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Ind. Eng. Chem. Res. 2009, 48, 11162–11167
Supercritical Antisolvent Micronization of Cyclotrimethylenetrinitramin: Influence of the Organic Solvent Byoung-Min Lee,† Jin-Seong Jeong,† Young-Ho Lee,† Byung-Chul Lee,‡ Hyoun-Soo Kim,§ Hwayong Kim,† and Youn-Woo Lee*,† School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National UniVersity, 599 Gwanangno, Gwanak-gu, Seoul 151-744, Korea, Department of Chemical Engineering and Nano-Bio Technology, Hannam UniVersity, 461-6 Jeonmin-dong, Yuseng-gu, Daejeon 305-811, Korea, and High ExplosiVes Team, Agency for Defense DeVelopment, 462, Jochiwon-gil, Yuseng-gu, Daejeon 305-600, Korea
A supercritical antisolvent (SAS) process was used to prepare micronized cyclotrimethylenetrinitramin (RDX). This study examined the influence of different solvents at a fixed temperature (50 °C) and pressure (13.7 or 15 MPa) on the morphology, particle size (PS), and particle size distribution (PSD) using a semicontinuous SAS process. Dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetone (AC), acetonitrile (ACN), n-methyl 2-pyrrolidone (NMP), and cyclohexanone (CHN) were used as solvents. The recrystallized RDX particles were characterized by scanning electron microscopy (SEM), particle size analyzer (PSA), Fourier transform infrared (FT-IR) spectroscopy, and differential scanning calorimetry (DSC). Depending on the solvent used, the recrystallized RDX particles showed a variety of morphologies, particle sizes, and particle size distributions (PSD). The mean particle size of the recrystallized RDX ranged from 2.6 to 17.7 µm. The enthalpy change (∆H ) 583.4, 847.7, 967.1, 823.9, 1131, and 1620 J/g) for the exothermic decomposition of recrystallized RDX was much higher than that of the unprocessed RDX (∆H ) 381.5 J/g). Introduction In the case of high explosives, it is essential to manufacture these substances with micron-sized particles to enhance their performance. Various technologies for preparing micron-sized particles have been developed and are currently used in many processes, as shown in Table 1.1 However, because materials, such as high explosives, are vulnerable to heat and impact, it is not only dangerous to prepare micron-sized particles by the milling, recrystallization from solution, and spray-drying process, but it is also very difficult to manufacture nanosize particles due to the problems associated with the residual solvent.2,3 As an alternative to these techniques, supercritical fluid precipitation processes are used widely to produce micron-sized particles of various organic and inorganic compounds.4 These techniques can potentially overcome all of the previously described limitations of classical micronization processes and hasve been extended to the formation of a wide variety of substances, such as explosives,5 pharmaceuticals,6 polymer microcapsules,7 and superconductor precursors.8 However, the crystallization of explosives with supercritical fluid is still in the very early stages of development. The first application of the supercritical antisolvent (SAS) process to explosive compound processing was reported by Gallagher et al. in 1989.9 Teipel et al. in 2001 reported that cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX) can be recrystallized into micronized particles through precipitation by a compressed fluid antisolvent (PCA process) using a modifier.10 In addition, Teipel et al. prepared the micronized trinitrotoluene (TNT) and 3-nitro-1,2,4-triazole-5-one (NTO) particles using the rapid expansion supercritical solution (RESS) process.10 TNT is recrystallized into particle sizes of 14 and 10 µm * To whom correspondence should be addressed. Tel.: +82-2-8801883. Fax: +82-2-883-9124. E-mail:
[email protected]. † Seoul National University. ‡ Hannam University. § Agency for Defense Development.
depending on the change in nozzle diameter. In the case of NTO, agglomerated needle-shaped particles with a mean particle size of 540 nm were observed. Stepanov et al. in 2005 manufactured RDX nanoparticles with a narrow particle size distribution of 110-220 nm using the RESS process.11 Among the supercritical micronization processes for explosives, Table 2 summarizes the SAS results on explosives. However, all SAS experiments described were carried out using a liquid batch operation mode, as shown in Table 2. A semicontinuous process can produce smaller particles, and continuous operation allows higher productivity. Therefore, among the many processes for micronization using supercritical fluid, the semicontinuous SAS process using supercritical carbon dioxide was employed in this study. The SAS process is one of the most promising technologies for preparing micron-sized explosive particles.4,16-18 There are some reports on the preparation of explosive particles using the SAS process, but there are few reports of the effect of various solvents in the preparation of micronized explosive particles using a semicontinuous SAS process, as shown in Table 2. Therefore, RDX was recrystallized using a semicontinuous SAS process. This study examined the influence of different solvents on the morphology, particle size, and particle size distribution (PSD) in the semicontinuous SAS process and characterized its properties. Experimental Section Materials. The substance used in this experiment was RDX, which is widely used for military and industrial purposes. RDX was purchased from Hanwha Co. (Korea). Figure 1 shows the
Figure 1. Chemical structure of cyclotrimethylenetrinitramin (RDX).
10.1021/ie900448w CCC: $40.75 2009 American Chemical Society Published on Web 10/26/2009
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Table 1. Supercritical Fluids Recrystallization Techniques and Conventional Methods for Some Particle Size Redistribution Ranges
Table 2. Experimental Results on the Supercritical Antisolvent Micronization of Explosives and Propellants5,9,12-15 compound NG
RDX HMX RDX HMX HMX NTO
solvent DMF cyclohexanone NMP acetone γ-butyrolactone acetone cyclohexanone acetone acetone γ-butyrolactone DMF DMSO methanol
process
morphologies and particle size (µm)
liquid batch particles: sphere, snowballs, starbursts 1-100 liquid batch particle >200 liquid batch liquid batch particle
