Effect of Pressure of the Casting Vessel on the Solidification Characteristics of a DNAN/RDX Melt-Cast Explosive

This study experimentally and numerically investigated the effect of pressure of the casting vessel on the solidification characteristics of a DNAN (2,4-dinitroanisole)/RDX(cyclotrimethylenetrinitramine) melt-cast explosive. The solidification and cooling time was recorded by a thermocouple, the distribution of the shrinkage porosity was observed and analyzed, and the density of the explosive charge was measured. The primary solidification characteristics were verified by numerical simulation achieved with ProCAST casting software, and results were in qualitative agreement with experimental results. Both the experimental and numerical results demonstrate that increasing pressure of the casting vessel can effectively diminish the shrinkage porosity, reduce the solidification time, and increase the relative density of the DNAN/RDX melt-cast explosive.     

Measurements of reaction zone and determination of the equation of state parameters of DNAN-based melt-cast aluminized explosive

ABSTRACT

To investigate the detonation characteristics and the detonation wave profiles of the melt-cast aluminized explosive RBOL-2 (DNAN/HMX/aluminum/other additives), the particle velocity of the interface between the explosive and the Lithium-Fluoride (LiF) window and the free-surface velocity of the copper plate accelerated are measured by using the DISAR interferometer system. It is found that, for RBOL-2 explosive, the reaction zone length is 202 ± 2.5 ns/1.073 ± 0.013 mm, the particle velocity and the pressure at the Von Neumann spike are 2364 ± 3 m/s and 41.80 ± 0.07 GPa, respectively, and the particle velocity and the pressure at the Chapman-Jouguet point are 1564 ± 3 m/s and 25.42 ± 0.05 GPa, respectively. Moreover, the parameters of the Jones-Wilkins-Lee (JWL) equation of state of the RBOL-2 explosive are also determined by the genetic optimization algorithm and the measured free surface velocity of the copper plate.     

Thermokinetic decomposition and sensitivity studies of 4,4ˊ-diamino-3,3ˊ-azoxy furazan (DAAF)-based melt cast explosive formulations

4,4ˊ-diamino-3,3ˊ-azoxy furazan (DAAF) is an insensitive high explosive. DAAF has safety characteristics (impact, friction) similar to triaminotrinitrobenzene and shock sensitivity similar to HMX. The present article describes the thermal analysis and sensitivity study of DAAF with RDX and 2,4,6-trinitrotoluene (TNT). DAAF has been evaluated as a possible replacement for RDX in TNT-based, aluminized as well as nonaluminized melt cast formulation. DAAF-based melt cast formulations were characterized for their sensitivity to mechanical stimuli, bomb calorimetric analysis, and thermal decomposition behavior. The thermal analysis reveals the compatibility of DAAF with benchmark explosives like RDX and TNT in explosive formulations. The composition DT (DAAF + TNT) and DTA (DAAF + TNT+ Al) is more friction and impact insensitive as compared to RT (RDX + TNT) and RTA (RDX + TNT+ Al) compositions. The bomb calorimetric values of DT composition as well as DTA composition are higher than RT and RTA compositions. The result shows that DAAF can be effectively used as a RDX replacement in melt cast explosive formulations. DT-based aluminized composition showed more thermal stability than RT- and RTA-based control compositions, which clearly revealed the usefulness of DAAF for enhanced blast effect.     

Ignition and Growth Modeling of Shock Initiation of Different Particle Size Formulations of PBXC03 Explosive

The change in shock sensitivity of explosives having various explosive grain sizes is discussed. Along with other parameters, explosive grain size is one of the key parameters controlling the macroscopic behavior of shocked pressed explosives. Ignition and growth reactive flow modeling is performed for the shock initiation experiments carried out by using the in situ manganin piezoresistive pressure gauge technique to investigate the influences of the octahydro-1,3,5,7–tetranitro-1,3,5,7-tetrazocine (HMX) particle size on the shock initiation and the subsequent detonation growth process for the three explosive formulations of pressed PBXC03 (87% HMX, 7% 1,3,5-trichloro-2,4,6-trinitrobenzene (TATB), 6% Viton by weight). All of the formulation studied had the same density but different explosive grain sizes. A set of ignition and growth parameters was obtained for all three formulations. Only the coefficient G₁ of the first growth term in the reaction rate equation was varied with the grain size; all other parameters were kept the same for all formulations. It was found that G₁ decreases almost linearly with HMX particle size for PBXC03. However, the equation of state (EOS) for solid explosive had to be adjusted to fit the experimental data. Both experimental and numerical simulation results show that the shock sensitivity of PBXC03 decreases with increasing HMX particle size for the sustained pressure pulses (around 4 GPa) as obtained in the experiment. This result is in accordance with the results reported elsewhere in literature. For future work, a better approach may be to find standard solid Grüneisen EOS and product Jones-Wilkins-Lee (JWL) EOS for each formulation for the best fit to the experimental data.     

