Abstract:
An explosive suitable for low energy initiation is made by coating epsilon structure one to five micron crystals of CL-20 with a polymeric binder where the polymeric binder is one to three percent by weight of the coated crystals and a method of applying such polymeric binders by either a slurry method or an alternate method of using a nonaqueous liquid to suspend CL-20 crystals then adding a lacquer of polymeric binder.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0001] The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Exploding foil initiators have been developed as an alternative to hot wire detonators. Hot wire detonators are extreme safety hazards due to their initiating charges being made from primary explosive powders. Hot wire detonators are susceptible to accidental initiation due to minor electrostatic variations and are not considered to be safe to use on ordnance without mechanisms to provide misalignment with boosters and warhead explosives EFI&#39;s can be made to function with secondary explosives and are generally considered safe to use embedded in warhead fills. Hot wire detonators suffer from repeatability and reliability failures.  
           [0003]    To solve these problems exploding foil initiators use a high amplitude electrical pulse into a narrow section of a bridge material which cause it to vaporize, shearing out a disc which accelerates down a barrel driven by a plasma wave front and impacting a pellet of consolidated secondary explosive powder. This impact shock initiates the secondary pellet due to the kinetic energy of the flyer. The use of high voltages have permitted repeatability and reliability for initiation. The risk of inadvertent initiation is removed with this technology. Due to the high kinetic energy of the plasma, the secondary pellet does not have to be made of primary explosive powders which further increase safety.  
           [0004]    Past explosive foil initiators, EFI&#39;s, still presented problems. They required large complicated components to generate and execute the initiating energy reliably. They have required high-voltage components which challenge space constraints in ordnance devices. The material used, HNS (hexanitrostilbene), for the pellet in EFI&#39;s has numerous problems. HNS, specifically version HNS-IV, which is the best known version of HNS to use for EFI&#39;s, is extremely expensive. It is environmentally unfriendly as it produces about one ton of toxic waste for every pound produced. HNS-IV has an approximate 50% loss rate during pelletization. A very fine mirror surface is needed as rough or cracked surfaces can permit the flyer to penetrate the pellet and bore a hole rather than detonate the pellet. HNS-IV sticks to the mold and yields fragile pellets which causes the high loss rate.  
           [0005]    After successful pelletization, HNS-IV pellets are extremely fragile unless confined. Thus, they need to go directly into a holder. Defects and failures in the holder increase the loss rate.  
           [0006]    An alternative to HNS-IV is CL-20 (hexanitrohexaazaisowurtizane) which has an energy output approximately 200 percent higher than HNS-IV. Higher output allows EFI&#39;s to be reduced in size with less expensive interfaces and with enhanced reliability. CL-20 has been found to require a binder. Without a binder, CL-20 is not suitable for in-line explosive handling. CL-20 without a binder tends to crack when subjected to standard military handling. The current best known configuration of CL-20 to use is the epsilon polymorph where the particle size is about 2-5 microns. A prior method of producing usable CL-20 is set forth in U. S. Pat. No. 5,750,921, issued May 12, 1998, by May L. Chan and Alan D. Turner. CL-20 is more thermally stable than PETN and needs a temperature of about 220° C. before it degrades. CL-20 also has a higher detonation velocity than HNS-IV. The difference of 9000 m/sec verse 7,000 m/sec is significant because it results in higher output energy per unit volume. Thus the shock wave front is more predictable and controllable.  
           [0007]    The next evolutionary step desired is to reduce the initiation energy needed to set off an EFI. Low Energy Exploding Foil Initiators, LEEFI, are highly desirable because the electronics package for current EFI usage consume significant volume and weight in ordnance. Further reduction in energy results in reduced cost, improve reliability and allows usage in a larger number of weapons systems.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the invention is to provide a suitable alternative for HNS-IV that will pass all of the safety tests and have comparable initiation characteristics.  
           [0009]    A further object of the present invention is to coat CL-20 particles with binders suitable for low energy EFI and conventional EFI application. A further object is to use binders that do not require organic solvents in coating processes. A still further object is to produce pellets with significant increase in mechanical strength compared to HNS-IV.  
           [0010]    One of the coating processes developed entails the steps of:  
           [0011]    1. Add water to a suitable binder. In this method commercially available aqueous latex-type polymeric preparation may be used as the binder material.  
           [0012]    2. Add the CL-20 at room temperature either by hand or via the slurry method.  
