US2498388A - Explosive composition - Google Patents

Description

Rttented Feb. 21, 1950 UNITED STATES 2,498,388 EXPLOSIVE COMPOSITION Alpheus M. Ball, Blacksburg, Va., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War No Drawing. Application June 8, 1945, Serial No. 598,420

3 Claims.

This invention relates to an improved propellent powder and to an improved method for the preparation of propellent powders. More particularly, the invention relates to an improved method of producing smokeless powder contain-v ing noncolloidable, nonexplosive constituents, and to the improved powdersproduced-thereby.

Powders produced for use in guns must release their useful energy during the very short interval in which the projectile is being moved through the barrel of the gun. It is necessary, therefore, that these powders be capable of burning rapidly and of developing relatively high pressures under normal conditions of use.

Powders produced for use in jet-propulsion devices, such as rockets and the like, wherein the propulsion is obtained by the rapid escape of gaseous products of explosion of the powder through a narrow orifice in the rear of the device, should yield their useful energy in a short time, also, as

this type of burning aids in minimizing dispersion of the devices and accomplishes other desirable effects. However, because of practical limitations on the weight and resulting strength of the powder chamber, the pressure developed by the jet-propulsion propellent charge must be kept relatively low. Thus, slower-burning forms of powder are required for this use.

An important way of increasing the burning time of smokeless types of powder is to mold or extrude or otherwise shape the powder, in colloided form, into large grains which afiord a relatively low ratio of surface (burning) area per weight of powder.

An important type of propellent powder for use in guns has been the double-base nitroglycerin/nitrocellulose powder colloided by the use of volatile solvents. The use of these powders, in the form of large grains, in rockets and the like has not been satisfactory in a number of respects. One important fault has been that instead of generating a substantially constant pressure throughout the interval of burning, the charges have developed a marked pressure peak of short duration immediately after ignition. Consequently, the rocket chambers have had to be built stronger and heavier than otherwise would have been necessary. Also, it has been observed in firings with the propellent charges relatively cold, that the burning has progressed in irregular surges or spurts. The fluctuation in thrust produced thereby has tended to make the rockets unstable in flight and, consequently, poor in accuracy.

It has been known to incorporate certain salts 2 in smokeless powders, which are to be used in large caliber guns, in order to reduce the flash produced at the muzzle end of the gun. In such cases, of course, the purpose is to prevent ignition of the powder gases as the high pressure is released and combustible material contacts the air. In the case of many of the salts used for the purpose of reducing muzzle flash, the decomposition of the salt during the early stages of the ignition results in cavities within the powder grain, thus increasing substantially the burning area within the powder and insuring complete burning'of the powder before it reaches the muzzle of the gun. In the case of jet-propulsion devices the conditions of burning and the like are quite different,

as has been indicated above. Furthermore, no attempt is made to reduce the flash.

Now, in accordance with thisinvention an improved large grain propellent powder has been discovered which comprises a noncolloidable, nonexplosive material in a highly subdivided form uniformly incorporated throughout the matrix of a double-base smokeless powder colloid. Also, in accordance with this invention a method of producing such propellent powder has been discovered. 7

It has been found that the hereinbefore-discussed faults of jet-propulsion powders can be reduced by incorporating in the powder a small percentage of certain noncolloidable, nonexplosive ingredients, such as potassium nitrate or "potassium sulfate.

The presence of such salts in the propellent increases the burning rate and makes it more uniform. Thus, the initial pressure peak is eliminated, and the weight of the charge which can be used safely in a combustion chamber of a given design is increased. This extends the range of the jet-propulsion device. Also, because the burning is more uniform, the accuracy of the device is improved. Furthermore, the nature of the burning is otherwise desirably altered, for the temperature below which spurt burning or chuffing occurs isreduced materially.

The salts incorporated in propellent powders to reduce flash have been of commercial grades having the common powdery or finely-granular, crystalline form. Now, although the incorporation of these salts, in the same form asused in making flashless smokeless powders, gives advantageous results in jet-propulsion powders, the resulting finished powders are not entirely satisfactory, for they are subject to a type of heterogeneity shown by the presence of numerous small voids throughout the powder matrix. When powder grains containing such voids are burned, the burning successively uncovers the voids, which when reached present to the flame a surface area greater than that for which the propellent and projectile were designed. The burning rate becomes excessive and irregular, and the flight of the projectile is unpredictable. In the event the pressure becomes too high the rocket chamber bursts.

