GREASE-TYPE LUBRICANTS COMPATIBLE WITH MISSILE FUELS AND OXIDIZERS J O S E P H M E S S I N A A N D
H E N R Y G I S S E R
U. S. Army Munitions Command, Pitman-Dunn Institute for Research, Frankford Arsenal, Philadelphia 37, Pa. Thickening of mixed perfluorotrialkylamines [alkyl = Cd to C,) with tetrafluoroethylene polymers (molecular weights 2000 t o 30,000) was studied in connection with the development of grease-type lubricants for liquid fuel-powered missiles. Grease-type mixtures were stable to shear stresses and showed no separation on standing (up to one year) and little separation in the cone tests at 100' C. The greases were unreactive with and insoluble in ethyl alcohol, JP-4, unsyrn-dimethylhydrazine, diethylenetriamine, a 60: 40 mixture of the last two, a 50:50 mixture of unsyrn-dimethylhydrazine and hydrazine, 90% hydrogen peroxide, and inhibited red fuming nitric acid. There was no explosive reactivity in impact tests with liquid oxygen or nitrogen tetroxide. A typical grease exhibited antiwear and extreme pressure properties (four-ball tests) comparable to conventional petroleum greases, and did not attack most conventional elastomers. Average particle size of the polymers was 5 microns. HE MOVISG PARTS which require lubrication in a typical Tliquid fuel-powered rocket motor include reduction gears, turbine pumps, gimbal bearings, and valves. The single most important property of a lubricant (grease or oil) for rocket motors is complete unreactivity with the liquid fuels and oxidizers used. Otherwise there is danger of malfunction-either interference with operation of a lubricated part or initiation of a destructive chemical reaction. I n either event, the entire rocket system may be lost. Solubility of the lubricant in the fuel or oxidizer is also undesirable, but precise solubility limits have not been established and the extent to which very slight solubility may cause malfunction depends on the particular materials system combination. Consideration must also be given to lubricity, antiwear, load-carrying capacity, and other physical and chemical properties which determine longterm stability and usefulness over a wide temperature range. Since conventional lubricants such as petroleum, dicarboxylic acid esters, silicates, silicones, and polyglycols are either miscible with the fuels. 01- reactive or explosive a t high impact energy levels with the oxidizer (3),they may not be used as lubricants for rocket engines. The development of liquid-fueled rocket motor technology has led to considerable \%ark on development of lubricants, mostly for a particular liquid fuel rocket motor system (70, 72, 74). The Lvork on compatibility (solubility and reactivity) of organic compounds with liquid fuels and oxidizers has been reviewed (2). More rrcent work (3) showed that one liquid, perfluorotributylamine, and two solids, polytetrafluoroethylene (Du Pont Teflon - TFE) and tetrafluoroethylene-hexafluoropropylene copolymer (Teflon -FEP), were inert with all fuels and oxidizers tested, which included diethylenetriamine (DETA), unspm-dimethylhydrazine (UDMH), hydrocarbon fuel (JP-4), ethyl alcohol, 90% hydrogen peroxide, inhibited red fuming nitric acid (IRFNA), and liquid oxygen (LOX). The work described in this paper included all of the above fuels and oxidizers, as well as nitrogen tetroxide and a 1 to 1 mixture of U D M H and hydrazine. Perfluorotributylamine, in addition to inertness, exhibits lubrication properties comparable to conventional petroleumbase fluids, but its volatility is too high for general use. Although homologs u p to perfluorotrioctylamine have been reported in the patent literature (5), the available material of lowest volatility was a mixture of perfluorotrialkylamines (alkyl = Cq to CJ. Perfluorotributylamine evaporates com-
