Insensitive high performance explosive compositions
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to insensitive high performance explosive compositions. More specifically, the present invention relates to the use of 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.0 5 ,9 0 3 ,11 ]dodecane as an explosive ingredient in compositions useful in high performance, low sensitivity explosive applications. 
     2. Technology Background 
     It is a continuing objective in the design and production of explosives to provide explosives which are highly energetic when intentionally initiated, but in which the risk of unintentional detonation is minimized. It is preferable that the mass and confinement effects of the explosive case be negligible on the probability of initiation or the transition from burning to detonation in either transport or storage. It is also preferred that if such explosive is unintentionally initiated it will be incapable of propagating to another explosive. Such explosives are termed insensitive high-explosives (IHE). Standards for IHE are discussed, for example, at pages 3-5 to 3-12 of the July 1984 DoD 6055.9-STD &#34;Ammunition and Explosive Safety Standards&#34; and in draft DoD-STD-2105A of Oct. 18, 1988 &#34;Military Standard Hazard Assessment Tests For Non-Nuclear Ordnance&#34;. 
     Conventional IHE compositions have comprised a curable elastomeric binder in which particles of high-energy material, particularly explosive particles, oxidizers, and reactive metals, are dispersed throughout the binder. The elastomeric binder has generally been a cured elastomer, such as hydroxy-terminated polybutadienes, polypropylene glycols and the like. More recently, efforts have been made to use thermoplastic resin binders to produce insensitive high-explosives. 
     3-nitro-1,2,4-triazol-5-one, known in the industry as nitrotriazolone or &#34;NTO&#34; is a currently preferred insensitive explosive ingredient. Although explosive formulations based on NTO are less sensitive than those based on the widely used high performance explosives RDX and HMX, their poor explosive performance, compared to RDX and HMX formulations, limit the usefulness of NTO. Nitroguanidine (NQ) is another insensitive explosive ingredient commonly used in the industry, but NQ, like NTO lacks the explosive performance of conventional high performance explosives. 
     It would, therefore, be a substantial advancement in the art to provide a high performance explosive which was also insensitive during storage and transportation. It would be a further advancement in the art to provide an explosive ingredient useful in compositions which are high in performance, low in sensitivity and which may be used in a wide variety of explosive formulations. 
     Such insensitive high performance explosive compositions are disclosed and claimed herein. 
     SUMMARY OF THE INVENTION 
     The invention is directed to the use of 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.0 5 ,9 0 3 ,11 ]dodecane (herein referred to as TEX) as a major explosive ingredient in high performance, low sensitivity explosive compositions. The structure of TEX is shown below: ##STR1## Characterization of TEX shows it to be thermally stable and significantly less impact or friction sensitive than conventional high performance solid nitramines such as RDX (1,3,5-trinitro-1,3,5-triaza-cyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane). 
     TEX may be used either alone or in combination with conventional or novel solid explosive ingredients such as RDX, HMX, NTO, NQ, ADN (ammonium dinitramide), HNIW (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.0 5 ,9 0 3 ,11 ]dodecane), TATB (1,3,5-triamino-2,4,6-trinitrobenzene), DADNE (1,1-diamino-2,2-dinitro ethane), and similar compounds obvious to those skilled in the art, as the basis for formulating very high performance insensitive explosive compositions. For example, TEX may be used in combination with a binder, metal, and oxidizer to prepare low cost, castable explosives. TEX and a small amount of binder may also be used to prepare high solids (&gt;90% TEX) pressable or extrudable explosives. Melt cast explosives may be prepared by combining TEX with an energetic or inert material having a relatively low melt temperature (&lt;120° C.). These melt cast explosives may also contain a metal, oxidizer and other nitramine. TEX may also be used as the primary solid in simple castable formulations (up to 90% solids) using energetic or inert polymeric binders. This class of explosives may contain another nitramine (i.e., RDX, HMX), but will not normally contain an oxidizer or metal. 
     The explosive compositions of the present invention exhibit greatly improved explosive performance relative to current insensitive explosive compositions comprising nitrotriazolone (NTO), nitroguanidine (NQ), and various nitrate salts, all of which suffer from low performance in order to satisfy sensitivity criteria. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to compositions using 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.0 5 ,9 0 3 ,11 ]dodecane (herein referred to as TEX) as an explosive ingredient in compositions useful in high performance, low sensitivity explosive applications. The structure of TEX is shown below: ##STR2## A synthesis of TEX is reported by Joseph H. Boyer and colleagues in Heterocycles, vol. 31, pp. 479-80 (1990), which is incorporated herein by reference. TEX may be prepared quicker and easier by reaction of diformyltetrahydroxypiperazine with nitric acid in concentrated sulfuric acid. Ammonium nitrate may also be used instead of nitric acid as a nitrate source. 
     The TEX as prepared often contains small measurable amounts of trioxa-trinitro-triazaisowurtzitane or tetraoxa-mononitro-monoformyl-diazaisowurtzitane and analogous isomers with progressively higher nitramine content up to and including HNIW. The inclusion of these compounds in the TEX formulations does not significantly affect the sensitivity of performance of the explosive and should be considered typical of the compositions containing TEX. Characterization of TEX shows it to be thermally stable and significantly less impact or friction sensitive than conventional high performance solid nitramines such as RDX, HMX, or HNIW. Theoretical performance calculations utilizing TEX indicate that the explosive performance of the TEX exceeds that of RDX in the neat form and is comparable to compositions containing RDX in a simple energetic binder-based plastic bonded explosive (PBX) composition. 
     The TEX nitramine may be used either alone or in combination with other conventional or novel solid explosive ingredients such as RDX, HMX, NTO, NQ, ADN, HNIW, TATB, DADNE, and similar compounds obvious to those skilled in the art, as the basis for formulating very high performance insensitive explosive compositions. The explosive compositions of the present invention exhibit greatly improved theoretical explosive performance relative to current insensitive explosive compositions based on NTO, NQ, and various nitrate salts, all of which suffer from low performance in order to satisfy sensitivity criteria. 
     The TEX nitramine may be used in the development of different types of insensitive high performance explosives. For example, TEX may be included in castable explosive formulations using a binder and optionally a metal and an oxidizer. The binder may be inert or energetic. Typical formulations may contain from about 5% to 90% TEX, preferably from about 30% to 90% TEX; from about 10% to 30% binder; from about 0% to 50% oxidizer; and from about 0% to 30% reactive metal. 
