US3236702A

US3236702A - Manufacture of densified spheroidal fine particle nitrocellulose

Info

Publication number: US3236702A
Application number: US358366A
Authority: US
Inventors: John J Sapiego
Original assignee: Hercules Powder Co
Current assignee: Hercules Powder Co
Priority date: 1964-04-08
Filing date: 1964-04-08
Publication date: 1966-02-22
Anticipated expiration: 1983-02-22
Also published as: FR1436182A; CH431946A; BE662278A; DE1446879A1; GB1075193A; NL6504498A

Description

United States Patent 3 236,702 MANUFACTURE OFDENSIFIED SPHEROIDAL FINE PARTICLE NITROCELLULOSE John J. Sapiego, New Brunswick, N.J., assignor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Apr. 8, 1964, Ser. No. 358,366

7 Claims. (Cl. 149-2) This invention relates to the manufacture of tiny particles of densified nitrocellulose and more particularly to the production of tiny particles of densified nitrocellulose which are well suited for use in casting large propellant grains by the slurry casting process and for use in other applications requiring such a product.

Heretofore, large grains of propellant powder suitable for propelling military rockets and missiles or for actuating sizable jet devices have been manufactured by various in-situ casting techniques. The distinguishing characteristic of all such in-situ casting methods is that the casting powder, consisting of precolloided, fully densified, solid particles or granules of nitrocellulose composition, is first loaded dry into a suitable casting mold and then a predetermined quantity of casting liquid comprising nitroglycerin and/or suitable densensitizing plasticizers, such as triacetin, with or without stabilizers and/ or other adjuvants included to control or regulate burning rate of the resulting propellant grain when fired, is then introduced into the mold to completely fill all voids between the dry casting powder granules and to just cover the casting powder granules. The mixture of prec-olloided nitrocellulose particles and casting liquid is then cured in the mold, usually at a suitable elevated temperature until the particles of nitrocellulose casting powder absorb all of the casting liquid and the whole mass consolidates and welds together into a solid, unitary, colloided structure conforming to the shape of the mold.

It has been recognized by those skilled in the art of manufacturing propellant grains, however, that in-situ casting of propellant grains is subject to certain restrictions and limitations. For example, in-situ casting methods have required a relatively large inventory of casting powder formulations. Moreover, in-situ casting requires relatively long curing times and does not lend itself to the manufacture of grains of irregular configuration such as may be necessary to fit in certain types of rocket motors to provide the desired type of ballistic functioning. Furthermore, known methods for preparing casting powder formulations are tedious and expensive since these methods involve all of the steps necessary in the manufacture of smokeless powder granules, since, in reality, casting powder granules heretofore have been smokeless powder granules.

In attempts to overcome limitations and disadvantages of in-situ casting it has been proposed to prepare large propellant grains by slurry casting, employing very small particles of colloided densified nitrocellulose. In slurry casting the very small casting powder particles and casting liquid are mixed together to form a pourable slurry which is then poured into the mold, and the mix ture is then cured in the usual way in the mold to form the propellant grain.

3,236,702 Patented Feb. 22, 1966 Experience has demonstrated that in order to realize the advantages of slurry casting, the small particles of colloided densified nitrocellulose employed for this purpose should desirably have certain characteristics as to shape, size and density. More particularly, the tiny particles should desirably be smooth and rounded, preferably into substantially ellipsoidal and/or spheroidal shape for uniform dispersal and distribution in the casting liquid, and formation of a slurry with smooth, uniform pouring properties. The particles of nitrocellulose desirably should be densified at least to the point where there is a smooth, hard, dense shell of colloided nitrocellulose forming the surface of each tiny particle. The interior of the tiny particles may be, but need not necessarily be, as completely colloided and densified as the surface shell; in fact, experience indicates that some porosity associated with only a partially colloided or partially gelled state in the interior of the particles may be desirable.

The hard, dense shell of colloided nitrocellulose forming the surface of the particles is an important physical characteristic which imparts satisfactory "pot life to the slurry, for this dense shell resists rapid absorption of the casting liquid and, therefore, delays swelling of the nitrocellulose and thickening of the slurry during the period necessary to prepare and uniformly mix the slurry and then pour it into the mold. Fibrous nitrocellulose particles, or partially densified nitrocellulose particles having a substantially porous surface layer, on the other hand, absorb the casting liquid much too rapidly for satisfactory slurry casting techniques, for such particles swell and thicken the slurry too rapidly for satisfactory mixing and pouring of the slurry into the mold. Such fibrous or porous-surfaced particles, therefore, impart an unsatisfactorily short pot life to the slurry. Some porosity in the interior of the particles, on the other hand, is believed to facilitate a more rapid and uniform cure after casting, which is desirable.

In general, other factors remaining substantially the same, pot life of the slurry decreases with decreasing size of the nitrocellulose particles, and experience has shown that when a substantial majority of the casting powder particles are less than about 10 microns in size, pot life of the slurry is unsatisfactorily short, even though the particles are substantially completely densified. Experience has also shown that when a majority of the casting powder particles are substantially larger than about microns in size, especially when highly densified, pot life of the slurry is undersirably long. Such large, dense particles tend to settle from the slurry after casting and also unduly prolong curing of the grain, both effects being obviously undesirable.

Although numerous methods for preparing tiny particles of densified nitrocellulose have been proposed, very few of these methods are capable of producing particles having a desirable combination of characteristics as to shape, size, and density for use in slurry casting of large propellant grains. Moreover, those few methods which can produce satisfactory particles for this use have been handicapped by being unduly tedious and expensive.

It is, therefore, an object of this invention to provide an improved process for manufacturing tiny particles of densified nitrocellulose especially suited for use in casting large propellant grains by the slurry casting process.

It is another object of this invention to provide an improved process for manufacturing tiny particles of densified nitrocellulose having a desirable combination of shape, size and density for use in slurry casting of large propellant grains, and for use in other applications requiring tiny particles of densified nitrocellulose.

It is a further object of this invention to provide a simplified and improved process for preparing tiny particles of densified nitrocellulose, which has economic and procedural advantages over prior art processes.

