Formation and characterization of core-shell CL-20/TNT composite prepared by spray-drying technique

Chang-gui Song, Xiao-dong Li, Yue Yang, Hui-min Liu, Ying-xin Tan, Jing-yu Wang

School of Environment and Safety Engineering, North University of China, Taiyuan, 030051, China

Keywords:Energetic materials CL-20(2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane)TNT(2,4,6-Trinitrotoluene)Spray-drying method Core-shell structure

ABSTRACT The core-shell 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane/2,4,6-Trinitrotoluene (CL-20/TNT) composite was prepared by spray-drying method in which sensitive high energy explosive(CL-20)was coated with insensitive explosive(TNT).The structure and properties of different formulations of CL-20/TNT composite and CL-20/TNT mixture were characterized by scanning electron microscopy (SEM),Transmission electron microscopy (TEM), Laser particle size analyzer, X-ray photoelectron spectroscopy(XPS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), impact sensitivity test and detonation performance.The results of SEM,TEM,XPS and XRD show that ?-CL-20 particles are coated by TNT.When the ratio of CL-20/TNT is 75/25,core-shell structure is well formed,and thickness of the shell is about 20-30 nm.And the analysis of heat and impact show that with the increase of TNT content,the TNT coating on the core-shell composite material can not only catalyze the thermal decomposition of core material (CL-20), but also greatly reduce the impact sensitivity. Compared with the CL-20/TNT mixture (75/25) at the same ratio, the characteristic drop height of core-shell CL-20/TNT composite(75/25) increased by 47.6% and the TNT coating can accelerate the nuclear decomposition in the CL-20/TNT composites. Therefore, the preparation of the core-shell composites can be regarded as a unique means, by which the composites are characterized by controllable decomposition rate, high energy and excellent mechanical sensitivity and could be applied to propellants and other fields.

1. Introduction

Energetic materials,due to the difference of material properties,have different application fields, which mainly include explosives,pyrotechnics and propellants[1].And high energy and insensitivity have been the research topics of energetic materials at present[2-6]. However, high energy and insensitivity are irreconcilable contradictions for single energetic material, and researchers are committed to solving and balancing these contradictions [7-11].

2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), as a widely applicable energetic material with the highest energy and density, is considered to have a wide development prospect by researchers at home and abroad[12].However,due to its high mechanical sensitivity, shock wave sensitivity and electrostatic sensitivity,its application in various fields is limited,which has caused researchers committed to solving this problem [13]. In order to improve the properties of CL-20,researchers have done the following research: the improvement of crystal quality [14,15],the formation of cocrystal [16-19] and the preparation of composite[20-23]. Among them, some studies show that hot spots often appear on the surface of energetic materials by effect of external force [24-26]. Gradually, many researchers tend to study the structure of composite particles,especially the core-shell structure of composite particles. Because core-shell composites have three obvious advantages,i.e.the close contact between components,the combination of multiple functions and the improvement of material properties [27]. And 2,4,6-Trinitrotoluene (TNT), as a low melting point explosive and an insensitive explosive,is often used as coating material [28]. The application of coating materials can reduce the sensitivity of sensitive explosives and decrease the risk of accidental explosion caused by accidental impact. What’s more,the preparation of the composite can not only effectively adjust its decomposition rate but also broaden its application fields.

At present, there are many preparation methods of core-shell composite particles, such as cooling crystallization method [29],solvent/nonsolvent process [30,31], ultrasonic method [23,32]mechanical ball milling method [33], and spray-drying method[34]. Among them, spray-drying method, as a facile and preferred technology,is widely adopted.And this process can realize that the solid (core) in the suspension is coated by the shell material according to the principle of rapid contraction of liquid droplets on the effects of evaporation. The composite particles prepared by spray-drying method have access to realize continuous and largescale production by dint of less agglomeration phenomenon and higher material utilization rate [27]. Therefore, spray-drying method is considered to be the most effective method to prepare core-shell composites.

In this study, different formulations of CL-20/TNT composites were designed and studied, which were prepared by spray-drying method. The formation mechanism, crystal morphology, structure, thermal properties, sensitivity characteristics and detonation performance of core-shell composite were systematically investigated in detail.

