ARTICLE pubs.acs.org/IECR
Preparation of Micronized β-HMX Using Supercritical Carbon Dioxide as Antisolvent Byoung-Min Lee,† Soo-Jung Kim,† 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, Yuseong-gu, Daejeon 305-811, Korea § High Explosives Team, Agency for Defense Development, 462, Jochiwon-gil, Yuseng-gu, Daejeon 305-600, Korea ABSTRACT: This study investigated the micronization of HMX (cyclotetramethylenetetranitramine) using two types of recrystallization methods with supercritical carbon dioxide as an antisolvent, namely, the aerosol solvent extraction system (ASES) and gas antisolvent (GAS) processes. The effects of experimental variables on the recrystallization of β-HMX particles in the GAS process were also studied. In particular, we varied the temperature from 303 to 323 K, used CO2 feeding rates of 20 and 50 mL 3 min 1, and varied the agitation speed from 200 to 900 rpm in the GAS process. The precipitated HMX particles were characterized by field-emission scanning electron microscopy (FE-SEM), particle size analysis (PSA), and Fourier transform infrared (FT-IR) spectroscopy. Depending on the organic solvent used and the type of recrystallization method, the precipitated HMX particles showed a variety of morphologies, particle size distributions, and crystal phases. The HMX precipitated by the ASES process was transformed into micronized γ- or δ-HMX with a volume-mean particle size (D50) of 6.3 32.1 μm. On the other hand, in the case of the GAS process, micronized γ- or β-HMX was formed, with a volume-mean particle size (D50) of 5.3 37.45 μm. In particular, HMX dissolved in acetone was successfully precipitated to micronized β-HMX with a narrow particle size distribution through the GAS process.
’ INTRODUCTION In the case of explosives, product quality, including properties such as performance and insensitivity, can be significantly influenced by particle morphology and particle size.1 The best way to enhance the performance and insensitivity is to synthesize a new explosive material. However, it is very difficult to synthesize new explosives that have higher performance and lower sensitivity. Thus, attempts have been made to change the endproduct properties such as crystal phase, particle size, particle size distribution, and morphology. In general, grinding and crystallization from solution are largely used as micronization processes for explosives in industry. However, these processes have some limitations: It is not only difficult to control morphology and particle size of explosives, but also dangerous to prepare fine particles because of their vulnerability to heat and impact. Therefore, interest has increased in developing technologies that allow the production of nano- or micrometer-sized explosives with controlled morphology, crystal phase, and particle size distribution. Supercritical antisolvent (SAS) processes have received a great deal of attention for the formation of nano- or micrometer-sized particles of various kinds of materials such as polymers, pharmaceutical substances, explosives, and inorganic materials.2 5 Among these processes, the aerosol solvent extraction system (ASES) and gas antisolvent (GAS) processes have been demonstrated to be effective for the micronization of various substances.6 The ASES process induces a degree of supersaturation by injecting a solution through a nozzle into a high-pressure vessel where supercritical fluid flows continuously. On the other hand, the GAS process involves the r 2011 American Chemical Society
injection of a supercritical fluid into a high-pressure vessel that is partially filled with a solution containing the desired solute where the volume expansion in solution reduces its solubility, thus precipitating the desired solute. Indeed, a few reports have been published comparing the GAS and ASES processes in terms of particle formation. Adami et al. used both the ASES and GAS processes for the micronization of poly(vinyl alcohol).7 The GAS process generated submicro- and microparticles with mean sizes between 0.4 and 2.0 μm. However, the ASES process was able to harvest nanoparticles in the range of 50 250 nm. Steckel et al.8 and Gallagher-Wetmore et al.9 used the same two processes to study the precipitation of the steroid dexamehasone and found significant differences in morphology by hydrodynamics. The GAS process produced discrete micrometer-sized particles, whereas the ASES process produced a film precipitated on the vessel wall. In this work, HMX (cyclotetramethylenetetranitramine or octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) was selected as a target explosive that is widely used not only for military purposes but also in industrial applications. It is a white crystalline powder that is practically insoluble in water and highly soluble in organic solvents such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and cyclohexanone. General chemical information and some physical properties of HMX are listed in Table 1, and Received: December 29, 2010 Accepted: June 23, 2011 Revised: June 23, 2011 Published: June 23, 2011 9107
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Table 1. Physical Properties of HMX property
data value
molecular formula
C4H8N8O8
molecular weight
296.2
crystal density at 20 °C (g 3 cm 3) R-phase
1.87
β-phase
1.96
γ-phase
1.82
δ-phase
1.78
melting point (°C) deflagnation point (°C)
275 287
Figure 1. Chemical structure of HMX.