     When inert binders are desired, the binder is preferably hydroxy terminated polybutadiene (HTPB), although other inert binders could also be used such as PBAN (butadiene-acrylonitrile-acrylic acid terpolymer), PPG (polypropylene glycol), PEG (polyethylene glycol), polyesters, polyacrylates, polymethacrylates, CAB (cellulose acetate butyrate), and mixtures thereof. Typical energetic binders which may be used include PGN (polyglycidyl nitrate), poly-NMMO (nitratomethyl-methyloxetane), GAP (polyglycidyl azide), 9DT-NIDA (diethyleneglycol-triethyleneglycol-nitraminodiacetic acid terpolymer), poly-BAMO (poly(bisazidomethyloxetane)), poly-AMMO (poly(azidomethyl-methyloxetane)), poly-NAMMO (poly(nitraminomethyl-methyloxetane)), copoly-BAMO/NMMO, copoly-BAMO/AMMO, NC (nitrocellulose), and mixtures thereof. The binder may also contain 0% to 75% of a plasticizer such as DOA (dioctyladipate or (2-ethylhexyl)adipate), IDP (isodecylperlargonate), DOP (dioctylphthalate), DOM (dioctylmaleate), DBP (dibutylphthalate), oleyl nitrile, or mixtures thereof. Energetic plasticizers may also be used, such as BDNPF/BDNPA (bis(2,2-dinitropropyl)acetal/bis(2,2-dinitropropyl)formal), TMETN (trimethylolethanetrinitrate), TEGDN (triethyleneglycoldinitrate), DEGDN (diethyleneglycoldinitrate), NG (nitroglycerine), BTTN (butanetrioltrinitrate), alkyl NENA&#39;s (nitratoethylnitramine), or mixtures thereof. 
     The reactive metal is preferably aluminum or magnesium, although other reactive metals could be used such as boron, titanium, zirconium, or mixtures thereof. The oxidizer is preferably AP (ammonium perchlorate) or AN (ammonium nitrate), although other oxidizers could be used such as HAN (hydroxylammonium nitrate), ADN, or mixtures thereof. Some of the TEX may be replaced with another energetic solid such as RDX, HMX, NQ or NTO. Those skilled in the art will appreciate that the sensitivity of the final explosive formulation will depend upon its explosive ingredients; therefore, as a general rule, as the TEX concentration replacing HMX or RDX in the explosive increases, the sensitivity of the explosive decreases, while if the TEX concentration replacing NQ or NTO increases, the performance increases. This family of explosive formulations provides insensitive high performance explosives for general purpose bomb fill and underwater applications. 
     Another class of explosives containing TEX are the high solids, pressable or extrudable explosive formulations. The pressable or extrudable explosives have a high solids content and contain up to about 98.5% TEX, preferably from 50% to 98.5% TEX and most preferably form 80% to 98.5% TEX, or a combination of TEX and another high explosive. Various polymeric binders, including halogenated, inert and energetic binders, may be used in pressable or extrudable explosives. The polymeric binder is preferably present up to 20% by weight of the explosive composition. Although not commonly included in pressable or extrudable explosive formulations, a reactive metal and/or an oxidizer may be included in the pressable or extrudable explosive formulations of the present invention. 
     Halogenated polymers are preferred where high density is important. Typical halogenated polymers which may be used include Viton A® (fluorinated ethylene propylene copolymer) sold by DuPont and Kel-F® (copolymer of chlorotrifluoroethylene and vinylidene fluoride) sold by 3M. Other halogenated polymers which may be used include polyvinylidene fluoride, polydifluorochloroethylene, fluorinated polyethers, PVC (polyvinyl chloride), polytetrafluoroethylene, and mixtures thereof. Inert binders are sometimes desirable in pressable explosives because of stability and improved explosive processing. Typical inert binders which may be used include EVA (ethylenevinylacetate) and Estane® (a poly-urethane resin) sold by B. F. Goodrich. Other inert binders which may also be used include HTPB, PBAN, PPG, PEG, polyesters, polyacrylates, polymethacrylates, and CAB. Energetic binders may also be used in pressable explosives to maximize energy production and to reduce solids loading without reducing explosive performance. Typical energetic binders which may be used include PGN (polyglycidyl nitrate), poly-NMMO, GAP and NC (nitrocellulose) or CAB (cellulose acetate butyrate) plasticized with an energetic plasticizer such as a liquid nitrate ester, BDNPA, or BDNPF. BDNPA and BDNPF are sometimes blended together or made as a mixture synthetically. 
     Combinations of energetic solids, such as 50% TEX/40% HNIW/10% binder or 60% TEX/35% HMX/5% binder, etc. may be made to achieve the desired performance and sensitivity goals. It will be appreciated that by using TEX in this class of explosives, it is possible to achieve a wide range of performance and sensitivity goals. These explosive compositions are ideal for shaped charge or explosively formed projectile applications where a more insensitive fill is required than achievable with RDX or HMX. They are also useful for applications where more energy is required than possible with NTO or NQ, but insensitivity is still mandatory. 
     An additional class of explosives are those containing up to 90% TEX with the balance being a liquid polymer. These formulations may also contain a plasticizer. Both inert and energetic polymers and plasticizers may be used in this class of explosives, and another energetic solid such as HMX or RDX may replace part of the TEX to tailor the explosive properties. The polymers and plasticizers may be inert or energetic depending on the specific properties required for the particular application. The energetic binder systems may be used to maximize energy and/or to reduce solids loading while the inert binder systems may be used to increase processability and reduce sensitivity. Examples of inert polymers include HTPB, PPG, and acrylates. Typical inert plasticizers include DOA, IDP, and DOM. Typical energetic polymers useful in this class of explosives include PGN, poly-NMMO, and GAP. Energetic plasticizers include BDNPA/BDNPF, NG, DEGDN, and other nitrate esters. 
     Yet another class of TEX explosives are the melt cast explosives which contain TEX, or TEX in combination with another high explosive, and an energetic or inert meltable material with a relatively low melt temperature (about 120° C. or below). The meltable material acts as a &#34;binder&#34; to the solid explosive. Two currently preferred meltable energetic materials which may be used in the present invention are TNT (2,4,6-trinitrotoluene) and TNAZ (1,3,3-trinitroazetidine). Other meltable energetic materials which may be used include AN/NQ eutectic or alkylammonium nitrate salts. Inert meltable materials such as polyethylene and hydrocarbon wax may also be used to prepare melt cast explosive compositions within the scope of the present invention. Inert thermoplastic materials may also be used in these compositions to reduce the sensitivity. Oxidizers (AP, AN, HAN, ADN, etc.) and reactive metals (Al, Mg, B, Ti, Zr, etc.) may also be added to these melt case explosives to achieve the desired properties in a given explosive. 
     Typical melt cast explosive formulations within the scope of the present invention include a meltable material in the range from about 15% to 40%; TEX or blends of TEX/RDX, TEX/HMX, etc. in the range from about 5% to 85%, preferably 30% to 85%; a reactive metal from about 0% to 30%; and an oxidizer from about 0% to 30%. The reactive metal is preferably aluminum, magnesium, boron, titanium, or zirconium, although other reactive metals could be used. The oxidizer is preferably ammonium perchlorate or ammonium nitrate, although other oxidizers could be used. 
     For example, Composition B is a well known melt cast explosive formulation comprising 59.5% RDX, 39.5% TNT, and 1% D2 wax (84% polyethylene, 14% nitrocellulose, and 2% lecithin). A high performance, insensitive explosive may be prepared by replacing the RDX in Composition B with TEX. It will be appreciated that the percentages of TNT and TEX may be varied in such a manner that the explosive performance of Composition B is maintained or even improved. 
     The following examples are offered to further illustrate the present invention. These examples are intended to be purely exemplary and should not be viewed as a limitation on any claimed embodiment. 
    
