US3057012A - Process of preparing dense non-fibrous nitrocellulose - Google Patents

Description

Oct. 9, 1962 J. E. LUFKIN 3,057,012

' PRocEss oF PREPARING DENsE NoN-FIBRoUs NITRocELLuLosE Filed May 27, 1959 2 Sheets-#Sheet 1 FIG. I.

JAMES E. LUFKIN .Mum

ROLL NIP PRESSURE Il PSI.

OC 9, 1962 J. E. Lul-'KIN 3,057,012

PROCESS OF PREPARING DENSE NON-FIBROUS NITROCELLULOSE Filed May 27, 1959 2 Sheets-Sheet 2 I6 z DENSE N R0 cEL uLosE clon |4 l2 s l0 UD L 8 e 3 IB ou NIIR CELL L0 E w 4 REGION :L 2

lo l2 I4 I6 la 20 22 24 2628 3052 54 36 5a 4o 42 44 BULK DSIITY LBS. DRY/cu. FT.

DENSE NITRO CELLULOSE REGION Fl ROUS NITRO ELLULOSE RE I0 INVENTOR JAMES E. LUFKIN WKK-Lm ATT NEY (WIDE OPEN) o 20o 40o 600 800 MEDIAN PARTICLE SIZE-MICRONS United States The present invention relates to a method of treating nitrocellulose to render it more safe for shipment and storage. More particularly, the present invention pertains to a method for treating nitrocellulose which, in addition to making it safer, has the further advantages of greatly raising its bulk density, enhancing its dissolution properties, and greatly improving its ability to flow freely from containers in which it is shipped and/or stored.

The present invention is a continuation-in-part of my prior copending application Serial No. 682,581, filed September 9, 1957, now abandoned, which, in turn, is a continuation-in-part of my prior copending application Serial No. 607,255, filed August 31, 1956, now abandoned.

`Nitrocellulose is a cellulose derivative which has found extensive use in a Wide variety of industrie-s. It is prepared commercially by the direct nitration of cellulose in any convenient form, such as purified woodpulp or cotton linters. The nitration is usually performed with an acid mix consisting essentially of nitric acid, sulfuric acid, and water in suitable proportions, although other nitrating media are sometimes used.

Nitrocellulose is rarely, if ever, shipped or stored in a dry form because of its greatly increased sensitivity to ignite when dry. For this reason, nitrocellulose is almost always maintained in a wet form. Where the presence of moisture can be tolerated in the end use, the nitrocellulose is wetted with water. Commercial water-wet nitrocellulose characteristically contains about Ztl-25% water. For some end uses, however, the presence of moisture in nitrocellulose is extremely objectionable and in such cases the nitrocellulose is wetted with alcohol; usually ethanol, isopropanol, or butanol. Commercial alcohol-wet nitrocellulose normally contains about 30 to 35% total volatiles; the latter being primarily alcohol with small amounts of moisture. Throughout this speciiication, the term wet nitrocellulose is intended to designate nitrocellulose which has been wetted with water, alcohol, or other suitable liquid.

The density of nitrocellulose which has not been compressed or compacted in any way is in the neighborhood of about l pounds per cubic foot (dry basis). For storage and shipment, this 'material is rammed into cylindrical drums to a density of 20-25 pounds per cubic foot. 'Ihe commercial nitrocellulose drum contains on the average of about 135 to about 160 pounds of nitrocellulose (dry basis) per drum.

Though wetting the nitrocellulose, as described above, clearly minimizes the hazards of storing and shipping nitrocellulose, some danger still remains. For example, if an open standard commercial drum of alcohol-wet nitrocellulose should ignite, a violent eruption will ensue which will send a ball of fire upwards for a distance of 25 to 50 feet and may propel a shower of burning particles and sparks a considerable distance laterally outward from the drum. In the event of accidental ignition, any personnel in the immediate vicinity are likely to be seriously injured. In addition, the great shower of burning particles and sparks represents an extreme hazard in that other drums of nitrocellulose or other inflammable material in the area may also be ignited, leading to an even more widespread and dangerous conagration.