A still further object of this invention is to provide an improved process for manufacturing tiny particles of densified nitrocellulose wet with hydrocarbon liquid in stead of with alcohol or with water.

A further object of this invention is to provide a hydrocarbon-wet nitrocellulose of improved form for use in slurry casting of large propellant grains and other applications where tiny particles of densified nitrocellulose are necessary or desirable.

These objects and others are accomplished in accordance with the present invention which, generally described, comprises forming a slurry with agitation of water-moist nitrocellulose fibers substantially free of fiber aggregates in a volatile organic liquid mixture of hydrocarbon diluent and nitrocellulose solvent which is miscible with said diluent in the presence of a water-soluble protective colloid, said organic liquid mixture being only a softening and swelling agent for said nitrocellulose fibers incapable of dissolving said fibers, said solvent being soluble in water to the extent of at least about 2.5% by weight, subjecting the slurry of nitrocellulose fibers to high shear agitation, whereby said fibers become softened and swollen and break down into tiny smooth surfaced, rounded particles of substantially ellipsoidal and spheroidal shapes, initiating hardening of the comminuted and shaped particles of softened and swollen nitrocellulose by diluting the slurry with hydrocarbon diluent while continuing high shear agitation, and removing substantially all of the nitrocellulose solvent and water by distillation with agitation in the presence Olf excess hydrocarbon diluent to complete hardening and densification of the nitrocellulose particles.

The above process produces a slurry of hard, tiny, rounded and smooth-surfaced particles of densified nitrocellulose of substantially ellipsoidal and spheroidal shapes in hydrocarbon diluent, from which the densified nitrocellulose product is readily recovered in a hydrocarbonwet state by any convenient means, such as by gravity drainage, centrifugation, suction filtration, or the like.

In the preferred practice of this invention, water-wet fibrous nitrocellulose, after conventional treatments for stabilization and viscosity adjustment, is slurried with water and beaten in a Jordan engine or similar fiber beating device to break up fiber aggregates and shorten the fibers and produce an aqueous slurry of nitrocellulose fibers substantially free of fiber aggregates. Excess water is then drained off, preferably by centrifugation or suction filtration to produce water-moist nitrocellulose fibers containing between about 35% and about 50% by weight of sorbed water, based on total weight of water-moist nitrocellulose fibers. The terms sorb and sorbed are used herein in the usual sense to mean the ability of the nitrocellulose to take up and hold a liquid, either by adsorption or absorption, or by a combination of adsorption and absorption, substantially independent of gravitational forces. The resulting water-moist nitrocellulose fibers are then introduced and dispersed at substantially incapable of dissolving the fibers, and having a watersoluble protective colloid dispersed therethrough to [form a stirrable slurry of nitrocellulose fibers substantially free of fiber aggregates. The water-moist nitrocellulose fibers are introduced into the organic liquid mixture as rapidly as the agitating device can disperse the fibers, either in small increments or continuously, as desired. The time required to accomplish addition and dispersal of the water-moist nitrocellulose fibers in the organic liquid mixture is usually quite short, amounting at most to only a few minutes. Softening, swelling and breakdown of the nitrocellulose fibers commences as soon as the fibers have been introduced into the organic liquid mixture.

When all of the nitrocellulose fibers have become uniformly dispersed, the slurry is then subjected to high shear agitation with a Kady dispersion mill (Kady Dispersion Corporation, Botsford Place, Buffalo 16, New York), or the like, in a jacketed vessel with cooling water circulating through the jacket to remove at least some of the heat generated in the slurry by the high shear agitation.

This high shear agitation is important and necessary for it definitely contributes to softening and swelling of the nitrocellulose fibers and comminution of the softened and swollen fibers into tiny fragments, a majority of which surprisingly are relatively uniform in size. High shear agitation also is an important feature in helping to shape the comminuted fragments into smooth surfaced, rounded particles of substantially ellipsoidal and spheroidal shapes and to facilitate solution and uniform distribution of the water-soluble protective colloid over the surfaces of the comminuted particles. Although it is not known exactly What causes size reduction, it is believed that all features of the invention are significant. High shear agitation is continued until substantially all of the fibers have been comminuted and shaped, and observation of samples taken at intervals, aided by magnification, indicates no further comminution and shaping taking place, and that a substantial equilibrium exists in the slurry with respect to size and shape of the comminuted particles. Depending somewhat on the ease of fragmentation, this may require anywhere from a few minutes to /2 hour or more.

The protective colloid which is present in the slurry is believed to form a thin protective coating or layer around each fragmented particle, and thus keep the fragments from sticking together and agglomerating into substantially larger aggregates. The protective colloid also appears to aid in the rounding and shaping of the fragmented particles into desirable ellipsoidal and spheroidal shapes. However, irrespective of whether the above explanation of the action of the protective colloid is correct or not, the fact remains that the protective colloid is a necessary element of this invention, for in the absence of protective colloid, the fragmented particles tend to stick together and agglomerate into undesirably larger aggregates and do not shape properly into rounded ellipsoidal and spheroidal shapes.

When observation indicates a substantial equilibrium condition to exist with respect to size and shape of the comminuted particles, the slurry is diluted with hydrocarbon diluent while continuing high shear agitation to initiate hardening of the comminuated and shaped articles. High shear agitation may, if desired, be continued for several minutes, permitting solvent, water, and hydrocarbon vapors to escape from the slurry, to promote further initial hardening of the particles.

After initial hardening of the comminuted particles has been accomplished by the dilution of the slurry with hydrocarbon diluent, nitrocellulose solvent and water are substantially all removed from the slurry by distillation to complete the hardening and densification of the particles. The slurry desirably should be agitated during the distillation step, but such agitation need not necessarily be high shear agitation; any agitating means will suffice.

Since a portion of the hydrocarbon diluent also distills off with the nitrocellulose solvent and water, sufficient hydrocarbon diluent should be added to the slurry either prior to or during distillation to insure a large excess of hydrocarbon diluent at all stages of the distillation. The resulting hardened and densified particles of nitrocellulose are readily recovered in a hydrocarbon-wet state by draining off excess hydrocarbon liquid by any convenient means, such as by gravity drainage, suction filtration, centrifugation, or the like.

