FIRE EXTINGUISHANT COMPOSITIONS, METHODS AND SYSTEMS UTILIZING BROMODIFLUOROMETHANE

Abstract: Fire extinguishing methods, compositions and systems are disclosed which provide for efficacious fire extinguishment with minimal environmental hazard. The methods employ compositions comprising bromodifluoromethane alone or in combination with at least one other fluorocarbon fire extinguishant. Many of the extinguishant mixtures display synergistic fire fighting efficacy, and representative fluorocarbons for use with the bromodifluoromethane include bromotrifluoromethane, bromochlorodifluoromethane, heptafluoropropane, 1,1,1,2,3,3-hexafluoropropane, 1,1,1,3,3,3-hexafluoropropane, pentafluoroethane, 2-chloro-1,1,1,2-tetrafluoroethane, dibromodifluoromethane, dibromotetrafluoroethane, chloropentafluoroethane, 2-bromo-1,1,1,2-tetrafluoroethane, C2F6, C3F8, C4F10, 1-bromo-1,1,2,2-tetrafluoroethane, 1-chloro-1,1,2,2-tetrafluoroethane and mixtures thereof. In addition, a compatible propellant may be employed. The compositions themselves and systems charged with such compositions are also disclosed. The compositions are characterized by surprising fire extinguishing efficacy, low toxicity and ozone depletion potential, and minimal production of deleterious decomposition products.

Description: FIRE EXTINGUISHANT COMPOSITIONS, iMETHODS AND SYSTEMS UTILIZING BROMODIFLUOROMETHANE REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of the following United States Patent Applications: Serial No. 419,132, filed on October 10, 1989 and entitled FLUOROCARBON COMPOSITIONS FOR USE AS FIRE EXTINGUISHANTS; Serial No. 488,295, filed on March 2, 1990 and entitled FIRE EXTINGUISHING METHODS AND COMPOSITIONS UTILIZING 2-CHLORO-l,l,l,2-TETRAFLUOROETHANE; and Serial No. 439,738, filed on November 21, 1989, and entitled FIRE EXTINGUISHING METHODS AND BLENDS UTILIZING HYDROFLUOROCARBONS, the last mentioned Application being a continuation-in-part of United States Patent Application Serial No. 396,841, filed on August 21, 1989 and entitled FIRE EXTINGUISHING METHODS AND BLENDS UTILIZING HYDROFLUOROCARBONS. BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to the field of fire extinguishant compositions, and particularly to extinguishant mixtures including bromodifluoromethane. These compositions display a surprising efficacy in fighting fires, and also are characterized by low ozone depletion potential (ODP) , low toxicity and minimal decomposition during extinguishment. Description of the Prior Art: Several properties are desirable for fire fighting chemicals. In addition to efficacy for fire extinguishing, the compositions should be relatively inexpensive, readily discharged onto the fire, and low in both toxicity and ozone depletion potential. Various materials have been considered in the past for use as fire extinguishants, and some are in widespread commercial use. However, these chemicals generally have drawbacks in respect to one or more of these desired properties. Numerous halocarbons have been reviewed for possible fire extinguishant use. A comprehensive study was conducted at Purdue University in the late 1940*s, in which numerous halocarbons were examined. Purdue Research Foundation and Department of Chemistry, "Fire Extinguishing Agents, Final Report Sept. 1, 1947 to June 30, 1950", discussed in Larsen, E.R., "Mechanism of Flame Inhibition I: The Role of Halogen", JFF/Fire Retardant Chemistry, Vol. 1 (February 1974). In a first phase of the Purdue experiments, forty-six halocarbons were tested for their capacity to render a heptane/air mixture non-flammable. However, these tests were indicated in the report as not correlating with fire extinguishing efficacy. Also, after this preliminary review, several compounds including bromodifluoromethane performed poorly. Following this initial test, certain compounds were selected to be evaluated for suitability as fire extinguishants. The selected compounds were tested as extinguishants in terms of efficacy at different temperatures and pressures and with different flammable materials, use in binary mixtures, stability, resistivity and toxicity. Materials not performing well in the initial tests, including bromodifluoromethane, were not among the several chosen to be tested for fire extinguishing utility. The efficacy of bromodifluoromethane is surprising from a consideration of the prior art. The prior art teaches that the order of effectiveness of the halogens for fire extinguishing is Br > Cl >> F, as stated for example in Ford, in Haloσenated Fire Suppressants. ACS Symposium Series 16, ashington, DC, 1976. Ford and others further teach that adding a second atom of halogen produces a marginal additional increase in effectiveness. These trends are apparent in the fluoromethane series where the flame inhibiting properties have been shown to be in the order CF3Br > CFC13 > CF2C12 > CF3C1 > CF3H > CF , as discussed by da Cruz, et al., in Bull. Soc. Chi . Belσ. , 97, 1011 (1988). Hence, one would predict that the replacement of Cl with H in Halon 1211 (CF2BrCl) would produce a less effective agent, bromodifluoromethane, CF2HBr. We have surprisingly found however that bromodifluoromethane is a very effective extinguishing agent, and in fact on a weight basis is superior to Halon 1211 (CF2BrCl) for the extinguishment of a variety of fire types. Also surprising is the low level of decomposition products HX formed during the extinguishment of fires by bromodifluoromethane. As noted in the Fire Protection Handbook (NFPA, 1981), decomposition of halogenated agents can take place on exposure to flame or to surface temperatures, and in the presence of H (from water vapor or the combustion process itself), the main decomposition products including HF and HBr. It is well known that bromodifluoromethane can undergo alpha elimination of HBr at elevated temperatures, as described for example in US 3,210,430 to Knight, and one might expect that the use of bromodifluoromethane for the extinguishment of flames would be characterized by the production of high levels of HBr. In contrast we have found that very low levels of HBr are formed during extinguishment. For example, in the extinguishment of n-heptane pool fires with Halon 1301 (CF3Br) or bromodifluoromethane, smaller amounts of both HF and HBr are formed from the bromodifluoromethane. The low toxicity of bromodifluoromethane is also somewhat surprising. As noted by Clayton in Fluorine Chemistry Reviews, v. 1, 1967, the substitution of H for Cl in general leads to increased inhalation toxicity. Hence one might expect the toxicity of bromodifluoromethane to be much higher than that of Halon 1211. Bromodifluoromethane is also known to undergo hydrolysis to form HBr and difluorocarbene, CF2, as described in Hine and Langford, J_j_ Amer. Chem. Soc. , 79, 5497 (1957), and it might be expected that such behavior would also render bromodifluoromethane toxic. However, we have found that the four hour LC50 values (the concentration of agent required to cause death in half of the sample population) in rats for bromodifluoromethane (CHF2Br) and Halon 1211 (CF2BrCl) are 108,000 and 131,000 ppm, respectively. Thus, bromodifluoromethane is of approximately the same toxicity as Halon 1211, and the concentrations required for extinguishment with bromodifluoromethane are at levels safe to living organisms. The result of past studies such as the one at Purdue University has been to direct the art to certain compounds, primarily bromochlorodifluoromethane, bromotrifluoromethane and dibromotetrafluoroethane. The effectiveness of these three compounds in extinguishing fires has been described in United States Patent No. 4,014,799 issued to Owens. Bromotrifluoromethane and bromochlorodifluoromethane, and a few other compounds, have been extensively utilized as fire extinguishing agents because of their cleanliness, relatively low toxicity, moderate cost and effectiveness. There is considerable concern, however, about the apparently high ozone depletion potential of certain totally halogenated compounds in commercial use, including bromotrifluoromethane and bromochlorodifluoromethane. These compounds are asserted by some to be capable of destroying the earth's ozone layer, which forms a protective shield against harmful ultraviolet radiation. Clearly a need exists for an effective agent that presents a reduced threat to the earth's protective ozone layer. The use of emulsified sludges of halogenated methane hydrates as fire extinguishing agents is described in United States Patent No. 3,106,530, issued to Glew on October 8, 1963. The hydrates are dispersed, with an emulsifier, in either water or liquid halogenated methanes. Flame extinguishing compositions are also discussed in United States Patent No. 3,479,286, issued to Gambaretto. This patent describes a two component system combining a completely halogenated alkane and a chlorofluorohydrocarbon. Japanese Patent No. 58078677 describes fire extinguishing compositions which comprise three component systems including bromotrifluoromethane, dibromotrifluoroethane and a cyanamide derivative. Other prior art references also fail to teach the use of bromodifluoromethane as a fire extinguishant, and instead teach away from this use. For example, numerous bromomethanes, typically perhalogenated compounds, are mentioned as fire extinguishants in each of United Kingdom Patent No. 1,247,919, Poland Patent 