2. Experimental section

2.1. Materials

Refined CL-20 was provided by Liaoning Qingyang Chemical Industry Ltd. Raw TNT was provided by Modern Chemistry Research Institute of China. 1, 2-dichloroethane was purchased from Tianjin fuchen Chemical Industry Ltd.

2.2. Preparation of CL-20/TNT samples

2.2.1. Spray-drying method

In order to investigate the influence of CL-20/TNT ratio on coreshell structure and retain the good characteristics of high energy,samples were prepared according to the formulations in Table 1according to the detonation velocity at different charge density(8000-9000 m/s). The experimental device is shown in Fig. 1.Firstly, the raw material TNT was dissolved in 1, 2-dichloroethane.Secondly, with the effect of magnetic stirring and ultrasonic, a certain amount of refined CL-20 was added into the above solution to form a uniform suspension (the ratio of 1, 2-dichloroethane to material was 99:1). Thirdly, the inlet temperature, nitrogen flow rate and feed rate were set to 85°C, 350 mL/min and 4.5 mL/min,respectively. Finally, the suspension was dried rapidly via a spray drying process, from which the core-shell structured CL-20/TNT composites were obtained. And these were marked as CL-20/TNT-1, CL-20/TNT-2, CL-20/TNT-3 and CL-20/TNT-4, respectively and were collectively named as CL-20/TNT composites.

Table 1 Formulations of CL-20/TNT composites.

2.2.2. Physical-mixing method

According to the ratio of CL-20/TNT-3 mentioned above,refined CL-20 and raw material TNT were mixed in aqueous solution with stirring and ultrasonically dispersing for 30 min. The physicalmixing CL-20/TNT sample can be obtained by washing, suction filtering and drying.which was named as CL-20/TNT mixture.And CL-20/TNT mixture and CL-20/TNT composites were named as CL-20/TNT samples.

2.3. Characterization

2.3.1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)

The morphology and structure of refined CL-20,raw TNT and CL-20/TNT samples were examined by using a scanning electron microscope (SEM SU8010 Oxford) and a transmission electron microscopy (TEM,Tecnai G2 F20, FEI, America).

2.3.2. Laser particle size analyzer

The size distribution of all samples was characterized using a laser particle size analyzer (Bettersize2000E, Dandong Bettersize Instruments Co., Ltd., Liaoning, China).

2.3.3. X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) measurements were recorded on an ESCALAB 250Xi (Thermo scientific, America) to analyze the element content on the sample surface.

2.3.4. X-ray diffraction (XRD)

Fig.1. Schematic diagram of the spray-drying device.

XRD was used to visualize the changes in the crystal structure of the samples.XRD patterns were recorded using the DX-2700 X-ray diffractometer(Dandong Haoyuan Corporation,Liaoning,China)at a voltage of 40 kV and a current of 30 mA using Cu Kα radiation at λ= 1.5418 ?. And the 2θ angle varied from 5°to 50°at a scanning rate of 5°/min.

2.3.5. Differential scanning calorimetry (DSC)

Thermal analysis was performed on a differential scanning calorimeter ((DSC-131, France SETARAM Corporation, Shanghai,China) by heating 0.5 mg of samples in a hermetically sealed aluminum crucible with a pinhole in the lid at a heating rate of 10°C/min and under the pressure of 0.1 MPa and 2 MPa (static atmosphere).

2.3.6. Impact sensitivity test

The impact sensitivity was determined via HGZ-1 impact instrument, according to GJB-772A-97 standard method 601.2 [35].The testing conditions: drop weight of 2.500 ± 0.002 kg, sample mass of 35 ± 1 mg, ambient temperature of 20°C, and relative humidity of 30%. The test was carried out 25 times to characterize the impact sensitivity of the samples. Finally, the critical drop height (H50) is used to characterize impact sensitivity.

2.3.7. Detonation performance

According to the different formulation, the EXPLO5 software was used to estimate detonation performance of samples.