It is an object of the present invention to treat nitrolarent O 3,657,012 Patented Oct. 9, 1962 cellulose in such a way as to greatly minimize the effect of accidental ignition. It is a further object of the present invention to provide such a treatment which, in addition, will increase the bulk density of the nitrocellulose and enhance its dissolution properties. It is a still further object of the present invention to provide such a treatment which is convenient and economical. Other and additional objects Will be readily apparent from a consideration of the following specification and claims:

Broadly stated, my invention involves subjecting wet fibrous nitrocellulose to severe compressive pressures of a magnitude hereinafter specified in order to compact the wet nitrocellulose into a compact sheet or into a particu late form consisting of irregularly-shaped, flat particles which may be thereafter broken up into smaller particles by a mild granulating action if necessary. The compression may be performed in any suitable way, and the particular apparatus which is used forms no part of my invention.

For example, the wet fibrous nitrocellulose may be placed on a simple roll mill of the type which is conventional in the rubber industry for compounding rubber stocks prior to curing. Such a roll mill characteristically consists of a pair of cooperating rollers spaced a short distance apart and driven in opposite directions. One roller might be driven in a clockwise direction at a given speed, and the other might be driven in a counterclockwise direction at the same or a diiferent speed, or the second roller may idle. The space between the rollers is not generally critical insofar as my invention is concerned and may be varied widely; the preferred setting in any instance depending upon the type of nitrocellulose, the rate of nitrocellulose feed, and various other factors. Similarly, the thickness of the nitrocellulose disk-like particles or sheet formed by the rollers may vary from exceedingly thin films a few thousandths of an inch thick to relatively thick particles or sheets.

After the wet nitrocellulose has been compressed, as described above, it may be crumbled by subjecting it to a mild granulating action. I use the latter term in its broadest sense to include any mechanical working or agitation which tends to break up the flat particles or sheets into a smaller particulate form. In many cases the mere dropping of the particles or sheet from the rollers to the surface on which the rollers are mounted is sufficient to crumble all or a significant proportion of the compressed nitrocellulose. Depending upon the thickness of the sheets or particles and the type of nitrocellulose, it may be desirable to subject them to -a mild tumbling or agitation `or to the action of slowly rotating teeth to insure that the pressed nitrocellulose is reduced to a particulate form for packing into a drum. In this connection, any mild mechanical working is operable including, for example, shaking, crumbling, pulverizing, vibrating, chewing, comminuting, tumbling or the like. In many cases, however, the flat disk-like particles which result from the compression are only several inches wide on the average and may themselves be loaded into a drum without rst breaking them up into still smaller particles.

The invention will be better understood from a consideration of the attached drawings in which FIGURE l represents an elevational schematic view of one form of apparatus suitable for carrying out the process of invention. FIGURES 2 and 3 show in graph form the relationship between the pressure applied to the bulk density of the finished product, and to the particle size of the nitrocellulose starting material, respectively.

In FIGURE l, 1 and 2 represent a pair of abutting rollers; roller 1 idling on its shaft and roller 2 being driven in the direction indicated by any suitable drive means (not shown). Roller 2 is journalled in the end of a pair of hydraulic ram arms 3 by means of which roller 2 may be forced to bear against roller 1 under great pressure. Beneath the rollers 1, 2 is a shredder or comminuter containing two sets of parallel, rotating, intermeshing teeth 6, 7.

The wet nitrocellulose from hopper 4 feeds into the roller set 1, 2 and is there severely compressed into a hard, dense, compact, non-fibrous, sheet-like form or the like, which, in turn, falls through the comminute 5 Where the compressed material is broken up; the resultant particles falling for collection, for example, onto conveyor 8.

The process of the present invention is primarily applicable to the treatment of so-called industrial nitrocellulose, i.e., conventional fibrous nitrocellulose products having a nitrogen content of 10.8% to 12.3% and used industrially for lacquers, coatings, plastics, and the like, as distinct from guncotton and other nitrocellulose propellant products having a higher nitrogen content. The latter varieties of nitrocellulose, having a nitrocellulose content greater than 12.3% nitrogen are usually referred to in the trade as military grade or powder grade nitrocellulose. Though the invention may have some beneficial effects in connection with treatment of military grade nitrocellulose, the greatest benefits are derived in the industrial nitrocellulose field and this represents by far the most significant and preferred embodiment of this invention.

