SYNTHESIS, THERMAL STABILITY, FLAMMABILITY, AND VISCOSITY OF SOME PARTIALLY FLUORINATED AND PERFLUORINATED AROMATIC AND POLYAROMATIC ETHERS G. A. RICHARDSON A N D E. 5. BLAKE
Monsanto Research Corp., Dayton, Ohio
A number of perfluorinated and partially fluorinated phenyl and polyphenyl ethers were synthesized, characterized for thermal stability, fire resistance, and viscosity, and compared with their hydrogen analogs to assess the potential use of this class of compounds as functional fluids. Without exception, polyfluorination and perfluorination lower thermal stability; the decrease in stability depends on the position and number of fluorine substituents. The autoignition temperature and fire resistance are not improved over the hydrogen analog, and viscosity is degraded. These data coupled with the comparatively high melting points do not suggest a bright future for this class of compounds as useful functional fluids.
THEpossibility of hydraulic fires in military and commercial aircraft always is a hazard. The fire-resistant hydraulic fluids now used for commercial jet transports are adequate but, because they are not thermally stable above 225'F., they cannot be employed above 400"F., the temperature projected for supersonic aircraft. There also exists a challenge to develop lubricants improved in oxidative, thermal, and fluid properties to meet the severe performance demands of future turbine engines. Since the advent of the polyaromatic ethers, interest has been widespread (L. S. Air Force, 1963) in synthesis and characterization of perfluorinated polyphenyl ethers as a base stock potentially improved in thermal and oxidative stability and in fire resistance. Because all bonds are chemically strong, such molecules would seem to be ideal. Free rotation about the oxygen atom should improve viscosity and, since all hydrogen is replaced with halogen, the molecule should be stabilized oxidatively and be fireresistant. A number of selected partially fluorinated polyphenyl ethers and two perfluorinated ethers were synthesized in an effort to establish the thermal stability level and other physical properties of this broad class of compounds. Experimental
Synthesis. Intermediates and reaction conditions for the synthesis of the fluorine-substituted aromatic ethers are listed in Table I. Diglyme was used as the solvent for all reactions except for those of compounds 1 and 5 , which used dimethylformamide, and compound 10, which used triglyme. I n general, for a 0.1-mole reaction, the aromatic halide was dissolved in 50 to 100 ml. of solvent, and added dropwise to the nucleophilic reagent in 100 to 150 ml. of solvent a t the reaction temperature. The phenoxides were prepared by azeotropic distillation (150" to 200" C.) with toluene of a stoichiometric mixture of the respective phenol and potassium hydroxide. 22
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On completion of the reactions the solvent was removed in vacuo and the residues were taken up in ether and washed with dilute hydrochloric acid, dilute caustic, and water to neutrality. I n the preparation of compound 9 the potassium m-(m-phenoxyphenoxy)phenoxide solution was added dropwise to the pentafluorobenzene solution (reverse addition) and no disubstitution products were found. The compound numbers of the fluorine-bearing ethers are identified with structures in Table 11. All distillations of the fluoroethers were conducted using a 42-inch Todd Vigreux column, except compounds 20, 22, and 23, which were distilled using a 10-inch heated Vigreux column. The solid ethers could be recrystallized from either methanol or hexane. All melting points, except that of decafluorodiphenyl ether, are uncorrected. Copper powder was used as a catalyst in the synthesis of compounds 11 and 12. Preparation of Decafluorodiphenyl Ether (Compound 2 ) and p-Di(pentafluorophenoxy)tetrafluorobenzene (Compound 6). A mixture of anhydrous potassium pentafluorophenoxide (Forbes et al., 1959) (53.1 grams, 0.24 mole), hexafluorobenzene (59.3 grams, 0.32 mole), and 80 ml. of diglyme was heated a t 160" to 165" C. for 4 days in a 300-ml. stainless steel, Aminco rocking autoclave. The slurry was diluted with one-half volume of ether, filtered, and distilled a t a 40 to 1 reflux ratio up to a boiling point of 165°C. per atm. Two products were obtained on distillation of the residue (30 to 1 reflux ratio): 12.0 grams of decafluorodiphenyl ether (b.p. 105°C. a t 10 mm., m.p. 70" to 70.8"C.) (compound 2 ) ; and 9.0 grams of p-di(pentafluorophenoxy) tetrafluorobenzene (b.p. 165' to 190"C. a t 0.1 mm., m.p. 137" to 145" C.) (compound 6). Decafluorodiphenyl ether was recrystallized three times from methanol to yield 8.4 grams of product [m.p. 71" to 72°C. (corr.), (lit. m.p. 67" to 69"C.)] (Wail et al., 1963). p-Di(pentafluorophen0xy)tetrafluorobenzene (compound 6) was redistilled through an 11-inch Vigreux column.
