Research on the Size of Defects inside RDX/HMX Crystal and Shock Sensitivity

Intragranular defects inside RDX/HMX were studied by optical microscopy with matching refractive (OMS), sink‐float method (SFM), and micro‐focus CT (μCT) techniques. OMS results revealed the phenomenon that RDX/HMX had more defects and cracks than RS‐RDX/RS‐HMX. μCT results indicated that RDX/HMX had more defects with larger volume than RS‐RDX/RS‐HMX. The gap test showed that critical shock pressure/gap thickness was 6.4 GPa/19.4 mm for PBX based on RDX, while they were 7.5 GPa/17.5 mm and 8.6 GPa/16.2 mm for PBX based on M‐RDX and RS‐RDX, respectively. Meanwhile, an analysis of the relationship between defects inside RDX/HMX crystal and shock sensitivity was made. Finally, the shock pressure response under impact loading was investigated by discrete element method.     

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.     

Shock Sensitivity of Pressed RDX‐Based Plastic Bonded Explosives under Short‐Duration and High‐Pressure Impact Tests

The shock sensitivities of plastic bonded explosives were studied with a thin flyer impact test by using two types of pressed RDX. The thin flyer, driven by an electrically exploding plasma, exerts a short‐duration, high‐pressure pulse to the samples to trigger a shock‐to‐detonation process. It was found that the duration and magnitude of the incident shock strongly influence the dominant mode of hot‐spot formation, promoting a fast pore collapsing mechanism while suppressing other slower shear or friction mechanisms, as proposed by Chakravarty et al. [1]. The pressed PBX based on reduced sensitivity RDX had higher shock threshold pressure, compared to the pressed PBX based on commercial RDX. The difference was observed even with a certain portion of external extragranular defects. It is postulated that the internal crystal defects are more efficient than the external porosity in terms of the rapid reaction of hot spots.     

Preparation and Characterization of Nanoenergetics Based Composition B

Explosive compositions employing nanoscale crystals of high explosives (i. e., nanoenergetics) have demonstrated reduced sensitivities to external stimuli. Until recently, the investigated formulations were limited to plastic bonded explosives. Explosives that are normally melt‐cast also would benefit from the use of nanoenergetics. However, the integration of nanoenergetics into the melt‐cast process is challenging due to the large surface area and solubility associated with nanoenergetics. In this work, we explored the preparation of nanoenergetics‐based Composition B (Comp B), a widely used melt‐cast explosive, by spray drying followed by mechanical compaction. The Comp B molding powder obtained from spray drying was characterized by scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The structure and the shock sensitivity of the compacted nanoenergetics‐based Comp B (N‐Comp B), both as‐prepared and thermally cycled, was also studied using melt‐cast Comp B as the reference material. The characterization shows that N‐Comp B consisting of nanoscale cyclotrimethylenetrinitramine (RDX) and trinitrotoluene (TNT) contains mostly nanoscale voids but has a large number density. Reduced shock sensitivity was observed from N‐Comp B, attributed to the elimination of large voids. But the decrease seems to have been constrained by the large number density of voids. Thermal cycling induced significant structural change, i. e., the increase of both void size and the crystal size, causing an increase in sensitivity. Procedures are proposed to further reduce the sensitivity and enhance the thermal stability of N‐Comp B.     

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.     

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.     

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 and Properties of a nRDX‐based Propellant

The incorporation of nano‐scaled cyclotrimethylene trinitramine (nRDX) in nitrocellulose (NC)‐based propellants poses processing problems when following conventional methods. Hence, a new preparation method containing a pre‐dispersion process was developed, by which 30 mass % RDX (290 nm) was incorporated in the propellant. Meanwhile, the corresponding 290 nm, 12.85 μm and 97.76 μm RDX‐based propellants were prepared for comparison using a conventional method. The morphology, structure, ballistic and mechanical properties of the prepared propellants were characterized by scanning electron microscopy (SEM), density analyzer, closed vessel (CV), uniaxial tensile tester and impact tester. The results indicate that the nRDX particles were uniformly dispersed in the NC/NG/TEGDN matrix using the novel method, while agglomerated and recrystallized into large particles with the conventional method. The propellant density increased with decreasing RDX particle size. In particular, the 290 nm RDX‐based propellant exhibited a higher burning rate and lower average pressure exponent (α =0.958) compared to the 12.85 μm RDX‐based propellant (α =1.043). The tensile strength, elongation at break and impact strength of the RDX‐based propellant at −40 °C, 20 °C and 50 °C were dramatically improved by using 290 nm RDX with the novel method.     

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).     