Research on Equation of State For Detonation Products of Aluminized Explosive

The secondary reaction of the aluminum powder contained in an aluminized explosive is investigated, from which the energy loss resulted from the quantity reduce of the gaseous products is demonstrated. Moreover, taking the energy loss into account, the existing improved Jones-Wilkins-Lee (JWL) equation of state for detonation products of aluminized explosive is modified. Furthermore, the new modified JWL equation of state is implemented into the dynamic analysis software (DYNA)-2D hydro-code to simulate numerically the metal plate acceleration tests of the Hexogen (RDX)-based aluminized explosives. It is found that the numerical results are in good agreement with previous experimental data. In addition, it is also demonstrated that the reaction rate of explosive before the Chapman-Jouget (CJ) state has little influence on the motion of the metal plate, based on which a simple approach is proposed to simulate numerically the products expansion process after the CJ state.     

Characterization of RDX-Based Thermosetting Plastic-Bonded Explosive by Cone-Beam Microfocus Computed Tomography

In this study microfocus computed tomography (μCT), a nondestructive technique, was used for 3D characterization of cyclotrimethylenetrinitramine (RDX)-based thermosetting plastic-bonded explosives. Quantitative information on RDX crystals including the packing status of the crystal and binder system, as well as the inner structure characteristics of partially solidified and fully solidified samples, was obtained using cone-beam μCT technology. The CT images show that the CT value of RDX crystal grains was obviously higher than that of crystal powder/binder. The results also show that after vacuum casting and thermosetting, the RDX grains were dispersed evenly and there were no air pores or cracks observed in the sample. Small pores exist inside the RDX grains with a porosity of less than 0.3%. The analysis results indicate high quality of the explosive part after employing thermosetting molding. The binder system and RDX crystals were integrated sufficiently, yet the grain packing was not maximum. The estimated average density and maximal difference in density of the sample were both in accordance with the testing results by Archimedes’ method and it was proven that complete curing may enhance the overall density and density uniformity of the product.     

Effect of RDX powders on detonation characteristics of emulsion explosives

ABSTRACT

To obtain an explosive suitable for the explosive welding of foils and produce a new way to reuse demilitarized explosives, RDX powders and high amounts of hollow glass microballoons (GMs) were introduced into an emulsion matrix to reduce detonation velocity and critical thickness. The effect of different percentages of RDX on the detonation performance of the mixtures was systematically investigated. The results showed that the critical thickness decreased significantly with increasing RDX contents, and the detonation velocity at the critical thickness was almost unchanged for the different RDX contents. Thus, the as-created mixtures were suitable for the explosive welding of foils due to their low detonation velocities and low critical thicknesses. The brisance test results indicated that the brisance of the composite explosives increased with increasing RDX contents, and this trend was more remarkable at low RDX contents. All the energy output parameters of the underwater explosions also increased with increasing RDX contents.     

Microstructure effects on the detonation velocity of a heterogeneous high-explosive

ABSTRACT

Although numerous methods exist for the theoretical calculation of detonation parameters of explosives, few thermodynamic-hydrodynamic-based theoretical codes take into account particle size. The basis for their computational analysis is primarily focused on the equation of state of the detonation products, heat of formation, and density of the explosive composition. This study utilized regression analysis to model the relationship between the microstructure characteristics and detonation velocity of a heterogeneous high-explosive composition containing cyclotrimethylene-trinitrmaine (RDX). The principal characteristics examined were the average particle size of RDX, amount of HMX impurity within the RDX particles, method of RDX manufacture, and compositional density. Statistical analysis demonstrated the relevancy of the microstructure influence on the detonation velocity of the developed experimental compositions of 73 wt. % solids and 27 wt. % polyurethane binder. An equation is developed that accurately predicts detonation velocity based on average particle size, density, and manufacturing process for RDX. The model underscores the significance of the relationship between the average particle size and detonation velocity. Compositions containing smaller average particle sizes of RDX generate higher detonation velocities. A 100 micron increase in the average particle size was shown to decrease detonation velocity by 161 m/s for the monomodal polyurethane compositions used in this study. The relevance of using statistical models for selecting characteristics that result in optimum explosive performance is addressed.     

