I
WILLIAM POSTELNEK Wright Air Development Center, Wright-Patterson Air Force Base, Dayton, Ohio
Search for High Temperature Elastomer The Air Force's Pioneer moon rocket which shot out 80,000 miles into space last month underscores the fact that the space age is here. The plastics reported in the bulletin below and the polymers described in the article will help scientists to construct vehicles to carry man into space and eventually to the moon
WHEN the first synthetic oil for the J-47 gas turbine engine was developed, it was apparent that a new high temperature- and oil-resistant polymer would have to be developed. The engine oil, based on diester fluids, swelled all the
contemporary rubber compounds, including the silicone types. In addition to excessive swell, these rubbers showed a loss in tensile strength, decrease in mechanical properties, and deterioration in general.
The high temperature polymer research program aimed at the solution of this problem was begun in 1949 and is still continuing. A number of elastomers have resulted from this program, all based on organic fluorine intermediates.
BULLETIN Newly Developed High Temperature Plastics Give Desiin Engineers New Tools Items: General Electric has just revealed a new series of modified polyester resins-for reinforced plastic applications at high temperatures. Plastics will take continuous exposure at 425'F. and intermittent exposure a t 500'F. Reichhold Chemicals has come up with a missile age plastic. Company says the plastic-an unmodified phenolic resin-can be used in laminates that can stand temperatures up to 4500'F. for short periods and up to 500OF. for over 100 hours. Shell Chemical and Atlas Powder are both active in researching possible new uses of epoxy resins. Shell's 1310 is said t o make a reinforced plastic that can stand extreme heat for extended periods. Atlas is seeking epoxy-polyester resins with high resistance to chemicals. Hastings Plastics has developed a series of low density plastics with extreme temperature resistance. In recent tests, the new formulation was exposed to the 6500'F. heat of an oxy-acetylene torch for thirty seconds. Damage was limited to slight melting. 1 602
INDUSTRIAL AND ENGINEERING CHEMISTRY
The original problem of a 500' F. stable elastomer, resistant to fuels and oils, had been solved satisfactorily by these elastomers. However, year by year since 1949, as gas turbine engines of higher performance were developed, the requirements for more thermally stable rubber for various aircraft components became more apparent. I n addition, newer gas turbine lubricants and fluids for hydraulic systems were developed for still higher temperature use, imposing another burden on rubber performance. Also, exotic high energy fuels posed additional problems in connection with rubber for fuel systems. Radiation resistance of high temperature elastomers is also ag important consideration. Fluorinated Acrylic Elastomers
The first clue in the search for a high temperature elastomer was obtained from experience gained with the fluorinated polymers Teflon and Kel-F during and shortly after World War 11. The remarkable chemical inertness, oxidative stability, and thermal stability imparted by the substitution of fluorine for hydrogen in these polymers was the important factor in deciding to embark upon a fluorine-containing elastomer research program. The initial Air Force-sponsdred program with the Minnesota Mining and Manufacturing Co. was on polyacrylic esters, in which the alcohol portion contained a high percentage of fluorine. After an exhaustive study of homo- and copolymers of various fluorinated esters of acrylic acid, a rubbery polymer was ultimately developed-poly(1,l -dihydroperfluorobutyl)acrylate, first known as Poly FBA and subsequently as 1F4.
1F4 (Poly FBA) The full potential of the basic polymer was not wholly realized by the initial use of metal oxides as cross-linking agents. Various recipes based on the oxides of calcium, magnesium. and lead were utilized in an extensive cross-linking study. Superior physical and mechanical properties were ultimately realized by the use of polyamines as curing agents. A typical recipe is shown in the box a t right. The excellent physical and mechanical properties as well as the unusual resistance to fuels and high temperature of this typical compound are shown in Tables I, 11,and 111. The only deficiency noted in this system was with respect to low temperature properties. Various studies were made on the synthesis of new monomers and the polymerization of these new materials. Evolved from this effort was a
T h e need for a high temperature elastomer for Air Force use became apparent shortly after World W a r I1when the gas turbine aircraft engine became a reality. The immediate forecast of sonic and supersonic speeds and the realization that high performance aircraft would require more thermally stable engine lubricating oils and hydraulic fluids, were the important factors in stimulating an extensive research and development program for new high temperature oil resistant elastomers. The first synthetic engine oil developed for the J-47 gas turbine engine was based on diester type fluids. This combination of jet engine and lubricant was developed for the operation of the B-47 medium bomber. The operational temperature of the oil sump was 21 2' F. but hot spots up t0500' F. were anticipated. The diester lubricant was expected to operate at an average temperature of 350-400" F. for 1000 hours. None of the commercial rubber compounds available in the late 1940's were suitable for use as seals, hose, packing, or gaskets under these conditions. The data,below shows the volume increase at 21 2" F. in diester type fluids of these elastomer compounds.