60068, West German Patent 1,913,405 and United States Patent Nos. 2,837,891 and 2,885,450, but bromodifluoromethane is not included in any of these references. Instead, bromodifluoromethane is identified in Japanese Patent No. 59221375 as useful in an aerosol composition, and in United States Patent Nos. 2,639,301 and 4,810,403 as a refrigerant for air conditioning units. There remains a need and demand for fire extinguishing agents which are highly effective and which have minimum ozone depletion potential and low toxicity. Preferably, such a composition could be utilized interchangeably with current fire extinguishing agents in existing equipment. Contrary to the teachings of the prior art, we have discovered that bromodifluoromethane and mixtures including bromodifluoromethane are effective fire extinguishing agents which satisfy the foregoing criteria. SUMMARY OF THE INVENTION Briefly describing one aspect of the present invention, there is provided an effective and low ozone depleting method for extinguishing a fire which includes introducing to the fire a fire extinguishing concentration of bromodifluoromethane or mixtures of bromodifluoromethane and a compatible propellant. Representative propellents include nitrogen, carbon dioxide, trifluoromethane, carbon tetrafluoride, argon and mixtures thereof. The invention also includes fire extinguishant compositions comprising bromodifluoromethane and a compatible propellant, which are characterized by desirable fire fighting efficacy and other advantageous physical properties, such as low ozone depletion potential. In a further aspect of the present invention, there is provided an effective and low ozone depleting method for extinguishing a fire which includes introducing to the fire a fire extinguishing concentration of a composition including a mixture of bromodifluoromethane and at least one other fluorocarbon fire extinguishant. Representative fluorocarbon extinguishants include bromotrifluoromethane, bromochlorodifluoromethane, chlorodifluoromethane, chlorotrifluoromethane, heptafluoropropane, 1,1,1,2,3,3-hexafluoropropane, 1,1,1,3,3,3-hexafluoropropane, pentafluoroethane, 2-chloro-l,l,l,2-tetrafluoroethane, dibromodifluoromethane, dibromotetrafluoroethane, chloropentafluoroethane, 2-bromo-l,1,1,2-tetrafluoroethane, C 2 F 6' C 3 F 8' C 4 F 10' 1-bromo-l,1,2,2-tetrafluoroethane, 1-chloro-l,1,2,2-tetrafluoroethane and mixtures thereof. The invention also provides fire extinguishant compositions, characterized by high fire extinguishing efficacy and low ozone depletion potential, and comprising mixtures of bromodifluoromethane and at least one other fluorocarbon fire extinguishant. Such mixtures may further include a compatible propellant. In particular embodiments the compositions of the present invention have a phase equilibrium pressure of between about 45 psig and about 600 psig at 70°F. The compositions in certain embodiments comprise ozone depletion potentials of less than about 3.0, and even below about 1.0. Other embodiments of the inventive compositions consist essentially of bromodifluoromethane and propellant, or of bromodifluoromethane, propellant and at least one other fluorocarbon fire extinguishant. The foregoing fire extinguishants have low ozone depletion potential and good fire extinguishing efficacy. The present invention is also directed to fire extinguishing systems charged with the foregoing compositions. Also covered are methods for extinguishing fires comprising discharging from a pressurized container into the combustion zone of a fire, a fire extinguishing amount of a fire extinguishant composition comprising any of the compositions defined hereinabove. It is an object of the present invention to provide methods and compositions for extinguishing fires rapidly and effectively, and with lessened depletion of the atmospheric ozone layer. Another object of the present invention is to provide fire extinguishant compositions which include bromodifluoromethane, alone or in mixture with one or more other fluorocarbon components, the bromodifluoromethane itself and the resulting mixtures being surprisingly effective as fire extinguishants and having relatively low ozone depletion potential and toxicity. Further objects and advantages of the present invention will be apparent from the description which follows. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a perspective view of a room enclosure having a total flooding system in accordance with one aspect of the present invention. FIG. 2 is a schematic, cross-sectional view of a portable fire extinguisher useful in accordance with the present invention. FIG. 3 schematically illustrates a connection between the container of a fire extinguisher and a valve for discharge of extinguishant composition from the extinguisher, the connection including an O-ring seal of EPDM rubber. DESCRIPTION OF THE PREFERRED EMBODIMENT For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiment of the invention and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations, modifications and further applications of the principles of the invention being contemplated as would normally occur to one skilled in the art to which the invention relates. The present invention provides surprisingly effective methods and compositions for fire extinguishing. The fire extinguishant compositions comprise bromodifluoromethane as well as mixtures of bromodifluoromethane and other fluorocarbon fire extinguishants. Prior to this invention, the prior art has taught away from the use of bromodifluoromethane for fire extinguishing applications. However, it has been discovered that bromodifluoromethane is an effective fire extinguishant, and moreover that it has a desirably low ozone depletion potential (ODP) . In addition, blends of bromodifluoromethane with other extinguishants also exhibit superior fire extinguishing qualities and low ozone depletion potentials. It has further been discovered that certain of the extinguishant mixtures of the present invention generally have even greater efficacy than would be predicted based upon the combination of the separate compounds. This synergistic result makes the inventive mixtures unexpectedly useful as fire extinguishants. The various inventive compositions containing bromodifluoromethane have also been demonstrated to have low toxicity and minimum temperature dependency, and to be capable of effective use in a wide variety of fire extinguishing applications. Importantly, it has been demonstrated that such compositions containing bromodifluoromethane can replace high ozone depletion potential fire extinguishing agents in existing equipment. Bromodifluoromethane has the molecular formula CHF„Br and is understood to have the following structural formula: H I Br - C - F 1 F Bromodifluoromethane may be prepared by a number of methods, including fluorination of bromoform with HF in the presence of chromium-type catalysts, as described in United States Patent No. 3,210,430 issued to Knight. One of the notable advantages of bromodifluoromethane is the combination of efficacy, low ODP and acceptable toxicity. Prior art fire extinguishants typically are less desirable as to one or more of these properties. Bromodifluoromethane is therefore a superior fire extinguishant in appropriate settings. By comparison, known extinguishants such as bromotrifluoromethane (Halon 1301) are slightly more effective on a weight basis, but their ODP's are substantially higher, rendering them environmentally unacceptable. The bromodifluoromethane compositions of the present invention have reduced ODP ratings, but are surprisingly effective at levels safe to humans, i.e. particularly at concentrations less than about 10% (v/v) . In view of the concern with ozone depletion in the earth's atmosphere, the present invention provides a distinct advantage resulting from the use of bromodifluoromethane. While not being bound to a particular theory, it is believed that the hydrogen substituent of bromodifluoromethane makes it less stable than fully halogenated methanes such as bromotrifluoromethane and bromochlorodifluoromethane, thereby making it less likely to penetrate into the stratosphere, or to persist in that region. Accordingly, its potential for ozone depletion is dramatically lower than more stable compounds, such as CHF-Br, or CF_Br. For example, bromodifluoromethane has been determined to have an ozone depletion potential less than one tenth of that of bromotrifluoromethane. The fire extinguishant compositions of this invention generally exhibit an ozone depletion potential of less than about 3.0, and preferably less than about 1.0. For purposes herein, the ODP of a pure compound may be calculated using the following algorithm developed by G. Dana Babson of the University of Virginia: ODP = A E P [(#C1) B + C(#Br)] D (#C_1) In this expression, P is the photolysis factor. P = 1.0 if there are no special structural features which make the molecule subject to tropospheric photolysis. Otherwise, P = F, G or H, as indicated in the following Table of Constants: TABLE OF CONSTANTS CONSTANT NAME VALUE A Normalizing constant 0.446 B Exponent for chlorine term 0.740 C Multiplier for bromine term 32.000 D Constant for carbon term 1.120 E Hydrogen factor [=1.0 for no H's] 0.0625 F Photolysis factor for geminal Br-C-Cl 0.180 G Photolysis factor for geminal Br-C-Br 0.015 H Photolysis factor for adjacent Br-C-C-Br 