Results
In Tables I1 and I11 the results obtained in batchwise reactions, in solution and in emulsion, are summarized. As these runs were of a n exploratory nature, no special attempt was made to keep temperature and ethylene pressure entirely constant during the run. The purpose of these reactions was to find the optimum conditions for subsequent continuous reactions. I n batches of 650 grams of carbon tetrachloride and ethylene pressure up to 215 atm., the reaction could be carried out safely a t temperatures of up to 140’ C. About 200 runs have been completed in the emulsion system without a mishap. I n the batchwise runs, several aliphatic and aromatic amines have been tried, in a solution system: monoethanolamine, triethanolamine, morpholine, triethylamine, aniline, dimethylaniline. and tetramethylenepentamine. Monoethanolamine proved to be by far the most effective. Conversions of 60 to 70%, calculated on carbon tetrachloride, were regularly attained. This value could be increased to about 90% by injecting a solution of monoethanolamine in the used solvent, in three equal parts during the reaction (Table 11: 4 and 5). Table I V gives the results of continuous telomerization I uns in emulsion. The aim was to test the amine initiator in a flow system. It proved possible, in the apparatus used. to control pressure, temperature, and molar ratio of ethylene to carbon tetrachloride. Reactions at 160’ showed no corrosion, and that the distribution of the resulting telomers differed only slightly from that in runs a t 140’ and 120’, with the same ethylene concentration.
in emulsion. Analysis by gas chromatography shows that reactions even a t 140’ and 160’ yield pure telomers. Even a t high temperatures, the continuous emulsion system is reliable and safe, being fully controllable with regard to temperature and pressure. The present initiation system is well adaptable to a continuous process when used in emulsion, and is therefore feasible on an industrial scale, as equipment does not corrode. The initiator giving the best results, monoethanolamine, is considerably less expensive than any initiator previously used for this reaction. literature Cited
(1) Asahara, T., Takagi, Y., Bull. Japan Petrol. Znst. 2, 70 (1960). (2) Asscher, M., Levy, E., Vofsi, D., Atti del XI1 Congress0 Inteinazionale delle Materie Plastiche e Conferenze IUPAC, Torino, September 1960, p. 151. (3) Asscher, M., Vofsi, D., J . Chem. SOL.1961, 2261. (4) Bolt, R. O., Chem. Eng. News 25, 1866 (1947). (5) Calkins, K. W., Howley, R. W., Corrosion 15, 447 (1959). (6) David, C., Gosselain, P. A., Tetrahedron 18, 639 (1962). (7) Farbwerke Hoechst A. G., Brit. Patent 803,463 (Oct. 29, 1958). (8) Freidlina, R. Kh., Karepetyan, Sh. A., “Telomerization and New Synthetic Materials,” p. 35, Pergamon Press, New York, 1961. (9) Hanford, T.V. E., Joyce, R. M. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,440,800 (May 4, 1948). (10) Joyce, R. M., Chem. Eng. News 25, 1866 (1947). (11) Joyce, R. M., Hanford, W. E., Harmon, J., J . Am. Chem. Soc. 70, 2529 (1948). (12) Karapetyan, Sh. A., Pichugina, L. A., Proc. Acad. Sci. U.S.S.R. 3. 114 (19571. (13j Katihalsky, A,, Vofsi, D., Asscher, M., Levy, E., Israeli Patent 13,845 (Nov. 23, 1961) ; C. A. 56, 9961b (1962). (14) Nesmeyanov, A. N., Frei dlina, R. Kh., Tetrahedron 17, 65 I1 063) \-,-I’
Conclusions
High conversions are achieved in average holdup times between 15 and 30 minutes, giving a high space-time yield for the present system. The telomerization can be carried out a t a wide range of temperatures: from room temperature in solution, u p to 160’ C.
