Concomitant crystallization for in situ encapsulation of organic materials

Concomitant crystallization leads to process intensification through the synergistic combination of the partial processes of particle formation and encapsulation within a single process step. Both cooling and electrospray crystallization in multi-component solutions were used to create (sub-)micron sized particles of different crystalline materials. Concentrations were varied to control core and shell material. Depending on the relative initial concentrations used, concomitant electrospray crystallization of isonicotinamide and caffeine leads to encapsulated particles. Only limited encapsulation was achieved during concomitant cooling crystallization. Concomitant cooling crystallization of cyclotrimethylenetrinitramine (RDX)–2,4,6-trinitrotoluene (TNT) resulted in separate RDX and TNT particles. Using electrospray crystallization, spherical nano-particles were produced, for which the component distribution within the particles could not be determined. Whereas crystallization from bulk solvent starts with a nucleus that grows gradually outward, whereby heterogeneous growth of a coating material on this core particle is not guaranteed, it appears that crystallization from evaporating solvent droplets starts at the surface of the droplets, and moves gradually inward. The resulting RDX–TNT powders have been tested for impact and friction sensitivity. The impact sensitivity has decreased compared to the raw materials, and the friction sensitivity did not change.     

Preparation and Properties of Submicrometer‐Sized LLM‐105 via Spray‐Crystallization Method

Submicrometer‐sized 2,6‐diamino‐3,5‐dinitropyrazine‐1‐oxide (LLM‐105) crystals were prepared by spray‐crystallization method with dimethyl sulfoxide (DMSO) and ultra‐pure water with surfactant as the solvent and anti solvent, respectively. Submicrometer‐sized LLM‐105 particles were characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD), and particle size analysis. The thermal stability and sensitivity properties of submicrometer‐sized LLM‐105 were also investigated. The results revealed that the submicrometer‐sized LLM‐105 particles are spherelike in morphology with a narrow particle size distribution at the range of 100–600 nm. The submicrometer‐sized LLM‐105 has a lower exothermic peak at about 343.7 °C compared with the synthesized material. Sensitivity tests showed that submicrometer‐sized LLM‐105 is more insensitive under impact stimulus with a drop height (H₅₀) of 102 cm. The submicrometer‐sized LLM‐105‐based PBX is more sensitive for short impulse shock wave that can be initiated at lower initiation current.     

The Effect of RDX Crystal Defect Structure on Mechanical Response of a Polymer‐Bonded Explosive

An explosive composition, derived from AFX‐757, was systematically varied by using three different qualities of Class I RDX. The effect of internal defect structure of the RDX crystal on the shock sensitivity of a polymer bonded explosive is generally accepted (Doherty and Watt, 2008). Here the response to a mechanical non‐shock stimulus is studied using an explosion‐driven deformation test as well as the ballistic impact chamber. No correlation between RDX crystal quality and deformation sensitivity is observed. The DDT behavior (Deflagration to Detonation Transition) of the three plastic bonded explosives, although similar in composition, is distinct regarding the rate of diameter increase in the explosion‐driven deformation test. Recovered polymer bonded explosive from the explosion‐driven deformation test responds equally fast or slower in the ballistic impact chamber. Based on our experimental results the shear rate threshold as a single parameter describing mechanical sensitivity is challenged, and preference is given to the development of an ignition criterion based on inter‐granular sliding friction under the action of a normal pressure.     

Formation and Characterization of Nano‐sized RDX Particles Produced Using the RESS‐AS Process

It has been shown that nano‐sized particles of secondary explosives are less sensitive to impact and can alter the energetic performance of a propellant or explosive. In this work the Rapid Expansion of a Supercritical Solution into an Aqueous Solution (RESS‐AS) process was used to produce nano‐sized RDX (cyclo‐1,3,5‐trimethylene‐2,4,6‐trinitramine) particles. When a saturated supercritical carbon dioxide/RDX solution was expanded into neat water, RDX particles produced from the RESS‐AS process agglomerated quickly and coarsened through Ostwald ripening. However, if the pH level of the suspension was changed to 7, particles were metastably dispersed with a diameter of 30 nm. When the supercritical solution was expanded into air under the same pre‐expansion conditions using the similar RESS process, RDX particles were agglomerated and had an average size of approximately 100 nm. Another advantage of using a liquid receiving solution is the possibility for coating energetic particles with a thin layer of polymer. Dispersed particles were formed by coating the RDX particles with the water soluble polymers polyvinylpyrrolidone (PVP) or polyethylenimine (PEI) in the RESS‐AS process. Both PVP and PEI were used because they have an affinity to the RDX surface. Small and well‐dispersed particles were created for both cases with both PVP and PEI‐coated RDX particles shown to be stable for a year afterward. Several benefits are expected from these small polymer coated RDX particles such as decreased sensitivity, controlled reactivity, and enhanced compatibility with other binders for fabrication of bulk‐sized propellants and/or explosives.     