its chemical structure is shown in Figure 1. Until now, few studies on the precipitation of HMX by supercritical fluid technology have been published in the literature, and all reported experiments were carried out in liquid batch GAS mode. Cai et al.10 and Teipel et al.11 obtained micronized β-HMX with number-based mean diameters ranging from 2 to 5 μm and volume-based mean diameters between 65 and 90 μm. HMX exists in four solid-phase polymorphs: R-, β-, γ-, and δ-phases (see Table 1). β-HMX is the most stable thermodynamically and mechanically because of its highly symmetric configuration of molecules and has the highest density. Consequently, it is stable at room temperature and has excellent explosive power.12 Most of all, control of polymorphic phase is very important for the performance and stability of HMX particles, which should be precipitated to β-HMX. The primary aim of this study, therefore, was to investigate the feasibility of the ASES and GAS processes for the precipitation of micronized β-HMX. We tested both processes utilizing several kinds of organic solvents and then investigated the effects of experimental variables for the organic solvent and process to obtain micronized β-HMX particles with a narrow particle size distribution.
’ EXPERIMENTAL SECTION Materials. The HMX samples were provided by the Agency for Defense Development (Daejeon, Korea). Figure 2 shows a SEM image and the particle size distribution of the unprocessed HMX particles used in this work. Cyclohexanone (purity 99.8%), acetone (purity 99.5%), and dimethyl formamide (DMF, purity 99%) were supplied by Samchun Chemical (Seoul, Korea). CO2 (purity 99%) was purchased from Hyoup-sin Gas Co. (Seoul, Korea). All materials were used as received. ASES Apparatus. Figure 3a shows a schematic diagram of the experimental apparatus used for the semicontinuous ASES
Figure 2. Unprocessed HMX particles: (a) SEM image, (b) particle size distribution (D10 = 17.1 μm, D50 = 100.0 μm, D90 = 231.2 μm).
recrystallization of HMX. A detailed description of the experimental apparatus and procedure was given in our previous publication.13 The apparatus consists of carbon dioxide (CO2) and solution feeding parts, a precipitator, a particle collecting part, and a separator. Liquefied CO2 from a cylinder (1) was produced in an overcooled liquid state using a low-temperature thermostat (2) and then injected into a preheater using a highpressure pump (3). The CO2 was heated to a supercritical state in the preheater (5) and moved to the precipitator (6). The precipitator (6), made of stainless steel 316, had dimensions of 115 mm in width, 95 mm in length, 240 mm in height, and 30 mm in diameter, with an internal volume of 60 cm3. Sapphire windows were inserted into the precipitator for visual observation of the interior of the precipitator. A coaxial tube was used to feed the solution and CO2, with solution provided inside the inner tube (capillary tube) and CO2 introduced outside the inner tube. The system temperature was controlled and kept constant by a jacket surrounding the precipitator and a forced-convection air bath. The precipitator was placed in the center of the air bath, and its temperature was measured by thermocouples (K-type) inserted into the interior of the precipitator. The inner pressure of the precipitator was set and adjusted using a back-pressure regulator (Tescom model 26-1721-24). The experiments for precipitating HMX particles were performed by the following procedure: First, the HMX sample was dissolved in a solvent. Then, CO2 was delivered into the precipitator 9108
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Figure 3. Schematic diagrams of the experimental apparatus used for the (a) semicontinuous ASES and (b) batch GAS processes.
