US2946673A - Manufacture of double-base propellant grains - Google Patents

Description

2,946,673 MANUFACTURE OF DOUBLE-BASE PROPELLANT GRAINS Vernon R. Grassie, Kennett Square, Pa., assignor to Hereules Powder Company, Wilmington, Del., a corporaiion of Delaware No Drawing. Filed Dec. 29, 1955, Ser. No. 556,058

7 Claims. (11. s2-.s

This invention relates to cast double-base propellant grains and to the manufacture thereof.

Relatively large grains 'of solid propellant suitable for propelling military rockets or for actuating sizeable jet devices such as jet-assisted take-oft units for airplanes are currently produced by one of several casting techniques, since solventless extrusion, although quite feasible for grains up to about 7 or 8 inches in diameter, requires excessively massive and expensive equipment to produce larger diameter grains. Briefly, the casting technique involves bringing together in a mold, with or without a beaker, depending on the type grain desired, predetermined amounts of a suitable single-base propellant casting powder in dense granular form and of casting liquid comprising nitroglycerin and suitable desensitizing plasticizers, and then curing the resultant mixture to produce a cast grain of double-base propellant. During the curing operation the casting liquid swells and colloidizes the granules of single-base propellant casting powder and consolidates and welds the whole mass in the mold into single-base'propellant, a s heretofore required. Other must then be subjected-'to removal of solvent before they'are suitable'for use as castingpowders.

Therefore, the primary object of this invention is to provide an improved and simplified method ofproducing cast double-base propellant grains, which eliminates the necessityof initially converting fibrous nitrocellulose into fully colloided uniformly shaped andsi zed grains of objects of the invention will appear hereinafter, the novel features and combinations being set forth in the appended claimsg f 7 ,Generally, the present invention comprises agitating fibrous nitrocellulose 'inan aqueous bath containing organic liquid solvent having active-solvent-power for said nitrocellulose,- the solvent being present in amount to soften and destroy the fibrous structure of the nitro- 3 cellulose without dissolution thereof, continuing agitation a solid unitary homogeneous mass conforming-to the shape of the mold. The casting is cured in the mold for a suitable length of time, usually at an elevated temperature on the order of 140 F., and after curing is ready for use in the rocket assembly for which it is designed.

When the grain design requires restricted outer surwith removal of substantially all the solvent until smooth,

hardened, densified and irregular particles of nitrocellulose are obtained, removing substantially all water from the. resulting densified nitrocellulose particles, bringing together in a mold predetermined amounts of the resultingdry densified nitrocellulose particles and of casting liquid to produce a solid unitary cast double-base propellant'grain, and curing the resulting'propellant grain.

More specifica1ly, in carrying out this invention, waterwet nitrocellulose, after stabilization treatment, is mixed and agitated with alarge quantity-of water and preferably asmallquantity of an organic liquid having active solvent power for nitrocellulose and having an' appreciable vapor pressure at or below the boiling point, of

water; The organic liquid solventis preferentially absorbedby the nitrocellulose whereby the nitrocellulose is softened and the fibrous structure thereof is destroyed. However, the nitrocellulose is not dissolved and the particles thus modifiedare hardenedand densified by boiling oil the solventor by dilution of themixture with water. Agitation is maintained until hardening is, complete to prevent: agglomeration. Substantially all r the remaining solvent isremoved from the nitrocellulose bytdistillation.

. The resulting smooth, hardened, densifiediand irregular faces, as for example, in case the grain is to burn only' from the ends or from axial perforations, it is customary to cast in a beaker. This beaker consists of a cellulose acetate plastic tube that is prefabricated by extrusion ,or wrapping on a mandrel. V t the mold before adding casting powder. During the curing operation the beaker and propellant fuse together, and the beaker becomes an integral part of the cured propellant grain. 7 V

The principal disadvantage of castingfmethods employed heretofore resides in the tedious and expensive process of producing suitable initial single-base propellant casting powders in dense granular form. It is well known, of course, that cast propellant grains cannot be manufactured directly from fibrous nitrocellulose because a great enough mass of fibrous nitrocellulose cannot be forced into the casting mold, and because the solvent action of the casting liquid on fibrous nitrocellulose is so rapid that dissolving and plugging occur before uniform distribution of casting liquid through the The beaker. is assembled to particles of nitrocellulose are then dried, and apredeter mined quantity 'of the dry dense particles, preferably A predetermined quantity of casting liquid comprising nitroglycerin and suitable desensitizing plasticizers, with or without stabilizers} and/or other adjuvants included to control orregulate burning rateof the resulting propellant'grain when 'fired, is then introduced into the mold to completely fill all voids between the particles of nitro-..

cellulose and to just cover the nitrocellulose particles. The mixture of densified nitrocellulose particles and casting ,liquidis, then cured in the mold, preferably at an elevated temperature, for a suitablelength of time, such whole mass can be achieved. Accordingly, it has been necessary heretofore to convert fibrous. nitrocellulose into small, dense, uniform, fully colloidized grains of single-base propellant of approximately 25 'mesh' size.

This involves alcohol dehydration of the water-wet fibrous I nitrocellulose, mastication of the dehydrated nitrocellulose with volatile solvents to form av completely colloided mass, extrusion of the uniformly colloided ma'ss'to form uniformly thin strands which are then cut'into uniform lengths. The fully colloided particles thus obtained j as, forexample, 96 hours at F. During the curing operation the particles of nitrocellulose absorb all of the casting liquid and, due to the, solvent action of the cast- I ing liquid .on the nitrocellulose, become swollen and fully colloidized, and thewhole mass in themold con solidates and welds together into a solid,-unitary, homogeneous colloided structure conforming to the shape of the mold. Aftercuring, the cast propellant grain is ready for use in the rocket assembly for which it is designed.