0.370 The Babson method provides a reasonable approximation of the ozone depletion potential (ODP) as calculated by the more complex method described in "The Relative Efficiency of a Number of Halocarbons for Destroying Stratospheric Ozone", by D. J. Wuebbles, Lawrence Livermore Laboratory Report UCID-18924, issued January, 1981. The various compositions of this invention have been found to exhibit excellent fire extinguishment properties, a good combination of physical properties, such as boiling point and vapor pressure, a good combination of handling and use properties, such as cleanliness and superior discharge pattern configurations, reasonable toxicity characteristics, and low ozone depletion potential. Appropriate selection of the components of the composition permit utilization of existing equipment without making expensive hardware changes, and may also minimize the temperature dependency of the discharge agent, thus broadening the range of the composition's potential fire fighting applications. Also, the mixtures of the present invention may be formulated to have a desirably low ODP or other optimized physical characteristics by selected proportioning of the bromodifluoromethane and the other component(s) . Comparative tests have indicated that the bromodifluoromethane compositions of the present invention provide a uniform discharge over a slightly longer discharge period than comparable fire extinguishant compositions based on bromochlorodifluoromethane (Halon 1211) , despite the fact that bromodifluoromethane has a higher vapor pressure than Halon 1211 at a given temperature. The bromodifluoromethane compositions also exhibit good throw characteristics, thus providing a significant advantage over bromotrifluoromethane (Halon 1301), which tends to volatilize too quickly. Because of the lower equilibrium pressure of bromodifluoromethane compositions relative to the comparable Halon 1301 compositions at the same temperature, the compositions of the present invention may be contained and discharged using containers and fittings of lower pressure rating than those required for comparable compositions based on Halon 1301. Comparative tests have also indicated that the bromodifluoromethane compositions provide a discharge stream more cohesive than that provided by either Halon 1211 or Halon 1301 compositions. This affords a particular advantage in efficiency. A good cohesive discharge pattern can achieve quick "knock down" of fire by physically displacing high temperature combustion products and oxygen supply from the fuel. This allows the fire to be extinguished more quickly and/or with a lesser amount of fire extinguishant. In a first aspect of the present invention, there are provided fire extinguishant compositions comprising bromodifluoromethane, alone or in combination with a propellant. In a related embodiment the fire extinguishants consist essentially of bromodifluoromethane, or of bromodifluoromethane and propellant. To produce an effective composition for conventional use as a fire extinguishing agent, bromodifluoromethane is preferably blended with a propellant gas to produce a gas and liquid phase mixture of fire extinguishant component and compatible propellant. Compatibility contemplates that the propellant be non-reactive, stable and non-interfering with bromodifluoromethane and that it provide suitable pressure, at moderate weight percents, to propel the bromodifluoromethane adequately into the fire for the given fire extinguishing systems and conditions. Suitable gases for use in such a blend include nitrogen, carbon dioxide, trifluoromethane, carbon tetrafluoride, argon and mixtures thereof, or other gases including air. Depending on the application for which the fire extinguishant composition is desired, a wide range of blends of bromodifluoromethane and propellant gases may be suitable. In order to provide a composition that is effective for delivery under autogenous pressure from conventional fire extinguishing apparatus the relative proportions of bromodifluoromethane and propellant are preferably such that the equilibrium pressure at 70°F of the two phase mixture is between about 45 psig and about 600 psig, and more preferably between about 100 psig and about 400 psig. For many applications, this translates for binary mixtures of bromodifluoromethane and propellant into a bromodifluoromethane weight fraction of at least about 70% by weight. Proportions in these ranges provide highly effective fire extinguishment qualities, including effective function over a range of ambient temperature, yet provide an ozone depletion potential that is substantially lower than common prior art compounds, including either bromotrifluoromethane or bromochlorodifluoromethane. In accordance with another aspect of the present invention, the fire extinguishing compositions comprise a mixture of bromodifluoromethane and a fluorocarbon fire extinguishant. The fluorocarbon fire extinguishant may comprise any of a wide variety of known, fluorine-containing compounds which form stable, efficacious mixtures with bromodifluoromethane. For example, known fluorocarbon fire extinguishants typically are those effective at less than about 12% (v/v) . Such fluorocarbons include bromotrifluoromethane, bromochlorodifluoromethane, chlorodifluoromethane, chlorotrifluoromethane, heptafluoropropane (1,1,1,2,3,3,3-heptafluoropropane and 1,1,1,2,2,3,3-heptafluoropropane) , 1,1,1,2,3,3-hexafluoropropane, 1,1,1,3,3,3-hexafluoropropane, pentafluoroethane, 2-chloro-l,1,1,2-tetrafluoroethane, dibromodifluoromethane, dibromotetrafluoroethane, chloropentafluoroethane, 2-bromo-l,1,1,2-tetrafluoroethane, C 2 F 6 , C 3 F 8 , C 4 F 1Q , 1-bromo-l,1,2,2-tetrafluoroethane, l-chloro-l,l,2,2-tetrafluoroethane and mixtures thereof. More generally, bromodifluoromethane may be used as a partial replacement for halogenated hydrocarbons otherwise used in fire extinguishing compositions. In other words, for a single component fire extinguishant of the prior art, bromodifluoromethane may be added thereto to form a fire extinguishant mixture. For a composition including two or more of the prior art extinguishants, bromodifluoromethane may be used to partially or fully substitute for one of those components. This has the advantage of modifying the overall characteristics of the prior art fire extinguishant composition, such as by increasing efficacy and/or reducing ODP or toxicity. Bromodifluoromethane is therefore seen to be extremely flexible in its manner of use in preparing mixtures for use as fire extinguishants. For the bromodifluoromethane mixtures, the relative amounts of the bromodifluoromethane and fluorocarbons mixed therewith are not critical, but rather are dictated by the characteristics desired for the overall composition. Thus, certain applications may require lower toxicity, while other instances may call for maximized efficacy. Therefore, no particular ratios of compounds are required. Importantly, it has also been discovered that bromodifluoromethane has the characteristic of forming synergistic mixtures with other fluorocarbons. Specifically, the efficacies of various extinguishant compositions utilizing bromodifluoromethane have been found to exceed the predicted efficacies based simply on a weighted average of the components. Specific results are described in the Examples hereafter. By way of example, substantial synergism has been found for the combination of bromodifluoromethane and heptafluoropropane. As determined by tests (see Table 11A) , the percentage (v/v) of bromodifluoromethane in air necessary to extinguish a cup burner flame was about 3.9%, compared to the 6.0% necessary for heptafluoropropane alone. The combination of these two components was significantly more effective than the simple weighted average for the two. For example, at a mixture of 50% (by mole) bromodifluoromethane and 50% heptafluoropropane, the volume percent of the mixture in air needed to extinguish the flame was only 4.2%, contrasted to the predicted level of 5.0%, representing an increase in efficiency of 16%. Particularly desirable mixtures according to the present invention are those which have at least about a 10% increase in fire extinguishing efficiency over the predicted efficiency for the weighted average of the two components. Effective mixtures for fire extinguishment may be prepared by mixing bromodifluoromethane with a variety of known fire extinguishing agents, such as Halon 1301 (CF„Br) , Halon 1211 (CF 2 BrCl), Halon 1202 (CF 2 Br 2 ) or Halon 2402 (BrCF 2 CF 2 Br) . Such mixtures optionally include a compatible propellant such as nitrogen, carbon dioxide, trifluoromethane (CF_H), tetrafluoromethane (CF.) or argon. Compositions which include bromochlorodifluoromethane (Halon 1211) preferably have an ozone depletion potential of less than about 1.5 and contain between about 5% and about 35% by weight bromochlorodifluoromethane. Compositions which include bromotrifluoromethane (Halon 1301) preferably have an ozone depletion potential (calculated by the Babson method) of less than about 3.0 and contain between about 0.9% and about 15% by weight bromotrifluoromethane. For some flooding systems or other applications, it may be preferable for the compositions to contain between about 5% and about 15% by weight bromotrifluoromethane. Effective compositions for fire extinguishment are also prepared by mixing