US 3153606 A
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3,153,606 AQUEOUS EXPLOSIVE COMPOSITION CONTAIN- ENG FLAKE ALUMINUM AND AMMONIUM NITRATE Cyril .i. lireza, Thorofare, NJ., and Charles H. Noreu, Aurora, Colo, assignors to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware No Drawing. Filed Nov. 13, 1562, Ser. No. 237,370 11 Claims. (Cl. 149-41) This invention relates to explosive compositions. More particularly, this invention relates to water-bearing explosive compositions free of self-explosive ingredients.
For many years water-soluble, inorganic, oxygen-supplying salts such as nitrates, chlorates, and perchlorates have been used in explosive compositions. Ammonium nitrate and sodium nitrate, in particular, find extensive use in commercial high explosives and blasting agents. Of necessity, such explosives often are stored under conditions of high relative humidity and are used under wet conditions. The hygroscopicity and water-solubility of the inorganic oxygen-supplying salts are undesirable characteristics which may lead to deterioration of the explosive compositions and to failures in use.
Because of its deteriorating action, extensive precautions are taken to exclude water from such compositions not only during formulation but also in packaging, storage and use, or at least to minimize the amount of water entering the compositions. Hence, these products have been packed in waterproof, closed metal containers; in highly water-resistant, laminated fiberboard containers; in heavily waxed paper cartridges; or in water-resistant plastic films.
One class of explosives has been made water-resistant or appreciably so by formulating them with nitrocellulose and explosive liquid nitrate esters such as nitroglycerin, nitroglycol and the like which, with nitrocellulose, form water repellant gel structures. These compositions are relatively expensive and sensitive, thus requiring storage under rigorously specified conditions.
Another method employed to protect substantially water-free explosive compositions against densensitization and failure under moderately severe conditions of exposure to water is to include in the compositions finely divided, water-dispersible starches and gums which on first contact wtih water form a paste in the outer part of the explosive, which paste retards the rate of water penetration into the interior of the charge and prevents segregation of the original components of the mixture and detonation failures.
Such efforts as described above have been directed toward excluding water from the explosive compositions, or minimizing the amount of water which enters the compositions, or minimizing the deleterious efiects of such water as might enter the compositions. More recently, however, as in US. Patent No. 2,930,685, the presence of substantial amounts of water has been recognized as being beneficial in certain explosives rich in ammonium nitrate and containing substantial amounts of a self-explosive such as coarse granular TNT. In addition to containing a substantial amount of self-explosive, said compositions detonate satisfactorily only in columns of relatively large cross section and, additionally, require the use of a suitable high explosive booster to insure elfective priming.
In spite of advantages associated with the use of waterbearing explosives, the difiiculties, drawbacks, and limitations characteristic of presently known water-bearing explosive compositions effectively define a need for a novel and improved explosive composition.
3,153,606 Patented Oct. 20, 1964 EQQ The recognized difiiculties of the compositions of the prior art may be overcome in accordance with the instant invention which provides a thickened, water-bearing, blasting explosive composition capable of propagating a detonation in columns having diameters as small as one-half inch in the absence of substantial confinement, said explosive composition comprising at least 45% by weight of oxygen supplying salts, at least two-thirds of said oxygen-supplying salts being ammonium nitrate and the balance being an oxygen-supplying salt selected from the group consisting of alkali and alkaline earth metal nitrates, ammonium perchlorate, and alkali and alkaline earth metal perchlorates, the total amount of oxygensupplying salts in the composition being such as to provide an oxygen-balance of between +10 and -10%; from about 0.5 to about 4% by weight of water-dispersible thickening agent; from about 2 to about 20% by weight of non-explosive fuel consisting essentially of carbonaceous material; and about 1 to about 8% by weight of aluminum powder; all solid components of said explosive composition being uniformly dispersed in a thickened substantially saturated aqueous solution of said oxygen-supplying salts, the water in the composition representing from about 10 to about 45 of the total weight of the composition, said explosive composition being free of self-explosive ingredients and having a density of at least 0.95 gram per cubic centimeter. V
The nature of the present invention will become more apparent from the non-limiting examples below which also illustrate the manner in which improved explosive compositions of the present invention can be made. Unless otherwise indicated, all parts are by weight.
