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Control of Crystal Density of E-Hexanitrohexaazaisowurzitane in Evaporation Crystallization Myung-Ho Lee,† Jun-Hyung Kim,‡ Young-Chul Park,‡ Ji-Hwan Hwang,† and Woo-Sik Kim*,† Department of Chemical Engineering, Kyunghee UniVersity, Yongin Kiheung Seochun 1, Kyungki-do 449-701, Korea, and Agency for Defense DeVelopment, P. O. Box 35-4, Yuseong, Taejon 305-600, Korea
In the case of evaporation crystallization, the density of -hexanitrohexaazaisowurtzitane (-HNIW, C6H6N12O12) crystals was controlled using the evaporation rate, initial HNIW concentration, and temperature as the operating parameters. When crystal growth was promoted, the crystal density was found to deviate from the theoretical density, probably due to the inclusion of voids in the crystals that amplified the decrease in the crystal density. Thus, the crystal density was significantly reduced when the evaporation rate and initial HNIW concentration were increased. The influence of the evaporation rate on the crystal density was also found to be much stronger than that of the initial HNIW concentration. However, the crystal density was enhanced when the temperature was increased, due to a retardation of the crystal growth process. It was also interesting to find that the β-form of HNIW crystals was obtained with a high evaporation rate above 0.125 mL/h at 70 °C. 1. Introduction Crystals with a high density and stable structure have the essential physical properties required for highly energetic materials (HEMs) used as propellants and explosives, since these qualities predominantly dictate the detonation velocity, stability (shelf life), and sensitivity as the key characteristics determining the quality and performance of HEMs. For example, 2,4,6,8,10,12hexanitrohexaazaisowurtzitane (HNIW) crystals containing nitramine groups composed of high oxygen content and no halogen components in the molecular formula have the highest crystal density (>2 g/cm3) with an -form structure and produce the highest detonation velocity (9800 m/s) among HEMs. Therefore, controlling the crystal density and structure of HNIWs has attracted a great deal of attention in order to produce high-performance HEMs.1,2 There are four structural isomers (R-, β-, γ-, and -forms) of HNIW crystals. The R- and β-forms of crystals have the space groups of Pbca and Pb21a, respectively, and γ- and -HNIW crystals are same in the space group of P21/n. R-HNIW always exists in hydrate form. Among the four crystals, the -form is thermodynamically and mechanically the most stable due to its highly symmetric molecular configuration, which also provides better density, solubility, and thermal stability compared to the other forms (R, β, and γ).3,4 According to Foltz et al.,3,4 β-form crystals can be transformed into γ-form crystals, and then finally into -form crystals, by adjusting the solvent and temperature of the recrystallization. Plus, the crystal density is shifted from 1.98 g/cm3 (β-form) to 2.044 g/cm3 (-form) along with the structural transformation. Hoffman5 also measured the influence of synthetic methods on the crystal density of HNIW, and found that the HNIW prepared from tetraacetyldiaminohexaazaisowurtzitane (TADA) had a lower -form crystal density than that prepared from tetraformyldiaminohexaazaisowurtzitane (TADF) due to different byproducts from the synthesis. Furthermore, in the crystallization of RDX, Borne and Patedoye6 * To whom correspondence should be addressed. Tel.: +82-31-2012576. Fax: +82-31-202-1946. E-mail:
[email protected]. † Kyunghee University. ‡ Agency for Defense Development.