bromodifluoromethane with hydrofluorocarbons such as CF 3 CHFCF 3 , CF 3 CF 2 CF 2 H, CF 3 CHFCF 2 H, CF 3 CH 2 CF 3 and CF 3 CF 2 H, optionally in the presence of a compatible propellant such as nitrogen, argon, carbon dioxide, CF_H or CF.. Mixtures prepared by the combination of bromodifluoromethane and these saturated, higher-fluorinated C. , C„ and C„ hydrofluorocarbons have been found to be particularly effective fire extinguishants. Because these hydrofluorocarbons contain no bromine or chlorine, they have an ozone depletion potential of zero. Furthermore, since the compounds contain hydrogen atoms, they are susceptible to breakdown in the lower atmosphere and hence do not pose a threat as greenhouse warming gasses. Mixtures -n- of CHF„Br and heptafluoropropane are especially preferred because the compounds have similar vapor pressures over a wide range of temperatures and therefore the composition of the mixture remains relatively constant during discharge or other application. Specific hydrofluorocarbons of this type are compounds of the formula C H F , where x is 2 or 3; y is 1 or 2; and z is 5, 6 or 7; where y is 1 and z is 5 when x is 2; and where z is 6 or 7 when x is 3. Hydrofluorocarbons in this class include heptafluoropropane (CF„CHFCF 3 ), 1,1,1,3,3,3-hexafluoropropane (CF.,CH 2 CF 3 ) , 1,1,1,2,3,3-hexafluoropropane (CF 3 CHFCHF 2 ) , and pentafluoroethane (CF_CHF 2 ) . These hydrofluorocarbon compounds are non-toxic and are economical to manufacture. For example, heptafluoropropane may be conveniently produced via the reaction of commercially available hexafluoropropene (CF_CF=CF_) with anhydrous HF as described in U.K. Patent 902,590. Similarly, 1,1,1,3,3,3-hexafluoropropane may be synthesized by reacting anhydrous HF with pentafluoropropene (CF 3 CH=CF_). 1,1,1,2,3,3-hexafluoropropane may be obtained by hydrogenation of hexafluoropene (CF 3 CF=CF 2 ). Pentafluoroethane may be obtained by the addition of hydrofluoric acid to tetrafluoroethylene (CF 2 =CF 2 ) . Blends of bromodifluoromethane with these hydrofluorocarbons desirably include the hydrofluorocarbon at a level of at least about 10 percent by weight of the blend. The use of hydrofluorocarbons at higher levels in such blends further minimizes the ODP and resultant adverse environmental effects. Compositions containing hydrofluorocarbons preferably have an ozone depletion potential of less than about 0.9 and contain between about 5% and about 90% by weight hydrofluorocarbon. Heptafluoropropane, having a boiling point of about -17°C, is highly miscible with bromodifluoromethane. Consequently, mixtures may contain substantially any proportion of heptafluoropropane to bromodifluoromethane. Such compositions preferably include between about 1% and about 99% by weight heptafluoropropane. The ODP for compositions including heptafluoropropane is preferably below about 0.9. Effective extinguishing compositions are also prepared by mixing bromodifluoromethane with a hydrochlorofluorocarbon such as CF 3 CHFC1, CF 2 HCF 2 C1, CF 3 CHC1 2 or CHF 2 C1, optionally in the presence of a compatible propellant such as nitrogen, trifluoromethane, carbon tetrafluoride, argon or carbon dioxide. Compositions containing hydrochlorofluorocarbons preferably have an ODP of less than about 0.9, and contain about 5% to 90% by weight hydrochlorofluorocarbon. Similarly efficacious blends are achieved by the combination of bromodifluoromethane and 2-Chloro-l,1,1,2-tetrafluoroethane (CF g CHFCl) , a halogenated hydrocarbon also known as CFC 124. CFC 124 has a molecular weight of 136.48 and a boiling point of -12°C. Methods for the preparation of CFC 124 are known in the prior art. For example, 2-Chloro-l,1,1,2-tetrafluoroethane may be prepared by fluorination of CC1 2 =CC1 2 with HF, as described in European Patent Application No. 313,061 (1989). An alternative preparation is by reaction of CF =-CFCl with KF/formamide, as reported in the Journal of the American Chemical Society, vol. 82, p. 3091 (1960). Blends of bromodifluoromethane and 2-Chloro-l,1,1,2-tetrafluoroethane (CFC 124) are effective in low concentrations, and of course at high concentrations as well. The concentration employed may depend to some extent on the nature of the fire, the combusting material and the circumstances of application. The similarity of boiling points for the two compounds allows the composition discharged or otherwise applied to remain essentially constant. Blends having from about 5% to about 99% by weight CFC 124 and from about 95% to about 1% by weight bromodifluoromethane are particularly preferred. Effective fire extinguishant mixtures are also obtained by the combination of bromodifluoromethane and one or more of C 2 F 6' C 3 F 8' C 4 F 10' 1-bromo-i,1,2,2-tetrafluoroethane, 1-chloro-l,1,2,2-tetrafluoroethane. Ratios of the components and the concentrations of use are similar to the previously described blends. A further desirable aspect of the blends of bromodifluoromethane and 2-Chloro-l,1,1,2-tetrafluoroethane is that the mixtures are especially attractive due to their low ODP. 2-Chloro-l,1,1,2-tetrafluoroethane has an ODP of 0.03 as calculated by the Babson model. It is believed that the presence of the hydrogen in 2-Chloro-l,1,1,2-tetrafluoroethane makes the compound less stable and contributes to the lower ODP, since the molecules are susceptible to breakdown in the lower atmosphere. A consideration in selecting a concentration for the compositions of this invention is the maintenance of the area in a non-toxic and non-anesthetic condition. A 50% lethal concentration (LC50) for a compound is that concentration of the compound (volume of compound per volume of air) at which 50% of a test population is killed; a 50% anesthetic dose (AD 50) is that concentration at which 50% of a test population is anesthetized. For example, 2-Chloro-l,1,1,2-tetrafluoroethane has an LC50 of 44.7% v/v, and an AD50 of 15.5% v/v, as reported by Davies, et al., Int. J. Quantum Chem: Quantum Biology Symp No. 3, 171 (1976). Selection of the appropriate usage rate of bromodifluoromethane/CFC 124 mixtures will therefore be affected by these properties. For example, a usage rate where humans may be present is preferably below about 15% v/v, and more preferably below about 10% v/v. Mixtures are also provided in the present invention which comprise bromodifluoromethane and a selected fluorocarbon component, all as previously described, and a third component comprising a propellant. Such propellant may be any which is compatible and useful in combination with the other ingredients, also as previously described. Such propellants include nitrogen, carbon dioxide, trifluoromethane, carbon tetrafluoride and argon. Other compatible propellants or other compounds may also be mixed with the fire extinguishants of the present invention. For minimum ozone depletion allowance, compositions containing a non-fluorocarbon propellant such as nitrogen, argon or carbon dioxide are preferred. Among such propellants, nitrogen is particularly advantageous because it provides a favorable balance of reasonable cost and low dielectric constant. Carbon dioxide is also advantageous from a cost standpoint, but is less preferred in applications where conductivity of the fire extinguishing agent is or may be a problem. Compositions containing such non-fluorocarbon propellants preferably have a low proportion of propellant, typically in the range of 0.1% to 5% by weight. Such compositions typically exhibit an ozone depletion potential of less than 1.0. The bromodifluoromethane compositions blended with a propellant gas produce a gas and liquid phase mixture of fire extinguishant component and propellant. Depending on the application, a wide range of blends of bromodifluoromethane compositions and propellant gases may be suitable. In order to provide a composition that is effective for delivery under autogenous pressure from present fire extinguishing apparatus, the relative proportions of bromodifluoromethane composition and propellant are such that the equilibrium pressure of the two phase mixture at 70°F is preferably between about 45 psig and about 600 psig, and more preferably between about 100 psig and about 400 psig. Higher pressures can be used, although there will be an increase in equipment costs. For most applications, this translates into a bromodifluoromethane weight fraction of at least about 70% by weight. Proportions in these ranges provide highly effective fire extinguishment qualities, including effective function over a range of ambient temperature. At the same time, the ozone depletion potential is substantially lower than many prior art extinguishants such as bromotrifluoromethane or bromochlorodifluoromethane. Among the preferred compositions of the invention are mixtures containing bromodifluoromethane, nitrogen, and one of the following: Halon 1211, Halon 1301, heptafluoropropane and CFC 124. In such compositions, the ratios of the components are as already described. For mixtures containing Halon 1211, the nitrogen content is typically between about 0.3% and about 10% by weight. For mixtures containing Halon 1301, the nitrogen content is typically between about 0.1% and about 10% by weight. Particularly preferred compositions of the invention include those formulated for replacement of Halon 1211 and Halon 1301 in existing systems. Compositions adapted to replace Halon 1211, e.g., in hand held fire