(15) Nesmeyanov, A. N., Freidlina, R. Kh., Zakharkin, L. I., Quart. Rev. 10, 330 (1956). (16) Takagi, Y . , Asahara, T., J . Chem. SOC.Japan, Znd. Chem. Sect. 64, 1634 (1962). (17) Takehisa, M., Yasumoto, M., Hosaka, J., Ibid., 65, 53 (1962). RECEIVED for review December 5, 1962 ACCEPTEDFebruary 25, 1963
E L A S T O M E R S FOR LIQUID ROCKET FUEL A N D OXIDIZER APPLICATION JOSEPH GREEN,NATHAN
B. LEVINE, AND
ROBERT C. KELLER
Chemistry Department, Reactton Motors Division, Thiokol Chemical Cofp., Denuillz, W . J . The resistance of commercially available and experimental polymers to hydrazine-type fuels, nitrogen tetroxide, and fluorine-containing oxidizers was investigated. On the basis of static and dynamic tests several materials have been recommended for application in each of these propellants. Techniques for encapsulating elastomer O-rings with inert resins and metals are discussed. HE need of greater specific impulse for the propulsion Tsystems of advanced aerospace craft and missiles has been accompanied by increasingly severe problems of materials compatibility. It is desirable to prepare such end items as seals, gaskets, and flexible connectors from elastomeric polymers because of their simplicity and low weight. The fuels and oxidizers of requisite energetic qualities are generally chemically reactive with organic materials of construction. Hence, research has been undertaken on elastomeric and .
126
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
compliant materials resistant to hydrazine-type fuels, nitrogen tetroxide, and fluorine-containing oxidizers. Several approaches toward developing materials resistanr to various fuels and oxidizers were investigated : Commercially available polymers and small quantities of experimental polymtrs were subjected to formulation and evaluation studies. Resin-coated elastomers were prepared and their resistance to nitrogen tetroxide was determined. Metal-clad elastomers were investigated for oxidizer application.
Experimental Procedure
PROTECTIVE DOOR CHECK VALVE
The materials investigated were evaluated for chemical resistance to the propellants, permeability of the polymer, and dynamic evaluation of O-rings prepared from the materials which looked most promising on the basis of static test data. The chemical resistance was measured by volume swell and change in physical properties after immersion. Chemical Resistance. Throughout this study volume swell data were obtained in accordance with ASTM D 1460 (3). T h e more widely used method, ASTM D 471 ( Z ) , requires weighing the swollen samples in water after immersion, and results in data which are often misleading because of the high volatility and the water solubility of the various fuels and oxidizers used in this study. I n the test used, the change in length of the sample is measured while still immersed in the fluid, by use of a graph paper background. As the original sample dimensions are known, the change in length can be related to volume swell. As a safety measure the size of the ASTM sample was reduced to half of the length specified, so that smaller quantities of propellant were required. Enough test liquid was placed in the tube to reach a level double the test specimen length. T h e resistance to hydrazine-type fuels was evaluated in immersion tubes capped with air condensers. Evaluation of resistance to nitrogen tetroxide required the use of either ground-glass stoppered tubes or sealed tubes, depending upon the evaluation temperature. Determination of behavior in the more volatile fluorine-containing oxidizers required the use of vacuum rack technology. Physical properties after immersion in hydrazine-type fuels were obtained using the swollen specimens. T h e high volatility of nitrogen tetroxide and the fluorine-containing oxidizers made it difficult in the former case and unrealistic in the latter case to obtain physical property data on the swollen specimens. The high corrosivity of these propellants also made it impractical to test swollen specimens. As a result the specimens were outgassed before tensile testing. T h e data obtained are a measure of polymer degradation and combined with the volume swell data give an indication of the utility of the compound. Micro-dumbbell tensile specimens, 2 inches long with a necked-down area of '/8 X 5/8 inch, were used because of safety considerations. T h e nitrogen tetroxide-swollen specimens were outgassed before testing by a three-step process: (1) 4-hour suspension in air a t ambient room temperature, (2) 24-hour vacuum treatment a t room temperature, and (3) 24-hour rest period a t room temperature. T h e specimens
Table 1.
Formulation KO. cis-l,4-Polybutadiene Butyl 218 Ethylene-propylene rubber Hydropol ( 18yo unsaturation) Hydropol (8% unsaturation) Carbm black (HAF) Zinc oxide Sulfur Stearic acid Tetramethyl thiuram disulfide Amber01 ST-137 Dioctyl sebacate Hypalon 20 Polyethylene (low density) Dicumyl peroxide (Di-Cup 40C) Flexamine Curing conditions, min./" F. Postcure, hr./ O F.
SHATTERPROOF GLASS WINDOW
Figure 1 .
