US3376040A - Compacted frangible target of agglomerated particulate material - Google Patents

Description

April 2,- 1968 .v c, MOEHLM ET AL 3,376,040

COMPACTED F GIBLE TAR 0 GGLOMERATED TICU E ERI Filed 1964 INVENTORS 5 VERNON c. EHLMAN ROBERT KLEIN BY ATTORNEY United States Patent 3,376,040 0MPACTED FRANGIBLE TARGET OF AGGLOM- ERATED PARTICULATE MATERIAL Vernon C. Moehlman, St. Louis, and Robert J. Klein,

Florissant, M0,, assignors t0 Olin Matliieson Chemical Corporation, a corporation of Virginia Filed Jan. 16, 1964, Ser. No. 338,031 6 Claims. (Cl. 273-4054) This invention relates to frangible targets of the type adapted to be launched from throwing devices, such as that described in US. Patent No. 2,125,812, granted Feb. 23, 1960, and which once in flight are adapted to be broken to register a hit by at least one shot pellet.

An object of this invention is an improvement in the composition of such a. target which may be produced more economically than targets heretofore available.

Another object is the fabrication of such projectable and frangible targets from an improved composition of molding powder enabling target production not only with speed and economy, but also with reproducible reliability as to their required conformity to accepted standards of projectability and frangibility.

Still another object is an improved process for making frangible targets by compaction of a powder in a tableting press rather than by the pouring and molding of a fairly viscous but fluid mix of hot bituminous material and filler in devices of the type shown in US. Patent Nos. 1,238,980 and 2,300,290, accompanied by evolution of considerable fumes and vapors.

Other objects and advantages will come to mind from a description of a preferredembodiment when taken in connection with the accompanying drawing in which:

FIG. 1 is an elevational view in cross section taken on a diameter of a target made by this invention;

FIG. 2 is an enlarged cross sectional view of some typical granules constituting the agglomerate used to make the target; and

FIG. 3 is an enlarged cross sectional view diagrammatically showing the internal structure of a representative portion III of the target of FIG. 1.

According to this invention filler material, such as limestone of suitable fine granulation, has all of its particles at least partially and many thoroughly coated with a brittle binder such as a pitch of the proper character in the least amount found necessary to achieve some particle encapsulation and substantially complete agglomeration and to produce a relatively coarse highly cellular molding powder the granules of which are adapted to be dry charged and readily fiowunderpressure in the forming die of a tableting press where the technique of briquetting is used in such a way that the granules are compacted to the size and shape of a target. In compaction substantially complete limestone particle encapsulation is achieved in a matrix of pitch. Although it may be done with some success using a hot mix, compaction preferably is done cold and is followed by sintering to produce targets having acceptable appearance and vulnerability to fracture by impact with shot pellets while at the same time having resistance to undue breakage in handling, shipping, and launching under all conditions of climate encountered.

Briefly, the particles of limestone while hot are thoroughly mixed with coarse pitch granules in a minimum amount found suitable. This mixture is then agglomerated and processed to a desired particle size distribution to make a powder which can be molded to shape and size of the target by pressure in contradistinction to melting and casting.

The targets produced in this way are saucer-shaped (FIG. 1) having a weight of from about 95 to 105 grams, a diameter of about 4 /2 inches at the rim 1 and a height of about 1 inch at the annular ridge 2 formed at the top of arch 3 about a central portion 4 known as a poker chip upraised with respect to the rim but depressed with respect to the ridge. These have a somewhat thinner wall but a density of about 2.1 grams per cc. as compared to the wall thickness and 1.9 grams or less per cc. presently usual in targets of this type. Each is quite a special kind of briquette having very critical properties of both coherence and frangibility.

Specifically, the pitch content of the target is at least about 8% by weight but not more than about 25%. Preferably, the pitch content is kept in a range between about 8 and 20%. A pitch content of about 8 to 15% by weight was found to be the best in a standard target weighing around 100 grams. For targets of the miniature class used with pellets shot from a .22 cartridge, the proportion of pitch is increased by not more than about 5 to 10%, not to exceed about 25%.

A 12 to 15% pitch: to 88% limestone mixture is preferred where fine limestone of size distribution all less than about 20 mesh and largely of less than 200 to 230 mesh is used. A 15% pitch:85% limestone mixture appears to be one of the best. This, however, does not preclude smaller amounts of pitch in the target, especially when the limestone is somewhat coarser because of the presence of a higher proportion of particles in the 200 to mesh range, for example, largely less than 100 mesh.