extinguisher applications, preferably are formulated to have a phase equilibrium pressure of between about 100 psig and about 200 psig at 70°F. Compositions adapted to replace Halon 1301, typically in large stationary flooding systems, preferably are formulated to have a phase equilibrium pressure of between about 300 psig and about 400 psig at 70°F. The bromodifluoromethane compositions may be applied in the variety of ways employed for other halogenated hydrocarbon extinguishants, including application in flooding systems, specialized systems and portable systems, described hereafter in more detail. The methods for application of the described fire extinguishing compositions include those known to be useful for other halogenated hydrocarbons, such as Halon 1211 and Halon 1301. In broad terms, these methods utilize application systems which typically include a supply of agent, a means for releasing or propelling the agent from its container, and one or more discharge nozzles to apply the agent into the area of the hazard or directly onto the burning object. A system may also contain other elements, such as one or more detectors, remote and local alarms, a piping network, mechanical and electrical interlocks to shut down ventilation, etc., directional control valves, etc. Such systems may be stationary or portable, and typically the fire extinguishant may be pressurized with propellant gas at up to about 600 psig at ambient temperature. For example, referring to Figure 1 there is shown a typical system for a room 11 having a raised floor 12 and ceiling 13. Automatic fire detectors 14 are installed in the ceiling and floor and activate the fire extinguishing system when needed. The extinguishing system comprises storage tanks 15, piping 16 and discharge nozzles 17. A control panel 18 operatively connects the detectors with the fire extinguishing system to activate it. Thus, the compositions of the present invention may be used in a total flooding fire extinguishing system in which the agent is introduced to an enclosed region (e.g., a room or other enclosure) to surround a fire at a concentration sufficient to extinguish the fire. Total flooding systems are used, for example, for computer rooms, control rooms, special storage areas, machinery spaces and the like. In a total flooding system apparatus, equipment or even rooms or enclosures may be provided with a source of agent and appropriate piping, valves, and controls so as automatically and/or manually to be introduced at appropriate concentrations in the event that fire should break out. Local application systems discharge fire extinguishing agent in such a manner that the burning object is surrounded locally by a high concentration of agent to extinguish the fire. Local systems are often employed because the enclosure may not be suitable to provide for total flooding. Examples include use for presses, tanks, spray booths, and electric transformers. Specialized systems are frequently used for specific applications or hazards, such as for aircraft, military vehicles, emergency generators, etc. By way of further example, the compositions of the present invention can be conveniently employed in local application systems through the use of conventional portable fire extinguishing equipment, such as those described in the Fire Protection Handbook. For such systems, the supply of fire extinguishing agent is typically directed by hand at the fire, or larger mobile units with hoses and nozzles are employed for directional spraying. It is usual to increase the pressure in portable fire extinguishers with suitable propellant gases in order to insure that the agent is completely expelled from the extinguisher. Systems in accordance with this invention may be conveniently pressurized at any desirable pressure up to about 600 psig at ambient conditions. In accordance with the fire extinguishing method of the present invention, a combustion suppressing amount of extinguishant composition is discharged into the combustion zone of a fire for a time sufficient to suppress the fire. It will be appreciated that the amount of extinguishant applied may desirably be enough to fully extinguish or simply to suppress the fire. In the case of suppression of a fire, the fire may be limited to a controllable volume, and thereafter be fully extinguished with additional use of the bromodifluoromethane compositions or with other extinguishant materials or methods. In either circumstance, the fire fighting efficacy of the bromodifluoromethane will be usefully employed, and as used herein the term "extinguishing amount" encompasses both suppression and extinguishment. Minimum concentrations employed may be substantially any at which a given fire may be suppressed or extinguished, the exact minimum level being dependent on the nature of the fire, the combusting material, the particular extinguishant composition, the combustion conditions, and the circumstances and manner of application. In general, however, best results are achieved where the compositions are present at a level of at least about 2% (v/v) , and more preferably at least about 4% (v/v) . The maximum amount to be employed will be governed by matters such as economics and potential toxicity to living things. About 15% (v/v) provides a convenient maximum concentration for the compositions in occupied areas. Higher concentrations including those up to about 25% (v/v) may be employed in unoccupied areas, with the exact level again being determined by the particular combustible material, the extinguishant employed and the conditions of combustion. The preferred fire extinguishing concentrations of the compositions in accordance with this invention are in the range of about 4% to about 10% (v/v) . Initially, i.e., when discharge of extinguishant is commenced, the extinguishant composition within the container is a liquid and gas phase mixture of the fire extinguishant components and propellant having an equilibrium pressure typically of about 45 psig to 600 psig, preferably about 100 psig to 400 psig, at the ambient temperature that may prevail at a fire scene. As the extinguishant is expelled, the pressure falls. However, the initial composition within the container is preferably such that a substantial pressure is maintained as long as there is any significant residue of liquid phase within the container. A substantial pressure should be maintained until an effective amount of extinguishant has been applied to the fire. Illustrated schematically in Figure 2 is a portable fire extinguisher which includes a cylinder 21 containing a charge 23 of the composition of the invention. The bromodifluoromethane composition is maintained at a pressure of 45 to 600 psig, preferably 100 to 200 psig, at ambient temperatures contemplated for use in fighting fires. An amount of extinguishant is provided as will be sufficient for blanketing a fire, and thereby extinguishing it or suppressing it to a controllable level. The charge is a gas and liquid phase mixture which is substantially at equilibrium at the aforesaid pressure and ambient temperature. A discharge valve 25 operated by a trigger mechanism 27 provides for release of the fire extinguishant composition from the container, and a discharge horn 29 is adapted to direct the extinguishant to the combustion zone of the fire. Alternatively, the extinguisher discharge valve may be fitted with a hose and spray nozzle for direction of the extinguishant. Arrangements similar to that of the drawing, but having relatively large containers for the extinguisher charge and typically containing a charge having a phase equilibrium pressure of 200-500 psig, are provided for mobile or stationary systems designed for fighting large fires. There are four basic types of extinguisher system triggering devices: manual (such as a pull pin), thermatic (similar to a heat activated sprinkler head), electronic (activating a solenoid), and rupture disc. Discharge temperatures typically range from -40° to 120°F (50°C) . Large stationary or mobile systems typically also include means such as a filling valve for charging the system with fire extinguishant. The mixtures of the invention may be delivered from a single cylinder or other suitable container containing the bromodifluoromethane and the adjuvant, and optionally also containing a propellant. Alternatively, the components may be stored in separate containers and premixed through common piping or mixture devices prior to delivery to the fire. Hence, any desired blend may be effectively delivered to the fire. The invention will be further described with reference to the following specific Examples. However, it will be understood that these Examples are illustrative and not restrictive in nature. In the following Examples, percents indicated are percents by weight unless indicated otherwise, and all ODP's are calculated by the Babson method. Example 1 Fire extinguishant compositions of the present invention were prepared by combining bromodifluoromethane and nitrogen in the proportions shown in nos. B-1, B-2 and B-3 in Table 1. These compositions have low ODP, suitable phase equilibrim pressure for use in conventional systems, and are effective fire extinguishants. Similar mixtures of bromodi luoromethane with carbon dioxide, argon and carbon tetrafluoride yield similarly good compositions. As also shown in Table 1, compositions were prepared by