The'gene'ralnature of the invention having been set forth, the following examples are presented a's specific illustrations thereof. It will be understood, of course,

that the inventionis not limited to the examples butis susceptible to different modified embodiments which 7 come within the scope of the claims.

Patented July 26, 1960 V I 3 EXAMPLE 1 Five thousand grams (5,000), dry basis, of water-wet fibrous nitrocellulose of 12.6% nitrogen content, after conventional preliminary stabilization treatment, but before jordaning, was 'slurried with 45,000 grams of water in a 20-gallon jacketed vessel fitted with a condenser and a 12-inch diameter turbine agitator driven at 200 rpm. The mixture was heated to 91 C. by employing steam in the jacket of the vessel. Nine thousand five hundred ten grams (9,510) of methyl isobutyl ketone was then added to the heated aqueous nitrocellulose slurry under reflux conditions with agitation, requiring about 15 minutes. Distillation was then carried on with continued agitation for 90 minutes, during which time the distillation temperature gradually increased to the boiling point of water. Distillation with agitation was continued for an additional 140 minutes. Substantially all of the solvent was removed by distillation, and the nitrocellulose was transformed, without being dissolved, from its initial fibrous state into smooth, hardened, densified, irregular particles having a bulk density (dry basis) of 41 pounds per cubic foot. Nitrocellulose in its initial fibrous state, derived from either cotton linters or wood pulp, has a bulk density of about 8 to about 15 pounds per cubic foot.

The densified product thus obtained was screened to remove particles larger than 20 mesh, and that portion of the product which passed through a 20 mesh screen was tray dried in warm air at about 140 F. to remove substantially all water. The dried product contained substantially less than 1% total volatile by weight. Approximately 150 grams of the resulting dry densified nitrocellulose particles was then loaded into a tubular cellulose acetate beaker 12 inches long and 1 /2 inches inside diameter, suitably supported in a readily removable mold of rigid material equipped with a valved Vacuum line at the bottom and a valved casting liquid line at the top. The loaded mold was placed under pressure of less than mm. of mercury absolute for 16 hours. While maintaining vacuum on the loaded mold, casting liquid under a pressure head of 1 to 3 pounds per square inch gauge was introduced into the loaded evacuated mold until all interstices between particles of nitrocellulose were filled with casting liquid and some casting liquid had overflowed into the vacuum trap, whereupon the supply of casting liquid was cut ofi. Vacuum was-kept on -the mold during the entirecasting process, and when the casting was complete the casting liquid and vacuum lines were disconnected from the mold. The casting was then cured in the mold at 140 F. for 96 hours.

The finished casting upon removal from the-mold was hard and well consolidated, and contained approximately 150 grams of casting liquid, so that the ratio of nitrocellulose to casting liquid was essentially l to 1 ona weight basis. The composition of the casting liquid used was as follows:

Parts by weight Nitroglycerin 72 Triacetin (glyceryl triacetate) 27 Z-nitrodiphenylamine l The cured propellant grain after inspection was cut into 1% inch lengths which were fired successfully in a conventional vented vessel having a nozzle diameter of P; designates pressure taken one 'se'e'ond after ignition. P, designates pressure at midpoint of burning. P; designates pressure taken one second beibre end of burning.

The vented vessel employed in these tests and in other tests hereinafter set forth was a conventional stationary rocket motor equipped with automatic devices for making a continuous record in the form of a graph of pressures developed during burning as a function of time. Such vented vessels are standard equipment in all ballistic laboratories engaged in evaluation of rocket or gas generator solid propellant grains.

Attention is directed at this point to the fact that the nitrocellulose employed in this example was taken after preliminary stabilization treatment, but before jordaning. Accordingly, this means that the nitrocellulose was not subjected to the usual jordaning, followed'by the customary four-hour soda boil, a water wash, a two hour neutral boil, a water wash, a one-hour neutral boil, 2. water wash, a one-hour neutral boil, and final washing with water. The full customary stabilization treatment of nitrocellulose intended for use in solid propellant grains or smokeless powder may be summarized briefly as follows:

The nitrocellulose, after centrifuging and drowning in water, is subjected to a 24-hour acid boil in an aqueous bath containing approximately 0.35% acidity calculated as H 50 This is followed by a water wash, an eighthour neutral boil in water, a water wash, another eighthour neutral boil, a water wash, followed by jordaning, a four-hour soda boil, a water wash, a two-hour neutral boil, a water wash, a one-hour neutral boil, a water wash, a one-hour neutral boil, and final washing with water. This conforms with IAN-N-244, July 31, 1945, Joint Army-Navy Specification, Nitrocellulose, page 3. The fact that jordaning and final stabilization after jordaning can be dispensed with in accordance with this invention is an important advantage of this invention, as will be evident from Example 2 following.

