Evaluation of Organic Fluorine Compounds for Use in Military Aircraft HAROLD ROSENBERG AND J. C. MOSTELLER Wright Air Development Center, Wright-PattersonAir Force Base, Ohio
I
N ORDER to obtain materials possessing unusual properties
and capable of withstanding the extremes of temperature and operating conditions encountered in new high-speed aircraft and guided missiles, the U. S. Air Force during recent years has been conducting investigations in various fields of chemistry. One class of substances which has been studied extensively and in which work is continuing is that of organic fluorine compounds. As a result of the investigations on fluorine-containing materials, many of the desirable properties and advantages of these compounds have been further exploited to permit application in military aircraft. Unfortunately, certain limitations of this class of compounds have been uncovered which prevent, in part, their full utilization by the Air Force. Of the many potential uses considered for fluorine compounds and for which evaluation studies have been carried out, only the more significant applications are discussed here. These include use of fluorine-containing materials as fire-extinguishing agents, acid-resistant coatings and protective materials, nonflammable hydraulic fluids and lubricants, rubberlike materials, components of electronic equipment, and fungicides. FIRE-EXTINGUISHING AGENTS
bromotrifluoromethane, to be highly successful in extinguishing full scale rocket propellant fires ( 2 ) . The Aerojet Engineering Corp., in work performed under Air Force sponsorship, has demonstrated that it is feasible by use of either of these two materials to protect piloted rocket-powered aircraft from fireand explosion hazards involving such propellant combinations as white fuming nitric acid (WFNA) and JP-3(formula assumed as CgHla)fuel. The two fluorocarbons, as can be seen from the data presented in Table 11, are completely effective in extinguishing fires of this type; none of the other agents tested extinguished propellant fires. Although bromotrifluoromethane appears to be slightly more efficient in terms of amount of agent required, dibromodifluoromethane is believed to be the more practical agent for use in rocket aircraft because of its high density, relatively high boiling point, and relatively low vapor pressure at elevated temperatures. ACID-RESISTANT COATINGS AND PROTECTIVE MATERIPlLS
Fluorine-containing materials are being used with much success in sealing and gasketing materials in aircraft reaction motors. They are also now employed as lubricants as well as for gaskets and seals in nitric acid-refueling trailers. Polymers of monochlorotrifluoroethylene and tetrafluoroethylene, such as Kel-F and Teflon, are utilized as flanged dynamic seals and as static gaskets in the oxidizer, fuel, and air systems of the reaction motor. The lubricants used in these motors are primarily fluorocarbon oils and greaselike materials such as Fluorothene G and 0 or Fluorolube derivatives, all polymers of fluorinated ethylenes. The fluorocarbon oils are used where lubrication is required for moving parts in contact with nitric acid-type oxidizers over the temperature range of -65" to 160" F. The heavier greaselike substances, such as Fluorothene G, are used particularly for sealant purposes where i t is desirable to prevent entry of oxidizer fumes or necessary to protect materials lacking somewhat in corrosion resistance to such fumes.
Following World War I1 the Air Force employed methyl bromide (and more recently, bromochloromethane) as the fire-extinguishing agent in military aircraft. This compound is, in general, more effective than the agents previously used, such as carbon tetrachloride, but possesses certain significant limitations. The toxicity and corrosiveness of methyl bromide were sufficiently great to cause the Air Force to seek other materials as effective as the halocarbon but less toxic and corrosive. As a result of an extended investigation conducted by the Purdue Research Foundation under the sponsorship of the Army Engineers Research and Development Laboratories and, later, under the Air Force, a number of fluorocarbons were shown to be more effective than methyl bromide as fire-extinguishing agents (4). Of this group, TABLE I. FLUORINE-CONTAIMNG AND NOXFL~ORINE-CONT.4INING EXTINGUISHING AGENTS several compounds, presented Order of with certain of their properties Effectiveness in Table I in comparison to Agent Mol. B.P., Curve, Volume Weight Toxicity, Formula Name Wt. C. %" basis basis P.P.M.b other common extinguishers, showed exceptional promise. These were evaluated further as extinguishing agents for rocket fuel fires in a second study cona Per cent of extinguisher required t o reduce flammability of mixtures of air and n-heptane. b Average lethal concentration for rats on 15-minute exposure. Data obtained from toxicity studies carried ducted by Purdue under conout by Army Chemical Center ( 1 ) . tract to the Air Force ( 3 ) . I n this investigation, 1,2-dibromoTABLE 11. RESULTSOF WFNA-JP-3 SPRAYFIRE-EXTINGUISHMENT TESTS WITH tetrafluoroethane was found to VARIOUS EXTINGUISHING AGENTSa be, in general, the most effecAgent Discharge Total tive extinguisher, while dibroAgent Rate Agent, m o d i f 1u o r o m e t h a n e ranked Formula Name Lb./S$c. Lb. Result second. The latter compound, CHaBr Methyl bromide 9.0 9.0 Fire not affected by agent CHtBrCl Bromochloromethane 10.6 5.52 Fire not affected by agent shown to be the most effective CFgBrt Dibromodifluoromethane 5.7 1.5 Fire extinguished immediately CFaBr Bromotrifluoromethane 5.5 0.7 Fire extinguished immediately of all compounds tested in the a Conducted with "fire size," or propellant flow rate, of approximately 0.2 pound per second. earlier Purdue studies, was recently found, together with 2283
