Abstract:
A method of in-situ remediation of chlorinated and fluorinated reaction by-products resulting from energetic detonations and/or burning of energetic mixtures comprising adding a quantity of Calcium disilicide (CaSi 2 ), Calcium silicide (CaSi), Magnesium disilicide (MgSi 2 ), Magnesium silicide, or Aluminum Calcium (Al 2 Ca) compounds to the energetic mixture prior to its detonation and/or burning. Advantageously, the in-situ production of more inert by-products results from this addition thereby preventing the formation of any less-desirable by-products.

Description:
U.S. GOVERNMENT INTEREST 
     The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to the field of energetic materials. More particularly, it pertains to a method for in-situ remediation of detonation and/or burning by-products containing chlorinated or fluorinated components while maintaining system performance. 
     BACKGROUND OF THE DISCLOSURE 
     An undesirable effect of the detonation and/or burning of energetic materials is the production of by-products containing Chlorine or Fluorine. Such by-products may include acids such as HF and HCl, gases (Fluorine and Chlorine), and—if metalized energetic are used—gaseous metal fluorides. Accordingly, methods and/or systems which mitigate the production of these undesirable by-products would represent a significant advance in the art. 
     Recent work has shown that combustion of calcium disilicide in polytetrafluoroethene (PTFE) did form calcium fluoride (A. Huczko, H. Lange., G. Chojecki, S. Cudzilo, Y. Q. Zhu, H. W. Kroto, D. R. M. Walton, “Synthesis of Novel Nanostructures by Metal-PTFE Thermolysis”,  J. Phys., Chem. B,  2003, 107, 2519-2524). Other work shows that metal addition of metals to fluorinated binder systems show improvements in propellant systems (C. J. Fawls, J. P. Fields, K. L. Wagaman, U.S. Pat. No. 6,843,868, Jan. 15, 2005.). 
     Accordingly, a continuing need exists in the art for methods and/or systems which mitigate the production of undesirable detonation and/or combustion by-products. 
     SUMMARY OF THE DISCLOSURE 
     An advance in the art is made according to an aspect of the present disclosure directed to methods that provide in-situ remediation of Chlorine and Fluorine by-products. Advantageously, energetic materials formulated according to the present disclosure exhibit significant reductions in Chlorine and/or Fluorine containing compounds while maintaining their energetic and/or overall system performance. In sharp contrast to contemporary remediation methods which generally remediate after the formation/depositing of these materials, the present disclosure is an in-situ method which effectively prevents their formation at the time of reaction. Such advantages—according to an aspect of the present disclosure—are accomplished by the addition of calcium disilicide (CaSi 2 ), calcium silicide (CaSi), or other group silicides (Mg, Sr, etc.), or aluminum calcium (Al 2 Ca) to the energetic formulation in the proper amounts and particle sizes 
     Viewed from a first aspect, the present disclosure is directed to a method of providing in-situ remediation of Chlorine and Fluorine by-products of the detonation and/or burning of energetic mixtures by preparing the energetic mixtures such that they contain a quantity of metallic components—and in particular Calcium disilicide CaSi 2 . Advantageously—and as a direct result of this addition—the formation of undesirable Chlorine and/or Fluorine containing by-products is substantially prevented without loss of measurable energetic effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       A more complete understanding of the present disclosure may be realized by reference to the accompanying drawings in which: 
         FIG. 1  is a plot of an X-Ray diffraction analysis of a power residue from combustion of a formulation according to an aspect of the present disclosure; and 
         FIG. 2  is a plot showing detonation product residue from Calcium disilicide impregnated explosive experiments wherein * denote CaF 2  peaks and Δ denotes silicon peaks. 
     
    
    
     DETAILED DESCRIPTION 
     The following merely illustrates the principles of the disclosure. 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 disclosure 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 disclosure 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 disclosure, 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. 
     By way of some additional background, it is noted that the use of fluorinated binder systems in explosive, pyrotechnic, and propellant applications are energetically-beneficial due—in part—to the highly oxidative nature of the fluorine atom. Chlorinated binder systems have also been used in energetic formulations resulting in similar enhancements in performance. One such chlorinated binder system which has found widespread use in explosive and propellant systems is ammonium perchlorate (AP). 
     More recently, concerns about the environmental effects/impacts of these chlorinated or fluorinated compounds have arisen. Consequently, attempts to remediate Chlorine and/or Flourine containing reaction by-products have been undertaken. 
     And while much contemporary literature discusses bioremediation and other remediation efforts of these by-products, nothing is reported regarding the in-situ prevention of their formation. Advantageously, the present disclosure addresses this deficiency in the art. 
     With this background in place, it is known to those skilled in the art of pyrotechnics that burning metals with fluorinated binders, such as magnesium and Teflon, produce magnesium fluoride:
 