    
     EXAMPLE 1 
     TEX was prepared as follows: 250 mL of concentrated sulfuric acid were placed in a 1 liter Erlenmeyer flask. The flask was placed in a water bath and stirred. To the stirred contents were added 250 mL of 90% nitric acid followed by 100 g 1,4-diformyl-2,3,5,6-tetrahydroxypiperazine at such rate that the temperature remained below 60° C. The contents were stirred for 1.5 hours over which time the temperature of the reaction dropped to ambient. The pale yellow reaction mixture was poured on to ice and the resultant precipitate collected by filtration on a glass frit. The wet solid was then returned to the flask and washed with saturated sodium bicarbonate until no more gassing was apparent. The solid was then recollected on a glass filter and washed several times with methanol to remove impurities and to facilitate drying. The product was dried in a vacuum oven overnight at 50° C. The yield was 37.62 g. The product displayed the proton and carbon NMR resonances as well as FTIR absorbances reported by Joseph H. Boyer and colleagues in Heterocycles, vol. 31, pp. 479-80 (1990). The chemical reaction is shown below: ##STR3## 
     EXAMPLE 2 
     TEX prepared according to the procedure of Example 1 was characterized by standard chemical methods.  1  H and  13  C NMR and FTIR spectra were identical to data provided by Boyer. The material was further characterized for safety data. Safety tests were run using standard methodologies common the those skilled in the art. It should noted that TC tests are 50% values and ABL numbers are threshold initiation values. The results are as follows: 
     
