I
A.
F.
RATZERl
Pyrene Manufacturing Co., Newark 1,
N. J.
History and Development of Foam as a Fire Extinguishing Medium -
Use of foam for fighting fim has grown o major mportance through deve1op ment of
b
Efficient foam-forming concentrates
b
Apparatus for producing foam readily, with minimum manpower
b
Mobile equipment and portable appliances
FOAMS
as used for fire extinguishing purposes are divided into two main classes, known as chemical foam and mechanical foam; the latter is also known as air foam. Chemical foam is obtained by the interaction of sodium bicarbonate and aluminum sulfate in aqueous solution in the presence of a foaming agent. The carbon dioxide evolved is trapped by the foaming agent to form a mass of bubbles, the walls of which are strengthened by the precipitated aluminum hydroxide. Mechanical foam is formed by the mixing of a n aqueous solution of a liquid foaming agent with a gas under pressure or by other mechanical means. I n both cases the principle of operation is the same; aeration of the aqueous medium reduces its density and increases its surface area. This enables the foam to float on the surface of a burning liquid, thus preventing evolution of flammable vapors. I t also presents a large area for the absorption of radiant heat which helps cool the surface and surrounding edges with consequent lowering of the vapor pressure. Early History
The earliest suggestion for the use of foam as a fire extinguishing agent appears in an English patent to Johnson (33) in 1877. He records, as a communication from F. Girard & Co., of Paris, that “the object of said inventionis to form with aluminous, siliceous, calcareous, or other analogous products, a viscous incombustible composition cf small density in consequence of its frothy condition, which enables it to float on the surface of fatty bodies, such, for example, as petroleum and other essences when on fire, and to cover solid objects when on fire with a n incombusPresent address, Baker Industries, Inc., Newark 12, N. J.
tible varnish in such manner as to suddenly arrest all combustion and to prevent the rekindling which might be occasioned by the proximity of flames.” Johnson described a n apparatus consisting of four interconnected receptacles. T h e first contained a n acid ; the second, a n excess of a concentrated solution of sodium bicarbonate; the third, a solution of aluminum, sodium, and ammonium sulfates; and the fourth, a solution of alkaline silicates and sulfides to which was added organic matter, such as soaps, mucilages, or albuminous material. T o operate the extinguisher the acid bottle was inverted to discharge into the bicarbonate which pressurized the equipment. This forced the carbon dioxide and excess bicarbonate into the third compartment which, in turn, flowed into the fourth chamber. The resultant foaming mixture was discharged through a hose and nozzle. This work was apparently not developed, and credit for the original fire foam experiments has been given to Laurent (39),who, in 1904, demonstrated the extinguishment of a naphtha fire in a 35-foot diameter tank with foam generated from two solutions, one containing sodium bicarbonate together with saponin as a foam stabilizer, and the other containing aluminum sulfate. The solutions were delivered separately by the use of twin pumps to a chamber a t the tank, and there allowed to react before flowing onto the liquid surface. Laurent also suggested that the foam could be made by the addition to water of a powder formed by mixing the chemicals used in his two-solution experiments. Actually, Gates (27) in 1903 had patented a method for extinguishing fires using mechanical foam. H e proposed a vessel containing a solution of ammonium soap with dissolved borax
or ammonium sulfate connected to a cylinder containing ammonia, nitrogen, or carbon dioxide under pressure. The gas forced the solution out of the container and into a cup where additional gas bubbled through to form foam. The salts in the soap solution made the resultant product more fire resistant. Development of Chemical Foam
