US 3715438 A
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INVENTOR (1,4 rro/v M #066277 BY gt!) C. M. HUGGETT Filed July 22, 1970 HABITABLE COMBUSTION-SUPPRESSANT ATMOSPHERE COMPRISING- AIR, A PERFLUOROALKANE AND OPTIONALLY MAKE-UP OXYGEN FL (/OIPOC'AFBON ADDED 7'0 A/R-MOLE Pf/FCf/VT w w v w w m Q wvkQk b6 .wwQSXw. vRv 0 3,715,433 HABETABLE CQMRUSHUNSUPPRESSANT A'EMtld- PHERE COIVilPiillifillNG AER, A PERFMJQRO- ALKANE AND U FTIONALLY MAKEUP UXYGEN Clayton M. Huggett, Burke, Va, assignor to The SusquehannaCorporation, Fairfax County, Va. Filed .luly 22, 11970, Ser. No. 57,255 int. (Cl. Adllr 13/00, 27/00 ELS. Cl. 424366 13 Cla ms BACKGROUND OF THE INVENTION Fluoroalkanes of the type containing one or more Cl and/or Br substituents have been used very effectively as fire extinguishing agents, namely as agents to put out fires after combustion is actively under way. The chloroand/or bromo-substituted fluoroalkanes, such as bromochlorodifiuoromethane, bromotrifiuoromethane and dibromotetrafiuoroethane, are generally, in fact, considerably more effective as fire extinguishing agents than the perfiuoroalkane gases because, unlike the stable lower-pel fluoroalkanes, the former decompose at elevated temperatures and form products, such as chlorine and bromine atoms, which are highly effective in quenching combustion by such mechanisms as interrupting free radical reaction propagation. Care, however, must be exercised in the storage and use of the chloroand/or bromo-fiuoroalkanes, particularly in a confined environment, such as aircraft, because of their undesirable side effects on mammalian life, such as anaesthetic action in relatively small concentrations, and toxicity at higher concentrations. The undesirable side effects and toxicity of the chloroand/or bromo-iuoroalkanes make them impossible to utilize in a habitable atmosphere.
Although the perfluoroalkanes have fire extinguishing properties, they have generally been discarded as fire extinguishing materials, despite their non-toxicity and freedom from undesirable side effects, because of their substantially lesser effectiveness as compared with the chloroand/or bromofluoroalkanes.
The perfluoroalkanes, because of their non-toxicity, have found principal use as aerosol-forming vehicles in the food and cosmetic art and as refrigerants. Inhalation tests have been made to determine and demonstrate the non-toxicity of the perfiuoroalkanes for such uses. W. S. Clayton et al., Toxicity Studies With Octafiuorocyclobutane, Industrial Hygiene Journal, October 1960, pp. 382-8, reports on the non-toxicity of octafiuorocyclobutane on mammalian test animals. W. S. Clayton, Fluorocarbon Toxicity: Past, Present, Future, Journal of the Society of Cosmetic Chemists, May 27, 1967, pp. 333-50,
Patented Feb. ti, 1973 reports on the non-toxicity of carbon tctrafiuoride, hexafiuoroethane and octafiuorocyclobutane.
None of the known art has taught or suggested the present discovery that per-fluoroalkane gases can be utilized to convert normal air into a still-habitable atmospherc which prevents combustion of normally ignitable materials of the non-self-sustaining type present in the environment, thus substantially eliminating the hazard of fire. A non-self-sustaining material is one which does not contain an oxidizer component capable of supporting combustion.
By habitable atmosphere is meant an atmosphere which supports mammalian life and permits the normal activities of such life for extended periods of time without disabling side effects or other forms of acute toxicity.
The object of this invention is to suppress the normal combustion-sustaining properties of air in an enclosed environment while maintaining its mammalian lifesustaining properties without substantial interference with the normal activities of such life.
Still another object is to provide a habitable atmosphere which comprises air so modified that it does not sustain the burning of ignitable non-self-sustaining combustible materials and, thereby, substantially eliminates fire hazards.