until a desired pressure was reached. Once the pressure and temperature had equilibrated, the HMX solution was injected into the precipitator through a capillary tube (i.d. = 254 μm) using a solution feeding pump (Minipump model NSI-33R). When the HMX solution was sprayed into the CO2 phase, the solvent was rapidly extracted by supercritical CO2, resulting in the precipitation of solid dispersion particles. CO2 continued to be added during solution spraying. The precipitated particles were collected on a membrane filter (0.5 μm). CO2 was supplied continuously for 20 min during drying of the particles. Two filters (7) were connected to enable continuous serial experiments under different conditions. Along the line from the back-pressure regulator (8) to the separator (9), a small buffer tank (13) was installed to prevent the freezing of moisture and to provide for smooth discharge of CO2 because the temperature decreased rapidly as a result of the adiabatic expansion of CO2. GAS Apparatus. A schematic diagram of the experimental apparatus for the GAS recrystallization of HMX is shown in Figure 3b. The apparatus consists of a CO2 cylinder (1), solution (7), CO2 feeding pumps (3), a precipitator (9), membrane filters (11), and a separator (13). First, HMX solution of a fixed concentration in an organic solvent was introduced into the precipitator (9) using a solution feeding pump (Minipump model NSI-33R). CO2 from the cylinder (1) was subcooled by a cooling bath (2) (Jeiotech model MC-11), delivered to a preheater using a high-pressure metering pump (3), and then delivered to the precipitator. The precipitator (9) with an internal volume of 150 cm3 was equipped with a window that can withstand high pressure for observation of the crystallization behavior in the
precipitator. The temperature was controlled easily and kept constant by installing a heat-transfer unit with a water-circulated jacket, and a stirrer (8) (∼1000 rpm) regulated with an electric controller ensured that the solution was well-mixed with CO2. The temperature in the precipitator was measured by thermocouples (K-type) inserted into the precipitator. The pressure in the precipitator was adjusted using a back-pressure regulator (5, 12) and measured with a pressure gauge (Millipore, 500 bar maximum). Precipitated HMX particles were collected on a membrane filter (11) (0.5 μm). The separator (13) was used to collect organic solvent from vented CO2. Characterization of Precipitated Particles. The morphology of the precipitated HMX particles was examined with a fieldemission scanning electron microscope (Carl Zeiss model Supra 55VP, Oberkochen, Germany) operated at an accelerating voltage of 2 kV. Secondary electron signals were detected using either a lateral detector (Everhart-Thornley type, below-lens) or an axial annular in-lens detector. Particle sizes and their distribution were evaluated by a particle size analyzer (Sympatec model HELOS/ BF, Clausthal-Zellerfeld, Germany) that could measure in a size range from 0.1 to 875 μm depending on the lens (R1, R3, R4, and R5). The powder was placed in the particle size analysis (PSA) system and was allowed to flow into the PSA instrument by the RODOS/M ASPRIOS disperse system. Fourier transform infrared (FT-IR) spectra were measured by an attenuated total reflectance method (Thermo Scientific model Nicolet 6700, Waltham, MA). A resolution of 8 cm 1 was used, and 32 scans were coadded to each spectrum over a frequency range of 4000 650 cm 1. 9109
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Table 2. Experimental Conditions and Results for Selection of Process and Solvent for Obtaining β-HMX Particles results and observations particle size P (MPa)
distribution (μm) solution concentration
T (K) initial final
CO2 feeding
solution feeding rate
agitation
rate (mL 3 min 1) (g 3 min 1) speed (rpm)
crystal phase
3.0
2.3
14.3
40.1
irregular
γ-phase
2.4
1.1
6.3
22.9
needle-like
δ-phase
11.5
32.1
68.6
aggregated
δ-phase
2.72
16.54
108.4
aggregated
γ-phase
1.99 1.52
19.88 5.30
prism-like irregular +
β-phase β + γ-phase
A
cyclohexanone
313
20
20
4.00
40
B
acetone
313
20
20
2.17
32
C
DMF
313
20
20
1.00
32
2.4
D
cyclohexanone
313
0.1
20
4.00
20
400
E F
acetone DMF
313 313
0.1 0.1
20 20
2.17 1.00
20 20
400 400
D90
shape
D50
solvent
(wt %)
particle D10
run
ASES Process
GAS Process 36.80 12.36
prism-like
Table 3. Experimental Conditions and Results for Preparing β-HMX Particles in the GAS Process results and observations particle size P (MPa)
distribution (μm) solution concentration
CO2 feeding
agitation
particle
crystal
run
solvent
T (K)
initial
final
(wt %)
rate (mL 3 min 1)
speed (rpm)
D10
D50
D90
shape
phase
G
acetone
303
0.1
8
2.17
50
400
2.63
12.90
23.02
prism-like
β-phase
H
acetone
313
0.1
8
2.17
50
400
4.26
14.81
23.50
prism-like
β-phase
I
acetone
323
0.1
8
2.17
50
400
4.16
15.48
24.31
prism-like
β-phase
J
acetone
303
0.1
8
2.17
20
400
5.22
33.08
66.50
prism-like
β-phase
K L
acetone acetone
313 323
0.1 0.1
8 8
2.17 2.17
20 20
400 400
0.86 5.66
36.68 37.45
56.73 76.95