combination of bromodifluoromethane, nitrogen and either of Halon 1211 (H-1211) or Halon 1301 (H-1301) . These compositions, listed as T-l through T-6, also provide advantageous fire extinguishants, although the ODP's are slightly higher. Table 1 T-5 85.0 T-6 76.6 -2 ? - Example 2 Test systems were prepared by filling containers with the compositions set forth in Table 2. In each case the test container was filled about half way on the basis that this would be the minimum fill for a commercial system, maximizing the propellant content. Table 2 also indicates the phase equilibrium pressure exhibited by each composition at 70°F. These compositions perform well as extinguishing materials in terms of discharge and efficacy. Table 2 Concentrations of bromodifluoromethane (CHF-Br) required to extinguish diffusion flames of n-heptane, n-butane and methanol were determined using the cup burner method described by Ford in Haloσenated Fire Suppressants. ACS Symposium Series 16, ACS, Washington, DC, 1975, p. 16. Bromodifluoromethane vapor was mixed with air and introduced to a flame produced in a glass cup burner, with the concentration of bromodifluoromethane being slowly increased until the flow was just sufficient to cause extinction of the flame. The data are reported in Tables 3-5, which also show the amount of bromodifluoromethane required on a weight basis. Extinguishing concentrations, % v/v were calculated from the relationship flow agent (cc/min) % v/v = x 100 flow agent + flow air (cc/min) To convert volume data to mg/L, the following relationship was employed: (extinguishing cone, % vol) x (10) x MW mg/L = _—_ 24.5 where MW = the molecular weight of the agent. The results for Halon 1301 (CF3Br) and Halon 1211 (CF2BrCl) are also shown in Tables 3-5 for comparison purposes. Table 3 Extinguishment of n-Heptane Diffusion Flames -2 '9- Table 4 Extinguishment of n-Butane Diffusion Flames Table 5 Extinguishment of Methanol Diffusion Flames As evidenced by Tables 3-5, bromodifluoromethane is seen to be more efficient on a weight basis than Halon 1211 for the extinguishment of typical Class B type fuels. Example 4 Bromodifluoromethane (9 pounds) was charged into an Ansul Model SY 0941 portable extinguisher and sufficient nitrogen added to bring the total cylinder pressure to 125 psig. For purposes of comparison, Halon 1211 (CF2BrCl, 9 pounds) was charged into an identical extinguisher and pressurized with nitrogen to a total of 125 psig. Discharge testing was conducted in accordance with Section 26 of UL Standard 1093, and results are shown in Table 6. Table 6 Discharge Duration Test As indicated in Table 6, the average discharge durations of CHF2Br and CF2BrCl were 11.7 and 11.1 seconds, respectively, and comparable stream ranges were also obtained. The results indicate comparable discharge times and ranges for the two agents. By comparison, 9 lbs. of bromodifluoromethane was charged at 195 psig (with nitrogen) to an Amerex 1211 extinguisher having an orifice smaller than the Ansul unit, and provided with a short discharge horn. 100% of the agent discharged in 11.3 seconds, yielding a stream range of 16.5 feet. The present discharge requirement for the Ansul SY 0941 extinguisher filled with 9 pounds of Halon 1211 at a charging pressure of 125 psig is 12.0 + 1.0 seconds, and it is seen that CHF2Br also meets these requirements. The stream from CHF2Br was observed to be more cohesive than that from Halon 1211. Example 5 Fire tests for Class A fires were conducted in accordance with ANSI/UL 711, Standard for Rating and Fire Testing of Fire Extinguishers. The Class A fire tests were conducted in 5 accordance with the procedures and requirements of paragraphs 4.6 through 4.16 inclusive of that standard. Crib construction and ignition arrangements were conducted in accordance with Table 4.2 and 4.3 of that standard. All tests were conducted in a fire test house constructed of prestressed 10 concrete measuring 30x40 ft. on the base and 50 ft. in height. A cupola with adjustable louvers to control drafting conditions was mounted on the top of the roof. Bromodifluoromethane was charged into an Ansul SY 0941 extinguisher and the extinguisher pressurized with nitrogen to 15 125 psig. Results of these tests are shown in Table 7. Table 7 Extinguishment of Class A Fires 5 Analysis of video tapes shows that CHF2Br extinguished the Class A fire almost instantaneously and had no difficulty in preventing reignition during the required 10 minute wait period. In addition, the stream from CHF2Br was observed to be more cohesive than that from Halon 1211, which results in an advantage in efficiency. The increased cohesiveness of the discharge pattern results in rapid "knockdown" of the fire, in which the high temperature combustion products and oxygen are physically separated from the fuel, allowing the fire to be extinguished with a relatively small amount of fire extinguishant. This is seen in the last examples of Table 7, where a reduced charge of CHF2Br is seen to be capable of extinguishing the fire as rapidly as the larger charge. This increased knockdown renders CHF2Br more effective than Halon 1211 for a Class A fire. In another test using the Amerex 1211 extinguisher on a Class B fire, a discharge of 6 lb. 13 oz. (75.6%) of the bromodifluoromethane agent successfully extinguished the fire. This Class B fire test was conducted in accordance with Section 5 of the ANSI/UL 711 standard. Example 6 Tests were conducted comparing the performance of bromodifluoromethane to that of Halon 1211 in an Argus type total flooding fire extinguishment system, used primarily for fire protection of machinery in textile mills. Tests were conducted in accordance with established Factory Mutual criteria for approval of Halon 1211 for this application. The system was charged with CHF2Br or Halon 1211 and pressurized with nitrogen to 150 psig. All trials were conducted outdoors by discharging the fire extinguishant agent through varying piping configurations into enclosed boxes of varying size. Discharge time and total elapsed time until extinguishment were recorded. The two products were tested using equal amounts by weight. A summary of the results is set forth in Table 8. Letter prefixes in the test numbers of this table designate particular box sizes and piping configurations utilized in the test. Table ii Performance Testing in Argus System DISCHARGE EXTIN. ESTIMATED TIME(S) TIME(S) CONCEN. COMMENTS 11 8 7.OX 4.8 4.8 4.7% 2.42 4.39 4.7% 5.34 4.1 4.7% 3.9 3.19 3.5% 4.1 9.05 2.4% Pedestal used 4.32 NE 1.2% Pedestal used 3.69 7.02 1.9% Pedestal used 2.39 NE 0.9% Pedestal used 4.46 4.73 4.8% 3 4.09 3.6% 2.69 20.51 2.4% 2.57 6.66 2.4% Cl CHF2Br 22 16.12 30.91 22.0% Cotton fire. I 18.37 9.93 22.0% Cotton fire. 20 15.11 8.73 22.0% Cotton fire. 10.92 9.92 5.3% 4.26 37.28 2.4% 3.99 NE 1.9% 4.99 9.48 5.2% 25 15.83 2.6% 6.83 7.27 4.5% 2.87 38.83 2.4% 9.39 3.99 4.8% 4.33 15.44 2.4% 30 3.82 NE 1.9% 18.1 12 4.7% 6.98 NE 2.2% = -3 ' 4- No residues of extinguishant were detected in the fire extinguishant containers after discharge of the product in accordance with the test. There were no discernible differences in cylinder filling other than some differences in the equilibrium pressure of the charge. Flow of the bromodifluoromethane composition was judged to be as good as that of the bromochlorodifluoromethane composition, both in balanced and unbalanced systems. No trials were made to determine the effect of nozzle design on discharge of the CHF_Br composition, but nozzle coverage was judged better than that afforded by Halon 1211. Both products mixed well in the test box enclosure. No equipment or valve problems were noted. As indicated in Table 8, for the various configurations tested, CHF 2 Br was found to be equal to or more efficient than Halon 1211 on a weight basis. For configuration A, Halon 1211 was seen to be only slightly more efficient on a weight basis. For equipment configurations E and H, equal weights of Halon 1211 failed to bring about extinguishment. Tests 12 and 13 indicate that the bromodifluoromethane charge achieved results superior to that of the bromochlorodifluoromethane charge despite the fact that the weight of the former was only 85% that of the latter. Example 7 This example demonstrates the use of CHF2Br in a total flooding system and demonstrates the low level of decomposition products, HX, produced during the extinguishment of fires with bromodifluoromethane. Decomposition products were determined by a procedure similar to that described by Sheinson, et al., in Fire &. Fla mabilitv, vol. 12, p. 229 (1981). Pool fires of n-heptane were extinguished by CHF2Br total flooding at 5% in a 116 cubic foot enclosure. A distribution system allowed delivery of 5 to 7 volume percent agent concentration within 10 to 20 seconds following manual initiation of the discharge. A nozzle located at the top of the chamber distributed the agent throughout the enclosure. The fuel was ignited, allowed a preburn and then sufficient CHF2Br discharged to achieve a 5% calculated concentration at 21° C. HF levels were monitored with Sensidyne real-time continuous HF analyzers located at different heights in the enclosure. Gas samples were withdrawn via a 6 mm O.D. metal tube from the center of the enclosure through a porthole. Samples were collected in a Teflon trap containing a pH 5 buffer, and HBr determined via an Orion bromide specific electrode. The