Studies on Advanced RDX/TATB Based Low Vulnerable Sheet Explosives with HTPB Binder

Hydroxyl‐terminated polybutadiene (HTPB) based sheet explosives incorporating insensitive 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) as a part replacement of cyclotrimethylene trinitramine (RDX) have been prepared during this work. The effect of incorporation of TATB on physical, thermal, and sensitivity behavior as well as initiation by small and high caliber shaped charges has been determined. Composition containing 85% dioctyl phthalate (DOP) coated RDX and 15% HTPB binder was taken as control. The incorporation of 10–20% TATB at the cost of RDX led to a remarkable increase in density (1.43→1.49 g cm⁻³) and tensile strength (10→15 kg cm⁻²) compared to the control composition RDX/HTPB(85/15). RDX/TATB/HTPB based compositions were found less vulnerable to shock stimuli. Shock sensitivity was found to be of the order of 20.0–29.2 GPa as against 18.0 GPa for control composition whereas their energetics in terms of velocity of detonation (VOD) were altered marginally. Differential scanning calorimeter (DSC) and thermogravimetry (TG) studies brought out that compositions undergo major decomposition in the temperature region of 170–240 °C.     

RDX and HMX with Reduced Sensitivity Towards Shock Initiation–RS‐RDX and RS‐HMX

Reduced Sensitivity RDX (RS‐RDX) has received a lot of attention and interest from the explosive community in the recent years. There are several producers of RS‐RDX, most of them using a direct nitration (Woolwich process) for the RDX synthesis, while Chemring Nobel uses the Bachmann process. The processes for obtaining the RS properties probably differ between the various producers. Chemring Nobel has also developed an HMX quality that shows Reduced Sensitivity (RS‐HMX) of different particle size distributions. The shock sensitivity is at the same level as for RS‐RDX in comparable compositions. Reduced shock sensitivity has been obtained for RS‐RDX and Reduced Sensitivity (RS‐HMX) in both pressable and cast‐cured compositions. By using a pressable composition, it is possible to get the results from a BICT gap test faster than from a cast‐cured composition that has to go through a curing process. Chemring Nobel in cooperation with FFI have performed an extensive accelerated ageing testing of RS‐RDX produced by the Bachmann process. The samples have been aged at 60 and 70 °C and the shock sensitivity tested by two different gap tests. The results demonstrate that the Chemring Nobel RS‐RDX retain the insensitivity towards shock during ageing and show no degradation at all. Accelerated ageing testing of RS‐HMX has also been performed and shows no degradation in the shock sensitivity.     

Preparation and Properties of 2,6‐Diamino‐3,5‐dinitropyrazine‐1‐oxide based Nanocomposites

With estane as binder, a new nanocomposite energetic material based on 2,6‐diamino‐3,5‐dinitropyrazine‐1‐oxide (LLM‐105) was successfully prepared by the spray drying method. Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and X‐ray diffraction (XRD) was employed to characterize the nanocomposite samples. The impact sensitivity and thermal decomposition properties of the nanocomposites were also measured and analyzed. The results show that the nanocomposite particles are spherical in shape and range from 1 μm to 10 μm in size. The composite is aggregated of many tiny granules with nucleus/shell structure, in which the shell thickness and crystal size of LLM‐105 are about 20 nm and 50–100 nm. The crystal type of LLM‐105 in the nanocomposite is similar to that of raw LLM‐105, however, the diffraction peaks become weaker and wider mainly due to decreasing of particle size. The nanocomposite has lower impact sensitivity and better thermal stability.     

Reduced Sensitivity RDX Obtained From Bachmann RDX

In recent years much interest has been generated in a quality of reduced sensitivity RDX (RS‐RDX), like I‐RDX^® which, when incorporated in cast cure and even pressable plastic bonded explosives (PBX compositions), can confer reduced shock sensitivity as measured through gap test. At crystal level, lot of work has been done to try to determine which property or properties may explain the behaviour of the corresponding cast PBX composition. But up to now, and despite an international inter‐laboratory comparison (Round Robin) of seven lots of RDX from five different manufacturers conducted from 2003 to 2005, even if some techniques lead to interesting results, there is no dedicated specification to apply to RS‐RDX. This quality (I‐RDX^®) has proved to retain its low sensitivity even after ageing, which does not seem to be the case for standard RDX produced by the Bachmann process (when re‐crystallized under I‐RDX conditions in order to obtain RS‐RDX). It has been shown that the higher sensitivity of RDX produced by the Bachmann process, or the evolution of sensitivity after ageing of RS‐RDX produced from Bachmann RDX may be linked to the presence of octogen (HMX) during the crystallization process. In order to check such hypothesis, low HMX content RDX produced by the Bachmann process has been prepared and evaluated in cast PBX composition (PBX N 109). Results of the characterization of such quality of RDX and its evaluation in cast PBX composition as well as ageing behaviour are presented and discussed; there are indications that removal of HMX from Bachmann RDX may lead to RS‐RDX, which retains its RS character even after ageing.     

