transmitted from the ball clamped in the spindle to the three balls clamped in the ball pot increased suddenly by a factor of 2 or more. The data in Table VI1 indicate that the transition temperatures for mm-5P4E are about the same as those for the esters of pentaerythritol and higher than those for the mineral oils and silicones. Judging from the results of a limited number of experiments, the addition of 57'0 tricresyl phosphate to the mmm-5P4E will raise the transition temperature under the 40-kg. load another 60' F.
The "pump stand" (Figure 4) utilizes a triplex homogenizing pump with a discharge pressure of 3000 p.s.i.g. and circulates fluid at about 5 gallons per minute. This circuit includes a filter used to detect sludge-forming tendencies, a lacquer indicator, and a corrosion indicator. Because the pump has no sliding metal-to-metal contacts, the lubricity of the fluid being pumped is of little concern. Experiments can be run with fluid temperatures as high as 800" F. Sormally these experiments are conducted continuously for 100 hours at a constant temperature. The "high temperature hydraulic circuit" (Figure 5) utilizes a variable flow rate piston-type aircraft hydraulic pump. The pumps used for the work reported here were the New York Air Brake Co. Model 69WO3006-2 seven-piston pumps. Normally, a fluid is evaluated by running it at 400' F. for 50 hours, 500' F. for 50 hours, 550' F. for 50 hours, etc., until either the fluid degrades or the pump fails. During these runs, the pump is driven at 3750 r.p.m. and the discharge pressure is cycled for 1 minute at about 2700 p.s.i.g. (maximum driving horsepower and flow of about 9 gallons per minute) and 1 minute a t about 3000 p.s.i.g. and 0.5 gallon per minute.
75 200 75
84
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
Table VI.
Test Temp., F. 167
Four-Ball Wear Evaluations of m-Bis(m-phenoxyphenoxy) benzene
Spindle Speed, R.P.M. 600
Steel Ball Material 52,100
1200
52,100
(2-hr. duration) Load, 10 40 10 40 10 40 10 40 10 40 40 10 40 10 40
M-1 0 600
400
52,100 M-10
1200 600
167
52,100
M-1 0
400
Table VII.
Wear Scar Diameter, Mm. 1.100 1.675 1.578 2.282 2.362 3.592 0.992 1.166 1.751 2.004 1.488 0.483 0.795 0.781 1.100
Fluid mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E mmm-5P4E 5Y0 TCP mmm-5P4E 570 TCP mmm-5P4E 5% TCP mmm-5P4E 5% TCP
4.
+ + + +
Transition Temperatures for Various Fluids (' F.)
(Spindle Speed, 574 r.p.m.) Load, Kg. MLO 56-839 DC 560 MLO 8200 MLO 7277 MLO 60-294 MLO 56-848 MLO 57-426 MLO 58-432 MLO 63-29
Fluid Phenyl methyl silicone Chlorinated phenyl methyl silicone Hexa(2-ethylbutoxy) disiloxane Naphthenic mineral oil Deep dewaxed mineral oil Pentaerythritol ester Tetracaproate ester of pentaerythritol Bis(p-phenoxyphenyl) ether m-Bis(m-phenoxyphenoxy )benzene
10
20
230 250 >650 >675 >700 >750 >800 >800 >725
