Abstract:
The present invention provides an explosive composition of substantially reduced sensitivity and low flammability, being definable as an extremely insensitive detonating substance (EIDS) according to UN Regulations for the Transport of Dangerous Goods and comprising one or more explosive material 42-58% vol., one or more fire retardant material 15-26% vol. and a binder 20-36% vol.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an explosive composition of substantially reduced sensitivity and low flammability and a method for its manufacture. More specifically, the present invention is concerned with an explosive composition definable as an extremely insensitive detonating substance (EIDS) according to UN Regulations for the Transport of Dangerous Goods (often referred to as the ‘orange book’), classified in Class 1.5D. 
       BACKGROUND OF THE INVENTION 
       [0002]    High performance explosive compositions are long known. It has been a goal for researchers to find new explosive compositions which can be defined as low impact and shock sensitivity and low flammability, nevertheless offer high energy explosive performance. A combination of these properties offers higher survivability when applied in armor modules as well as greater transportation and handling safety, whilst not deteriorating the overall performance, when compared to readily available explosive compositions of similar energetic properties. 
         [0003]    One example of low-flammability explosive compositions is disclosed in U.S. Pat. No. 4,861,397 to Hillstrom disclosing a material comprising an explosive in an amount of 41-85%, an additive selected from the group consisting of zinc borate, hexabromobiphenyl molybdenum flame suppressant, triaryl phosphate ester, calcium formate, antimony oxide, ammonium phosphate, aluminum oxide trihydrate, and organophosphorous diols in an amount of 9-41% and a binder component selected from the group consisting of polyurethane, acrylic polymers, phosphate ester-vinyl chloride latexes, cellulose acetate butyrate, vinyl esters, styrene-ethylene butylene block copolymers fluorinated elastomers, and Plaster of Paris rubberized with acrylic latexes in an amount of 6-39%, all of proportions being on a % by weight basis. 
         [0004]    U.S. Pat. No. 5,080,735 to Wagner discloses a cap-sensitive flexible explosive composition of reduced flammability comprising a finely divided cap-sensitive explosive in a flame resistant polymeric binder system which comprises a fluorinated elastomer, or mixture of fluorinated elastomers, admixed with from about 10% to about 30% by weight of a compatible flame retardant material, a drip suppressant, and optionally a cross-linking activator whereby the binder system when exposed to heat from an ignition source will crosslink and harden at a rate which is faster than the rate at which the explosive composition will burn. This material is commercially available and is known as LF-2. 
         [0005]    The composition disclosed in the &#39;735 patent is concerned, as stated above, with a cap-sensitive composition. The term ‘cap-sensitive’ composition denotes a substance detonable when subjected to ignition by a so-called No. 8 detonator at unconfined substance conditions, i.e. a substance classified in Class 1.1D according to UN Regulations for the Transport of Dangerous Goods. 
         [0006]    The explosive composition, provided between plates of a cassette of a reactive armor module, causes the plates to displace as result of detonation, and thus scatter (break) the jet caused by a warhead hitting the reactive armor module. Some major problems associated with reactive armors arise from the use of excessively sensitive and flammable explosive compositions. Excessive flammability can lead to ignition of the explosive even by small or medium caliber threats. In case the explosive composition burns, a potentially dangerous result may ensue e.g. for a vehicle&#39;s crew, because burning of the confined explosive composition may cause detonation, creating some serious hazards for personnel in the vicinity of the vehicle. Moreover, burning increases both the visual and thermal signature of the protected vehicle and further, the fire is likely to consume the entire explosive in the particular armor tile and may even spread to adjacent tiles. Obviously, a reactive armor tile in which the explosive composition has burnt, offers no protection against shaped charge threats. 
         [0007]    Other explosive compositions are not fully satisfactory and are either too impact sensitive or shock sensitive and thus may burn and/or detonate at some undesired conditions and further may have unsatisfactory physical or mechanical properties, or suffer from difficulties and limitations in their preparation and application. 
         [0008]    It is thus the purpose of the present invention to provide an explosive composition of substantially reduced impact/shock sensitivity and low flammability, which composition is classified in Class 1.5D according to UN regulation, i.e. a composition definable as an extremely insensitive detonating substance (EIDS), and the preparation of which is substantially simple. 
       SUMMARY OF THE INVENTION 
       [0009]    According to the present invention there is provided an explosive composition having significantly low sensitivity, low flammability and a high self-extinguishing rate, such that it is detonable only under extremely high pressure/energy conditions, e.g. striking of a jet warhead, and offers high performance as compared to readily available explosive compositions of similar energetic properties. The present invention is also concerned with a method for manufacturing such an explosive composition and applications for use of said composition. 
         [0010]    The composition according to the present invention is of substantially low flammability, i.e. it is not easily ignitable by conventional means (such as black powder, small arms, projectiles, shrapnel, etc.), however once ignited its burning rate is significantly low (almost as a passive material) and its self extinguishing rate is significantly high. In any event, the probability of burning-to-detonation-transition is negligible. 
         [0011]    The composition according to the present invention is of substantially low sensitivity, i.e. complies with test series  3 ,  5 , and  6  according to the UN regulations. For example, the composition according to the present invention is not cap-sensitive (test  5 ( a )) even under confined conditions, non-ignitable in deflagration-to-detonation-transition (DDT) test (test  5 ( b )) and reacts as a passive material in external fire test (test  6 ( c )). 
         [0012]    In light of the above, the composition according to the present invention is classified in Class 1.5D (EIDS) according to UN regulation. 
         [0013]    According to the present invention the explosive composition comprises one or more explosive materials, one or more fire retardant materials and a binder and optionally, some other additives for obtaining various desired properties. 
         [0014]    In accordance with the present invention a volumetric percentage of the components in the explosive composition are in the following ranges, as follows: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Explosive material/s 
                 42-58% vol. 
               