Another object is to provide a method for preventing and controlling fire in an enclosed compartment which can nevertheless sustain mammalian life.
Other objects and advantages will become obvious from the following detailed description.
DRAWING The figure is a graph showing the relationship between fluorocarbon concentration added to air, the heat capacity of the modified air in cal./ C. per mole of total oxygen at 25 C. and constant pressure, and the mole percent of oxygen in the fluorocarbon-modified air.
SUMMARY OF THE INVENTION The invention comprises a habitable atmosphere, which does not sustain combustion of combustible materials of the non-self-sustaining type and which is capable of sustaining mammalian life, consisting essentially of air; a perfiuoroalkane selected from the group consisting of carbon tetrafiuoride, hexafluoroethane, octafiuoropropane, and mixtures thereof; and make-up oxygen in an amount from 0 to the amount required to provide, together with the oxygen present in the air, sufiicient total oxygen to sustain mammalian life. The perfiuoroalkane should be present in an amount suiiicicnt to impart to the atmosphere a heat capacity per mole of total oxygen which is sulficient to suppress combustion of the flammable matcrials present in an enclosed compartment containing said atmosphere.
The invention also comprises a method for preventing and controlling fire in a confined air-containing compartment while maintaining the compartment habitable by mammalian life, which comprises, introducing into the air carbon tetrafiuoride, hexafiuoroethane, octailuoropropane, or mixtures thereof, in an amount sufiicient to provide a heat capacity per mole of total oxygen which is sufiicient to suppress combustion of the flammable materials present in the compartment and additionally introducing oxygen, if and as required, to make up with the oxygen available in the air, sufiicient total oxygen to sustain mammalian life.
3 PREFERRED EMBODIMENTS The perfluoroalkanes CR CQF and C F when added in adequate amounts to the air in a confined space, eliminate the combustion-sustaining properties of the air and suppress the combustion of flammable materials, such as paper, cloth, wood, [flammable liquids, and plastic items, which may be present in the enclosed compartment, without detriment to normal mammalian activities.
The perfluoroalkanes P C F and C F are extremely stable and chemically inert. They do not decompose at temperatures as high as 400 C. to produce corrosive or toxic products and cannot be ignited even in pure oxygen so that they continue to be effective as flame suppressants at the ignition temperatures of the combustible items present in the compartment. They are also physiologically inert. Although they may cause some discomfort due to the increased density and reduced thermal conductivity which they impart to the air, such discomfort is not such as substantially to impair normal activity and can be counteracted by physical means, such as air cooling, or by the addition of an inert low-molecular weight gas, such as helium.
The C to C prefiuoroalkanes are additionally advantageous because of their low boiling points, the highest being that of C F which has a boiling point at normal atmospheric pressure of 36.7 C. Thus, at any low environmental temperature likely to be encountered, these perfiuoroalkane gases will not liquity and will not, thereby, diminish the fire preventive properties of the modified air.
To eliminate the combustion-sustaining properties of the air, the perfluoroalkane gas should be added in an amount which will impart to the modified air a heat capacity per mole of total oxygen present, including any make-up oxygen required, sufiicient to suppress or prevent combustion of the flammable, non-self-sustaining materials present in the enclosed environment. The minimum heat capacity required to suppress combustion varies with the combustibility of the particular flammable materials present in the confined space. It is well known that the combustibility of materials, namely their capability for igniting and maintaining sustained combustion under a given set of environmental conditions, varies according to chemical composition and certain physical properties, such as surface area relative to volume, heat capacity, porosity, and the like. Thus, thin, porous paper such as tissue, is considerably more combustible than a block of Wood.