If desired, various additives such as nitrocellulose stabilizers, carbon black, and other desirable additives which are soluble or dispersible in the organic liquid mixture can be introduced into the slurry at any convenient point in the process and become very uniformly distributed into the nitrocellulose product.

The hydrocarbon-wet product of this invention is composed of tiny, hard, densified, substantially rounded, particles of nitrocellulose, preferably of substantially ellipsoidal and spheroidal shapes having a diversity of particle sizes, a substantial majority of which are between about 20 microns and about 80 microns, preferably between about 40 microns and about 75 microns in size, with an overall range of particle sizes between about 1 micron and about 200 microns. The absolute density of these tiny particles, dry basis, is at least about 1.3 gms./cc., the absolute density of completely densified nitrocellulose being approximately 1.65 gms./ cc. When magnified, these particles are seen to have smooth, dense, glazed surfaces, and, generally, to have some degree of porosity in the interior of the particles. The bulk density of these particles is at least about 40 pounds per cubic foot, preferably at least about 45 pounds per cubic foot, dry basis. While some irregularity in particle shape diverging from ellipsoidal and spheroidal shapes is acceptable, no more than about 5% of such irregular shaped particles are in the form of elongated fiber-like particles.

The tiny densified particles of this invention are readily dispersed in aqueous or organic liquid mixtures to form hydrosol, organosol, or plastisol compositions which are uniformly smooth, pourable slurries of relatively high solids content. A plastisol mixture of 30 parts by weight of the densified particles of this invention uniformly mixed with 70 parts by weight of triacetin, which is a solvent-type plasticizer for nitrocellulose forms a smooth, pourable slurry which, after one hour at room temperature, is still pourable and has a viscosity less than 4000 centipoises. This demonstrates an eminently satisfactory pot life for employment of the tiny densified particles in the slurry casting of large propellant grains, and this characteristic combined with the other described properties with respect to particle shape, size, density, surface, and bulk density characteristics make the tiny densified particles of this invention especially well suited for use in slurry casting of large propellant grains.

The tiny densified nitrocellulose particles of this invention are wetted with between about and about 30% by weight of sorbed hydrocarbon liquid and are further characterized by being free flowing and relatively incompressible. The chemical characteristics of the product of this invention are apparently the same as conventional nitrocellulose, since no chemical action is involved in the process of producing this product.

While production of tiny particles of densified nitrocellulose especially suited for use in casting propellant grains by the slurry casting process is among the objects of this invention, it is emphasized that the utility of the densified particles of this invention is by no means limited to use in the slurry casting of large propellant grains, for these densified particles can be used either in the hydrocarbon-wet state or dried in any application where commercial nitrocellulose is now used, such as lacquers, plastics, paints, adhesives, coatings, inks, impregnations, and the like. In view of the tiny particle size and shape,

combined with high bulk density, the densified nitrocellulose particles of this invention are also especially well suited for formulating nitrocellulose wood fillers substantially free of a tendency to shrink when overcoated with nitrocellulose lacquers and other coating materials having solvent vehicles which are solvents for nitrocellulose. This use will be more fully described hereinafter.

The general nature of the invention has been set forth and the following examples are presented as specific illustrations thereof. All parts and percentages are by weight unless otherwise stated.

EXAMPLE 1 Water-wet fibrous nitrocellulose, 12.6% nitrogen by weight, 5 seconds A -inch falling ball viscosity measured in accordance with Military Spec. JAN-N-244, after conventional treatments for stabilization and viscosity adjustment, was slurried with water and jordaned to break up fiber aggregates and shorten the fibers and produce a slurry of nitrocellulose fibers substantially free of fiber aggregates. This slurry was then dewatered by centrifugal drainage to 63.83% total solids.

Twenty-three and one-half (23.5) parts of this watermoist nitrocellulose containing 15 parts nitrocellulose, dry weight, and 8.5 parts water were then introduced and dispersed at room temperature with agitation in an organic liquid mixture of 42.8 parts of lactol spirits, a petroleum hydrocarbon fraction having a boiling range between 201 and 217 F. and a kauri-butanol value of 41, and 33.6 parts of methyl ethyl ketone, having dispersed therethrough 0.1 part of water-soluble methyl cellulose, in a stainless steel vessel equipped with a Cowles dissolver. This required about 5 minutes of agitation with the Cowles dissolver operating at approximately 4,000 rpm.

The charge was then transferred to a jacketed Kady dispersion mill provided with a cover, with cooling water circulating through the jacket, and was subjected to high shear agitation at approximately 16,000 rpm. in the covered Kady dispersion mill for approximately 30 minutes. The temperature in the slurry reached, and was then maintained by jacket cooling, in the range between about 60 C. and 64 C. during this period of high shear agitation. The slurry was then diluted with 11.2 parts by weight of lactol spirits, the cover of the Kady mill was removed, and the high shear agitation was continued for an additional 30-minute period. During this latter period of high shear agitation there was appreciable evaporation of the methyl ethyl ketone, water, and lactol spirits components from the slurry and additional lactol spirits were added as needed to maintain the liquid level in the mill.

The charge was then further diluted with lactol spirits and transferred to a distillation kettle provided with a conventional propeller type agitator, where the remaining water and methyl ethyl ketone were removed by distillation while agitating the charge. A tot-a1 of approximately 48.5 parts by weight of additional lactol spirits were employed for dilution of the slurry during the latter period of high shear agitation with the cover removed from the Kady dispersion mill and prior to removal of remaining water and methyl ethyl ketone in the distillation kettle. Distillation was discontinued when the distillation temperature reached the boiling point of lactol spirits, approximately .C. The resulting slurry of tiny, densified nitrocellulose particles in petroleum diluent was then deliquefied on a suction filter to 80% total solids by Weight.