EXAMPLE 2 A sample of fibrous nitrocellulose of 12.6% nitrogen content after nitration, centrifuging, and drowning in water was given a 12-hour acid boil (0.3 to 0.4% acidity as H water washed, boiled in water for eight hours, and water washed. The 134.5 C. heat stability test was 10 minutes. Viscosity was 25.2 seconds. Fully stabilized nitrocellulose intended for use in smokeless powder or propellant grain manufacture normally has a stability of 30 minutes in the 134.5 C. heat stability test. Five thousand grams (5,000) of the above partially stabilized nitrocellulose Was slurried with 45,000 grams of water in a 20-gallon jacketed vessel fitted with a condenser and a 12-inch diameter turbine agitator driven at 200 r.p.m. The slurry was heated to 91 C. by employing steam in the jacket of the vessel. Nine thousand five hundred grams (9,500) of methyl is'obutyl ketone containing 50 grams of Z-nitrodiphenylamine was then added to the heated aqueous nitrocellulose slurry under reflux conditions with agitation, requiring about four minutes. Distillation was then carried on with continued agitation for 45 minutes, during which time the distillation temperature gradually increased to C. Distillation was continued for an additional ten minutes. Substantially all of the solvent was removed by distillation and the nitrocellulose was transformed, without being dissolved, from its initial fibrous state into smooth, hardened, densified irregular particles having a 134.5 C. heat stability test of 73 minutes and a viscosity of 11 seconds. Ordinary single-base propellant, prepared from nitrocellulose fully stabilized by customary procedure has a 134.5" C. heat stability test of 45 to 50 minutes.

EXAMPLE 3 Five thousand grams (5,000), dry basis, of water-wet fibrous nitrocellulose of 12.6% nitrogen content, after jordaning and final stabilization as hereinbefore set forth in the final paragraph in Example 1, was slur-ried with 45,000 grams of water in a 20-gallon jacketed vessel fitted with a condenseri and a 12-inch diameter turbine agitator driven at 200 r.p.m. The mixture was heated to 91 C. by employing steam in the jacket of the vessel. Ten thousand two hundred 'forty grams (10,240) of methyl isobutyl ketone containingSO grams of 2-nitro- EXAMPLE .4 a Five thousand grams (5,000), .dry. basis, .of waterwet fibrous. nitrocelluloseof' 12.6% nitrogen content, after jordaning and final stabilization as hereinbefore set forth d1phenyl'amme was then added to the heated aqueous m the final paragraph.m Examp 1e Slumgd wlth nitrocellulose Slurry under reflux onditions with a 45,000 grams. of water ma 20-gallon acketed vessel fitted flon re r g1 with-a condenser and a 12-inch turbine agitator driven quirmg about 15 minutes. DlStlHatlOn'WaS then I v 3 carried 0 at 200 r.p.m. The mixture was heated to 91 C. by n with continued agitation for 90 minutes, durin employing steam 1n the acket ofthe vessel. Eight thoug which time the dlstillatlon temperature gradually m- Y creased to 100 C Distillation with agitation was con- Sand elght hundreq cieventy grams (837.0) i methyl tinued for an additional 30 minutes Substantially all of butyl ketone contalmng 50 grams of z'mtrodnihenylamine the solvent was removed by distillation and the nitrowas then'added to the heated aqueous nitrocellulose cellulose was transformed without being dissolved from slurry nude? reflux cqnliifiqns with agitation" requu. g r r about six minutes. Distlllationwas then carried on with 1ts lmtlal fibrous state into smooth, hardened, densified, 15 7 continued agitation for 85 minutes, during which tnne the irregular particles. The water-Wet denslfied product was tin t t d all d 100 C screened with the following results: v a 9 empera i gTa u y mclieiase Distillation was continued for an additional 30 minutes.

Substantially all of the solvent was removed by distil- Scteen Mesh size g gg lation, and the nitrocellulose was transformed, without Classified Sample No. Weight Pounds being, dissolved, from its imtial fibrous state mto smooth, c fg hardened, densified irregular-particles. :Ihe;water-wet Through On Foot product was divided into three portions. The first portion was screened through a 10 mesh screen, and all material 10 14 29 39 passing through the screen was designated as Sample 1. $3 i; ii The second portion was screened through a 14 mesh 35 65 3 1 48 screen, and all material passing through the screenwas 7 designated as Sample 2. The third portion was screened The various screened fractions were then each tray through a 20 mesh screen, and all material passing dried in warm air at about 140 F. to a total volatile through the screen was designated as Sample 3. content substantially less than 1% by weight, and castings These three samples were then dried and cast into were made following the procedureset forth in Example propellant grains substantially as set forth in Example 1. l, employing the diiferent classified samples individually, Data relative to particle size distribution within each and also blends of these. Pertinent data on the castings sample, bulk density, and vented vessel firing data appear made, including vented vessel firing data, appear in Table in Table 2 following; 1 following: 1 V Table 1 Vented Vessel firing Data Lengthoi Bulk Castin Density, Grain Out DensitiedNitrocelluloselurticles LbJOu. PressureinLb./Sq. In. BurningRate Noteson Castings Employed Ft. a Burning lWeb Vented Time (Inches), essel Fir- P Pz' P; Max. (Seconds) Burning Time ing (Inches) I (Seconds) 10-14 Mesh 39 Not tested for firing characteristics Solidgn te I DOIOS 14-20 Mesh... 41 348 301 235 610 4.10 0.122 Solid and'well 1' consolidated. 20-35 Mesh 44 344 304 336 632 8.14 '0.123 do 2 -65 Mesh 48 Samples too small 0 make casting Equal parts by wt. of 10-14 224 151 151 224 11.33 0.088 Solid and well 2 Mesh; 14-20 Mesh. consolidated. Equal parts by wt. of 14-20 43 233 178 202 233 10.93 0.091, .-.--do 2 Mesh; 20-35 Mesh. 7 Equal parts by wt. of 10-14 44 286 211 233 286 9.25 0.108 -..-do 2 Mesh; 20-35 Mesh.

