Abstract:
A method for the continuous production of finely ground particulates coated with a barrier, or other desirable film wherein the coated particulates exhibit a diameter of less than 10 microns. In an exemplary embodiment, large coated particulates are introduced into a fluid energy, or jet mill, along with smaller, uncoated particulates. As the particulates collide within the mill they are comminuted, and an amount of coating is transferred from the coated particulates to the uncoated ones such that they become sufficiently coated and size-reduced to a desired size. Alternatively, uncoated particulates are milled and coated during their milling. Still alternatively, uncoated particulates are milled and subsequently directed through an atomized mist of coating material wherein the size of the mist droplets are as large, or larger than the directed particulates.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 USC 119( e ) of U.S. Provisional Patent Application No. 60/596,777 filed Oct. 20, 2005, the entire file wrapper contents of which provisional application are herein incorporated by reference as though set forth at length. 
    
    
     UNITED STATES GOVERNMENT INTEREST 
     The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes. 
    
    
     FEDERAL RESEARCH STATEMENT 
     The invention described herein may be made, used, or licensed by or for the United States Government for government purposes without payment of any royalties thereon or therefore. 
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of finely ground materials preparation and in particular how such processes combine with continuous methods for coating extremely-fine particulates with polymeric material or other coating materials. 
     2. Background of the Invention 
     Finely ground particulates are known to have widespread applicability in a number of industries including pharmaceuticals, cosmetics, industrial coatings and energetics (i.e., propellants, explosives). In a number of these applications, the particulates must be coated with certain other material(s) that impart desirable physical and/or mechanical and/or chemical characteristics to the particulates. Such particulate coatings include lubricants, barrier films, wetting agents, polymers and/or monomers. 
     One particularly important coated fine particulate is the extremely energetic, high explosive cyclotrimethylenetrinitramine (RDX). Finely ground RDX is currently being employed as an ingredient in new Insensitive Munition (IM) explosive formulations and as an energetic enhancer in propellant formulations. Coating these materials generally enhances processability, safety and shelf life. 
     Presently, super-fine, coated RDX is manufactured in a multi-step process which unfortunately exhibits a significant rework potential. Manufacturing methods that do not suffer from this rework infirmity would therefore represent a significant advance in the art and, in the particular case of energetics, the safety of the end product(s). 
     SUMMARY OF THE INVENTION 
     The present invention provides a continuous process for reducing the size of particulate materials and coating the size-reduced particulates with a barrier or other desirable film. The process continuously processes the particulates in a fluid-energy, or jet mill. 
     According to the present invention, relatively large, coated particles are introduced into the fluid-energy mill along with smaller, uncoated particles. As the particulates collide within the mill they are reduced in size and comminuted, and an amount of coating is transferred from the coated particulates to the uncoated ones such that they become sufficiently coated. 
     In an alternative embodiment of the present invention, relatively large uncoated particulates are introduced into the fluid-energy mill along with smaller uncoated particulates. During the milling operation, a coating material is introduced which coats the particulates. Subsequent polymerization and/or curing of the coating overlying the particulates may be initiated or catalyzed through the effect of a wide-variety of known mechanisms including, but not limited to, ultraviolet radiation, heat, and time-dependent curing. 
     Although the types of fluid-energy and/or jet mills employed are well known and readily available, their particular utility for simultaneously producing ultra-fine, coated particles—and in particular ultra-fine, coated, energetic compositions—was not previously recognized. Furthermore—and due in part to the extreme difficulty to produce atomized coating materials in sizes smaller than ten (10) microns—consistently coating particles of that small size was equally difficult. Finally, coating particles on the order of one (1) micron in size—prior to the present invention—was unknown. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Various features and advantages of the present invention and the manner of attaining them will be described in greater detail with reference to the following description, claims and drawing wherein: 
         FIG. 1  is a sectional illustration of a MICRONIZER® mill suitable for use in the present invention; 
         FIG. 2  is a schematic illustration of equipment suitable for carrying out the process of the present invention in-situ; 
         FIG. 3  is a schematic illustration of equipment suitable for carrying out an alternative process of the present invention in-situ; and 
         FIG. 4  is a schematic illustration of equipment suitable for carrying out another alternative process of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. 
     Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
     Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
     Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the invention. 
       FIG. 1  is a sectional illustration of a widely known, MICRONIZER (Sturtevant Mill Corp.) fluid-energy or jet mill which is one class of mill employed in the present invention. With such mills, fluid energy is admitted in fine, high-velocity streams at an angle at the periphery of a grinding and classifying chamber. While different configurations of fluid-energy mills are known, what all have in common is that particle size reduction is achieved by particles colliding with other particles, as well as by collisions between the particles and grinding surfaces of the mill. 
     The mill illustrated in  FIG. 1  is representative of a commercially available MICRONIZER mill manufactured by Sturtevant Inc., Hanover Mass. Operationally, particulate material is introduced via feed funnel  110  and subsequently directed into grinding chamber  150  through the effect of compressed feed air applied to feed air/gas inlet  120 . Compressed grind air or gas, applied to grind air gas manifold  130 , causes the introduced particulates to rotate about the grinding chamber. The rotational particulate flow generates high-speed collisions, creating increasing smaller particulates as a result of the particulate-on-particulate and particulate-to-wall impacts. 
     While not specifically shown in the illustration of  FIG. 1 , such a mill will normally have a number of feed inlets spaced around the periphery of the grinding chamber  150 . Similarly, a series of air jets, which are supplied with air by the air manifold  130 , are also spaced around the periphery of the grinding chamber. The air jets cause entering particulates to move in high speed rotation, so that they violently impact each other and with the wall. 
     As a further result of the rotation, larger particulates—due to centrifugal forces—are kept at the periphery of the grinding chamber, where most of the grinding occurs. Smaller particulates—due to centripetal forces—are driven toward the center of the grinding chamber where a centrally located outlet  170  permits their discharge. 
     As can be readily appreciated by those skilled in the art, mills such as that shown in  FIG. 1  are relatively simple and generally contain no moving parts. In addition, they provide an efficient, one-step grinding and classifying operation, which advantageously lends itself to the present invention. 
     As noted, one particularly important application for the present invention is the preparation of energetic materials (such as the high explosive composition RDX) coated with one or more of a variety of coating materials. By way of example only, the types of coating materials may be broadly categorized as follows. 
     Solid Coating Materials: Waxes exhibiting various sizes and melting points and fumed silica; 
     Solvent-Borne Coating Materials: di-octyl adipate (DOA), polyisobutylene (PIB), Estane, etc; 
     Non-Reactive Organic Liquids: oils, lubricants, and plasticizers; and 
     Monomeric and Pre-Polymeric Coating Materials: including heat curable compositions, UV curable compositions as well as smooth coat monomers and pre-polymer solutions. 
     As will become apparent to those skilled in the art, the present method is advantageously compatible with these enumerated coating materials (and others) and accommodates the nearly instantaneous curing of applied coatings via ultra-violet (UV) mechanisms or longer cure times associated with solvent flash-off or chemically induced polymerization. 
       FIG. 2  illustrates an arrangement of equipment suitable for carrying out the present invention wherein the milling and coating proceed simultaneously inside the FEM grinding chamber (in-situ). Large, coated particulates  210  and smaller, uncoated particulates  215  are fed into feed funnel  220  of fluid energy mill  225 . The particulates are directed into grinding chamber  225  through the effect of feed air applied at feed air input  229 . Compressed grind air applied through grind air manifold  227 , causes particulates to violently circulate around grinding chamber  225 . 
     While in the grinding chamber  225 , the flow induced centrifugal forces assist the transfer of coating material from the larger, pre-coated particulates to the smaller uncoated ones as the particulates undergo milling and the resultant size reduction in size. Advantageously, the present method may produce coated particulates &lt;10 microns in size—including those on the order of one (1) micron in size. 