Heretofore, targets have been made of a bituminous material, such as coal tar pitch and a filler material, such as clay or ground limestone, but to obtain a mixture liquid enough to pour and also the requisite properties in the finished target it has been considered necessary to incorporate a relatively high proportion of pitch in the.

mixture despite the fact that the pitch is more costly than the filler. By the present invention the proportion of pitch can be considerably reduced. This is possible since the targets are briquetted from composite porous granules made by pressureless pelletization.

Furthermore, molding by compaction of powder agglomerate according to this invention has been found to further reduce the cost of the targets as compared to molding of a fluid admixture by the prior process comparable to injection molding or die casting. The compact is brought to a finished condition by sintering, which may be done in any suitable way but preferably by radiant heating such as by infrared quartz lamps, thus producing a fusion and capillarity of the pitch and a harder, rougher, and more cohesive but more brittle surface layer 5 on the finished product.

This occurs because sint-ering drives off exess volatiles mostly from the surface, without either melting or embrittling the whole target. The whole target is thus heat treated most intensely at the surface.

MATERIALS A. The limestone particles used have a varied granulation preferably in a range of size distribution such that at least about 45% and as much as about 65% pass through from about a 200 mesh to about a 230 mesh screen but all through about a 20 mesh screen (US. Sieve Series). Too fine a limestone gives t-oo fine a mixture too dead to mold well. Too coarse a limestone crushes excessively upon compaction and causes pitch exudate as well as poor mobility.

B. The pitch used is usually coal tar pitch having a suitable melting point and a substantially full content of naturally occurring low-boiling volatiles; this pitch is added in the form of relatively coarse particles having a granulation finer than about 10 mesh but mostly about 20 mesh (US. Sieve Series).

By pitch is meant a friable binder such as a hard coal tar pitch with (1) high resin content imparting the necessary hardness, brittleness, and cohesiveness and (2) relatively low tar oil content constituting around 20% of the pitch and which, if not lost, gives a content of low boiling oils found useful in the compaction process. Therefore, admixture of the pitch and limestone is done at as low as possible an elevated temperature in as short a time as possible. The resulting agglomerate should not be permitted to age. To avoid excessive crystal growth and/ or loss of these volatile oils, residence time from mixing to compaction should not exceed about 75 hours. Pitch having a melting or softening temperature in the range of about 170 F. to about 350 F. may be used but a pitch having a melting point of at least 190 F. and still better up around 250 F. is preferred so as to provide volatiles and to produce a target stable enough at temperatures up to about 140 F. on the shelf and in the field. The 250 F. pitch spreads well not only when heated for agglomeration but also when pressed for molding.

C. The molding powder, developed by agglomeration includes considerable encapsulation of the limestone and also many pitch-rich nuclei; it consists of rather coarse granules of agglomerate mostly of about 20 to 50 mesh size, few, if any, finer than about 100 mesh and only a few of to 20 mesh size. This makes for better flow under pressure than a fine powder in which the limestone may be more uniformly and more thinly coated with pitch but which is too dead to move properly in the die even when heated. Pelletization is used to advantage.

The approximate structure of the rough granules of the resulting molding powder as shown in FIG. 2 may be described as involving (1) a somewhat depleted nucleus or core 10 high in pitch surrounded by (2) a mantle of many thinly pitch-encapsulated limestone particles 11 attracted to the core and each entrained in a matrix 12 of pitch becoming thinner the farther it runs from the core. The mantle also includes (3) ports 13 and (4) many partially encapsulated particles 14, particularly where the agglomerate has become a large cluster as in a large granule 20. A smaller granule 21 may consist of the core 10 of pitch coated with what amounts to a single layer of pitch encapsulated limestone particles 11 except for an occasional partly encapsulated limestone particle 14, many of which exist in the form of an appendage, which occurs on all the granules and which also contributes to the desired roughness of the granules. Granules such as 20 and 21 are those which have remained pretty much as they were agglomerated in the mixture. Other granules such as 22 are those which were fractured either in mixing, pulverizing, or screening so that the pitch rich area does not appear exactly as a nucleus or core because it is offset to one side, being a subdivision of the core of a larger cluster originally.

All of the molding powder granules are characterized by much surface roughness and irregularity of shape. The mantle portion, particularly in the larger clusters, is characterized by the presence of fairly high proportion of open voids 13. These, together with the surface roughness, make the density of the resulting agglomerate granule and molding powder quite low despite the high pro portion of limestone.

The bulk density of the molding 1.5 grams per cubic centimeter.

