Patent Grant
8252370
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.
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).
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.
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:
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.
The mill illustrated in
While not specifically shown in the illustration of
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
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.
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 <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.
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
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.
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.
The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5512403 | Tyagi et al. | Apr 1996 | A |
| 6004717 | Creatura et al. | Dec 1999 | A |
| 6197369 | Watano et al. | Mar 2001 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 60596777 | Oct 2005 | US |