prism-like prism-like
β-phase β-phase
M
acetone
313
0.1
8
2.17
50
200
4.61
22.82
43.77
prism-like
β-phase
N
acetone
313
0.1
8
2.17
50
900
2.16
15.03
24.86
prism-like
β-phase
’ RESULTS AND DISCUSSION Effect of Organic Solvents in ASES and GAS Processes. Previous work in our laboratory has already demonstrated that the type of organic solvent affects the size and morphology of particles precipitated in supercritical antisolvent processes.13 Thus, in the ASES and GAS processes, the effects of the organic solvent on the morphology, particle size, and particle size distribution were investigated. Tables 2 and 3 summarize the experimental conditions and results for the preparation of HMX particles using the ASES and GAS processes. To evaluate the effects of the organic solvent in both of the supercritical processes, all experiments for each solvent were conducted at the same experimental conditions (refer to the data for runs A F in Table 2). Figure 4 shows SEM images of precipitated HMX particles produced by the ASES and GAS processes in several organic solvents. The precipitated HMX particles show a variety of morphologies such as irregular (images a and f), aggregated (images c and d), needle-like (image b), and prism-like (image e). Figure 5 shows
the particle size distributions of HMX particles obtained from both processes. Depending on the organic solvent used and the recrystallization process, the precipitated HMX particles show a variety of particle sizes. The mean particle sizes (D50) of the precipitated HMX particles ranged from 5.3 to 32.1 μm. It is noteworthy that, even though HMX was precipitated from the same organic solvents at fixed experimental conditions, the morphology and particle size distribution of its particles were dramatically modified depending on the type of supercritical process. For example, when acetone was used as the solvent in both processes, the GAS process produced particles of prism-like habit (Figure 4e), but the ASES process generated particles of needle-like habit (Figure 4b). The difference in the morphology can be attributed to the solvent solute interaction. In the case of ASES, when the solution is sprayed out to the antisolvent through the capillary nozzle, the degree of supersaturation and miscibility at that moment is a major factor in inducing the nucleation and crystal growth of the solute. However, the driving force for nucleation and crystal growth in the GAS process is a 9110
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Figure 4. SEM images of precipitated HMX particles produced by the (a c) ASES and (d f) GAS processes from solutions of HMX in (a,d) cyclohexanone, (b,e) acetone, and (c,f) DMF at 313 K, 20 MPa, and a CO2 feeding rate of 20 mL 3 min 1 with (a c) a nozzle diameter of 2.54 μm for the ASES process or (d f) an agitation speed of 400 rpm for the GAS process.
volume expansion by the interaction between the solvent and antisolvent. Actually, the ASES process has a higher possibility of precipitating smaller particles with some irregular morphologies6,7,14 because of the higher degree of supersaturation and faster interaction between solvent and antisolvent, compared with the GAS process (see Figure 4a c). The polymorphs of the HMX particles were further assessed by performing the infrared spectral analysis of the particles.15 The FT-IR spectra of HMX show transmittance bands indicating the presence of N N bonds and NO2 groups associated with cyclic nitro compounds.15 A general perusal of the spectra shows that the band positions of the γ- and δ-phases are very similar to each another, even though they were distinctly different from those of the β-phase. Generally, the β-phase shows no transmittance band in the wavenumber range between 700 and 750 cm 1. However, the γ-phase has transmittance bands at 709, 732, 907, and
1014 cm 1, and the δ-phase has bands at 713, 739, 910, and 1028 cm 1 with a slight difference.15 Figure 6 shows FT-IR spectra of the HMX particles before and after precipitation using the ASES and GAS processes. There was no transmittance band between 700 and 750 cm 1 in the case of unprocessed HMX, which indicates that the unprocessed HMX is the β-phase. However, the precipitated HMX showed transmittance bands between 700 and 750 cm 1, indicating that the HMX was precipitated as other phases. For example, when cyclohexanone was used in the ASES and GAS processes, the precipitated HMX was modified into the γ-phase (spectra a and d in Figure 6). In the cases of acetone and DMF in the ASES process, δ-phase HMX was formed (spectra b and c in Figure 6). On the other hand, as shown in spectrum e in Figure 6, in the case of acetone in the GAS process, the crystal phase of the precipitated HMX was β-phase, showing the same spectrum as the unprocessed HMX. 9111
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Figure 5. Cumulative size and volume density distributions of precipitated HMX particles obtained by the (a c) ASES and (d f) GAS processes from solutions of HMX in (a,d) cyclohexanone, (b,e) acetone, and (c,f) DMF.
