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Effect of a polymer binder on the extraction and crystallization-based recovery of HMX from polymer-bonded explosives
Journal of Industrial and Engineering Chemistry 79 (2019) 124–130
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effect of a polymer binder on the extraction and crystallization-based recovery of HMX from polymer-bonded explosives Dongwoo Kima , Hyejoo Kimb , Eugene Huhb , Sewon Parkb , Chang-Ha Leeb , Ik-Sung Ahnb,* , Kee-Kahb Kooa , Keun Deuk Leec a b c
Department of Chemical and Biomolecular Engineering, Sogang University, Seoul, 04107, Republic of Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, 120-749, Republic of Korea Agency for Defense Development, Daejeon, 305-600, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 March 2019 Received in revised form 16 May 2019 Accepted 8 June 2019 Available online 15 June 2019
Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) was separated out from polymer-bonded explosives (PBXs) by extraction with DMSO and crystallization with ethanol. The model PBX used herein was DXC-57, which is composed of HMX and Estane, a polyurethane-based binder. Crystallization at a higher degree of supersaturation (S, the ratio of concentration to solubility) lowered the possibility of the formation of the thermodynamically stable β-form HMX. Estane was found to retard the nucleation and to cause crystallization at high S. β-form HMX with purity greater than 99% was recovered from DXC57 in over 90% yield via crystallization at S 6.26 followed by washing with THF. © 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Octahydro-1,3,5,7-tetranitro-1,3,5,7tetrazocine Polymer-bonded explosive Recovery Crystallization Nucleation
Introduction A polymer-bonded explosive (PBX) is a highly energetic material, in which molecular explosives like octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine (C4H8N8O8, HMX) and 1,3,5-trinitro1,3,5-triazine (C3H6N6O6, RDX) are bound together in a matrix with small amounts of a synthetic polymer [1]. The chemical structures of HMX and RDX are shown in Fig. 1. When the polymer is an elastomer like Estane, which is a polyurethane-based binder, it tends to absorb shocks and help make the PBX insensitive to accidental detonation. Therefore, PBXs are ideal for use in insensitive munitions. Since their first development in 1952, as RDX embedded in a polystyrene matrix using a dioctyl phthalate plasticizer, PBXs have been developed and produced worldwide as the main explosive filling used in warheads. Since ammunition has limited service life, it must be disposed of at a certain stage, and thus large quantities of obsolete ammunition are currently stored throughout the world [2]. Besides the potential hazards of soil and water contamination due to the accidental release of explosives, the storage of unused ammunition is undesirable due to cost and space requirements [3].
* Corresponding author. E-mail address: [email protected] (I.-S. Ahn).
Therefore, stockpiles of obsolete ammunition have conventionally been destroyed using a variety of techniques including: dumping at sea, open burning, and open detonation [4,5]. When the ammunition is dumped at sea, the leakage of explosives is unavoidable and marine ecosystems would be seriously damaged. Open burning and open detonation are also inappropriate for the disposal of munitions due to the environmental damage caused by the contamination of soil with heavy metals [6], and by the emissions of NOx, acidic gases, and particulate matter [3,7,8]. Therefore, due to environmental pollution and safety issues, the disposal of unused ammunitions by these conventional techniques has been made illegal in many countries [3,7,8]. The recovery and recycling of molecular explosives have gained interest in demilitarization activities because valuable energetic materials like HMX are not compromised, and can thereby be reused as commercial explosives, propellants, military explosives, or fuel supplements [8,9,10]. HMX, in particularly, is a highly valuable explosive although its value is lost when ammunitions containing HMX are dumped or destroyed. HMX particles are classified as either α-, β-, g-, or d-form depending on their morphology (needle, prism, plate, or rod/needle, respectively) [11]. Among these polymorphs, the β-form has the highest density and is most thermodynamically stable at room temperature. Furthermore, as it has the lowest explosive sensitivity, it has been attractive to military applications and is used in the production of PBXs.
https://doi.org/10.1016/j.jiec.2019.06.014 1226-086X/© 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
D. Kim et al. / Journal of Industrial and Engineering Chemistry 79 (2019) 124–130
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Table 1 Solubilities (g/100 g) of HMX and Estane in selected organic solvents at 25 C a . Component
Organic solvent DMSO
THF
Ethanol
Acetone
HMX Estaneb
58 50
<1 50
<1 insoluble
2.8 [16] 50
Determined at 25 C in this study except the solubility of HMX in acetone. Roughly determined from the evident formation of a transparent and homogeneous solution. a
b
Fig. 1. Chemical structures of HMX and RDX.