               
                   
                 Fire retardant/s 
                 15-26% vol 
               
               
                   
                 Binder 
                 20-36% vol 
               
               
                   
                   
               
             
          
         
       
     
         [0015]    The explosive material/s may be selected from a group comprising pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX and reduced sensitivity RDX), cyclotetramethylene tetranitramine (HMX and reduced sensitivity HMX), trinitrotoluene (TNT), nitrotriazolone (NTO), CL-20, FOX-7 and any other such explosive, or mixtures thereof. The one or more raw explosive materials may be in several granulations chosen according to the desired final properties of the explosive composition. 
         [0016]    The fire retardant may be selected from a group comprising boron containing compounds such as zinc borate, boric acid, ammonium fluoroborate etc.; phosphorus containing compounds such as phosphate esters, ammonium polyphosphate, etc.; halogen containing inorganic compounds or hydrocarbon compounds, such as decabromodiphenyl oxide (DBDPO), with or without radical stabilizers, such as antimony oxide; hydrated materials, such as alumina trihydrate (ATH), magnesium dehydrate, etc., and any other such fire retardants known in the art or mixtures thereof. 
         [0017]    The binder may be a thermoplastic or thermosetic polymer. The binder may be selected from a group comprising thermoplastic polymers, such as Estane™, Kraton™, Fluorel™, Viton™ etc., or a group of thermosetic polymers, such as polyurethanes (e.g. HTPB, Desmophen™, etc.), polydimethylsiloxanes (PDMS), etc. 
         [0018]    In compliance with US laws and regulations any plastic bonded explosive (PBX) must contain a taggent agent (for detection by security sniffers) and thus, the explosive composition according to the present invention may contain a taggent agent selected from a group comprising Ethylene glycol dinitrate (EGDN), 2,3-Dimethyl-2,3-dinitrobutane (DMDNB), para-Mononitrotoluene (p-MNT), and ortho-Mononitrotoluene (o-MNT), etc. 
         [0019]    Optionally, coloring agents (pigments) may be added to the composition for differentiation purposes between different compositions. The pigment may be in powder or liquid form. 
         [0020]    It is also possible to add to the compositions of the invention cross-linking inhibitors (or pot-life extenders), such as aliphatic phosphates. A suitable pot-life extender is, e.g. tris(2-ethylhexyl) phosphate (EHP). 
         [0021]    Depending on the selection of the fire retardant materials, the composition according to the present invention produces burning and explosive products which are not more toxic than burning and explosion products of conventional explosives. For example, acidic gasses such as HCl or HBr are not emitted during burning or explosion of the explosive composition but rather, regular burning gasses are emitted such as NOx, carbon oxides, etc. 
         [0022]    Furthermore, the raw materials used for preparation of the explosive composition are substantially non toxic, and in any case those materials which may be considered as hazardous, such as the cross-linking inhibitor and the catalyst, are at significantly low quantities in the composition. 
         [0023]    An important feature of the explosive composition in accordance with the present invention is that quantitative reverse analysis for providing the exact content of the raw materials in the composition is practically not possible or substantially complicated. 