3. Result and discussion

3.1. Morphology and particle size characterization

The SEM images of refined CL-20, raw TNT, and CL-20/TNT samples are shown in Fig. 2, which displays the microstructure and morphology of the particles. As shown in Fig. 2(a, b) and Table 2, refined CL-20 particles are densely crystallized and have less crystal defects.Compared with raw TNT,refined CL-20 particles(D50= 57.16 μm) appear spindle shape,whose size is smaller than the particle size of TNT(D50=101.5 μm).

As shown in Fig.2(g),CL-20/TNT mixture basically maintain the morphology and particles size of the pristine materials that have spindle shape of CL-20 and structure of raw TNT. It is found that a small part of raw TNT are scattered onto the surface of refined CL-20, and many TNT particles and CL-20 are distributed in a disordered manner.

Fig. 2. SEM images of (a) refined CL-20, (b) raw TNT, (c, d, e, f) CL-20/TNT composites (CL-20/TNT-1, CL-20/TNT-2, CL-20/TNT-3 and CL-20/TNT-4), (g) CL-20/TNT mixture, TEM images of (h and j) CL-20/TNT-3.

Table 2 Median particle diameter of samples.

However, it can be seen from Fig. 2(c-f) that there are a large number of small TNT particles on the surface of the refined CL-20 particles. As shown in enlarged figure of Fig. 2(c), the surface of refined CL-20 particles cannot be coated by TNT particles completely and some tiny TNT particles(nanoscale)are attached to the surface of refined CL-20 particles. As can be seen from Fig. 2(c-f) and Table 2, with the increase of TNT content, the amount of TNT particles coated onto the surface of CL-20 are increasing and core-shell structured CL-20/TNT composites are formed gradually and median particle diameter of composites become larger. Compared with CL-20/TNT-4 composites (Fig. 2(f)),CL-20/TNT-3 composites (Fig. 2(e)) are smoother, with smaller particle size and almost no obvious particles and cracks on the surface.The reason why is that with the increase of TNT content in suspension system,more and more TNT particles are adsorbed onto the surface of solid particles (CL-20) and TNT particles are accumulated on the surface in large quantities,which makes the surface of sample coarse. It indicates that TNT has been continuously distributed on the surface of CL-20 particles in CL-20/TNT-3.At the same time,in order to confirm the structure of the composite,TEM of CL-20/TNT-3 was carried out in Fig. 2(h, j). The results confirm the existence of core-shell structure,and the thickness of the shell is about 20-30 nm. A schematic illustration of the formation process of core-shell CL-20/TNT composite is shown in Fig. 3.

The mechanism of the preparation of core-shell CL-20/TNT composites is analyzed as follows. On account of special morphology of TNT particles, it is difficult to form core-shell structured composites composing with refined CL-20 particles coated by insensitive explosive TNT particles, such as CL-20/TNT mixture. As shown in Figs. 1 and 3, first of all, a uniform CL-20/TNT suspension was prepared with the magnetic stirring and ultrasonic dispersing. Then, by effect of peristaltic pump, the suspension was atomized through a 1.4 mm spray nozzle to form many small droplets according to the atomization mechanism of droplet[36]. In the process of evaporation and adsorption, solid particles(refined CL-20) were regarded as core, and increasing number of tiny TNT particles adhered to the surface of CL-20 to form an outermost shell after volatilizing 1, 2-dichloroethane at the spray dry tower. According to the adsorption theory [37] and wetting behavior of the liquid on the solid surface[38],there existed some the interactions between molecules, which made that refined and wetted CL-20 particles were coated by tiny TNT particles to form a core-shell CL-20/TNT composite. Finally, under the function of cyclone separator, CL-20/TNT composites were collected into the sample collector, and the volatile gas entered into the condensing unit,so as to avoid environmental pollution and respond to the call of green technology.

Fig. 4. Molecular structures of CL-20 and TNT. (a) CL-20, (b) TNT.

3.2. XPS analysis

The molecular structure of CL-20 and TNT is shown in Fig.4.In order to confirm the structure of CL-20/TNT composites,the surface elementary content of refined CL-20, raw TNT and CL-20/TNT samples was characterized by XPS. The XPS spectra are shown in Fig. 5.