The purpose of this study was to develop a process for the extraction and crystallization-based recovery of β-form HMX from PBXs. DXC-57, which has been reported to be made of HMX and Estane in a mass ratio of 0.95:0.05, was used as a model PBX [12]. Nucleation is known to be an essential step in determining the polymorphic composition during crystallization [13]. The effect of Estane, which is used as a polymer binder in DXC-57, on the morphology of the recovered HMX crystals was explained via its effect on the nucleation kinetics. To our knowledge, this is the first study showing that a polymer binder, which has been used in the preparation of PBX, may inhibit the recovery of the molecular explosive to its thermodynamically stable form. Hence it is believed that results of this study are applicable not only to the recovery of molecular explosives from PBX but also to the preparation of ammunition via the crystallization of explosives. Experimental Materials DXC-57, HMX, and Estane were supplied by the Agency for Defense Development (ADD) of Korea. HPLC–grade DMSO, THF,
Table 2 Solubility (g/100 g) of HMX in the mixture of DMSO and ethanol at 25 C and the maximum concentration of HMX (g/100 g) when the solvent was saturated with DXC-57. Saturated with
Mass fraction of DMSO in the mixture of DMSO and ethanol 0.25
0.50
0.75
1.0
DXC-57 Pure HMX
0.95 0.7
3.8 4.9
14.2 19.5
56.1 58
acetone, and 95% ethanol were purchased from Daejung Chemicals & Metals Co., Ltd. (Siheung, Korea). HPLC–grade methanol was purchased from J.T. Baker (Center Valley, PA, USA). DMSO-d6 and acetone-d6, which were used in the NMR analysis, were purchased from Sigma–Aldrich Co. (St. Louis, MO, USA). Solubility measurement The solubility of HMX was determined by gravimetric analysis [14,15]. An excessive amount of HMX was added to a different organic solvent in a vessel whose temperature was maintained at 25 C. After 2 h of incubation with agitation, the undissolved solute was separated by filtration through a 0.2-mm PTFE membrane filter (Toyo Roshi Kaisha, Ltd., Tokyo, Japan). The filtered solution was freeze-dried, and weight measurements before and after freeze-drying allowed the solubility of the solute
Fig. 2. XRD patterns of the 4 HMX polymorphs, obtained by simulation using the Reflex module of Material Studio Software 7.0 and crystallographic information files provided by the Cambridge Crystallographic Data Center. Miller indices for major peaks are indicated in the plot.
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Fig. 3. Solubility curves of HMX in mixtures of DMSO and ethanol saturated with (A) DXC-57 and (B) pure HMX. The dotted line shows the content of HMX in the solvent mixture, which started from conditions of saturation with (A) DXC-57 and (B) HMX in DMSO and changed with the addition of ethanol during the nucleation experiment. The nucleation times were determined at various flow rates of ethanol using a FBRM probe. The degree of supersaturation (S) and the mass fraction of DMSO (indicated beside the flow rate) at the moments of nucleation are also shown in the plots.
drying was the mass of the solvent, the concentration of HMX in a solution saturated with DXC-57 could be determined. Crystallization of HMX In order to separate HMX from DXC-57, an anti-solvent drowningout crystallization was performed by adding ethanol as an anti-solvent to the solution of DXC-57 in DMSO in a jacket vessel which was continuously agitated. The resulting HMX crystals were collected by filtration and washed with THF 4 times prior to structural analysis. To study the nucleation kinetics, ethanol was added drop-bydrop to the saturated DMSO solutions of DXC-57 or pure HMX (i.e., HMX used in the production of DXC-57 and supplied by ADD) at 25 C. The moment of nucleation and the number density of crystals were determined using a focused beam reflectance measurement (FBRM) probe (G400, Mettler-Toledo, OH, USA). Analysis
Fig. 4. XRD patterns of the HMX crystals obtained from the one-pot addition of ethanol to the DMSO solution of DXC-57 (0.5 g DXC-57 in 1 mL DMSO). The volumes of ethanol added are shown in the plot.
in the specific organic solvent to be estimated. As Estane is a sticky polyurethane-based polymer and its mass fraction in DXC57 is only 0.05, its solubility in various organic solvents was roughly determined from the evident formation of a transparent homogeneous solution at 25 C. In this study, it was necessary to determine the concentration of HMX when the solution was saturated with DXC-57. An excessive amount of DXC-57 was added to an organic solvent. After 2 h of incubation and filtration at 25 C, the filtrate was freeze-dried. The mass after freeze-drying was multiplied by 0.95 (i.e., the mass fraction of HMX in DXC-57), and was taken to be the mass of HMX in the solution. Since the difference in mass before and after freeze-
To confirm the removal of the polymer binder Estane, the recovered HMX particles were analyzed by 1H-NMR spectroscopy (600-MHz nuclear magnetic resonance spectrometer, Bruker BioSpin Corp., MA, USA) [16] and thermogravimetric analysis (TGA; Q50, TA Instruments, DE, USA) [17]. The morphology of the recovered HMX particles was determined by field emission scanning electron microscopy (FE-SEM; JSM-7800F, JEOL Ltd. MA, USA) and X-ray diffraction analysis (XRD; Ultima IV, Rigaku, Tokyo, Japan) [18]. XRD patterns of the 4 HMX polymorphs (see Fig. 2) were simulated using the Reflex module of Material Studio Software 7.0 and crystallographic information files (CIFs). The CIFs were provided by the Cambridge Crystallographic Data Center (CCDC) [19–21]. The recovery yield of HMX was estimated via gravimetric analysis. Results and discussion Determination of solvent and anti-solvent for HMX The solubilities of HMX and Estane in various organic solvents are shown in Table 1. Based on these data, DMSO and ethanol were
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Fig. 5. SEM images of the HMX crystals. In cases when the crystals were obtained from the one-pot addition of ethanol to the DMSO solution of DXC-57 (0.5 g DXC-57 in 1 mL DMSO), the volume of ethanol was varied as follows: (A) 1 mL; (B) 1.5 mL; (C) 2 mL; (D) 2.5 mL; and (E) 3 mL. The prism morphology characteristic of β-form HMX can be clearly seen in (A), (B), and (C). In cases when the crystals were obtained from the continuous addition of ethanol to the DMSO solution saturated with DXC-57, the flow rate of ethanol was varied as follows: (E) 1.5 mL/min; (F) 1.75 mL/min; (G) 1.8 mL/min; and (H) 1.85 mL/min. The prism morphology characteristic of β-form HMX can be clearly seen in (E) and (F).