As shown in FIG. 2 the irregular granules are of various sizes in a distribution range largely within mesh and 50 mesh; preferably practically all pass on a 10 mesh screen and all are retained in a 100 mesh screen. In any event, the molding powder is coarse compared to the granulated limestone so that about 90% or more by weight is retained on a 200 mesh screen, when some fines are included.

The surface roughness, irregularity of shape, presence of voids, and the size of granules in the agglomerate make for a good molding powder of superior cold flow and cohesiveness under pressure.

powder is about 1.0 to

4 PROCESSING I. Agglomeration of the limestone filler particles with pitch binder is done in a pelletizer or mixer. First, the limestone is placed in the mixer where it is heated to at least about 260 to 300 F. but above the 170 to 350 F. softening point of the pitch at a temperature depending not only upon the pitch melting point but also upon the fineness of the filler. The limestone may be heated before mixing, which is done, however, preferably around 400 to 450 F. Granules of pitch are then added and mixing, preferably in a closed system so as not to lose volatiles, is continued until all of the white of the limestone disappears to the naked eye, and clusters of black and dark grey appear. The hot limestone sticks to and sinks into the pitch until agglomerates are formed about nuclei of pitch. Each nucleus is a core which during mixing heats up, oozes out to form a limestone rich mantle, and feeds the mantle of the growing clusters for further agglomeration after which some pulverizing of the agglomerate occurs in the mixer. As the mixture becomes more homogeneous upon further mixing, thinning out of the matrix and depletion of the nuclei occurs, and the growth of the clusters stops and fines may occur. Further mixing, hence, is stopped when practically all of the limestone particles are at least partially encapsulated, but before too many fines develop.

II. The agglomerated mxture is granulated in a desired size. Most of this may occur in the mixer as the agglomerate is cooled and then only the coarse granules separated are pulverized and sized as by screening through about a 10 to 20 mesh screcn on about a 50 to mesh screen. Also, all the agglomerate formed may thus be pulverized and sized to effect desired granulation and separation. The fines, if produced, are separated for reuse to make more agglomerate.

III. The molding powder is fed to the die of a tableting press where compaction to size and shape is done at pressures of from about 6 /2 to about 8 /2 tons per square inch at temperatures Well below the melting point of the pitch, preferably at from normal room temperatures to about F., using 250 F. pitch. A press force of from about 100 to about tons applied over a target of a diameter of 4% inches is satisfactory for cold compaction, done in a suitable briquetting press.

IV. Upon ejection of the green target from the press, it is heated for a short time of from half a minute to about two minutes and thus sintered. The surface is most thoroughly sintered by heating to a temperature of at least 350 F. but not in excess of 600 F. Where the molding powder and die are used at room temperature, sintering is conducted for about one and a half minutes at temperatures approaching 550 F.

It will be appreciated that for achieving desired factors of appearance, stability, cohesiveness, and frangibility, it is necessary to adjust the proportions of ingredients, the granulation, temperature of the molding powder, the press force, and the temperatures and time of sintering until exactly the right balance of these factors is obtained.

An alternate processing procedure is to feed the hot molding powder as soon as agglomerated, and before too many fines form, to a hot die in the press to be compacted hot without further need for granulating and sintering. Cold compaction followed by suitable sintering is preferred; however, this gives better control of the composition and flow in dies allows screening out fines and avoids high loss of violatiles and growth of graphite-like crystallites too soon. For either hot or cold compaction it is contemplated that the same novel composition of agglomerate and target be used.

In compaction the voids 13 in the agglomerate (FIG. 2) tend to be expressed from or greatly reduced in the clusters 20, 21, and 22 which in turn are deformed and flow under pressure so as to close up the irregular gaps occurring between them as the granules of the molding powder move in the die. In this action the pitch helps drive out and reduce the voids both in the clusters and the closing gaps between them as it is extended to complete the encapsulation of all particles and to form the network of matrix 32 and its cells 33, each closed.

As shown in FIG. 3, this makes in the target a fairly dense structure of limestone particles 31 crushed together except for a cellular matrix 32 of pitch which substantially completely fills the interstices between the particles except for an occasional small void or cell 33 in the matrix. In the surface 5, particles 31 are crushed together most closely and some even broken and the pitch 32 is finally sintered to cause fusion and enrichment of the brittle resin content there.

The finished target is so densified that no less than about 85 to 95% of its volume is made up of the limestonepitch mixture and the balance voids 33.