EXAMPLE 1 This exemplifies a. typical composition of the instant invention and a procedure for its manufacture. The percentage composition as Well as parts by weight of ingredients used in its manufacture are given as Composition 1 in Table I.
A solution of 51.7 parts of ammonium nitrate and 15.2 parts of sodium nitrate in 22.1 parts of water is pre pared, the pH is adjusted to about 5, and the temperature is adjusted to substantially ambient room temperature. To said solution is added a mixture containing 4 parts of flake aluminum powder, 1.9 parts of guar gum, and 5.1 parts of powdered coal. The total mixture is stirred for about 5 minutes at room temperature, or until sufficient thickening occurs to hold the solids in suspension in the thickened mixture. The mixture then is pumped or extruded into polyethylene tubing of 1%" diameter closed at one end and, as filled, clamped shut by flat metal bands placed at approximately 8-inch intervals along the otherwise fully filled polyethylene tubing to form a series or string of linked cartridges having the conventional 1%" x 8" size. By severing the string of cartridges across the metal bands, individual sealed cartridges are separated from the string for packing or for use. Cartridges made of other materials, as Well as cartridges of other diameters and lengths, may be loaded equally well by the same general procedures and, in practice, may be preferred to the 8" long cartridges which are common for granular or semigranular explosives. Other compositions made in a similar manner also are shown in Table 1.
EXAMPLE 2 By a process similar to that used in Example 1, a number of compositions are made containing perchlorates, or other nitrates, in place of sodium nitrate. The composition of these explosives is shown in Table II. All of the explosive compositions shown in Table II are primed by a standard No. 6 blasting cap.
Table I Composition 1 2 3 4 5 6 6 8 Alum. powder, percent 4.0 6.0 2.0 4.0 4. 3 0 4.0 4. O Ammonium Nitrate, percent 51. 7 50. 0 62.0 70. 4 34.0 61 0 41.0 63.0 Sodium Nitrate, percent 15.2 40. 0 20.0 11.0 Water, percent 22. 1 41.0 30.0 20.0 16.0 31.0 29.0 15.0 Guar gum, percent 1. 9 3.0 3.0 2.0 1. 7 2.0 2.0 1. Coal powder (through 50 mesh), percent. 5. 1 3.0 3.6 7. 3 3.0 4. 0 5. 5
Oxygen balance, percent 1. 3 +1. 3 0. 5 1.0 1.0 0. 5 +1.5 1. 7 Density, g./cc 1. 25 0.98 1. 04 1.33 1.43 1. 21 1.26 1.34 Impact sensitivity +56 +56 Triton value ('IN'I=10)* 9.1 8. 8 8.3 10. 4 8 2 9. 4 8. 3 10. 1 No. 6 Cap (Scnsit. Test.) (114 D. ctg.) Det. Det. Det. Det. Det Det Det Det Det. Velocities, m./sec.:
(1% D. ctg.). 3, 400 8, 700 2, 700 3, 500 3, 200 3, 500 3,100 3, 400 D. ctg.) 2, 500 Min. dia. for det. (unconfined) 9g" The Triton Value (TV) is an expression of the strength of an explosive with reference to the strength of pure trinitrotoluene (triton) as measured in a ballistic mortar.
The apparatus and testing procedure used are described in the U.S. Department of Commerce, Bureau of Mines Bulletin 346 titled, Physical Testing of Explosives.
Table II Composition n} 9 10 11 12 I 13 Aluminum powder, percent 4 4 4 4 4 Ammonium nitrate, percen 52 52 52 52 52 Ca(NOa)2-3H2O, percent Water, percent. Guar, percent- Powdered coal Sensitivity to No 6 cap Density, g./cc
On an anhydrous basis, equivalent to 15% net of metal nitrate; balance added to water content.
As an alternative in the procedure of Example 1, the guar gum or other thickening agent may be added separately to the solution of ammonium and sodium nitrates and the solution agitated until thickening occurs, after which the other ingredients are added and thoroughly mixed into the thickened nitrate solution. As another alternative, all of the solid ingredients may be mixed togcthcr and the water added to said mixture, after which agitation is continued until the solids are uniformly dispersed and sufiicient thickening occurs to hold the solids in suspension.