Fig. 6. Number densities of crystals formed upon the continuous addition of ethanol to the DMSO solution saturated with (A) DXC-57 and (B) pure HMX. Nucleation times at different flow rates of ethanol are shown above the plots.
Table 3 Nucleation kinetics for the anti-solvent crystallization of HMX in 4 mL of the DMSO solution saturated with DXC-57.
Table 4 Nucleation kinetics for the anti-solvent crystallization of HMX in 4 mL of the DMSO solution saturated with HMX.
Flow rate of ethanol (mL/min)
tnuc (s)a
VEtOH (mL)b
Xc
S1d
Flow rate of ethanol (mL/min)
tnuc (s)a
VEtOH (mL)b
Xc
S2d
0.5 1.0 1.5 1.75 3.0
425 239 170 148 92
3.54 3.98 4.25 4.32 4.60
0.612 0.584 0.568 0.563 0.548
5.20 5.79 6.15 6.26 6.62
0.5 1.75 3.0 6.0 12.0
298 96 66 36 20
2.48 2.8 3.3 3.6 4.0
0.692 0.666 0.628 0.608 0.582
2.83 3.07 3.45 3.66 3.95
a
a
tnuc denotes the nucleation time. VEtOH denotes the volume of ethanol added up until tnuc. c X is the mass fraction of DMSO in the solvent mixture at tnuc, as calculated from Eq. (3). d S1 is the degree of supersaturation and was calculated from Eq. (4).
tnuc denotes the nucleation time. VEtOH denotes the volume of ethanol added up until tnuc. c X is the mass fraction of DMSO in the solvent mixture at tnuc, as calculated from Eq. (3). d S1 is the degree of supersaturation and was calculated from Eq. (6).
selected as a solvent and an anti-solvent, respectively, for HMX. Water was not used as an anti-solvent because of the possible formation of g-form HMX. As THF was found to preferentially dissolve Estane over HMX (see Table 1), the precipitates obtained
from the anti-solvent crystallization were washed with THF to remove the remaining Estane. For the study of nucleation kinetics, mixtures of DMSO and ethanol were prepared and saturated with DXC-57 or pure HMX.
b
b
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The concentrations of HMX were then measured at 25 C, and are summarized in Table 2. When these concentrations were plotted against the mass fractions of DMSO, the points were fitted with the following exponential functions: C1 ¼ eð1:49þ5:52XÞ
ð1Þ
C2 ¼ eð0:519þ4:58XÞ
ð2Þ
where C1* and C2* are the solubilities of HMX when the solvent mixtures were saturated with DXC-57 and pure HMX, respectively, and X is the mass fraction of DMSO (see Fig. 3). Anti-solvent crystallization of HMX from DXC-57
Fig. 7. XRD patterns of the HMX crystals obtained upon the continuous addition of ethanol to the DMSO solution saturated with DXC-57. The flow rates of ethanol are shown in the plot.
Prior to crystallization, DXC-57 was dissolved in DMSO at a ratio of 0.5 g DXC-57 to 1 mL DMSO. Anti-solvent crystallization was then performed via the one-pot addition of ethanol to the above solution. The volume of ethanol was varied from 1 mL to 8 mL (i.e., the volumetric ratio of ethanol to DMSO was varied from 1:1 to 8:1). The temperature and the agitation speed were maintained at 25 C and 250 rpm, respectively. The particles were then washed with THF to remove the co-precipitated Estane. The results of XRD
Fig. 8. 1H-NMR spectra of (A) HMX supplied by ADD, (B) Estane, (C) DXC-57, and (D) HMX recovered from DXC-57. Each sample was prepared by dissolving 0.1 g of the solid particles in 1 mL of DMSO-d6.
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and SEM analyses are shown in Figs. 4 and 5. The formation of βform HMX crystals was evident when ethanol volumes of 2 mL or less were used. At lower ethanol concentrations, a larger amount of HMX was dissolved in the mixture of DMSO and ethanol. It is thus believed that crystallization at the lower S increased the probability of the formation of β-form HMX, which is known to be most thermodynamically stable at ambient temperature [18]. When the experiments were performed at 50 C, β-form HMX crystals could be obtained in the mixtures containing up to 4 mL of ethanol (data not shown). The higher solubility of HMX at 50 C allowed the formation of β-form HMX crystals in the presence of greater volumes of ethanol. Influence of Estane on the crystallization and morphology of HMX The dissolution of both HMX and Estane in DMSO and their coprecipitation in ethanol led us to suspect an influence of Estane on the crystallization process, especially in the nucleation step. The effect of Estane on nucleation was investigated by comparing the nucleation kinetics during crystallization in the DMSO solutions saturated with DXC-57 and pure HMX. While ethanol was added dropbydrop, changes in the number density of crystals were monitored and are shown in Fig. 6. When VEtOH is the total volume of ethanol added to the 4 mL DMSO solution up until the moment of nucleation, the mass fraction of DMSO in the solvent mixture at that moment is determined by: X¼
1:1 4 1:1 4 þ 0:789V EtOH
ð3Þ
where the unit of VEtOH is mL. The density of DMSO at 25 C is 1.10 g/ mL [22]. The density of ethanol at 20 C was reported to be 0.789 g/ mL [23], and was assumed to be the same at 25 C. Starting with the DMSO solution saturated with DXC-57, S was defined as follows: ð4Þ S1 ¼ C 1 C1 where C1 is the content of HMX in the mixture of DMSO and ethanol, and: C1 ¼ 56:1X
ð5Þ
where 56.1 g/100 g DMSO is the solubility of HMX in the DMSO solution saturated with DXC-57 (see Table 2). For the DMSO solution saturated with pure HMX, S was defined as follows: ð6Þ S2 ¼ C 2 C2 where C2 is the content of HMX in the mixture of DMSO and ethanol, and: C2 ¼ 58X
ð7Þ
where 58 g/100 g DMSO is the solubility of HMX in the DMSO solution saturated with pure HMX (see Table 2). Tables 3 and 4 summarize the nucleation times, the volumes of ethanol added to the 4 mL DMSO solution before nucleation, the mass fractions of DMSO in the solvent mixture at the moments of nucleation, and the S. Fig. 3 shows the curves of C1 and C2 with the values of S1 and S2 at the flow rates of ethanol employed in this experiment. In the DMSO solution saturated with DXC-57, β-form HMX crystals were obtained at flow rates 1.75 mL/min (see Figs. 5 and 7). At such flow rates, nucleation began at 148 s or later, and S1 was 6.26. In the DMSO solution saturated with pure HMX, β-form HMX crystals were obtained at all flow rates tested in this study. The S2 was 3.95 at most. The nucleation times at flow rates of 0.5 and 1.75 mL/min were 425 s and 148 s, respectively, in the DMSO solution saturated with DXC-57 (see Table 3), while they were 298 s and 96 s, respectively, in the DMSO solution saturated with pure HMX (see Table 4). Hence, it is believed that Estane retarded the
Fig. 9. TGA results of (A) DXC-57 and (B) the recovered β-form HMX.