There does not appear to have been much practical application of this early work until about 1912, when foam equipment of the type used by Laurent was introduced into England for fire department use (65). The apparatus employed two tanks, one containing a 13% solution of aluminum sulfate and the other a n 8% solution of sodium bicarbonate with 3y0 saponin, licorice, or Turkey Red oil as a foam stabilizer. Small hand extinguishers were also used. About this time fixed systems, similar to the above, began to be installed for the protection of oil storage tanks, and by the early 1920’s two-solution systems were used in various parts of the world, including the United States, Germany, and E>ngland. I n 1925, Urquhart (82) developed a foam generator consisting of a hopper containing a dry mixed foam powder, which led into the throat of a n injector through which water under pressure was passed, thus producing in a practical form Laurent’s earlier suggestion. Various other types of foam generators were developed by Burmeister (8-70), Graaff (28), Timpson (70), and others, and foam systems using chemical foam powder, either mixed or as separate sodium bicarbonate and aluminum sulfate gradually replaced two-solution systems. Although the foam generator was the most successful piece of apparatus VOL. 48, NO. 1 1
*
NOVEMBER 1956
20 13
o r producing large volumes of foam, trouble was experienced due to caking of the foam powders which prevented their flow. This was caused by premature reaction between the sodium bicarbonate and aluminum sulfate because of the presence of moisture, and various methods were suggested for improving the storage and flow properties of the powder. Schmidt (59) dried the sodium bicarbonate, aluminum sulfate, and saponin at 60’ C., and then mixed them with pumice or ground stone. Dunlap and Ewer (79) recommended the addition of inert materials, such as talc, China clay, and flour to improve the flow of the powders and prevent decomposition. I t is now general practice to use an aluminum sulfate dried to the equivalent heptahydrate as the acid component in a mixed foam powder. Starting with saponin, a wide variety of foaming agents have been suggested and employed as chemical foam stabilizers, including plant extracts, hydrolyzed proteins, and synthetic compounds. Numerous patents have been granted for different substances-e.g., Money (44)and Phillips (53)for licorice; Van Leuven and Van Leuven (85)for saponin and quebracho extract; Urquhart (80)for soybean protein; Walker (87) for sulfite cellulose waste liquor; Kent (35),Chapman (72),and Daimler (74) for sulfonated compounds; and Berghausen (3)for alfalfa extract. Until World War 11, most chemical foams used plant extracts as stabilizers, chiefly saponin or licorice. Owing to high cost, however, saponin has been replaced by other extracts, by-products of the paper industry, or protein hydrolyzates. Chemical foam powders and charges for chemical foam extinguishers are now generally produced to comply with the requirements of government specifications (84). In general, the expansion of the foam produced by the various types of equipment now in service varies from 8 to 1 to 16 to 1, based on the solution or water volume used. low-Temperature Chemical Foam
The foams described above are only suitable for operation at normal temperatures. At temperatures below 40’ F., the reaction between the bicarbonate and aluminum sulfate is too slow to be effective. For operation in cold weather Mork (45) suggested the use of ammonium bicarbonate and sodium acetate, and Thomas and Hochwalt (68, 69) employed potassium carbonate and halogenated sulfonic acids. Special formulations using potassium bicarbonate and aluminum chloride have also been prepared but are not much in use.
20 14
Alcohol-Resistant Chemical Foam
Although chemical foam has excellent stability in contact with gasoline and other hydrocarbon fires, it is rapidly broken down by alcohols and other polar solvents. The addition of saturated fatty acid soaps to a chemical foam powder imparts alcohol stability to the foam because of the formation of a n insoluble aluminum soap in the bubble walls. Blakeborough and Garratt ( 5 ) and Boyd (7) disclosed suitable soaps for this purpose, but they could not be retained in solution in chemicalfoamextinguishers. Ratzer (55)showed that this property could be obtained by the use of sodium ricinoleate with the saturated soaps. Perri (49)achieved the same result by the use of an alkanolamino soap colloidally dispersed with lecithin. Development of Mechanical Foam