Mg+—[C x F y ]—→MgF 2 +CO 2 +heat
 
     Accordingly, if one could apply these reaction principles to chlorinated and/or fluorinated energetic systems, then reaction by-products exhibiting a greater (more favorable) inertness may be produced. While arguably intuitively obvious, it is nevertheless a perplexing challenge is to obtain the reaction of the metal fluoride (as in the above example) at rates that can be used in energetic mixtures such as high explosives and/or propellants, where pressure rise times and reaction rates oftentimes exceed the burn propagation front with speeds approaching and/or exceeding the speed of sound. 
     More recently, a number of aluminized explosives have been produced that show evidence of an aluminum reaction within a short timeframe characteristic of a detonation event. These aluminized explosives are generally referred to as “combined effects” explosives, where the metal pushing ability (Gurney constant) is nearly the same as highly loaded nitramine formulations without the aluminum additive while also exhibiting a sufficiently high blast. 
     According to an aspect of the present disclosure then, a metal additive is formulated with energetic materials such that it reacts upon the rapid pressure/temperature rise of detonation/ignition and preferentially forms certain reactive by-products. Caclium disilicide CaSi 2  is one example of such an additive according to the present disclosure and it advantageously binds the Fluorine and Chlorine toxic by-products with the silicon while not significantly degrading the energetic output. 
     EXAMPLES 
     Example 1 
     A combustion study of CaSi 2  in Viton binder was used to evaluate the feasibility of calcium chloride formation during a burning event. 0.69 g of Viton was dissolved in acetone to which 0.55 g of CaSi 2  was added. The mixture was stirred and acetone slowly evaporated. The dried solids were placed in a ceramic crucible (0.872 g) and combusted in a furnace preheated at 800° C. and then ramped to 950° C. for 1 hour. The solids collected were 0.619 g of grey and white material. The material was digested in sulfuric acid and the liquid tested for fluorine ions. It was calculated that the sample contained about 7% calcium fluoride. While only performed at the combustion level at atmospheric pressure, this test confirmed the formation of calcium fluoride. It is anticipated that at higher pressures and temperatures of a detonation event will further drive the oxidation of calcium with fluorine to completion. 
     Example 2 
     A study using LX-07 (10% Viton/90% HMX) was performed using Cheetah 5.0 thermochemical code and the bkws library. Upon optimization by statistical design of experiments, it was found that a formulation of 10% Viton/83% HMX/7% calcium silicide, HF in the final combustion products was advantageously reduced by nearly 300 times. For lkg of explosive, this brings the level of potential free HF in the reaction by-products to below 3 ppm at equilibration with atmospheric pressure and room temperature, which is within the OSHA permissible limit value (PML) for HF. A product analysis of the Cheetah detonation is shown below in Table 1. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Product analysis from Cheetah analysis of detonation 
               
             
          
           
               
                   
                 Performance and detonation products  
               
               
                   
                 (mol/kg) at full combustion 
               
             
          
           
               
                   
                   
                 Detonation 
                 Total  
                   
                   
                   
               
               
                 Formula- 
                 CJ 
                 velocity 
                 Energy 
                   
                   
                   
               
               
                 tion 
                 (Mpa) 
                 (km/s) 
                 (kJ/mol) 
                 HF 
                 CAF 2   
                 SiO 2   
               
               
                   
               
               
                 LX-07 
                 33.88 
                 8.82 
                 10.542 
                 0.03473 
                 — 
                 — 
               
               
                 83%  
                 34.61 
                 9.05 
                 11.441 
                 0.0001  
                 0.0103 
                 0.0068 
               
               
                 HMX/10% 
                   
                   
                   
                   
                   
                   
               
               
                 Viton/7%  
                   
                   
                   
                   
                   
                   
               
               
                 CaSi 2   
               
               
                   
               
             
          
         
       
     
