US2853450A - Extinguishing agents for magnesium fires - Google Patents

Description

Sept. 23, 1958' s. 1. RlcHMAN ET AL 2,853,450

EXTINGUISHING AGENTS FOR MAGNESIUMFIRES.

Filed Aug. 15, 1955 Tuvf y!!! .prevention of 'reignition of the metal.

EXTINGUtsmG AGENTS non MAGNESIUM Frans Application August 15, 1955, Serial No. 528,266

6 Claims. (c1. 252-8) This; invention relates to liquid ycompesitions useful for extinguishmg magnesium res and fires of magnesium 7 'I alloys.

Magnesium and its alloys, as well as other metals, such as calcium, are extremely reactive. Since molten magnesiur'n reacts with water, this most common of extinguishing liquids cannot be used on magnesium res. Indeed, the reaction between magnesium and water produces hydrogen and the use of water adds the dangers-of explosion to those of the magnesium iii-e. Even exhaustion of V'available oxygen in the air is not suicient to extinguish inagnesiur'n tires, since nitrogen supports the combustion 'of the metal with the production of magnesium nitride.

Carbon dioxide cannot be used as the extinguishing agent, since it too reacts with magnesium. Carbon tetrachloride similarly is y(not a suitable extinguishingY agent because it reacts violently with the Vburning magnesium. Y

Because of the unsuitability of these commonrextinguishing materials, many of the agents whichr have been proposed have been solids. These generally depend on mixtures which exclude air mechanically. A mixture of potassium acid sulfate, trisodium phosphate, bauxite, and pumice issaid to fuse around the magnesium mass athigh temperatures, thereby excluding air. Another extinguishing agent consists of powdered graphite containing a `.small amount of high boiling liquid, Ytri(.ptertbutylphen.

yl.).phosphate. The vapor-ization of the phosphate in the interstices between graphite particles forces out 'the air, ywhile the graphite provides 'a cover over the vhot metal mass. Solid extinguishing agents are best suited Vto 'small fires, Where the powlderV Ycan b'e applieilto the burning Ysurface manually, but arenot Well adapted to large lires which cannot be closely approached. Sincegsolid extinguishing agents for magnesium must be coated over the Ventire exposed surface,- such agents are :usually not suitablefor use on large outdooriires.` Y Y VThe duid extinguishing agents which have vbeenpproposed may be divided into two classes. In the iirsthare A'those materials which protect kthe .surface magnesium against further progress of the oxidation, but which provide little or no cooling action.Y An example is boron trichloride liquid o or vapor. 'A second class n includes Yagents ywith high cooling capacity, butwhich offerhttle in the way of protection against further oxidation or ignition. High flashrpoint petroleum or fatty oils fallin this class. Y Y l A It is accordingly a'basic objectof this Invention to provvides nre-extinguishing compositions for magnesium lires which display a high coolingrc'a'pacity with good protection against additional oxidation of the magnesium and Itis another object of the invention to provide extinguishing'agen'ts in the form of liquids Whichare free-flowing and which can therefore be applied 'tores from t standard VVfire-fighting spray nozzles, pumps fand, similar equipment. Y

Patented sept. 2s, 195s Other objects and advantages of the invention will in part be obvious and in partappear hereinafter. p

The invention accordingly is embodied in a nre-extinguishing agent inliquid form, preferably of a low viscosity so that it is free-flowing like water or light lubricating'oil, the agent being a mixture of liquid substances with at least one ingredient from Veach of two groups, the iirst group of materials being characterized as non-aqueous organic compounds, which are capable of absorbing large quantities of heat and may bey defined as having a high heat absorbing capacity, preferably comparable atleast with that of ethyl alcohol, and the second lingredient being selected from a group consisting of `halogenated hydrocarbons which have the` characteristic that when applied to the burning metal, they will form halogen compounds of the metal, thereby forming a protective coating and even a fusible coating at the burning surface. t

The drawing is a diagrammatic representation of the apparatus used in evaluating the compositions of this invention.

