Spherical 3-nitro-1,2,4-triazol-5-one (NTO) based melt-cast compositions: heralding a new era of shock insensitive energetic materials

Abstract

3-Nitro-1,2,4-triazol-5-one (NTO) is an unique candidate among military high explosives and is explored as a potential bomb filler with TNT in melt cast formulations. In the present study, we attempted to replace sensitive RDX with NTO with spherical morphology to develop a less hazardous, thermally stable shock insensitive composition. A bimodal mixture (150 and 25 μm in 70/30 ratio) of spherical NTO powders with superior flowability was chosen for the formulations and achieved 60% solid loading. The temperature sensitivity of the formulations was assessed by calculating the activation energy for the flow. The velocity of detonation and shock sensitivity of the composition were also determined. The study demonstrated that the spherical-NTO/TNT (60 : 40) was found to be 2.5 times more shock and 2 times more friction insensitive than composition B which consisted of RDX/TNT (60 : 40). The activation energy for thermal decomposition was determined to assess the thermal hazards and a vacuum stability test was carried out to ensure the storage life of the composition.

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1. Introduction

Energetic materials are inherently hazardous in nature and sensitive to numerous hazard stimuli leading to accidents involving explosive filled munitions which have come at a high cost in terms of weapons platforms, personnel and materials. The intention of this research work on energetic materials is to develop safer, less hazardous and high or moderate performance melt-cast formulations compared to composition B which typically consists of 60% RDX and 40% TNT. Composition B has been universally employed for over 60 years in munitions in several anti-armor warheads and shells as an explosive filler due to its high performance, ready availability and ease of processability. The high sensitivity to various stimuli and catastrophic explosions associated with this composition limit the realization of shock insensitive munitions.¹ Alternatively, in order to meet insensitive munitions (IM) requirements, shock sensitive RDX can be replaced in the existing formulations. The use of intrinsically insensitive candidates such as TATB, NTO, and FOX-7 etc., which are stabilized by extensive inter- and intra-molecular hydrogen bonding are promising replacements for RDX.²

3-Nitro-1,2,4-triazol-5-one (NTO) is a unique candidate among military high explosives for effective insensitive munitions and a potential bomb filler in the admixture with TNT under investigation.³ Various NTO based melt-cast formulations such as Picatinny Arsenal Explosives (PAX), Ordnance System Explosives (OSX) and Insensitive Munitions Explosives (IMX)-101 (containing DNAN, NTO and NQ) and IMX-104 (containing 40% DNAN or TNT, 40% NTO and 20% RDX) etc., have been reported using TNT or DNAN as a binder.⁴ Among these, the IMX-104 composition involves the part replacement of RDX and the VOD was determined to be 7190 ± 200 m s⁻¹ and 7410 ± 100 m s⁻¹ for DNAN and TNT respectively. Accordingly the impact sensitivity of the TNT based composition is higher than the DNAN based composition. Further, the biodegradation and phytoremediation of IMX-101 formulations were also investigated by Richard et al.^5,6 It is also inferred from the toxicological studies that NTO is non-toxic compared to RDX and TNT, and Table S1 of ESI† lists the LD₅₀ values of the studied explosives. Cliff and Smith et al. developed a RDX-free ARX-4002 melt-cast formulation consisting of NTO/TNT (50 : 50). The lower amount of solid explosive (NTO) in the reported formulation may be due to the usage of non-spherical NTO and also the solid loading can be significantly increased by the use of spherical NTO.⁷ Large scale preparation of NTO involves the crystallisation of NTO from water and this yields irregular rods and jagged crystals.^8,9 This irregular and undesired crystal morphology leads to high viscosity, poor processability and hence reduced solid loading. Our previous studies reported the crystallisation process for the preparation of spherical-NTO (SNTO) of various particle sizes and the characterisation of the prepared powders. It was demonstrated that the use of spherical-NTO improves the mix fluidity of the composite explosives significantly and hence the solid explosive loading.^10,11 Further, it was also revealed that spherical crystals of explosives can improve their insensitivity towards a sudden shock, performance, processability and packing density compared to non-spherical crystals.^12–14

The present study is aimed to develop less hazardous shock insensitive melt-cast explosive formulations based on NTO by complete replacement of RDX (60%) in composition B. In order to achieve higher solid loading, spherical NTO with good flow characteristics was identified and employed in the formulations. An anchor blade mixer was used for processing and the explosive charges were made using suitable moulds. Performance and hazard assessments like shock, friction and impact sensitivities were carried out using these charges. The study also reports the activation energy of the thermal decomposition of SNTO/TNT and RDX/TNT (60 : 40) melt-cast formulations using Ozawa and Kissinger methods to determine the thermal hazards of compositions. A thermogravimetric study and vacuum stability tests have been carried out to asses the storage stability of the compositions.