gziz
O
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Thc nitiic acid-servicing trailers employ the same fluorocarbon oils and grertselike materials that are used in the rocket motors. However, greases used to lubricate the pump bearings in these trailers are required to be resistant not only to the concentrated oxidizers but also to various aqueous mixtures as well as to watei itself. The fluorocarbon greases have thus far been deficient as lubricants, owing to their poor stability in remaining as greaselike materials under applied load and their lack of water resistance, n hich results in rusting of the pump bearings. External portions of rocket motors or of the aircraft housing the rocket, assembly are primarily constructed of materials inherently resistant to corrosion, such as corrosion-resistant steel or aluminum alloys. Paints and chemical coatings for corrosion protection have not been applied very successfully t o rockets using nitric acid-type oxidizers. Teflon and Kel-F paints have been used experimentally, but their applicability appears to be marginal. Such coatings ale not impervious to nitric acid attack and, in addition, require that the part coated be subjected to temperatures of 450' F. and highei in order for the paint t o "spt." Some nonfluorine-containing paints which resist nitric acid fumes, such as vinylite or chlorinated rubber-base paints, arc fairly satisfactory, except that they are readily soluble in the rocket fuel used in combination ~ i t the h oxidizer. Thus, spilling of the fuel often results in dissolution of the paints arid lack of protcction to metal burfaces against nitlic acid NONFLAMMABLE HYDRAULIC FLUIDS AND LUBRICANTS
In the developnient of hydraulic fluids for military aircraft, certain properties or characteristics are desired. These may be classifird as: Nonflanunability High viscosity index Wide liquid range Stability 1. Thermal 2. Hydrolytic 3. Low-t.emperature 4, Oxidw t,ion-corrosion Lubricity Some of the disadvantages of t,he hydrocarbon-base hydraulic fluid currently used by the ilir Force are flammability and osidative instability a t elevated temperatures. Diester-base Iubricants, using di-2-ethylhexyI sebacate as the base stock oil, have now been developed by the Air Force which permit utilization a t temperatures above 400" F. However, these blends, which arc already employed in certain applications, are flammable and would be hazardous if used a8 hydraulic fluids in milit'ary ai:craft. To overcome these limitatione, the Air Force has been conducting a research and development program directed along two main paths. The first approach involves the development of nonflammable materials for addition as "snuffer" additives to flammable base stocks t,o produce less flammable fluids. The other course is concerned with the synthesis of nonflammable base stock oils for use with other addit,ives to obtain desired blends. Fluorinecorit'aining materials have been considered and studied in both connections because of their decreased flammability, exceptional stability, inertness to chemical attack, and excellent lubricity. Among the most promising substances examined as muff er additives are tetrachlorodecafluoroheptane, chlorofluoroheptenes, and low-molecular-weight polymers of monochlorotrifluorocthylene. These materials were found to be effective in snuffing such flammable base stocks as mineral (hydrocarbon) oils, silicates, and diestem. However, difficult,y has been encountered in preparing blends of these additives and base stock oils so as to obtain fluids having desirable viscosit,y-temperature characteristics, lowtemperaiure stability, and decreased volatility. On the other hand, it was not,ed early that certain polyhalo compounds poswssed excellent lubricity and could be used to improve the wear
Vol. 45, No. 10
characteristics of hydrocarbon-base oils. It has since been shom 11 that the antiwear propertics of the polychlorofluoro materials can also he extended to other base stocks. such as the silicones, which, by themselves, exhibit poor lubrication characteristics. Thi- i E particularly truein the case of such compoundsas the chlorofluoiaheptenes, which are effective antiwear additives for silicones, a i can be seen from the data in Table 111. These fluorinated alkenes may, in addition, prove of value as extreme-pressure additives. Lately, a ternary blend of a silicone, fluorocarbon, and diester was prepared which shows proniise as a hydraulic fluid from the ytandpoint of lubricity and der1rased flammability
TABLE 111. EFFECT O F A CHLOROFLLOROHEPTENE L-POX T%-ELR PROPERTIES OF A SILICOXE SHOVXB Y SHELA-BALL TESTS Composition of Fluid,
___ Parts by Weight ChloroSilicone 10 9 8 7 6 0
fluoroheptene
_,
.