1-methyl-2,4,5-trinitroimidazole (MTNI), a melt-cast explosive: synthesis and studies on thermal behavior in presence of explosive ingredients

ABSTRACT

Traditionally, TNT based melt-castable explosives have been used till date. Recently, energetics communities are looking for an alternate to TNT in melt-cast formulation because of its toxicity and environmental concerns. 1-Methyl-2,4,5-trinitroimidazole (MTNI) is an insensitive high energy material and potential for use as a melt-cast high explosive formulations in place of TNT. MTNI has superior explosive performance as compared to that of TNT. Typically MTNI is synthesized starting from imidazole by step-wise nitration using strong nitric acid mixture followed by N-methylation offers poor yield. The key step involves during synthesis of MTNI is substitution of third nitro group at fifth position of 2,4-dinitroimidaziole which is limiting for its large-scale preparation. We report herein, an improved method for synthesis of MTNI from 1-methyl-2,4-Dinitroimidazole (MDNI) under different nitration conditions in good yield. Heteropoly acid (HPA) is an efficient, mild and green catalysts used as nitric acid activator. The synthesized MTNI by this method was characterized by FT-IR, NMR & elemental analysis. Thermal behavior of MTNI was determined by differential scanning calorimeter (DSC), thermogravimetric analysis (TGA) and ignition temperature tester. Further, the effects of thermal energy on decomposition of formulations containing MTNI with solid high explosives such as RDX & CL-20 or polymeric binders like HTPB & GAP were investigated. Thermal decomposition mechanisms of MTNI and its precursor, MDNI based on Pyrolysis-GC/MS analysis were also described.     

Chemical Desensitization of Explosives. Part 1. Effect of Ethyl Centralite on the Properties of an RDX-Based PBX

The effects of the addition of ethyl centralite, a commonly used stabilizer in double-base propellants, on a cyclotrimethylenetrinitramine (RDX)-based polymer-bonded explosive (PBX) were investigated. A reduction in sensitivity to impact was observed with increasing ethyl centralite content along with a reduction in explosiveness. Small-scale gap tests also show a reduction in sensitivity to shock initiation. High-performance liquid chromatography (HPLC) analysis of impact-stressed samples showed traces of 4-nitro- and 2-nitro-ethyl centralite and N-ethyl-N-nitrosoaniline; these results indicate the possibility that the mechanism of desensitization may, at least partially, be chemical in nature.     

Influence of Small Change of Porosity on Shock Initiation of an HMX/TATB/Viton Explosive and Ignition and Growth Modeling

All solid explosives in practical use are more or less porous. Although it is known that the change in porosity affects the shock sensitivity of solid explosives, the effect of small changes in porosity on the sensitivity needs to be determined for safe and efficient use of explosive materials. In this study, the influence of a small change in porosity on shock initiation and the subsequent detonation growth process of a plastic-bonded explosive PBXC03, composed of 87% cyclotetramethylene-tetranitramine (HMX), 7% triaminotrinitrobenzene (TATB), and 6% Viton by weight, are investigated by shock to detonation transition experiments. Two explosive formulations of PBXC03 having the same initial grain sizes pressed to 98 and 99% of theoretical mass density (1.873 g/cm³) respectively are tested using the in situ manganin piezoresistive pressure gauge technique. Numerical modeling of the experiments is performed using an ignition and growth reactive flow model. Reasonable agreement with the experimental results is obtained by increasing the growth term coefficient in the Lee-Tarver ignition and growth model with porosity. Combining the experimental and simulation results shows that the shock sensitivity increases with porosity for PBXC03 having the same explosive initial grain sizes for the pressures (about 3.1 GPa) applied in the experiments.     

Reversal of particle size/shock sensitivity relationship at small particle size for pressed heterogeneous explosives under sustained shock loading

Abstract

Shock initiation of pressed heterogeneous explosives has been reviewed. The key processes of ignition and buildup and their relative importance under sustained and short duration shock are described. Particle size effects on shock sensitivity are shown to depend on both density (ZTMD) and shock duration. A series of RDX samples of narrow particle size range were subjected to sustained shock (gap test). Sensitivity increased over the series 250 < 138 < 100.0 < 21.2 μm median particle size but decreased for a 3.9 μm sample. These results combined with earlier published data support published modelling studies which predict decreased shock sensitivity at small particle size.     