           [0013]    3. Mix at a slow rate to a soft, muddy consistency.  
           [0014]    4. Mix until there is an even coating on the CL-20 crystals.  
           [0015]    5. Spread mixture in very thin layer (1-2 mm. thick) on aluminum foil or other suitable drying surface.  
           [0016]    6. Allow time to dry at a temperature, such as room temperature, which does not crack the binder coating.  
           [0017]    7. Vacuum dry.  
           [0018]    8. Once dried use a conductive spatula to separate the individual coated CL-20 crystals.  
           [0019]    In a second method binders can be dissolved in organic solvent to form lacquer. This alternative coating process entails the steps of:  
           [0020]    1. Suspend CL-20 crystals in a high shear slurry mixer filled with either aqueous or nonaqueous working fluid.  
           [0021]    2. Dissolve a polymeric binder in an organic solvent to form a lacquer.  
           [0022]    3. Slowly add lacquer to the slurry containing explosive solid while stirring.  
           [0023]    [0023] 4 . Continue to stir until CL-20 crystals are coated with binder.  
           [0024]    5. Let the organic solvent evaporate off by increasing the temperature of the working fluid.  
           [0025]    6. Filter the coated CL-20 solid from the slurry and vacuum dry. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a diagram of a standard exploding foil initiator. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    [0027]FIG. 1 shows an exploded diagram of a low energy exploding foil initiator. A base  10  provides electrical circuit which includes a bridge  12  which is embedded on the surface of Base  10 . Base  10  is made of a nonconducting material such as plastic except for conducting leads, not shown, which are attached to a voltage source, also not shown. Bridge  12  and the conducting leads may be made of any conducting material such as gold. Placement of bridge  12  on base  10  may be by a standard printed circuit operation well known in the art.  
         [0028]    When a predetermined current flows through bridge  12 , bridge  12  bursts detonating an explosive pellet  14  contained in pellet holder  16 . Explosive pellet  14  is made of CL-20 crystals, preferable crystals with the epsilon structure. Table 1 lists binders that have been used to coat the CL-20 crystals. Table 2 shows the performance tests for the different types of binder combinations. Table 2 has been normalized to compare the relative performance in the last column to HNS-IV. As defined here: Relative Performance=Firing Voltage CL-20 Mix/Firing Voltage HNS-IV  
         [0029]    Table 1 lists the type of commercially available aqueous latex preparations that can be used for coating the the first method above.  
         [0030]    Table 2 lists the type of polymers suitable for coating in the second method above. They are polyethyl acrylate, polyethyl and butyl acrylate, polystyrene acrylic copolymers, and fluorocarbon polymers.  
                           TABLE 1                                   CHEMICAL FAMILY   Wt % H 2 O                           Polyvinyl acetate ethylene   50-70           Vinylacetate vinyl chloride ethylene terpolymer   52           Ethylene vinyl chloride   ≧50           Vinylacetate ethylene acrylate terpolymer   60           Acrylic copolymer   50-70           Polyester-Polyurethane   40           Vinylacetate-dibutylmaleate   54-56           Vinylacetate, dibutymaleate, acrylic terpolymer   54-56           Polyvinylidene chloride terpolymer           Styrene butadiene itaconic acid   47           Vinylidene chloride-methyl-   54           methacrylate-acrylonitrile           Vinyl acetate-butylacrylate vinyl versatate   54-56           Vinylpyrrolidone/styrene   ˜40                       
 
         [0031]    [0031]                                                             TABLE 2                               Impact   Friction   Electrostatic           Mix   Percent   2.5 kg wt,   Above 250 lbs.   .25 Joule   Relative       Identification   Binder   cm, 50%   at 50% point   (Discharge)   Performance                                Zeon (Hytemp 4051 CG)   1.5   13   407   10/10 no fire   .95       Zeon (Hytemp 4051 CG)   2   15   417   10/10 no fire   .98       KEL-F   1.5   14   257   10/10 no fire       Calcium Sterate   1.5   13   245   10/10 no fire   .95       Airflex 9500   2   18   741   10/10 no fire   1       Bayhydrol   1   not done   not done   not done   .95       Bayhydrol   2   13   398   10/10 no fire   1       Daran SC112   2   13   617   10/10 no fire   1.03       Flex Bond   2   19   363   10/10 no fire   .92       Daratak MX   2   17   174   10/10 no fire       Flexbond 165   2   23   347   10/10 no fire   1.02       Polectron   2   18   562   10/10 no fire       CL-20        9-10   200-300   10/10 no fire   .85       RDX       16-20   500-600   10/10 no fire       HNS IV       44-61   &gt;1675      10/10 no fire   1       PETN       12   235   10/10 no fire   .83                    