.
Average Wear Scar Diameter, M m . , Steel on Steel Bearing Surfaces 4.0-kg. 40.0-kg. load load 0.376 2.415 0.244
1
0.252 0.300 0 346 0.318
0.758
0.854 1.006
1.086 1.102
Recently, the Air Force has undertaken the investigation of another class of fluorinated materials--the fluorine-containing diesters--as an additional source of hydraulic fluid oils or additivee. A number of these compounds have been prepared in the .4ir Force laboratories and are nom undergoing evaluation studies. Preliminary evidence indicates that certain of these materials may be less flammable than theii hydrocarbon analogs while still retaining many of the desirable lubricant properties of the nonfluorinated diesters. In the consideration of materials for use as nonflammable base stock oils, various halogenated compounds, such as hexachlorobutadiene and polymers of nionochlorotrifluoroethylene, have been investigated. While these substances are nonflammable, they possess certain undesirable properties. The butadiene derivative is exceedingly toxic and the fluorinated compounds exhibit rather poor viscosity-temperature characteristics. Other materials, such as brominated benzenes ( 5 ) , polyglycols, and the phosphate esters, show some promise, but no completely acceptable nonflammable base oil has been prepared to date. RUBBERLIKE MATERIALS
Major improvements must be realized in certain properties of elastomeric materials if they are to keep pace with advances in aircraft and missile designs. New fuels, coolants, lubricants, and hydraulic fluids, combined VI ith requirements for operation at both higher and lower temperatures than necessary heretofore. pose problems which cannot be satisfactorily resolved by selective copolymerization of conventional monomers, nor by thr judicious compounding of available synthetic rubbers. Since each of the latter methods has its ORTI peculiar vi1 tues and shortcomings, compromises are being accepted in compound: for many applications, even though such compromises may be of a most undesirable nature. Serviceability a t -65" F., for exaniple, is obtained in a Buna N rubber by the use of plasticizers but a t the expenRe of poorer ozone resistance as well aa decreaved tensilr strength, set, abrasion resistance, and other mechanical properties. If contact with organic solvents is also involved in the application (as i t usually is where Buna N compounds are concerned), the plasticizer will inevitably be leached out and Ion temperature flexibility lost. However, the rubber item may continue to funrtion satisfactorily at low temperatures as long as it i. swollen by the medium-Le., to the extent that the organic 601vent can penetrate the rubber and act as a plasticizer. Applications which require resistance to excessive swelling by varioub
October 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
synthetic oils and hydraulic fluids at high temperatures (500 F. and higher) present even more difficult problems. In recognition of the ever-increasing urgency of requirements for greatly improved elastomers in a variety of applications, the Air Force has for some time maintained an extensive program of exploratory research directed toward the development of socalled “super” elastomers. The major portion of this effort has been in the field of fluorine-containing polymers for reasons a hich are implicit from study of the structure and properties of commercial plastics of polymonochlorotrifluoro- and polytetrafluoroethylene. The development of any elastomer which would combine good mechanical properties with some of the thermal stability and chemical inertness of these materials would solve many pressing problems of the Air Force in the rubber field. However, i t is far easier to postulate than to realize experimentally such a fluoroelastomer (fluorine-containing polymer with rubberlike properties). Several classes of fluorine-containing polymers prepared for the Air Force by Minnesota Mining and Manufacturing Co. under this broad program are of considerable interest in this connection. Possibly the most promising class of new