2. Experimental

2.1 Materials and methods

All the reagents and chemicals used in the present study were of AR grade and used as such without any purification. Spherical NTO of specific particle size was prepared in-house using a cooling crystallization process from non-spherical NTO.¹⁰ The bimodal mixture was made from 150 and 25 μm powders in 70 : 30 weight ratio.

Calorimetric studies were undertaken on a Perkin Elmer DSC-7 instrument at four different heating rates 2, 5, 10 and 15 °C min⁻¹ under nitrogen atmosphere with 1 mg of sample. The Ozawa and Kissinger method was employed to calculate the activation energy (E_a) of the thermal decomposition reactions.^15–18 Thermogravimetric analysis (TGA) was carried out using a simultaneous thermal analyzer (TA instruments SDTQ600) at 5 °C min⁻¹ under a nitrogen atmosphere using an open platinum cup. Morphological characterization was carried out using an Optical Microscope (RAX-vision Y-coo series). The extent of sphericity is expressed by means of circularity which was determined by the following formula: (4πarea)/(perimeter²).

Particle size distribution was measured with a Sympatech particle size analyzer in dry mode by applying a laser diffraction method and volume mean dia, D[4,3] is reported in the present study. The true density was determined by measuring the changes in pressure with helium gas displacement using a gas pycnometer of Thermo Scientific Instruments. USP-II standard procedure was adopted to measure the tapped bulk density with the defined set of the tapping procedure using Veego Industries, India Tap density apparatus. Flowability properties like Carr’s index (CI) and Hausner ratio (HR) were also calculated from the above measurements. All viscosity measurements were made using a Brookfield viscometer (RV-DVII + Pro) equipped with a small sample adapter (SC4-27) where shear rate and stress can be measured. The sample was placed in a chamber which was heated through a circulator. All measurements were made at 81, 83, 86 and 90 °C and at the shear rates varied from 17 to 59.5 s⁻¹.

In order to determine the chemical and thermal stability under extreme conditions and also to verify the compatibility among the explosives in the melt-cast compositions, a vacuum stability tester (Tirupati scientific industry, Calcutta) was used. A dried and weighed sample (5 g) of both RDX/TNT (60 : 40) and SNTO/TNT (60 : 40) was heated at 120 °C for 48 hours, the volume of the gases (mL g⁻¹) evolved was recorded and the experiments were repeated for consistency.

2.2 Preparation of melt-cast formulations

The compositions were processed by the standard, melt-cast technique involving the addition of bimodal spherical NTO to molten TNT under continuous stirring in a steam jacketed 7 liter anchor blade mixer at a temperature of 95 °C (Fig. 1). The speeds of the anchor blade and side propeller were kept at 50 and 70 rpm respectively. The mixture was stirred for about 20–25 min and then transferred to a suitable mould. The inner diameter of the mould in which the casting of the VOD charge was carried out was 35 mm and length of the mould was 300 mm. After cooling to ambient conditions, the charge was removed from the mould and machined to the required dimensions. The inner and outer diameters of the mild steel shock sensitivity tube which contained the main acceptor charge were 44 and 55 mm respectively.

Fig. 1 Steam jacketed anchor blade mixer.

2.3 Determination of sensitivity characteristics

The sensitivity and performance of the melt-cast compositions have been determined using standard methods. Using the fall-hammer method with a 2 kg drop weight and by employing the Bruceton staircase approach, the impact sensitivity of the explosive compositions was determined. The results are expressed statistically in terms of 50% probability of explosion (h_50%). Julius Peter’s apparatus was used to determine the friction sensitivity and measurements were carried out in duplicate for confirmation.