To ob-tain the principal advantages of the invention, as described more particularly hereinafter, it is essential that the compressive forces to which the brous nitrocellulose is subjected be of a certain critical minimum magnitude. This critical minimum pressure will vary somewhat on a case-to-case basis depending upon the nature of the nitrocellulose starting material.

The graph which appears in FIGURE 2 depicts the variation in bulk density of the compressed nitrocellulose product as a function of the pressure applied to effect the compression. The pressure was applied by feeding the nitrocellulose (alcohol-wet) continuously, at the rate of 3,000 pounds/hour, through equipment of the type illustrated in FIGURE l. The rolls were metal, 15 inches in diameter and 39 inches long, and were rotated at about 20 r.p.m. The family of curves shown in this graph relate to three different nitrocellulose materials. Curve A represents an industrial grade of nitrocellulose known to those skilled in the art as 5-6 second regular soluble. It contains 12% N2 and has a steel ball viscosity of 5-6 seconds measured in 5012 solution at 25 C. `Curve B represents a 1/2 second regular soluble grade containing 12% N2 and having a steel ball viscosity of 3-4 seconds in 5020 solution at 25 C. Curve C represents a 1A second regular soluble grade containing 12% N2 and having a steel ball viscosity of 2.5-4.0 seconds in 5025 solution at 25 C.

It will be noted from FIGURE 2, that as the pressure on the nitrocellulose feed is increased from zero, the bulk density of the resultant product increases fairly rapidly until the pressure reaches the vicinity of 6000-10,000 p.s.i. In this area, the slopes of the curves reverse and the rate of increase in bulk density increases more gradually as the pressure rises. Somewhere in the 6000-10,000 p.s.i. range the nitrocellulose starting material undergoes a basic change in character. At some point above about 6000 p.s.i., the nitrocellulose begins to lose its fibrous nature in favor of a hard compact essentially non-fibrous form, which I refer to as dense nitrocellulose in view of its greatly increased density compared with the ordinary fibrous material. This conversion from the conventional brous to the non-fibrous dense form, of course, does not occur suddenly, but rather occurs gradually starting at some point above a pressure of about 6000 p.s.i. The conversion is generally complete for all industrial grades of nitrocellulose when the applied pressure reaches about 10,000 p.s.i., and at some point intermediate between these two pressures, the nitrocellulose may be considered to be essentially the dense product having little or no fibrous components.

Thus, in FIGURE 2, points 1, 2, and 3 on curves A, B, and C, respectively, designate the lowest pressures at which the nitrocellulose was found to have passed from the primarily fibrous state into the essentially non-fibrous dense form. The line 1-23 connecting these points divides the graph into two general zones; the area below the line representing the conventional fibrous nitrocellulose region and the area above the line representing the non-fibrous dense nitrocellulose region with which the present invention is concerned.

One of the principal ways of characterizing nitrocellulose (aside from N2 content) and differentiating one industrial grade from another is in terms of inherent viscosity. The viscosity of different nitrocellulose grades is directly dependent upon the degree of polymerization of the nitrocellulose. In commercial practice, a reduction in viscosity, i.e., lowering the degree of polymerization, is accomplished by means of a high-temperature digestion. As is well-known to those skilled in the art, a reduction in degree of polymerization, by digestion or otherwise, is invariably accompanied by a corresponding reduction in the lengths of the individual fibers, i.e., fiber particle size. It is possible, therefore, to identify the nitrocellulose products represented by the family of curves in FIGURE 2 in terms of iiber particle size rather than by specific nitrocellulose types. This is particularly convenient inasmuch as the pressure-density relationship shown in FIGURE 2 has been found to hold true for liber particle size variations even within a single nitrocellulose species. That is to say, a family of curves similar to A, B, and C of FIGURE 2 can be drawn for three different nitrocellulose starting materials which differ only in fiber particle size and in no other way. Fiber lengths can be reduced substantially by purely mechanical means, eg., in an attrition mill, without significant effect on the degree of polymerization of the product.