Now, in accordance with this invention, an improved powder is made by the method which, generally described, comprises reducing a noncolloidable, nonexplosive salt 1301a degree of fineness such that at least 80% by "weight of the salt has a particle mean diameter of less than 60 microns, the remainder consisting of particles such that the maximum diameter is less than 80 microns, and uniformly incorporating the highly subdivided salt, in small percentage, in the matrix of a colloided nitrocellulose-nitr-oglycerin powder, the salt being mixed with the nitrocellulose and/or nitroglycerin prior to completely colloiding the nitrocellulose-nitroglyceri.nmixture.

The method and resulting product produced thereby in accordance with this invention are illustrated in the following examples, which are given for the purposes of illustrating the invention and showing the improvement therein over the art.

Example 1 As a basis of comparison a batch of double-base smokeless powder was prepared according to the following procedure: 1345 parts of nitrocellulose (13.22% N) was wet with 275 parts of 23 alcohol and placed in a sigma-blade mixer together with 5 parts of diphenylamine, 25 parts f ethyl contralite, 125 parts of potassium sulfate of the fineness commercially supplied and '61 parts additional of 2B alcohol. (All parts "are by weight.) Particle size measurements of the potassium sulfate gave these results expressed as per cent by weight: 1%, 0-10 microns; 25%, 10-501riicrons; 60%, 50-100 microns; 14%, 100-300 microns. The ingredients were mixed for 5 minutes, then 1000 parts of nitroglycerin and 264 parts of acetone were added. Mixing was continued for 4 hours, after which the powder was extruded in a 4-inch press through a LOGO-0.312 inch die, and'cut into 5.25-inch length grains.

. Within 24 hours after extrusion of the powder, numerous small voids, about 30 per square inch, were seen just beneath the surface of each grain. These voids were approximately 1mm. long, and 0.3 mm. in diameter.

Ea'ample 2 A double-base smokeless powder was prepared identical to the one described in Example 1, except that 125 parts of potassium sulfate of the fineness supplied commercially was ground with 1.25 parts of lead stearate (antis'etting agent) for 48 hours in a ball mill prior to incorporation in the powder mix. The particle size of the ground potassium sulfate was measured as follows: 30% by weight, 0-10 microns; 68% by weight, -50 microns; 2% by weight, 50-100 microns.

After extrusion, the powder grains were dried for 2 days in air at20 C., then for 14'daysin air at '50-55 C. to remove excess solvents. 'No voids formed during'drying.

Example 3 A double-base smokeless powder was prepared identical to'the o'ne'described in Example 1 except that the 125 parts of potassium sulfate of the I fineness supplied commerciall was well blended with 1.25 parts of lead stearate prior to incorporation in the powder mix. The particle size of the potassium sulfate was of the same distribution as that used in Example 1.

Within 24 hours after extrusion of the powder numerous small voids, about 30 per square inch, were seen just beneath the surface of each grain. These Voids were similar in every respect to those obtained in Example 1.

Example 4 A double-base smokeless powder was prepared substantially identical with the powder of Example 2. However, in this example, the potassium sulfate was prepared by mixing it first in a ball mill with 1.25 parts of magnesium carbonate for 1 hour, then grinding it in a Bantam Mikro-Pulverizer which was equipped with a 0.027-inch mesh screen and operated at a hammer speed of 16,000 R. P. M. Microscopic examination of the ground salt indicated that 90% by weight was less than 50 microns and the remainder between 50 and microns. The extruded grains were dried in air at 20 C. for 2 days and in air at 50 to-55 C. for 14 days. No voids'appeared in the :powder.

Example 5 A smokeless powder was prepared according to Example 4, except that in preparing the finelyground potassium sulfate, zinc oxide was used in this example instead-of the magnesium carbonate of Example 4. Microscopic examination indicated that by weight, of the potassium sulfate had a particle size under 50 microns, and the remainder between 50 and 80 microns. .No voidsappeared in the finished grains.

Example 6 The following ingredients were incorporated into a double-base smokeless'pow'der in the manner described in Example 1: 1220 parts of nitrocellulose (13.22% N), wet with 250 parts of 2B alcohol, 1000 parts of nitroglycerin, '5 parts of diphenylamin'e, 25 parts of ethyl centralite, 264 parts of acetone, 86 parts additional -of 2B alcohol, and 250 parts of potassium sulfate of commercial fineness (particle size measurements same as given in Example I). Mixing time was 3 /2 hours. (All parts areby weight.)