pletely in one hour a t 98.8' C. (ASTM iMethod D 972), while the mixed perfluorotrialkylamines lose approximately S5%, by evaporation in 22 hours under the same conditions. The mixed perfluorotrialkylamines are also compatible (insoluble and unreactive) with all of the fuels and oxidizers previously mentioned (8) except for a moderate solubility in nitrogen tetroxide. The development of a grease that is compatible with fuels and oxidizers requires that both the thickening agent and base fluid be unreactive. The two inert fluorinated resins mentioned above appeared to be promising candidates as thickening agents; however, a mechanically stable grease could not be prepared using the T F E or F E P fluorinated resin. Conventional and special preparation techniques described by Irwin ( 4 ) )including modifications in inert atmospheres a t high pressures and temperatures, were tried but no satisfactory mixture having a greaselike consistency was obtained (8). The inertness of the perfluorinated polymers led to a n investigation of a polymer of lower molecular weight and it was found that a polytetrafluoroethylene of approximately 5000 molecular lveight was a n effective thickening agent for the mixed perfluoroalkylamines. This led to a further investigation of polytetrafluoroethylenes of low molecular weight as thickening agents. This paper describes the preparation and properties of a series of grease-type lubricants made from the mixed perfluorotrialkylamine fluid and tetrafluoroethylene polymers in the molecular weight range from 2000 to 30.000. Grease Preparation
Materials. T h e fluid used for all greases was a mixture of perfluorotrialkylamines (Minnesota Mining and Manufacturing Co., FX-45) in which the alkyl groups varied from C4 to Cg. Typical properties of this fluid are: boiling range 230' to 300' C.; density at 25' C., 1.9; viscosity a t 25' C., 141 cs.; pour point, -20.6' C. T h e thickening agents were all tetrafluoroethylene polymers (Du Pont) (Table I). All these materials had average particle size of 5 microns, and were received as suspensions in trichlorotrifluoroethane (in concentrations varying from 3 to 20%). Dispersion Procedure. T h e dispersion of resin in trichlorotrifluoroethane was heated on a steam bath until 50 to 7S% of the solvent evaporated, leaving a viscous dispersion of the consistency of glycerol. Approximately 757, of the required quantity of mixed perfluorotrialkylamines was then added, the mixture was stirred, and heating was continued VOL. 2
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SEPTEMBER 1963
209
Table 1.
Code
Au. M o l . W t .
T-20 T-30 T-45 T-50 T-57 T-100 T-100.A T-120 T-200 T-250 T-300
2,000
c4
3,000
Table II.
Physical Stability. A sample of G-50, after storage in the laboratory for approximately one year, and samples of G-100 and G-300 stored for several weeks, showed no fluid separation. Shear stability tests \yere run using the one-quarter (ASTM D 1403) grease worker and oil separation was determined a t 100" C. for 30 hours using Federal Standard Test Method 321.2. Table I11 shows that the greases are shear-stable and that fluid separation is low.
%
0.6 1 .o 1 .o 0.0
4,500 5,000 5,700 10,000 10,000 12,000 20,000 25,000 30,000
0.5 0.20 0 0 0.0 0.03 0.03 0.01
Reactivity with Fuels and Oxidizers and Other Materials
Fuels and Oxidizers. Contact compatibility tests were run a t 25' i 1' C. ( 3 ) . Preliminary experiments were run o n approximately 0.03 gram of thickening agent to ensure that there was no explosive reactivity; tests were continued using suitable precautions against later reactivity. To 1.0 ml. of test reagent (fuel or oxidizer) in a 5-ml. graduated cylinder was added approximately 0.03 gram of thickening agent with thorough shaking. If there was no evidence of solubility or reactivity, further increments were added with shaking until 1 gram of gelling agent had been used. If no reaction occurred, a 1-gram sample of the grease was placed in a 5-ml. graduate and 1 ml. of fuel or oxidizer added. Visual observations preceded by shaking were made after 5 minutes and 1, 24, 48, and 72 hours. The fuels and oxidizers used were EtOH, JP-4, U D M H , DETA, a 60:40 mixture of U D M H and DETA, a 50:50 mixture of U D M H and hydrazine, 90% Hz02, IRFNA, and Nz04. T h e tests using N204 were run in closed, pressure glass jars, 1 X 5 inch.