         ______________________________________                                    
                           Allegheny                                      
            Thiokol Corp.  Ballistics Lab.                                
______________________________________                                    
Impact      &gt;46 inch       33 cm                                          
Friction    &gt;64 lb         25 psi @ 8 ft/sec                              
Electrostatic                                                             
            0.43 J         0.075 J                                        
Discharge (ESD)                                                           
Simulated Bulk                                                            
            onset 486° F. burned                                   
Autoignition                                                              
Temperature                                                               
(SBAT)                                                                    
______________________________________                                    
Heat of combustion (ΔH.sub.c) = 2527.5 cal/g                        
Heat of formation (ΔH.sub.f) = -106.5 Kcal/mol                      
______________________________________                                    
 
    
     EXAMPLE 3 
     Card gap testing of explosive compositions containing TEX, NTO, and RDX were conducted. In the standard &#34;card gap&#34; test, an explosive primer is set off a certain distance from the explosive. The space between the primer and the explosive charge is filled with an inert material such as PMMA (polymethylmethacrylate). The distance is expressed in cards, where 1 card is equal to 0.01 inch such that 70 cards is equal to 0.7 inches. If the explosive does not detonate at 70 cards, for example, then the explosive is insensitive at 70 cards. 
     The explosive compositions tested included 70% solids (e.g., 70% TEX, 70% NTO, and 70% RDX) in a PGN binder system comprising PGN, diethylene glycol dinitrate, and an isocyanate curative in which the plasticizer to polymer ratio was 2:1. The TEX used was prepared according to the procedure of Example 1. 
     The density of each explosive composition was measured and compared with its theoretical density. From this information, the theoretical detonation velocity at that density was also calculated using the standard BKW equation of state. The results are shown below: 
     