Investigation of CL‐20 and RDX Nanocomposites

Nanocrystalline explosives offer a number of advantages in comparison to conventional energetics including reduced sensitivity and improved mechanical properties. In this study, formulations consisting of 90 % hexanitro‐hexaazaisowurtzitane (CL‐20) or cyclotrimethylene trinitramine (RDX) and 10 % polyvinyl alcohol (PVOH) were prepared with mean crystal sizes ranging from 200 nm to 2 μm. The process to create these materials used a combination of aqueous mechanical crystal size reduction and spray drying. The basic physical characteristics of these formulations were determined using a variety of techniques, including scanning electron microscopy, X‐ray diffraction, and Raman spectroscopy. Compressive stress‐strain tests on pressed pellets revealed that the mechanical properties of the compositions improved with decreasing crystal size, consistent with Hall‐Petch mechanics. In the most extreme case (involving CL‐20/PVOH formulations), crystal size reduction from 2 μm to 300 nm improved compressive strength and Young’s modulus by 126 % and 61 %, respectively. These results serve to highlight the relevance of structure‐property relationships in explosive compositions, and particularly elucidate the substantial benefits of reducing the high explosive crystal size to nanoscale dimensions.     

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.     

Investigation of HMX‐Based Nanocomposites

Advanced munition systems require explosives which are more insensitive, powerful, and reactive. For this reason, nano‐crystalline explosives present an attractive alternative to conventional energetics. In this study, formulations consisting of 95 % octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) and 5 % polyvinyl alcohol (PVOH) were prepared with mean crystal sizes ranging from 300 nm to 2 μm. The process to create these materials used a combination of mechanical particle size reduction and spray drying, which has the advantages of direct control of crystal size and morphology as well as the elimination of ripening of crystals (which occurs during slurry coating of nanomaterials). The basic physical characteristics of these formulations were determined using a variety of techniques, including scanning electron microscopy and X‐ray diffraction. Compressive stress‐strain tests on pressed pellets revealed that the mechanical properties of the compositions improved with decreasing crystal size, consistent with Hall‐Petch mechanics. The 300 nm HMX/PVOH composition demonstrated a 99 % and 129 % greater strength and stiffness, respectively, than the composition with 2 μm HMX. The formulations were subjected to the Small Scale Gap Test, revealing a significant reduction in shock sensitivity with decreasing crystal size. The formulation containing 300 nm HMX registered a shock initiation pressure 1.6 GPa above that of the formulation with 2 μm HMX, a 44 % improvement in sensitivity. These results serve to highlight the relevance of structure‐property relationships in explosive compositions, and particularly elucidate the substantial benefits of reducing the high explosive crystal size to nano‐scale dimensions.     

Investigation of Crystal Morphology and Shock Sensitivity of Cyclotrimethylenetrinitramine Suspension by Rheology

Crystal morphology and shock sensitivity of a series of cyclotrimethylenetrinitramine (RDX) particles suspended from ethylene glycol were investigated. Flow rheology was employed to measure the rheological properties of the suspensions at constant temperature; it was observed that the stress‐shear rate and viscosity behavior of the suspensions were controlled by the particle morphology. The viscosity of the RDX suspensions changed with the roundness/smoothness of RDX crystals at all applied shear rates. The suspensions containing crystals with smoother morphology showed reduced viscosity. When the viscosity data was compared to the shock sensitivity results from the RS‐RDX Round Robin study, a good correlation was obtained. This study has validated the use of flow rheology to indicate the morphology and shock sensitivity of crystalline particles.     

Characterization of Granular and Polymer‐Embedded RDX Grades: Floret Tests

In an attempt to further contribute to the characterization of explosive compositions, small scale Floret tests were performed using four RDX grades, differing in product quality. A Floret test provides a measure – by indentation of a copper block – of detonation spreading or the initiability and shock wave divergence and is applied in particular to explosives used in initiation trains. Both as‐received RDX and PBXs (based on the AFX‐757 composition, a hard target penetrator explosive) containing these RDX grades were tested in the Floret test set‐up. It was found that the Floret test method, when applied to granular, as‐received RDX, was not able to discriminate between the overall RDX product qualities on the basis of the resulting volume of the indentation in the copper block. For the Floret test data of the PBX samples, a division into two parts, where one of the RDX lots shows a lower dent volume compared to the other RDX lots tested. Based on the results presented in this paper with granular RDX and a PBX composition and earlier results with a different type of PBX (based on PBXN‐109, an insensitive high explosive used in a wide range of munitions), the Floret test could be developed into a screening test for shock sensitivity and product quality, without the need for complex and large volume casting of specific PBX compositions.     

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.