The shock sensitivity was determined with the standard card gap test, using a cellulose acetate sheet as an attenuator and a CE pellet (tetryl) as a donor charge. The sheet thickness of cellulose acetate was varied until No-Go was observed on the witness plate while carrying out experiments with the RDX and SNTO charges. The shock sensitivity of the melt cast explosive composition is expressed in terms of the minimum pressure of the shock wave which can initiate detonation. The critical pressure (P in kbar) developed across the cellulose acetate sheets which can detonate the explosive composition with 50% probability was determined from the following equation:

P (kbar) = 105e^−(0.0358x)

where ‘P’ is the critical pressure in kbar and ‘x’ is the thickness of the cellulose acetate sheet as an attenuator in mm. The repeatability of the shock sensitivity results were confirmed with consistent No-Go observations.

2.4 Determination of velocity of detonation (VOD)

The velocity of detonation is the performance parameter in melt-cast formulations and it was determined by employing a pin ionization probe technique. Explosive charges were pre-inserted with pin type ionization probes which were twisted with enamel copper wire at predetermined points and made ready for detecting the arrival time of the detonation wave. An oscilloscope instrument (make: YOKOGAWA DL9140, 1 GHz) was used for data acquisition. Three charges of each formulation were subjected to the test and fired for the determination of the velocity of detonation. VOD is reported after three firings of each composition and expressed as an average of three trials.

3 Results and discussion

3.1 General characterization and selection of powders for compositions

The particle size and its distribution are essential and play an important role in the field of high explosives and rocket propellant formulations. The size distribution of particles is well understood with the help of span. There are several measures of absolute width one can derive from a given cumulative distribution. One common measure is the span, D₉₀ − D₁₀. A dimensionless measure of width is the relative span defined as span/D₅₀. The narrower a distribution is the more closely the absolute measures of width approach zero. Optical microscopic images of spherical-NTO with 150 and 25 μm particle sizes obtained from the controlled cooling crystallization with water and NMP as the solvent system are shown in Fig. 2. Table 1 presents the particle size and span data. It is 0.63 and 0.69 for spherical-NTO of 150 and 25 μm respectively which indicates the narrow particle size distribution. Fig. 3 shows the morphology of RDX (185 μm) and the span (0.73) which was observed to be a relatively broad distribution. Similar to the particle size, shape is also a fundamental property of the material in powder technology which affects many processing parameters in final formulations. In order to measure the shape of a particle, circularity is a measure of the sphericity of particles. Circular objects will have a circularity of 1 while other shapes will have less than 1 and deviation from one gives the degree of sphericity. Spherical-NTO with D(4,3) 25 and 150 μm was found to have a circularity between 0.91 to 0.86 which indicates that the particles are very close to a spherical shape.

Fig. 2 Optical microscopic images of spherical-NTO 150 and 25 μm.

Table 1 Particle size and its distribution of RDX and NTO

Product	D(4,3)	Span^a
a Span = (D90 − D10)/(2D50).
RDX	185 μm	0.73
Spherical NTO	150 μm	0.63
Spherical NTO	25 μm	0.69

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Fig. 3 Optical microscopic image of RDX.

It is important to control the size distribution of particles in attaining high density and also to achieve the maximum packing density in the formulations which further increases the amount of solids per unit volume.^19,20 Further, the size along with the shape of the particle plays a key role in obtaining the flowability of the powders. Our earlier studies on spherical NTO powders demonstrate the flowability parameters such as Carr’s Index (Compressibility Index) and Hausner ratio (HR) which are the simple measurements used to describe the complete flow properties of a material.^21–24 To attain the specific density many combinations of distribution are preferable. In order to achieve efficient packing various ratios of coarse (150 μm) to fine (25 μm) powders were screened and based on the high density and flowability, a 70 : 30 ratio (coarse to fine) was chosen, analysed for particle size and further used for the formulation studies (Fig. 4). The optimized bimodal mixture obtained maximum tapped bulk density i.e.1.09 g cc⁻¹ with a Carr’s index less than 15 and Hausner ratio less than 2 (Fig. 5). True density is a fundamental parameter contributing to the characterization of a product, directly proportional to the performance of an explosive and also helps to identify different polymorphs of a particular molecule.²⁵ The true densities of virgin explosives were determined and are presented in Fig. 5. The combination of the bimodal mixture of SNTO (70 : 30) resulted to give 1.892 g cc⁻¹ which is in between that of SNTO 150 μm (100%) and SNTO 25 μm (100%) indicating that efficient packing has occurred.

