US2230100A - Smokeless powder - Google Patents

Description

Patented Jan. 28, 1941 UNITED STATES PATENT OFFICE SMOKELESS POWDER No Drawing. Application December 8, 1939, Serial No. 308,219

Claims.

This invention pertains to smokeless powder of low gravimetric density, and more particularly to such powder having a cell-like structure.

' The ballistic advantages of smokeless powders possessing a cellular structure have long been recognized by those skilled in the powder art. As a result, various methods for the manufacture of powders-of this general type have been developed. These methods usually involve the addition, to the explosive mass, of a stable body which is readily soluble in water, with subsequent dissolution of this substance after granulation of the smokeless powder. Thus, for example, inorganic salts such as saltpetre have been added as a constituent of the explosive mass, which is colloided by means of an ether-alcohol mixture and then granulated to the desired form, following which removal of the saltpetre-is effected by leaching the powder granules in hot water.

These known processes, however, are characterized by certain disadvantages, because they are uneconomical, both from the standpoint of complexity of control of operation as well as difliculty of recovery of ingredients. Thus it is 25 known that, if the freshly-cut smokeless powder grains be accorded the solvent recovery treatment deemed normal for the production of a pressed and cut powder, the cellular formation of the powder is reduced to a minimum. In addition, the shrinkage of the powder efiected as a result of the removal of solvent renders it exceedingly diflicult to leach out the stable soluble material, because it is firmly encased within the relatively dry nitrocellulose. Conse-' quently, if the recovery of the solvent from the green powder be deemed essential, it becomes necessary to incorporate an excessive quantity of soluble material therein, in order to cause a decrease in the gravimetric density of the pow- 40 der. In turn, this excessive quantity of soluble solid can be removed from the hardened grain only with increased difficulty, so that the recovery of said solid becomes an onerous and expensive procedure, if not a completely imprac- 45 ticable one.

The present invention has as one of its objects a novel method for the production of a smokeless powder having a cellular structure. Another object is a facile and economical process for pro ducing cellular powder. A further object is a method for producing a cellular powder, which method permits recovery of the solvent employed for colloiding said powder. Further objects will become apparent as the invention is is described hereinafter.

We have found that the foregoing objects can be accomplished by the process of our invention, which comprises colloiding nitrocellulose by means of a solvent having a boiling point of at least 100 C., granulating said colloid,- and dis- 5 placing and removing the solvent from said grains by heating in the presenlce of water. Preferably, the solvent employed is substantially immiscible with water. By selecting a solvent of this type'we are able to regulate readily the rate of displacement of solventfrom the freshly grained powder. That is, we employ a solvent of lower vapor pressures at the temperatures occurring under ordinary atmospheric conditions than those of solvents commonly used for the production of a pressed and cut smokeless powder. This selection has the effect of reducing the solvent loss during the colioiding of the powder, said colloiding including the mixing, macerating, blocking, and pressing operations. The reduction in solvent loss is quite important in view of the fact that it permits more exact formulation while insuring the production of a colloid of the proper consistency. Likewise, since-the amount of solvent present in the powder after it has been pressed and cut is a. factor in the cell formation, it is highlydesirable to be able to predict the total quantity of solvent present at this state of the operation. In greater detail, our process is as follows: I Nitrocellulose and the various other constituents aretreated with a predetermined quantity of the low vapor pressure solvent, for example, isobutyl propionate, and the mass thoroughly intermixed by means of equipment and in the manner currently employed for the production of smokeless powder. The colloided mass is then extruded through dies of proper size and the extruded strings [are granulated to the desired length. The smokeless powder grains are then 40 introduced into a still, water is added, and the mixtureis. agitated vigorously. while the temperature is gradually increased. The rate oftemperature rise, of course, is not fixed but is determined empirically for each type of powder. The degree of cell formation desired, the solvent employed, and the size of the grains. are among the factors to be considered in establishment of rate. of temperature rise. Sumcientheat is supplied to cause distillation of the solvent and water, which are removed from the system. Where the solvent and water are immiscible, separation of substantially all the solvent from the condensate presents no difliculty.