         ______________________________________                                    
Ener-                                                                     
getic Measured   Theoretical       Theoretical C.J.                       
Solid Density g/cc                                                        
                 Density g/cc                                             
                            % TMD  Velocity m/s                           
______________________________________                                    
TEX   1.68       1.764      95.2   7889                                   
TEX   1.65       1.764      93.5   7776                                   
NTO   1.48       1.719      86.1   7039                                   
NTO   1.47       1.719      85.5   7006                                   
RDX   1.63       1.667      97.8   8042                                   
RDX   1.61       1.667      96.6   7973                                   
______________________________________                                    
 
    
     The pipes were instrumented for detonation velocity. Two tests were conducted for each explosive composition, the first test at zero cards and the second at 70 cards. The results of these tests are summarized below. 
     
         ______________________________________                                    
                                Measured                                  
Energetic                       Detonation                                
Solid   Cards     Reaction Type Velocity (m/s)                            
______________________________________                                    
TEX      0        Sustained detonation                                    
                                6811                                      
TEX     70        No detonation, No                                       
                  deflagration                                            
NTO      0        Sustained detonation                                    
                                6263                                      
NTO     70        Sustained detonation                                    
                                5571                                      
RDX      0        Sustained detonation                                    
                                7844                                      
RDX     70        Sustained detonation                                    
                                7790                                      
______________________________________                                    
 
    
     These results support theoretical calculations that the performance of TEX is better than NTO. They also indicate reduced sensitivity for the composition based on TEX relative to both NTO and RDX. 
     Of the compositions tested, TEX was the least sensitive because it did not detonate at 70 cards (0.7 inches). Also pieces of the card gap pipe from the 70 cards TEX test were recovered along with pieces of unburned explosive. In the other tests, all of the explosive was consumed. In the tests with sustained detonations, a clean hole was punched through the witness plate. 
     EXAMPLE 4 
     The safety profile for TEX was compared with that of HMX and RDX and is summarized below. Safety tests were run using standard methodologies common the those skilled in the art. It should noted that TC tests are 50% values and ABL numbers are threshold initiation values. 
     