The resulting densified nitrocellulose particles had an absolute density, dry, of 1.357 gins/cc. and a bulk density, dry basis, of 52 pounds per cubic foot. The overall particle size was in the range from 40 microns to microns with a majority of the particles in the range from 50 microns to 75 microns. Visual examination with the aid of magnification showed the particles to be smooth and rounded with dense glazed surfaces, and for the most part to be of ellipsoidal and spheroidal shapes. The dried material flowed easily like dry sand.

Three (3) parts by weight of the dried particles were mixed with 7 parts of triacetin to produce a smooth, pourable slurry which was still readily pourable after hours. The viscosity build-up of this slurry with time at 20 C. follows:

Viscosity Time (minutes): (centipoises) 15 102 30 112 45 127 60 145 75 170 90 188 105 210 120 235 180 460 240 1,040 300 2,920

These particles were satisfactory and well suited for use in slurry casting of large propellant grains.

The A inch falling ball viscosity characteristic of the nitrocellulose was measured in accordance with Military Spec. JAN-N-244 on a 10% by weight solution of the nitrocellulose in a solvent composed of 11.1% denatured ethyl alcohol and 88.9% acetone by weight at 25 C., noting the time in seconds for a W -inch steel ball to fall freely ten inches through the solution.

EXAMPLE 2 Water-wet fibrous nitrocellulose, 11% nitrogen by weight, approximately 154 seconds ASTM inch falling bal-l viscosity, after conventional treatments for stabilization and viscosity adjustment, was slurried with water and jordaned to break up fiber aggregates and shorten the fibers and produce a slurry of nitrocellulose fibers substantially free of fiber aggregates. This slurry was then dewatered by centrifugal drainage to 64.1% total solids.

The ASTM ;-incl1 falling ball viscosity characteristic of the nitrocellulose was measured on a 12.2% by weight solution of the nitrocellulose in a solvent composed of 55% toluene, 25% denatured ethyl alcohol and 20% ethyl acetate by weight at 25 C., noting the time in seconds for a -inch steel ball to fall freely ten inches through the solution.

Twenty-five and one-tenth (25.1) parts of this watermoist nitrocellulose containing 16.1 parts nitrocellulose, dry basis, and 9 parts water were then introduced and dispersed at room temperature with agitation in an organic liquid mixture of 43.3 parts of lactol spirits and 31.5 parts methyl ethyl ketone, having dispersed therethrough 0.1 part of water-soluble methyl cellulose, in a stainless steel vessel equipped with a Cowles dissolver. This required about 1 minute of agitation with the Cowles dissolver operating at approximately 4,000 r.p.m.

The charge was then transferred to a jacketed Kady dispersion mill provided with a cover, with cooling water circulating through the jacket, and was subjected to high shear agitation in the covered Kady dispersion mill operating at approximately 16,000 rpm. for approximately 30 minutes. The temperature in the slurry reached, and was then maintained by jacket cooling, in the range between about 60 C. and 64 C. during this period of high shear agitation. The slurry was then diluted with 7.45 parts by weight of lactol spirits, and high shear agitation was continued for an additional 2 minutes, approximately, with the cover removed.

The charge was then further diluted with about 48.5

parts by weight of iactol spirits and transferred to an agitated distillation kettle where the remaining water and methyl ethyl ketone were removed by distillation. Distillation was discontinued when the distillation temperature reached the boiling point of lactol spirits, approximately C., and .the slurry of hardened, densified nitrocellulose particles in lactol spirits was returned to the Kady dispersion mill and subjected to about 1 minute of high shear agitation to break up some loose aggregates formed in the distillation kettle. The resulting slurry of tiny, densified nitrocellulose particles in petroleum diluent was then deliquefied on a suction filter to 80% total solids by weight.

The resulting densified nitrocellulose particles had an absolute density, dry, of 1.570 -gms./cc., and a bulk density, dry basis, of 51.6 pounds per cubic foot. The overall particle size was in the range from 25 microns to microns, with a majority of the particles in the range from 40 to 60 microns. Visual examination with the aid of magnification showed the particles to be smooth and rounded with dense glazed surfaces. A few fiberlike particles, substantially less than about 1% in number, were observed.

The hydrocarbon-wet nitrocellulose particles of this example were employed in a wood filler application, as follows:

The following ingredients were mixed and blended together in a pony mixer to prepare the wood filler composition.

Ingredients: Parts by weight Hydrocarbonwet nitrocellulose particles (80% nitrocellulose by weight) 14.2

V.M. & P. naphtha 16.1

Methyl abietate -s 11.2

Silver bond silica (filler grade) 42.0

3x asbestine (magnesium silicate) 10.0

Brown iron oxide 0.2

Lamp black 0.3

Total 94.0

This composition was thinned to 60% solids with V.M. & P. naphtha and brushed on open-grained mahogany panels, which had previously been given a nitrocellulose wash coat with a 6% solution of nitrocellulose in a conventional nitrocellulose solvent mixture and dried. The wood filler composition was allowed to flash dry for about 10 minutes, after which the excess filler composition was wiped off the panels in conventional manner across the wood grain with a rag, and the panels were allowed to dry for about an hour. A conventional system of nitrocellulose lacquer coatings was then applied by spray coating to the filled panels, consisting of one coat of nitrocellulose lacquer sealer and two finish coats of nitrocellulose lacquer, allowing each coat to dry tackfree before application of the next coat. The lacquered panels were then allowed to air dry overnight and were observed, both before and after polishing, for filling efficiency of the above nitrocellulose filler in direct comparison to similar panels filled with a conventional commercial oil-type filler prior to application of the nitrocellulose lacquer coats.

The above nitrocellulose filler composition filled the pores of the wood effectively, and the degree of filling observed was equivalent to that obtained with the conventional commercial oil-type filler.

