Recrystallization of CL-20 to ϵ-polymorphic form

Abstract

High explosive CL-20 (2,4,6,8,10,12 - Hexanitro - 2,4,6,8,10,12 - hexaazaisowurtzitane) exists in four polymorphic forms i.e. α, β, γ and ϵ polymorphs at ambient conditions and polymorph ζ exists only at high pressure (1.44 GPa). CL-20 is the only single compound having highest energy among all the organic substances. But polymorphic form α- of CL-20 is generally obtained during most of the synthetic processes which has low density and high sensitivity. Thus, there is requirement of further recrystallization to obtain most stable ϵ-polymorphic form. Owing to the highest density, superior thermal stability and better insensitivity of ϵ- polymorph over other three polymorphs (α-, β- and γ- form) which are highly sensitive towards impact, friction and shock, ϵ- CL-20 is most desirable for use in defence applications. The present work studies the effect of various variables which influence the crystallisation process like effect of moisture, nature of solvent/anti-solvent and time on the preparation of ϵ- form of CL-20. Ethyl acetate and n-butyl acetate as solvents and toluene as anti-solvent resulted in the formation of desired ϵ- polymorphic form successfully. Polymorphic form was confirmed by FTIR and XRD analysis. SEM analysis was carried out for the morphological analysis of prepared samples.