Composition of casting liquid in all cases was identical to that set forth in Example 1, and in all cases the ratio of densifled nitro cellulose particles to casting liquid in the cast propellant grains was approximately 1 to 1 by weight. The orifice employed in the vented vessel firing tests was 0.100 inch diameter in all tests set forth above.

Table 2 Particle Size Distribution Vented Vessel Firing Data lgngttih oi as 11 Sample 1 a Grain Out Desig- Through Through Through Bulk Pressure in I la/Sq. In. Burnigg Rate Notes on Employed nation Me 14 Mesh 20 Mesh Throu h Density, I Burning 96 sh Castings Invented on 14 on 20 on 35 '35 Mes Lb./Cu. Time (Inches) Vessel. Mesh, Mesh, Mesh, Percent Ft. (Seconds) Burning T me Firing. Percent Percent. Percent 4 P1 7 P: .P Max. 1 (Sec) (Inches) 25 3s 34 3 44 325 436 331 570 6. s2 0.147 solid and well: Q 2 i consolidated. 1 7 None 51 45 4 46 341 451 3562 626 6.96 t 0.144 do; 2 None None 92 8 46 352 400 188 512 7.71 0.130 do 2 Composition of casting liquid in all cases was identical to that seti'orth in Example 1 and in cases the ratio of densiiied nitrocellulose particlesto casting liquid in the cast propellant grains was approximate in diameter in all tests set forth above.

1 to 1 by weight.

The orifice employed in the vented vessel'firing tests'was 0.080 inch 7 EXAMPLE A'sa'mple'of water-wet fibrous nitrocellulose of 12.6% nitrogen content, 'densified in accordance with Example 1, was then beaten in a laboratory beater (D'owningtown Duplex Laboratory Beater, Downingtown Iron Works, Downingtown, Pennsylvania) and that portion of the beaten sample which passed through 35 mesh and was retained on 65 mesh was selected for further testing. The bulk density of this sample was 30.4 lb./cu. ft. (dry basis). Upon drying and casting in accordance with Example 1, a relatively soft grain having some porosity was obtained. A 2 inch length of the resulting grain when fired in the customary vented vessel firing test, employing a nozzle 0.100 inch diameter, gave the following normal ballistic data:

Pressure in Lb./Sq. In. Burning Rate Burning Web Time (Inches), (Seconds) Burning Time P1 P: F; Max. (Seconds) EXAMPLE 6 Six hundred eighty grams (680), dry basis, of water-wet fibrous nitrocellulose of 12.6% nitrogen content, after conventional preliminary stabilization treatment, but before jordaning, was slurried in 3275 grams of water in a vigorously agitated open vessel, and the slurry was heated to 75 C. by sparging steam into the slurry. One thousand two hundred grams (1200) of methyl ethyl ketone was then added to the heated agitated aqueous nitrocellulose slurry in about one minute, and heating was continued with agitation by' sparging steam into the slurry until the temperature in the slurry reached 100 C. Substantially all of the solvent was removed by distillation, and the nitrocellulose was transformed, without being dissolved, from its initial fibrous state into smooth, hardened, densified, irregular particles having a bulk density (dry basis) of 25 pounds per cubic foot. The resulting densified product, after drying, was suitable for use in casting grains of double-base propellant in accordance with Example 1.

EXAMPLE 7 One hundred'fifty grams (150), dry basis, of water-wet fibrous nitrocellulose of 12.6% nitrogen content, after conventional preliminary stabilization treatment, but before jordaning, was slurried with 1350 grams of water in a vigorously agitated 2-liter vessel fitted with a condenser. The mixture was heated to 100 C. with an electric heating mantle around the vessel. Three hundred eight grams (308) of butyl acetate was then added to the heated agitated aqueous slurry of nitrocellulose in about 'two minutes, depressing the temperature to about 91 C. Heating and agitation were continued until the tempera; ture of the slurry reached 100 C., requiring about 90 by distillation, and the nitrocellulose was transformed, without being dissolved, from its initial fibrous state into smooth, hardened, densified, irregular particles having a bulk density (dry basis) of 44 pounds per' 'cubi resulting densified product, after drying, was suitable for use .in casting grains of double-base propellant in accordance with Example 1.

c foot. 'The' 8 EXAMPLE 8 Forty parts (40), dry basis, of water-wet fibrous'nitrocellulose, 12% nitrogen type, and containing 17 parts water, was added to an agitated mixture containing 442 parts of Water and 442 parts acetone at room temperature, and the slurry was agitated for four minutes. One thousand (1000) parts of water was then added and agitation was continued for one more minute. The liquid was drained and the product was washed with water, then boiled in water for one hour, whereby substantially all solvent was removed, and the nitrocellulose was transformed, without being dissolved, from its initial fibrous state into smooth, hardened, densified irregular particles having a bulk density (dry basis) of 34.1 pounds per cubic foot. The resulting densified product, after drying, was suitable for use in casting grains of double-base propellant in accordance with Example 1.

EXAMPLE 9 One hundred five parts dry basis, of water-wet fibrous nitrocellulose, 12% nitrogen type, and containing 45 parts water, was added to an'agitated mixture containing 893 parts of water and 96 parts of ethyl acetate at room temperature, and the resulting slurry was agitated for five minutes. The liquid was drained off and 'the product quickly washed with 2000 parts of hot water. The product was then boiled in water for two hours to remove any residual solvent. The nitrocellulose was transformed, without being dissolved, from its initial fibrous state into smooth, hardened, densified, irregular particles having a .bulk density, dry basis, of 26 pounds per cubic foot. The resulting densified product, after drying, was suitable for use in casting grainsofdoublebase propellantin accordance with Example 1.