Thus, the HMX particles were changed from the β-phase to either the γ- or δ-phase by the ASES process and to either the β- or γ-phase by the GAS process. It was found that various polymorphs could be obtained simply by the choice of organic solvent and process type. The choice of organic solvent is, therefore, very important for the control of HMX polymorphs in the supercritical antisolvent process. Consequently, micronized β-HMX particles were generated by the batch-mode GAS process with acetone used as the solvent and liquid CO2 used as the antisolvent. Effect of Temperature in the GAS Process. It was found that the proper organic solvent and process for the preparation of micronized β-HMX were acetone and the GAS process, respectively, which produced particles of small size, good morphology, and desirable crystal phase. Thus, the effects of various process variables in the GAS process using acetone were investigated.
The experiment that takes precedence in the GAS process is to measure the volume expansion of the organic solvent. Gallagher et al.16 measured the volume expansion of the organic solvent by CO2 depending on the temperature and pressure. During the injection of CO2, the volume expansion of the organic solvent was measured by reading the liquid level through the view window and calculated from the equation reported by Kordikowski et al.17 The volume expansion curves for acetone, as shown in Figure 7, were measured for the determination of pressure temperature volume behavior at 303, 313, and 323 K in a vessel of 150 cm3 volume equipped with sight glasses. The volume expansion of the organic solvent was strongly affected by temperature. At lower temperature, a higher volume expansion was measured at a given pressure. For example, when the pressure was 5.5 MPa, the volume expansion of acetone at 303 K increased to 300%. As the 9112
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Figure 6. FT-IR spectra of precipitated HMX particles obtained by the (a c) ASES and (d f) GAS processes from solutions of HMX in (a,d) cyclohexanone, (b,e) acetone, and (c,f) DMF.
Figure 8. SEM images of precipitated HMX particles produced by the GAS process from a solution of HMX in acetone for a concentration of 2.17 wt %, an agitation speed of 400 rpm, and a CO2 feeding rate of 50 mL 3 min 1 at (a) 303, (b) 313, and (c) 323 K.
Figure 7. Volume expansion curves of acetone by CO2 at different pressures and temperatures.
temperature increased, the volume expansion decreased, reaching 48% at 323 K. However, it was found that, at these temperatures, acetone could be fully expanded with CO2 and became single phase with CO2 at 8 MPa or below. The full volume expansion of an organic solvent by CO2 means that the solvent power of the organic solvent reduces dramatically, leading to solute precipitation from the solvent. Experiments on the effect of temperature were performed in the range of 303 323 K at the CO2 feeding rate of 50 mL 3 min 1. Pressure was controlled to a maximum of 8 MPa, which led to a perfect volume expansion of the organic solvent. The amount of HMX dissolved in acetone was fixed at 2.17 wt % to maintain an initial saturation concentration in the solution. At a fixed pressure, as the temperature increased, the density of CO2 decreased, and the solubility of CO2 in acetone decreased, thus resulting in a lowering of the degree of supersaturation in the solution. The results of the temperature effect are reported in Figure 8. The volume-mean particle sizes (D50) of HMX were 12.9, 14.8, and
15.48 μm at 303, 313, and 323 K, respectively (runs G I in Table 3). The precipitated HMX did not show a significant difference in the particle size depending on the precipitation temperature. However, the volume-mean particle size (D50) increased slightly, which was induced by a higher degree of supersaturation at lower temperature. Similar results have been reported in several studies.18 20 Especially for 313 K (image b in Figure 8), desirable micronized β-HMX particles with a small particle size and narrow size distribution were formed. The particle population was fairly unimodal with a volume-mean particle size (D50) of 14.81 μm. Effect of CO2 Feeding Rate in the GAS Process. The effect of the CO2 feeding rate on the particle size of precipitated HMX was studied at two levels of feeding rate, namely, 20 and 50 mL 3 min 1, at 303, 313, and 323 K. In all cases, the agitation speed of the solution was 400 rpm, and the maximum pressure was 8 MPa. Figure 9 shows the particle size distributions for experimental runs G L listed in Table 3. For the CO2 feeding rate of 20 mL 3 min 1, the particle sizes were relatively large. However, at the higher rate of 50 mL 3 min 1, smaller particles were obtained. For example, at 303 K, the mean particle size (D50) decreased from 33.08 to 12.90 μm as the CO2 feeding rate was increased. At all temperatures, as the CO2 feeding rate increased, the mean particle sizes (D50) of precipitated HMX decreased by more than half, as reported in Table 3 and Figure 9. The rate of supersaturation is proportional to the rate of volume 9113
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Figure 9. Effect of the CO2 feeding rate on the particle size distribution for HMX particles precipitated by the GAS process at 303, 313, and 323 K for an initial concentration of 2.17 wt % and an agitation speed of 400 rpm: (a) 20 and (b) 50 mL 3 min 1.