nucleation, and that the resulting crystallization at the higher S lowered the possibility of forming the thermodynamically stable βform HMX crystals. Recovery of β-form HMX crystals from DXC-57 The process suggested for the recovery of stable β-form HMX from DXC-57 consisted of dissolution of DXC-57 in DMSO, crystallization with ethanol, and 4 consecutive washings with THF. The volumes of the organic solvents needed for 0.5 g of DXC57 were 1 mL DMSO, 4 mL ethanol, and 4 mL THF. The HMX crystals obtained through this process were analyzed by NMR spectroscopy and TGA, the results of which are shown in Figs. 8 and 9, respectively. The 1H-NMR spectrum of the recovered HMX crystals was compared to those of pure HMX, Estane, and DXC-57. To detect peaks corresponding to Estane (if any) in the recovered HMX crystal, all the NMR samples were prepared by dissolving 0.1 g of solid particles in 1 mL of DMSO-d6. A single peak corresponding to HMX was detected at 6.02 ppm (compare Fig. 8A and C). In the 1H-NMR spectrum of Estane, five peaks were detected (see Fig. 8B). Peaks corresponding to Estane were also
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detected in the 1H-NMR spectrum of DXC-57 (see Fig. 8C). None of the five peaks characteristic of Estane were detected in the 1H-NMR spectrum of the recovered HMX (see Fig. 8D). In the thermogravimetric analysis of DXC-57, the mass portions of HMX and Estane were measured to be 96.5% and 3.5%, respectively (see Fig. 9A). After extraction with DMSO, anti-solvent crystallization with ethanol, and washing with THF, the portion of HMX increased to 99.9% (see Fig. 9B). Based on these results, the purity of the recovered β-HMX crystals was determined to be higher than 99%. The yield of β-HMX was above 90%, including losses in the washing and filtration steps. Conclusions HMX was recovered from DXC-57 via extraction with DMSO and drowning-out crystallization with ethanol as an anti-solvent. In this study, S for HMX was defined as the ratio of its concentration to its solubility. Crystallization at higher S lowered the possibility of formation of the thermodynamically stable β-form HMX. Estane, the polymer binder in DXC-57, was found to both retard the nucleation and cause the crystallization at high S. It was found that the nucleation of HMX at S 6.26 enabled the formation of β-form HMX. Washing with THF to remove the co-precipitated Estane yielded β-form HMX crystals of purity 99% in a yield of 90%. Acknowledgements The authors gratefully acknowledge support from the Defense Acquisition Program Administration and the Agency for Defense Development.