         [0032]    Binder materials by chemical nomenclatures may be matched to trade name commercially available as follows:  
         [0033]    A. Polyethyl acrylate, polyethyl and butyl acrylates, Hytemp 4004, 4054, 4404, 4454, 4051 CG  
         [0034]    B. Polystyrene acrylic copolymers, i.e. Kraton D, Kraton G  
         [0035]    C. Fluorocarbon polymers, i.e. KEL-F or Viton  
         [0036]    D. Polyester ethylene copolymer, i.e. Airflex 400, 465 (polyvinyl Acetate ethylene Copolymer)  
         [0037]    E. Polyethylene vinyl chloride, i.e. Airflex 4500  
         [0038]    F. Polyester polyvinyl chloride ethylene terpolymer, i.e. Airflex 728  
         [0039]    G. Acrylic Copolymer, i.e. Flexbond 165, 185  
         [0040]    H. Polyester-polyurethane, i.e. Bayhydrol DLN;  
         [0041]    Poly Styrene butadiene itaconic acid Copolymer, i.e. Darex 537 LNA  
         [0042]    I. Poly urethane esters, i.e. Everflex GT  
         [0043]    J. Polyester polyacrylic terpolymer; i.e. Flexbond 153, Daratak MX  
         [0044]    K. Polyvinylidene chloride terpolymer, i.e. Daratak XB-3631  
         [0045]    Neat CL-20 pellets crack when subjected to normal handling and will not pass required safety tests. Ordnance will be subject to jolting and jarring so coating with a binder is required to safely allow it to function in a military environment. The epsilon structure size preferred is 1-5 microns although different sizes do not prevent function. CL-20 does not degrade at temperatures below 190° C. CL-20 also has a detonation velocity of about 9,000 m/sec.  
         [0046]    To coat the CL-20 crystals with any of the binders in Table 1, water is added to the binder. CL-20 is then added per the percentage ratios shown in Table 2 via the slurry method at room temperature using 1% to 3% of binder by weight is preferred for the coated crystal. The amount of water is limited so the slurry has a muddy consistency. Mixing continues until there is an even coating on the CL-20 crystals. The mixture is then spread in a thin layer on a suitable drying surface, such as aluminum foil. The mixture is dried at a low enough temperature to avoid cracks in the binder coating. Room temperature of about 72° F. has been found to be both practical and reliable as a drying temperature. Once dried a conductive spatula is used to separate the individually coated CL-20 crystals that have adhered to one another during the drying stage. A conductive spatula and a conductive drying surface are used to avoid static discharge that might arise during the separating of the coated crystals. The material is then vacuum dried at 100° F. for 24 hours. The last step is to break up lumps before usage.  
         [0047]    An alternate way to coat the crystals is to suspend the CL-20 crystals in a nonaqueous liquid such as fluorinated oil. Then add a lacquer of polymeric binder in an organic solvent. Stir until all crystals are evenly coated. Vacuum dry the coated crystals. Vacuum drying will remove all the working fluid and water, if aqueous working fluid is used.  
         [0048]    Returning to FIG. 1, a flyer  18 , such as a metal disc, is placed over explosive pellet  14  and pellet holder  16 . Pellet holder  16  is optional when CL-20 is coated. The plasma created by the detonation of explosive pellet  14  accelerates flyer  18  to a velocity that provides enough kinetic energy to allow flyer  18  to shock initiate a secondary pellet  20 . Secondary pellet  20  may be any suitable material adequate to detonate a warhead. To allow flyer  18  distance to be accelerated to an adequate velocity, a spacer  22  held in place by a support washer  24  provides the adequate distance for flyer  18  to accelerate. The complete assembly can be contained in a sealing cup  26 . Sealing cup  26  may be made of plastic or metal which is designed to rupture when secondary explosive pellet  20  detonates.  
         [0049]    The higher detonation velocity of CL-20 accelerates flyer  18  to desired velocity in a shorter distance. The shorter distance also means that the shape of the plasma wave accelerating flyer  18  is more predictable because it has less time to degrade. The binder used to coat the CL-20 crystals permits the coated crystals to detonate when relatively low energy is used to burst bridge  12 .