Preparation of Spherical HMX/DMF Solvates, Spherical HMX Particles, and HMX@NTO Composites: A Way to Reduce the Sensitivity of HMX.

To reduce the sensitivity of HMX (HMX = high-melting explosive-cyclotetramethylenetetranitramine), spherical HMX/DMF (DMF = dimethylformamide) solvates, spherical HMX particles, and HMX@NTO (NTO = 1,2,4-triazol-5-one) composites are prepared by crystallization. The structure and performance of spherical HMX crystals, HMX particles, and HMX@NTO composites are characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, accelerating rate calorimetry, and mechanical sensitivity test. The results show that the space group of the spherical HMX/DMF solvate is R̅3c with the lattice parameters of a = 15.9159(4) Å, b = 15.9159(4) Å, and c = 30.5136(8) Å. The non-isothermal stability and adiabatic thermal stability of HMX/DMF solvates are similar to those of HMX particles. The non-isothermal stability of HMX@NTO composites is lower than that of NTO and HMX particles, while the adiabatic thermal stability of HMX@NTO composites is higher than that of NTO but lower than that of HMX particles. The mechanical sensitivities of spherical HMX/DMF cocrystals, spherical HMX particles, and HMX@NTO composites are lower than that of raw HMX. This study can provide some guidance for desensitizing HMX and other energetic materials.

Establishing the Interface Layer on the Pentaerythritol Tetranitrate Surface via In Situ Reaction.

Pentaerythritol tetranitrate (PETN) was coated by tannic acid (TA), polydopamine (PDA), and melamine-formaldehyde (MF) resins via in situ reaction to prepare PETN@TA, PETN@PDA, and PETN@MF microcapsules for reducing sensitivity and enhancing thermal stability of PETN. The coating effects of TA, PDA, and MF shells on PETN surfaces are characterized by scanning electron microscopy and atomic force microscopy. The structures of PETN@TA, PETN@PDA, and PETN@MF microcapsules are characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier-transform infrared spectra. The performances of PETN@TA, PETN@PDA, and PETN@MF microcapsules are characterized by differential scanning calorimetry, accelerating rate calorimetry, explosion point, vacuum deflation volume, and mechanical sensitivity. The study results show that TA, PDA, and MF shells can coat the PETN surface well. Compared with pure PETN, the explosion point has an increase while the vacuum deflation volume and mechanical sensitivity have a decrease for PETN@TA, PETN@PDA, and PETN@MF microcapsules, illustrating that the safeties of PETN@TA, PETN@PDA, and PETN@MF microcapsules are enhanced. In addition, the initial decomposition temperature (T0) and peak decomposition temperature (Tp) of PETN@TA, PETN@PDA, and PETN@MF microcapsules have a slight increase, demonstrating that the thermal stabilities of PETN@TA, PETN@PDA, and PETN@MF microcapsules are better than that of pure PETN. The obtained method can provide some guidance for the desensitizing of other energetic materials with high sensitivities.

The influence of temperature environmental on performance of HNIW/FOX-7 based PBXs.

During application, energetic materials may suffer different temperature environmental stimulation. In order to study the influence of temperature environmental on performance of HNIW/FOX-7 based PBXs, HNIW/FOX-7 based PBX modeling powders and PBX columns were treated by LT (low temperature), HT (high temperature), HLC (high-low temperature cycle) and HLS (high-low temperature shock). Then scanning electron microscope (SEM), infrared spectra (IR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to study the variation of PBX modeling powders after LT, HT, HLC and HLS treatments; in addition, the mass, size and mechanical properties of PBX columns were characterized after different temperature adaptability treatments as well. The results indicate that the change ratios of mass and size of HNIW/FOX-7 based PBX columns are less than 1%, illustrating that mass and size of PBX columns are at acceptable level after different temperature adaptability treatments. The unevenness degree of the surface of PBX modeling powders followed the order of HLC > HT > LT > HLS, which agrees well with mass loss order. Moreover, IR and XRD results indicated that the molecular structure and crystal form of HNIW and FOX-7 did not change after different temperature adaptability treatments. Additionally, thermal stabilities of PBX modeling powders are decreased after different temperature adaptability treatments, among which HLS has the largest influence on HNIW/FOX-based PBX modeling powders. The compression strengths and elastic moduli of HNIW/FOX-based PBX columns are enhanced after different temperature adaptability treatments, among which the strength of PBX columns after HLC has the maximum increase, indicating that HLC has more significant effect on mechanical property.

Study on Cellulose Acetate Butyrate/Plasticizer Systems by Molecular Dynamics Simulation and Experimental Characterization.

Cellulose acetate butyrate (CAB) is a widely used binder in polymer bonded explosives (PBXs). However, the mechanical properties of PBXs bonded with CAB are usually very poor, which makes the charge edges prone to crack. In the current study, seven plasticizers, including bis (2,2-dinitro propyl) formal/acetal (BDNPF/A or A3, which is 1:1 mixture of the two components), azide-terminated glycidyl azide (GAPA), n-butyl-N-(2-nitroxy-ethyl) nitramine (Bu-NENA), ethylene glycol bis(azidoacetate) (EGBAA), diethylene glycol bis(azidoacetate) (DEGBAA), trimethylol nitromethane tris (azidoacetate) (TMNTA) and pentaerythritol tetrakis (azidoacetate) [PETKAA], were studied for the plasticization of CAB. Molecular dynamics simulation was conducted to distinguish the compatibilities between CAB and plasticizers and to predict the mechanical properties of CAB/plasticizer systems. Considering the solubility parameters, binding energies and intermolecular radical distribution functions of these CAB/plasticizer systems comprehensively, we found A3, Bu-NENA, DEGBAA and GAPA are compatible with CAB. The elastic moduli of CAB/plasticizer systems follow the order of CAB/Bu-NENA>CAB/A3>CAB/DEGBAA>CAB/GAPA, and their processing property is in the order of CAB/Bu-NENA>CAB/GAPA>CAB/A3>CAB/DEGBAA. Afterwards, all the systems were characterized by FT-IR, differential scanning calorimetry (DSC), differential thermogravimetric analysis (DTA) and tensile tests. The results suggest A3, GAPA and Bu-NENA are compatible with CAB. The tensile strengths and Young's moduli of these systems are in the order of CAB/A3>CAB/Bu-NENA>CAB/GAPA, while the strain at break of CAB/Bu-NENA is best, which are consistent with simulation results. Based on these results, it can be concluded that A3, Bu-NENA and GAPA are the most suitable plasticizers for CAB binder in improving mechanical and processing properties. Our work has provided a crucial guidance for the formulation design of PBXs with CAB binder.