While 12.6% nitrogen nitrocellulose is customarily employed in the manufacture of cast double-base propellant grains, the present invention is by no means limited there- 'to, since all commercial types of nitrocellulose can be densified 'by solvent granulation as set forth herein for the purpose of this invention. It is understood, of course, that both burning rate and temperature of burning are correlatable with the nitrogen content of the nitrocellulose (other factors remaining constant) with both burning rate and burning temperature increasing with increase of nitr0- gen content in the "nitrocellulose. Thus, choice'of nitrocellulose with respect to' nitrogen contentis one means for regulating and controlling the burning characteristics of cast propellant grains.

It has been found that this invention can be practiced with nitrocellulose-water slurries containing from about 3% to about 15% by weight of fibrous nitrocellulose and preferably from about 7% to about 12% by weight of nitrocellulose. From an economic viewpoint it is, of course, desirable to practice the invention with an aqueous slurry containingas much fibrous nitrocellulose as practicable. Although slurry consistencies of less than 3% by weight of nitrocellulose can be employed, it is not presently considered to be economical to do so. The upper practical limit of nitrocellulose in the slurry is governed by the ability to agitate the slurry effectively. The fibrous nitrocellulose to be densified may be jordaned,or-othersomewhat smaller particlesare produced when jord aned nitrocellulose is employed.

Solvent granulation and densification of nitrocellulose minutes. Substantially all of the solvent was'rernoyed in accordance with this invention depends upon the action of an active servant 'upo'rf'fibrous nitrocellulose suspended in water. The desired action is a softening'of the nitrocellulose fibers by the solvent toeliminatenthe fibrous character thereof, witho'ut' actually dissolving-any measurable amount of "the nitrocellulose. Organic liquid solvents suitable for use in' practicing this invention. are those having an active solvent powerior nitrocellulose,

and preferably having an appreciable vaporpressure at or below the boiling point of water; Actually any active nitrocellulose solvent capable of subsequent removalby steam distillation can be employed for densifying nitrocellulose for the purpose of this invention. The solvents may be soluble in water in all proportions or may have limited solubility in water. Suitable solvents include, by way of example, various ketones such as acetone, methyl ethyl ketone, methyl propyl ketone-methyl isobutyl ketone, and the like, various esters such as ethyl acetate, propyl acetate, butyl acetate, and the like, and mixed ether-alcohols such as mo'nomethyl ether of ethylene glycol, monoethyl ether of ethylene glycol, and the like. Butyl acetate and methyl isobutyl ketone are presently preferred for densifying nitrocellulose in accordance with this invention for use in casting double-base pro'pellant grains. The solvent treatment for densifying nitrocellulose can be accomplished by addition of the solvent to an agitated slurry of nitrocellulose in water, or by addition of nitrocellulose to an agitated mixture of water and solvent, or by simultaneous addition of all ingredients, with agitation, as in a continuous process.

In the solvent granulation and densification of nitrocellulose in accordance with this invention, the ratio of active organic liquid nitrocellulose solvent to water is controlled in a range where enough solvent is partitioned to the nitrocellulose to soften and gel the tiny fibrous projections which are characteristic of nitrocellulose fibers without actually dissolving any measurable portion of the nitrocellulose. Too little solvent leaves the physical form of the nitrocellulose unaltered or not sufliciently altered, thus resulting in a product'having'low bulk density which causes fo'rmation of soft porous castings. Too much solvent causes the product to'agglomcrate into large hard lumps which produce nonuniform castings. Still more solvent, of course, causes undesirable dissolving of the nitrocellulose. The desired "degree of alteration of the physical structure of the nitrocellulose is to obtain a product which, after solvent removal, consists of smooth, hardened, densified, irregular shaped particles having a bulk density of at least about 25 pounds per cubic foot (dry basis). It will be apparent that solvent requirement to obtain thisobjective will vary somewhat with slurry consistency, initialphysical form of the nitrocellulose, temperature, degree of agitation, and solvent chosen for densification purposes. In general, however, it has been found that suitable products for the purposes of this invention are obtained by using between about 15% and about 22% of either butyl acetate or methyl isobutylketone, based on combined weight of solvent and water in the aqueous slurry, and preferably between about 16% and 19% by weight. The minimum suitable quantity of methyl ethyl ketone is on the order of approximately one part by weight of methyl ethyl ketone for each 2.73 parts of water in the aqueous nitrocellulose slurry. Similarly, the minimum suitable quantity of ethyl acetate is on the order of approximately one part by weight ethyl acetate for each 9.5 parts of water in the aqueous nitrocellulose slurry. When acetone is employed, the ratio is approximately one part by weight of acetone to each part of water in the slurry. However, regardless of the solvent employed for densification purposes, it-is 'a simple expedient to carry out a preliminary densification trial, using the hereinabove indicated quantities as a guide, and determine the degree of densification obtained, based on bulk density requirements. It will'be' apparent, of course, that densification, as measured by increased bulk density of the densified product, will improve with increasing .quantity of active nitrocellulose solvent employed until that point is reached Where the softened particlesbegin to agglomerate into large lumps; This phenomenon governs the upper useful limit of active solvent which can beemployed to alter the physical structure of the nitrocellulose in accordance with thisinvention.