Fig. 4 Particle size analysis of the bimodal mixture of SNTO (70 : 30).

Fig. 5 Flowability parameters of virgin SNTO and the bimodal mixture (70 : 30).

3.2 Physical and flow characteristics of the composition

In the case of melt cast formulations, pourability is the deciding processability criterion and our earlier studies confirmed the superior flowability of NTO/TNT over the benchmark RDX/TNT composition.²⁶ The physical appearance of the prepared melt-cast compositions of SNTO/TNT and RDX/TNT (60 : 40) was found to be smooth. The void free solid blocks were further witnessed by true density measurements as presented in Table 2 and these compositions have been used for further detail characterization. The true density of the SNTO/TNT (60 : 40) is higher than the composition B.

Table 2 True density of virgin explosives as well as melt-cast compositions

Composition	True density (g cc^−1)
RDX (100%)	1.798
TNT (100%)	1.616
RDX/TNT (60 : 40)	1.717
SNTO/TNT (60 : 40)	1.741

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The rheological behaviour of a material is greatly affected by the temperature and the precise control of temperature is of major importance in viscosity measurements. It is also vital for safety during handling and production. Our previous study reported the temperature dependent flow phenomenon of melt-cast formulations which can be described by the Arrhenius equation of ideally viscous materials.²⁶ It was observed that, the SNTO/TNT (60 : 40) composition exhibited temperature independent behaviour (Fig. 6) (no significant variation in viscosity). A viscous flow phenomenon involves a thermally activated rate process and in order to move molecules to an adjacent vacant site they must overcome an energy barrier. By applying the Arrhenius relationship, the activation energy for flow has been obtained. Fig. 7 compares the activation energy for the flow of the compositions. The RDX/TNT based composition requires about a three times higher activation energy for flow than SNTO/TNT (60/40). This clearly indicates the role of chemical composition and the nature of the material in the activation energy for flow and also indicates the relative temperature susceptibility of the different compositions. From this study, it can be inferred that the SNTO/TNT composition was found to be insensitive to temperature and hence, processing can be done nearly at the melting temperature of TNT. In contrast to SNTO, the dependency is high in the case of the RDX based benchmark composition, which demands a higher processing temperature.

Fig. 6 Temperature dependant flow behaviour of melt-cast compositions at a shear rate of 59.5 s⁻¹.

Fig. 7 Activation energy for flow of melt-cast compositions.

Sedimentation of a solid in any liquid matrix plays a crucial role especially in melt-cast compositions. The sedimentation rate of the solid explosive was studied for both compositions at 86 °C under a shear rate of 59.5 s⁻¹ for a period of 1 h. Viscosity was noted at an interval of 5 min and a plot of time versus viscosity is shown in Fig. 8. The study reveals that the rate of increase of viscosity is high for the RDX based composition and it may be due to the non-uniform distribution of RDX. The increase in viscosity is low in the case of SNTO/TNT (60 : 40) and it is mainly due to the stronger interaction of NTO and molten TNT which kept the dispersion more stable and hence gave a lower sedimentation rate.

Fig. 8 Plot of time versus viscosity of RDX/TNT and SNTO/TNT compositions.

3.3 Hazard assessment and vacuum stability studies

3.3.1 Thermal assessment of SNTO/TNT and RDX/TNT (60 : 40) compositions. The importance of understanding the thermal behaviour of explosives is prime in the explosive field for handling and safety during the manufacture of compositions. In order to review the thermal behaviour of the final compositions, a decomposition study was carried out with a differential scanning calorimeter and the maximum decomposition temperature (T_max) of both compositions at a heating rate of 10 °C min⁻¹ is reported in Table 3. It is clearly understood from the data that the spherical NTO based composition is thermally more stable than composition B. The activation energy for the melt-cast compositions, RDX/TNT (60 : 40) and SNTO/TNT (60 : 40) has been determined using the generated data at different heating rates 2, 5, 10 and 15 °C min⁻¹ (Fig. S1 and S2†). The profiles of the thermal curves of the RDX/TNT (60 : 40) and SNTO/TNT (60 : 40) compositions were similar. Notably, in all the thermal experiments, both samples exhibited melting of TNT initially at about 78 °C, while an exothermic peak was observed from 254 °C to 272 °C for NTO/TNT and 227 to 249 °C for RDX/TNT. The DSC data showed that the maximum decomposition temperature (T_max) of the material was increased with the increase in heating rate.