Within the class of organic materials having a high heat capacity are a rather large group which includes ethyl and methyl alcohol and Vsimilar low molecular weight alcohols, esters of these alcohols and others such as di-Z-ethylhexyl phthalate for diisodecyl adipate,` fatty acid amides and amines, and high boilingethers, such as vdioctyl ether, which may be mixed with fluid halogenated hydrocarbons 'such as diuorodibromomethane and monochloromonobromomethane `(CHzCliSr.) Fundamentally, the compound having the high heat absorbing capacity is employed for the purpose of providing 'a rapid cooling ofthe burning magnesium 'andit isin a relatively non-viscous solution so that it can be easily applied to the burning mass of material. Since such'compounds can Ybe sprayed on the burning metal, their cooling capacity'is Vvaporize freely enough so that its heat absorbing capacity is effectively utilized. f

.In all re extinguishing applications, it is desirable to remove heat as quickly as possible, in order'to reduce the ilammable materials to temperatures below theirignition points. Water, as used lin ordinary fire extinguishing, is the'most eflicient heat absorber, because of its -high heat capacity and its very high heat of vaporization. jNo other common substance approaches its heat of vaporization of 540 calories per gram. Non-aqueous substances which are commonly used as vaporizing liquid Vextinguishers, such as carbon tetrachloride, derive Itheir efectiveness from the exclusion of oxygen, and* from breaking the chain of Areactions involved in combustion, rather than from the absorption of heat; carbon tetrachloride, for example, has a heat of vaporization of only 46 calories per gram. v

Since water cannot be used as a .heat absorber for magnesium, it is necessary to turn to organic liquids withrlhigh heat absorbing capacity which do not react violently with the metal. Of the fairly volatile common organic compounds, methanol and ethanol have 'the highest' heat for" vaporization, 263 and 204 calories per gram, respectively. Total heat absorbing capacity, however, depends not only `on the Vheat of vaporization, but onthe heat capacity of the liquid as well, since this Vvproperty determines'ahow much heat is absorbed in raising the liquid from ordinary temperatures to its boiling point. Thus a liquid with a very high boiling point may absorb more heat in attaining that temperature than is required to etect conversion from the liquid to the vapor state. With lower boiling compounds, onthe other hand, it is likelythat vaporization causes the absorption of more heatthan thewarming of the liquid to the boiling temperature.

By way of illustration, one pound of ethanol at 25 C. will absorb 16,000 calorieswhile` being raised to the boiling point, and 93,000 calories in vaporization, making a' total of 109,000 calories. One pound of di2 ethylhexyl phthalate at 25 C. will absorb an estimated total of 122,000 calories, 98,000 in attaining its boiling point of approximately 386 C., and 24,000 calories in vaporization. In the alcohol, the major part of the heat absorption is in vaporization, and in the phthalate in i heating to the boiling point.

Among the extinguishing agents of this invention, the materials of high heat absorbing capacity (group A) which we have shown to be effective as components of magnesium tire extinguishing agents include ethanol, methanol, di 2 ethylhexyl phthalate (DOP), dibutyl phthalate (DBP), diisodecyl phthalate (DIDP), diisodecyl adipate (DIDA), and di(ptertbutylphenyl)mono-' phenyl phosphate. These group A substances are examples of materials of high heating absorbing capacity, and the invention is not limited to these specific illustrations. For example an alcohol or ester or other organic cornpound containing a halogen or other substituent may be used because the essential property sought is high heat absorbing capacity. i

The heat absorbing substances which have been described are all ammable, although the compounds with high boiling points also have high ash points. One of the functions of the group B, the halogenatedhydrocarbon, component of our extinguishing composition is the control of the secondary re resulting from the ignition of the group A compounds. VFor example, the secondary lire which results from the application of a mixture of 50% DOP and 50% monochloromonobromomethane (CB) is much more moderate than that which is obtained with DOP alone; in fact asV the metal tire extinguishing proceeds and the magnesium is cooled, the presence` of CB in the agent causes the secondary tire to be extinguished as well. The presence of CB in this agent has, however, a second function: if the application of extinguishing agent is discontinued when the magnesium teml perature is still above the ignition point of approximately 650 C., nolreignition occurs. Presumably,` some CB reacts with the surface of the burning magnesium to produce a thin impervious protective lm of compounds involving magnesium and halogens. In the absence of CB or other halogenated hydrocarbon, reignition of the hot mass takes place; the magnesium oxide and nitride coating which form during combustion is too' porous to exclude additional air.