Although both Gates (27) and Laurent (39) described methods of producing mechanical foam in their early experiments, the commercial application of this type did not come until some time after chemical foam was in general use on a large scale. To produce a mechanical foam satisfactory for fire-fighting purposes, both equipment and foamforming concentrates were needed. Schnabel (67-63) devised several pieces of apparatus involving the mixing under pressure of air or other gas and a foam-making solution, and also restated (60) one of Laurent’s suggestions in which a foaming agent was supersaturated with gas under pressure so that on release of pressure the mixed gas and !iquid issued from the container as a foam. Schnabel used saponin as the foaming agent, and apparatus employing his principles was built and sold in Germany and England. Its main use, however, was in the production of foam for dust allaying in coal mines. At about the same time Wagener (86), using the principle of the open water-jet air pump, produced the first injector-type air foam apparatus which was the forerunner of most of the air foam equipment in general use today. Wagener was, however, handicapped by the lack of suitable foaming agents which would give a sufficient volume of foam having a stability comparable to that of chemical foam, and the results of his work languished for several years. I n 1929 Schroeder and Van Deurs (64) and Ellehammer (20) in Denmark developed foam pumps in which air, water, and a foaming agent were drawn into the suction side of a rotary pump and then forced through mixing chambers, which produced a mass of fine bubbles, and discharged through a hose and nozzle. Both the Schroeder-Van Duers and Ellehammer pumps produced foams using saponin solution that were
INDUSTRIAL AND ENGINEERING CHEMISTRY
sufficiently stable for them to gain recognition as fire-fighting equipment. Licorice extract and potassium soap solutions could also produce suitable foams and the Ellehammer pump using saponin and later potassium coconut oil soap w a s adopted by the British Government for use by the Royal Air Force on aircraft crash fire-fighting trucks. Meanwhile, Friedrich (23-25) was working on air injzction-type foammaking apparatus and by 1933 had developed svzveral pieces of equipment for use with high pressure water hose lines, but he was also confronted with lack of suitable foaming agents. Although saponin and licorice would give excellent foams when used as ingredients in chemical foam or in the foam pump where considerable energy was available and work done in converting to the foam state, the amount of energy that could be utilized from the high velocity water jets was insufficient to form a stable foam with air-injection playpipes. Wetting Agent Base Foam-Forming Concentrates
I t became evident that, in order to exploit this new method of foam making, which held numerous advantages over chemical foam production for fire fighting owing to simplicity of equipment and continuity of operation, it would be necessary to devise a different type of foaming agent that would foam more readily than those previously employed. Sulfonated lorol (sodium lauryl sulfate) was tried with only partial success. Considerable work was then done by Daimler and others (76, 77. 37) and Gross and others (29), who developed mixtures consisting of concentrated aqueous solutions of synthetic organic wetting agents. albumen degradation products, and glycols or glycol ethers. The wetting agent acted as the foamant, the protein as the stabilizer, and the glycol as a liquefying agent and freezing point depressant. Work was also done on these lines by Moilliet and Todd (43)in England. The most successful wetting agents were found to be the alkali metal salts of the alkylated naphthalene sulfonic acids and in particular sodium butyl naphthalene sulfonates. The proteins found most suitable were the nitric acid hydrolysis products of albumen and glue. Concentrated aqueous solutions of these products were manufactured and used on a n increasing scale in Germany and England during the middle 30’s. in conjunction with foam-making playpipes of the Wagener or Friedrich type or modifications of them (23-26, 38, 54.71, 73). Other workers during this period developed different products as foamforming concentrates; among them may be mentioned Treichel (76), who proposed a mixture of licorice extract and
A Q U E O U S FOAMS
,
lime in powder form for use in foam pumps; Hood (30), who suggested the use of sulfite cellulose liquor and glycerol foots; and Timpson (72, 7 4 75), who used various alkali, phosphated fatty acid, and amino soaps. Timpson’s soap formulations were employed to a limited extent in this country. By the middle 193O’s, air foam had become established as a recognized fire-fighting medium in Germany and England and appreciable amounts of equipment and foam-forming concentrate of the kind described above were being produced. Solutions of 2.5 to 4y0 of a wetting agent-proteinate-type concentrate in fresh water would readily produce foams having expansion ratios between 12 and 18 to 1 and the foam obtained was satisfactory for many flammable liquid and other fires. The foam was not, however, so stable as chemical foam, and broke down more rapidly under heat. The foam was also not too satisfactory when it was used with salt water, nor was it resistant to alcohol fires. Protein-Base Foam-Forming Concentrates