Preparation and Properties of an AP/RDX/SiO2 Nanocomposite Energetic Material by the Sol‐Gel Method

A nanocomposite energetic material was prepared using sol‐gel processing. It was incorporated into the nano or submicrometer‐sized pores of the gel skeleton with a content up to 95 %. AP, RDX, and silica were chosen as the energetic crystal and gel skeleton, respectively. The structure and its properties were characterized by SEM, BET methods, XRD, TG/DSC, and impact sensitivity measurements. The structure of the AP/RDX/SiO₂ cryogel is of micrometer scale powder with numerous pores of nanometer scale and the mean crystal size of AP and RDX is approx. 200 nm. The specific surface area of the AP/RDX/SiO₂ cryogel is 36.6 m² g⁻¹. TG/DSC analyses indicate that SiO₂ cryogel can boost the decomposition of AP and enhance the interaction between AP and RDX. By comparison of the decomposition heats of AP/RDX/SiO₂ at different mass ratios, the optimal mass ratio was estimated to be 6.5/10/1 with a maximum decomposition heat of 2160.8 J g⁻¹. According to impact sensitivity tests, the sensitivity of the AP/RDX/SiO₂ cryogel is lower than that of the pure energetic ingredients and their mixture.     

Nanoparticle generation by intensified solution crystallization using cold plasma

In this study, atmospheric pressure cold plasma (surface dielectric barrier discharge) was used as an alternative energy form to intensify solution crystallization and produce nano-sized organic crystals. Nano-sized particles can have beneficial product properties such as improved internal quality and dissolution rate, compared to conventionally sized crystalline products. In cold plasma intensified crystallization a nebulizer system sprays the solution aerosol into the plasma with the help of a carrier gas. The plasma simultaneously heats and charges the droplets causing fast solvent evaporation and Coulomb-fission to occur, after which nucleation and crystal growth commence within the small, confined volume offered by the small droplets. In this manner, nano-sized crystals of the energetic material RDX and the pharmaceutical niflumic acid were produced.     

Physicochemical Parameters of Nitramines Influencing Shock Sensitivity

TNO Prins Maurits Laboratory has actively followed and contributed to the research on the development of insensitive munitions (IM). One of the initial research topics at TNO focused on the improvement of the shape of RDX crystals and its relation to the shock sensitivity. The variation of crystal shape has been studied by crystallization from different solvents and/or by post‐treatment of the crystals. The role of the mean particle size on shock sensitivity was also included in these analyses. The decrease in shock sensitivity is even more pronounced when controlling the internal quality of crystals. In the meantime research has shifted to other energetic materials as well – in particular HMX and CL‐20 – in this way revealing step by step the important physicochemical parameters which play a role in determining the shock sensitivity of formulations containing these types of nitramines. Various characterization techniques, to determine the internal and external quality of crystals will be discussed, and their relation to shock sensitivity in PBXs will be shown. Two different grades of I‐RDX have been subjected to different characterization tests. The objective is to gain more understanding about which of the physicochemical parameters enables one to discriminate between a reduced sensitivity RDX and normal RDX.     

Preparation of Nano‐Structured RDX in a Silica Xerogel Matrix

RDX is preferred as explosive in munitions due to its balance of power and sensitivity that is known to be dependent on its particle size and size distribution. In this study, we prepared nano‐sized RDX in a silica xerogel matrix using a sol‐gel method and investigated its sensitivity for explosive properties. The presence of RDX in composite xerogel was confirmed by TG‐DSC and FTIR techniques. Microstructure and porosity were characterized by transmission electron microscopy (TEM), small angle X‐ray scattering, and N₂‐physisorption techniques. TEM results showed that the size of RDX particles in the RDX‐silica composites is in the range of 10–30 nm. The sensitivity to impact and friction was found to be higher for the composites compared to raw RDX. It was also found to be significantly dependent on the acetone/TMOS ratio used in the preparation.     