EXAMPLES 3-9 A series of finely divided, densified nitrocellulose particles was prepared following substantially the same procedure described in Example 1. The same water-moist nitrocellulose prepared in the same manner described in Example 1 was employed in this series. In Examples 9 3-6, inclusive, lactol spirits was employed as the hydrocarbon diluent with methyl ethyl ketone as the nitro cellulose solvent, and the ratio of hydrocarbon diluent to nitrocellulose solvent was varied. In Examples 7-9, inelusive, heptane was employed as the hydrocanb on diluent 10 cedure described in Example 2. =Fibr=ous nitrocellulose of 11.0% nitrogen by weight and 45seeond ASTM falling ball viscosity was employed in Examples 10 and 11, and of 11% nitrogen and l54-second ASTM viscosity 5 in Example 12, and fibrous nitrocellulose of 11.0% nitrorg P g' g i ketgne g mtr 8 8 i and gen by weight and A-second AS [M y -inch falling ball g j g y mean uent to Use so vent viscosity was employed in Example 13. In Examples and 11, part of the methyl ethyl ketone solvent nor- Table I fOlIOW mg presents pertinent data relative to many em 10 ed for the m 0568 of this invention was the kind and amount of ingredients employed, operational 10 re lac d z dr S b details relative to dispersal agitation, high shear agitation p e y y u ano and dilution with hydrocarbon diluent, and properties of m 2 followmg pr esents Ferment data relatlve to the particles Obtained. the kind and amount of ingredients employed, operational h d ifi d particles of any f Examples 3 9 were details relative to dispersal agitation, high shear agitasatisfactory and well suited for use in slurry casting of H011 and dilution with hydrocarbon diluent, and P p large propellant grains, and also for use in formulating es 0f the particles obtained. 7 nitrocellulose wood fillers. The densified particles of any of Examples 10-13 were Table 1 Ingredients (Parts by Weight) Operational Details Ratio Additional Dilu- Ex. Hydro- High Initial Dilution High Shear Agition of Slurry No. Nitro- Methyl Watercarbon Cowles Shear of Slurry with tation. after Iniwith Hydrocellulose Water Ethyl Hydrocarbon Sol. Diluent Agita- Kady Hydrocarbon tial Dilution carbon Diluent (Dry Ketone Diluent Methyl to tion, Agita- Diluent to with Cover for Distillation weight) Cellulose Methyl Minutes tion, Initiate Removed, and Final Ethyl Minutes Hardening Minutes Hardening Ketone Properties of Densified Particles Ex. Particle Size Viscosity of 30% by N 0. Absolute Bulk Weight Slurry of Dried Notes on Particle Shape, Surface, Etc,

Density, Density, 1 Particles in Triacetin Dry, gm./cc. Dry Basis, Overall Majority After 2 Hours at 1bs./cu.1t. Range, Range, 0., Centipoises Microns Microns 3 1. 540 47.0 -150 -70 1,000 Smooth, rounded, with dense glazed surfaces; majority of gai'tifiles irregular diverging from ellipsoidal and spheroii a s apes.

4 1. 502 44.0 20-100 40-60 1, 925 Smooth, rounded, with dense glazed surfaces; majority of particles ellipsoidal and spheroidal in shape.

5 1.520 25-150 40-70 Smooth, rounded, with dense glazed surfaces; almost all particles ellipsoidal and spherical in shape.

6 1. 357 52 40-200 45-75 235 Smooth, rounded, with dense glazed surfaces; almost all particles ellipsoidal and spheroidal in shape.

7 1. 631 41.6 25-150 3060 780 Smooth, rounded, with dense glazed surfaces; majority oi particles irregular diverging from ellipsoidal and spheroidal shapes; a very few, less than 1% by number, of fiberlike particles.

8 1.621 40.2 25-150 30-60 2, 550 Smooth, rounded, with dense glazed surfaces; majority of particles either ellipsoidal and spheroidal in shape, or approaching ellipsoidal and spheroidal shapes.

9 1. 638 40. 8 25-150 30-60 435 Smooth, rounded, with glazed surfaces; almost all particles ellipsoidal and spheroidal inshape.

l L.S. designates Lactol Spirits. I H designates Heptaue.

EXAMPLES 10- 13 A series of finely divided, densified nitrocellulose particles was prepared following substantially the same pro- Table II Ingredients (Parts by Weight) Operational Details N trocellulose Ratio Lactol Additional Ex. Spirits to High Initial Dilu- Dilution of No. Water- Methyl Cowles Shear tion of Slurry w th ASTM Methyl Buta- Lactol Sol. Ethyl Agita- Kady Slurry with Lactol Percent ere-Inch Amount Water Ethyl n01 Spirits Methyl Ketone or tion, Agita- Lactol Spirits for Nitro- Falling (Dry Ketone Oellu- Mixture Minutes tion, Spirits to Distillation gen by Ball Vis- Weight) lose of Methyl Minutes Initiate and Final Weight cosity, thyl Hardening Hardening Seconds Ketone and Butanol 10 11.0 45 16.6 9. 3 19. 8 49. 6 0.1 2.04 2 30 7. 45 48. 5 11 11. 45 16. 6 9. 3 16. 3 49. 6 0. 1 2.04 2 30 7. 45 48. 12 11. O 154 16. 1 9. 0 31. 5 43. 2 0. 1 1. 37 2 50 7. 45 48. 5 13. ll. 0 16.1 9.0 16.0 58.8 0.1 3. 67 2 3O 7. 45 48. 5

Properties of Dcnsified Particles Ex. No. Absolute Bulk Particle Size Density, Density. Notes on Particle Shape, Surface, Etc.

Dry, Dry Basis, gn1./cc. lbs/cu. it. Overall Majority Range, Range, Microns Mierons 1.556 41.6 25-150 40-70 Smooth, rounded, with dense glazed surfaces; almost all particles ellipsoidal and spheroidal in shape. 11 l. 564 50. 0 25-150 40-70 Do. 12 l. 564 50. 7 -150 40-70 Do. 13 1.540 55.7 15-150 40-60 Smooth, rounded, with dense glazed surfaces; almost all particles ellipsoidal and spheroidal in shape; a very few, substantially less than 1% by number, oi fiber-like particles.