Grease Composition
Fluoroaminr, Cod?
Thickening Agent
CI /o
G-20 G-45 G-50 G-100 G-300
Table 111.
ature treatment was omitted, a stable greaselike product could not be obtained.
Polytetrafluoroethylenes
71.8 72.4
T-20 T-45 T-50 T-100 T-300
80.0
80,6 82.0
Shear Stability and Fluid Separation Unaorked Penetration
Worked Penetration'"
Separation,
Grease
G-50 G-100 G-300
286 294 266
286 286 256
4.7 3.1 3.1
After 500 strokes in
%
The results of the contact compatibility tests are given in Tables IV and V. Earlier data (3)indicated that compounds containing appreciable chlorine are not inert with the amines. This was again observed here. Compounds having 0.5% or more chlorine showed some reactivity with the amines. However, compounds T-1004, T-120, and T-250, although having no chlorine (0.03% for T-250), showed some reactivity with U D M H and some swelling with E t O H and JP-4. I t is not known whether this is due to a n impurity or some preparation variable-Le.. polymerization catalyst, terminal group. etc. Although the perfluoroamine is soluble to the extent of 25%
scale worker.
until all the trichlorotrifluoroethane had evaporated. T h e remainder of the amine was then added with stirring and the stirring continued until a homogeneous greaselike product was obtained. The mixture was cooled to room temperature with stirring, immersed in a bath a t -75' C. for 30 minutes, then permitted to remain a t room temperature overnight. Greases were prepared in 100-gram batches (Table 11). If the solvent was permitted to evaporate completely from the thickening agent before addition of the fluoroamine, o r the low temper-
Material
a
EtOH
T-20 T-30 T-45 T-50 T-57 T-100.4 T-ioo T-120 T-200 T-250 T-300 I , no apparent change;
I I I I I
Table IV. Contact Compatibility Tests of Thickening Agents" JP-4 L'DMH DETA C - - D E T A b Aniline L;-HC S2H4
C C C
I
I I I I I I I
M
M
C
M
C, color change;
I C C
I
C
R C 1
I I I I
M ,swell; R, gas formation.
C R C
I
C C
I C
b
I I C
I I I I I
C R C
R R C
I
I
C
C 1
I I I
I I
H,Oz
I I I I I I I I
IRF.A R C
unsym-Dimethylhydrazine-dzethylenetriamine60:40.
I I I
1
I I
Nz04
I C
I I I I I I
unsym-Dimethyl-
hydrazine-hydrazine 50350.
Material
G-20 G-45 G-50 G-100 G-300
EtOH 1 S
I I I
Table V. JP-4
I S
I I 1
Contact Compatibility Tests of Greases and Base Fluid" U D M H D E T A U-DETAb Aniline U-He AY?H, C C I S C C C C C C C S I 1 I I I I
I I
I
I
I
1
I
1
Perfluoro-
a C
I I
IRF-VA
I I I I
I I I I I
1
.v?o, s S
I I I
trialkylamine I I I I I I I I I I Y I , no aflparent changb; C, color change; S, separation of thickening agent; E; soluble (2.5W). b unsym-Dimethylhydrazine-diethylenetriamine 60t40.
~nsym-Dimeth~vlhydrazine-hydrazine50:50.