observed that the inclusion of void defects in the crystal structure reduced the crystal density and shock sensitivity of the explosive formulation. Although previous studies have implied that the properties and performance of HEMs are dictated by the crystal density and structure, controlling such density and structure in the course of crystallization has not been extensively studied. Accordingly, the present study investigated the factors influencing crystallization as a way to control the density of HNIW crystals. When evaporation crystallization is used to obtain HNIW crystals, the evaporation rate, concentration, and temperature are considered the key operating factors influencing the crystal density. 2. Experimental Section 2.1. Preparation of E-HNIW Powder. Pure HNIW was prepared from raw HNIW powder supplied by the Korean Agency for Defense Development. With the use of cooling crystallization, the raw HNIW was dissolved in dimethyl carbonate (ACS grade, Fluka, Switzerland) at 85 °C and then cooled at 25 °C to crystallize-out the HNIW. As a result, β-form crystalline HNIW powder with a purity greater than 99.9% was obtained. The structure of the β-HNIW powder examined by use of X-ray diffraction (XRD; M18XHF-SRA, Mac Science, Japan). The β-HNIW powder was then transformed into an -form powder by drowning-out crystallization, with an ethyl acetate solution (ACS grade, Fluka, Switzerland) and dichloromethane (ACS grade, Fluka, Switzerland) as the solvent and antisolvent agents, respectively. For the drowning-out crystallization, the weight ratio of HNIW, ethyl acetate, and dichloromethane was fixed at 2:5:20. The resulting crystalline powder was then filtered and dried in a desiccator before use, and the structure of the -form crystalline powder was confirmed with the use of Fourier transform infrared spectroscopy (FT-IR; FTS-60, Bio-Rad) and XRD. 2.2. Evaporation Crystallization for Crystal Density Control. Evaporation crystallization was used to control the density of the -HNIW crystals. Initially, 0.15 g of the -HNIW powder was dissolved with 5 mL of ethyl acetate in a vial (20 mL volume) that was completely sealed with aluminum foil. To control the solvent evaporation, the foil was punctured with
10.1021/ie0608152 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/27/2007
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Figure 1. Influence of evaporation rate on crystal density and degree of deviation from ideal crystal density for -HNIW crystals prepared by evaporation crystallization.
pinholes and the vial was placed in a water bath, where the temperature was accurately controlled within a fluctuation of (0.1 °C. During the experiment, the evaporation rate and evaporation temperature were varied from 0.02 to 0.15 mL/h and from 25 to 70 °C, respectively. In addition, the initial HNIW concentration in the ethyl acetate changed from 0.03 to 0.15 g/mL. Generally, it took from 1 day to 2 weeks to carry out the evaporation crystallization, depending on the evaporation rate. The experimental system for the evaporation crystallization was set on the floor in an isolated space to prevent any effects of vibration on the crystallization. 2.3. Analysis of Crystals. The single crystals resulting from the evaporation crystallization were analyzed with regard to their density and structure using a pycnometer, density gravity gradient column, XRD, and FT-IR. A zinc bromide aqueous solution was used with the density gradient column based on a density range from 1.97 to 2.05 g/cm3 at 25 °C, while the crystal density was finely checked with the pycnometer to an accuracy of 10-3 (g/cm3), and the crystal structures were confirmed using XRD and FT-IR. 3. Results and Discussion In the present study, the HNIW crystals exhibited a change in density relative to the evaporation rate when an initial HNIW solution of 0.05 g/mL was evaporated at a fixed temperature of 25 °C. As shown in Figure 1, at a low evaporation rate of 0.02 mL/h, the crystal density reached almost 2.04 g/cm3 and then was rapidly reduced to below 2.02 g/m3 when the evaporation rate was increased. Since all the above crystal densities were much higher than the ideal density for β-HNIW crystals (1.98 g/m3), this implied that the crystals obtained by the evaporation crystallization were -form, which was confirmed with XRD and FT-IR. However, it was interesting to note that -HNIW crystals were only obtained with the evaporation crystallization when -HNIW powder was used as the solution. In crystallization, void defects in the crystals frequently occur during the crystal growth process due to the supersaturation gradient on the crystal surface,7-9 which leads to growth layers with different heights and velocities. That is, the outer layer on the surface has an advantage of high supersaturation for the growth to the inner layer and meets to seal the surface with the void. According to Myerson and Saska10 and Miki et al.,11 the formation of the voids in the crystal is promoted by the conditions of a high growth rate. Similarly, in the present study, when the evaporation rate was increased, this seemingly
Figure 2. Influence of initial concentration of HNIW on crystal density and degree of deviation from ideal crystal density for -HNIW crystals prepared by evaporation crystallization.