Table 3 Performance and sensitivity data of melt-cast compositions

Composition	Theo. max density (g cm^−3)	Experimental density (g cm^−3)	VOD (m s^−1)	Sensitivity to various stimuli
				Tmax (°C)	Shock (kbar)	Friction (kg)	Impact h50 (cm)
SNTO/TNT (60/40)	1.79	1.65	7100	266	51.3	36	72
RDX/TNT (60/40)	1.74	1.68	7900	241	18.7	14.8	99

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The activation energy for the decomposition of the compositions was computed using the Ozawa and Kissinger method.^15–18 Arrhenius plots of these compositions are shown in Fig. 9a and b and 10a and b and the calculated data are given in Tables 4 and 5. The activation energies of these compositions are 185 and 259 kJ mol⁻¹ for the RDX and SNTO based compositions respectively. These values did not significantly vary through calculating them from the above methods. The higher activation energy of the SNTO/TNT composition indicates a high thermal stability and that it is relatively safe at elevated temperatures compared to composition B. This increased thermal stability may be attributed to the existence of the hydrogen bonding stabilised layered structure of NTO.

Fig. 9 (a and b) Activation energy of decomposition (RDX/TNT 60 : 40) calculated using the Ozawa and Kissinger method.

Fig. 10 (a and b) Activation energy of decomposition (SNTO/TNT 60 : 40) calculated using the Ozawa and Kissinger method.

Table 4 Activation energy of decomposition of RDX/TNT (60 : 40) calculated using the Ozawa & Kissinger method

Heating rate (β) (°C min^−1)	Tm (K)	Tm^2	1/Tm (K)	log β	ln β	ln (β/Tm^2)
2	500.52	2.50 × 10^5	1.99 × 10^−3	0.3010	0.6932	−11.7381
5	505.62	2.55 × 10^5	1.97 × 10^−3	0.6990	1.6094	−10.8421
10	514.17	2.64 × 10^5	1.94 × 10^−3	1	2.3026	−10.1825
15	522.31	2.72 × 10^5	1.91 × 10^−3	1.1761	2.7081	−9.8085

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Table 5 Activation energy of decomposition of SNTO/TNT (60 : 40) calculated using the Ozawa & Kissinger method

Heating rate β (°C min^−1)	T (°C)	Tm (K)	Tm^2	1/Tm (K)	log β	ln β	ln(β/Tm^2)
2	254.02	527.02	2.77 × 10^5	1.89 × 10^−3	0.3010	0.6932	−11.8413
5	261.93	534.93	2.86 × 10^5	1.86 × 10^−3	0.6990	1.6095	−10.9548
10	266.47	539.47	2.91 × 10^5	1.85 × 10^−3	1	2.3026	−10.2786
15	272.21	545.21	2.97 × 10^5	1.83 × 10^−3	1.1761	2.7081	−9.8943

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The thermogravimetric analysis (TGA) of the melt-cast compositions is given in Fig. 11. The RDX/TNT (60 : 40) composition starts to show weight loss within a temperature range of 94.8 to 241.7 °C in two steps. The loss in weight for the composition in the first step is found to be 28.8% in the temperature range of 94.8 to 178.4 °C which corresponds to the loss of TNT, while the 58.9% loss in weight observed in the second step corresponds to RDX in the temperature range of 178.4 to 241.7 °C. The SNTO/TNT (60 : 40) composition was also decomposed in two steps as shown in Fig. 11. It starts to show weight loss within a temperature range of 104.5 to 264.2 °C. The first step shows a 38.9% weight loss in the temperature range of 104.5 to 185.6 °C which corresponds to TNT, whereas the 63% loss in weight observed in the temperature range of 185.6 to 264.2 °C corresponds to NTO. This study clearly examines that the weight mixtures of SNTO/TNT and RDX/TNT from the temperature ranges 104.5 to 264.2 °C and 94.8 to 241.7 °C indicated that each species enhanced the decomposition of the other. This shows that the NTO containing formulations are found to be thermally stable as brought out by the thermal decomposition studies and this composition is less susceptible to storage temperature compared to RDX/TNT compositions. Both the calorimetric and weight loss studies brought out that the SNTO/TNT (60 : 40) composition shows good thermal and storage stability.