Although we normally use monochloromonobromomethane and difluorodibromomethane as group B substances, other halogenated compounds are also eifective.

The performance of magnesium tire extinguishing agents can be expressed in terms of the results of two re testing techeniques. The first test, designed to measure cooling eiciency, determines the quantity of agent necessary to reduce 15 pounds of burning magnesium to a temperature below its ignition point. Since the burning metal generally attains a temperature of about 1050 C., a drop of 440 C. is suicient to bring the temperature below 650 C. Thus M440 is defined as the weight oi agent necessary to reduce thetemperature of` 15 pounds of burning magnesium by 440 C., and thereforerepresents the quantity required for complete extinguishing of the charge. Y

In the drawing all elements of the apparatus used for making fxneutests are identified.

In the re test, 15 pounds of magnesium or magnesium alloy are ignited in a cylindrical cavity 11.5 inches in diameter and 6 inches deep, in slightly moistened sand. A chromel-alumel thermocouple, which has been buried in the earth, emerges at the center ofthe cavity, so that the bimetallic junction is 1.5 inches above the floor of the cavity. The magnesium is ignited by means of an oxyacetylene torch. As the metal melts and burns, it lls the cavity uniformly. Forced air may be used to bring the metal to a temperature in the neighborhood of 1050 C. more rapidly. When this point is reached, the extinguishing agent is applied in a slow stream or spray at a rate of about one pound per minute. The agent is distributed over the surface of the burning mass by manipulation of the delivery pipe. Because of the rapid heat transfer from theburning magnesium to the agent, the distribution is not critical. The temperature is measured while the agent is applied; from the known rate of application, M440 may be calculated.

To test for protection against reignition, `an essentially similar procedure is followed, lwith the exception that application of the extinguishing agent is discontinued when the temperature has fallen to 900 C., which is still 250 .above the ignition temperature of magnesium. Furthermore, the sand cavityis lined with a cylindrical steel shell (24 gauge) open at both ends; the additional rigidity is necessary to maintain an approximately constant metalair interface from test to test. The temperature measurements are continued after application has ceased. If there is adequate protection against reignition, the metal gradually cools. In the absence of such protection, the mass ,reignites, and the temperature increases. To put the results on a semiquantitive basis, it is convenient to cite the change in temperature twenty minutes after the application is discontinued; this figure is presented as ATM. The more negative the value of ATz, the more elective is protection against reigntion.

In Table I are given M440 and AT20 values for a number of extinguishing agents.

TABLE I Efjiciency of extinguishing agents on magnesium fires Agent vMlm (1b.) ATzo O.)

50% DIDP in CB 18 -270 50% DIDP in DFDB 24 -40 DIDP 11 |-20 50% DOPvill CB 20 -200 507 DOP in DFDB 2l -170 50 a DIDA in C -..1.-- 21 -190 50% EthanOl 111 CB 22 -170 *DFDB-Dinuorodibmmoniethane.

.to be self-extinguished as excess extinguishingagent was rapidly added. This does not occur in the absence of halogenated hydrocarbon.

Although all the mixtures of group `A and group Bl `compounds which are given in the table are solutions involving equal parts by weight of the two components, the invention is by no` means confined to this concentration. The compositions of extinguishing agents tabulated represent compromises between heat absorbing capacity, protection against reignition, and reduction in intensity of the secondary lire. Although a change in the ratio of group A and group B components can place the .emphasis on one or another of these properties, solutions `covering a wide range of concentrations are very effective re extinguishing agents for magnesium, as shown in VTable II, in which the proportion of denatured ethanol and CB is varied.

It should be noted that the group B halogenated hydrocarbon, CB, when used alone is ineffective as either a cooling or as a protective agent. In the absence of a group A heat absorber, the reaction between magnesium and CB proceeds at so great a rate that the liquid cannot be used as an extinguisher.

Whereas ethanol in the absence of CB has no protective effect, the addition of as little as CB is sufticient to impart some protection, as shown by the change of ATZ() from +60 to -80 C. When the CB content is increased to 50%, ATZ() becomes 170 C. M440 is lower at the lower concentrations of CB, however, since, in this mixture, it is the alcohol which provides the heat absorbing capacity.