fluorine-containing polymers is represented by poly-1,l-dihydroperfluorobutylacrylate, [-CH2CH( COOCH2CF2CF2CF8)-],. This particular polymer is under intense study a t the present time as a possible fueland oil-resistant elastomer. It is almost completely indifferent, as a homopolymer, to the normal swelling action of both aliphatic and aromatic-type fuels and t o various synthetic oils of interest in specialized applications. Table I V illustrates the order of SOL vent resistance and low-temperature flexibility offered by important members of this fluoroalkyl acrylate class as compared with Buna N synthetic rubbers. The butadiene copolymers of fluoroacrylates are much less sensitive to aromatics than are the corresponding Buna N compounds. Such properties encourage the further investigation and development of these and other fluoroelastomers despite the unique difficulties which have been encountered in connection with the curing and reinforcing of highly fluorinated polymers of a saturated nature. There has been some indication that the polyperfluoroalkyl acrylates have potentialities as fuel- and oil-resistant elastomers for high temperature applications somewhat beyond the capabilities of the Buna N rubbers; preliminary investigation of certain other fluoropolymers also offers encouragement in this respect. O
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elastomers that will be serviceable far beyond the limitations of the known rubbers, In general, the considerations which served as original justification for this work have been confirmed, although it appears that satisfactory low-temperature properties will ultimately be obtained despite, rather than because of, the presence of fluorine in the molecule, especially when the fluorine atoms are located on the “backbone” of the polymer. No significant attempts have been made to extend low-temperature flexibility by the use of plasticizers. However, it is believed that fluorocarbon plasticizers can be developed which will be compatible with “fluoroelastomers,” yet will resist extraction by hydrocarbon solvents. It would be still more desirable, of course, to synthesize an elastomer which would be plasticized internally by virtue of its chemical composition and molecular architecture.
TABLEIT. SOMECLASSES OF FLUORINE-SUBSTITUTED MONOMERS INVESTIGATED AS POSSIBLE SOURCE M.4TERIALS FOR SPECIALTY SYNTHETIC RUBBERS~ Fluorinated Olefins Fluorinated Olefinic Esters Dihydroperfluoroalkyl acrylates Perfluoro-olefins Dihydroperfluoroalkyl crotonates Fluorinated dienes Perfluoronitriles Dihydroperfluoroalkyl fumarates Perfluoroalkyl-substituted olefins Dihydroperfluoroalkoxy acrylates Vinyl esters of perfluoro acids Perfluoroalkyl-substituted dienes Allyl esters of perfluoro acids Perfluoroalkyl-substitutedacrylonitriles a Studies conducted under Air Force contract by Minnesota Mining and Manufacturing Co.
TABLEVI. VOLTAGE BREAKDOWN STRENGTHS OF SOME F L U O R I N A T E D AND NONFLUORINATED DIELECTRICS AT 25’ c.
Formula (CZF5)d (CdF~)20 (CaT1)sN C8FinO (CeHu)zO (CdF~)ar\’ CCla
....
Dielectric Material Name Perfluorotriethyl amine Perfluorodibutyl ether Perfluorotripropyl amine Perfluorocyclooctyl ether Perfluorodihexyl ether Perfluorotributyl amine Carbon tetrachloride Hydrocarbon oil (aliphatic-aromatic)
D.C. Breakdown with 0 025-Inch Gap (Approx. 0.5-Inch Diam Spheres), Kv. 24 27.5 29 29 30 5 32 1s
1s
ELECTRONIC EQUIPMENT
In the field of electronic equipment, including items such as insulation materials, motor and transformer leads, coaxial cable, high-voltage cable, magnet wire, thermocouple wire, and ignition T A B L E Iv. COMPARATIVE PROPERTIES O F EXPERIMENTAL wire, the requirements of thermal stability, good heat transfer ELASTOMERS~ characteristics, and low dielectric properties have led to the invesSwell of tigation of fluorine compounds. The properties of some fluorineVulcanizate, % FBAb containing materials thus far examined explain the reamn for the FHAC,’ 70:30 AN4 The, T1d Beniso-octane/ interest in fluorine compounds for such applications. This may Polymer Mole % ‘ C. C. zene toluene be seen from the data in Table VI, where voltage breakdown Poly-FBA 100 FBA - 15 -6.5 12 0-10 Buu FBA 11 FBA -71 -59.0 310 310 strengths of several fluorinated compounds are compared with Bu/FBA 28 FBA - 62 150 -41.0 250 those of a hydrocarbon and a halocarbon. Other gaseous, liquid, 43 FBA Bu/FBA - 46 -320.0 70 71 -5 1 82 FBA Bu/FBA -12.0 5 and solid fluorine-containing compounds are of interest for elecn 10-20 100 FHA - 39 -6.5 Poly-FHA .., 126 24 FHA -4.2 Bu/FHA - 75 tronic applications and many such materials, particularly of the 45 FHA Bu/FHA -36.0 < -43 72 poiyfluoroethylene type, are being employed in present-day airQ30 530 0 AN < -72 -72.0 Polybutadiene 12 AN - 72 - 4 2 . 0 520 230 Bu/AN craft. - 60 - 3 1 . 0 95 23 AN 510 Bu/AN 195 -27 35 43 AN -7.0 Bu/AN Although fluorine compounds are being utilized with much suca Data compiled under Ail Force contract by Minnesota Mining and cess in aircraft today, materials with still better properties are deManufacturing Co. sired. Many requirements stem from the fact that suitable coolb 1,l-Dihydroperfluorobutylacrylate. C 1,l-Dihydroperfluorohexyl acrylate. ing systems are essential in miniaturized design &s the result of d Acrylonitrile. e ASTM brittle temperature. efforts to reduce weight and space aboard aircraft, Thus, dielecI Gehman temperature. trics must serve a dual purpose-coolant as well as dielectric. 0 Butadiene. The conventional cooling systems incorporating fans and blowers impose additional weight which nullifies the advantages of miniaturization. Therefore, means to circulate a low-electricalHomopolymers and/or selected heteropolymers have also been loss, stable, high-temperature liquid in such equipment are being prepared from representative members of a number of classes of sought. The fluorine compounds tested thus far are limited by new or relatively new monomeric materials. In Table V is pretheir boiling point. I n addition, only materials of extremely high sented a list of the major classes of monomers which have been purity are desired for dielectrics. As can be seen from Table screened to date in the continuing quest for fuel- and oil-resistant O
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 45, No. 90
VII, in which data on the dissipation factor of a fluorinated dielectric are presented, purity of product is exceedingly important in this type of applicational use.
effective as fungistatic agents (6). This effectiveness is &own markedly in Table VIII. The advantages of the fluorobenzenes over the fungicides currently employed for protection of Air Force fabrics are significant. They include greater solubility in organic solvents, relatively low concentration in fabric required to give protection, greater e a , ~ e T~~~~~~ 7 1 1 , E~~~~~OF pURITY O K D~~~~~~~~~~ pACToR OF PERFLUORODIHEXYL ETHER .4T hfICROlVAVE FREQUENCIES of application to fabric, and nonrestrictive coloring properties. They do not promote the oxidation of the materials in which Frequency a t 25." C. 10 1000 3000 8500 they are impregnated, as is the case with some of the fungicides Purification Stage m.c.s. m.c.s. n1.c.s. m.c.s. now in use. The fluorobenzenes are relatively similar in toxicity 0.0005 0.016 .... After 1st disti!latipn and corrosiveness to the other fungicides in use, but t,hese disad0,0005 0:0037 0.0045 0,004 After 2nd distillation After 2nd distillation and vantages are far outweighed by their beneficial silica gel treatment 0.0006 0,0023 0.0028 0.0029 properties and can perhaps be circumvented il-i t h e TABLEL'III. BREAKIKG STRENGTHS O F C O T T O N PAR.4CHUTE ~ ~ ' E B B I S G future. THREAD TREATED WITH FUNGICIDES A X D EXPOSED TO Xyrothecium verrucaria Formula CsHe(NOI)2F2
Fungicide
C&(XO~)~F CaH2(SO2)2FC1 C6H2(S02)2FBr
ClaH,202y2Cu
Name 1,3-Difluoro-4,6-dinitrobenzene l-Fluoro-2,4-dinitiobenzene l-Fluoro-3-chlo1o-4,6dinitrobenzene l-Fluoro-3-bromo-4,6dinitrobenzene Copper-8-guinolinolate
Concen- Fungicide tration, in Thread, yo ;\Ig./Liter 1500 0.31 1000 0.15 1500 0 17 1000 0 11 2000 0 22 1000 0 16 1000 0.129 730 0.129
...
.
....