Recognizing this drawback of air foam, various workers were attempting t o obtain a n improved product. I n 1937, Weissenborn (66, 88),working for Sthamer in Hamburg, produced a concentrate which he called “Schaumgeist,” or ghost foam, because of its power to persist in skeletal form for hours after the water had drained or evaporated away. This material consisted initially of two solutions, one a concentrated protein hydrolyzate and the other a solution of ferrous sulfate, mixed together just before they were used. The proteinate itself was the foaming agent and the iron salt acted as a stabilizer for the foam. Weissenborn later produced Schaumgeist as a single mixed concentrate. A 5y0 solution in fresh water or salt water produced a foam having an expansion of approximately 8 to 1, which was as effective and stable as chemical foam on hydrocarbon fires and also had a certain measure of resistance to alcohol fires. Several years previously, Jennings (32) had used mixtures containing glue and ferrous sulfate to form a permanent foam to act as a blanket to prevent evaporation losses in gasoline storage tanks, but this was the first time that a protein hydrolyzate and an iron salt had been used to make a fire-fighting foam. Weissenborn did not disclose the details of the protein hydrolyzate he made, but claimed as his invention the use of a dissociated protein in combination with the salt of polyvalent metals. He theorized that the ferrous
salts of the protein degradation products were soluble, but that on dilution and formation of foam, they were oxidized to the ferric salts by the oxygen in the bubbles. I n this latter state they were insoluble and precipitated out in the bubble walls. Shortly after Weissenborn, and working independently from him, Friedrich (22) and Ratzer (56) also developed protein-base air form concentrates. Friedrich obtained a proteinate solution by the lime hydrolysis of keratin and added a n aluminum salt to achieve the same result as Weissenborn; Ratzer disclosed the controlled hydrolysis of keratins, albumens, and globulins by alkaline earth hydroxides. Concentrates made by the above processes were being produced in substantial quantities in Germany and England by 1939. Because of the shortage of suitable proteins in Germany during the war years, the majority of the foam concentrate used in that country in the period 1939-45 was of the sulfonate wetting agent type, although there was some production of a protein-base material mixed with sulfite cellulose liquor. The results from this latter product were inferior to those from the original Schaumgeist, however. In England, protein concentrate, made from hoof and horn meal and stabilized with ferrous sulfate, was accepted by the government, and several modifications and improvements to the original formula were made (57, 77). Davies and Clark (78) developed a concentrate to be made from indigenous raw materials by the caustic soda hydrolysis of blood and this was also used to the limit of availability of material. When the United States entered World War 11, the Armed Forces also adopted mechanical foam for fighting oil and gasoline fires, development of both protein-base and wetting agenttype foam forming concentrates was accelerated. Urquhart (80) had already been working on a concentrate derived from soybean protein and this was further developed by his later work ( 8 7 ) and by Perri (48, 50). Bagley and Levin ( 2 ) produced a concentrate of the type previously described by Daimler and others (76) by the use of a wetting agent and a protein hydrolyzed with sodium nitrite, and Levin (40) later disclosed a process for the production of a protein-type concentrate by hydrolysis of peanut or cottonseed meal. Mattin and Timpson (42) obtained foam concentrates by the hydrolysis of fish scale proteins and Kiel and Ingraham (36) made a concentrate from blood on the lines of the Davies and Clark patent (78). Busse and others (77) suggested the use of wetting agents in combination with karaya gum to form a heat-stable