Solutions of nitrates may be utilized in the preparation of said explosive compositions. Thus, the ammonium nitrate solution known as neutral liquor often is available at explosives manufacturing plants and may be used in place of solid ammonium nitrate and water. Likewise, solutions of sodium nitrate; other alkali metal and alkaline earth metal nitrates; and ammonium, alkali metal and alkaline earth metal pcrchoratcs may be used with the ammonium nitrate, and are considered to be within the scope of this invention. The well-known hygroscopicity of some of these nitrates and perohloratcs presents no problem in explosive compositions of the instant invention since the explosive compositions include substantial amounts of water, and are packaged and used in such a manner that substantially no water is lost from the compositions. Generally, however, sodium nitrate (from 0 to by weight) is preferred for use with ammonium nitrate in compositions of the instant invention because use of these nitrates results in the lowest cost, effective explosive compositions. If solutions of salts instead of the solid salts are used in the manufacture of explosivecompositions of the instant invention, the amounts and concentrations of said solutions must be adjusted to give an overall gross composition corresponding to the percentages specified herein.
In the compositions of the instant invention, at least about 0.5% and preferably about 1% of a thickening agent is required to prevent solids from segregating from the mixtures. The compositions exemplified in Tables I and II employ commercial grades of powdered guar gum as thickening agent. Locust bean gum or karaya gum may be used in place of guar gum, the amount being adjusted to attain the desired degree of thickening. If desired, orosslinking agents may be incorporated into the compositions with the vegetable gums to increase the viscosity of the resulting mixtures.
Guar gum and other thickening agents provide some of the carbonaceous fuel. Additional non-explosive solid carbonaceous fuels, however, are required for compositions of the instant invention. Powdered coal, as exemplified in the tables, is a preferred carbonaceous ingredient because of its ready availability and its relatively high density. Other carbonaceous materials also may be used such as starch, finely divided parafiin wax, powdered polyethylene, calcium stearate, ground ivory nut meal, ground oat hulls, ground rice hulls, apricot seed meal, walnut shell meal, petroleum and the like. In order to minimize formation of noxious gases in the detonation products, the relative amounts of ingredients are adjusted so that the oxygen balance of the explosive composition lies between 10 and +10%, but preferably is close to zero, as exemplified in Tables I and II.
Oxygen balance is an expression which describes the relationship between the oxidizing and reducing components of an explosive or of an explosive composition. As pointed out by Meyer (The Science of Explosives, published by Thomas Y. Crowell Co., New York, 1943, pp. 99 and 100), an explosive like trinitrotolucne (TNT) which does not contain enough oxygen to give completely oxidized final reaction products is said to have an oxygen deficiency (or to have a negative oxygen balance). On
the other hand, nitroglycerol has an oxygen excess, i.e., it has a positive oxygen balance. Both substantial oxygen deficiency and oxygen excess may represent losses in explosive power. Commonly, explosive compositions comthe data of Table 111, where compositions 3 and 4 are of high-aluminum-content type.
The relatively high negative oxygen balance of highaluminum-content compositions 3 and 4 is apparent in prise mixtures of fuels and oxidizing agents which are ad- 5 Table III. The detonation test results for compositions justed to bring the oxygen balance to some desired level 1 to 4 illustrate that inclusion of flake aluminum powder which usually is expressed as the percent excess contributes to the sensitivity of both highand lowor deficiency of oxygen as related to that chemically aluminum-content compositions, although much less of required for complete combustion (i.e., zero oxygen balthe relatively more expensive flake aluminum is required ance). Thus, TNT has a negative oxygen balance, exfor sensitization of compositions of the instant invention pressed as 74%. The distribution of the oxygen among than of the compositions of the prior art. The hidden the products of the explosion reaction, as is well known, hazard characteristic of the high aluminum content comis related to the chemical reactions which take place under positions is disclosed by examining the gaseous products explosion conditions. But an explosion reaction does not produced by detonation of compositions of both types. always lead to the same end products. Common, stable This quantitative examination is possible by use of the end-products of