5‐Amino‐3,4‐dinitropyrazole as a Promising Energetic Material

The nitrogen‐rich energetic compound 5‐amino‐3,4‐dinitropyrazole (5‐ADP) was investigated using complementary experimental techniques. X‐ray diffraction indicates the strong intermolecular hydrogen bonding in 5‐ADP crystals. Compound exhibits low impact sensitivity (23 J) and insensitivity to friction. The activation energy of thermolysis determined to be 230±5 kJ mol⁻¹ from DSC measurements. Accelerating rate calorimetry indicates the lower thermal stability (173 °C) of 5‐ADP than that of RDX, which is probably the main concern about using this compound. 5‐ADP also exhibits good compatibility with common energetic materials (viz. TNT, RDX, ammonium perchlorate), including an active binder. The burning rate of 5‐ADP monopropellant is higher than that of benchmark HMX, while the pressure exponent 0.51±0.04 is surprisingly low. Addition of ammonium perchlorate does not affect the pressure exponent of 5‐ADP, while the burning rate increases. The 5‐amino‐3,4‐dinitropyrazole exhibits a notable combination of combustion performance, low sensitivity, and good compatibility, which renders it as a promising energetic material.     

Energetic Materials: Crystallization,Characterization and Insensitive Plastic Bonded Explosives

The product quality of energetic materials is predominantly determined by the crystallization process applied to produce these materials. It has been demonstrated in the past that the higher the product quality of the solid energetic ingredients, the less sensitive a plastic bonded explosive containing these energetic materials becomes. The application of submicron or nanometric energetic materials is generally considered to further decrease the sensitiveness of explosives. In order to assess the product quality of energetic materials, a range of analytical techniques is available. Recent attempts within the Reduced‐sensitivity RDX Round Robin (R4) have provided the EM community a better insight into these analytical techniques and in some cases a correlation between product quality and shock initiation of plastic bonded explosives containing (RS‐)RDX was identified, which would provide a possibility to discriminate between conventional and reduced sensitivity grades.     

Surface Coating of RDX with a Composite of TNT and an Energetic‐Polymer and its Safety Investigation

To improve the safety of RDX (hexogen), an energetic polymer (HP‐1) was introduced to coat RDX with 2,4,6‐trinitrotoluene (TNT) by combining the solvent–nonsolvent and the aqueous suspension‐melting method. Scanning electron microscope (SEM), transmission electron microscope (TEM), and X‐ray photoelectron spectrometry (XPS) were employed to characterize the samples, and the role of HP‐1 in the coating process was discussed. The impact sensitivity, friction sensitivity, and the thermal stability of unprocessed and coated RDX were investigated, and the explosion heat of samples was also estimated. Results indicate that HP‐1 improves the wetting ability of the liquid coating material on RDX surface and reinforces the connection between RDX and the coating material. By surface coating, the impact and friction sensitivity of RDX decrease obviously; the drop height (H₅₀) is increased from 37.2 to 58.4 cm, and the friction probability is reduced from 92 to 38%. The activation energy (E) and the self‐ignition temperature increase by 10457.38 J⋅mol⁻¹ and 1.8 K, respectively. The explosion heat is reduced merely by 0.93%.     

Nanostructuring of Pure and Composite‐Based K6 Formulations with Low Sensitivities

The aim of this work was to desensitize keto‐RDX, respectively 2‐oxo‐1,3,5‐trinitro‐1,3,5‐triazacyclohexane (K6). For this purpose, two different methods were employed. First, nano‐K6 was produced by means of the Spray Flash Evaporation process. Particles with a median size of 74 nm were obtained. Sensitivity to friction and electrostatic discharge were reduced by downscaling particle size of K6. Second, due to their molecular analogy, the mixing of K6 and RDX was studied. For that reason, a physical nanometric mixture of K6 and RDX was produced by the same technique. In the latter case, an inter‐particular synergy between both compounds was noticed but without forming a cocrystal. The median particle size of the mixture is about 82 nm, and its sensitivity is between the ones of raw nano‐materials concerning friction and electrostatic discharge. Moreover, the mixture is less sensitive to impact (3.03 J) than nano‐K6 (<1.56 J) and nano‐RDX (threshold is 2.0 J).     