Thermal Behaviors and Their Correlations of Mg(BH4)2-Contained Explosives

In order to explore the effect of metal hydride on energetic materials’ thermal behaviors and their correlations, we studied the heats of combustion and detonation of RDX, TNT, and Mg(BH₄)₂-containing explosives both theoretically and experimentally. The results showed that Mg(BH₄)₂ can significantly improve the energy of explosive. As the mass fraction of Mg(BH₄)₂ increases, the combustion heat of composite explosives increases gradually, while the combustion efficiency decreases. When its mass fraction is about 30%, the theoretical heats of detonation of RDX/Mg(BH₄)₂ and TNT/Mg(BH₄)₂ reach maximum, which are 7418.47 and 7032.46 kJ/kg, respectively. When we compared the errors between calculation and experimental values, we found that L-C method is more accurate in calculating oxygen-enriched and oxygen-balanced explosives, and that minimum free energy method is more suitable for seriously negative oxygen-balanced explosive. For single explosive, there are three kinds of relationships between heat of combustion and detonation according to the oxygen balance. For Mg(BH₄)₂-containing explosives, the relationship is in accordance with Boltzmann function.     

Preparation and Safety of Well-Dispersed RDX Particles Coated with Cured HTPB

HTPB/IPDI (hydroxyl terminated polybutadiene & Isophorone diisocyanate) and TNT (2,4,6-trinitrotoluene) were successively coated on RDX (hexogen) particles by solvent evaporation and aqueous slurry melting, respectively. When HTPB coated on RDX particles cured completely, TNT was removed by solvent dissolution and the well-dispersed RDX particles coated with cured HTPB were obtained successfully. SEM (scanning electron microscopy), TEM (transmission electron microscopy), XPS (X-ray photoelectron spectrometry), and laser granularity measurement were employed to characterize the coated samples, and the mechanical sensitivity and thermal stability were measured and analyzed. Results show that TNT on the outer layer effectively hinders the adhesion among the particles resulting from the curing of inner layer (i.e., HTPB and IPDI). The final coating particles disperse well and their mechanical sensitivity decreases significantly. When the covering amount of HTPB is 2 wt.%, drop height (H₅₀) of RDX increases from 37.2 to 66.5 cm and explosion probability (P) decreases from 92 to 16%. Compared with that of uncoated samples, the activation energy and self-ignition temperature of coated samples do not vary.     

Shock initiation of TATB/FEFO formulations

Abstract

The shock initiation properties of transferable insensitive explosive (TIE) formulations based on the solid high explosive, triaminotrinitrobenzene (TATB), and the liquid explosive, bis(2-fluoro-2,2-dinitroethyl) formal (FEFO), are measured by wedge test, embedded particle velocity gauge and embedded manganin pressure gauge techniques and calculated using the Ignition and Growth reactive flow model. These extrudable formulations are demonstrated to be slightly more shock sensitive than the TATB/inert binder explosive, LX-17. However, the TIE formulations are much less sensitive than HMX-based explosives and still qualify as insensitive explosives in safety and hazard tests. The wedge tests showed a very steep dependence of run distance to detonation on the input shock pressure. Embedded gauge and reactive flow modeling results imply that shock initiation begins when a small amount of the solid TATB decomposes rapidly enough to heat the surrounding FEFO to decomposition temperature. The FEFO then reacts rapidly, raising the pressure and temperature sufficiently to cause surface decomposition of the TATB particles at rates comparable to those measured in other TATB-based explosives. An Ignition and Growth reactive flow model for TIE based on these assumptions yields reasonable agreement with the experimental shock initiation data.     

An Investigation into the Effects of Additives on Crystal Characteristics and Impact Sensitivity of RDX

Additives are one of the most important factors that greatly affect the crystal characteristics of the high energy compound hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX, C₃H₆N₆O₆) and they have an influence on impact sensitivity. In this article, a growth morphology method was applied to obtain the crystal habit of RDX in a vacuum as well as the morphologically important faces, and molecular dynamics simulations were applied to calculate the interaction energy between these crystal faces and additive molecules for prediction of the additive-effect crystal habits of RDX. On this basis, crystal characteristics including crystal morphology, aspect ratio, and total surface charge were investigated. Then the particle size and surface electrostatic voltage of the samples from recrystallization were analyzed experimentally. The impact sensitivity test indicated that acrylamide, which could enhance the regularity and degree of sphericity of RDX crystals and effectively reduce the surface static electricity of RDX, was successful in reducing the impact sensitivity of RDX as an additive for crystallization. The above experimental results were in good agreement with the conclusions based on the theoretical calculations.     