results of these tests are shown in Table 9. For purposes of comparison, Halon 1301 was examined employing the same system. Table 9 HF and HBr Production During Total Flooding * sq ft of fire per 1000 cu ft of enclosure As indicated in Table 9, for a given fire size and discharge time, less decomposition products HF and HBr are observed with CHF2Br than with Halon 1301. Example 8 This example demonstrates the efficacy of blends of bromodifluoromethane with hydrofluorocarbons. Various mixtures of bromodifluoromethane and a hydrofluorocarbon were tested with n-heptane fuel in a cup burner apparatus as described in Example 3, and the results are shown in Tables 5 10-13. In Tables 10A, 11A and 12A, the amount of agent required for extinguishment is expressed as the volume % in air, i.e., ext % = [cc of agent/(cc of agent + cc of air)] x 100. In Tables 10B, 11B and 12B, the amount of agent required for extinguishment is expressed as the % added to air, i.e., 10 % added to air = (cc of agent/cc of air) x 100. In addition to the molar composition of the blends in percent, we report in Tables 10-13 the volume percents of each agent and the total volume percent of the mixture at extinguishment, and the concentrations of each agent and the total concentration on a i5 weight basis (mg/L) at extinguishment. The Tables also illustrate the range of ODP values obtainable with the various mixtures. The ODP of a given mixture is calculated as the sum of the weight percent of each agent multiplied by its ODP, as calculated by the Babson method. For example a 50:50 (by 0 weight) blend of CHF2Br and CF3Br has an ODP of (0.5x0.89)+(0.5xl4.26) = 7.6. The data demonstrate that effective flame extinguishment may be obtained with mixtures of bromodifluoromethane and various hydrofluorocarbons, and that the already low ODP of 5 CHF2Br can be further materially reduced without significant reduction of the extinguishing capability of the mixture. For example, as shown in Table 11B, a 43:57 by weight mixture of CHF2Br and CF3CHFCF3 extinguishes an n-heptane flame at a concentration of 4.4 % by volume, compared to 4.0 % by volume 0 for pure bromodifluoromethane. This represents a 10 % increase in the total volume (a 25 % increase in the total mg/L delivered) required for extinguishment with a 56 % reduction in ODP. Table 10A CHF2Br / CF3CF2H Blends TABLE 10B Extinguishment of n-Heptane Diffusion Flames CF 3 CF 2 H / CHF 2 Br Mixtures Flow at Extinguishment cc/min Added to Air Weight \ CF 3 CF 2 H CHF 2 Br CF 3 CF 2 H CHF 2 Br Total CF 3 CF 2 H ODP Table 11A CHF2Br / CF3CHFCF3 Blends TABLE 11B Extinguishment of n-Heptane Diffusion Flames CF3CHFCF3 / CHF 2 Br Mixtures TABLE 12A CHF2Br / CF3CHFCF2H Blends Extinguishment of n-Heptane Diffusion Flames CF 3 CHFCF 2 H / CHF 2 Br Mixtures Blends of CHF2Br with CF3H are particularly useful, and a large synergistic effect is observed. For example, a 52:48 by mole mixture of CHF_Br and CF3H affords a 34 % reduction in ODP, with only a 1 % increase in the total weight of agents required for extinguishment. In fact, for compositions containing between ca. 60 to 95 % by weight CHF2Br, the total weight of agents required is actually less than that required for either pure agent. For most of the hydrofluorocarbons, the volume percent of the mixture required for extinguishment is significantly less than that calculated on the basis of the known extinguishing capacity of the single components. Table 14 shows the percentage difference between the quantity of composition theoretically necessary, and the quantity found to be actually necessary for a selection of mixtures. Table 14 Mixture Composition Extinguishing Concentration, Vol mole % Theoretical Actual Difference 8.0 5.1 36 6;2 5.0 21 5.0 4.2 16 4.2 3.8 Example 9 This example demonstrates the efficacy of mixtures of bromodifluoromethane with the extinguishing agents Halons 1301 and 1211. Mixtures were examined according to the procedure of Example 8, and the results are shown in Tables 15 and 16. The data demonstrate that effective flame extinguishment may be obtained with mixtures of bromodifluoromethane and the fire extinguishment agents Halons 1301 and 1211, and that the ODP can be significantly reduced materially without loss of efficiency. For example, employing a 76:24 by mole mixture of CHF2Br and CF3Br affords a 69% reduction in ODP compared to pure CF3Br, and the total weight of agents required is only 5% higher compared to CF3Br by itself. TABLE 16 CHF2Br / H1301 Blends Example 10 This example demonstrates the efficacy of mixtures of CHF2Br with hydrochlorofluorocarbons, for example CFC 124 (CF3CHFC1) or CHF 2 C1. Mixtures were tested according to the procedure of Example 8, and the results are shown in Table 17 and 18. The data demonstrate that effective flame extinguishment may be accomplished by employing mixtures of bromodifluoromethane with hydrochlorofluorocarbons, while also providing significantly reduced threat to depletion of ozone. For example, as shown in Table 17A a 71:29 by mole mixture of CHF2Br and CF3CHFC1 provides a 28% reduction in ODP compared to pure CHF2Br, and is capable of extinguishment of n-heptane diffusion flames at a concentration of 3.8% by volume, well within the limit for occupied areas of ca. 10%. TABLE 17A CHF2Br / CF3CHFC1 Blends Table 17B Extinguishment of n-Heptane Diffusion Flames CF3CHFCl/CHF2Br Mixtures TABLE 18 CHF2Br / F22 Blends The present invention provides compositions which release from a contained charge in a uniform, cohesive stream over a sustained period. The compositions yield the distinctive throw characteristics with utilization in existing fire extinguishing equipment with little or no modification to the equipment required. The discharge patterns achieved with the inventive compositions impact the ability to obtain quick "knock down" of a fire, which in turn will affect the amount of extinguishant required to extinguish a given fire. The fire extinguishing systems containing the inventive compositions include means for sealing the components thereof against leakage. Elastomers used in gaskets and O-rings, for example, are vital to assure proper containment and discharge of an extinguishant. It has been found that there is a distinct difference in compatibility of elastomers with bromodifluoromethane, as opposed to other fire extinguishants such as bromochlorodifluoromethane and bromotrifluoromethane. Systems according to the present invention using the bromodifluoromethane compositions preferably include compatible sealant elastomers, such as ethylene propylene diene terpolymer (EPDM) . Compatible elastomers will in general be those which have dissimilar solubility parameters. Solubility parameters for certain identified fire extinguishants and elastomers are listed in Table 19, and are consistent with observations as to elastomer compatibility, particularly when it is taken into consideration the range of polymers which can be produced within each classification. Table 19 Solubility Parameter (cal/cm--)l/2 Material Total Dispersive Polar Hydrogen Bonding Halon 1201 9.42 8.31 3.42 2.84 Halon 1211 8.42 8.31 1.37 0.00 Halon 1301 4.86 4.85 1.22 0.00 Solubi lity Parameter (cal/cm 3 ) 1 /2 6.2% ENB* 8.46- 24% propylene 8.55 Buna N 8.8- poor 9.29 *ENB - Ethylidenenorbornene in another aspect, compatible elastomers will generally be those which are non-polar and are therefore less susceptible to attack from the more polar bromodifluoromethane (as compared for example to the more polar bromochlorodifluoromethane) . By contrast, the more polar elastomers such as acrylonitrile-butadiene copolymers (nitrile rubber, Buna-N) are suitable for use with Halon 1211 and Halon 1301, but are not desirable for use with the bromodifluoromethane compositions of the present invention. Fig. 3 schematically shows the detail of a connection between a container and discharge valve in an extinguisher of the type depicted in Fig. 2. A nozzle 31 at the top of the container 21 comprises a flange 33 having a circular groove 35 in its face concentric with the nozzle. A flange 37 on discharge valve 25 also has a circular groove 39 that matches the groove in flange 33 to define a space for receipt of an O-ring gasket 41 that seals the connection. It has been found that selection of the material of sealing means such as O-ring 21 is a matter of importance in the fire extinguishant system of the invention. Many conventional gasket materials have been found to swell and deteriorate when exposed to Halon 1201. This is a deterrent to the commercial use of bromodifluoromethane as a fire extinguishant. However, in accordance with the invention, it has further been discovered that certain particular gasket materials are unusually resistant to Halon 1201. In particular, it has been found that ethylene-propylene terpolymers can be used effectively as materials of construction for sealing materials exposed to Halon 1201. These preferred sealing materials are stereospecific linear terpolymers of ethylene, propylene and small amounts of a nonconjugated diene, for example a cyclic or aliphatic diene such as hexadiene, dicyclopentadiene, or ethylidene norbornene. Backbones of these polymers are saturated, but a pendant moiety derived from the unconjugated diene is unsaturated. The polymers are vulcanized with conventional vulcanizing materials. EPDM rubber is an especially preferred sealing material. Example 11 Samples of elastomeric materials, established to have the dimensions 2.54 cm x 5.08 cm x .178 cm, were immersed in Halon 1201 in a pressurized container at ambient temperature (20°-25°C) for 70 hours. Immediately upon removal from the container, the dimensions were again measured, and a computation was made of the linear swell of the largest dimension and the percent change in volume of the specimen. Results of the tests of this example are set forth in Table 20. oo I A linear swell of 4.7% is generally considered acceptable in a gasket material, but a linear swell of over 5% is considered unacceptable. Accordingly, it may be seen that, of the various gasket materials tested, only the EPDM materials were found to be feasibly useful in a commercial fire extinguisher. In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. As various changes could be made in the above process, compositions and systems without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims: What is claimed is: 1. A method for extinguishing a fire with high efficacy and with reduced ozone depletion, which method comprises the steps of: a. introducing to the fire a fire extinguishing concentration of a composition consisting essentially of bromodifluoromethane or of a mixture of bromodifluoromethane and a compatible propellant; and b. maintaining the extinguishing concentration of the composition until the fire is extinguished. 