Iodine Pentoxide Nano‐rods for High Density Energetic Materials

The iodine pentoxide is one of the most advanced oxidizers for nanostructured energetic formulations among the thermites due to the highest energy release per volume 25.7 kJ cm⁻³. The size and shape of iodine pentoxide particles have a strong impact on the pressurization rates during the reaction. Although micro‐sized iodine pentoxide particles are commercially available, nano‐sized particles, which are desired for various nano‐energetic applications, are not available on the market. Conventional wet chemical methods are unable to produce iodine pentoxide nanoparticles due to high solubility in water. In this study, we demonstrate fabrication of iodine pentoxide nano‐rods by high energy mechanical treatment of micro‐sized I₂O₅ particles. Tuning the energy dose in high‐energy ball milling is allowing to produce I₂O₅ nano‐rods with diameter of 50–100 nm and length of 300–600 nm. The produced nano‐rods exhibited 10 % smaller decomposition energy compared to the precursor of micro particles. The experiments showed that the nano‐energetic materials prepared with nano‐sized I₂O₅ rods have pressure discharge value of 43.4 MPa g⁻¹ which is two times higher than using commercial iodine pentoxide particles.     

Preparation and Properties Study of Core‐Shell CL‐20/TATB Composites

The insensitive high explosive 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) was selected for coating and desensitization of hexanitrohexaazaisowurtzitane (CL‐20), another high explosive, after surface modification. About 2 wt‐% polymer binder was adopted in the preparation process to further maintain the coating strength and fill the voids among energetic particles. The structure, sensitivity, polymorph properties, and thermal behavior of CL‐20/TATB by coating and physical mixing were studied. Scanning electron microscopy (SEM) and X‐ray photoelectron spectroscopy (XPS) results indicate that submicrometer‐sized TATB was compactly coated onto the CL‐20 surface with coverage close to 100 %. The core‐shell structure of CL‐20/TATB was confirmed by observation of hollow TATB shell from the CL‐20 core dissolved sample. X‐ray diffraction (XRD) analysis revealed that the polymorph of CL‐20 maintained ε form during the whole preparing process. Thermal properties were studied by thermogravimetry (TG) and differential scanning calorimeter (DSC), showing effects of TATB coating on the polymorph thermal stability and exothermic decomposition of CL‐20. Both the impact and friction sensitivities were markedly reduced due to the cushioning and lubricating effects of TATB shell. The preparation of explosive composites with core‐shell structure provides an efficient route for the desensitization of high explosives, such as CL‐20 in this study.     

Preparation and characterization of RDX based composite energetic materials with a cellulose matrix

Cyclotrimethylenetrinitramine (RDX)-based high-energy explosives are widely used in weapon warheads, propellants, and ammunition. Many studies have explored different supporting structures for RDX; however, the use of natural materials have rarely has been reported. Natural cellulose is widely known for its excellent compatibility and loading capacity. In this study, cellulose was used as a supporting structure and insensitive material for RDX composites. Cellulose/RDX composite aerogels (RCAs) were prepared using 1-allyl-3-methyl imidazole (AMIMCl) as the solvent, and their properties were characterized. The results show that the content of nitrogen in cellulose/RCAs was 34.5%, and the content of RDX was as high as 94.3%. Moreover, RDX particles were attached to the fibers inside the cellulose aerogels (CAs), forming a homogeneous protective layer on the surface of the cellulose matrix. Compared with the raw RDX material, the thermal stability of the cellulose/RDX energetic aerogels was greatly increased. The porosity of the CAs was reduced due to RDX particles growing inside the CAs. The impact sensitivity increased from 35 to 78 cm.     