In general a heat capacity of about cal./ C. and constant pressure per mole of oxygen is more than adequate to prevent or suppress the combustion of materials of relatively moderate combustibility, such as wood and plastics. More combustible materials, such as paper, cloth, and some volatile flammable liquids, generally require that the perfluoroalkane be added in an amount suflicient to impart a higher heat capacity. It is also desirable to provide an extra margin of safety by imparting a heat capacity in excess of minimum requirements for the particular flammable materials. A minimum heat capacity of cal./ C. per mole of oxygen is generally adequate for moderately combustible materials and a minimum of about cal./ C. per mole of oxygen for highly flammable materials. More can be added if desired but, in general, amounts imparting a heat capacity higher than about cal./ C. per mole of total oxygen add substantially to the cost and may create unnecessary physical discomfort without substantial further increase in the fire safety factor.
Heat capacity per mole of total oxygen can be determined by the formula:
The boiling points of CR C F and C F and the mole percents of each required to impart to air heat capacities (C of 40 and 50 ca1./ C. at 25 C. and constant pressure while maintaining a 21% oxygen content are tabulated in Table I:
TAB LE I Boiling point, 0;, =40, C,,=50, 0 percent percent C 1 and C F are preferred embodiments because of the relatively small amounts of each required to impart fire suppressant properties to air. C 1 is particularly preferred because its boiling point is sufficiently low to make it usable under the most extreme temperature conditions. It has the additional advantages of substantial commercial availability and relatively low cost.
The concentration of oxygen available in the confined air space should be sufiicient to sustain mammalian life. "lhe amount of make-up oxygen, if required, is determined by such factors as degree of air dilution by the perfluoroalkane gas and depletion of the available oxygen in the air by respiration. The amount of oxygen required to sustain human, and therefore mammalian life in general, at atmospheric, subatmospheric, and superatmospheric pressures, is well known and the necessary data are readily available. See, for example, Paul Webb, Bioastronautics Data Book, NASA SP-3006, National Aeronautics and Space Administration, 1964, p. 5. The mini mum oxygen partial pressure is considered to be about 1.8 p.s.i.a., with amounts above about 8.2 p.s.i.a. causing oxygen toxicity. At normal atmospheric pressures at sea level, the unimpaired performance zone is in the range of about 16 to 36 volume percent 0 Preferably the amount of oxygen maintained is at or close to that which maintains optimum comfort, namely at least about 18% and preferably about 21% at normal atmospheric pressure with adjustments as required for different pressures, particularly if the confined space is maintained or subjected to reduced pressures.
In some applications, little, if any, make-up oxygen will be required initially, particularly where the periluoroalkane volume requirement, e.g. C F and C 1 is relatively small. However, habitation for extended periods of time will generally require addition of oxygen to makeup depletion caused by respiration.
The figure shows the heat capacity per mole of total oxygen imparted to air by different mole percentages of OR, C 1 and C 1 Without the addition of make-up oxygen. The resulting percentages of oxygen present in the fluorocarbon-modified air is also shown.
It will be seen that between about 4 to 9 mole percent 0 E; provides a heat capacity in the 40 to 50 cal. range and the oxygen content drops only to about 20.1 to 19%. This is well within the habitable range. Similarly, for C F from about 5.5 to 12.3 mole percent are required for a heat capacity range of 40 to 50 cal. and the corresponding oxygen concentrations are about 19.8 to 18.3%, again well within the habitable range. In the case of 0B,, the concentrations required for a heat capacity range of 40 to 50 cal. are about 9.3 to 18.7%. The corresponding oxygen concentrations are about 18.9 to 17.1%. At concentrations appreciably above a 10% addition of CF.,, it is desirable to introduce make-up oxygen initially.
Introduction of the perfluoroalkane gas and make-up oxygen as required is easily provided for by metering appropriate quantities of the gas or gases into the enclosed air-containing compartment.
The air in the compartment can be treated at any time that it appears desirable. The modified air can be used continuously if a threat of fire is constantly present or the particular environment is such that fire hazard must be kept at an absolute minimum, or it can be used as an emergency measure if a threat of fire develops.