         [0024]    The present invention is further concerned with a method for manufacturing of an explosive composition in accordance with the invention as will be disclosed hereinafter. It is appreciated that granulations and ratios of the powdered materials have significant influence on the mechanical properties of the final explosive composition, namely flexibility, strength, strain hardness, etc. The granulation ratio defines the compactability of the powdered components in the composition and thus reflects on the mechanical properties of the final product. 
         [0025]    The explosive composition in accordance with the present invention is thus characterized by the following features:
       the composition offers similar explosive performance (efficiency) as of other known explosive compositions (e.g. for Explosive Reactive Armor (ERA))   the explosive composition is classified in Class 1.5D according to UN Classification, i.e. the material is defined as an extremely insensitive detonating substance (EIDS);   the burning time is shorter (i.e. the extinguishing rate is high) than heretofore known low flammability compositions;   the composition is cheaper than heretofore similar compositions owing to the ingredients used and the method for its manufacture;   the explosive composition in accordance with the present invention is easily machined (by hand or by machine) to cut, pierce, fold, etc., and is easily applied;   the explosive composition in accordance with the present invention is substantially durable to environmental conditions such as temperature and humidity changes. It is water and other liquid substances resistant (e.g. oil, fuel, etc.), and retains its properties also after long periods of time at room temperature (above 10 years), and at 70° C. for at least three months;   the explosive composition according to the present invention may be manufactured into flexible or rigid form, depending on the intended use and purpose.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    In order to better understand the invention, some non-limiting illustrations are provided herein, wherein: 
           [0034]      FIG. 1  is a schematic representation comparing efficiency versus survivability/safety of various energetic/explosive compositions; 
           [0035]      FIG. 2  is a schematic representation of a bullet impact sensitivity test setup; 
           [0036]      FIGS. 3 to 5  are photographs of several safety test setups and their respective results, according to the UN regulations, wherein: 
           [0037]      FIGS. 3A and 3B  are setup and result of a cap sensitivity test (test  5 ( a )); 
           [0038]      FIGS. 4A to 4B  are setup and result of deflagration-to-detonation transition (DDT) test (test  5 ( b )); and 
           [0039]      FIGS. 5A and 5B  are setup and result of external fire test (test  6 ( c )); 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    The composition according to the present invention is of substantially low flammability, i.e. it is not easily ignitable by conventional means (such as small arms, projectiles, shrapnel, black powder, etc.), however once ignited its burning rate is significantly low (almost as a passive material) and in any case its self extinguishing rate is significantly high. In any event, the probability of burning-to-detonation-transition is negligible. 
         [0041]    The following are examples of compositions of explosive compositions in accordance with the present invention, wherein percentages of ingredients are indicated in percent volume. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Explosive material 
                 42-58% vol.; 
               
               
                   
                 Fire retardant 
                 15-26% vol.; 
               
               
                   
                 Binder 
                 20-36% vol. 
               