' into relatively narrow fractionswith respect to particle In practicing this invention it has been found that softening of the nitrocellulose fibers by the activenitrocellulose solvent occurs almost instantaneously 'upon bringing the nitrocellulose and solvent into contact with each other in the aqueous slurry. Accordingly, it is important to maintain vigorous. agitation in the aqueous slurry from the moment that the nitrocellulose is contacted by the active solvent in the aqueous slurry until the softened nitrocellulose'particles have been hardened and densified by removal of substantially all of the solvent. Agitation prevents agglomeration of the softened nitrocellulose particles and assures that the individual nitrocellulose particles retain their identity. Any desired additives, such as stabilizers, which are soluble in the solvent used for densification and which are insoluble in water can be introduced with the solvent and thus are distributed very uniformly into the nitrocellulose product.

Densification of nitrocellulose by solvent granulation can be practiced at any ordinary temperatures. However, addition of solvent to the aqueous nitrocellulose slurry at or near the boiling point of the solvent or its water azeotrope is desirable and is preferred because the partition of solvent to the nitrocellulose is aided by reduced solubility of the solvent in water. Moreover, solvent removal is greatly accelerated at elevated temperatures, and there is no tendency for the softened particles of nitrocellulose to putt up or popcorn during solvent removal, thus aiding materially in attaining a highbulk density in the densified nitrocellulose product.

Hardening of the softened nitrocellulose is carried out by boiling off the solvent or by diluting the aqueous slurry with water. Boiling is preferred because in this manner particle hardening and solvent removal are accomplished simultaneously. When hardening is produced by dilution, subsequent solvent removal by draining and boiling is necessary. Solvent removal is carried out until substantially all solvent is removed from the nitrocellulose. This may require rom about 20 minutes to about 4 hours, depending upon equipment used and the solvent which has been employed. Economical operation,

of course, dictates eflicient recovery of solvent for re-use in the process, and such recovery is readily achieved by distillation.

When the solvent strength 'is reduced by dilution with water or by boiling olf the solvent, the softened and gelled portions of the nitrocellulose harden to produce irregularly shapeddense granular particles lackingthe fibrous projections which give ordinary fibrous nitrocellulose low bulk density and high compressibility.

The densified granular nitrocellulose produced in accordance with this invention should have a bulk density of at least about 25 pounds per cubic foot, and preferably higher, since in general harder better consolidated cast double-base propellant grains are produced from higher density nitrocellulose particles. The ultimate upper limit for bulk density appears to be in the neighborhood of about 60 to 65 pounds per cubic foot. Generally, the densified granular nitrocellulose product will consist of particles having a relatively Wide diversity of sizes, as determined by screen analysis. It is presently believed that this is a desirable feature of the densified nitrocellulose particles of this invention, since the smaller particles help to fill in the spaces between larger particles,

thus leading to a-product of higher bulk density. :If desired, of course, the densified nitrocellulose particles produced bythis invention can be classified by screening size, but the examples clearly indicate that such classifica tion is not necessary forthe production of cast doublebase propellant grains having satisfactory ballistic char-.

acteristics. It is advisable, however, to screen out any particles larger than 10 mesh, since such particles usual- 1y lead to nonuniformity in the cast double-base propellant grains. Particles larger than 10 mesh are conveniently 11 ground up and reworked through the solvent densification steps of this invention.

Following solvent removal, the water associated with the densified granular nitrocellulose product is substantially all removed. This is usually accomplished by simple draining followed by conventional tray drying the moist product in warm air at about 140 F. The densified nitrocellulose particles drain freely to about 20 to 40% (usually 20 to 30%) moisture content, thus eliminating the need for dewatering centrifuges.

The dry densified nitrocellulose particles are then charged into a mold of suitable configuration to conform with the shape of the propellant grain desired and casting liquid is run into the mold following conventional casting technique, employing either top or bottom filling, until all interstices between particles have been filled with casting liquid, whereupon the casting is cured, usually for several days at about 140 F. to cause the mixture to set up and consolidate and weld together into a solid unitary homogeneous mass conforming to the shape of the mold. This is brought about by the solvent action of the casting liquid on the nitrocellulose particles which absorb all of the casting liquid with resultant swelling and collodizing of the nitrocellulose into a homogeneous mass. After curing, the propellant grain is ready for use in the rocket motor for which it is designed.

The proportion by weight of nitrocellulose and casting liquid in the cast propellant grain is governed largely by the bulk density of the granulated nitrocellulose particles employed, the casting liquid employed consisting of nitroglycerin and stabilizer, with or without various desensitizing plasticizers in accordance with conventional cast double-base propellant practice.

The primary advantage of employing nitrocellulose densified in accordance with this invention in the preparation of cast propellant grains resides in the elimination of the tedious and expensive prior art process of converting fibrous nitrocellulose into uniformly shaped and sized grains of fully colloided material, requiring alcohol dehydration of the nitrocellulose, colloiding, extruding, cutting, and solvent removal. Solvent densification practiced in accordance with this invention is much more economical than the conventional propellant powder'manufacturi-ng processes which fireplaces. Other important advantages of solvent-densificatoin-of nitrocellulose reside in the elimination of the usual dewater'ing centrifuges from the nitrocellulose manufacturing process, and in a marked simplification in the nitrocellulose stabilization process. -Further. advantages where the nitrocellulose is to be shipped reside in the reduced shipping costs resulting from the higher bulk densities and the much greater ease of unloading the granular material from shipping containers as compared with tightly packed fibrous nitrocellulose.

This application is a continuation-in-part of my copending application, Serial No. 448,156, filed August 5, 1954, now abandoned.