combustion such as carbon dioxide and Bichel Pressure Gage as described in US. Department of water may not appear in the expected amounts in explo- Commerce, Bureau of Mines Bulletin 345, and the prosion systems which contain limited amounts of oxygen cedure described therein except that, to avoid pumping and fuel components other than carbon and hydrogen; oil water and to insure complete detonation of composiother important end products of the explosion reaction intions of both types, each charge is fired in a Bichel bomb stead may appear in the explosion gases. Although, confilled with pure nitrogen gas at atmospheric pressure, ventionally, the calculation of oxygen balance even for rather than being fired in a bomb evacuated to a low such systems still is based on complete combustion, the no residual pressure. The nitrogen originally present in the tual products of the reaction must be examined experibomb then is allowed for in making the mass spectrommentally and evaluated independently. eter analysis of the gaseous detonation products. The
It is preferred that the aluminum powder used in comdetonation products from a preferred composition of the positions of the instant invention be a finely divided flake prior art (No. 4 in Table III) contain about 75-80% of form of aluminum such as conforms to the requirements hydrogen gas, equivalent to the generation of about 6.3 of specification ASTM D-962-49, Type 1, Classes A, B, standard cubic feet of hydrogen for each pound of charge or C, or of Federal Specification TT-A468a, Type I. which is detonated. This quantity of hydrogen if dis- Aluminum powder of the size and shape but free of the charged into an air-filled mine tunnel Would provide stearic acid or other coating agent shown in said speciflammable hydrogen airmixtures in a tunnel volume up fications also may be used in compositions of the instant to about 100 cubic feet (6% minimum lower concentra- Table III Composition No 1 2 3 4 5 6 7 8 9 Composition:
Flake Aluminum, percent 4. 4 4.0 4.0 4.0 8. 0
Atomized Aluminum, percent.
Ammonium Nitrate, percent 52. 5 62. 5 51. 7 40. 9 50. 6
Sodium Nitrate, percent.-. 14.4 10. 8 15. 2 19. 6 14. 6
Water, percent 21.0 15.7 22.1 28. 5 21. 3
Coal Powder (through mesh), percent. 5. 5 5.0 6. 0 5.0 3. 5
Guar Gum, percent 2.2 2- 0 2. 0 2.0 2. 0
Total 100.0 100.0 100.0 100.0 100.0
Oxygen Balance (3) Density, g./cc 1. 9 1.38 1.48 1.09 Ballistic Mortar TNT Value--. 0.3 4.0 11.3 10.2 9 8 as s s 6 9 Detonation Tests in 1% Diam. Ctgs.:
(a) Primed with No. 6 cap Dct. Failed Failed Det.
(b) Primed with 25 g. RDX Failed Failed Test 1 2 1 2 1 2 Gaseous Products on Detonation:
Vol. Percent Hydrogen in Gas prod 80 76 0.1 0.1 0.1 0.1
On. it. of Hydrogen generated per one pound of charge 6.8 5.9 .01 .01 .01 01 1 A commercial flake aluminum powder known as 3XD grade; maximum retained on a 325 mesh screen15%. 1 A commercial atomized aluminum powder known as grade 140; granular particles, 100% through 325 mesh screen, avg. size 4-6 microns. 3 Commercial product; some as used in compositions of Tables I and II.
invention. The content of aluminum powder will range tion of hydrogen in air saturated with water vapor; see
between 1 and 8% of the total weight of the composition, but preferably will be in the range of 2 to 5% by weight of the total composition.
The prior art teaches that water-bearing explosive compositions free of self-explosive ingredients, but containing a much higher percentage of finely divided aluminum in combination with a low percentage of a stable oxidizing salt, are feasible. However, the high aluminum content of such a composition creates a high negative oxygen balance. We have found that while such explosive compositions have desirable features, they do, on detonation, liberate substantial quantities of hydrogen; and hence their use under ground or in any confined space introduces the very great hazard of disastrous secondary gas U.S. Bureau of Mines Bulletin 503, p. 17), and a. violently detonating stoichiometric composition when mixed with about 16 cubic feet of air.
The presence of these large quantities of hydrogen in the detonation products of the high-alnminum-content compositions confirms that the aluminum not only reacts preferentially with oxygen available from the oxidizing salts and thus prevents formation of water, but also decomposes water originally present in the composition thereby liberating the large amounts of gaseous hydrogen which are found by analysis.
In marked contrast with the above results are those obtained by detonating explosive compositions of the instant invention (compositions 7 and 9 in Table III) in explosions. This is illustrated in quantitative manner by the Bichel bomb under strictly comparable conditions.
A completely innocuous and non-hazardous quantity of hydrogen is liberated, incapable of leading to secondary gas explosions in mine tunnels or other enclosed spaces.