Fig. 11 TGA profile of RDX/TNT and SNTO/TNT (60 : 40).

3.3.2 Vacuum stability assessment of melt-cast compositions. The vacuum stability test was carried out primarily to find out the compatibility of the ingredients used in the compositions. The test was performed for both the RDX/TNT (60/40) and SNTO/TNT (60/40) compositions at 120 °C for 48 hours. The volume of the gases evolved from the composition was determined to be 1.56 and 1.08 ml per five gram of test material for the RDX/TNT and SNTO/TNT compositions respectively. This is well within the acceptable limits that clearly indicated spherical-NTO and RDX compositions are very compatible with TNT. Hence they are expected to possess good storage stability.

Thus the SNTO/TNT (60 : 40) melt-cast compositions exhibit better thermal and storage stability even under extreme conditions. The vacuum stability results were also corroborated with the TGA and DSC analyses.

3.3.3 Sensitivity assessment of melt-cast compositions. To realise less hazardous and insensitive munitions (IM’s), shock sensitivity is an important criterion in melt-cast formulations. The typical set-up used for the determination of shock sensitivity is shown in Fig. 12. A 10 mm thickness mild steel sheet was used as a witness plate on which an acceptor charge consisting of SNTO/TNT was placed. 136 g of booster donor charge separated by an attenuator sheet which consisted of cellulose acetate was assembled over the acceptor charge. The shock sensitivity of the SNTO/TNT composition is 51.3 kbar which is significantly higher than the corresponding RDX/TNT based composition (Table 3). In explosive formulations, friction insensitivity plays an important role and the spherical NTO based compositions are almost two times more friction insensitive than the benchmark composition. However, the SNTO/TNT composition was found to be slightly more sensitive to impact stimuli compared to composition B.

Fig. 12 Trial set up for shock sensitivity test.

Overall, a melt-cast composition with spherical-NTO possessing 60% solid loading was developed and exhibited to be 2.5 times more shock insensitive and 2 times more friction insensitive compared to the RDX based composition B. The insensitivity of the spherical NTO based compositions may be attributed to the layered crystal structure of NTO unlike RDX.

3.4 Evaluation of explosive properties of SNTO/TNT and RDX/TNT compositions

The explosive characteristics were also determined for these formulations and compared with the standard explosive TNT. The results are given in Table 3. The velocity of detonation was determined using an ionisation probe method and is significantly lower than the benchmark composition B. This may be due to the poor charge density achieved in the case of the NTO composition compared to its estimated theoretical maximum density. Since the morphology of NTO is spherical, it may be possible to increase the solid loading up to 75% with efficient vacuum mixing and casting. Hence, we believe that the NTO composition with increased solid loading will have enhanced performance combined with superior insensitivity. This study further recommends using the combination of RDX (or reduced shock sensitive RDX) and NTO to achieve better performance along with shock insensitivity.

4. Conclusions

A thermally less hazardous and shock insensitive melt-cast composition is developed by replacing sensitive RDX with spherical NTO. Because of the spherical morphology, NTO loading is proved to be 60% by a simple processing methodology. The activation energy for the thermal decomposition of NTO/TNT is determined to be high compared to RDX/TNT and hence relatively safer than the existing RDX/TNT composition. Superior processability is also realised with the NTO based composition. Further, the weight loss and vacuum stability studies also indicate that the SNTO/TNT composition exhibits less susceptibility to storage temperature and possess good storage life. The SNTO/TNT composition is found to be shock insensitive (51.3 kbar) and is 2.5 times more shock insensitive compared to RDX/TNT (18.7 kbar) composition B. The composition is also found to be friction insensitive. Though the velocity of detonation of SNTO/TNT is relatively lower than the benchmark, it can be improved by increasing the NTO content or by part replacement with reduced sensitivity RDX. Overall, this study on a NTO/TNT based melt-cast composition concludes the realisation of a less hazardous and thermally stable shock insensitive composition for future insensitive munitions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19010j

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