EXAMPLES 14-18 A series of finely divided, densified nitrocellulose particles was prepared following substantially the same procedure described in Example 2. In this series nitrocelluloses differing both in nitrogen content and 'y -inch falling ball viscosity characteristics were employed, and the resulting particles in each example were smooth and rounded, with dense glazed surfaces, and for the most part the particles were substantially ellipsoidal and spheroidal in shape. The hydrocarbon-wet nitrocellulose particles of each of these examples were employed in a wood filling application as described in Example 2, with substantially the same desirable results noted in Example 2. Table 3 following presents pertinent data relative to the nitrogen content and falling ball viscosity characteristic of the nitrocelluloses employed, together with absolute density, bulk density, and particle size range of the particles obtained, and observations on the filling efficiency of the wood filler compositions prepared from the densified nitrocellulose particles.

EXAMPLE 19 Finely divided, densified nitrocellulose particles were prepared following substantially the same procedure described in Example 1, with the exceptions that acetone was employed as the nitrocellulose solvent in place of the methyl ethyl ketone used in Example 1, and there was no high shear agitation after initial dilution of the slurry with hydrocarbon diluent to initiate hardenin". The same water-moist nitrocellulose prepared in the same manner described in Example 1, and the same petroleum hydrocarbon diluent used in Example 1, Were employed in this example.

Pertinent data relative to the kind and amount of ingredients employed, operational details relative to dispersal agitation, high shear agitation, and dilution with hydrocarbon diluent, and properties of the particles obtained are summarized in Table IV following. The particles produced were satisfactory and Well suited for use in slurry casting of large propellant grains.

Table III Nitrocellulose Particle Properties Particle Size Wood Ex. Nitrogen, fe-Il'l0ll Absolute Bulk Filling No. Percent by Falling Ball Density Density, Efliciency Weight Viscosity, Dry, Dry Basis, Overall Majority Seconds GmsJcc. Lbs/cu. It. Range, Range.

Microns Mierons 11 45. 7 10-80 11-30 Excellent. 12. 6 47 -150 40-70 Do. 11 51. 6 25-150 40-60 D0. 11 55. 7 15-150 40-60 Do. 11 50 150 -60 Do.

1 Viscosity measured in accordance with ASTM procedure as set forth in Example 2. 1 Viscosity measured in accordance with Military Spec. JAN-N-244, as set forth in Example 1.

13 Table IV Nitrocellulose, dry weight 15 parts. Water 8.5 parts. Acetone parts. Lactol spirits 56.4 parts. Water-soluble methyl cellulose 0.1 parts. Ratio: Lactol spirits/acetone 2.82 parts. Cowles agitation 2 minutes. High shear Kady agitation 30 minutes. Initial dilution of slurry with lactol spirits to initiate hardening 11.2 parts. Additional dilution of slurry with lactol spirits for distillation and final hardening 48.5 parts.

1.459 gms./cc. 44.2 lbs./ cu. ft.

1,875 ceutipoises. 30-450 microns. 50-75 microns.

The tiny densified nitrocellulose particles of this invention can be produced from any fibrous nitrocellulose, obtained by nitrating natural or artificial cellulose fibers, such as cotton, purified cotton linters, purified wool pulp, regenerated cellulose fibers, and the like, in such forms as picked linters, shredded wood pulp, fiuffed bulk linters, finely' ground or cut fibers, fiber aggragate particles, and the like. However, it is important that the fibrous nitrocellulose be substantially free of fiber aggregates for use in this invention, for such aggregates interfere with proper comminution and lead to formation of undesirably large particles.

Accordingly, it is both desirable and preferable to initially subject the fibrous nitrocellulose to a conventional jordaning, or similar fiber beating treatment, to break up fiber aggregates and generally shorten the fibers prior to use in this invention. To accomplish this, water-wet fibrous nitrocellulose, after conventional treatments for stabilization and viscosity adjustment, is slurried with water to the consistance of a conventional pulping slurry and is beaten in a jordan engine, or similar fiber heating device, to break up and disintegrate fiber aggregates and generally shorten the fibers. Excess water is then drained oif by any convenient means, preferably by centrifugation or suction filtration to produce water-moist nitrocellulose fibers containing between about 35% and about 50% by weight of sorbed water, based on total weight of the water-moist nitrocellulose fibers, and these water-moist fibers are used directly in the process of this invention, since water is necessary to dissolve and uniformly distribute the water-soluble protective colloid over the surfaces of the comminuted fiber fragments. These water-moist nitrocellulose fibers feel deep damp, and contain sufiicient water for the purposes of this invention. Although nitrocellulose fibers containing a greater quantity of water than indicated above could be employed, there is no advantage to be gained thereby. Moreover, since water is removed in the process of this invention, it is economically undesirable to use more water than indicated above.

Nitrocellulose fibers dehydrated with one of the lower alkanols such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, or tertiary butanol can be employed in this invention. In these cases, however, the alcohol of dehydration does not take the place of the water indicated to be necessary for the purposes of this invention, and an amount of water, as pointed out above, should be added to the alcoholdehydrated nitrocellulose, preferably before dispersing the nitrocellulose in the organic liquid swelling and softening medium. Instead, the alcohol of dehydration replaces active nitrocellulose solvent, approximately part for part by weight.

Substantially all commercial types and grades of fibrous nitrocellulose are suitable for the purposes of this invention, having nitrogen contents from about 10.9% to about 13.5% nitrogen by Weight, and of any viscosity characteristic from the very low viscosity, 10 centipoise type to exceedingly high viscosity types as exemplified by dynamite grade nitrocellulose.

The water-moist nitrocellulose fibers are introduced and dispersed in the organic liquid softening and swelling medium of this invention with vigorous agitation to form a uniformly smooth, readily stirrable slurry; and the upper practical limit for the amount of nitrocellulose fibers in the slurry is governed by the ability to agitate the slurry effectively. Generally, slurries containing from about 10% to about 20% by weight of fibrous nitrocellulose, dry weight, have been employed and preferably between about 14% and about 18% by weight of nitrocellulose. Although slurry consistencies of any concentration below about 10% by weight can be employed, it is not considered economical to do so.