enhanced the supersaturation rate in the solution and increased the crystal growth, thereby lowering the crystal density from the ideal value due to the formation of voids and cavities in the crystals. Upon comparison of the apparent density with the ideal density for -HNIW crystals, the degree of deviation from the ideal crystal density was quantitatively estimated, as defined by [(dideal - dreal)/dideal] × 100 (%).6 Considering the crystal density drop by voids etc. in the crystal, the deviated density of crystal from ideal might implicitly indicate the degree of crystal defects. According to Nielsen et al.,12 the crystal system of -HNIW is monoclinic (P21/n) with lattice parameters of a ) 8.852 Å, b ) 12.556 Å, c ) 13.386 Å, R ) γ ) 90°, and β ) 106.82°, and its group number is 4. Therefore, the ideal crystal density ()dideal) was calculated as 2.0446 g/cm3, which is consistent with the maximum density for -HNIW crystals suggested by Johnston and Wardle13 and Bescond et al.14 The initial HNIW concentration of the solution was varied from 0.03 to 0.13 g/mL to modify the supersaturation rate at a fixed temperature of 25 °C and evaporation rate of 0.03 mL/h. As shown in Figure 2, when the initial HNIW concentration was increased, the crystal density was reduced and the degree of deviation from the ideal crystal density was increased due to the promotion of crystal growth by the enhanced supersaturation rate. Here, it is interesting to note that the degree of deviation from the ideal crystal density varied from 0.25 to 0.5% when the initial HNIW concentration was increased from 0.03 to 0.13 g/mL, whereas increasing the evaporation rate from 0.02 to 0.06 mL/h caused a significant increase in the degree of deviation from the ideal crystal density from 0.25 to 1.2% (Figure 1), implying that the evaporation rate had a greater influence on controlling the crystal density than the initial HNIW concentration. The influence of the evaporation rate and initial HNIW concentration was compared in relation to the initial supersaturation rate, as defined by (d ln S)/(dt), where S is the supersaturation ratio (C/Cs) and t is the time scale. Until crystal nucleation occurred in the evaporation crystallization, the solute material (HNIW) was conserved in the solution (CV), where C is the solute concentration and V is the solution volume. Thus, the material balance before nucleation occurred was described by
V
dC dV ) -C dt dt
(1)
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Figure 3. Correlation of crystal density and degree of deviation from ideal crystal density for -HNIW relative to initial supersaturation rate.
Figure 4. Influence of temperature on crystal density and degree of deviation from ideal crystal density for -HNIW crystals prepared by evaporation crystallization.
For the evaporation crystallization in a cylindrical vial, eq 1 was rearranged as
d ln S 1 dL )dt L dt
(2)
where L is the level height of the solution in the vial. Thus, using the measurable quantities of the evaporation rate (dL/dt) and the solution level at the saturation point (L), the generation rate of supersaturation at the initial point ((d ln S)/dt), where the first supersaturation was induced, could be estimated. When the evaporation began with unsaturated solution, the saturation point of the solution could easily be calculated from the solubility. Thus, the generation rate of supersaturation estimated from eq 2 is called the initial supersaturation rate in the present study. As shown in Figure 3 based on eq 2, the wide range of initial HNIW concentrations from 0.03 to 0.13 g/mL produced only a small variation in the initial supersaturation rate from 0.006 to 0.008 [1/h], whereas modifying the evaporation rate from 0.02 to 0.06 mL/h changed the initial superatuartion rate from 0.004 to 0.013 [1/h]. Therefore, this result may explain why the evaporation rate had a more predominant impact on the crystal density than the initial HNIW concentration. The influence of the crystallization temperature on the crystal density was investigated with a fixed evaporation rate (0.04 mL/ h) and initial HNIW concentration (0.05 g/mL), as shown in Figure 4. In the crystal growth process, the sticking probability
Figure 5. Influence of evaporation rate on crystal density and degree of deviation from ideal crystal density for HNIW at various evaporation temperatures: (a) 32 and (b) 60 °C. (c) Influence of temperature on ratio of crystal density and evaporation rate.