The product boiling a t 120" to 130°C. a t 0.10 mm. was recrystallized two times from n-hexane to give 7.5 grams of product [m.p. 152.4" to 153.1"C. (lit. 145" to 148" C . ) ] (Wall et al., 1963). Thermal 'Stability. The apparatus employed for the majority of the thermal stability ( T D= temperature a t which d p dt = 0.014 mm. of H g per second) determinations (Table 11) has been described in detail (Blake et al., 1961). For a few of the smaller molecules (compounds 2, 5, and 7) where the T n was appreciably above the boiling point, the d p dt data were determined as described by Johns (Johns et a l , 1962, Method D ) , converted to millimeters of mercury per second, plotted (Blake et d., 1961), and extrapolated to the temperature a t which the d p dt value is 0.014 mm. of Hg per second. The decomposition temperatures were determined for compounds 19 and 24 (Table 11) using both methods. The d p dt data by the Johns method converted to the temperature a t which the dp dt value is 0.014 mm. of Hg per second gave T D values 13" and 20°F. higher, respectively, than were obtained by the Blake et al. procedure. Flammability. The autogenous ignition temperatures (Table 11) were measured by ASTM procedure D 215565T and the flash and fire points by the ASTM procedure D 92-57, using a semimicrocup (Watson, 1955). Viscosity. Kinematic viscosity was determined by the ASTM D 445-T 1961 procedure using Cannon-Manning viscometers calibrated and supplied by the Scientific Development Corp., State College, P a . Measurements a t 100" to 210" F. were conducted in a Scientific Development Corp. constant temperature bath, Model 11-MS. For temperatures of 300" F. and above, the thermal stability nitrate bath was used.
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Results and Discussion
Thermal Stability. The fluorinated ethers prepared from hexafluorobenzene, although of high purity by VPC, N M R , P M R , infrared, molecular weight, and elemental analysis, generally could be purified further by an 18-hour heat treatment a t a temperature 50" to 70°F.higher than initial decomposition temperature followed by redistillation. This purification increased the T D by as much as 10O0F. Certain generalizations can be made from the To data of Table 11. Without exception, polyfluorination and perfluorination of a polyphenyl ether lower the T Dbelow that of the hydrogen analog. The magnitude of the decrease in T n is dependent upon the position and number of fluorine substituents. Polyphenyl ethers with a perfluorinated terminal ring are thermally less stable than their hydrogen analog by 70" to 80°F. (compare compounds 8 and 17 with compounds 13 and 18) and possibly slightly less stable than ethers with a perfluorinated center ring (compare compound 8 with compound 15). The lower stability of a terminal ring, in comparison with a 2,3,5,6tetrafluoro terminal ring, suggests that the p-fluorine may be eliminated as a radical (compare compounds 8 and 9). Fluorine atoms in the 2,3,5,6-position of the terminal ring of a polyphenyl ether decrease the Tn of the hydrogen analog by 40°F. (compare compound 9 with compound 13). Tetrafluorophenyl and octafluorobiphenyl moieties in the center o f a polyphenyl ether lower the thermal stability by 30" to 50" F. compared to the hydrqgen analog (com-
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pare compound 15 with compound 19, compound 10 with compound 14, and compound 20 with compound 21). A monofluorinated ether has a higher thermal stability than a monochlorinated ether, as would be predicted from the difference in aromatic C- C1 and C- F bond energies (compare compounds 11 and 1 2 ) . Since no partially fluorinated polyphenyl ether, regardless of the position or degree of substitution, has a higher T o than that of the hydrogen parent, it appears that the molecular forces in a polyphenyl ether molecule are so well balanced that when any substitution is made, even though a stronger chemical bond may be formed, the balance is disturbed and the stability is reduced. Flammability. The flash and fire properties were determined on compound 10 (flash 258°C.; fire 346'C.) and on compound 14 (flash 291" C., fire 328" C.).As evidenced in these and the respective AIT data, partial fluorination of an aromatic ether up to 31% fluorine (compound 10) does not increase the A I T values over that of the unfluorinated ether (compound 14) nor appreciably alter the fire point. Further, from a comparison of the A I T data of compound 2 (1040°F.) and of compound 3 (1145OF.1, one can conjecture that perfluorination of an aromatic ether will not improve, and may degrade, the fire resistance. Viscosity. A comparison of the viscosity data of Table I1 shows that the fluorine-bearing aromatic ethers without exception have poorer ASTM viscosity slopes than their unfluorinated analogs.
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Conclusions
The lowered thermal stability, unimproved fire resistance, and degraded viscosity of the polyfluorinated and perfluqrinated aromatic ethers in comparison with their hydrogen analogs do not suggest a bright future for this class of compounds as useful functional fluids.
Acknowledgment
The authors acknowledge the assistance of J. Satanek in the thermal stability, viscosity, and flammability measurements, and J. V. Pustinger and J. E. Strobe1 in the spectral analysis.
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literature Cited
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Blake, E. S., Hammann, W. C., Edwards, J. W., Reichard, T. E., Ort, M. R., J . Chem. Eng. Data 6, 87 (1961). Forbes, E. J., Richardson, R. C., Stacey, M., Tatlow, J . C., J . Chem.Soc. 1959, pp. 2019-21. Johns, I. B., McElhill, E. A., Smith, J . O., IND. ENG. RES.DEVELOP. 1 , 2 (1962). CHEM.,PROD. U. S. Air Force Contract AF 33(616)-7458. Monsanto Research Corp., Dayton, Ohio, 1963. Wall, F. A., Pummer, W. J., Fearn, J. E., Antonucci, J. M., J . Res. Natl. Bur. Std. A67, 481 (1963). Watson, F. J., Lubrication Eng. 11, 86 (1955). RECEIVED for review August 3, 1967 ACCEPTED December 14, 1967 VOL. 7 N O . 1 M A R C H
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