The organic liquid mixture which is employed as the medium for the comminution and densification of the nitrocellulose is a mixture of hydrocarbon diluent and nitrocellulose solvent which is completely miscible with said diluent. It is important and necessary for the hydrocarbon diluent and nitrocellulose solvent to be proportioned in the mixture so that the mixture is only a swelling and softening agent for nitrocellulose fibers, incapable of dissolving said fibers. Suitable proportions of hydrocarbon diluent and nitrocellulose solvent to obtain this object varies depending principally on the particular hydrocarbon diluent and nitrocellulose solvent selected, and on the nitrogen content and viscosity characteristic of the nitrocellulose to be comminuted and densified. For example, when employing mixtures of lactol spirits and methyl ethyl ketone as the medium for comminution and densification of nitrocellulose fibers of 12.6% nitrogen content and about 5 seconds -inch falling ball viscosity, the ratio of lactol spirits to methyl ethyl ketone in the mixture can vary between about 1.25 and about 2.0 parts by weight. Substituting heptane for lactol spirits and employing methyl ethyl ketone and the same nitrocellulose, the ratio of heptane to vmethyl ethyl ketone in the mixture can vary between about 1.0 and about 1.2 parts by weight. Employing mixtures of lactol spirits and acetone as the medium for comminution and densification of nitrocellulose of 12.6% nitrogen and 5 seconds viscosity, the ratio of lactol spirits to acetone in the mixture can vary between about 2.6 and about 3.3 parts by weight. Employing mixtures of lactol spirits and methyl ethyl ketone as the medium for comminution of nitrocellulose of 11% nitrogenand 45 seconds viscosity, the ratio of lactol spirits to methyl ethyl ketone in the mixture can vary between about 1.3 and about 2.1 parts by weight. Employing nitrocellulose of 11% nitrogen and A second viscosity with mixtures of lactol spirits and methyl ethyl ketone, the ratio of lactol spirits to methyl ethyl ketone in the mixture can vary between about 3.3 and about 3.9 parts by weight.

However, with any particular selection of hydrocarbon diluent and nitrocellulose solvent, it is a simple expedient to carry out a preliminary trial by slurrying the desired nitrocellulose in the selected hydrocarbon diluent and then progressively adding the selected nitrocellulose solvent with agitation until the point is reached where the mixture begins to swell the fibers, using the above 1 5 examples as a guide. It is then a simple matter in practicing this invention to make minor increases or decreases in the ratio of hydrocarbon diluent to nitrocellulose solvent to obtain the desired shapes, density, bulk density, range of particle sizes, and the like.

Any volatile hydrocarbon which is liquid at ordinary temperatures and atmospheric pressure may be employed for the purposes of this invention including aliphatic, cycloaliphatic, aromatic, arylaliphatic and aliphaticaryl hydrocarbons, and mixtures of any of these. Some typical hydrocarbons include, by way of example, hexane, heptane, octane, isooctane, nonane, and the like, various proprietary petroleum distillate cuts such as textile spirits, mineral spirits, lactol spirits, V.M. & P. naphtha, gasoline, kerosene, and the like, cyclopentane, cyclohexane, methyl cyclohexane, benzene, toluene, xylene, ethyl benzene, styrene, a-methyl styrene, various proprietary aromatic hydrocarbon distilate cuts, mixtures of aliphatic and aromatic hydrocarbons, and the like. The preferred hydrocarbons for the purposes of this invention are aliphatic hydrocarbons which boil in the range of heptane or higher.

Nitrocellulose solvents suitable for the purposes of this invention are the lower molecular weight ketones, esters, gylcol ether-alcohols and glycol ether-esters which are soluble in water to the extent of at least about 2.5% by weight. Some typical nitrocellulose solvents which are suitable for practice of this invention include, for example, methyl formatc, ethyl formate, methyl acetate, ethyl acetate, isopropyl acetate, methyl propionate, acetone, methyl ethyl ketone, diethyl ketone, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, methoxyethyl acetate, and the like. Preferably, the nitrocellulose solvent should have a boiling point below the boiling point of the hydrocarbon diluent, or should form a minimum boiling azeotropic mixture with the hydrocarbon diluent and/or with water. Methyl ethyl ketone is especially preferred for the purposes of this invention since, in addition to having suitable solubility in water and forming a minimum boiling mixture with water, it is also free of any tendency to hydrolyze.

It has been pointed out hereinabove that a Watersoluble protective colloid is necessary for practice of this invention, and substantially any water-soluble protective colloid can be employed in this invention. Some typically suitable water-soluble protective colloids include, by way of example, water-soluble alkyl ethers of cellulose, hydroxyalkyl ethers of cellulose, mixed alkyl hydroxyalkyl ethers of cellulose, such as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose, and the like; watersoluble alkyl ethers of starch, hydroxyalkyl ethers of starch, mixed alkyl hydroxyalkyl ethers of starch, such as methyl starch, ethyl starch, hydroxyethyl starch, hydroxypropyl starch, methyl hydroxyethyl starch, methyl hydroxypropyl starch, and the like; alkali metal and ammonium salts of alginic acid, such as sodium alginate, ammonium alginate, potassium alginate, and the like; alkali metal and ammonium salts of naphthalene sulfonic acid, and lignosulfonic acid; alkali metal and ammounium salts of glucuronic, galacturonic, manuronic, and related acid structures present in various vegetable muciliages, pectins and gums; polyoxyalkylene gylcols, such as polyoxyethylene glycol, polyoxypropylene glycol, and the like; adducts of ethylene oxide and nonyl phenol, etc. Water-soluble methyl cellulose is presently preferred as the water-soluble protective colloid.

The water-soluble protective colloid can be added either in dry powdered or granulated form, or as a Water solution thereof. However, it is preferred to add the protective colloid in dry form, since the water present in the water-moist nitrocellulose fibers is sufficient to effect solution and uniform distribution of the water-soluble protective colloid over the surfaces of the comminuted particles. f.

The amount of water-soluble protective colloid employed should be no more than is consistent with the ability of the colloid to coat the comminuted particles with a thin but effective protective film, and will generally be between about 0.1% and about 0.5% by weight, based on the total slurry weight. Amounts substantially less than about 0.1% are generally insufficient for the purpose of this invention, and amounts greater than about 0.5%, although effective for coating the comminuted particles with a protective layer, unnecessarily increase the presence of a water-sensitive ingredient in the final product.