% Volume' Increase
Rubber Compound Natural Butyl
590 106 102 142 32 30-94
GRS Neoprene Type Acrylate Type Nitrile Type ' Silicone Type
50
new polymer designated as 2F4, a poly(fluoroalkoxyalky1)acrylate.
[ g. H
l
LCHY-C
.
J
CH2CF2CF20CFa x The properties of this polymer, when compounded with the recipe shown in the box, compare with 1F4 as follows: have lower modulus, slightly kower tensile strength, slightly higher elongation, similar resistance to swell, but a Tlo of -30' C. instead of -7O'C. (73).
Kel-F Elastomer
Shortly after the Air Force program on fluorinated acrylates began, the U. S. Army Quartermaster Research and Development Command began a research program on fluorocarbon elastomers at the M. W. Kellogg Co. Although the initial objective of this work was to develop a fuel-resistant Arctic rubber, it soon became apparent that thermal stability was also an inherent feature of the polymers which evolved. Many homopolymers and copolymers of a variety of halogenated olefins and dienes were investigated, and eventually there emerged a copolymer of vinylidine fluoride and chlorotrifluoroethylene which eventually
Typical lF4 Recipe (74) Polymer Stearic acid Sulfur Carbon black Triethylaminetetramine Cure. 60 minutes at 310' F:
1. 50, NO. 11
Parts 100 1 1
35 ,
1
NOVEMBER 1958
1603
became known ds Kel-F elastomer. The probable structure is f CF&FCI-CHZCF,~~
Table I.
Original Properties of 1F4 (5)
100% modulus, p.s.i. Tensile strength, p.8.i. Elongation, % Set at break, % Shore A hardness 2'10, C.
Table 11.
Cooperative compounding studies were performed by the Quartermaster Corps, Kellogg Co., and WADC Materials Laboratory. The results of these studies indicated that Kel-F elastomer has outstanding strength, toughness, abrasion resistance, and resistance to high temperature, fuels and oils, and nitric acid. These characteristics of the polymer as according to the recipe shown in the box, are given in Tables IV and V. The exceptional nitric resistance of Kel-F elastomer indicated its use for tank sealants, seals, and hose in contact with nitric acid as well as for protective clothing. The poor resistance to polar fluids such as diesters precluded its use in some lubricating systems, but its resistance to hydrocarbon fuels and silicate esters implied utility in fuels and hydraulic systems.
315 1250 300
9 52 - 7 (23)
Effect of Diester-Type Fluids on 1 F 4 (5) Modulus, P S I . 587 440 448
Condition 250 hr. at 350' F. 1000 hr. at 350° F. 24 hr. at 500° F.
Tensile, P.S.I.
Elongation, %
1060
186 22 1 141
945 495
In 1951, the Air Force started an alternative approach in the search for a high temperature elastomer. The objective was to attempt to place fluoroalkyl groups on silicone rubber in place of methyl groups, with the hope of retaining the excellent high temperature properties of the silicone rubber but greatly improving the resistance to fuels and oils. The initial effort was that of McBee and others at Purdue University (7, 8). These workers were able to synthesize fluoroalkyl silicone monomers for the first time. Two of these monomers are:
3
Table Ill. Fluid Diester-type fluid Diester-type fluid Silicate ester fluid Hydrocarbon oil
Swell Data on 1 F 4 ( 7 )
Time, Hr. 500 168 750 500
% ' Swell
Temp., F. 350 400 350 350
0 +3
i-15 0
Typical Kel-F Elastomer Recipe (6) Parts Polymer Zinc oxide Benzoyl peroxide Dibasic lead phosphite Silica Cured. 1 hour at 300' F. Aged. 24 hours at 300' F.