Relationship Between RDX Properties and Sensitivity

An interlaboratory comparison of seven lots of commercially available RDX was conducted to determine what properties of the nitramine particles can be used to assess whether the RDX has relatively high or relatively low sensitivity. The materials chosen for the study were selected to give a range of HMX content, manufacturing process and reported shock sensitivity. The results of two different shock sensitivity tests conducted on a PBX made with the RDX lots in the study showed that there are measurable differences in the shock sensitivity of the PBXs, but the impact sensitivity for all of the lots is essentially the same. Impact sensitivity is not a good predictor of shock sensitivity for these types of RDX. Although most RDX that exhibits RS has low HMX content, that characteristic alone is not sufficient to guarantee low sensitivity. A range of additional analytical chemistry tests were conducted on the material; two of these (HPLC and DSC) are discussed within.     

Shock-sensitive cyclotrimethylenetrinitramine (RDX)

A form of RDX with a shock sensitivity comparable with that of PETN has been produced by sublimation. However, the material is as insensitive to heat, impact, friction and static discharge as are the normal forms of RDX, and it has a similar storage life. The shock sensitivity tests were performed with these explosives either cast in TNT or bonded in silicone rubber.     

Nanocomposites: An Ideal Coating Material to Reduce the Sensitivity of Hydrazinium Nitroformate (HNF)

Hydrazinium nitroformate (HNF), an energetic and eco‐friendly oxidizer, was desensitized successfully by changing its crystal size and shape and by coating with a nanocomposite. During crystallization, various methods were tried out which include mechanical stirring, ultrasound and using crystal shape modifiers (CSMs). Experimental results showed that the multi‐pronged approach to control the shape and size of the crystals by a careful choice of CSMs and crystallization method worked well to counter the preferential axial crystals growth, thus shifting the long needles to near cubical shape. Among various coating agents, a hydroxy‐terminated poly butadiene (HTPB)‐based clay nanocomposite was found to be most effective for reducing the friction and impact sensitivity of HNF. A Scanning Electron Microscope (SEM) coupled with Energy Dispersive Spectroscopic (EDS) analysis was found to be a very reliable and most useful technique to assess the extent of coating on HNF so as to achieve a consistent figure of insensitivity. The coating material was stable as well as compatible with the HNF crystals as revealed by thermal analysis.     

Nanomaterials for Heterogeneous Combustion

Nanophase materials and nanocomposites, characterized by an ultra fine grain size (less than 100 nm) have attracted wide spread interest in recent years by virtue of their unusual mechanical, electrical, optical, magnetic, and energetic properties. Studies have shown that the thermal behavior of nano‐scaled materials is quite different from micron‐sized powders. Nanosized metallic and explosive powders have been used as solid propellant and explosive mixtures to increase efficiency. At the same time recent studies reveal that the presence of nanosized metals in propellants does not necessary translate into an increased burning rate and burning temperature. The reasons of this effect are far from being clear. This paper presents a new approach to the production of nanocomposites of some energetic materials – ammonium nitrite, cyclotrimethylene trinitramine (RDX), and aluminum – by the vacuum co‐deposition technique. The thermal behavior of the synthesized nanopowder and nanocomposites is investigated. A substantial difference in burning rate of RDX nanopowder has been found in comparison to micron‐sized material. Experimental results allow investigating the effects of nanosized materials on the combustion characteristics.     

Synthesis and Characterization of 4,4′,5,5′‐Tetranitro‐2,2′‐Bi‐1H‐imidazole (TNBI)

We synthesized 4,4′,5,5′‐tetranitro‐2,2′‐bi‐1H‐imidazole (TNBI), which may serve as a new energetic filler for high explosive formulations. TNBI was synthesized by treating an excess amount of sodium nitrate with 2,2′‐bi‐1H‐imidazole (BI), which was produced from glyoxal and ammonia gas. The overall synthetic yield was 32%. The synthesized TNBI was characterized by performing various chemical analyses including NMR, IR, and CHN analyses. Small scale sensitivity tests were carried out at both research institutes (ADD and ARDEC). The sensitivity results varied from ‘more sensitive than RDX’ to ‘substantially less sensitive than RDX’ according to the purity and conditions of the test samples. Based on our careful characterizations, this large variation in sensitivity was attributed to the moisture content that was present in the test samples due to a hygroscopic nature of TNBI. We also found that the hygroscopic nature of TNBI changed significantly due to the amount of impurities, especially sulfates.