foam, and Swift (67) disclosed a process for the treatment of maize protein. During the war the U . S. Government standardized the use of a foam-forming concentrate consisting basically of a hydrolyzed protein containing iron salts as a stabilizer for the foam state and ultimately issued Specification JAN-C266 (83),which is still in effect at this date. This specification details the requirements for a material to be used at a 6yo concentration in water, and relates to the production of a free-flowing stable foam. Other products are now available in more concentrated form for use a t 3y0 dilution and containing depressants so that they will remain liquid a t subzero temperatures. While the bulk of foam-forming concentrates produced today are of the protein type, there is still a requirement for the synthetic wetting agent or high expansion material, The wetting agents give foams of much higher expansion than the protein variety when used in air injection-type apparatus. Although not so stable, they are more rapid in action because of greater fluidity. They are particularly suitable for spill fires and where water supply is limited. Alcohol-Resistant Mechanical Foam
The first development of an alcoholresistant air foam was in a patent to Bohme, A. G. (6),which claimed the use of lauryl pyridinium sulfate and other high molecular quaternary ammonium or phosphonium compounds. The first successful product is due to Daimler and Paquin (75), who produced a concentrate consisting of a protein hydrolyzate (or other protective colloid) together with a zinc fatty acid soap held in solution by excess ammonia. However, this product gave a low expansion, was corrosive, and deteriorated on storage. More recently, Kimber and Clifford (37) obtained a suitable product by the admixture of alginates with a wetting agent, and Tuve and Peterson (79) have suggested the use of egg protein solids in conjunction with a hydrolyzed protein. A development project of Perri and Sloviter (57) describes a n all-purpose foam-forming concentrate prepared from hydrolyzed soybean protein and aluminum soaps, and Ratzer and Levin (58)also worked on this problem. General Application of Foam
The use of foam for fire fighting has grown from an experimental exercise at the beginning of this century to a place of major importance today. Fixed air foam systems, mobile equipment, and portable appliances are used throughout the world for the protection of refineries, oil storage depots, crashed aircraft, on VOL. 48, NO. 11
NOVEMBER 1956
20 15
ships, and wherever there is a flammable liquid fire hazard. Foam has also been successfully employed on domestic fires, where its ability to spread and cover has enabled extinguishment to be effected with much less water than would be possible with water alone. Another application of this property has been in exposure protection, where the water content of the foam can be efficiently used for heat absorption. These applications show considerable merit from the viewpoint of water conservation and possibility of more rapid control; it is hoped that they will be studied further. Additional References
I t has not been possible in a historical review of this nature to go into detail concerning the production and evaluation of the different fire-fighting foams, the theories underlying their behavior, and the standards regarding their use. For fuller information on these matters, reference may be made to the studies of mechanical foam by Clark (73), Tuve and others (52, 78), Amsel (7), and Friedrich (27); the relevant chapters in books by Kausch ( 3 4 , and Bikerman ( 4 ) ; NBFU and NFPA standards (46, 47), and the “Manual of Firemanship” (47), in addition to the references cited above. literature Cited (1) Amsel, O., Oel u. Kohle 38,291 (1942). (2) Bagley, T. J., Levin, D. (to R. M. Hollingshead Corp.), U. S. Patent 2,365,619 (Dec. 19, 1944). (3) Berghausen, A. (to E. Berghausen Chemical Co.), Ibid., 1,848,042 (March 1,1932). (4) Bikerman, J. J., “Foams: Theory and Industrial Application,” Reinhold, New York, 1953. (5) Blakeborough, R. A,, Garratt, W. R., Brit. Patent 369,012 (March 30, 1931). (6) Bohme, A.-G., Ibid., 434,856 (Sept. 10,1935). (7) Boyd, F. L. (to National Foam Systems, Inc.), U. S. Patent 2,269,426 (Jan. 13,1942). ( 8 ) Burmeister, H. (to Pyrene-Minimax Corp.), Ibid.,1,823,559 (Sept. 15, 1931). ( 9 ) Ibid.,-l,907,773 (May 9, 1933). (10) Ibid.,2,143,138 (Jan. 10, 1939). (11) Busse, . W.. F., Lambert, J. hl., Debye, P. P. (to General Aniline & Film Corp.), Ibid.,2,529,211 (Nov. 7,1950). (12) Chapman, E. (British Dyestuffs Corp., Ltd.), Brit. Patent 289,630 (April 23,1927). (13) Clark, N. O., Dept. Sci. Ind. Research, Chemistry Research Special Rept. 6 , London, 1947. (14) Daimler, K. (to General Aniline Works), U. S. Patent 1,914,406 (June 20, 1933). (15) Daimler, K., Paquin, M. (to General Aniline & Film Corp.), Ibid., 2,232,053 (Feb. 18, 1941). (16) Daimler, K., Paquin, M., Riedelsberger, W. (to I. G. Farbenindustrie A. G.), Zbid., 2,154,231 (April 11, 1939).