1 .,0o05
Thread exposed 31.03 23.48 31 59 18 16 28 85 2 5 48 31, 58 31.17 23 90 . 6 8
Most fluorine compounds employed as dielectrics undergo voltage breakdown but recover and do not prevent the continual build-up of energy. Certain gaseous fluorinated compounds are being used as dielectrics, Freon and sulfur hexafluoride being the predominant members of this group. However, sulfur hexafluoride, although an excellent dielectric, gives rise under voltage breakdown to toxic and corrosive decomposition products. Some solid fluorocarbons are being used as coatings for electrical wire and as insulators. Bonding to metallic surfaces, extrusion processes, and coatings free from pin holes have been improved over the past several years by use of this type of material. One disadvantage, however, is still apparent and may be characmaterials break down owing to teristic of fluorocarbons-the bombardment of electrons. FUNGICIDES
Deterioration, weakening, and destruction by rotting of cloth and other woven fabrics, thread cordage, leathers, and plastics, as a result of the growth of molds or fungi, constitute important factors in the storage, and in use before and after storage, of such materials. Prevention of such deterioration and destruction depends upon protection of fabrics from invasion by fungi. One method of providing protection is treatment of these materials with fungicidal or fungistatic chemicals. For this purpose no ideal or perfect chemical has as yet been discovered. Some compounds supply only partial protection, vhile others disintegrate quickly or cause weakening of the fabric. Other fungicides, such as copper-8-quinolinolate, a compound extensively employed by the Air Force, impart undesirable colors, corrode attached metal articles, or prove toxic in contact Kith human skin. Hence, i t is highly desirable that there be available a number of materials with different characteristics, from ~3 hich may be chosen one adapted to known or anticipated conditions. One class of materials recently investigated for fungicidal properties and which now shows excellent promise for use as fungicides is the fluorobenzenes. A number of such compounds, including fluorinated phenols, quinones, anilines, and nitrobenzenes have been prepared and evaluated for the Air Force by the Illinois State Natural History and Geological Surveys. Of the various groups of compounds tested, the dinitrobenzenes proved most
CONCLUSIONS
Lb.
Threadunexposed 30.55 30.76 30 75 30 70 28 86 28 67 28.61 28.93
Definite progress has been made in the appiicxtion of fluorine-containing compounds for use in military aircraft. Fluorocarbons, polyfluorocarbony fluorobenzenes, and other fluorine-containing materials are being employed or will shortly find use as fire-extinguishing agents, acid-resistant cos:ings and greases, components of electronic e q u i p ment, and fungicides. However, much work re30.4 30.6 mains in the field of fluorine chemistry before fuller utilization of fluorine compounds can be ma,de in military aircraft, particularly in connection with the development of nonflammable hydraulic fluid6 and rubberlike materials. Liquids having wider liquid range.. improved viscosity-temperature characteristics, and lower vapor pressures are desired. Rubberlike materials with more satisfactory cold flow, resilience, permanent set and hardnese, and high temperature stability must be prepared. However, it 1s safe to predict that aircraft and guided missiles will, in future years, utilize fluoroelastomers as well as the other fluorinated materials described in a number of applications of critical operational importance. .4CKNOW LEDGM ENT
The authors wish to express their appreciation to Maj. k?*~ H. Ebelke, A. E. Prince. R. R. Stasiak, and H. A. Pearl of the Wright A4irDevelopment Center for t,heir generous aid i n the prepareiion of this paper. LITERATURE C I T E D
(1) Downing,
R. C., Eisenian, B. J., Jr., and Malcolm, J. E., .\'?i~i;tZ,
Fire Protect. Assoc.. Quart., 44 [No. 21, 119 (1951).
(2) Harrold, hZ. C., Wright Air Deveiopment Center, CS.AF, .4P Tech. R e p t . 5876 (May 19.52). (3) hIcBee, E. T., Welch, 2. D., and Rosenberg, H., Wright Air Development Center, US-kF, AF Tech. Rept. 5872 (September (1951). ( 4 ) McBee, E. T., et al., "Fire Extinguishing Agents," Filial Report on Contract KO.W-44-009eng.507, Army Engineers Research and Dei-elopment Laboratories, June 1950. ( 5 ) Pennsylvania Refining Laboratory Rept. P E L 2.57 (Febrxzy 1951). (6) Tehon, L. R., and Kelcyra, S., Wright Air Development Ctntr-ir, USAF, AF Tech. R e p t . 6518 (Part 11) (August 19.52). RECEIVED for review November 3, 1952. ACCEPTED June 33. 11253. Presented in part before the Fluorine Symposium, Division of 1ndu;trial arid Engineering Chemistry, a t the 122nd 3leeting of the - 4 n r ~ ~ r CHEMICAL c~r SOCIETY,Atlantic City, . ' h J. Opinions expressed are those of the n,ithors and d o not necessarily express the official opinion of the U. E. Air Pcrce c: Wright .4ir Development Center.