References [1] R. Fossey, D. Summers, L. Tyler, L. Craig, J. Short, WOMBAT, A Waterjet Ordnance and Munitions Blastcleaner with Automated Tellurometry, (1988) . [2] J. Wilkinson, D. Watt, N. Headquarters, MSIAC/NATO/PfP Editor, Report L-118, (2006) . [3] S. Hloch, H. Tozan, M. Yagimli, J. Valí9cek, K. Rokosz, Tehni9 cki Vjesnik 18 (2011) 267. [4] D. Benchoff, Global Demilitarization Symposium, American Defense Preparedness Association (1995). [5] G. Carton, A. Jagusiewicz, Mar. Technol. Soc. J. 43 (2009) 16. na, L. Knap9 ák, M. [6] J. Kme9 c, P. Hreha, V. Kube cíková, T. Ma9cej, P. Hlavá9 cek, M. Zelen Harni9 cárová, M. Duspara, J. Cumin, Tehni9 cki Vjesnik 17 (2010) 383. [7] J.B. Morris, Separation of RDX from Composition B via a Supercritical Fluid Extraction Process, DTIC Document, 1997. [8] C. Ferreira, J. Ribeiro, R. Mendes, F. Freire, Propellants Explos. Pyrotech. 38 (2013) 296. [9] K.B. Arcuri, D.A. Goetsch, S.J. Schmit, R.M. Smith, P. Miller, Recovery of TNT and RDX from Bulk Composition B Explosives, Google Patents, 2008. [10] C.A. Myler, W.M. Bradshaw, M.G. Cosmos, J. Hazard. Mater. 26 (1991) 333. [11] C.-K. Kim, B.-C. Lee, Y.-W. Lee, H.S. Kim, Korean J. Chem. Eng. 26 (2009) 1125. [12] H.S. Kim, B.S. Park, Propellants Explos. Pyrotech. 24 (1999) 217. [13] J.-W. Kim, J.-H. Park, H.-M. Shim, K.-K. Koo, Cryst. Growth Des. 13 (2013) 4688. [14] J.W. Lewicki, Separation of RDX and HMX, Google Patents, 1988. [15] M.E. Sitzmann, S. Foti, C.C. Misener, Solubilities of High Explosives: Removal of High Explosive Fillers from Munitions by Chemical Dissolution, NAVAL ORDNANCE LAB, WHITE OAK MD, 1973. czyk, Cent. Eur. J. Energ. Mater. 11 (2014) 129. [16] M. Szala, L. Szyman [17] E.C. Mattos, M.F. Diniz, N.M. Nakamura, R.D.C.L. Dutra, J. Aerosp. Technol. Manag. 1 (2009) 167. [18] R. Kumar, P.F. Siril, P. Soni, J. Energ. Mater. 33 (2015) 277. [19] H.H. Cady, A.C. Larson, D.T. Cromer, Acta Crystallogr. 16 (1963) 617. [20] P. Main, R. Cobbledick, R. Small, Acta Crystallogr. Sect. C Cryst. Struct. Commun. 41 (1985) 1351. [21] R. Cobbledick, R. Small, Acta Crystallogr. Sect. B Struct. Cryst. Cryst. Chem. 30 (1974) 1918. [22] A. Muszkiewicz. [23] L. Zhang, S.-L. Jiang, Y. Yu, Y. Long, H.-Y. Zhao, L.-J. Peng, J. Chen, J. Phys. Chem. B 120 (2016) 11510.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effect of a polymer binder on the extraction and crystallization-based recovery of HMX from polymer-bonded explosives Dongwoo Kima , Hyejoo Kimb , Eugene Huhb , Sewon Parkb , Chang-Ha Leeb , Ik-Sung Ahnb,* , Kee-Kahb Kooa , Keun Deuk Leec a b c
Department of Chemical and Biomolecular Engineering, Sogang University, Seoul, 04107, Republic of Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, 120-749, Republic of Korea Agency for Defense Development, Daejeon, 305-600, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 March 2019 Received in revised form 16 May 2019 Accepted 8 June 2019 Available online 15 June 2019
Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) was separated out from polymer-bonded explosives (PBXs) by extraction with DMSO and crystallization with ethanol. The model PBX used herein was DXC-57, which is composed of HMX and Estane, a polyurethane-based binder. Crystallization at a higher degree of supersaturation (S, the ratio of concentration to solubility) lowered the possibility of the formation of the thermodynamically stable β-form HMX. Estane was found to retard the nucleation and to cause crystallization at high S. β-form HMX with purity greater than 99% was recovered from DXC57 in over 90% yield via crystallization at S 6.26 followed by washing with THF. © 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Octahydro-1,3,5,7-tetranitro-1,3,5,7tetrazocine Polymer-bonded explosive Recovery Crystallization Nucleation
Introduction A polymer-bonded explosive (PBX) is a highly energetic material, in which molecular explosives like octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine (C4H8N8O8, HMX) and 1,3,5-trinitro1,3,5-triazine (C3H6N6O6, RDX) are bound together in a matrix with small amounts of a synthetic polymer [1]. The chemical structures of HMX and RDX are shown in Fig. 1. When the polymer is an elastomer like Estane, which is a polyurethane-based binder, it tends to absorb shocks and help make the PBX insensitive to accidental detonation. Therefore, PBXs are ideal for use in insensitive munitions. Since their first development in 1952, as RDX embedded in a polystyrene matrix using a dioctyl phthalate plasticizer, PBXs have been developed and produced worldwide as the main explosive filling used in warheads. Since ammunition has limited service life, it must be disposed of at a certain stage, and thus large quantities of obsolete ammunition are currently stored throughout the world [2]. Besides the potential hazards of soil and water contamination due to the accidental release of explosives, the storage of unused ammunition is undesirable due to cost and space requirements [3].
* Corresponding author. E-mail address: [email protected] (I.-S. Ahn).
Therefore, stockpiles of obsolete ammunition have conventionally been destroyed using a variety of techniques including: dumping at sea, open burning, and open detonation [4,5]. When the ammunition is dumped at sea, the leakage of explosives is unavoidable and marine ecosystems would be seriously damaged. Open burning and open detonation are also inappropriate for the disposal of munitions due to the environmental damage caused by the contamination of soil with heavy metals [6], and by the emissions of NOx, acidic gases, and particulate matter [3,7,8]. Therefore, due to environmental pollution and safety issues, the disposal of unused ammunitions by these conventional techniques has been made illegal in many countries [3,7,8]. The recovery and recycling of molecular explosives have gained interest in demilitarization activities because valuable energetic materials like HMX are not compromised, and can thereby be reused as commercial explosives, propellants, military explosives, or fuel supplements [8,9,10]. HMX, in particularly, is a highly valuable explosive although its value is lost when ammunitions containing HMX are dumped or destroyed. HMX particles are classified as either α-, β-, g-, or d-form depending on their morphology (needle, prism, plate, or rod/needle, respectively) [11]. Among these polymorphs, the β-form has the highest density and is most thermodynamically stable at room temperature. Furthermore, as it has the lowest explosive sensitivity, it has been attractive to military applications and is used in the production of PBXs.
https://doi.org/10.1016/j.jiec.2019.06.014 1226-086X/© 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
D. Kim et al. / Journal of Industrial and Engineering Chemistry 79 (2019) 124–130
125
Table 1 Solubilities (g/100 g) of HMX and Estane in selected organic solvents at 25 C a . Component
Organic solvent DMSO
THF
Ethanol
Acetone
HMX Estaneb
58 50
<1 50
<1 insoluble
2.8 [16] 50
Determined at 25 C in this study except the solubility of HMX in acetone. Roughly determined from the evident formation of a transparent and homogeneous solution. a
b
Fig. 1. Chemical structures of HMX and RDX.