     Silicon tetrafluoroide (SiF 4 ) was also a detonation product at 0.0035 mol/kg, which translates to 1000 ppm. Further increasing the amount of CaSi 2  in the formulation does lead to nearly an order of magnitude decrease in the amount of this toxic chemical and a corresponding increase in amount of SiO 2 . 
     Of particular note is the increase in detonation velocity, Chapman-Jougot (CJ) pressure, and total energy of the detonation event. This is due to the highly exothermic formation of calcium fluoride, assuming this compound is formed in the detonation reaction zone. Considering the melting point of calcium disilicide is on the order of about 1070° C., and it readily reacts with air by 800° C., it is likely this reaction would occur at volume expansions &lt;15 V/V o . 
     A formulation from this Example 2 was made on the 50 gram scale. The dry ingredients were slowly added to a solution of Viton A dissolved in methyl ethyl ketone while mixing in a Ross ½ pint mixer with counter rotating blades. The granules were dried overnight at 60° C. in a vacuum oven, and then pressed into a small pellet and combusted in a Parr 1590 Semi-micro combustion calorimeter. 
     The solid products from the burn event were analyzed in a Rigaku Miniflex x-ray diffractometer (XRD). Peaks consistent with calcium fluoride were identified at 2θ=28.6°, 47°, and 56.8° ( FIG. 1 ). The small broad peak size is consistent with either nanocrystalline domains or amorphous structure, most likely carbon centered at ˜25. 
     Example 3 
     Two 50 g batches of explosive were made; one with Viton A, RDX, and calcium disilicide and the other without calcium disilicide. The materials were mixed in a slurry of Viton A in MEK and slowly evaporated while mixing in a Ross planery blade Pint Mixer. 
     The two energetic formulations were tested in a closed bomb detonation. Both formulations included a nitramine, fluorinated binder and burn suppressant, however one contained CaSi 2 . After detonation of the samples both solid and off gasses were analyzed. The HF was collected in a polyethylene loop cooled in a liquid nitrogen/ethanol bath and the gases bubbled through ice water. The tubing was thoroughly rinsed and combined with the ice water and analyzed with anion chromatography. Results of HF formation are summarized in Table 2. As can be seen the Percent Theoretical HF production was reduced by nearly 30 percent with the presence of CaSi 2 . The theoretical HF production is calculated such that all F in the binder is converted to HF as an end product which yields a theoretical maximum amount. XRD was run on the solid residues which confirmed the formation of CaF 2  ( FIG. 2 ). 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Detonation products analysis 
               
               
                 from two formulations one with and one without calcium disilicide 
               
             
          
           
               
                   
                 Mass of 
                 Theoretical 
                 Total HF 
                 Percent 
               
               
                 Sample 
                 Sample  
                 HF Production 
                 Production 
                 Theoretical 
               
               
                 Identification 
                 (g) 
                 (mg) 
                 (mg) 
                 HF Produced 
               
               
                   
               
               
                 Formulation 
                 6.0318 
                 1257.0 
                 758.7 
                 60.36 
               
               
                 WithOut CaSi 2   
                   
                   
                   
                   
               
               
                 Formulation 
                 5.7927 
                 1005.6 
                 421.3 
                 41.89 
               
               
                 With CaSi 2   
                   
                   
                   
                   
               
               
                 Formulation 
                 5.7059 
                  990.54 
                 418.1 
                 42.21 
               
               
                 With CaSi 2   
               
               
                   
               
             
          
         
       
     
     At this point, while we have discussed and described exemplary embodiments directed to methods and compositions for the remediation of Chlorine and Fluorine by-products of energetic reactions. And while the exemplary compositions have been disclosed, those skilled in the art will appreciate that the techniques and compositions according to the present disclosure are not so limited. For example, numerous chlorine and/or fluorine containing binder systems are anticipated as benefiting from the present disclosure. More particularly, it is anticipated that Calcium disilicide employed will include a quantity of one or more of the following impurities, i.e., iron, iron silicide, free silicon, and carbon. No negative effects from their inclusion are anticipated. In addition, in a preferred embodiment the Calcium disilicide will advantageously exhibit an average particle size varying from 0.01 microns to 1000 microns. Consequently, no special Calcium disilicide is required to practice the disclosure. In addition, silicides/disilicides of Magnesium as well as AluminumCalcium (AlCa2) compositions are contemplated by the present disclosure in a similar manner to those described with respect to Calcium compositions. This includes impurities, like compounds and particle sizes—to name a few. 
     And while we have generally use the terminology of energetic mixtures, those skilled in the art will appreciate that such energetic mixtures may include high explosives, propellants and/or thrusters as those terms are generally known and used in the art. Finally, the energetic mixtures may comprise one or more known halogenated monomers, polymers, waxes, or elastomers as well as quantities of known chlorine and/or fluorine containing compounds used in the energetic arts including—but not limited to—ammonium perchlorate, potassium perchlorate, ammonium chlorate, potassium chlorate, sodium perchlorate and sodium chlorate. It was noted in the art that by addition of calcium disilicide to energetic formulations containing fluorinated and chlorinated binder systems or ammonium perchlorate that the toxic by-products upon reaction are substantially reduced or eliminated. Additionally, the useful work energy of the system, either in propellants, thrusters, or high explosives, remains with little or no loss in performance responses provided the rate of calcium disilicide reaction is such that the reaction enthalpy to calcium chloride or fluoride is realized within the timeframes of the desired reaction. This can vary from microseconds for secondary high explosive formulations to milliseconds for propellant and thruster applications. Such performance remains in spite of the loss of energetic moieties due to the high heats of formation of calcium fluorides or chlorides which further promotes gas expansion. 
     Accordingly, the present disclosure should only be limited by the scope of the claims that follow.