20 1 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ibid.,2,165,997 (July 11, 1939). Davies, G. R., Clark, N. O., Brit. Patent 571,686 (Sept. 5, 1945). Dunlap, F. L., Ewer, N. T. (to Amdyco Corp.), U. S. Patent 1,624,398 (April 12, 1927). Ellehammer, J. C. H., Brit. Patent 347,048 (April 23, 1931); U. S. Patent 1,970,082 (April 14, 1934). Friedrich, K., Foam Conference Report, Apolda, Germany, 1943. Friedrich, K., U. S. Patent 2,212,470 (Aug. 20, 1940). Friedrich, W., Ibid., 2,003,184 (May 28. 1935).
Graaff, W. (to Pyrene-Minimax Corp.), Ibid., 1,908,141 (May 9, 1933). Gross, E., Wappes, H., Hilgenfeldt, B., Petz, A. (to I. G. Farbenindustrie A. G.), Ger. Patent 635,884 (Oct. 7, 1933); U. S. Patent 2,088,085 (July 27, 1937). Hood, 0. E., Ibid., 2,122,883 (July 5. 1938’1. I. 6. Farbenindustrie A. G., Brit. Patent 460,596 (Feb. 1, 1937). Jennings, J. M. (to Standard Oil Development Co.), U. S. Patent 1,423,719 (July 25, 1922). Johnson, J. H., Brit. Patent 560 (July 20, 1877). Kausch, Oskar, “Das chemische Feuerloschwesen,” pp, 102-47, Hirzel, Leipzig, 1939. Kent, G., U. S. Patent 1,441,728 (Jan. 9, 1923). Kiel, H. L., Ingraham, J. S. (to Armour Co.), Ibzd.,2,431,256 (Kov. 18,1947). Kimber, K. G., Clifford, MI. J., Brit. Patent Appln. 23,716/50 (Sept. 27, 1950). Komet Kompagnie fur Optik, Mechanik, und Elektro-Technik, Brit. Patent 403,291 (Dec. 21, 1933). Laurent, Imp. Russ. Tech. Inst.,Chem. Sec. (1904). Levin, D. (to Chemical Concentrates Corp.), U. S. Patent 2,405,438 (Aug. 6, 1946). Manual of Firemanship, Part 6B, DD. 140-60. H. M. Stationerv Office, London, 1945. Mattin, H. E., Timpson, L. G. M. (to Pyrene-Minimax, Ltd.), Can. Patent 417,315 (Dec. 21, 1943). Moilliet, J. L.. Todd, W. (to Imperial Chemical Industries: Ltd.), Brit. Patent 469,325 (Jan. 22, 1936). Money, G. J., Ibid.,170,390 (July 19, 1920). Mork, H. S. (to American LaFrance Fire Engine Co.), U. S. Patent 1,393,237 (Oct. 11, 1921). National Bureau of Fire Underwriters, Pamphlet 16, “Standards for Combined Foam and Water Spray Systems,” 1954. National Fire Protection .4ssociation, Pamphlet 11, “Standard: for Foam ExtinguishingSystems, 1954. Perri. J. M. (to National Foam Svstems, Inc.),‘U. S. Patent 2,470,719 (May 19, 1949). Ibid.,2,594,985 (April 29, 1952). Perri, J. M., Hazel, J. F., IND.ENG. CHEM.38, 549 (1946). Perri, J. M., Sloviter, B., Air Force Tech. Rept. 6130 (November 1952). Peterson, H. B., Neill, R. R., Jablonski, E. J., Tuve, R. L., Naval Research Laboratory, Memorandum Rept. 92 (December 1952). I I