210
I I
H202
l & E C P R O D U C T RESEARCH A N D DEVELOPMENT
in iY2O4?XZO4had no apparent effect on G-50, G-100, and (3-300 (Table V). After evaporation of Nz04 the grease appeared completely unchanged. Evidently, the structure of the grease dispersion is such that the fluid is not readily accessible for solution, but this point needs further work. Liquid oxygen (LOX) impact compatibility tests were run a t a n impact level of 72.3 foot-pounds (7). Materials were considered nonreactive with L O X if they withstood 20 separate impact trials without reaction (flashes, explosions, o r other indications of sensitivity). Nitrogen tetroxide impact tests were run using the method of Kopituk ( 6 ) starting a t 400 footpounds per sq. inch; if there were any failures in 10 trials, tests were repeated a t lower energy levels. (Different units were used for impact levels in the LOX and P i 2 0 4 tests and the numbers are not directly comparable.) Materials were considered nonreactive Lvith N ? 0 4 if they withstood 10 separate impact trials a t 200 foot-pounds per sq. inch minimum without reaction. Pu'one of the materials tested are reactive with LOX and N204 within the mentioned limits (Table VI). (The reactivity of grease G-50 is somewhat lower than its thickening agent, T-50. T o the extent that this difference is significant, the fluid appears to exert a shielding effect on the thickening agent.) Other Materials. The reactivity of the mixed perfluorotrialkylamines and G-50 was determined (ASTM D 471) a t 70' C. for a 6-month contact period with neoprene; butyl, buna N, and natural rubbers; polyfluorosiloxane ; polychlorotrifluoroethylene ; and vinylidene fluoride-hexafluoropropylene copolymer. Determinations were made of changes in dimension, hardness, and color. S o change was observed. Explosive reactivity occurs \\hen aluminum surfaces are coated with polytetrafluoroethylene ( 9 ) or polymers of chlorotrifluoroethylene ( 7 7 ) and subjected to mutual shear a t high loads. T o determine irhether similar reactivity pertains to the greases described here, tests were run on G-50 ( 7 7 ) . Approximately 5 grams of grease were placed in a cylindrical hole ('/?-inch diameter X '/2 inch deep) in a block of the test metal. A doM-el of test metal ('/l-inch diameter, flat end) was rotated into the block a t 1760 r.p.m. under a load of 100 pounds. The metal combinations tested are given in Table \ T I . The only combination which showed explosive reactivity !vas 52100 steel on magnesium. These results are important in the use of the lubricants as thread lubricants, in addition to other applications. lubricant Properties
In Table VI11 are given the physical and chemical properties of the greases. The limiting property may be volatility, which may limit the use of the grease to relatively short timehigh temperature combinations. (Precise limits for any particular application would have to be determined.) Although no low temperature viscosity data are yet available on the greases, the pour point of the fluoroamine fluid is -20.6' C., M hich gives a first approximation of low temperature limit. \Year and extreme pressure (EP) properties were determined I n the Shell four-ball wear and EP testers, respectively. Wear scar diameters mere measured on the three stationarv balls after 1 hour a t 600 r.p.m. a t 75' C. with IO-kg. and 40-kg. loads using a tiaveling microscope at 40X (Table IX). The values are the average of the readings taken parallel and normal to the scuff marks. The E P properties were determined bv measuring the incipient seizure and weld loads. The former is the load at which a sudden sizable increase in the scar diameter occurs, while the latter is the load a t which rotation o f the upper ball in relation to the other three is no longer
Table VI. Material
Impact Compatibility Tests
LOX Kot reactive Not reactive hTotreactive h-ot reactive Not reactive Not reactive Not reactive
T-50 T-100 T-300 G-50 G-100 G-300 Mixed perfluorotrialk ylamines
N204' 340 400 400
400 400
400 400
.Waximum impact level (ft.-poitnds per sq. inch) ,for no reaction in 10 trials.
Table VII. Reactivity of Grease G-50 with Metals at High Sheara Explosive React ionslh'o. of Trials
52100 steel
Titanium Magnesium6 304 stainless steel 2024 aluminum 6061T6 aluminum 7075 aluminum 2024 aluminum 606lT6 aluminum 7075 aluminum
2024 aluminum 606lT6 aluminum 7075 aluminum
a Load 100 16. at 1760 r.p.m. with same results.
b
0/6 6 /6 0 /6 0/6 0/6 0/6 0 /6 0 /6 0 /6
Also tested at 60 lb. and 500 r.p.m.