The pH of the solutions of nitrate salts and the pH of the finished explosive compositions of the instant invention will be between 3 and 8. Outside these limits, the sensitivity of the compositions to initiation by commercial blasting caps is lessened owing to the tendency to gassing and blowing. The pH may be adjusted by adding suitable quantities of aqueous nitric acid or aqueous ammonia during the mixing operations. Preferably the pH of the finished explosive compositions of the present invention will be in the range of 4 to 6.
The method of packaging and type of package are not critical factors for the practice of the instant invention so long as the package or container is durable and prevents substantial loss of water during storage and use of said explosive compositions. The use of polyethylene containers has been exemplified, but other substantially water-impermeable films such as those made of polyvinyl chloride, polypropylene, polyethylene terephthalate, cellulose acetate, and the like may be used, as well as waxed or plastic coated paper shells, metal cartridges, and the like. The explosive compositions of the present invention likewise may be pumped directly into the borehole without being packaged at the point of manufacture.
Even the presence of water in the borehole will not prohibit use of an explosive of the present invention. Said composition, preferably of density greater than 1.0, can be pumped into the bottom of a borehole through a retractable tube to displace the water and fill the borehole with explosive to the desired depth, or a packaged composition may be used, especially if the borehole water is not at rest.
Explosive compositions of the instant invention mark a significant contribution to the explosives art with respect to safety in manufacture and use, freedom from undesirable physiological efiects, controlled variability of selected physical properties, and performance as blasting agents.
Several factors contribute to the outstanding safety characteristics of the explosive compositions of the present invention. The presence of a substantial amount of water and the absence of all self-explosive ingredients, and the absence of substantial quantities of hydrogen gas in the detonation products all contribute greatly to the inherent safety of the compositions. The compositions are surprisingly insensitive to mechanical shock as determined in a US. Bureau of Mines type drop test apparatus using a 5 kilogram Weight. Whereas compositions of the present invention are not initiated by a 56-inch drop (maximum for the equipment) of the weight, conventional nitroglycerin dynamites have a 50% detonation point at -15 inches drop. Compositions of the present invention will not ignite from flames nor will they burn unless the water is lost from the compositions. On the other hand, the explosive character conveniently may be destroyed completely by dilution or flushing the compositions with large quantities of water. No high explosive residue can segregate and accumulate, and after flushing away the oxygen-carrying salts, the residue will not be sensitive upon evaporation of the residual water.
Complete freedom from self-explosive organic nitrates such as nitroglycerin, nitroglycol, nitromannite, pentaerythritol tetranitrate, and the like represents an additional benefit for compositions of the present invention because said compositions are incapable of causing headaches to users who handle the explosive composition or are exposed to nitrate fumes remaining after a blast in a confined area such as a tunnel. As a further advantage, said compositions are easily oxygen-balanced so that the detonation products are substantially free of noxious gases such as carbon monoxide and oxides of nitrogen.
The consistency of the compositions may be varied broadly from a soft, pasty, viscous mixture to a moldable,
tough, plastic mass. The resulting handling properties will permit loading or packaging by pumping on the one hand, or by extrusion on the other, into cartridges which can be tamped to take the shape of and to fill the irregularities of the borehole. The attainment of the moldable tough plastic character is facilitated by including a small amount of cross-linking agent with the thickener, as noted above.
The excellent safety characteristics and good handling properties of the explosive compositions of the instant invention are accompanied by very favorable performance characteristics as blasting agents. Unlike some of the water-bearing compositions heretofore known, the present explosive compositions are dependably initiated or primed by the commonly available No. 6 and No. 8 blasting caps and, unlike the high-aluminum-content compositions heretofore known, are free of the hazard associated with secondary gaseous explosions. The compositions of the instant invention may be used in a wide range of diameters of the powder column. Thus, detonation is propagated through a column of explosive /8" in diameter without special confinement other than that afforded by the paper or plastic walls of the container, and a velocity of detonation of 2500 meters/ second is measured for a /2" diameter column of explosive, as shown in Table I. The velocity of detonation is observed to be somewhat greater for columns of larger cross-section. In blasting action, the explosive compositions of the present invention are comparable with 40% to 60% ammonia dynamites.