The purpose of this study was to develop a process for the extraction and crystallization-based recovery of β-form HMX from PBXs. DXC-57, which has been reported to be made of HMX and Estane in a mass ratio of 0.95:0.05, was used as a model PBX [12]. Nucleation is known to be an essential step in determining the polymorphic composition during crystallization [13]. The effect of Estane, which is used as a polymer binder in DXC-57, on the morphology of the recovered HMX crystals was explained via its effect on the nucleation kinetics. To our knowledge, this is the first study showing that a polymer binder, which has been used in the preparation of PBX, may inhibit the recovery of the molecular explosive to its thermodynamically stable form. Hence it is believed that results of this study are applicable not only to the recovery of molecular explosives from PBX but also to the preparation of ammunition via the crystallization of explosives. Experimental Materials DXC-57, HMX, and Estane were supplied by the Agency for Defense Development (ADD) of Korea. HPLC–grade DMSO, THF,
Table 2 Solubility (g/100 g) of HMX in the mixture of DMSO and ethanol at 25 C and the maximum concentration of HMX (g/100 g) when the solvent was saturated with DXC-57. Saturated with
Mass fraction of DMSO in the mixture of DMSO and ethanol 0.25
0.50
0.75
1.0
DXC-57 Pure HMX
0.95 0.7
3.8 4.9
14.2 19.5
56.1 58
acetone, and 95% ethanol were purchased from Daejung Chemicals & Metals Co., Ltd. (Siheung, Korea). HPLC–grade methanol was purchased from J.T. Baker (Center Valley, PA, USA). DMSO-d6 and acetone-d6, which were used in the NMR analysis, were purchased from Sigma–Aldrich Co. (St. Louis, MO, USA). Solubility measurement The solubility of HMX was determined by gravimetric analysis [14,15]. An excessive amount of HMX was added to a different organic solvent in a vessel whose temperature was maintained at 25 C. After 2 h of incubation with agitation, the undissolved solute was separated by filtration through a 0.2-mm PTFE membrane filter (Toyo Roshi Kaisha, Ltd., Tokyo, Japan). The filtered solution was freeze-dried, and weight measurements before and after freeze-drying allowed the solubility of the solute
Fig. 2. XRD patterns of the 4 HMX polymorphs, obtained by simulation using the Reflex module of Material Studio Software 7.0 and crystallographic information files provided by the Cambridge Crystallographic Data Center. Miller indices for major peaks are indicated in the plot.
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Fig. 3. Solubility curves of HMX in mixtures of DMSO and ethanol saturated with (A) DXC-57 and (B) pure HMX. The dotted line shows the content of HMX in the solvent mixture, which started from conditions of saturation with (A) DXC-57 and (B) HMX in DMSO and changed with the addition of ethanol during the nucleation experiment. The nucleation times were determined at various flow rates of ethanol using a FBRM probe. The degree of supersaturation (S) and the mass fraction of DMSO (indicated beside the flow rate) at the moments of nucleation are also shown in the plots.
drying was the mass of the solvent, the concentration of HMX in a solution saturated with DXC-57 could be determined. Crystallization of HMX In order to separate HMX from DXC-57, an anti-solvent drowningout crystallization was performed by adding ethanol as an anti-solvent to the solution of DXC-57 in DMSO in a jacket vessel which was continuously agitated. The resulting HMX crystals were collected by filtration and washed with THF 4 times prior to structural analysis. To study the nucleation kinetics, ethanol was added drop-bydrop to the saturated DMSO solutions of DXC-57 or pure HMX (i.e., HMX used in the production of DXC-57 and supplied by ADD) at 25 C. The moment of nucleation and the number density of crystals were determined using a focused beam reflectance measurement (FBRM) probe (G400, Mettler-Toledo, OH, USA). Analysis
Fig. 4. XRD patterns of the HMX crystals obtained from the one-pot addition of ethanol to the DMSO solution of DXC-57 (0.5 g DXC-57 in 1 mL DMSO). The volumes of ethanol added are shown in the plot.