               
                   
                   
               
             
          
         
       
     
         [0042]    The explosive material may be a homogenous explosive material or may consist of two or more materials. For example, RDX and HMX in 10:1 ratio may be used. 
         [0043]    The fire retardant may be selected from a group comprising of boron containing compounds such as zinc borate, boric acid, ammonium fluoroborate etc.; phosphorus containing compounds such as phosphate esters, ammonium polyphosphate, etc.; halogen containing inorganic compounds or hydrocarbon compounds, such as decabromodiphenyl oxide (DBDPO), with or without radical stabilizers, such as antimony oxide; hydrated materials, such as aluminum trihydrate (ATH), magnesium dehydrate, etc., and any other such fire retardants known in the art or mixtures thereof. 
         [0044]    The binder may be a thermoplastic or thermosetic polymer. The binder may be selected from a group comprising of thermoplastic polymers, such as Estane™, Kraton™, Fluorel™, Viton™ etc., or a group of thermosetic polymers, such as polyurethanes (e.g. HTPB, Desmophen™, etc.), polydimethylsiloxanes (PDMS), etc. According to one particular embodiment, the binder comprises PDMS (between about 88-95% weight), a cross-linking agent (between about 5-10% weight) and a catalyst (between about 0.05-0.2% weight). The binder typically comprises a cross-linking inhibitor such as tris(2-ethylhexyl)phosphate (EHP) in a typical amount of 0.3-1.5% weight. 
         [0045]    The taggent agent may be selected from a group comprising, among others, Ethylene glycol dinitrate (EGDN), 2,3-Dimethyl-2,3-dinitrobutane (DMDNB), para-Mononitrotoluene (p-MNT), and ortho-Mononitrotoluene (o-MNT), etc. 
         [0046]    The method for manufacturing of an explosive composition in accordance with the present invention comprises the following steps:
       a) drying powdered explosive materials in a specially designed explosive proof oven for explosive powders;   b) weighing raw materials;   c) mixing the raw materials, e.g. in a sigma mixer or a planetary mixer, as known per se, thereby obtaining a dough;   d) removal of residual air from the dough to thereby avoid presence of air voids in the final product. This is obtained under vacuum and the composition is then compressed through a nozzle having a desired shape, e.g. cord shaped, flat paste form, etc.;   e) cross-linking the composition in an oven at 50-70° C. to harden the composition. This stage, however, may be omitted to thereby retain the composition as a dough for different applications e.g. where the final product is to be injected in its use;   f) forming the hardened composition to a desired final shape.       
 
         [0053]    It is appreciated that different levels of cross-linking may be performed in order to obtain different levels of flexibility of the final product. While cross-linking, the chemical, sensitivity and energetic properties of the composition are not affected, but rather only the mechanical properties of the resultant product. 
         [0054]    It is further appreciated that the explosive composition obtained after the compressing stage through the shaped nozzle (step d) may be of any desired form. For example, for use in reactive armor modules, sheets of material are required. In this case, the paste obtained after step d) is pressed using spacers so as to obtain material at a desired and uniform thickness. 
         [0055]    Furthermore, forming the final material, i.e. cutting and piercing of the final composition obtained at step e) may be carried out by mechanical means, e.g. using a water jet or different types of presses or rollers, or, alternatively, it may be carried out manually, using simple means such a scissors, knives, mold templates punches, etc. 
         [0056]    When the composition is to be used as an exploding cord for example, to cut/sheer pillars, pipes, etc., the explosive composition is formed in the shape of a flexible cord. 
         [0057]    The explosive composition according to the present invention may be modified, and the following are different examples: 
       EXAMPLE 2 
       [0058]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 explosive material - RDX 
                 55% vol, 
               
               
                   
                 ATH 
                 20% vol, 
               
               
                   
                 PDMS 
                 25% vol, 
               
               
                   
                 pigment 
                 0.05% vol,   
               
               
                   
                 taggent (e.g. DMDNB) 
                 0.18% vol.   
               
               
                   
                   
               
             
          
         
       
     
       EXAMPLE 3 
       [0059]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 explosive material - RDX 
                 46% vol, 
               
               
                   
                 explosive material - HMX 
                  4% vol, 
               
               
                   
                 boric acid 
                 25% vol, 
               
               
                   
                 PDMS 
                 25% vol, 
               
               
                   
                 pigment 
                 0.05% vol,   
               
               
                   
                 taggent (e.g. DMDNB) 
                 0.2% vol.  
               