The particle size distribution of fibers may be readily determined in an accurate manner, for example, by means of a four-screen Clark Classifier, manufactured by the Thwing-Albert Instrument Co. This latter instrument is widely used and relied on in the paper and pulp industry. See A. E. Reed and I. dA. Clark, An Instrument for Rapid Fractionation of Pulp, TAPPI (published by the Technical Association of the Paper and Pulp Industry), vol. 33, No. 6.

By means of a four-screen Clark Classifier, the median fiber size for the three nitrocellulose products represented by curves A, B, and C in FIGURE 2 was determined to be 2000, 650 and 160 microns, respectively. The median fiber size represents that fiber length which is smaller than the half of the fibers in the lot classified and larger than the remaining half.

It is therefore possible to plot the minimum critical pressure required to produce an essentially non-fibrous dense nitrocellulose in accordance with the invention as a function of median nitrocellulose particle size. Such a graph 1s depicted in FIGURE 3 for the range of median particle sizes encountered in ordinary industrial nitrocellulose products. At either end of the straight line shown, a complete plot would show a curve approaching a vertical asymptote, but this is of no real significance insofar as ordinary industrial nitrocellulose products are concerned. With regard to the latter, the relationship is essentially a linear one. For ordinary industrial nitrocellulose products, i.e., those having a nitrogen content of 10.8% to 12.3% and a median fiber length of to 3000 microns, the minimum critical pressure required to convert the ordinary fibrous material to an essentially non-fibrous dense product may be taken as the slope of the line shown in FIGURE 3, or expressed mathematically as:

where P is pressure in pounds per square inch and M 1s the median fiber particle size in microns. The

pressure P, determined in accordance with this relationship, represents the critical minimum pressure which must be applied to the conventional fibrous nitrocellulose composition in order to convert it to the improved essentially non-fibrous, dense form having the many novel characteristics and advantages hereinafter described.

The invention is further illustrated =by the following examples.

Example 1 200 pounds per hour of alcohol-wet nitrocellulose (12%N) containing 19% isopropyl alcohol and having a median liber particle size of 650 microns were fed continuously to a pair of cooperating metal rollers 6 inches in diameter and l2 inches in length. One of the rollers was rotated in a clockwise direction at about 20 r.p.m. and the other roller was rotated in a counter-clockwise direction at about 28 r.p.m. The peripheries of the rollers were spaced about 0.015 inch apart. The force applied to the rollers was such that the pressure on the nitrocellulose in the nip was about 17,000 p.s.i. The compressed wet nitrocellulose disk-like sections which emerged from the rollers were permitted to fall 8 inches to the oorpan beneath the rollers where a substantial proportion of them crumbled due to the impact of the fall.

Example 2 Approximately 3000 pounds per hour of alcohol-wet 5-6 second regular soluble nitrocellulose (12% N2, containing 21% ethyl alcohol) having la median liber particle size of about 2000 microns, were fed continuously to a pair of cooperating metal rolls inches in diameter and 39 inches in length. One of the rolls was rotated in a clockwise direction at about r.p.m. and the other was rotated in a counter-clockwise direction at approximately 20 r.p.m. A total pressure of about 50 tons was applied by means of hydraulic cylinders bearing upon both ends of one roll; the second roll being lixed. This total pressure was equivalent to a pressure on the nitrocellulose of about 16,500 p.s.i. The dense, compact, compressed wet nitrocellulose disk-like sections which emerged from the rolls were permitted to fall 10 inches into a cutting device, comprised of a horizontal rotating shaft (150 r.p.m.) and a stationary Shaft, each iitted with a series of intermeshing T-shaped blades. The final products consisted of iiakes averaging about 1 to 2 in.2 in area and 0.040" thick. The free-flowing product was readily packed into `a standard ICC-6J shipping container at 200 lbs. dry nitrocellulose and adjusted to a iinal alcohol content of Example 3 Approximately 3000 pounds per hour of water-wet nitrocellulose (11.6% N2), containing 23% water, were fed continuously to the same pair of cooperating metal rollers utilized in Example l. The roller speeds and loading pressure, as well as the method of disintegrating the disk-like sections produced were identical to those specilied in Example 1. The final water-wet, dense nitrocellulose product consisted of akes averaging about l to 2 in.2 in area and 0.040 thick. The free-iiowing product was readily packed into a standard ICC-6J shipping container at 200 lbs. dry nitrocellulose and adjusted to a iinal water content of 23%.