Assembly drawing of O-ring tester
Temperature controller, oxidizer or fuel reservoir Temperature controller, test fixture Valves for controlling reservoir pressure P I & Pz. SI. Reservoir heater Sq. Solenoid valve Si. Cylinder heater Ss. Cycle timer SB. Blower Ss. Strain g a g e assembly
TI. Tz.
swollen with the fluorine-containing oxidizers were outgassed under vacuum in the vacuum rack. Permeability. Cured samples, 2 X 2 X 0.075 inches, were evaluated in a permeability cup apparatus having an opening of approximately 1.5 sq. inches. T h e cup was filled with propellant and held at ambient room temperature when hydrazine-unsp-dimethylhydrazine ( U D M H ) was the propellant and a t 60" & 1 ' F. when nitrogen tetroxide was the propellant. The cup, propellant, and specimen were tveighed immediately after filling and a t specific time intervals thereafter. T h e per cent loss of propellant was reported for various time intervals. Dynamic Testing. O-rings were prepared and dynamically evaluated in the Thiokol-RMD dynamic 0-rinq tester shown in Figure 1. This tester consists of a piston-cylinder combination with the fuel or oxidizer under test sealed between two O-rings located on the piston. The piston is cycled in the cylinder a t a rate of 3600 cycles per hour with a stroke of inch. Additional propellant is stored in a reservoir above the tester and pressurized from a nitrogen cylinder through a threeway valve. The entire apparatus fits into a temperature control box, so that the O-rings can be tested over a range of operating temperatures. Polymer Resistance to Hydrazine-Type Fuels
Chemical Resistance. Hydrocarbon elastomers were reported to be best suited for hydrazine-type fuel application ( 7 , 4, 5). The present study quantitatively evaluated the resistance of cis-l,4-polybutadiene, butyl rubber, ethylenepropylene rubber (EPR), and partially hydrogenated polybutadiene (Hydropol) (Table I) in hydrazine-UDMH blends,
Elastomer Formulations Evaluated for Hydrazine-Type Fuel Application 7 100
...
2
3 ...
...
100
100
100
...
...
...
...
... 50 5 2.5
. . .
75 5
75 5
...
...
...
, . .
, . .
...
0.4
...
...
, . .
12
12 10 5 25
...
10 5
... ...
...
...
...
... ...
60/287 ...
45/350
45/350
, . .
...
...
6
100 80 20
65 5
1 ... ...
5
4
, . .
50 ... ... ... ... ... ... ... ...
50 ...
5 ... ...
... 45/320 16/300
VOL. 2
40 5 1.75 2
0.75 , . .
... ... ... ...
10 ...
0.1
40/320
45/307
...
NO. 2
JUNE 1963
127
monomethylhydrazine ( M M H ) , and mixed hydrazine fuels (MHF-1 and MHF-4). Physical property data were obtained after immersion in 50/50 hydrazine-UDMH a t room temperature and 160' F. The compositions studied generally showed excellent resistance for a t least 30 days' immersion a t 160' F. The ethylenepropylene rubber formulation swelled only 16% after 7 days immersion a t 160' F., as compared to approximately 40% for the other compositions. Physical properties of the above vulcanizates were determined after immersion in M M H , MHF-1, MHF-4 for 7 days at 160' F. Again these hydrocarbon polymers showed little degradation in the fuels. All the compounds swelled less than 10% in MHF-1 and MHF-4. In MMH &-I ,4-polybutadiene swelled lo%, EPR swelled 18%, and butyl composition 3 swelled 26%. Dynamic Testing. One of the objectives of the program was to evaluate the above compounds dynamically as O-ring seals for hydrazine-type fuel application. O-rings were prepared from all compounds listed in Table I with the exception of compound 4, and dynamically tested. T h e 0rings were cycled a t 3600 cycles per hour at -60" to 160' F. for 10,000 cycles or until failure as evidenced by propellant leakage. The appearance of the O-ring after testing was recorded. All the samples tested in 50/50 hydrazine-UDMH went the full 10,000 cycles a t 160' F. with little or no surface wear on the O-ring, with the exception of compound 2, which exhibited excessive surface wear. The polyethylene in compound 3 apparently functioned as a lubricant in this dynamic test. .4lthough the Hydropol O-rings (compound 6) functioned well, they were considered too hard (Shore A = 95) and stiff for practical seal applications. Permeability. There is considerable interest within the aerospace industry in the fabrication of positive expulsion bladders for application in a zero g environment. Since this requires highly flexible materials of low permeability, the most promising elastomer, EPR compound 5, was evaluated for permeability to hydrazine-UDMH. T h e results of this investigation show no fuel loss in 5 days and 0.770weight loss after 7 days. Weight losses of 2.2 and 7.4Y0 were obtained after 14 and 30 days of testing, respectively. Polymer Resistance to Nitrogen Tetroxide ( N p 0 4 )
Chemical Resistance. Fluorocarbon elastomers such as Viton A and fluorosilicone elastomer Silastic LS-53 had been
Table II.