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Fluorinated Silicone Rubber
INDUSTRIAL AND ENGINEERING CHEMISTRY
100
10 3 10 10
These monomers were prepared by the reaction of fluoroalkyl organometallic reagents on alkoxysilanes. Homopolymerization studies of these monomers at WADC failed to produce any high polymers; viscous oils were the only product ( 3 ) . However, copolymers with dimethyl silicone monomer produced elastomers which exhibited volume increases of one third to two thirds that of silicone rubber in fuel, but still above the useful range. Because the methods of monomer preparation were tedious and difficult and not particularly suitable for scaling up, a new monomer synthesis program was begun at Peninsular ChemResearch, Inc. Here Tarrant and others (76) demonstrated the feasibility of the preparation
HIGH-TEMPERATURE ELASTOMER of fluoroalkyl silicone monomers of the types shown previously, as well as other related types, by the catalytic addition of methyldichlorosilane to olefins as follows : RfCH=CH2
a + HSi-CHs c1
Pt/C --------+
or BzzOz
c1 RtCH2CH2Si-CHa c1
Polymerization studies on various monomers of these types were made at WADC without producing an elastomeric polymer (74). Through a cooperative program between WADC and the Dow Corning Corp., a successful elastomeric fluoroalkyl silicone polymer was ultimately obtained. This rubber, called Silastic LS-53, retains all of the excellent high temperature and low temperature properties of silicone rubber with the added feature of excellent fuel and oil resistance (Tables VI and VII).
I n 1954, another promising approach toward solution of the high temperature rubber problem was initiated as an Air Force-sponsored program with the Hooker Electrochemical Co. The best elastomeric polymer to emerge from this program was a polyester prepared from adipyl chloride and hexafluoropentanediol.
I/
Table IV.
Properties of Typical Kel-F Elastomer (6)
100% modulus, p.s.i.
340 3750 525 14 75 64 45 f28' - 15.8
Tensile strength, p.s.i. Elongation, % Set at break, % Shore A hardness Compression set, % after 70 hr. at 250' F. Brittle point, O F. TRIO,' F. Gehman TIO,O C.
-
Table V. Effect of Fluids on #el-F Elastomer (6) Conditibn Modulus, P.S.I.Tensile, P.S.I. Elongation, % Swell, %
Polyester Rubber
0
4
After 70 hr. at 400' F. in silicate ester fluid 70 hr. at 350' F., in diester-type fluid 70 hr. in 70130 isooctane-toluene at room temperature 70 hr. in red fuming HNOa at room temperature
553
2475
320
+6.9
102
554
510
i-105
289
1610
430
4-24
183
580
490
+22
0
+
C1CCH ?CHICH2CH zbC1 HOCHzCF&F&F&H20H -t Polvester rubber (72) Polymers in the molecular weight range of 17,000 to 25,000 were most suitable for compounding studies. A peroxide cure was found to be effective with either carbon black or calcium carbonate as reinforcing agent. These two reinforcing materials enhanced high temperature oil and air resistance, respectively, to the mutual exclusion of each other. An ultimate recipe incorporating both carbon black and calcium carbonate represented a compromise of balanced properties (Table VIII). This rubber more closely resembles natural rubber in physical prpperties than any of the other fluorinated rubbers, in that it is a snappy, elastic material with high tensile strength. Only limited swell data have been obtained, but preliminary results indicate low swell in jet fuel, but a slight shrinkage in diester-base oil at high temperatures.
/6\ Table
VI.
Typical Properties of Silastic LS-53 (15)" Tensile strength, p.s.i. ~1000 Elongation, % 170 Sho,re A hardness 55 Compression sei, yo after 22 hr. at 300' F. 22 Brittle point, F. - 90 Stiffening temp., ' F. - 78 After oven cure of 24 hr. at 300' F.