Because the agents of this invention are solutions, it is possible to choose a solid substance from either group A or group B, if the other component is a liquid, and if the solid has the desired solubility in the liquid. For example, Aroclor 1268 (a chlorinated biphenyl marketed by Monsanto Chemical Company) is a solid, but is soluble in CB to the extent of more than 50%. In this case, the compound of high heat absorbing capacity also happens to be a halogenated hydrocarbon. The fire extinguishing agent consisting of 50% Aroclor 1268 in CB has an M440 value higher than the agents shown in Table I, but has the advantage that there is no secondary lire, because of the relative non-ammability of the Aroclor. The high M440 value is apparently due to the fact that the efficiency of the agent is reduced by the exothermic nature of the reaction between the Aroclor and the magnesium.

It is also possible to improve the eiiectiveness of the agents by using more than one group A or group B compound. For example, ditluorodibromomethane imparts somewhat less protection against reignition than CB, but gives more control over the secondary lire. A mixture of the two gives an overall improvement. Similarly, a combination of group A compounds may be desirable. For storage in cold climates, ethanol is a very desirable component, since it reduces viscosity, while DOP is useful because of its higher ash point. Thus a mixture of the two is indicated.

It will be apparent therefore that the liquid lire extinguishing composition may be varied from a small proportion of the organic cooling agent with a large proportion of the halogenated hydrocarbon to a large proportion of the organic cooling agent with a small proportion of the halogenated hydrocarbon. Our tests indicate a range from about 10:1 by weight in the first instance to about 1:10 by weight in the second.

Magnesium, magnesium alloys, e. g., those of aluminum and calcium, and other metals of similar reactivity, all will burn rather violently once they are started. Such i'ires can be extinguished using the agents described with results paralleling tho-se shown for magnesium. That is, in each case the metal is cooled to a temperature below its ignition point, the fre is converted to a secondary one, and that is readily extinguished.

Though the invention has been described with reference to relatively few examples, it is to be understood that variations thereof may be devised without departing from its spirit or scope.

What is claimed is:

l. A liquid composition useful for extinguishing rires involving burning metal comprising a blend of a iirst ingredient which is a phthalate ester characterized by its high heat absorbing capacity and a second ingredient which is monochlo-romonobromomethane, said second ingredient reacting with the metal to form a protective coating to impede further oxidation, the proportions by weight of said ingredients being respectively from about three parts of the first to one part of the second to one part of the first to three parts of the second.

2. A composition in accordance with claim 1 in which said iirst ingredient is diisodecyl phthalate.

3. A composition in accordance with claim 1 in which said rst ingredient is di-2-ethylhexyl phthalate.

4. A liquid composition useful for extinguishing res involving burning metal comprising a blend of a irst ingredient which is a phthalate ester characterized by its high heat absorbing capacity and a second ingredient which is monochloromonobromomethane, said second ingredient reacting with the metal to form a protective coating to impede further oxidation, the proportions by weight of said ingredient being about equal.

5. A composition in accordance with claim 4 in which said first ingredient is diisodecyl phthalate.

6. A composition in accordance with claim 4 in which said first ingredient is di-Z-ethylhexyl phthalate.

References Cited in the le of this patent UNITED STATES PATENTS 1,622,809 Rodman Mar. 29, 1927 1,796,857 Barnard Mar. 17, 1931 2,195,171 Herbline Mar. 26, 1940 2,208,162 Prutton et al July 16, 1940 2,362,767 Morgan Nov. 14, 1944

Claims (1)

1. A LIQUID COMPOSITION USEFUL FOR EXTINGUISHING FIRES INVOLVING BURNING METAL COMPRISING A BLEND OF A FIRST INGREDIENT WHICH IS A PHTHALATE ESTER CHARACTERIZED BY ITS HIGH HEAT ABSORBING CAPACITY AND A SECOND INGREDIENT WHICH IS MONOCHLOROMONOBROMOMETHANE, SAID SECOND INGREDIENT REACTING WITH THE METAL TO FORM A PROTECTIVE COATING TO IMPEDE FURTHER OXIDATION, THE PROPORTIONS BY WEIGHT OF SAID INGREDIENTS BEING RESPECTIVELY FROM ABOUT THREE PARTS OF THE FIRST TO ONE PART OF THE SECOND TO ONE PART OF THE FIRST TO THREE PARTS OF THE SECOND.

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