expansion, which is regulated by CO2 feeding rate. For this reason, the high feeding rate of CO2 induced high supersaturation level in a short time, leading to rapid nucleation. Therefore, the higher CO2 feed rate produced smaller particles. This effect was more notable at low temperature, because volume expansion rapidly changes at lower temperature, as shown in Figure 7. All of the precipitated HMX particles exhibited the β-phase according to FT-IR analysis. Effect of Agitation Speed in the GAS Process. Experiments to study the effect of agitation speed in the precipitator (200, 400, and 900 rpm) on the particle size distribution of HMX were performed, with all other process variables held constant at the conditions reported in Table 3 (see runs H, M and N). Figure 10a shows the volume density distribution of the precipitated HMX for experimental runs H, M, and N in Table 3. The particle size distribution generated with a low agitation speed (i.e., 200 rpm) was quite broad. However, when the agitation speed increased to 400 rpm, a narrower particle size distribution with a mean diameter (D50) of 14.81 μm was obtained. A higher agitation speed in the precipitator can enhance mass transfer between CO2 and the HMX solution, thus allowing for a faster volume expansion, a higher degree of supersaturation, and hence smaller particles.
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Figure 10. Effect of agitation speed in the precipitator on the particle size distribution for HMX particles precipitated by the GAS process at 313 K and a CO2 feeding rate of 50 mL 3 min 1: (a) volume density distribution and (b) cumulative distribution.
However, at 900 rpm, the precipitated HMX did not exhibit a significant difference in particle size distribution, compared to that at 400 rpm. This is because the degree of supersaturation was not very high, because of the mixing power of CO2 and solvent at agitation speeds above 400 rpm. However, as shown in Figure 10b, the populations of precipitated HMX below 10 μm were 27% and 38% for 400 and 900 rpm, respectively, with a slight difference. All precipitated HMX particles exhibited the same crystal phase as the unprocessed HMX.
’ CONCLUSIONS The use of supercritical antisolvent processes has been shown to be effective for the recrystallization of HMX from solution in organic solvents. The polymorphic phases and particle size of HMX were significantly affected by the type of supercritical micronization process and organic solvent. Micronized β-HMX particles were generated in a batch-mode GAS process in which acetone was used as the solvent and liquid CO2 was used as the antisolvent. In particular, at the operating conditions of 303 K, a maximum pressure of 8 MPa, a CO2 feeding rate of 50 mL 3 min 1, and an agitation speed of 400 rpm, the precipitated HMX was found to show the smallest mean particle size (D50) of 12.90 μm with the desired morphology and β-phase. On the other hand, 9114
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’ AUTHOR INFORMATION Corresponding Author
*Tel.: +82-2-880-1883. Fax: +82-2-883-9124. E-mail: ywlee@ snu.ac.kr.
’ ACKNOWLEDGMENT This work was supported by the Agency for Defense Development and the High Energy Material Research Center.
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(17) Kordikowski, A.; Schenk, A. P.; Van Nielen, R. M.; Peters, C. J. Volume Expansions and Vapor Liquid Equilibria of Binary Mixtures of a Variety of Polar Solvents and Certain Near-Critical Solvents. J. Supercrit. Fluids 1995, 8, 205. (18) M€uller, M.; Meier, U.; Kessler, A.; Mazzotti, M. Experimental Study of the Effect of Process Parameters in the Recrystallization of an Organic Compound Using Compressed Carbon Dioxide as Antisolvent. Ind. Eng. Chem. Res. 2000, 39, 2260. (19) Bakhbakhi, Y.; Charpentier, P. A.; Rohani, S. Experimental Study of the GAS Process for Producing Microparticles of Beclomethasone17,21-dipropionate Suitable for Pulmonary Delivery. Int. J. Pharm. 2006, 309, 71. (20) Bakhbakhi, Y.; Rohani, S.; Charpentier, P. A. Micronization of Phenanthrene Using the Gas Antisolvent Process. 1. Experimental Study and Use of FTIR. Ind. Eng. Chem. Res. 2005, 44, 7337.