Example 4 Approximately 3000 pounds per hour of alcohol-wet, one-half second regular soluble nitrocellulose (12% N2, containing 21% ethyl alcohol), having a median fiber particle size of about 650 microns, were fed continuously to the same pair of cooperating metal rollers utilized in Example 1. The roller speeds and loading pressure, as well as the method of disintegrating the disk-like sections, were identical to those specified in Example 1. The rfinal alcohol-wet, dense nitrocellulose product consisted 5 of flakes averaging about l to 2 in.2 in area and 0.040" thick. The free-flowing product was readily packed into a standard ICC-6J shipping container at 240 lbs. dry nitrocellulose and adjusted to a final alcohol content of 25%.

The tests I have performed indicate quite conclusively that nitrocellulose which has been treated in acc-ordance with the present invention is much safer to ship and to store than is the conventional fibrous nitrocellulose currently available from commercial sources, as is illustrated by the following examples.

Example 5 An ICC-6] galvanized steel drum (inner diameter 221/2 inches, inner height 33% inches) was filled to the 50% level with ordinary commercial, isopropanol-wet, librouse nitrocellulose (12% nitrogen, 30% total volatiles). A second identical drum was iilled to the 50% level with the treated nitrocellulose material of Example l adjusted to a 30% total volatiles content. Both drums were ignited simultaneously with separate squibs. The results of both ignitions are indicated by the following table:

The procedure of Example 5 was repeated with both the regular and treated nitrocellulose being first adjusted to a 25% total volatiles content. The results of both ignitions are recorded in the following table:

Time after ignition (min.) Regular un- Treated material treated material of Example 1 lfm mild name mild name. Do. Do. Do. Do. D

small flare-up.

10 do Do. 12 (flame extinguished With Water no nitrocellulose a little nitrocellufrom rehose). left. lose left in bottom.

Example 7 The procedure of Example 5 was repeated with the treated nitrocellulose being first adjusted to a 20% total volatiles content. The results of both ignitions are recorded in the following table:

Time after ignition (min.) Regular untreated Treated material of material Example 1 liz mild ame mild flame.

V start eruption Do.

1 end eruption.. Do.

2 mild flame small flare-up.

t e gu ed with alittle nitrocellulose of the orlginal Water from rehose) left in bottom. nitrocellulose u nconsumed.

ln Examples 5-7, the term mild flame refers to a low quiet flame extending no more than about 1-2 feet into the air above the top surface of the nitrocellulose. By small are-up is means a modest flame of some vigor extending 5-10 feet in the air. The term eruption designates a vigorous and extremely active flame shooting upwards a distance of 25-50 feet in the air and propelling a shower of burning particles and sparks laterally outward for a considerable distance, 20-40 feet.

It will be readily apparent from the foregoing that in the event of an accidental or spontaneous ignition, drums containing applicants treated nitrocellulose are considerably safer than drums containing ordinary commercial material. Upon ignition, the latter burns with a vigor and intensity which in most instances is many times greater than that of the treated material. A further factor of equal importance is that the regular material ares up much more quickly than does the treated material. Upon ignition, the regular material erupts almost instantaneously giving little or no time for personnel in the area to escape or take steps to extinguish the flame. The treated material, on the other hand, either does not erupt at all or flares up only after a time lag sufficiently long to permit personnel to stand clear or to extinguish the blaze.