Table VIII. lubricant Properties Property (2-50 G-100 Dropping point. ' C.a >l50 117 Evaporation loss at 98.8' C.? 47.0 44.9
Neutralization numberc T\-ater washout at 37.3" C.,
0.00
19.5
G-300
>150 48.0
0.05 15.7
0.05 20.0
C d /O
Oxidation stability Pressure drop, 100 hr. at 121.2O C.e Copper strip, 20 hr. at 121.2' C.J
No pressure drop: no decomposition of grease Yo pressure drop, no discoloration of grease or strip a AST.M D ,566. b ASTAM D 972. c ASTAM D 974. d AST*M D 7264. e ASTXl D 942. J ASTM D 7402. Wear and Extreme Pressure Petroleum G-50 G-100 G-300 Greasea
Synthetic Grease
0.352 0.968
0.256 0.512
0.268 0.676
0.350 0.530
0.5505 0.770b
120
110
110
120
80c
240
250
Table IX.
Wear
Scar diam., mm. 10-kg. load 40-kg. load Extreme pressure Incipient seizure, kg. TYeld, kg. a
MIL-G-10924B.
b
MIL-G-3278A.
C
260 250 MIL-G-7178A.
282~
possible. Scar diameters were measured after 1 minute a t each load, which was increased in 10-kg. increments to the weld point. A fresh sample and new steel balls were used with each load. Wear and extreme pressure data were compared with corresponding data on typical conventional petroleum and synthetic greases ( 7 , 73) (Table IX). I t may be concluded that the properties in question are comparable. Discussion
The primary objective of the work was to prepare lubricants of a greaselike consistency that would be useful with a \.ride VOL. 2
NO. 3 S E P T E M B E R 1 9 6 3
211
spectrum of liquid fuels and oxidizers. I n the main, this objective has been achieved, but obvious limitations remain. The limiting variable may be volatility at high temperature. For the time and temperature conditions involved in rocket motor lubrication, however, the volatility of G-50, G-100, or G-300 may not be undesirably high. There are obvious further extensions of the work within the material system studied-Le., variation in thickener concentration should lead to greases having a range of consistencies. Structure and physical properties of the greases are probably dependent on particle size and shape of the thickening agent, in addition to the preparation techniques, and further work along these lines is desirable. No attempt was made to optimize the properties of the grease within the material systems studied. The data in Table V I I I show, for example, that the dropping point varies with the thickening agent (and, one would expect, with the thickener concentration). Further work is required on low temperature properties. I t has been established that a physically stable lubricant of greaselike consistency may be prepared by thickening mixed perfluoroalkylamines (alkyl = Cq to C,) with polytetrafluoroethylenes of relatively low molecular weight and that the product is inert with a wide variety of rocket motor liquid fuels and oxidizers-i.e., JP-4, ethyl alcohol, unqm-dimethylhydrazine, diethylenetriamine, 1 to 1 UDMH and hydrazine, 90% hydrogen peroxide, inhibited red fuming nitric acid, liquid oxygen, and nitrogen tetroxide (with the possible exception of N204, in which the base fluid is somewhat soluble). The lubricant is also inert to most elastomers used in missile and rocket motor systems and has antiwear and extreme pressure properties that are approximately the same as those of conventional petroleum and synthetic greases. Further testing in operating equipment is now required.