Within 24 hours-after extrusion of the powder, numerous small voids, about per square inch, appeared just beneath the surface of the powder grains. The voids were approximately 0.5 mm. long and 0.2 mm. in diameter.

Example 7 A smokeless powder was prepared identical to the one described in Exampleo, except that finely-divided potassium sulfate (see Example 2 for method of preparation and "particle size) was used.

Approximately "3 small voids per square inch appeared beneath the surface ofthegrains within 24 hours after extrusion. No additional voids appeared later. i

The improvement in the method and product thereof in accordance with this invention are shown in Examples 2, 4, 5 and 7 when compared with Examples 1, 3, and '6.

The powder in Example 3 is identical with that of Example 1 except for the addition of about 0.04% by weight of lead stearate. The results demonstrate that the lead stearate alone has -no beneficial effect with regard to the eliminationof voids in the finishedpowder.

The powder of Example 2 contained the same constituents as that of Example 3, and in the same proportions and having been mixed, colloided, and extruded in the same way. The only difference was that, whereas in Example 3, the potassium sulfate was of the degree of fineness normally used in making fiashless cannon powder and the resultant powder grain produced therefrom contained a large concentration of voids of appreciable dimensions, the powder of Example 2 contained finely-comminuted potassium sulfate of a much higher degree of fineness and the resultant powder grain produced therefrom was substantially free of voids. Examples 4 and 5 demonstrated a similar improvement for the same powder, except that magnesium carbonate and zinc oxide, respectively, were used as the antisetting agents in making the finely-comminuted salt. The powder of Example '7 demonstrated a similar improvement over the powder of Example 6, both of them being similar to those of the other examples except for a smaller proportion of nitrocellulose and about double the proportion of potassium sulfate, the potassium sulfate in Example 6 being the same in character as that of Examples 1 and 3, while that of Example '7 was the same-as that of Example 2.

Thus, in accordance with this invention, improved solvent-type, large-grain, thick-web. smokeless powders are produced by the method of finely comminuting noncolloidable, nonexplosive ignition-aid salts, such as potassium sulfate, potassium nitrate, barium nitrate, and the like, preferably in the presence of a minor proportion of an antisetting agent, such as lead stearate, magnesium carbonate, zinc oxide, and the like, and uniformly incorporating a minor percentage, in the order of 5%, of the finely-comminuted material with the powder constituents prior to complete colloiding, and substantially completely colloiding the colloidable constituents with the aid of volatile solvents to produce, after blocking, pressing and extruding, a large grain smokeless powder with a finely-comminuted noncolloidable, non-explosive material uniformly distributed throughout the powder matrix; after drying at large grain powder is obtained incorporating a finely-comminuted ignition aid and substantially free of voids.

The potassium sulfate used in the improved jet-propulsion powders prepared in the examples, was such that at least 85% by weight of the material was in the particle size range of to 50 microns, and in the remainder no particles had a mean particle diameter greater than 70 microns. Potassium sulfate of a fineness satisfactory for use in making thick-web, solvent-type, jet-propulsion powders should be such that at least 80% by weight of the material should have a particle size no greater than about 60 microns and the remainder should contain substantially no particles larger than 80 microns in diameter.

Although it is preferred to use potassium sulfate, other salts may be utilized in its place to regulate the ignition'characteristics of the powder. Similar advantages are obtained by reducing the particle size of such salts corresponding to the ranges given above for potassium sulfate.

The examples given included formulations containing and 10% potassium sulfate. The amount of salt used will usually be from about 1% to about 10%, however, larger amounts may be used, depending upon the purpose and characteristics of the salt and the powder.

The salts may be comminuted to the desired particle size either by ball milling'or grinding, or by any suitable equivalent method. In order to prevent caking of the finely-divided material prior to its use in powder manufacture, an anti-- setting agent or free-flowing agent, such as magnesium stearate, lead stearate, magnesium oxide, magnesium carbonate, zinc oxide or similar substances, may be added to the salt or noncolloidable ingredient before the latter is reduced to the desired fineness. The percentage of "free-flowing agent may range from about 0 to about 5%, but preferably from about 0.5% to about 2.0% by weight of the material to be reduced in particle size.