of molecules at a growth site is inversely proportional to the temperature (∝exp(E/kBT)), where T is the temperature, E is the binding energy between the solute molecules, and kB is the Boltzmann constant.15 Also, increasing the temperature smoothes the rough crystal surface, thereby decreasing the growth sites on the crystal surface.16-18 Therefore, in the present study, increasing the temperature retarded the crystal growth process, resulting in an increased crystal density. It should also be mentioned that the solubility of the HNIW was almost independent of the temperature.19
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the drop ratio was about 0.5 g/h at 25 °C, it diminished to 0.1 g/h above 40 °C. At a temperature of 70 °C, it was found that the crystal structure transformed from an -form to a β-form when the evaporation rate was increased above 0.07 mL/h, as shown in Figure 6. Due to the low boiling point of the solvent, ethyl acetate (77.1 °C), an evaporation rate below 0.07 mL/L was barely achievable at 70 °C, although this was still much higher than those at other low temperatures. Thus, a high initial supersaturation rate at a high temperature may provide the conditions for the formation of less stable β-form crystals rather than -form crystals. As a result of the structural transformation from an -form to a β-form, the crystal morphology shifted from a bulky shape to a long leaf one, which are the typical characteristics of - and β-HINW crystals, respectively, and the crystal density suddenly dropped from about 2.03 to 1.99 g/cm3 (Figure 6a,b). The structural transformation of the HNIW crystals was confirmed by the powder pattern XRD and FT-IR peaks, as shown in parts c and d, respectively, of Figure 6, which were identified using reference HNIW crystal structures.4,19,20 The stability of HNIW crystals depends on their crystal structure, i.e., -form, γ-form, R-hydrate, or β-form, which is predominantly determined by the thermodynamic conditions of the temperature and solvent.3,4 In addition, the solution pH, feed concentration, and additives are generally considered to be major thermodynamic factors changing the crystal structure in crystallization.21 However, in the present study, it was interesting to observe that transformation of the crystal structures was induced by varying the kinetic conditions, i.e., the evaporation rate. 4. Conclusions The crystal density of -HNIW was found to depend on the evaporation rate, HNIW concentration, and temperature, as the key operating parameters in evaporation crystallization. Increasing the evaporation rate and initial HNIW concentration led to a reduced crystal density, as promoting the crystal growth process caused the formation of voids and cavities in the crystals. However, increasing the temperature retarded the molecular sticking process for crystal growth, resulting in an enhanced crystal density. In addition, since a high evaporation rate and temperature favored the formation of an unstable crystal structure, the structural transformation from an -form HNIW crystal to a β-form HNIW crystal occurred when the evaporation rate was increased above 0.125 mL/h at 70 °C. Thus, a possibly unique result was obtained, where varying the kinetic conditions for crystallization induced a structural isomerism of the crystal. Acknowledgment Figure 6. Change in crystal density and structure with evaporation rate at 70 °C: (a) change in crystal density, (b) change of crystal habits, (c) change of crystal structure detected by XRD, and (d) change of crystal structure detected by FT-IR.
This study was financially supported by a research project (HM-33) of the Agency for Defense Development, Korea. Literature Cited
The contributions of the temperature and evaporation rate to the crystal density are compared in Figure 5. At a temperature of 32 °C (Figure 5a), the evaporation rate still had an influence on the crystal density. However, at a temperature of 60 °C (Figure 5b), the crystal density was almost independent of the evaporation rate, as the promotion of crystal growth by the evaporation rate was overtaken by the retardation at the high temperature. This trend was also evident in a quantitative description based on the drop ratio of the crystal density to the span of the evaporation rate causing a drop (Figure 5c). While
(1) Wardle, R. B.; Braithwaite, P. C.; Haaland, A. C.; Wallace, I. A. High Energy Oxetane/HNIW Gun Propellants. Int. Annu. Conf. ICT 1996, 27th, 1, 52. (2) Longevialle, Y. Low Vulnerability Minimum Smoke Rocket Propellants. International Symposium on Energetic Materials Technology, ADPA; 1995; p 125. (3) Foltz, M. F.; Coon, C. L.; Garcia, F.; Nichols, A. L. The Thermal Stability of the Polymorphs of Hexanitrohexaazaisowurtzifane. Propellants, Explos., Pyrotech. 1994, 19, 19. (4) Foltz, M. F. The Thermal Stability of -Hexanitrohexaazaisowurtzifane in an Estane Formulation. Propellants, Explos., Pyrotech. 1994, 19, 63.