     Of further advantage, while we have described this exemplary process using both coated and uncoated particulates, the present method will operate if only coated particulates are provided to feed funnel  220 . In either case, as the pre-coated particulates are circulated within the FEM, they collide with the chamber walls and other pre-coated particulates and smaller uncoated ones. During the collisions between the pre-coated and uncoated particulates, some of the coating material is transferred to or adsorbed by the uncoated particulates, thereby coating the uncoated particulates while simultaneously reducing the particulate size to that desired. 
     Once desired particulate size(s) are so produced, they are urged to the central region of the grinding chamber  225  and discharged to a collector  240  for storage and subsequent dispensation. 
     As can now be readily appreciated by those skilled in the art, the present invention advantageously employs the relatively high inherent kinetic energy associated with the milling process to transfer the coating material from the larger, coated particulates to smaller uncoated ones. Of particular significance, the coating does not agglomerate during the coating transfer. For particular groups of coating materials, the frequent, violent particulate collisions which occur during milling within the FEM facilitates the transfer of the coating materials from the coated surfaces to any fresh surfaces resulting from particulate attrition. 
       FIG. 3  shows an alternative arrangement for carrying out the present invention in-situ wherein only un-coated particulates  310 ,  315  are introduced into the FEM  330 . During milling, a coating material—for example a UV curable coating—is introduced into the grinding chamber and subsequently transferred to the particulates as described before. The introduction of sufficient energy, i.e., UV radiation into the grinding chamber cures the coating on the surface of the particulates. Any particulates exiting the FEM are coated and correctly sized for storage in collector  340 . 
     From these examples, those skilled into the art will readily appreciate that the present invention may continuously produce coated particulates of a desired size, without further processing or re-work (other than packaging for storage/transport). 
     An additional example of the present method is shown in  FIG. 4 . Shown therein are relatively large, uncoated particulates (˜20-200 microns)  410 , fed into FEM input funnel  420 . As with the earlier examples, the particulates so introduced into the FEM  430  are milled to a desired size. 
     As the properly sized particles are discharged from the FEM they are directed to a collector  440  via transfer duct  435  wherein they are sprayed with a desired coating material  423 , finely atomized into the particle stream flowing through transfer duct  435  from FEM discharge port to collector  440 . Although the finely atomized coating droplets may be larger than the milled particulates comprising the flowing particle stream, the milled particulates may be transferred through the duct at a sufficient high velocity to impart enough momentum to the particulates such that they penetrate the droplet(s) and become coated. The coated particulates may then flow through a portion of the transfer duct  435  wherein the coating may be cured on the surface of the particulates. 
     As before, any of a number of mechanisms may be employed (i.e., UV irradiation) to promote the cure of the coating during the transfer to the collector  440 . It should be noted that while we have used the term “cure” to describe the permanent affixation of the coating to the particulates, it is understood that such “cure” may include polymerization, solvent flash-off, catalysis, or other known mechanisms 
     Finally, and as can be appreciated, this exemplary embodiment of the present invention uses the velocity of the flowing particulates to penetrate the atomized coating droplets, thereby transferring the coating to the surface of the particulates. 
     Of course, it will be understood by those skilled in the art that the foregoing is merely illustrative of the principles of this invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. In particular, different FEM configurations may be employed or configurations wherein high-velocity jets of particulates are directed directly at one another to produce collisions of sufficient energy to impart a size reduction and coating. Additionally, the coatings may advantageously be applied dry, as liquids, or some combination as directed by the particular application. Furthermore, the applied coatings may advantageously be a mixture of various individual coatings in a variety of proportions. Finally, the particular particulates used may also be a mixture of various individual particulate types in any desired proportion. For example, a mixture of various particulate energetic materials such as HDX, RDX, etc. As can be readily appreciated by those skilled in the art, the invention of the present application may simultaneously mill and coat a mixture of particulates, depending upon the desired final end product. Accordingly, our invention is to be limited only by the scope of the claims attached hereto.