Rather numerous solvents for nitrocellulose l are suited for use when producing cellular powder in accordance with the present invention. Among these may be mentioned isobutyl propionate, the solvent mixture known to the trade as Pentacetate, mesityl oxide, n-butyl acetate secondary butyl acetate, isobutyl acetate, n-butyl propionate, amyl formate, amyl acetate, and the like. Well-known volatile solvents such as acetone, mixtures of ethyl alcohol and ether, methyl alcohol and the like are not within the purview of the invention, because their vapor pressures at temperatures occurring under ordinary atmospheric'conditions are too high, with the result that solvent loss during admixture of the powder constituents is excessive, thereby giving rise to the difliculties set forth in the foregoing. We prefer to employ solvents for nitrocellulose which are substantially immiscible with water, because the cell structure is formed by heating the, solvent-containing powder grain in the presence of water. In using a substantially waterimmiscible solvent, it is possible to control accurately the removal of the solvent from the powder grain, this result being more difficult to obtain in the case of a water-miscible solvent because of the extraction action which water exerts on such solvent.

It will be appreciated that water is used in the formation of the cell structure in the powder grain because it makes an excellent medium for the controlled displacement of the solvent. Likewise, various aqueous solutions are adapted for use in conjunction with the present invention. Thus, an aqueous solution of potassium sulfate may be employed advantageously where it is desired to add lesser amounts of this ingredient to the smokeless powder grains.

The use of solvents of low vapor pressure does not preclude the employment of solid stable substances as an aid to the formation of cells. Instead, such solid materials as sugar, saltpetre, barium nitrate, and the like can be employed advantageously when the rate of the removal of solvent is controlled. When said solids are employed as aids in the cell formation of the powder grain, we prefer to remove them simultaneously with the displacement of the solvent, although they may be removed separately by leaching the powder in water before the solvent removal treatment, provided a water-immiscible cell-forming solvent is employed.

In order to describe our process more clearly, reference is made to the following examples. They are, of course, cited as specific embodiments and are not intended as limiting the invention.

Example I 225 pounds of a nitrocellulose blend having a nitrogen content of 13.5 per cent., and 11.25 pounds of sugar sieved through a standard 150- mesh screen were charged into a dough type mixer. There was then added thereto a mixture comprising 83 pounds of isobutyl propionate, 90 pounds of toluene, and 1.35 pounds of diphenylamine, the toluene being present because of its power as a diluent. These substances were thoroughly mixed and blocked, following which the resulting partially colloided mass was extruded through dies having openings of 0.041 inch, the

powder string then being cut 200 cuts-to the inch. These powder grains, together with about 2800 pounds of water were then charged into a still and the green powder-water mixture agitated both mechanically and by means of air, while the temperature was brought to 100 C,

and there maintained for a period of 8 hours. At the end of this time, the powder was washed with water in order to remove the sugar which had been separated from the powder grains. The powder charge was given a preliminary drying and then glazed. Analyses of the finished powder revealed that it contained about 0.6% sugar. The gravimetric density of the finished product was 543.

The ballistic performance of this powder may be observed by an examination of the results obtained in comparison with the performance of a powder adopted as a standard for 22 calibre (short) ammunition, said standard powder having been produced according to known methods. The various components of the shot charge were, of course, identical for all the ballistic tests made. In the following tabulation, A represents the standard powder, whereas X represents the powder set forth in the foregoing example.

It will be appreciated, therefore, that the cel- I lular powder produces desirable velocity-pressure relationships and possesses advantageous characteristics such as low weight of charge, complete burning, and the like.

Example II 225 pounds of a blend of nitrocellulose having an average nitrogen content of 13.00% were charged into a mixer, and, after agitation of this nitrocellulose was commenced, there was added thereto a-mixture of 85 pounds of isobutyl propionate, pounds of toluene, and 1.35 pounds of diphenylamine. After these ingredients had been thoroughly intermingled, the mass was extruded through dies of .041" diameter, and the resultant string cut 150 times to the inch. The green powder grains, together with about 3000 pounds of water, were then charged into a still provided with means for both mechanical and air agitation, and the mixture heated up to 55 C., while agitating. This temperature was maintained for 3 hours and then it was increased to 75 C., said temperature being maintained for 6 hours, following which the temperature was raised to C. and maintained there for 6 hours, at which time the solvent displacement and removal was complete. then separated from the water, said powder being dried and glazed. Tests showed that it had a gravimetric density of 540, and that the ballistic performance was quite satisfactory.