Theoretical study on the weak interaction and energy performance of nitroformate salts and nitroformate-based propellant formulations.

Nitroformate energetic salts are potential high-performance oxidizers which can be used in a solid propellant. The geometric configuration, the weak interaction, and the energy characteristics of hydrazine nitroformate (HNF), ammonium nitroformate (ANF), aminotriazole nitroformate (ATNF), guanidinium nitroformate (GNF), and aminotetrazole nitroformate (ATTNF) were investigated. Analysis results show that there exist hydrogen bonds in all salts except GNF. The binding energies of salt are between 390 and 430 kJ/mol and are positively correlated with densities and thermodynamic stabilities of salts but show reverse trend on impact sensitivities. Binding energy decomposition indicates that the main interaction in anion and cation is electrostatic interaction. The detonation velocity and specific impulse of five nitroformate salts are in the range of 8.6~9.1 km/s and 2200~2600 N s/kg, respectively. Considering the five selected salts as oxidizers, several propellant formulations were designed and the performances of formulations were predicted. The calculation results show that nitroformate salts obviously reduce characteristic signals and improve specific impulse for propellant formulations.

Molecular dynamics simulations on miscibility, glass transition temperature and mechanical properties of PMMA/DBP binary system.

Polymethyl methacrylate (PMMA) and dibutyl phthalate (DBP) binary system was simulated by molecular dynamics (MD) simulations with the COMPASS force field to predict properties of PMMA/DBP blends such as the miscibility, the glass transition temperature (Tg) and mechanical properties of polymer/plasticizer blends. Results show that PMMA/DBP is a miscible system, which can be predicted by comparing the difference of the solubility parameters value (|Δδ|<2.0 MPa0.5) between PMMA and DBP. The free volumes (VF) and density (ρ) of PMMA/DBP system were simulated to study the Tg. It is found that the VF and ρ of PMMA/DBP change regularly along with the increase of DBP mass fraction and the transition occurred at the turning point. We also predicted the effects of temperature and DBP on the mechanical properties of PMMA including Young's modulus (E), Bulk modulus (K), Shear modulus (G) and Poisson's ratio (v). The mechanical properties can be effectively improved by the temperature and the addition of DBP plasticizer, which may provide a more flexible mixture with a lower E, K, G and an increased ductility. Accordingly, the method used in this work is not only a useful tool to provide properties of a given polymer/plasticizer blend but also a promising technique to help screen the formulations of polymer bonded explosive (PBX) and propellants before experiments.

Investigation of the effect of the CAB/A3 system on HNIW-based PBXs using molecular dynamics.

The influences of the temperature and the BDNPA/BDNPF (A3) content on the mechanical properties of and the binding energies between hexanitrohexaazaisowurtzitane (HNIW) and cellulose acetate butyrate (CAB)/A3 were studied via molecular dynamics simulations. The morphology of HNIW in acetone was simulated using an attachment energy (AE) model to elucidate the HNIW surfaces that are present under real-world conditions. The simulation results were consistent with the experimentally derived ones, and they indicated that the exposed HNIW surfaces were (0 0 1), (1 1 0), and (1 1 -1). The mechanical properties of CAB with different amounts of A3 were calculated at different temperatures, and the results showed that the amount of A3 was a stronger influence than the temperature on the mechanical properties. The binding energies between CAB/A3 and the exposed HNIW surfaces were calculated. Based on the binding energy and the area of each exposed surface, the weighted-average binding energy was calculated and then used instead of the total binding energy to evaluate the effect of the temperature and the A3 content on the binding energy. The average binding energy was found to be highest when the temperature was 313 K and the mass fraction of A3 was 0.15.

Prediction of crystal morphology of 3,4-Dinitro-1H-pyrazole (DNP) in different solvents.

Analytical grade ethyl acetate was used to recrystallize 3,4-Dinitro-1H-pyrazole (DNP), it was found that there were two different morphologies of the crystals. There is a possibility that ethyl acetate undergoes hydrolysis due to the absorption of moisture in the air. The influence of the hydrolysis products of acetic acid and ethanol on the morphology of DNP crystal was considered. In order to investigate the effect of solvents on DNP morphology, there is ongoing research to validate molecular dynamics (MD) simulation results with experiment data. The morphology of DNP in vacuum was predicted by the attachment energy (AE) model, and the growth morphology of DNP in different solvents was simulated by MD method. The modified AE model was successfully verified the phenomenon by the prediction of DNP crystals morphology in Ethyl acetate, H2O, H2O/EtOH and H2O/AcOH. The calculated results show that the two different crystal shapes are diamond and hexagon, respectively. The results are in agreement with the crystal morphology obtained by experiment.