Resistance of Elastomers to Temperature
N204at Ambient Room
Days Immersed 7 7 Volume Swell,. % .. 380 280 180-300 200-235 280-410 280-525 270 290 45 170 20 20 10 10
Vukanirates Silastic LS 53 Viton A Viton B Fluorel Nitroso rubbera Hydropol Cured polyethylene DYNH-peroxide) Butyl (various cure sys50 terns) , , , Butyl, high resin cure Ethylene-propylene rubber 20
50
Retention of Physical Properties, 7 Days Good Poor to Food Poor to-excellent Poor Poor Degraded No apparent change Reversionb
Good Good up to 3 days a TrEfIuoronitrosomethant-tetrajuoroethylenecopolymer. Degree of reversion depends upon cure system. 0 After 4 weeks, 35%, fair ProferQ retention.
128
recommended for Nz04 application (4, 5 ) . Silastic LS-53 was reported to swell approximately 40% in N204. During a recheck of these data it was noticed that the Silastic LS-53 while still in the immersion tube was swollen several hundred per cent. Viton vulcanizates also exhibited several hundred per cent swell in Nz04. One would expect the polar polymers to exhibit high volume changes in the polar &O, liquid, and nonpolar polymers to show relatively low volume changes. Table I1 illustrates this point; the polar polymers such as the fluorosilicones,Vitons, and Fluorel exhibit volume changes of approximately 300%, whereas the hydrocarbon polymers such as polyethylene, butyl rubber, and ethylene-propylene rubber show volume changes below 50%. ,Measurement of the rate of swell of LS-53 in Nz04 showed that most of the swelling occurred in the first hour (Figure 2). NzO4 2N02 began to diffuse from the swollen specimen, with concomitant shrinkage, immediately after its removal from the test liquid. After 10 minutes the sample recovered to a total swell of 26% and after 1 hour it exhibited a swell of only 6% (Figure 3). The low volume swell reported in the literature for LS-53 is obviously due to the loss of nitrogen tetroxide (b.p. 70.1' F.) during the measurement. This would be the result of using ASTM D 471 instead of ASTM D 1460, where linear change measurements are made while the specimen is still immersed in the test liquid. After the swollen specimens had returned to their original dimensions they showed good retention of strength and elasticity as measured qualitatively. Similar results were obtained with the \'iton elastomers (Table 11). Unsaturated polymers such as Hydropol degrade readily in Nz04 (Table 11), presumably because of the addition of NzO4 to the double bond (6). Polypropylene and cross-linked polyethylene were immersed in Nz04 for 7 days at room temperature; no change was apparent. However, neither polymer is sufficiently elastomeric to be considered for many applications requiring elasticity. Ethylene-propylene rubber (EPR) and resin-cured butyl rubber were both resistant to Nz04 for short periods of time and exhibited relatively low volume swell. With this in mind, efforts were concentrated on nonpolar polymers with little or no unsaturation. Low density polyethylene (DYNH-Bakelite) was unaffected by 7 days' immersion in Nz04 a t 60' and 70' F. This polyethylene was compounded with dicumyl peroxide (Di-Cup 40C), with and without 50 phr of HAF carbon black, and
40~
30
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
10
30
W
120
IS0
140
550
1-
TIME, MINUTES
Figure 2.
Swell of Silastic LS-53 in
,440 (1 d.si
N204
Table 111.
Elastomer Formulations Evaluated for Nz04Application
Formulation N o . 5 7 Ethylene-propylene rubbera 100 100 ... ... Butyl 21 8 50 Carbon black ( H A F ) 50 5 Magnesium oxide ... 10 Dicumyl peroxide (Di-Cup 40C) 10 ... Zinc oxide ... Stearic acid ... ... Amberol ST-137 ... ... Tin chloride (SnC12,2 H ? O ) ... ... Hypalon 20 ... ... Cure, min.!" F. 40/320 40/320 Postcure, hr./" F. ... ... a Enjay EPR-404. U. S. Rubber F-62-6-8 terpolymer.