Although a Kady dispersion mill was employed in the examples as the means for subjecting the nitrocellulose fibers to high shear agitation, the invention is by no means limited in this respect, since any high speed attrition mill is effective, as for example, a Premier colloid mill, or similar device.

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

1. A process for preparing a densified, fine-particle nitrocellulose product which comprises (a) forming a slurry with agitation of Water-moist nitrocellulose fibers substantially free of fiber aggregates in a volatile organic liquid mixture of hydrocarbon diluent and nitrocellulose solvent which is completely miscible with said diluent in the presence of a water-soluble protective colloid;

(b) said organic liquid mixture being only a softening and swelling agent for said nitrocellulose fibers incapable of dissolving said fibers;

(c) said solvent being soluble in water to the extent of at least about 2.5% by weight;

(d) subjecting the resulting slurry of nitrocellulose fibers to high shear agitation, whereby said fibers become softened and swollen and break down into tiny, smooth surfaced, rounded particles;

(e) initiating hardening of the resulting comminuted and shaped particles of softened and swollen nitrocellulose by diluting the slurry with hydrocarbon diluent while continuing high shear agitation;

(f) and removing substantially all of the nitrocellulose solvent and water from the slurry by distillation in the presence of excess hydrocarbon diluent to complete hardening and densification of the nitrocellulose particles.

2. A process for preparing a densified fine-particle nitrocellulose product which comprises (a) forming a slurry with agitation of water-moist nitrocellulose fibers substantially free of fiber aggre gates in a volatile organic liquid mixture of petroleum hydrocarbon diluent and methyl ethyl ketone in the presence of a water-soluble protective colloid;

(c) subjecting the resulting slurry of nitrocellulose fibers to high shear agitation, whereby said fibers become softened and swollen and break down into tiny, smooth surfaced, rounded particles;

(d) initiating hardening of the resulting comminuted and shaped particles of softened and swollen nitrocellulose by diluting the slurry with said petroleum hydrocarbon diluent while continuing higher shear agitation;

(e) and removing substantially all of the methyl ethyl ketone and water from the slurry by distillation with agitation in the presence of excess of said petroleum hydrocarbon diluent to complete hardening and densification of the nitrocellulose particles.

3. A process in accordance with claim 2 in Which the petroleum hydrocarbon diluent boils within the range between about F. and about 240 F.

4. A process in accordance with claim 2 in which the nitrocellulose is a smokeless type having a nitrogen content of at least about 12.6% by weight, the petroleum hydrocarbon diluent is a petroleum distillate fraction boiling between about 201 F. and about 217 F., the initial ratio of said hydrocarbon diluent to methyl ethyl ketone in the slurry is between about 1.25 parts and about 2.0 parts by weight, and the water-soluble protective colloid is methyl cellulose.

5. A process in accordance with claim 2 in which the nitrocellulose is a smokeless type having a nitrogen content of at least about 12.6% by weight, the petroleum hydrocarbon diluent is heptane, the initial ratio of said heptane to methyl ethyl ketone in the slurry is between about 1.0 part and about 1.2 parts by weight, and the water-soluble protective colloid is methyl cellulose.

6. A process in accordance with claim 2 in which the nitrocellulose has a nitrogen content of about 11% by weight, the petroleum hydrocarbon diluent is a petroleum distillate fraction boiling between about 90 F. and about 240 F., and the water-soluble protective colloid is methyl cellulose.

7. A process in accordance with claim 2 in which the nitrocellulose has a nitrogen content of about 12% by weight, the petroleum hydrocarbon diluent is a petroleum distillate fraction boiling between about 190 F. and about 240 F., and the water-soluble protective colloid is methyl cellulose.

References Cited by the Examiner UNITED STATES PATENTS 2,715,574 8/1955 COX 149-2 2,885,736 5/1959 ONeill 1492 X 2,946,673 7/ 1960 Grassie 1492 X CARL D. QUARFORTH, Primary Examiner.

Claims

1.A PROCESS FOR PREPARING A DENSIFIED, FINE-PARTICLE NITROCELLULOSE PRODUCT WHICH COMPRISES (A) FORMING A SLURRY WITH AGITATION OFWATER-MOIST NITROCELLULOSE FIBERS SUBSTANTIALLY FREE OF FIBER AGGREGATES IN A VOLATILE ORGANIC LIQUID MIXTURE OF HYDROCARBON DILUENT AND NITROCELLULOSE SOLVENT WHICH IS COMPLETLEY MISCIBLE WITH SAID DILUENT IN THE PRESENCE OF A WATER-SOLUBLE PROTECTIVE COLLOID; (B) SAID ORGANIC LIQUID MIXTURE BEING ONLY A SOFTENING AND SWELLING AGENT FOR SAID NITROCELLULOSE FIBERS INCAPABLE OF DISSOLVING SAID FIBERS; (C) SAID SOLVENT BEING SOLUBLE IN WATER TO THE EXTENT OF AT LEAST ABOUT 2.5% BY WEIGHT; (D) SUBJECTING THE RESULTING SLURRY OF NITROCELLULOSE FIBERS TO HIGH SHEAR AGIATION, WHERBY SAID FIBERS BECOME SOFTENED AND SWOLLEN AND BREAK DOWN INTO TINY, SMOOTH SURFACED, ROUNDED PARTICLES; (E) INITIATING HARDENING OF THE RESULTING COMMINUTED AND SHAPED PARTICLES OF SOFTENED AND SWOLLEN NITROCELLULOSE BY DILUTING THE SLURRY WITH HYDROCARBON DILUENT WHILE CONTINUING HIGH SHEAR AGITATION; (F) AND REMOVING SUBSTANTIALLLY ALL OF THE NITROCELLULOSE SOLVENT AND WATER FROM THE SLURRY BY DISTILLATION IN THE PRESENCE OF EXCESS HYDROCARBON DILUENT TO COMPLETE HARDENING AND DENSIFICATION OF THE NITROCELLULOSE PARTICLES.