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The outstanding high temperature resistance of this elastomer in air as well as in a variety of fluids is shown in Table X . This type of elastomer also shows outstanding nitric acid resistance. Advanced Polymer Research
Table VII. Fluid
Swell Data on Silastic LS-53 ( I 5) (Time = 3 days) Temp., ’F.
% Swell
++234
350 300 350 400
Jet fuel (JP-4) Petroleum lube Diester type oil Silicate ester oil
f 9
4-14
Typical Properties of Fluorinated Polyester Rubber (10) 70 Hr., 350’ F., 168 Hr., Physical Properties Original Diester Base Oil 400’ F..in Air
Table VIII.
Tensile strength, p.s.i. Elongation, yo Shore A hardness Set at break, % Weight loss, % Brittle point, F. Freezing point, a F. Gehman TIU TRIO,O F.
2200-2400 125- 225 70-80 0-1
940- 1400 150-175 75-78 18-37
6
- 98 -71 62 - 39
Fluorocarbon Elastomer
While the previously described elastomer systems were being developed, it was felt that a n elastomer, of structure more closely approaching that of Teflon, would have the best resistance to high temperatures and fuels and oils. A polymer which contained a maximum of carbonfluorine bonds, with only enough hydrogen to effect cross linking, would approach the Teflon structure and still be elastomeric. Kel-F elastomer was a step in the right direction, but it was felt that the presence of chlorine in the polymer chain slightly impaired real high temperature resistance. An almost simultaneous development of such an elastomer occurred at the M. W. Kellogg Co. and the Du Pont Co. The Kellogg Co. effort produced Kel-F Elastomer 214 on a program jointly sponsored by the U. S. Army Quartermaster Command and Wright Air Development Center and Du Pont’s development of Viton A resulted from a company-sponsored program. The elas-
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1000-1300 150-175 64-69 6-7
tomer in question, a copolymer of perfluoropropene and vinylidine fluoride, probably represents the ultimate properties obtainable from an organic elastomer. Although Kel-F 214 and Viton A are made from the same comonomer system : CF-CF
I
+ CHFCFZ
CF3 differences in polymerization techniques and monomer ratios result in some variation in physical properties. Curing and reinforcement of this polymer can be effected by carbon black, zinc oxide, dibasic lead phosphite, and polyamines. Silica can replace carbon black for increased stiffness and heat resistance ( 7 7), and benzoyl peroxide may be used instead of polyamines (70). The best physical properties are achieved by short press cure at 275 to 300’ F., followed by an oven cure of 400’ F. for 24 hours. Typical properties are shown in Table IX.
INDUSTRIAL AND ENGINEERING CHEMISTRY
All of the elastomers discussed up to this point probably represent the ultimate, thermal, and chemical stability available from a polymer system based on carbon-carbon linkages. However, marginal improvements in thermal and chemical resistance can probably be effected by optimization of curing and reinforcing techniques. The search for a thermally stable elastomer has been most successful in achieving the original objectives only because of the vast accumulation of data on organic synthesis, fluorine chemistry, and polymer science available in the world chemical literature. By designing the most desirable polymer structures and then devising techniques to obtain these structures, we have, in effect, tailor-made elastomers for high temperature applications. Unfortunately, there are no such guide lines to follow in designing elastomeric molecules which would be stable to 1000’ F. Consideration must be given to practically every chemical system, to the exclusion of carbon-carbon bonding. Recently, interest has been focused on ‘‘inorganic polymer” systems. The Air Force has been investigating several of these systems over the past 5 years. Some of the inorganic polymer systems which have been investigated or are currently undergoing investigation are as follows: -B-N-B-N -B-P-B-P-Al-P-4l-P-p-O-p-O-Si-0-B’ -Si-0-Sn-Si-0-P-
-Si-0-Al-P-N-P-N -C-N-C-X-S-hT-p-Sn-0-Sn-Sn-S-Sn Rf-X-Rf (X = 0, N, P, S, metal)
Chelate polymers
Although the potential area has scarcely been investigated, only one promising clue has been uncovered toward the discovery of an elastomer which might be stable to 1000’F. This system is worthy of note, although it cannot truly be classified as a n inorganic polymer. This elastomeric polymer, which has only recently been discovered and will be described more fully in the near future, resulted from the work of Brown (2), University of Florida, on a research program jointly sponsored by the Office of Naval Research and Wright Air Development Center. The polymer is formed by copyrolysis of a perfluoromonoamidine and a -diamidine.