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

l. The process for preparing cast double-base propellant grains which comprises agitating fibrous nitrocellulose in an aqueous bath containing organic liquid solvent which has active solvent power for said nitrocellulose and which has an appreciable vapor pressure at 100 C., said solvent being selected from the group consisting of solvents soluble in water in all proportions and solvents having limited solubility in water, said solvent being present in amount to soften and destroy the fibrous structure of the nitrocellulose without dissolution thereof, continuing agitation with removal of substantially all the solvent until smooth, hardened, densified and irregular particles of the nitrocellulose having a bulk density on a dry basis of at 'least about 25 pounds per cubic foot are obtained, removing substantially all Water from the resulting densified nitrocellulose particles, charging the resulting dry densified nitrocellulose 'into a propellant grain casting mold, evacuating air therefrom, introducing a nitroglycerin casting liquid into the mold to completely fill all voids and just cover the nitrocellulose particles, and curing the mixture of nitrocellulose and casting liquid in the mold until the particles of nitrocellulose absorb all of the casting liquid and become swollen and fully colloidized and the whole mass in the mold, due to the solvent action of the casting liquid on the nitrocellulose, consolidates and welds together into a solid, unitary, homogeneously colloided double-base propellant grain conforming to the shape of the casting mold.

2. The process for preparing cast double-base propellant grains which comprises agitating fibrous nitrocellulose in an aqueous bath at an elevated temperature up tothe boiling point of water, said bath containing organic liquid solvent which has active solvent power for said nitrocellulose and which has an appreciable vapor pressure at C., said solvent being selected from the group consisting of solvents soluble in water in all proportions and solvents having limited solubility in Water, said solvent being present in amount to soften and destroy the fibrous structure of the nitrocellulose without dissolution thereof, continuing agitation with removal of substantially all the solvent until smooth, hardened, densified and irregular particles of the nitrocellulose are obtained, drying the resulting densified nitrocellulose particles, charging the resulting dry densified nitrocellulose into a propellant grain casting mold, evacuating air therefrom, introducing a nitroglycerin casting liquid into the mold to completely fill all voids and just cover the nitrocellulose particles, and curing the mixture of nitrocellulose and casting liquid in the mold until the particles of nitrocellulose absorb all of the casting liquid and become swollen and fully colloidized and the whole mass in the mold, due to the solvent action of the casting liquid on the nitrocellulose, consolidates and welds together into a solid, unitary, homogeneously colloided double-base propellant grain conforming to the shape of'the casting mold.

3. The process for preparing cast double-base propellant grains which comprises agitating fibrous nitrocellulose in an aqueous bath containing organic liquid solvent which has active solvent power for said nitrocellulose and which has an appreciable vapor pressure at 100 C., said solvent being selected from the group consisting of solvents soluble in water in all proportions and solvents having limited solubility in water, said solvent being present in amount to soften and destroy the fibrous structure of the nitrocellulose without dissolution thereof, continuing agitation with removal of substantially all the solvent by distillation until smooth, hardened, densified and irregular particles of nitrocellulose having a bulk density on a dry basis of at least about 25 pounds per cubic foot are obtained, drying the resulting densified nitrocellulose particles having a bulk density on a dry basis of at least 25 pounds per cubic foot, charging the resulting dry densified nitrocellulose into a propellant grain casting mold, evacuating air therefrom, introducing a nitroglycerin casting liquid into the mold to completely fill all voids and just cover the nitrocellulose particles, and curing the mixture of nitrocellulose and casting liquid in the mold until the particles of nitrocellulose absorb all of the casting liquid and become swollen and fully colloidized and the whole mass in the mold, due to thesolvent action of the casting liquid on the nitrocellulose,

consolidates and welds together into a solid, unitary, homogeneously colloided double-base propellant grain conforming to the shape of the casting mold.

4. The process for preparing cast double-base propellant grains which comprises agitating fibrous nitr0cellulose in an aqueous bath containing organic liquid solvent which has active solvent power for said nitrocellulose and which has an appreciable vapor pressure at 100 C., said solvent being selected from-the group consisting of solvents soluble-inwater-in all-proportions and solvents having limited solubility in water, said solvent being present in amount to soften and destroy the fibrous structure of the nitrocellulose without dissolution thereof, continuing agitation with dilution of the aqueous bath by adding water thereto until smooth, hardened, densified and irregular particles of nitrocellulose having a bulk density on a dry basis of at least about 25 pounds per cubic foot are obtained and removing substantially all solvent by distillation, drying the resulting densified nitrocellulose particles, charging the resulting dry densified nitrocellulose into a propellant grain casting mold, evacuating air therefrom, introducing a nitroglycerin casting liquid into the mold to completely fill all voids and just cover the nitrocellulose particles, and curing the mixture of nitrocellulose and casting liquid in the mold until the particles of nitrocellulose absorb all of the casting liquid and become swollen and fully colloidized and the Whole mass in the mold, due to the solvent action of the casting liquid on the nitrocellulose, consolidates and welds together into a solid, unitary, homogeneously colloided double-base propellant grain conforming to the shape of the casting mold.

5. The process for preparing cast double-base propellant grains which comprises agitating fibrous nitrocellulose in an aqueous bath at an elevated temperature up to the boiling point of water, said bath containing organic liquid solvent which has active solvent power for said nitrocellulose and which has an appreciable vapor pressure at 100 C., said solvent being selected from the group consisting of solvents soluble in water in all proportions and solvents having limited solubility in Water, said solvent being present in amount to soften and destroy the fibrous structure of the nitrocellulose without dissolution thereof, continuing agitation with removal of substantially all the solvent by distillation until smooth, hardened, densified and irregular particles of nitrocellulose having a bulk density on a dry basis of at least about 25 pounds per cubic foot are obtained, drying the resulting densified nitrocellulose particles,'charging the resulting dry densified nitrocellulose into a propellant grain casting mold, evacuating air therefrom, introducing a nitroglycerin casting liquid into the mold to completely fill all voids and just cover the nitrocellulose particles, and curing the mixture of nitrocellulose and casting liquid in the mold until the particles of nitrocellulose absorb all of the casting liquid and become swollen and fully colloidized and the whole mass in the mold, due to the solvent action of the casting liquid on the nitrocellulose, consolidates and welds together into a solid, unitary, homogeneously colloided' double-base propellant grain conforming to the shape of the casting mold.