’ REFERENCES (1) Van der Steen, A.; Verbeek, R. Initiation and Detonation of RDX and HMX HTPB-based Plastic-Bonded Explosives. Propellants, Explos., Pyrotech. 1990, 15, 19. (2) Jung, J.; Perrut, M. Particle Design Using Supercritical Fluids: Literature and Patent Survey. J. Supercrit. Fluids 2001, 20, 179. (3) Shariati, A.; Peters, C. J. Recent Developments in Particle Design Using Supercritical Fluids. Curr. Opin. Solid State Mater. Sci. 2003, 7, 371. (4) Reverchon, E.; De Marco, I.; Torino, E. Nanoparticles Production by Supercritical Antisolvent Precipitation: A General Interpretation. J. Supercrit. Fluids 2007, 43, 126. (5) Chu, J.; Li, G.; Row, K. H.; Kim, H.; Lee, Y.-W. Preparation of Cefpodoxime Proxetil Fine Particles Using Supercritical Fluids. Int. J. Pharm. 2009, 369, 85. (6) Thiering, R.; Dehghani, F.; Foster, N. R. Current Issues Relating to Anti-Solvent Micronisation Techniques and Their Extension to Industrial Scales. J. Supercrit. Fluids 2001, 21, 159. (7) Adami, R.; Osseo, L. S.; Huopalahti, R.; Reverchon, E. Supercritical AntiSolvent Micronization of PVA by Semi-Continuous and Batch Processing. J. Supercrit. Fluids 2007, 42, 288. (8) Steckel, H.; Thies, J.; M€uller, B. W. Micronizing of Steroids for Pulmonary Delivery by Supercritical Carbon Dioxide. Int. J. Pharm. 1997, 152, 99. (9) Gallagher-Wetmore, P.; Coffey, M. P.; Krukonis, V. Application of Supercritical Fluids in Recrystallization: Nucleation and Gas AntiSolvent (GAS) Techniques. Respir. Drug Delivery 1994, 4, 287. (10) Cai, J.; Zhou, Z.; Deng, X. Microparticle Formation and Crystallization Rate of HMX with Supercritical CO2 Antisolvent Recrystallization. Chin. J. Chem. Eng. 2001, 9, 258. (11) Teipel, U.; F€orter-Barth, U.; Krause, H. H. Crystallization of HMX Particles by Using the Gas Anti-Solvent Process. Propellants, Explos., Pyrotech. 1999, 24, 195. (12) Svensson, L.; Nyqvist, J. O.; Westling, L. Crystallization of HMX from γ-Butyrolactone. J. Hazard. Mater. 1986, 13, 103. (13) Lee, B. M.; Jeong, J. S.; Lee, Y. H.; Lee, B. C.; Kim, H. S.; Kim, H.; Lee, Y.-W. Supercritical Antisolvent Micronization of Cyclotrimethylenetrinitramin: Influence of the Organic Solvent. Ind. Eng. Chem. Res. 2009, 48, 11162. (14) Warwick, B.; Dehghani, F.; Foster, N. R.; Biffin, J. R.; Regtop, H. L. Micronization of Copper Indomethacin Using Gas Antisolvent Processes. Ind. Eng. Chem. Res. 2002, 41, 1993. (15) Achuthan, C. P.; Jose, C. I. Studies on Octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine (HMX) Polymorphism. Propellants, Explos., Pyrotech. 1990, 15, 271. (16) Gallagher, P. M.; Coffey, M. P.; Krukonis, V. J.; Hillstrom, W. W. Gas Anti-Solvent Recrystallization of RDX: Formation of UltraFine Particles of a Difficult-to-Comminute Explosive. J. Supercrit. Fluids 1992, 5, 130. 9115
dx.doi.org/10.1021/ie102593p |Ind. Eng. Chem. Res. 2011, 50, 9107–9115