The foregoing example illustrates the formation of cellular powder without the incorporation of solid substances such as sugar, saltpetre, and the like in the powder composition, and then. subsequent removal by leaching in water. Instead, the cellular structure is obtained by displacement and removal of the low vapor pressure solvent.

It will be understood that the rate at which solvent can be removed from a pressed and cut powder is limited, because too rapid a removal causes distortion of the powder grains. over, regardless of how rapidly the solvent is removed in the normal solvent recovery process, no

The powder was" the displacement and removal of the solvent in accordance with the present description aifects the cellular structure is not entirely understood. However, it is believed that when an aqueous medium is employed in conjunction with the particular nitrocellulose solvents, said medium, by displacing said solvents, prevents to some extent the shrinkage that would otherwise occur. After a substantial percentage of the solvent has been removed, the material becomes sufficiently rigid and set so that subsequent removal of the solvent does not affect the'cells formed. This explana-- tion, however, is merely a theory, and we do not intend to be bound thereby.

Our process permits a controlled displacement and removal of solvent without causing distortion of the finished powder; hence consistent ballistic performances are assured. The solvent originally introduced into the powder can be recovered, except for solvent losses, which are considerably lower than is deemed normal for the manufacture of pressed and cut powder because they have lower vapor pressures at atmospheric temperatures than. do the solvents employed heretofore. When a solvent immiscible with water is employed, it can be recovered without resorting to expensive and complicated fractionation processes. Another important advantage which results when these high boiling point solvents are employed is the improvement wrought in the cutting operation. When solvents such as ether alcohol mixtures, acetone, and the like are used for colloiding the nitrocellulose, the loss of solvent during the different colloiding steps, that is, mixing, macerating, blocking, and pressing, is high because of the volatility of said solvents. Likewise this loss of solvent is reflected in the degree of porosity of the finished grain. Consequently, the string of colloid extruded from the press and being subjected to the cutting machine often has dried out sufiiciently to render cutting rather difficult. In such cases, the severed grain is not clean cut, but ragged and irregular. Such grains do not meet the specifications required and hence must be reworked as scrap material. Obviously, this disadvantage cannot be met satisfactorily by increasing the spirit or scope thereof.

solvent content, because such increase would alter the size of the finished grain. The adoption culty, because the low volatility of solvents such as isobutyl propionate necessarily means that 501- vent loss during the colloiding operation is correspondingly lower than in those cases where acetone or the like is used.

It will be evident to those skilled in the-smokeless powder art that variations may be made from our process without departing from the For example, in certain cases, nitrocellulose dehydrated with ethyl alcohol may be employed without serious disadvantage. The high boiling point solvents are added to the ethyl alcohol-containing nitrocellulose in the quantity found essential to yield a colloid of proper consistency. Upon removal of the high boiling point solvent by heating with water, the ethyl alcohol is also removed. We intend, therefore, to be bound only in accordance with the following claims.

We claim:

1. A method for producing a cellular smokeless powder, which comprises colloiding nitrocellulose by means of a solvent having a boiling point of at least 100 C., granulating the colloid to grains of definite dimensions by pressing and cutting, and then displacing and removing the solvent from said grains by heating in the presence of water.

2. A inethod for producing a cellular smokeless powder, which comprises colloiding nitrocellulose by means of a solvent having a boiling point of at least 100 (Land substantially immiscible with water, granulating the colloid to grains of definite dimensions by pressing and cutting, and then, displacing the solvent from said grains by heating in the presence of water, and removing the solvent vapors evolved.

3. The method of claim 2, wherein the solvent employed is isobutyl propionate.

' 4. The method of claim 2, wherein the solvent employed is secondary butyl acetate.

5. The method of claim 2, wherein the solvent employed is n-amyl formate.

JOHN HENRY AARON. J AIVIES JOSEPH McINTYRE.