The combination of safety characteristics, good handling properties, and unusual explosive properties makes the compositions of the instant invention suitable for use in many applications, and especially for underground work, notably in blasting operations which traditionally or by preference use smaller diameter cartridges of explosives, generally in the range of /8" to 1 /2 diameters, and which heretofore have been served only by organic nitrate-containing blasting agents. On the other hand, the present compositions are not so costly as to be excluded from use in blasting operations in which explosive columns of much larger diameter conventionally are employed.
The compositions of the instant invention are very stable in storage. Being very low-freezing because of the high concentration of salts dissolved in the aqueous phase, the compositions when suitably packaged as de scribed above are not deteriorated by storage at elevated temperatures, above those encountered in field storage and use, for extended periods of time.
This application is a continuation-in-part of our application, Serial No. 775,103, filed November 20, 1958.
The invention has been described at length in the foregoing. it will be understood, however, that compositions according to the instant invention are susceptible of considerable variation without departing from the features of the invention, and the invention is to be limited only by the following claims.
What is claimed is:
1. A cap sensitive explosive composition comprising at least 45% by weight of oxygen-supplying salts, at least two-thirds of said oxygen-supplying salts being ammonium nitrate and the balance being oxygen-supplying salts selected from the group consisting of alkali and alkaline earth metal nitrates, ammonium perchlorate, and alkali and alkaline earth metal perchlorates, the total amount of oxygen-supplying salts in the composition being such as to provide an oxygen balance of between +10 and l0%; from about 2 to about 20% by weight of nonexplosive fuel consisting essentially of carbonaceous material of which about from 0.5 to 4%, based on the total composition, is water-dispersible thickening agent; and from about 1 to about 8% by weight of finely divided flake aluminum; all solid components of said explosive composition being uniformly dispersed in a thickened, substantially saturated aqueous solution of said oxygen- 9 supplying salts, the water in the composition representing from about 10 to about 45% of the total weight of the composition, said explosive composition being free of selfexplosive ingredients and having a density of at least 0.95 gram per cubic centimeter.
2. The composition of claim 1 in which the alkali metal nitrate is sodium nitrate.
3. The composition of claim 1 in which the alkaline earth metal nitrate is calcium nitrate.
4. The composition of claim 1 in which the alkaline earth metal nitrate is magnesium nitrate.
5. The composition of claim 1 in which ammonium perchlorate is an oxygen-supplying salt.
6. The composition of claim 1 in which the alkali metal perchlorate is potassium perchlorate.
7. The composition of claim 1 in which the alkaline earth metal perchlorate is magnesium perchlorate.
8. A cap sensitive explosive composition consisting essentially of materials which per se are non-explosive and comprising at least 45 by weight of oxygen-supplying salts, at least two-thirds of said oxygen-supplying salts being ammonium nitrate and the balance being oxygen-supplying salts selected from the group consisting of alkali and alkaline earth metal nitrates, ammonium perchlorate and alkali and alkaline earth metal perchlorates, the total amount of oxygen-supplying salts in the composition being such as to provide an oxygen balance between +10 and l%; from about 2 to 20% by weight of non-explosive fuel consisting essentially of solid carbonaceous material of which about from 0.5 to 4%, based on the total weight of composition, is water-dispersible thickening agent; and from about 1 to 8% by weight of finely divided flake aluminum having a particle size of less than about 325 mesh; all solid components of said explosive composition being uniformly dispersed in a thickened, substantially saturated aqueous solution of said oxygen-supplying salts, the water in the composition representing from about 10 to about of the total weight of composition, said explosive composition having a pH of about from 3.0 to 8.0 and a density of at least about 0.95 gram per cubic centimeter.
9. The composition of claim 8 wherein the carbonaceous fuel, in addition to thickening agent, is powdered coal.
10. The composition of claim 8 wherein the water dispersible thickening agent is selected from the group consisting of guar gum, karaya gum, and locust bean gum.
11. A composition of claim 9 containing about from 11 to 20% by weight of sodium nitrate, at least 1% by weight of a water-dispersible thickening agent selected from the group consisting of guar gum, karaya gum and locust bean gum, and about from 2 to 5% by Weight of flake aluminum powder, said composition having a pH of about from 4 to 6.
References Cited in the file of this patent UNITED STATES PATENTS 2,768,073 Davidson Oct. 23, 1956 2,860,041 Grifiith et al. Nov. 11, 1958 2,930,685 Cook et al. Mar. 29, 1960 3,052,578 Davis et al. Sept. 4, 1962