| Nitramine explosives(RDX,HMX,CL-20)have been applied to navy,land and air force weapons and used as energy sources for explosive devices due to their high energy density.However,large safety hazards in the storage,transportation and usage caused by the high mechanical sensitivity have restricted their widespread application.Therefore,the development of new types of high-efficiency desensitization technologies is crucial for broadening the application range of nitramine explosives.To address shortcomings of the traditional coating technology,a novel coating desensitization method-emulsion polymerization-was proposed in this study.Herein,two emulsion polymerization systems of methyl methacrylate and melamine-urea resin were designed respectively.And finally spherical RDX,HMX and CL-20-based composite energetic microspheres with dense surface coating and significantly reduced mechanical sensitivity were successfully fabricated.The detailed contents of the research are as follows:(1)Compatibility analysis and simulation study of the binder and nitramine explosive.In order to study the compatibility of polymethyl methacrylate(PMMA)and urea-formaldehyde resin(UF)with three nitramine explosives(HMX,RDX and CL-20),firstly,the binding energy,which is used to measure the compatibility level between the two binders and three nitramine explosives,was calculated by molecular dynamics simulation.Secondly,differential scanning calorimetry(DSC)was employed to study the compatibility between the two binders and explosives.Finally,to explore the interaction ability between the binder component and the single explosive in different temperature systems,the binding energy of the binder and the explosive at various temperatures was simulated.Simulation and DSC results demonstrated that the binders PMMA and UF have good compatibility with the threekinds of explosives,and can be used as favorable coating materials for the desensitization of nitramine explosives;MD simulation showed that the optimum binding temperatures between molecules in HMX/PMMA,RDX/PMMA,CL-20/PMMA,HMX/UF,RDX/UF and CL-20/UF energetic composites were: 353 K,353 K,343 K,353 K,333 K and 343 K.(2)In this paper,the “green emulsification shearing technology” was proposed for the shortcomings of traditional refinement,and submicron HMX,RDX and CL-20 particles were prepared as the main explosives for the subsequent fabrication of spherical composite energetic microspheres.Moreover,the research focused on the effects of emulsification shearing time on the morphology and properties of HMX.It is found that as the emulsification shearing time increased,the particle size of HMX particles gradually became smaller.And noticeably the optimal emulsification shearing time was 6 h,the submicron HMX particles fabricated at this time had a narrow size distribution and the special height is35.62 cm,bad a good thermal stability with the particle size of about 300 nm.(3)In order to further investigate the differences in morphology and performance of composite particles prepared by emulsion polymerization and other methods,taking RDX/MUF as the research object,three different composite particles were prepared via three different techniques: physical mixing method,drying bath method and emulsion polymerization method.The morphology,crystal structure,thermal decomposition property and mechanical sensitivity of the as-prepared composites were characterized by scanning electron microscopy(SEM),X-ray powder diffraction(XRD),Fourier transform infrared(FT-IR),differential scanning(DSC),impact sensitivity test,and the like.At the same time,single factor experiments were carried out on the effects of three key factors(different ratio of melamine-urea,pH of urea-formaldehyde resin prepolymer solution,reaction temperature)on the crystal morphology in the process of preparing RDX/MUF via emulsion polymerization.Analyses showed that the crystal structures of RDX/MUF composite particles fabricated by different methods did not change,however,the morphologies and properties were quite different.Especially the RDX/MUF composite energetic microspheres prepared by emulsion polymerization possessed a spherical structure with smooth surface,and their thermal stability and mechanical sensitivity were optimal.The study also found that the optimum reaction temperature for preparing RDX/MUF composites by emulsion polymerization was 70 ?,the optimum pH value of urea-formaldehyde resin prepolymer solution was 3,and the optimum melamine-urea molar ratio was 0.4.Under these conditions,the resultant RDX/MUF composite energetic microspheres exhibited a solid spherical shape with uniform dispersion.(4)Aiming to verify the superiority of emulsion-polymerization method for the preparation of composite energetic microspheres when using PMMA as the binder,RDX/PMMA was used as the research object,and three different RDX/PMMA composites were prepared by water suspension coating method,one-step granulation method and emulsion polymerization method,respectively.And then the morphology and crystal structure of the as-prepared composites were further analyzed.Meanwhile,during the preparation of RDX/PMMA composite microspheres via emulsion polymerization,the influences of reaction temperature,initiator dosage and the amount of emulsifier on the particle size and forming effect of RDX/PMMA were conducted by orthogonal experiment.And a preliminary study on the sphere-shaping mechanism of RDX/PMMA was carried out as well.The experimental results indicated that the morphologies and properties of RDX/PMMA composite particles fabricated by different methods turned out rather differently.Among them,the RDX/PMMA composite energetic microspheres obtained through emulsion polymerization appeared as dense spherical structure.And the crystal structures of the composite particles prepared by the three methods did not change.Therefore,in the process of coating RDX with PMMA,emulsion polymerization is a reliable desensitization technology for the preparation of spherical composite energetic microspheres.In addition,the range analysis showed that the influence of the following three factors on the RDX/PMMA regularity was as follows: reaction temperature > initiator dosage > emulsifier dosage.And the preferred process parameters for preparing RDX/PMMA composite energetic microspheres were: reaction temperature 80 ?,initiator dosage 0.01 g,emulsifier dosage0.15 g.Furthermore,through the analysis of the section of RDX/PMMA microspheres,weestablished the “honeycomb spheroidization model” and explored the spheroidal mechanism of RDX/PMMA in detail.(5)The RDX/PMMA,RDX/MUF,HMX/PMMA,HMX/MUF,CL-20/PMMA,CL-20/MUF composite energetic microspheres with the same binder content were prepared via emulsion polymerization,and their crystal structures,morphologies and properties were made contrastive analysis.It is found that the six groups of composite particles obtained from emulsion polymerization all exhibited dense spheroidal structures.What's more,the addition of the two binders did not damage the crystal structure of the single explosive,meanwhile,the thermal stability and safety performance of the as-prepared microspheres were remarkably improved as well.On the whole,the emulsion polymerization method can effectively enhance the coating effect of the binder coating,and improve the thermal stability of the single explosive simultaneously,which means it is a highly efficient desensitization technology for preparing composite energetic microspheres with desirable morphology. |
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