Evaluation of small-scale combustion of an insensitive high explosive formulation containing 3-nitro-1,2,4-triazol-5-one (NTO), 2,4-dinitroanisole (DNAN), and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX)

ABSTRACT

Energetic materials are often disposed by open-burning or open-detonation as it is a cost-effective and efficient means of destroying explosive material, and often minimizes the need to transport hazardous explosives to treatment facilities. This practice is often scrutinized for the negative environmental impact of the odorous and unsightly toxic gaseous emissions as well as the resulting deposition residues, which often contain unburned energetic materials. With the increasing use of Insensitive High Explosive compositions in munitions, it is essential that the potential environmental impact of their disposal is assessed before their extensive use to prevent the kind of contamination incidents experienced with legacy explosives. Therefore, the aim of this work was to develop a controlled laboratory experiment to identify the gaseous emissions and the energetic material residues that are generated through the combustion of the IHE components 3-nitro-1,2,4-triazol-5-one (NTO), 2,4-dinitroanisole (DNAN), and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX). A sealed vial containing small (mg) quantities of energetic material was heated until the energetic material combusted. Gas chromatography/mass spectrometry (GCMS) was used to calculate the oxygen consumption and to identify the gases that were generated. The solid residues were analyzed by high-performance liquid chromatography (HPLC) to quantify unburned energetic material. Results showed that DNAN was the most resistant to burning, thus leaving significant quantities of unreacted starting material in the vial. An interesting observation for the IHE formulation was that DNAN also inhibited the combustion of NTO and RDX. The gases emitted during the open burning of IHE components and mixtures included CO, CO₂, and N₂O as expected, but the proportions differed when the components and mixture were compared, reflecting the influence of DNAN on the burning behavior. From our data, we concluded that open-burning DNAN-based formulations is an environmentally unfavorable waste-management practice for the disposal of IHEs mainly due to generation of solid residues as well as unburnt DNAN.     

A Study of Reduced-Sensitivity RDX

Five lots of RDX (1,3,5-trinitro-1,3,5,-triazinane) were believed to have different shock sensitivities based on large-scale gap test (LSGT) results. Laboratory investigations included direct-insertion mass spectrometry, liquid chromatography, powder X-ray diffraction, polarized light with hot-stage microscopy, infrared and Raman spectroscopy, and differential scanning calorimetry (DSC). DSC proved useful at distinguishing differences between lots. The RDX lots studied included three made in the United States by BAE Ordinance Systems (Holston), one from Eurenco (formally SNPE), and one from Dyno Nobel. Of Holston RDX, two lots had been reprocessed by Eurenco to reduce sensitivity. Increased sensitivity appeared to correlate with increased HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) content. Aging and heating of RDX with HMX content appeared to contribute to the formation of an RDX/HMX eutectic, which could explain the enhanced sensitivity observed in some samples.     

A Successive and Scalable Process for Preparing Spherical Submicrometer-Sized RDX by the SEDS Process

The preparation spherical and submicrometer-sized cyclotrimethylene-trinitramine (RDX) is described through the process of solution-enhanced dispersion by supercritical fluids (SEDS). Because SEDS is a successive and scalable process, about 40.5 g RDX particles can be obtained within 70 min and the total yield is about 90.0% under optimized conditions. Submicrometer-sized RDX and original samples were characterized and confirmed by different analysis methods. The results revealed that submicrometer-sized RDX particles are spherical with a mean particle size of 767.7 nm, have high purity, show no crystal structure change, and decompose easily. Mechanical sensitivity tests of submicrometer-size RDX were decreased in comparison to the original RDX.     

Composition Choice and Formulation of Three HMX-Based Research Explosives

The composition and formulation for three research explosives having similarities to military explosives are described. The primary energetic ingredient in each is cyclotetramethylene-tetranitramine (HMX), whose particle size is limited to a range of 125–210 μm to reduce variations in shock reactivity and performance. The binder in each explosive is hydroxy-terminated polybutadiene (HTPB). The first composition contains only these two components. Aluminum with a nominal particle size of 5 μm is incorporated into the second composition. The third composition contains ammonium perchlorate (AP) with a nominal particle size of 200 μm in addition to the aluminum. The explosives are designed with features to allow for comparisons in shock reactivity and performance and to elucidate the roles of HMX, Al, and AP.