From the available facts, it appears that a plausible explanation for the improvement in accordance with this invention, is that crystals or crystal aggregates having a particle size greater than approximately 80 microns, when incorporated in colloided smokeless powder containing a volatile solvent, represent points of weakness in the powder which permit the colloid to separate and form voids when shrinkage stresses'occur during drying; whereas, crystals or crystal aggregates of particle size no greater than about 80 microns mean diameter do not present a large enough area of weakness for initiation of material separation upon drying.

It will be understood that the advantages of this invention will be obtained when other noncolloidable, nonexplosive ingredients are incorporated in the matrix of a colloided powder, particularly when a drying or solvent removal operation follows the pressing, molding or otherwise shaping, of the powder, provided the noncolloidable, nonexplosive ingredient is sufficiently reduced in particle size so that the particles do not provide surfaces of sufficient area to function as nuclei for the initiation of material separation.

This invention is particularly advantageous in the manufacture of jet-propulsion, solvent-type stick (large web) powder containing potassium sulfate or potassium nitrate. The powders described in the examples contained about 40% nitroglycerin, about 48.8% to 53.8% nitrocotton (13.25% N), 5-10% finely-comminuted potassium sulfate and small percentages of stabilizers and plasticizer. Such formulations are highly suitable for use in jet-propelled devices; however, it will be understood that other formulations may be used while obtaining the advantages of this invention. The invention may be applied in both double-base and single-base powders for guns as well as in powders for use in jet-actuated devices.

Where, in the specification and appended claims, the term thick-web is used in connection with a powder grain, it is meant to designate a powder grain with a web thickness of not less than about 0.20 inch. By "web is meant the shortest distance through the powder between substantially opposite powder surfaces that approach each other during burning. Where the term 23 alcohol is used in connection with the formulation of a powder it is meant an alcohol having parts by volume of ethyl alcohol, 5 parts by volume of H20, and one-half part by volume of benzene.

What I claim and desire to protect by Letters Patent is:

1. A thick-web powder grain of web thickness of not less than about 0.20 inch comprising a colloided mixture of nitrocellulose and nitroglycerin, and from about 1% to about 10% by weight of a finely divided salt selected from the group consisting of potassium sulphate, potassium nitrate and barium nitrate uniformly incorporated therein, at least 80% by weight of said salt having a particle size no greater than about 60 microns mean diameter and substantially all of the remainder having a particle size no greater than about 80 microns mean diameter.

2. A thick-web powder grain of Web thickness of not less than about 0.20 inch comprising a colloided mixture of nitrocellulose and nitroglycerin, and from about 1% to about 10% by Weight of finely-comminuted potassium sulfate uniformly incorporated therein, at least 80% by weight of said potassium sulphate having a particle size no greater than about 60 microns mean diameter and substantially all of the remainder having a particle size no greater than about 80 microns mean diameter.

3. A thick-web powder grain of web thickness of not less than about 0.20 inch comprising a colloided mixture of nitrocellulose and nitroglycerin, and from about 1% to about 10% by weight of finely-comminuted potassium nitrate uniformly incorporated therein, at least 80% by Weight of said potassium nitrate having a particle size nogreater than about 60 microns mean (1'- ameter and substantially all of the remainde having a particle size no greater than about. 8

microns mean diameter.

ALPHEUS M. BALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

Claims (1)

1. A THICK-WEB POWDER GRAIN OF WEB THICKNESS OF NOT LESS THAN ABOUT 0.20 INCH COMPRISING A COLLOIDED MIXTURE OF NITROCELLULOSE AND NITROGLYCERIN, AND FROM ABOUT 1% TO ABOUT 10% BY WEIGHT OF A FINELY DIVIDED SALT SELECTED FROM THE GROUP CONSISTING OF POTASSIUM SULPHATE, POTASSIUM NITRATE AND BARIUM NITRATE UNIFORMLY INCORPORATED THEREIN, AT LEAST 80% BY WEIGHT OF SAID SALT HAVING A PARTICLE SIZE NO GREATER THAN ABOUT 60 MICRONS MEAN DIAMETER AND SUBSTANTIALLY ALL OF THE REMAINDER HAVING A PARTICLE SIZE NO GREATER THAN ABOUT 80 MICRONS MEAN DIAMETER.