Cl - 20 Recrystallization Crystal Structure Change In The Process Of Research

Hexanitrohexaazaisowurtzitane (CL-20) is a high energetic material, which is one of the highest density and energy single compound explosives. It has a wildly application prospect in propellants and composite explosives. Polymorphism is one of the most important aspects of explosive. The density, sensitivity, stability and detonation performance of explosive will be affected by the existence of different polymorphs. The pure polymorph crystals shall be prepared to satisfied the demand of applications. CL-20is a polymorphic explosive. Four polymorphs are exist in room temperature and pressure including α-, β-,γ-and ε-, in which The ε-form is the highest density, most stability, best detonation explosive and the most value in use. Because the polymorphic transformation of CL-20is easy to occur in recrystallizaiton that the mixed polymorphs are formed and then the performance of explosive are affected. So how to control the behaviors of polymorphic transformation and prepare the high purity polymorph and high quality ε-CL-20is the key problem in crystallization technique of CL-20. Therefore, in this paper we focus on the influence of crystallization conditions to the polymorphic transformation of CL-20and the optimization of crystallization technique to provide the instruction for the preparation of pure s-form and the crystallization mechanism of CL-20.In the paper, antisolvent crystallization method is used, and the factors of antisolvent property, solvent addition method, solvent/antisolvent ratio, the formation of solvation and temperature et al. are investigated. The PXRD and FTIR are used to identify the forms of CL-20in order to obtain the polymorphic transformation behaviors of CL-20, primary master the transformation rule and control technique and try to explain the transformation mechanism. The main results are as follows:(a) It was observed that the dipole moment of anti-solvent is related to the variations in polymorphic form. In high polarity antisolvent, the metastable β-form was first precipitated and then gradually transform to stables-form; but in nonpolarity antisolvent, the stable ε-form was directly obtained via metastable form.(b) The more rapid addition rate of antisolvent, the batter precipitation of β-form which adjoined the higher supersaturation of CL-20solution. When the CL-20solution was quickly plunged in antisolvent that the supersaturation instantaneity research maximum, resulted in the β-form always first appear. In this case, the influence of antisolvent property was minify.(c) The more volume proportion of antisolvent in the CL-20solution, the smaller solubility of CL-20which resulted in needing more time to transform the metastable form to stable.(d) Four new solvates were found, including CL-20-acetone, CL-20-acetonitrile, CL-20-DMC and CL-20-DES. CL-20-acetone solvate has a less stability, which would lose the acetone when storing in air and then formed β-CL-20; However, this solvate could stable exist in mother solution at room temperature, which would transform to ε-form when increase to40℃. C L-20-acetonitrile solvate has the different stability in mother liquid, and after dried the a-form are obtained.(e) The stability of a-form is related to the solvation water, the more solvation water, the higher stability of α-form. The a-form could not be precipitated, if there was not water exist in solution. The transformation process of β-â†'(α-+ε-)â†'ε-in acetone/benzene system was caused by a few water exist in this solution.(f) y-form is a high temperature stable polymorph, which is enantiotropically related to s-form. The transformation temperature Tc is related to the solvent system, above which two processes of form βâ†'(ε+γ)â†'γâ†'γ and form βâ†'γ can be observed, moreover, the higher temperature, the more favor to y-CL-20.Additionally, the method for preparing the four pure forms of CL-20respectly were obtained, and then provided materials for the research of CL-20performance. According to Ostwald’s rule of stages and Solvent-Mediated Phase Transition, in the view point of thermodynamics and kinetics the crystallization mechanism of CL-20was explained.