Abstract:
A method for producing particulate RDX is provided. RDX is dissolved and then precipitated with the precipitated RDX being separated from the mixture of solvent and anti-solvent and dried. The RDX has an increased insensitivity, i.e., it is more resistant to shock or impact stimuli and has a morphology characterized by a smooth surface and small particle size as formed.

Description:
STATEMENT REGARDING FEDERAL RIGHTS 
       [0001]    This invention was made with government support under contract number DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    RDX is a common explosive both as a single compound explosive and as mixed with other explosives and additives. The explosive is effective in most regards but it has been a desire for some time to make the explosive more resistant to shock or impact detonation. 
         [0003]    One of the desirable features of RDX explosives is that it can be molded in combination with a polymer, to fix the shape of the explosive for various purposes which are well known in the industry. Attempts have been made to reduce the sensitivity to shock or impact stimuli, hereinafter impact stimuli for convenience. For example, according to the article entitled  Reduced Sensitivity RDX  ( RS - RDX )  Part I: Literature Review and DSTO Evaluation , issued by DSTO Systems Sciences Laboratory located in Australia and dated July 2003, attempts were made to reduce the sensitivity of RDX to impact stimuli. However, according to this report, success was achieved only for reducing the sensitivity of the RDX when it was combined with the polymer for subsequent forming into shapes. The freestanding or natural RDX explosive was apparently not improved for impact resistance. 
         [0004]    It is therefore an object of the present invention to provide an improved RDX explosive that is more difficult to initiate an explosion from impact or shock. 
       SUMMARY OF INVENTION 
       [0005]    The present invention involves the provision of an improved RDX explosive that has a resistance to impact stimuli of at least 25 cm. The present invention also involves the provision of an RDX explosive in particulate form having a substantial portion of the particles with an average particle size of less than about 20 microns in length and preferably has an exterior surface that is characterized by a substantial absence of dimples or other surface imperfections. 
         [0006]    The present invention also involves the provision of a method of making an improved RDX explosive which involves dissolving an RDX explosive in a solvent. An anti-solvent and the RDX solution are then mixed to precipitate the RDX explosive out of solution in particulate form while the mixture is being agitated. The anti-solvent is miscible with the solvent. The precipitated RDX particles are separated and then dried. The dried RDX can be used as an explosive either as a freestanding explosive, mixed with other explosive material or in molded form using a polymer as a binder. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is a photograph of the inventive RDX explosive taken with a scanning electron microscope at a magnification of 5000×. 
           [0008]      FIG. 2  is a photograph of the inventive RDX explosive taken with a scanning electron microscope at a magnification of 200×. 
           [0009]      FIG. 3  is a photograph of prior art RDX explosive taken with a scanning electron microscope at a magnification of 200×. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The acronym RDX stands for Royal Demolition Explosive. The chemical name for RDX is 1,3-trinitro-1,3,5-triazacyclohexane. It is a white powder and very explosive. It can be used alone as an explosive or mixed with other ingredients and explosives. RDX, in the method of the present invention, is first dissolved in a suitable solvent for example, acetone. Preferably, during the dissolving step and thereafter, the solvent and the formed solution are agitated with a suitable mixer. In lab experiments, it was found that a blade mixer operating at about 400 rpms was adequate to produce the inventive product. Preferably, the process is conducted at a temperature of less than about 5° C. and preferably less than about 0° C. The solution forming and precipitation steps can be conducted at atmospheric pressure, i.e., one atmosphere absolute pressure simplifying the production of the inventive RDX. After the solution of RDX and solvent is made, the solution and an anti-solvent are combined and agitation of this mixture is maintained during precipitation of the RDX. The anti-solvent needs to be substantially miscible and preferably completely miscible in the solution of RDX and solvent. The anti-solvent effects precipitation of the RDX out of solution during agitation of the mixture of solvent, anti-solvent and precipitating RDX and the RDX remaining in solution. After precipitation, the precipitated RDX is separated from the liquid components of the mixture. Separation can be by filtration, centrifugation or other suitable separating methods. The separated RDX is then dried by any suitable method and preferably under vacuum of at least about 17 inches of hg. 
