Surface Coating Of Nitroamine Explosives And Its Effect On Performances Of Propellant

Although adding nitroamine explosives can increase the energy of solid propellants, but they also can worsen the safety, the mechanical performances and the processing properties of the propellants, which greatly restricts the application of high energy (HE) propellants. Some researches show that the safety and mechanical performances of propellants can be improved by surface-coating on the nitroamine explosives fillers. However, the energy properties of propellant decrease due to the addition of inert coating materials. Moreover, the formulation composition of propellants can be changed, thus the compatibility, storage transportation properties and service performances of propellants can also be affected by the addition of such coating materials. In order to eliminate the negative effects mentioned above, the ingredients of propellants are selected to coat the nitroamine explosives and the coated explosives are applied into propellants in this paper. The influence of surface coating on mechanical sensitivity and thermal stability of nitroamine explosive are studied. Moreover, the influence of coating of nitroamine explosives on mechanical sensitivity, thermal stability, tensile strength, percentage elongation and processing properties of propellants are also investigated. The research is of great and positive significance to exploit insensitive propellants and prevention of "dewetting" of nitroamine explosives. The details are described as follows:Firstly, NC (nitrocellulose) and centralite-1 (C-1) are selected to coat niroamine explosives together by phase separation and slurry-emulsion-distillation methods respectively, and the optimum technology conditions are obtained. The mechanical sensitivity of the coated samples prepared by the two methods both decreased. However, the insensitive effect of the sample prepared by slurry-emulsion-distillation is more evident than that prepared by phase separation, due to the denser coating layer. In addition, such coating method has the advantages of facility and safety. Concretely, when [NC+C-1]/RDX composite prepared by slurry-emulsion-distillation is filled in the CMDB propellant as the solid fillers, the safety, mechanical performances and processing technology of propellant are all improved.Secondly, TNT (2,4,6-trinitrotoluene) and an energetic material (HP-1) are used to coat RDX and HMX (octogen) by means of combining solvent-nonsolvent and aqueous suspension-melting. The research shows that unlike the coating mode of the sample coated with only TNT which belongs to "island coating", the mode of the sample coated with TNT/HP-1 changes to "film coating", in which the TNT/HP-1 capped the surface of RDX tightly. It is attributed to the decrease of surface tension of melted TNT owing to the introducing of HP-1. After coating, the impact and friction sensitivities of RDX decrease obviously, and the thermal stability and of the explosion heat do not vary obviously. In terms of the propellant made with RDX samples coated with 2.5% TNT and 0.5% HP-1, both impact and friction sensitivities are lower than that made with raw RDX. Moreover, the percentage elongation increases and the calendering times decrease distinctly.Thirdly, the burning catalysts (lead stearate and lead phthalate) of CMDB propellant are used to coat RDX and HMX by chemical precipitation. The insensitive effect of lead stearate on nitroamine explosive is in evidence. By surface coating with lead stearate, the mechanical sensitivity of RDX and HMX both decrease significantly. However, lead phthalate is no insensitive action on nitroamine explosives. The mechanical sensitivity of HMX coated with lead phthalate does not decrease but increase. The thermal decomposition properties of propellants are affected by methods of which way the burning catalysts added into. If the lead stearate and lead phthalate loaded upon the surface of explosive particles are filled into propellants, their catalytic activity are superior beyond the active of catalysts simple mixed in propellants, such as the decreasing of thermal decomposition peak temperatures of NC/NG and the increasing of the thermal decomposition heat. The mechanical sensitivity, mechanical performances and energy properties of propellants do not vary obviously, in despite of which way to add the catalysts into the propellant.Finally, HTPB (hydroxyl terminated polybutadiene) and its corresponding additives are used to coat nitroamine explosives by one-step phase separation (OSPS), two-step phase separation (TSPS) or double layer coating technology (DLCT), respectively. The performance testing results of samples prepared by OSPS show that the mechanical sensitivity of coated samples reduces greatly. However, lots of severe agglomerations formed during the curing process of HTPB, and the negative result limits its application. To improve the dispersibility of coated samples, TSPS is employed to coat the nitroamine explosive. Such method can effectively prevent the conglutination among the HTPB that attached on the surface of explosive particles. However, there are still some disadvantages in the process, such as high solvent dissipation, low preparation efficiency. As a result, HTPB/IPDI (Isophorone diisocyanate) and TNT are successively coated on RDX (hexogen) particles by DLCT. While HTPB coated on RDX particles cured completely and TNT is removed by solvent dissolution, the well-dispersed RDX particles coated with cured HTPB are obtained successfully. As the outer layer, TNT effectively hinders the adhesion among the inner layer of HTPB. The mechanical sensitivities of RDX decreased significantly by such surface coating. When the covering amount of HTPB is 2wt.%, drop height (H₅₀) of RDX increases by 79% and explosion probability (P) decreases by 76%. Compared with that containing uncoated RDX, the mechanical sensitivities of propellant slurry and products containing coated RDX samples decrease obviously, and the tensile strength also improves obviously. In addition, the energy properties of propellants do not decrease because the propellants filled with HTPB/RDX has the same formulation with the propellant filled with raw RDX.