               
                   
                   
               
             
          
         
       
     
       EXAMPLE 4 
       [0060]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 explosive material - RDX 
                 48% vol, 
               
               
                   
                 explosive material - HMX 
                  6% vol, 
               
               
                   
                 boric acid 
                 22% vol, 
               
               
                   
                 PDMS 
                 24% vol, 
               
               
                   
                 pigment 
                 0.05% vol,   
               
               
                   
                 taggent (e.g. DMDNB) 
                 0.18% vol.   
               
               
                   
                   
               
             
          
         
       
     
         [0061]    The following table illustrates the differences between the compositions of the above examples, as reflected in ballistic (armor) and bullet impact sensitivity test results. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Performance/efficiency 
                 +++ 
                 ++ 
                 +++ 
               
               
                   
                 (armor) 
               
               
                   
                 bullet impact 
                 ++ 
                 +++ 
                 +++ 
               
               
                   
                 sensitivity 
               
               
                   
                   
               
             
          
         
       
     
         [0062]    It is further appreciated that the granulation ratios of the raw powdered materials, in particular the explosive materials and the solid fire retardants, influence only the mechanical properties of the resultant composition, namely, flexibility, strength, stain, hardness, etc. 
         [0063]    Turning now to  FIG. 1 , there is illustrated a graph representing the efficiency (in an armor module) of different explosive compositions versus their survivability/safety. As may be noted the explosive composition in accordance with the present invention, identified as LBR-6, shows significantly high efficiency with reasonable survivability/safety as compared with other compositions for explosive reactive armor (ERA). The composition identified as LF-2, which is available in the market, shows similar survivability in an armor module as of the LBR-6 though its safety is lower than that of LBR-6: LF-2 is classified according to the aforementioned UN regulations under Class 1.1D, while LBR-6 is classified under Class 1.5D. 
         [0064]    Self-Limiting ERA (SLERA) comprises an energetic material/explosive layer in armor module, which can provide good multiple-hit capability in modular configuration. The energetic material/explosive used in SLERA is not as effective as fully detonable explosives. This material can be classified under Class 1.5D or potentially be excluded from Class 1 (not an explosive). 
         [0065]    Non-Explosive Reactive Armor (NxRA) has comparable efficiency to SLERA, though the energetic material in NxRA is not an explosive (not in Class 1). The survivability of NxRA is good, ant it has good multiple-hit capability against hollow charge warheads. 
         [0066]    Non-Energetic Reactive Armor (NERA) has limited efficiency against hollow charges and is totally passive, thus provides excellent survivability and maximal multiple-hit capability. In this type of armor module, the material layer in the cassette is not energetic at all, e.g. rubber, glass, etc. 
         [0067]    Turning now to  FIG. 2  there is a schematic representation of a bullet impact sensitivity test setup illustrating a barrel  14  aimed to fire rounds of 14.5 mm small arms at a sandwich-like element  16  containing an 8 mm thick layer of tested composition  18  (the energetic material or explosive) displaced between two steel plates  20  and  21  (2 mm and 6.4 mm thick, respectively). The sandwich-like element  16  is positioned at a typical standoff of 7 to 10 m in compliance with the UN regulations and is inclined horizontally at about 30°. The following Table 1 represents bullet impact sensitivity test results at room temperature for different explosives applied in such a sandwich-like assembly. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 bullet impact sensitivity tests 
               
             
          
           
               
                   
                 Explosive 
                 Burning probability 
                 Burning duration 
               
               
                   
                   
               
             
          
           
               
                   
                 C-4 
                 High (10/10) 
                 &gt;10 
                 min. 
               
               
                   
                 LF-2 
                 Low (4/10) 
                 5-10 
                 min. 
               
               
                   
                 LBR-6 
                 Low (4/10) 
                 1-2 
                 min. 
               