         __________________________________________________________________________
                                Differential                              
                       Electrostatic                                      
                                Scanning                                  
Impact       Friction  Discharge                                          
                                Calorimeter                               
TC       ABL TC  ABL   TC  ABL  Base Line                                 
(inch)   (cm)                                                             
             (lb)                                                         
                 (psi @ ft/s)                                             
                       (J) (J)  Departure                                 
__________________________________________________________________________
HMX 29   1.8 63  50/4  0.57                                               
                           0.075                                          
                                ˜240° C.                     
RDX 22   3.5 63  130/4 0.43                                               
                           0.075                                          
                                ˜240° C.                     
TEX &gt;46  33  &gt;64 25/8  0.43                                               
                           0.075                                          
                                ˜260° C.                     
__________________________________________________________________________
 
    
     With the exception of ESD, TEX appears to be quite insensitive to ignition stimuli compared to conventional nitramines. 
     EXAMPLE 5 
     Low cost HTPB binder system explosives having 85% solids were prepared according to the following formulations: 
     
         __________________________________________________________________________
Mix No.                                                                   
      Binder    Solids                                                    
__________________________________________________________________________
 A*   HTPB/DOA/IPDI                                                       
                Al-20%                                                    
                     AP-20%                                               
                           NTO-25%                                        
                                 RDX-20%                                  
B     HTPB/DOA/IPDI                                                       
                Al-20%                                                    
                     AP-20%                                               
                           TEX-25%                                        
                                 RDX-20%                                  
C     HTPB/DOA/IPDI                                                       
                Al-20%                                                    
                     AP-20%                                               
                           TEX-45%                                        
D     HTPB/DOA/IPDI                                                       
                Al-20%                                                    
                     AP-20%                                               
                           NTO-25%                                        
                                 TEX-20%                                  
__________________________________________________________________________
 *Baseline Mix                                                            
 
    
     Initial safety data is obtained for the above explosive formulations. The safety data suggests that the explosive formulations utilizing TEX provide insensitive general purpose bomb fill with performance superior to that obtained using NTO. 
     EXAMPLE 6 
     Computer modeling calculations comparing the theoretical explosive performance of several conventional explosive formulations with and without TEX were conducted utilizing the BKW equation of state. The calculations are summarized below: 
     
         ______________________________________                                    
            C-J Pres-   Detonation  Density                               
Composition sure (Katm) Velocity (m/s)                                    
                                    (g/cc)                                
______________________________________                                    
TEX/TNT 75/25                                                             
            328.52      8.306       1.893                                 
RDX/TNT 75/25                                                             
            324.67      8.435       1.773                                 
NTO/TNT 75/25                                                             
            288.21      7.938       1.839                                 
NQ/TNT 75/25                                                              
            276.86      8.146       1.695                                 
H-6  (RDX)  232.47      7.300       1.773                                 
H-6 (TEX)   232.77      7.236       1.852                                 
H-6 (NTO)   216.13      7.057       1.821                                 
H-6 (NQ)    215.16      7.142       1.735                                 
AFX-644  (NTO)                                                            
            242.44      7.312       1.903                                 
AFX-644 (TEX)                                                             
            260.24      7.487       1.934                                 
AFX-644 (NQ)                                                              
            239.81      7.384       1.818                                 
TEX         370         8.665       1.990                                 
NTO         306.99      8.120       1.910                                 
NQ          286.39      8.348       1.710                                 
RDX         359.58      8.802       1.820                                 
HMX         392.56      9.046       1.900                                 
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  H6 includes 74.2% Composition B, 20.6% Al, 4.7% D2 wax, and 0.5%        
 CaCl.sub.2.                                                              
  AFX644 includes 40% TNT, 40% NTO, 20% Al, and a trace of D2 wax.        
 
    
     These calculations suggest that TEX performs significantly better than NTO and NQ, about the same as RDX, and slightly lower than HMX. Given the insensitivity of compositions containing TEX, TEX and compositions based thereon, are an important additive to explosive compositions. 
     From the forgoing it will be appreciated that the present invention provides high performance explosives which are also insensitive during storage and transportation. The present invention further provides a new major explosive ingredient which may be used in a variety of explosive formulations for use in high performance, low sensitivity explosive applications. 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.