Numerous advantages accrue from the above-described burning properties of nitrocelulose treated in accordance with the present invention. The treated material is, of course, much safer in the event of ignition with respect to personnel and property in the vicinity since the blaze is much less severe and more easily extinguished if it Should occur. In view of these improved safety characteristics, it may very well be possible to safely reduce the total volatiles content, i.e., the alcohol with which the nitrocellulose is wetted, thereby effecting a substantial savings in the alcohol and in freight costs.

In addition to the safety and attendant advantages which are achieved by means of the present invention, several other incidental but extremely important advantages also result. For one thing, the bulk density of the treated material is much higher than that of the ordinary material. It is thus possible to pack the customary contents of a commercial nitrocellulose drum (about 230 lbs. of material) in a container which is 25-30% smaller in volume. On the other hand, if the container is not reduced in size, it is now possible to pack the container with up to 25-30% more nitrocellulose than has heretofore been possible. From the standpoint of economy in shipment and storage, this is obviously an extremely valuable achievement. The following table illustrates the differences in bulk densities between several types of regular commercially available nitrocellulose products before and after they have been treated in accordance with the process of the present invention:

Bulk Density (Dry Basis) Total Nitrogen, Wetting Agent Volatiles,

Percent Percent Regular Treated Product Product (lbs/cu. ft.) (lbs/cu. it.)

12.0 Isopropanol.-. 19 9. 1 16. 6 12.0--- do 20 14.2 24.0 21 6. 6 12. 1 6 23 9. 8 14. 0

In addition to a marked increase in the bulk density of the nitrocellulose, the process of the present invention also enhances the ability of the nitrocellulose product to enter into solution, as is indicated by the following example.

Example 8 30 seconds with a single-paddle stirrer, one inch from the bottom operating at 300 rpm. 989 grams of toluene was then added to each beaker and the contents of the beakers were then agitated for an additional 30 seconds with the stirrer. Thereafter, 359 grams of 88% ethyl acetate was added to each beaker and the agitator was turned on again. The agitator was permitted to run continuously except that it was stopped every l5 minutes to check the solution until the nitrocellulose was completely dissolved. On this basis, the regular commercial material was found to completely dissolve in 21/2 hours whereas the material which had been treated in accordance with the process of the present invention dissolved in a period of only 11/2 hours.

In addition to all of the foregoing advantages of the present process, nitrocellulose which has been treated in accordance with the present invention has the still further advantage of remaining free-liowable at all times even when stored in drums for extended periods. Regular commercial material when placed in a container invariably forms a matted mass known as hard-pack, and the resistance of this hard-pack to flow has plagued nitrocellulose consumers for many years. In order to empty a commercial container of ordinary wet nitrocellulose, it is necessary for the operator to tilt the container and to extract the material manually with a pitchfork or similar implement or to use some other mechanical aid to break the hard-pack and dislodge the nitrocellulose so that it will flow from the container. 'I'his is an inconvenient and money-wasting operation from the point of view of most industrial nitrocellulose consumers and one which is completely eliminated by the present invention. Nitrocellulose which has been treated in accordance with the present process will not form hardpack and will flow freely and quickly from any container in which it has been stored, even for long periods.

When treating nitrocellulose in accordance with the present invention, any suitable means for compressing the wet nitrocellulose into compact sheets or into flat, disk-like particles may be used in lieu of a roll mill. For example, intermittent ramming may be used, or a single heavy roller on a ilat surface. It will be readily apparent, however, that the continuous feature of a roll mill offers attractive economic advantages over other suitable methods, though the invention is by no means limited to this particular type of apparatus.