         [0011]    As seen in  FIG. 1 , the precipitated particulate RDX that has been separated and dried has somewhat rod like characteristics having a length to width ratio of at least about 2 to 1 and preferably at least about 3 to 1 on average in bulk. Preferably, at least a majority, by weight, of the dried particles are rodlike in shape. The size of the particles can be determined using scanning electron micrography where the particles are measured for size (two dimensionally) visually by comparing the particles to a scale. The particle size is determined by measuring the width or the minor dimension of the particles and the length or major dimension (as if the particles were straight). As seen in  FIG. 1 , the inventive particles are somewhat rodlike and can also be branched and/or curved. The RDX shown in  FIG. 3  is a fairly typical RDX and is comprised of particles having more of an egg (obovoid) shape or potato shape. It can also be noted in  FIG. 3  that the surface of the prior art RDX particles is rough having numerous so-called dimples or rather significant surface imperfections  1  with sharply defined edges. The surface of the inventive RDX seen in  FIGS. 1 ,  2  is smooth and characterized by a substantial lack of the just mentioned surface imperfections at a magnification of 5000×. 
         [0012]    The inventive RDX has a higher value of impact sensitivity, i.e., higher resistance to impact stimuli that is greater than or equal to currently available HMX (cyclotetramethylenetetranitramine) and at least 25 cm as measured by Type 12 impact sensitivity testing using an ERL testing machine and preferably at least about 30 cm. In bulk, the RDX particles have the following properties. A substantial portion of the particles, preferably at least about 75% and more preferably at least about 90% of the particles, have a size of about 5 microns, and preferably less than about 2 microns or less in width, W and a length L of less than about 20 microns and preferably less than about 15 microns on average as formed by precipitation (and without particle size reduction as by grinding) as measured on a branch  2  or stem  3 . Preferably, a substantial portion and at least the majority of the particles, more preferably at least about 75% and most preferably about 90% of the particles are characterized by having a smooth outer surface characterized by a substantial absence of surface imperfections as described above. With regard to the surface imperfections, it is preferred that the surface of at least a majority of the particles be substantially free of sharply edged visual imperfections which are detectable visually at a magnification of 5000× and have a size of at least about 2 microns for the major dimension thereof in width or length. The inventive RDX may be used in an explosive device freestanding, in conjunction with other explosive materials and/or molded to shape in conjunction with a polymer binder. 
         [0013]    The present invention is better understood by a description of a method used in the laboratory to produce the inventive particles. The process produced micron size particles as described with the unique as formed morphology described above. A solution of RDX in an acetone was prepared by dissolving 2.1 gram of conventional RDX (I-RDX) in 40 ml of acetone. The solution was set aside. A three necked one liter round bottom jacketed flask equipped with an overhead stirrer was charged with approximately 300 ml of hexane at 0° C. The jacketed flask can be used to maintain the temperature of the hexane at 0° C. The overhead stirrer was operating at approximately 400 rpm mixing the hexane. The solution of RDX in acetone was quickly poured through a funnel into the round bottom flask while maintaining agitation with the stirrer. Agitation was continued while the RDX precipitated out of the solution. After precipitation was completed, the mixture of hexane, acetone and precipitated RDX was filtered, and the precipitated RDX was collected on the filter media. The RDX was then dried under vacuum at a pressure of 17 mm Hg. This sample was then visually evaluated using a LEO Model 1525 field emissions scanning electron microscope. The photograph shown in  FIGS. 1 and 2  is a portion of the RDX produced in the above example.  FIG. 3  shows the I-RDX that was used to form the solution. This I-RDX had an impact resistance of 21 cm as measured by the above identified test. 
         [0014]    Thus, there has been shown and described several embodiments of a novel invention. As is evident from the foregoing description, certain aspects of the present invention are not limited by the particular details of the examples illustrated herein, and it is therefore contemplated that other modifications and applications, or equivalents thereof, will occur to those skilled in the art. The terms “having” and “including” and similar terms as used in the foregoing specification are used in the sense of “optional” or “may include” and not as “required”. Many changes, modifications, variations and other uses and applications of the present invention will, however, become apparent to those skilled in the art after considering the specification and the accompanying drawings. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.