Example I A closed chamber with a transparent plastic front and a volume of about 12 cubic feet was fitted with gas inlet and outlet connections and an oxygen meter. A small electric fan was placed in the bottom of the chamber to promote'gas mixing; Samples'of filter paper; cotton cloth and polyurethane foam, approximately one inch wide by six inches long, were suspended vertically from a metal rod in the center of the chamber. A small crucible containing IP-S hydrocarbon liquid and a filter paper wick was placed nearby. Ignition sources, consisting of small pieces of solid rocket propellant wrapped in short lengths of electric resistance wire and connected to an external power source, were placed at the base of the solid samples and on top of the wick in the JP-S.
With the fan turned on, hexafluoroethane was admitted to the chamber, displacing air through the exit tube, until the oxygen content of the atmosphere dropped to 16%. Pure oxygen was then admitted, displacing the air, fluorocarbon mixture until the oxygen content returned to 20.9%. At this point the composition of the atmosphere was approximately 20.9% oxygen, 57% nitrogren and 22.1% hexafiuoroethane and small amounts of the other normal air components. The heat capacity of the modified air was approximately 52.5 cal./ C. per mole of total At this point a rabbit was introduced into the chamber. The fan was stopped and an electric current was applied to each of the ignition circuits in sequence. The solid propellant pellets burned vigorously, charring the solid samples in close proximity and melting a portion of the polyurethane foam, but no sustained fires were obtained and the samples were undamaged at a short distance from the ignition source. The rabbit showed no signs of alarm or discomfort from the fluorocarbon-containing atmosphere during the approximately 15 minutes required to perform the experiment.
The chamber was opened and thoroughly purged with air and fresh fuel samples and igniters were introduced. The chamber was closed and the ignition sequence was repeated. This time sustained fires were obtained with each of the fuel samples. The samples were completely consumed except for a small amount of carbonaceous ash. The rabbit showed considerable alarm at the fire and smoke produced. The rabbit was observed for a period of thirty days following the experiment and showed no ill effects.
Example 11 Tests were made to determine the flammability limits of various flammable materials in fluoroalkanemodified air. A 3.2 cu. ft. test chamber was employed. The materials tested included 2" x 7" strips of cotton flannel, x 8" sheets of tissue paper, 1" x 7" strips of foamed plastic, and 1 cc. of kerosene in a small cup containing a saturated wick. Except for the kerosene, the samples were vertically suspended and ignition initiated from the bottom. The ignition sources were pieces of solid composite propellant that burned about .5 seconds. The test chamber contained an air atmosphere into which the fluoroalkane gas was introduced in varying amounts. No make-up oxygen was used.
All samples were first tested in the normal air atmosphere without fluoroalkane addition. All samples ignited readily and burned completely.
CaFa Air 0 0,, Result The following results were obtained with fiuoroalkane addition:
COTTON FLANNEL Mole percent of- CF4 All 0: Op Result 18.7 81.3 17.1 50.0 Noflame propagation.
17.5 82.5 17.3 48.4 Weak flame propagation along edges, then sell-extinguishment.
13.5 86.5 18.2 44.0 Burned completely.
Mole percent of- C2Fu Air 02 (3,, Result 12.0 88.0 18. 5 49. 5 N o flame propagation. 11. 0 89. 0 1S. 7 48. 0 Do.
9. 1 90. 9 19. 1 45. 1 Flame propagated slowly, high residue. 6. 6 93. 4 19. 6 41. 5 Burned completely.
ll/lole percent of- 9.4 90.6 19.0 50.5 No flame propagation.
8.1 91.9 19.3 47.8 Weak edgeflame forlsecond.
6.75 93.25 19.6 45.1 Burned [or several seconds, then selfextinguished.
5.4 94.6 19.9 42.8 Burned completely.
TISSUE PAPER Mole percent of CzFs Air 02 C Result 12.0 88. 0 1S. 5 49. 5 No flame propagation.
11.0 89. 0 18. 7 48. 0 Burned slightly for short time.
9. 1 90. 9 l9. 1 45. 1 Extingnished alter ignition. 6.6 93. 4 19.6 41. 5 Burned completely.
FOAM PLASTIC Mole percent of- CgFs Air 0; 0,, Result 12.0 88.0 18.5 49.5 No flame propagation.