HIGHPTEMPERATURE ELASTOMER
Possible structure
Table IX.
Typical CF3CF=CF2/CH2=CF2
Elastomer Properties 7 4)
Tensile strength, p s i . Elongation, % Shore A hardness Brittle point, O F. Compression set after 70 hr. ai 250' F., %
2000 215 70
'
- 30 . 17
Table X.
High Temperature Properties of CFaCF=CF2/CH2=CF2 Elastomer (4) Tensile Condition Strengtha Elongationa Hardness" Vol. Change, % After 70 hr. in air at
F. hr. in silicate-type fluid at 550' F. After 70 hr. in jet fuel at 550' F. a % ' of original property. 550' After 5
60
125
-15 f1.5
45
37
110
SO
100
a8
f 4
11
Table XI. Thermal Stability of C-F-N Temp., C. Time, Min. 470 475 (937'
F.)
30
30
Polymer (77) Wt. Loss, % 31.0 50.4
A possible and probable structure is a chain of triazine rings linked together by the perfluoropropene group. This polymer is the only known elastomer which does aot contain hydrogen. Although a very small amount of this polymer has been made available, work is being carried out on possible curing and reinforcing systems. The most interesting property of the raw gum is its thermal stability, as observed by Wall (77) (Table XI). The residues after pyrolysis were black rubbery solids. The search for an elastomer, stable at 1000° F., will probably not be successfully concluded until a great mass of preliminary data on new and unusual chemical'bonding systems is made available, new systems of polymerization catalysts are discovered, and a new technology of rubber reinforcing and curing is perfected. References
(1) Abere, J. F., Wright Air Develo ment
Center, Tech. Rept. 57-478 (19577. (2) Brown, H. C., Division of Polymer Chemistry, 134th Meeting, ACS, Chica 0,Ill., September 1958. (3) 81ark, R. T., Jr., Wright Air Develo ment Center, Tech. Rept. 5 4 2 1 3 (19547. (4) Griffin, W. R., Zbid., 57-478 (1957). (5) Hamlm, H. C., Zbid., 55-381 (1955). (6) Headrick, R. E., Zbid., 55-377 (1955). (7) McBee, E. T., Pierce, 0. R., Zbid., 52-191, Pt. I1 (1953). (8) McBee, E. T., Roberts, C. W., Judd, C. F., Chao, T. S., J. Am. Chm. SOG.77, 1292 (1955). (9) Postelnek, William, Chem. Eng. News 31,1958 (1953). (10) Postelnek, W., Rubber World 136,543 (July 1957). (11) Rugg, J. S., Wright Air Development Center, Tech. Rept. 57-487 (1957). (12) Schweiker, G. C., Robitschek, P., J. Polymer Sci.24,33 (1957). (13) Stedry, P. J., Abere, J. F., Borders, A. M., Ibid., 15, 558 (1955). (14) Stump. E. C.. Jr.. Wright Air Developmeit Cent&, Tkch. Eept. 56-493 (1957). (15) Talcott, T. D., Brown, E. D., Holbrook, G. W., Division of Rubber Chem-
-
istry, 132nd Meeting, ACS, New Yprk, N. Y . , September 1957. (16) Tarrant. P.. Dvkes. G. W.. Norris. F. F., OConnk, D. E.; Abstraits, 12btd Meeting, ACS, 1955, p. 47M. (17) Wall, L. A., National Bureau of Standards, Washin ton, D. C., private communication, 19&. RECEIVED for review July 10, 1958 ACCEPTED August 15, 1958 This article is from a paper presented by Major Postelnek at the Gordon Research Conference on Elastomers, New London, N. H., in August 1958. VOL. 50, NO. 11
NOVEMBER 1958
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