According to the present invention an essentially nonbrous dense nitrocellulose product having all of the various advantages and improved properties described above and may be prepared by vsubjecting a conventional fibrous nitrocellulose product to compressive forces of a minimum magnitude defined by the following equation:

wherein P equals the pressure in pounds per square inch and M equals the median particle size in microns, and thereafter mechanically Working or breaking up the resultant compressed product. It is vital, however, that the pressure applied be not less than the amount specified, as determined by the above-named relationship, if the improved properties and advantages of the present invention are to be obtained. I have found that the objectives of the present invention cannot be achieved or the principal advantages realized if the pressure to which the nitrocellulose feed is subjected in the course of the compression is not at least as great as the minimum specified. Lower pressures, though sometimes effecting a slight improvement in some of the properties of a nitrocellulose, will not accomplish the improvements described above, especially the improved safety characteristics, dissolution, and How properties, and the increased density. The lower pressures, short of the minimum specified, simply compact the nitrocellulose into large, thick, irregularly shaped plates which tend either to be converted `back to their original fibrous condition or to be broken into hard solid lumps when subjected to the use of a mechanical shredder.

As noted above, the pressure may be applied to the nitrocellulose in any suitable manner as, for example, by a pair of cooperating rollers, a single roll and plate, intermittent ramming, or the like. A pair of cooperating rollers arranged as illustrated in FIGURE 1 represents a most convenient way of carrying out the invention, and therefore, represents the preferred embodiment of the invention. Generally speaking, any pressure greater than that indicated above as the minimum requirement is suitable. I have found, however. that pressures in the vicinity of 15,000 p.s.i. or higher, which is substantially above the minimum required, yield consistently fine results inasmuch as such pressures consistently serve to convert the conventional fibrous nitrocellulose into a compact, dense, completely free-flowing product exhibiting all the properties described above. For most industrial nitrocellulose products, therefore, I prefer to operate in the range of 15,000 to 17,000 p.s.i. though lower pressures Within the limits defined above are definitely operable. The only upper limitation on the amount of pressure which may be employed is the pressure at which the nitrocellulose feed begins to char. As a practical matter, however, economic considerations would dictate that pressures above the preferred 15,000 to 17,000 p.s.i. range would rarely be used since they offer no particular advantages.

Throughout the specification, pressures to which the fibrous nitrocellulose is subjected are always referred to in pounds per square inch. It must be borne in mind that the precise pressures pertinent in any case may vary slightly from those figures mentioned depending on the size, efficiency, and surface conditions of the particular rollers or other equipment utilized to practice the invention. To some extent, even the age of the equipment will be a slight factor affecting the pressure employed. For example, the spacing between the rollers shown in the attached FIGURE 1 on one occasion was nil prior to the start of the nitrocellulose feed, but after the rollers were in operation for some time a spacing of about 0.040 inch Was noted when the feed was stopped due t flexing of the rollers and compression in the hydraulic loading system. These individual equipment characteristics will,

therefore, affect slightly the actual pressures required in any single instance, but the dierences from the figures mentioned will never be very great, running in the magnitude of a few percent at most from case to case.

The shape or dimensions of the compressed nitrocellulose sections which result from the compression step is not critical to the invention. The nitrocellulose may be pressed into the form of relatively long continuous sheets or it may be pressed into numerous relatively small (a few inches), individual, flat, irregularly-Shaped, disk-like particles. The latter are more likely to result if a roll mill is used and represents the preferred embodiment of the invention since it requires a minimum, if any, of subsequent mechanical working to break up the compressed nitrocellulose.

Having thus described my invention, I intend to be limited only by the following claim:

A method for improving the safety characteristics and dissolution and ow properties of Wet, fibrous, industrialgrade nitrocellulose having a nitrogen content of about from 10.8 to 12.3% by weight which consists of subjecting the wet, fibrous, industrial-grade nitrocellulose to severe compressive forces which are not less than that determined by the relationship:

wherein P equals pressure in pounds per square inch and M equals the median fiber particle size of the nitrocellulose starting material in microns, and thereafter mechanically breaking the resulting product into particles of smaller size.

References Cited in the file 0f this patent UNITED STATES PATENTS 1,896,642 ONeil Feb. 7, 1933 1,978,071 York Oct. 23, 1934 2,210,871 Boddicker Aug. 6, 1940 OTHER REFERENCES Bridgeman: The Compression of 46 Substances to 50,000 Kg./Cm.2, Am. Acad. of Arts & Science, vol. 47, No. 3, October 1940.

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