1.75 98.25 20.6 35.1 Burned completely.
KEROSENE Mole percent of- 021% Air 01 C Result It will be noted from the above results that the amounts of fiuoroalkane required for flame suppression varied with the degree of flammability of the generally highly flammable samples used. The degree of flammability of some of the test materials was increased by their physical forms, e.g. the high surface area of roughly textured flannel, the thinness and high porosity of tissue paper, and the high porosity of the formed plastic. It should also be noted that at fluorocarbon concentrations below those which completely prevent ignition of the fuel material, there are ranges of concentration in which flamming is weak and quickly suppressed, thus providing an additional safety factor against conflagration.
1. A habitable atmosphere which does not sustain combustion of combustible materials of the non-self-sustaining type and which is capable of sustaining mammalian life, consisting essentially of:
(b) a perfluoroalkane selected from the group consisting of carbon tetrafluoride, hexafluoroethane, octafluoropropane, and mixtures thereof in an amount sulficient to impart to said atmosphere a heat capacity per mole of total oxygen suflicient to suppress combustion of the combustible materials present in an- 7 enclosed compartment containing said atmosphere; and
(c) make-up oxygen in an amount from to the amount required to provide, together with the oxygen present in said air, suflicient total oxygen to sustain mammalian life;
said atmosphere containing sufiicient total oxygen to sustain mammalian life.
2. The habitable atmosphere of claim 1 wherein the heat capacity per mole of total oxygen is at least about 45 cal./ C. at 25 C. and constant pressure.
3. The habitable atmosphere of claim 1 wherein the heat capacity per mole of total oxygen is at least about 50 cal./ C. at 25 C. and constant pressure.
4. The habitable atmosphere of claim 1 wherein the perfluoroalkane is hexafluoroethane.
5. The habitable atmosphere of claim 2 wherein the per-fluoroalkane is hexafluoroethane.
6. The habitable atmosphere of claim 3 wherein the perfluoroalkane is hexafluoroethane.
7. The habitable atmosphere of claim 1 wherein the perfiuoroalkane is octafiuoropropane.
8. The habitable atmosphere of claim 2 wherein the perfluoroalkane is octafluoropropane.
9. The habitable atmosphere of claim 3 wherein the perfiuoroalkane is octafiuoropropane.
10. A process for preventing and controlling fire in an enclosed air-containing mammalian-habitable compartment which contains combustible materials of the nonself-sustaining type, which comprises:
(a) introducing into the air in said enclosed compartment a perfluoroalkane selected from the group consisting of carbon tetrafiuoride, hexafluoroethane, octarfiuoropropane, or mixtures thereof in amount sufficient to impart a heat capacity per mole of total oxygen suflicient to suppress combustion of the combustible materials present in said enclosed compartment; and
(b) introducing make-up oxygen in an amount from O to the amount required to provide, together with the oxygen present in said air, suflicient total oxygen to sustain mammalian life;
said atmosphere containing sufficient total oxygen to support mammalian life.
111. The process of claim 10 wherein the heat capacity per mole of total oxygen is at least about cal./ C. and constant pressure.
12. The process of claim 10 wherei per mole of total oxygen is at least abo constant pressure.
13. The process of claim alkane is hexafluoroethane.
14. The process of claim alkane is hexafluoroethane.
15. The process of claim 12 alkane is hexafluoroethane.
16. The process of claim 10 alkane is octafluoropropane.
17. The process of claim alkane is octafluoropropane.
It The process of claim alkane is octafluoropropane.
the heat capacity cal./ C. and
10 wherein the perfluoro- 11 wherein the perfiuorowherein the periluorowherein the perfluoro- 11 wherein the perfluoro- 12 wherein the perfluoro- References Cited UNITED STATES PATENTS Future, J. Soc. Cosmetic Chemists, May 27, 1967, pp. 333-350.
DANIEL J. FRITSCH, Primary Examiner US. Cl. X.R. 2522, 8, 8.1, 372
Yant et al. 424366