(53) Phillips, W. B., Brit. Patent 214,075 (April 3: 1923). (54) Pyrene Co., Ltd., Ibid., 441,441 (Jan. 20, 1936). (55) Ratzer, A.’F. (to Pyrene Co., Ltd.), Zbid.,474,479 (Nov. 2, 1937). (56) Ibid., 517,767 (Feb. 8, 1940); U. S. Patents 2,361,057 (Oct. 24, 1944), 2,324,951 (July 20, 1943). (57) Ratzer, A. F. (to Pyrene Co., Ltd.), Brit. Patent 587,701 (May 2: 1947); U. S. Patent 2,481,875 (Sept. 13, 1949). (58) Ratzer, A. F., Levin, D., U. S. Patent Appln. 280,640 (April 4, 1952). (59) Schmidt, K. (to Pyrene-Minimax Corp.), U. S . Patent 1,739,094 (Dec. 10, 1929). (60) Schnabel, R. (to Excelsior Feuerloschgerate), Brit. Patent 299,097 (Oct. 22, 1928); (to Minimax A. G . ) , U. S. Patent 1,669,213 (May 8, 1928). (61) Schnabel, R. (to Pyrene-Minimax Corp.), Zbid., 1,740,840 (Dec. 24; 1929). (62) Ibid.?1,829,715 (Oct. 27, 1931) (63) Ibid.,1,874,209 (Aug. 30, 1932). (64) Schroeder, E., Van Deurs, J. A. S., Ibid.,1,927,376 (Sept. 19, 1933). (65) Simonis, H. S.,,Proc. 70th Conf., pp. 47-53 (1933). (66) Sthamer. R.. Brit. Patent 476.552 * , (Dec. 30,1937). (67) Swift, C. K. (to hlachdrews & Forbes C o . ) , U. S. Patent 2,515,276 (July 18, 1950). (68) Thomas, C. A , , Hochwalt, C. A., Zbid.. 1.777.338 (Oct. 7. 1930). (69) Ibid.,1;777,339 (O‘ct. 7, 1930). (70) Timpson, L. G. M. (to PyreneMinimax Corp.), Ibid., 1,907,901 (May 9, 1933). (71) Ibid.,2,057,218 (Oct. 13, 1936). (72) Ibid.,2,135,365 (Nov. 1, 1938). (73) Ibid.,2,146,605 (Feb. 7, 1939). (74) Ibid.,2,193,541 (March 12, 1940). (75) Ibid.,2,196,042 (April 2, 1940). (76) Treichel, J. B. (to MacAndrews & Forbes Co.) Ibid., 1,971,997 (Aug. 28, 1934). (77) Tresise, D. J., Ratzer, A. F. (to Pyrene Co., Ltd.), Brit. Patent 559,503 (Feb. 22, 1944); (to Pyrene Development Corp.), U. S. Patent 2,368,623 (Feb. 6, 1945). (78) Tuve, R. L., Peterson, H. B., Naval Research Laboratory, Rept. 3725 (Aug. 23, 1950). (79) Tuve, R. L. Peterson, H. B., U. S. Patent 2,614,978 (Oct. 21, 1952). (80)Urouhart. G. G. (to National Foam Systems, Inc.),’ Zbid., 2,269,958 (Jan. 13, 1942). (81) Zbid.,2,413,667 (Dec. 31, 1946). (82) Urquhart, R. hf. (to American Fomon Co.), Ibid., 2,090,601 (Aug. 17, , . 1937). U. S. Joint Army-Piavy Specification JAN-(2-266 (Dec. 4, 1945) and Amendment 1 (Aug. 5,1947). U. S. Joint Army-Navy Specification JAN-C-267 (Dec. 4, 1945). Van Leuven, L. B.: Van Leuven, H. C. (to Vacuum Oil Co.). U . S. Patent 1,507,943 (Sept. 9, 1924). Wagener, C. (to Excelsior Feuerloschgerate), Brit. Patent 272,993 (June 2, 1927). Walker, W. W. (to MacAndrews & Forbes Go.), U. S. Patent 1,219,509 (March 20, 1917). Weissenborn, A. (to Fabrik Chemischer Praeparate von Dr. Richard Sthamer), Zbid., 2,151,398 (March 1939). ’
%
,
1
RECEIVED for review November 25, 1955 ACCEPTED August 17, 1956