in the specific organic solvent to be estimated. As Estane is a sticky polyurethane-based polymer and its mass fraction in DXC57 is only 0.05, its solubility in various organic solvents was roughly determined from the evident formation of a transparent homogeneous solution at 25 C. In this study, it was necessary to determine the concentration of HMX when the solution was saturated with DXC-57. An excessive amount of DXC-57 was added to an organic solvent. After 2 h of incubation and filtration at 25 C, the filtrate was freeze-dried. The mass after freeze-drying was multiplied by 0.95 (i.e., the mass fraction of HMX in DXC-57), and was taken to be the mass of HMX in the solution. Since the difference in mass before and after freeze-
To confirm the removal of the polymer binder Estane, the recovered HMX particles were analyzed by 1H-NMR spectroscopy (600-MHz nuclear magnetic resonance spectrometer, Bruker BioSpin Corp., MA, USA) [16] and thermogravimetric analysis (TGA; Q50, TA Instruments, DE, USA) [17]. The morphology of the recovered HMX particles was determined by field emission scanning electron microscopy (FE-SEM; JSM-7800F, JEOL Ltd. MA, USA) and X-ray diffraction analysis (XRD; Ultima IV, Rigaku, Tokyo, Japan) [18]. XRD patterns of the 4 HMX polymorphs (see Fig. 2) were simulated using the Reflex module of Material Studio Software 7.0 and crystallographic information files (CIFs). The CIFs were provided by the Cambridge Crystallographic Data Center (CCDC) [19–21]. The recovery yield of HMX was estimated via gravimetric analysis. Results and discussion Determination of solvent and anti-solvent for HMX The solubilities of HMX and Estane in various organic solvents are shown in Table 1. Based on these data, DMSO and ethanol were
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Fig. 5. SEM images of the HMX crystals. In cases when the crystals were obtained from the one-pot addition of ethanol to the DMSO solution of DXC-57 (0.5 g DXC-57 in 1 mL DMSO), the volume of ethanol was varied as follows: (A) 1 mL; (B) 1.5 mL; (C) 2 mL; (D) 2.5 mL; and (E) 3 mL. The prism morphology characteristic of β-form HMX can be clearly seen in (A), (B), and (C). In cases when the crystals were obtained from the continuous addition of ethanol to the DMSO solution saturated with DXC-57, the flow rate of ethanol was varied as follows: (E) 1.5 mL/min; (F) 1.75 mL/min; (G) 1.8 mL/min; and (H) 1.85 mL/min. The prism morphology characteristic of β-form HMX can be clearly seen in (E) and (F).
Fig. 6. Number densities of crystals formed upon the continuous addition of ethanol to the DMSO solution saturated with (A) DXC-57 and (B) pure HMX. Nucleation times at different flow rates of ethanol are shown above the plots.
Table 3 Nucleation kinetics for the anti-solvent crystallization of HMX in 4 mL of the DMSO solution saturated with DXC-57.
Table 4 Nucleation kinetics for the anti-solvent crystallization of HMX in 4 mL of the DMSO solution saturated with HMX.
Flow rate of ethanol (mL/min)
tnuc (s)a
VEtOH (mL)b
Xc
S1d
Flow rate of ethanol (mL/min)
tnuc (s)a
VEtOH (mL)b
Xc
S2d
0.5 1.0 1.5 1.75 3.0
425 239 170 148 92
3.54 3.98 4.25 4.32 4.60
0.612 0.584 0.568 0.563 0.548
5.20 5.79 6.15 6.26 6.62
0.5 1.75 3.0 6.0 12.0
298 96 66 36 20
2.48 2.8 3.3 3.6 4.0
0.692 0.666 0.628 0.608 0.582
2.83 3.07 3.45 3.66 3.95
a
a
tnuc denotes the nucleation time. VEtOH denotes the volume of ethanol added up until tnuc. c X is the mass fraction of DMSO in the solvent mixture at tnuc, as calculated from Eq. (3). d S1 is the degree of supersaturation and was calculated from Eq. (4).
tnuc denotes the nucleation time. VEtOH denotes the volume of ethanol added up until tnuc. c X is the mass fraction of DMSO in the solvent mixture at tnuc, as calculated from Eq. (3). d S1 is the degree of supersaturation and was calculated from Eq. (6).
selected as a solvent and an anti-solvent, respectively, for HMX. Water was not used as an anti-solvent because of the possible formation of g-form HMX. As THF was found to preferentially dissolve Estane over HMX (see Table 1), the precipitates obtained
from the anti-solvent crystallization were washed with THF to remove the remaining Estane. For the study of nucleation kinetics, mixtures of DMSO and ethanol were prepared and saturated with DXC-57 or pure HMX.
b
b
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The concentrations of HMX were then measured at 25 C, and are summarized in Table 2. When these concentrations were plotted against the mass fractions of DMSO, the points were fitted with the following exponential functions: C1 ¼ eð1:49þ5:52XÞ
ð1Þ
C2 ¼ eð0:519þ4:58XÞ
ð2Þ
where C1* and C2* are the solubilities of HMX when the solvent mixtures were saturated with DXC-57 and pure HMX, respectively, and X is the mass fraction of DMSO (see Fig. 3). Anti-solvent crystallization of HMX from DXC-57
Fig. 7. XRD patterns of the HMX crystals obtained upon the continuous addition of ethanol to the DMSO solution saturated with DXC-57. The flow rates of ethanol are shown in the plot.
Prior to crystallization, DXC-57 was dissolved in DMSO at a ratio of 0.5 g DXC-57 to 1 mL DMSO. Anti-solvent crystallization was then performed via the one-pot addition of ethanol to the above solution. The volume of ethanol was varied from 1 mL to 8 mL (i.e., the volumetric ratio of ethanol to DMSO was varied from 1:1 to 8:1). The temperature and the agitation speed were maintained at 25 C and 250 rpm, respectively. The particles were then washed with THF to remove the co-precipitated Estane. The results of XRD
Fig. 8. 1H-NMR spectra of (A) HMX supplied by ADD, (B) Estane, (C) DXC-57, and (D) HMX recovered from DXC-57. Each sample was prepared by dissolving 0.1 g of the solid particles in 1 mL of DMSO-d6.