8
100
... ...
Formulation N o . Elastomer-curative
...
50 10 ...
...
11
12
...
...
100 65 ...
100 65
100 65
5
5
5
... 5
... ...
... ...
50
2
...
60/350 16/300
5 45/320 16/300
...
...
...
1
12
...
... 40/320
10
...
50 ...
...
...
cured. Both compounds exhibited excellent retention of physical properties after 7 days' immersion at 60" and 70" F. T h e reinforced compound showed a small decrease in tensile strength after immersion ; however, its final tensile strength was greater than that of the nonreinforced compound. The volume svells were less than 10% after 7 days' immersion a t 60" F. Attempts to cure atactic polypropylene for a similar evaluation irere unsuccessful. High density, high molecular Lveight polyethylene (AC-X? Semet-Solvay) and isotactic polypropylene (Escon-Enjay) were evaluated after 7 and 30 days' immersion in Nz04 a t 70" F. Although the high density polyethylene exhibited low volume swell (0 to 6%), it showed considerable embrittlement within 1 week's exposure. The isotactic polypropylene, on the other hand, retained its tensile strength after 1 week's immersion. After 1 month's exposure the tensile strength \.vas reduced from 1840 to 850 p.s.i.
Table IV.
9 1OO*
...
12
12
...
...
5 45/320
5 45/320 16/300
...
The ethylene-propylene copolymer and terpolymer (EPT) were formulated with 50 phr HAF carbon black and cured with dicumyl peroxide and phenolic resin, respectively (Table 111). The immersion data indicate that the EPR cured with peroxide is usrful for a maximum of 7 days' total immersion in NZ04 at 60" F. (Table IV). At 70' F. the composition degrades much more rapidly, its useful life being lrss than 3 days. The rates of degradation are illustrated in Figures 4 and 5. T h e E P T cured with phenolic resin showed greater resistance to S20r degradation. This compound is useful for N ~ 0 4 application for greater than 7 d a y ' service a t 60' F.. although its volume s\vell is 50%. By contrast, a sulfur-cured E P T degrades even more rapidly than the EPR compound. NzO, contaminated with small quantities of water contains nitric and nitrous acids. Magnesium oxide evaluated as an acid acceptor in formulation 8 appears to inhibit the degrada-
Physical Properties of EPR and Butyl Vulcanizates after Immersion in NzO,
5 EPR-peroxide
9
10
11
72
EPT-phenolic
Butyl-excess phenolicpostcured
Butyl-phenolic
Butyl-phenolic post-cured
1960 525 490 66
1850 200 1850
1780 400 800 80
1800 250 1340 88
800 175
1730 275 1610 83 35
...
...
1500 350 930 72 35
... ... ... ...
1490 250 1320
...
40
900 650 400 65 70
1555 375 775 67 40
...
1135 350 905 76 33
... ... ... ... ...
...
Control Tensile,,p.s.i. 2000 Elongation, % 350 2007, modulus, p.s.i. 740 Shore A, points 62 Immersion, 7 days/60 F. Tensile, p.s.i. 230 Elongation, 70 950 200% modulus, p.s.i. 110 Shore A, points 37 Volume swell, yo 15 Immersion, 7 days/70 F. Tensile, p.s.i. 40a Elonqation, 225 200% modulus, p.s.i. ... Shore A, points 15 Volume swell, yo 30 Immersion, 30 days/7O0 (2. Tensile, p.s.i. ... Elon ation, yo 200& modulus, p.s.i. ... Shore -4, points ... Volume swell, 70 ... Immersion, 7 days/100 F. Tensile, p.s.i. ... Elon ation, 7c ... 200& modulus, p.s.i. ... Shore A, points , . . Volume swell, Y0c ... Degraded within 5 days: data listed for 3 days' immersion.
...
75 50
... ... ...
...
... ...
90
... ...
...
86
640
400 ... 475 ... 74 ... 65 Degraded within 7 days: data listed f o r 5 days'
... ... ... ...
290h 550 210 32 110 immersion.
VOL. 2
c
970 400 570 65 75 One-day immersion.
NO. 2
JUNE 1963
129
no
2000
z
215
K
4
i 4
W
i
6
AT
IY) W
5 W z c
500
z W
I-
500
115
B
1