6. The process for preparing cast double-base propellant grains which comprises agitating fibrous nitrocellulose in an aqueous bath at an elevated tempera'mre up to the boiling point of water, said bath containing methyl isobutyl ketone in an amount sufficient to soften and destroy'the fibrous structure of the nitrocellulose without dissolution thereof, continuing agitation with removal of substantially all the solvent by distillation until smooth, hardened, densified and irregular panticles of nitrocellulose having a bulk density on a dry basis of at least about 25 pounds per cubic foot are obtained, drying the resulting densified nitrocellulose particles, charging the resulting 'dry densified nitrocellulose into a propellant grain casting mold, evacuating air therefrom, introducing a nitroglycerin casting liquid into the mold to completely fill all voids and just cover the nitrocellulose particles, and curing the mixture of nitro-' cellulose and casting liquid in the mold until the particles of nitrocellulose absorb all of the casting liquid and become swollen and fully colloidized and the .whole mass in the mold, due to the solvent action of the casting liquid on the nitrocellulose, consolidates and welds together into a'solid, unitary, homogeneously colloided double-base propellant grain conforming to the shape of the casting mold.

7. The process for preparing cast double-base propellant grains which comprises agitating fibrous nitrocellulose in an aqueous bath at an elevated temperature up to the boiling point of water, said bath containing butyl acetate in an amount sufiicient to soften'and destroy the fibrous structure of the nitrocellulose without dissolution thereof, continuing agitation with removal of substantially all the solvent by distillation until smooth, hardened, densified and irregular particles of nitrocellulose having a bulk density on a dry basis ofat least about 25 pounds per cubic foot are obtained, drying the resulting densified nitrocellulose particles, changing the resulting dry densified nitrocellulose into a propellant grain casting mold, evacuating air therefrom, introducing a nitroglycerin casting liquid into the mold to completely fill all voids and just cover the nitrocellulose particles, and curing the mixture of nitrocellulose and casting liquid in the mold until the particles of nitrocellulose absorb all of the casting liquid and become swollen and fully colloidized and the whole mass in the mold, due to the solvent action of the casting liquid on the nitrocellulose, consolidates and welds together into a solid, unitary, homogeneously colloided double-base propellant grain conforming to the shape of the casting mold.

References Cited in the file of this patent UNITED STATES PATENTS 2,027,114 Olsen et a1. Jan. 7, 1936 2,035,471 Hale Mar. 31, 1936 2,292,469 Olsen Aug. 11, 1942 2,417,090 Silk et a1 Mar. 11, 1947

Claims (1)

1. THE PROCESS FOR PREPARING CAST DOUBLE-BASE PROPELLANT GRAINS WHICH COMPRISES AGITATING FIBROUS NITROCELLULOSE IN AN AQUEOUS BATH CONTAINING ORGANIC LIQUID SOLVENT WHICH HAS ACTIVE SOLVENT POWER FOR SAID NITROCELLULOSE AND WHICH HAS AN APPRECIABLE VAPOR PRESSURE AT 100*C., SAID SOLVENT BEING SELECTED FROM THE GROUP CONSISTING OF SOLVENTS SOLUBLE IN WATER IN ALL PROPORTIONS AND SOLVENTS HAVING LIMITED SOLUBILITY IN WATER, SAID SOLVENTS BEING PRESENT IN AMOUNT TO SOFTEN AND DESTROY THE FIBROUS STRUCTURE OF THE NITROCELLULOSE WITHOUT DISSOLUTION THEREOF, CONTINUING AGITATION WITH REMOVAL OF SUBAND IRREGULAR PARTICLES OF THE NITROCELLULOSE HAVING A BULK DENSITY ON A DRY BASIS OF AT LEAST ABOUT 25 POUNDS PER CUBIC FOOT ARE OBTAINED, REMOVING SUBSTANTIALLY ALL WATER FROM THE RESULTING DENSIFIED NITROCELLULOSE PARTICLES, CHARGING THE RESULTING DRY DENSIFIED NITROCELLULOSE INTO A PROPELLANT GRAIN CASTING MOLD, EVACUATING AIR THEREFROM, INTRODUCING A NITROGLYCERIN CASTING LIQUID INTO THE MOLD TO COMPLETELY FILL ALL VOIDS AND JUST COVER THE NITROCELLULOSE PARTICLES, AND CURING THE MIXTURE OF NITROCELLULOSE AND CASTING LIQUID IN THE MOLD UNTIL THE PARTICLES OF NITROCELLULOSE ABSORB ALL OF THE CASTING LIQUID AND BECOME SWOLLEN AND FULLY COLLOIDIZED AND THE WHOLE MASS IN THE MOLD, DUE TO THE SOLVENT ACTION OF THE CASTING LIQUID ON THE NITROCELLULOSE, CONSOLIDATES AND WELDS TOGETHER INTO A SOLID, UNITARY, HOMOGENEOIUSLY COLLOIDED DOUBLE-BASE PROPELLANT GRAIN CONFORMING TO THE SHAPE OF THE CASTING MOLD.