Acknowledgment
The authors express their appreciation to E. I. du Pont de Nemours & Co. for supplying the tetrafluoroethylene polymers; to \V. Riehl, Marshall Space Flight Center, for conducting the LOX impact compatibility tests; and to E. Palmer, Martin Co., Denver, Colo., for the reactivity tests with metals under shear. Literature
Cited
(1) Calhoun, S.F., ASLE Trans. 3, 208 (1960). (2) Fisch, K. R., Peale, L., Messina, J., “Lubricants for Missile Systems,” Frankford Arsenal Rept. R-1522 (December 1959). (3) Fisch, K. R., Peale, L., Messina, J., Gisser, H., ASLE Trans. 5 . 287 11962). (4f’IF&$, C.-F., U. S. Patent 2,576,837 (Nov. 27, 1951). (5) Kauch, E. A., Simons, J. H.. Zbzd., 2,616,927 (Nov. 4, 1952). (6) Kopituk, R. C., AST‘M Bull. No. 250, 51 (December 1960). (7) Marshall Space Flight Center Specification 106, “Compatibility Testing, Liquid Oxygen Systems and Materials.” (8) Messina, J., “Greases Nonreactive with Missile Fuels and Oxidizers,” Symposium Preprint, National Society of Aerospace Materials and Process Engineers, Covina, Calif., November 1962. (9) Morris, G., “Impact Testing of Nonmetallic Materials in Liquid Oxygen,” Martin Co., Baltimore, Md., Rept. ER10116-1 (March 1958). (10) Office of Director of Defense, Research and Engineering, JVashington, D. C., “Handling and Storage of Liquid Propellants,” 1961. (1 1) Reynales, C. H., “Compatibility of Materials with Oxygen,” Douglas Aircraft Co., Long Beach, Calif., Rept. D81-444 (October 1958). (12) Riehl, 1%’. A., “Sealants, Lubricants, Threading Compounds and Packings for Jupiter Missile Systems,” ABMA Tech. Note NR-N-61 (May 1957). (13) Smith, R. K., L L G I Spokesman 22, 435 (1958). (14) Woodward, J. L., Nielsen, C.: “Lubricity Study of Saturn Launch Vehicle Valves, Phase 11,” Chrysler Corp. Missile Division, NASA Contract No. NXS8-878, 1962. RECEIVED for review April 15, 1963 ACCEPTED July 15, 1963 Division”of Petroleum Chemistry, 144th Meeting, ACS, LoS Angeles, Calif., March 1963.
P R E P A R A T I O N OF A M A G N E S I U M - J P - 4 S L U R R Y FUEL E. G. F O C H T M A N , J.
F. B I T T E N , A N D S I D N E Y K A T Z
I Z T Research Institute, Chicago 76, Ill.
A potentially useful fuel consisting of magnesium powder suspended in JP-4 fuel has been prepared b y use of surfactants which induce compatibility between the solid metal and the liquid phase. Ball milling 200-mesh magnesium in the JP-4 containing a wetting agent was found to b e an effective method of slurry preparation.
s
1944, it was reasoned that certain nonhydrofuels could provide greater thrust than conventional hydrocarbons. The Lewis Laboratory, National Advisory Committee for Aeronautics, devoted considerable time and effort to developing slurries of metal powder in liquid hydrocarbons and evaluating their performance (3, 4). This work, which was started about 1950 and greatly curtailed in late 1955, did not result in a stable metal powder slurry suitable for jet aircraft systems. Although feasibility had been demonstrated, many problems remained, including the need for optimum stabilizing agents and wetting agents and even the properties of the metal powder to be used in the slurry. O u r studies in this particular area began in 1954 with the preparation and evaluation of stable slurries of elemental EARLY AS
A carbon
212
I & E C PRODUCT RESEARCH AND DEVELOPMENT
boron (2). Later efforts were devoted to the study of magnesium slurries; the results of these efforts are reported here.
Table 1. Magnesium-JP-4 Vapor Process Slurry Propert): Analysis Total Mg 4 0 . 0 wt. % MgO 5 . 3 wt. yo 0 .3-0.1 \ \ I t . % Free carbon (est.) Particle size 800-900 A.
Particle shape Brookfield viscosity at 12 r.p.m.
Spherical (0.5-micron clusters) 250,000 cp.