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(5) Hoffman, D. M. Void and Density Distributions in 2,4,6,8,10,12Hexanitro-2,4,6,8,10,12-hexaazaisowurzitane (CL-20) Prepared under Various Conditions. Propellants, Explos., Pyrotech. 2003, 28, 194. (6) Borne, L.; Patedoye, J. C. Quantitative Characterization of Internal Defects in RDX Crystals. Propellants, Explos., Pyrotech. 1999, 24, 255. (7) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann: London, 1993. (8) Murata, Y.; Honda, T. The Growth of m-Chloronitrobenzene Crystals. J. Cryst. Growth 1977, 39, 315. (9) Sato, K. Observation of Spatial Correlation between Growth Spirals and Inclusions in Stearic Acid Crystals Grown from Solution. Jpn. J. Appl. Phys. 1988, 19, 1257. (10) Myerson, A. S.; Saska, M. Formation of Solvent Inclusion in Terephthalic Acid Crystals. AIChE J. 1984, 30, 865. (11) Miki, H.; Terashima, T.; Asakuma, Y.; Maeda, K.; Fukui, K. Inclusion of Mother Liquor Inside KDP Crystals in a Continuous MSMPR Crystallizer. Sep. Purif. Technol. 2005, 43, 71. (12) Nielsen, A. T.; Chafin, A. P.; Christian, S. L.; Moore, D. W.; Nadler, M. P.; Nissan, R. A.; Vanderah, D. J. Synthesis of Polyazapolycyclic Caged Polynitramines. Tetrahedron 1998, 54, 11793. (13) Wardle, R. B.; Johnston, G.; Hinshaw, J. C.; Braithwaite, P. Synthesis of the cased nitramine HNIW (CL-20). Int. Annu. Conf. ICT 1996, 27th, 27. (14) Bescond, P.; Graindorge, H.; Mace, H. (Societe Nationale des Poudres et Explosifs, France). Antisolvent-Solvent Crystallization of
Hexanitrohexaazaisowurtzitane to Obtain the -Polymorph. Eur. Pat. Appl. 91337A1, 1999. (15) Elwell, D.; Scheel, H. J. Crystal Growth from High Temperature Solution; Academic Press: London, 1975. (16) Tempkin, D. E. Crystallization Processes; Consultants Bureau: New York, 1964. (17) Jackson, K. A. Liquid Metals and Solidification; American Society of Metals: Cleveland, 1959. (18) Ring, T. A. Fundamentals of Ceramic Powder Processing and Synthesis; Academic Press: San Diego, 1996. (19) Foltz, M. F.; Holtz, E. V.; Ornellas, D.; Clarkson, J. E. The Solubility of -CL-20 in Selected Materials. Propellants, Explos., Pyrotech. 1994, 19, 206. (20) Kim, J. H.; Park, Y. C.; Yim, Y. J.; Han, J. S. Crystallization Behavior of Hexanitrohexaazaisowurtzitane at 298K and Quantitative of Its Polymorphs by FTIR. J. Chem. Eng. Jpn. 1998, 31, 478. (21) Kang, S. H.; Hirasawa, I.; Kim, W. S.; Choi, C. K. Morphological Control of Calcium Carbonate Crystallized in Reverse Micelle System with Anionic Surfactants SDS and AOT. J. Colloid Interface Sci. 2005, 288, 496.
ReceiVed for reView June 27, 2006 ReVised manuscript receiVed October 23, 2006 Accepted November 20, 2006 IE0608152