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and SEM analyses are shown in Figs. 4 and 5. The formation of βform HMX crystals was evident when ethanol volumes of 2 mL or less were used. At lower ethanol concentrations, a larger amount of HMX was dissolved in the mixture of DMSO and ethanol. It is thus believed that crystallization at the lower S increased the probability of the formation of β-form HMX, which is known to be most thermodynamically stable at ambient temperature [18]. When the experiments were performed at 50 C, β-form HMX crystals could be obtained in the mixtures containing up to 4 mL of ethanol (data not shown). The higher solubility of HMX at 50 C allowed the formation of β-form HMX crystals in the presence of greater volumes of ethanol. Influence of Estane on the crystallization and morphology of HMX The dissolution of both HMX and Estane in DMSO and their coprecipitation in ethanol led us to suspect an influence of Estane on the crystallization process, especially in the nucleation step. The effect of Estane on nucleation was investigated by comparing the nucleation kinetics during crystallization in the DMSO solutions saturated with DXC-57 and pure HMX. While ethanol was added dropbydrop, changes in the number density of crystals were monitored and are shown in Fig. 6. When VEtOH is the total volume of ethanol added to the 4 mL DMSO solution up until the moment of nucleation, the mass fraction of DMSO in the solvent mixture at that moment is determined by: X¼
1:1 4 1:1 4 þ 0:789V EtOH
ð3Þ
where the unit of VEtOH is mL. The density of DMSO at 25 C is 1.10 g/ mL [22]. The density of ethanol at 20 C was reported to be 0.789 g/ mL [23], and was assumed to be the same at 25 C. Starting with the DMSO solution saturated with DXC-57, S was defined as follows: ð4Þ S1 ¼ C 1 C1 where C1 is the content of HMX in the mixture of DMSO and ethanol, and: C1 ¼ 56:1X
ð5Þ
where 56.1 g/100 g DMSO is the solubility of HMX in the DMSO solution saturated with DXC-57 (see Table 2). For the DMSO solution saturated with pure HMX, S was defined as follows: ð6Þ S2 ¼ C 2 C2 where C2 is the content of HMX in the mixture of DMSO and ethanol, and: C2 ¼ 58X
ð7Þ
where 58 g/100 g DMSO is the solubility of HMX in the DMSO solution saturated with pure HMX (see Table 2). Tables 3 and 4 summarize the nucleation times, the volumes of ethanol added to the 4 mL DMSO solution before nucleation, the mass fractions of DMSO in the solvent mixture at the moments of nucleation, and the S. Fig. 3 shows the curves of C1 and C2 with the values of S1 and S2 at the flow rates of ethanol employed in this experiment. In the DMSO solution saturated with DXC-57, β-form HMX crystals were obtained at flow rates 1.75 mL/min (see Figs. 5 and 7). At such flow rates, nucleation began at 148 s or later, and S1 was 6.26. In the DMSO solution saturated with pure HMX, β-form HMX crystals were obtained at all flow rates tested in this study. The S2 was 3.95 at most. The nucleation times at flow rates of 0.5 and 1.75 mL/min were 425 s and 148 s, respectively, in the DMSO solution saturated with DXC-57 (see Table 3), while they were 298 s and 96 s, respectively, in the DMSO solution saturated with pure HMX (see Table 4). Hence, it is believed that Estane retarded the
Fig. 9. TGA results of (A) DXC-57 and (B) the recovered β-form HMX.
nucleation, and that the resulting crystallization at the higher S lowered the possibility of forming the thermodynamically stable βform HMX crystals. Recovery of β-form HMX crystals from DXC-57 The process suggested for the recovery of stable β-form HMX from DXC-57 consisted of dissolution of DXC-57 in DMSO, crystallization with ethanol, and 4 consecutive washings with THF. The volumes of the organic solvents needed for 0.5 g of DXC57 were 1 mL DMSO, 4 mL ethanol, and 4 mL THF. The HMX crystals obtained through this process were analyzed by NMR spectroscopy and TGA, the results of which are shown in Figs. 8 and 9, respectively. The 1H-NMR spectrum of the recovered HMX crystals was compared to those of pure HMX, Estane, and DXC-57. To detect peaks corresponding to Estane (if any) in the recovered HMX crystal, all the NMR samples were prepared by dissolving 0.1 g of solid particles in 1 mL of DMSO-d6. A single peak corresponding to HMX was detected at 6.02 ppm (compare Fig. 8A and C). In the 1H-NMR spectrum of Estane, five peaks were detected (see Fig. 8B). Peaks corresponding to Estane were also
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detected in the 1H-NMR spectrum of DXC-57 (see Fig. 8C). None of the five peaks characteristic of Estane were detected in the 1H-NMR spectrum of the recovered HMX (see Fig. 8D). In the thermogravimetric analysis of DXC-57, the mass portions of HMX and Estane were measured to be 96.5% and 3.5%, respectively (see Fig. 9A). After extraction with DMSO, anti-solvent crystallization with ethanol, and washing with THF, the portion of HMX increased to 99.9% (see Fig. 9B). Based on these results, the purity of the recovered β-HMX crystals was determined to be higher than 99%. The yield of β-HMX was above 90%, including losses in the washing and filtration steps. Conclusions HMX was recovered from DXC-57 via extraction with DMSO and drowning-out crystallization with ethanol as an anti-solvent. In this study, S for HMX was defined as the ratio of its concentration to its solubility. Crystallization at higher S lowered the possibility of formation of the thermodynamically stable β-form HMX. Estane, the polymer binder in DXC-57, was found to both retard the nucleation and cause the crystallization at high S. It was found that the nucleation of HMX at S 6.26 enabled the formation of β-form HMX. Washing with THF to remove the co-precipitated Estane yielded β-form HMX crystals of purity 99% in a yield of 90%. Acknowledgements The authors gratefully acknowledge support from the Defense Acquisition Program Administration and the Agency for Defense Development.
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