               
                   
                   
               
             
          
         
       
     
         [0068]    The term burning probability denotes the likelihood of ignition of the explosive in the sandwich-like element  16  upon striking by a 14.5 mm round. The data presented in Table 1, under “burning probability” indicates the number of burning incidents out of 10 rounds fired. The term burning duration denotes the burning time of the 8 mm explosive layer in the sandwich-like element  16  once ignited upon striking by a 14.5 mm round. 
         [0069]    In case of striking element  16  by smaller arms, such as 0.5″ rounds, the burning probability of LF-2 and LBR-6 becomes zero (0/10), while for C-4 it is higher (4/10). 
         [0070]    Turning now to  FIGS. 3 to 5  there are illustrated photographs of several safety test setups and their respective results, according to the UN regulations. 
         [0071]      FIG. 3A  is the setup of cap-sensitivity test (standard UN regulations (test  5 ( a )) showing a polyurethane cylinder  22  of standard dimensions containing LBR-6 explosive  23  with a No. 8 detonator  24  received there within in the center. The cylinder is positioned on a steel witness plate  26  placed over supports  27  which in turn rest on a heavy steel plate (40 mm thick)  29 . A successful test result for a cap-sensitivity test is no penetration of the witness plate  26 , as can be seen in  FIG. 3B  after ignition of the detonator  24 . As can further be seen in  FIG. 3B  the explosive composition  23  is only mechanically scattered (i.e. no detonation occurred) such that most of the explosive remains intact in the cylinder. The other part of the cylinder was found outside of the cylinder after the test. Even more so the witness plate  26 , shown on the left side of the picture, remains un-indented. The cylinder  22  is partially ripped owing to the detonation of the No. 8 detonator  24 . The above results were repeated using a steel cylinder instead of polyurethane cylinder, considered as confined conditions which are much more severe (not shown). 
         [0072]      FIG. 4A  illustrates a deflagration-to-detonation transition (DDT) test setup (standard UN regulations (test  5 ( b )), wherein a steel cylinder  30  is filled with the tested LBR-6 explosive  32 . The bottom end of the cylinder  30  is welded to a steel whiteness plate  36 . A detonator  38  is received within 5 grams of black powder  40  supported by a plastic container  42  within the tested explosive composition, with an electric cord  44  extending from the detonator  38  through a sealing cap  34  screwed coupled to seal an opposed end of cylinder  30 . 
         [0073]    A successful test result for a DDT test is no penetration of the witness plate  36  due to detonation of the explosive  32 . As can be seen in  FIG. 4B  the witness plate  36  is sheered due to pressure built up in the cylinder  30  but no penetration occurred as result of detonation. Moreover, the cylinder  30  and the cap  34  remained whole (undamaged) and most of the explosive was found after the test, 50% remains intact in the cylinder and the rest of it was found beside the cylinder. 
         [0074]      FIG. 5A  illustrates an external fire test setup (standard UN regulations (test  6 ( c )) wherein five cardboard boxes  54  filled with 150 Kgs. of the tested LBR-6 explosive  56  (a total volume of 0.15 m 3  in compliance with the UN regulations) are mounted on a rack  58  placed in a Kerosene reservoir  60  of 1500 liters. The Kerosene is remotely ignited resulting in a total burning of the boxes  54  containing the explosive composition  56 . 
         [0075]    As is illustrated in  FIG. 5B  after complete burning of the explosive composition during 1 hour, remainders of the explosive composition  56  are visible. These results indicate a low burning rate of the LBR-6 explosive. Furthermore, in such a test the burning of the LBR-6 explosive was non-violent (calm), i.e. throwing flames at a diameter of approx. 30 cm. in average. 
         [0076]    The safety test results obtained in the tests exemplified in  FIGS. 3 to 5  indicate an explosive composition qualifying as a Class 1.5D according to UN regulations for the transport of dangerous goods. 
         [0077]    The composition obtained according to the present invention may be used, according to one of its applications, as an explosive composition in an explosive reactive armor (ERA) module, applied on combat vehicles etc. 
         [0078]    Whilst some particular embodiments have been illustrated and described, it will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown by the exemplary embodiments described hereinabove. Thus, it should be understood that numerous additional embodiments are within the scope of the invention, mutatis mutandis.