In the finished target, because of heating during agglomeration and sintering and discard of fines from the agglomerate, the proportion of pitch is not much difierent than at the start and changes, if at all, by no more than about 0.5 to 1.5%. For example, starting with pitch in the mixer, the pitch content in the target is around not less than about 14.5 to 13.5%.

The targets have a density of from about 1.95 grams per cc. to about 2.25 grams per cc. and both the limestone and the pitch become more uniformly distributed in the compacted target having a minimum of pitch for the purpose.

It will be recognized that the molding powder granules of pelletized limestone and pitch include (1) a fairly wide distribution of non-uniform sizes of the limestone, predominantly fine particles in the range set forth, (2) a skeletal matrix of the pitch, and (3) various sizes of irregular-walled voids or pores generally interconnected and open to give porosity. The granules of the molding powder itself are not of uniform size but of a continuous wide range of generally coarse size, although the powder includes many finer granules in this distribution range, which are comparable to some of the coarser limestone particles. This pelletized mixture is of advantageous mobility and formability in the target die. It produces targets which compare favorably to those heretofore made using fluid pitch vehicle filled with from as little as to as high as about 65% limestone by weight.

What is claimed is:

1. A projectable target of the frangible type having a body portion composed of a coherent but brittle compact of agglomerated granules comprising a filler material of limestone particles and particles of coal tar pitch binding said limestone particles, said pitch constituting from about 8 to about 25 percent of said target by weight and having a melting point of about 170 to 350 F., said body composed of not less than from about 85 to 95 percent by volume of said granules and the balance cellular voids.

2. The target of claim 1 wherein the pitch constitutes from 8 to 20 percent of the target weight.

3. The target of claim 2 wherein the filler material is fine ground limestone, the particles of which have a size distribution all less than about 20 mesh such that at least about 45 percent and as much as about percent are of less than 200 to 230 mesh and wherein the pitch constitutes from 8 to 15 percent of the target weight and has a melting point of about 250 F.

4. A projectable target of the frangible type having a body composed of a coherent but brittle compact of agglomerated granules briquetted in the size and shape of a target, said body consisting of from 12 to 15 percent by weight of coal tar pitch having a melting point of about 250 F. and the balance particles of ground limestone of a size distribution all finer than about 20' mesh such that about 65 percent are less than 200 to 230 mesh, substantially all completely encapsulated in said compact by said pitch filling the interstices between said particles to the extent that the target density is from about 1.95 grams to about 2.25 grams per cubic centimeter, said body includ ing from about 5 to about 15 percent by volume cellular voids.

5. A projectable target of the frangible type having a body composed of a compact of granules comprised of particles of limestone agglomerated with particles of coal tar pitch briquetted to the size and shape of a target and to a substantially uniform target density of from about 1.95 grams per cubic centimeter to about 2.25 grams per cubic centimeter, said target consisting of not less than about from to percent by volume of said limestone and pitch and the balance cellular voids, said granules consisting of not more than about 15 percent by weight of said pitch having a melting point of about 190 F. to about 250 F. and of not less than about 85 percent by weight of said limestone particles, said limestone particles in the target being each substantially encapsulated in said pitch and having a size distribution all of less than about 20 mesh and largely of less than about mesh.

6. A projectable target of the frangible type having a body composed of a pressed and sintered briquette of agglomerated granules of limestone particles and coal tar pitch, said pitch constituting about 15 percent of said target by weight, said agglomerated granules constituting from about 85 to 95 percent of said body by volume and the balance cellular voids, the density of said body being 1.95 to 2.25 grams per cubic centimeter.

References Cited UNITED STATES PATENTS 2,211,244 8/1940 Routledge 273105.4 2,418,936 4/1947 Hutchinson et al. 273-1054 X 2,831,778 4/1958 Allison et al. 106281 3,124,471 3/1964 Spetzler et al 10663 3,196,022 7/1965 Weaver 106-38 FOREIGN PATENTS 932,544 7/1963 Great Britain.

ANTON O. OECHSLE, Primary Examiner. DELBERT B. LOWE, Examiner. M. R. PAGE, Assistant Examiner.

Claims (1)

1. A PROJECTABLE TARGET OF THE FRANGIBLE TYPE HAVING A BODY PORTION COMPOSED OF A COHERENT BUT BRITTLE COMPACT OF AGGLOMERATED GRANULES COMPRISING A FILLER MATERIAL OF LIMESTONE PARTICLES AND PARTICLES OF COAL TAR PITCH BINDING SAID LIMESTONE PARTICLES, SAID PITCH CONSTITUTING FROM ABOUT

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