2. The method of claim 1 in which step a. comprises discharging the composition from a pressurized container. 3. The method of claim 2 in which the composition comprises bromodifluoromethane and a compatible propellant, said composition within the container initially comprising a liquid and gas phase mixture having a phase equilibrium pressure of between about 45 psig and about 600 psig at ambient temperature. 4. The method of claim 3 in which the composition within said container initially consists essentially of bromodifluoromethane and a propellant selected from the group consisting of nitrogen, carbon dioxide, trifluoromethane, carbon tetrafluoride, argon and mixtures thereof, said composition having an ozone depletion potential of less than about 1.0. 5. A method for extinguishing a fire with high efficacy and with reduced ozone depletion, which method comprises the steps of: a. introducing to the fire a fire extinguishing concentration of a mixture of bromodifluoromethane and a fluorocarbon fire extinguishant; and b. maintaining the extinguishing concentration of the mixture until the fire is extinguished. 6. The method of claim 5 in which the mixture consists essentially of bromodifluoromethane and the fluorocarbon fire extinguishant. 7. The method of claim 5 in which the mixture has a phase equilibrium pressure of between about 45 psig and about 600 psig at 70°F. 8. The method of claim 5 in which the extinguishing concentration is less than about 15% (v/v) . 9. The method of claim 5 in which the mixture comprises a mixture of bromodifluoromethane and a fluorocarbon fire extinguishant selected from the group consisting of bromotrifluoromethane, bromochlorodifluoromethane, chlorodifluoromethane, chlorotrifluoromethane, heptafluoropropane, 1,1,1,2,3,3-hexafluoropropane, 1,1,1,3,3,3-hexafluoropropane, pentafluoroethane, 2-chloro-l,1,1,2-tetrafluoroethane, dibromodifluoromethane, dibromotetrafluoroethane, chloropentafluoroethane, 2-bromo-l,1,1,2-tetrafluoroethane, C 2 F g , C-F-, C.F- _, 1-bromo-l,1,2,2-tetrafluoroethane, 1-chloro-l,1,2,2-tetrafluoroethane and mixtures thereof. 10. The method of claim 9 in which the fluorocarbon fire extinguishant is bromochlorodifluoromethane. 11. The method of claim 9 in which the fluorocarbon fire extinguishant is bromotrifluoromethane. 12. The method of claim 9 in which the fluorocarbon fire extinguishant is heptafluoropropane. 13. The method of claim 9 in which the fluorocarbon fire extinguishant is 2-chloro-l,1,1,2-tetrafluoroethane. 14. The method of claim 5 in which said introducing of step a. comprises: pressurizing the mixture within a closed container; and releasing the mixture under pressure from the container to disperse the mixture into the atmosphere in contact with the fire. 15. A fire extinguishant composition having a low ozone depletion potential and comprising at least about 70% by weight bromodifluoromethane in admixture with a propellant, the composition comprising a liquid and gas phase mixture having a phase equilibrium pressure of between about 45 psig and about 600 psig at 70°F. 16. The composition of claim 15 in which said propellant comprises a gas selected from the group consisting of nitrogen, carbon dioxide, trifluoromethane, carbon tetrafluoride, and argon. 17. A fire extinguishant composition combining the characteristics of high fire extinguishing efficacy and low ozone depletion potential, the composition comprising a mixture of bromodifluoromethane and a fluorocarbon fire extinguishant. 18. The fire extinguishant composition of claim 17 and which has a phase equilibrium pressure of between about 45 psig and about 600 psig at 70°F. 19. The fire extinguishant composition of claim 17 and which consists essentially of a mixture of bromodifluoromethane and the fluorocarbon fire extinguishant. 20. The fire extinguishant composition of claim 17 and which further comprises a compatible propellant. 21. The fire extinguishant composition of claim 17 in which the fluorocarbon fire extinguishant is selected from the group consisting of bromotrifluoromethane, bromochlorodifluoromethane, chlorodifluoromethane, chlorotrifluoromethane, heptafluoropropane, 1,1,1,2,3,3 hexafluoropropane, 1,1,1,3,3,3-hexafluoropropane, pentafluoroethane, 2-chloro-l,1,1,2-tetrafluoroethane, dibromodifluoromethane, dibromotetrafluoroethane, chloropentafluoroethane, 2-bromo-l,1,1,2-tetrafluoroethane, C 2 F 6' C 3 F 8' C 4 F 10' 1-bromo-l,1,2,2-tetrafluoroethane, 1-chloro-l,1,2,2-tetrafluoroethane and mixtures thereof. 22. The fire extinguishant composition of claim 21 in which the mixture consists essentially of bromodifluoromethane and a fluorocarbon fire extinguishant selected from the group consisting of bromotrifluoromethane, bromochlorodifluoromethane, chlorodifluoromethane, chlorotrifluoromethane, heptafluoropropane, 1,1,1,2,3,3 hexafluoropropane, 1,1,1,3,3,3-hexafluoropropane, pentafluoroethane, 2-chloro-l,1,1,2-tetrafluoroethane, dibromodifluoromethane, dibromotetrafluoroethane, chloropentafluoroethane, C-F , , C 3 F o' C 4 F 10' 1-bromo-l,1,2,2-tetrafluoroethane, 1-chloro-l,1,2,2-tetrafluoroethane and mixtures thereof. 23. The fire extinguishant composition of claim 22 in which the fluorocarbon fire extinguishant is bromochlorodifluoromethane. 24. The fire extinguishant composition of claim 22 in which the fluorocarbon fire extinguishant is bromotrifluoromethane. 25. The fire extinguishant composition of claim 22 in which the fluorocarbon fire extinguishant is heptafluoropropane. 26. The fire extinguishant composition of claim 22 in which the fluorocarbon fire extinguishant is 2-chloro-l,1,1,2-tetrafluoroethane. 27. The fire extinguishant composition of claim 21 and which further comprises a compatible propellant. 28. A fire extinguishant system comprising: a container; a fire extinguishant composition within said container; and means for releasing a fire extinguishing concentration of said composition from the container to a fire, said fire extinguishant composition combining the characteristics of high fire extinguishing efficacy and low ozone depletion potential and consisting essentially of: a. bromodifluoromethane, b. a mixture of bromodifluoromethane and a compatible propellant, c. a mixture of bromodifluoromethane and a fluorocarbon fire extinguishant selected from the group consisting of bromotrifluoromethane, bromochlorodifluoromethane, chlorodifluoromethane, chlorotrifluoromethane, heptafluoropropane, 1,1,1,2,3,3 hexafluoropropane, 1,1,1,3,3,3-hexafluoropropane, pentafluoroethane, 2-chloro-l,1,1,2-tetrafluoroethane, dibromodifluoromethane, dibromotetrafluoroethane, chloropentafluoroethane, 2-bromo-l,1,1,2-tetrafluoroethane, C 2 F g , C-.F-., C.F- _, 1-bromo-l,1,2,2-tetrafluoroethane, 1-chloro-l,1,2,2-tetrafluoroethane and mixtures thereof, or d. a mixture of bromodifluoromethane, a fluorocarbon fire extinguishant and a compatible propellant. 29. The fire extinguishant system of claim 28 in which the fire extinguishant composition consists essentially of bromodifluoromethane and a propellant selected from the group consisting of nitrogen, carbon dioxide, trifluoromethane, carbon tetrafluoride, argon and mixtures thereof, the composition having an ozone depletion potential of less than about 1. 30. The fire extinguishant system of claim 28 in which said releasing means includes means for pressurizing said fire extinguishant composition within the container with a compatible propellant. 31. The fire extinguishant system of claim 28 and which further includes sealing means for sealing said container and said releasing means against leakage of said composition therefrom, said sealing means comprising an ehtylene propylene diene terpolymer.

Inventors: FIDELLE THOMAS P, IIKUBO YUICHI, ROBIN MARK LESTER, ROSE RICHARD S, SWEVAL MARK A, FIDELLE, THOMAS, P., IIKUBO, YUICHI, ROBIN, MARK, LESTER, ROSE, RICHARD, S., SWEVAL, MARK, A.
Applicants: GREAT LAKES CHEMICAL CORP, GREAT LAKES CHEMICAL CORPORATION
Application year: 1990
Main classification: A62D1
IPC codes: A62D1