In this paper, a selection of the results obtained on the crystallization of the energetic materials RDX, HMX, and CL-20 will be briefly reported. Furthermore, the shock sensitivity of these explosives, when incorporated in a so-called plastic bonded explosive (PBX), will be discussed in more detail. One of the most important results is a direct correlation between the mean density of the energetic material and the shock sensitivity of the PBX containing this explosive. This implies that, similar to many other solid materials, the ability to control the product quality is also one of the major key factors playing a role during the crystallization of these energetic materials.

A method is described for determining the solubility of multicomponent crystalline compounds from clear points upon sample dilution at a constant temperature. Clear points are established by continuously adding a solvent mixture to a suspension of known composition until a clear solution appears. For validation, this solvent addition method is compared to the traditional equilibrium concentration method at constant temperature and the more recent temperature variation method with which clear point temperatures are determined upon increasing the sample temperature. Solubility data of binary systems (1 solute, 1 solvent) measured using the solvent addition method are obtained relatively quickly compared to the equilibrium concentration method. These solubility data are consistent with those of the temperature variation and the equilibrium concentration method. For the temperature variation method, the results are dependent on the heating rate. Likewise, for the solvent addition method, they are dependent on the addition rate. Additionally, for ternary systems involving antisolvent or cocrystals, solubilities are determined at a constant temperature using the solvent addition method. The use of the solvent addition method is especially valuable in the case of solvent mixtures and other complex multicomponent systems, in which the temperature variation method cannot be applied easily. ■ INTRODUCTIONIn production often a crystallization step is required for purification and final crystalline particulate product formation. 1,2 The solubility or phase diagram of such compounds is essential information for efficient and reliable crystallization process design and operation. 3−6 The phase diagram indicates the most stable phases at specific compositional and temperature conditions, 1,4−6 determines the achievable yield, 7 and enables the monitoring of the supersaturation during the crystallization process. 7,8 Traditionally the solubility is measured through equilibration of a suspension. 1 The solubility is then equal to the concentration in the equilibrated solution, which can be sampled and determined by, for example, a gravimetric method or HPLC (Figure 1 (left)). Although the Equilibrium Concentration (EqC) method is widely accepted and considered accurate, 1 it is laborious and time-consuming. Currently, commercial equipment from various suppliers is available that streamlines measurements through a temperature variation (TV) method in which clear points are measured. 9−11 In the TV method the solubility is changed by changing the temperature, until it matches the concentration. A clear point is then the temperature at which, upon increasing the temperature, a suspension turns into a clear solution. Figure 1 (center) shows the principle of a clear point measurement using the TV method. If the heating rate is sufficiently small, the crystal dissolution rate is fast and the clear point can be assumed to be equal to the saturation temperature. 10 This TV method is much less labor intensive, is much faster, and allows f...

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Nano-and submicron-sized crystals are too small to contain inclusions and are, therefore, expected to have a higher internal quality compared to conventionally sized particles (several tens to hundreds of microns). Using electrospray crystallization, nano-and submicron-sized crystals can be easily produced. With the aid of electrospray crystallization, a mist of ultrafine solution droplets is generated and subsequent solvent evaporation leads to crystallization of submicron-sized crystals. Using cyclotrimethylene trinitramine (RDX) solutions in acetone, the conditions for a stable and continuous jet were established. At relatively small nozzle diameters and relatively low potential differences, hollow spheres of RDX crystals were observed. At a higher nozzle diameter and potential difference and in the region of a continuous jet, RDX crystals with an average size of around 400 nm could be produced. In order to test the quality of the submicron-sized energetic material, impact and friction sensitivity tests were carried out. The test results indicate that the submicron-sized product had reduced friction sensitivity, indicating a higher internal quality of the crystalline product.

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.

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Nonphotochemical laser-induced nucleation (NPLIN) is a promising primary nucleation control method, yet its underlying mechanism remains elusive. To contribute to the discussion on whether the polarization of laser irradiation in NPLIN experiments influences the polymorphic outcome, we revisit NPLIN experiments with aqueous glycine solutions with supersaturations ranging between S = 1.5 and S = 1.7 irradiated by nanosecond pulses (∼7 ns) of near-infrared wavelength (1064 nm). Systematically altering laser light excitation properties, including the number of pulses and type of polarization, we quantified the nucleation kinetics and characterized the polymorphic form that crystallized upon laser irradiation. Due to the stochasticity of the nucleation process, a large number of samples (>100 per each experimental point) were studied under carefully controlled experimental conditions such as the ambient temperature, cooling rate, and aging period. We observed significant differences among laser-irradiated, spontaneously nucleated, and crash-cooled samples in terms of nucleation kinetics and polymorphic form. This result indicates that laser irradiation provides a different polymorph-forming pathway in comparison to crash-cooling and spontaneous nucleation. However, no clear dependence between the polymorphic form and the polarization of laser irradiation is observed. We discuss our results in the context of previous reports supported thorough quantification of sample heating in NPLIN experiments.

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.