It is found in Figs.4 and 5(a)that three elements(O 1s,N 1s and C 1s)are detected in each sample.By analysis of the C 1s spectra of refined CL-20 and raw TNT in Fig. 5(c) and (d), two peaks are assigned in CL-20 with binding energy of 288.63 and 284.7 eV,respectively, which arises from C-NNO2bond and C-C bond,respectively, and three peaks are assigned in TNT with binding energy of 292.3,286.5 and 284.7 eV,respectively,which arises from π-π* bond, C-NO2bond and C-C bond, respectively. Therefore,there are four peaks in the CL-20/TNT samples with binding energy of 292.3, 288.5, 286.5 and 284.7 eV. Owing to the different electronegativity of C,N and O elements [39], the C 1s peak illustrates the influence of the hydrogen atoms in C-NNO2bond, C-NO2bond,C-C bond and the benzene ring on electron excitation of C 1s.

As shown in Fig.5(a-i)and Table 3,the C 1s peak of the CL-20/TNT composites from each other is different to some extent,which is mainly attributed to the different proportion of CL-20 and TNT in the suspension,and it makes some differences in the coating effect of the composites and the difference in the proportion of elements on the surface of the composites.Compared with the refined CL-20,C 1s XPS results of CL-20/TNT composites show that the atomic concentration of C-NNO2decreases from 79.79%to 40.58%,and the atomic concentration of C

NO2increases from 0 to 29.17%. These changes could be attributed to the fact that with the increase of TNT content,refined CL-20 particles are coated by more TNT, which makes the content of C-NO2in the shell (TNT) increasing and C-NNO2in the core (CL-20) decreasing. This phenomenon indicates that core-shell CL-20/TNT composites are successfully prepared.Furthermore,it is found in Table 3 that C-NNO2atomic concentration of the CL-20/TNT composites C 1s spectra is much less than that of the same ratio of CL-20/TNT mixture. It suggests that there are some differences between the structure of CL-20/TNT mixture and self-assembly CL-20/TNT composite.

Fig. 3. Schematic illustration of the formation of core -shell CL-20/TNT composites.

Fig. 5. XPS full spectra and C 1s XPS spectra of samples.

Table 3 Atomic concentration of the samples C 1s spectra. (wt %).

3.3. Crystal structure of samples

The crystal structure of refined CL-20, raw TNT, and CL-20/TNT samples were characterized by X-ray diffraction for comparison.The XRD patterns of all samples are shown in Fig. 6.

The main characteristic peaks of the refined CL-20 appear at 12.580°,13.830°,25.789°,and 30.309°,corresponding to(11-1),(2 0 0), (0 2 2), and (2 0-3) crystal planes of ε-CL-20 (PDF#00-050-2045), respectively.

Fig. 6. XRD patterns of samples.

The main characteristic peaks of the raw TNT appear at 12.608°,17.711°, 22.992°, 29.741°, and 33.496°, respectively. And core-shell CL-20/TNT composites and physical-mixing CL-20/TNT mixture have characteristic peaks of refined CL-20 and raw TNT, indicating that CL-20 and TNT in CL-20/TNT composites have no change in the crystal form. From the diffraction curves of core-shell CL-20/TNT composites (CL-20/TNT 1-4), it can be observed that the characteristic diffraction peak intensity of core-shell CL-20/TNT composites (CL-20/TNT 1-4) decrease at 2θ = 25.789°and 30.309°and increase at 2θ=22.992°.The reason for the above phenomenon is that TNT is precipitated from the suspension at high temperature,which makes refined CL-20 coated due to the intermolecular van der Waals force.

Fig. 7. DSC curves and enlarge curves (125-175 °C) of samples at the heating rate of 10 °C/min: (a) and (c)Under unpressurized conditions (P = 0.1 MPa), (b) and (d)Under pressurized condition (P = 2 MPa).

Compared with physical-mixing CL-20/TNT mixture, the intensity of characteristic diffraction peaks of CL-20 into the coreshell CL-20/TNT composites at the same mass ratio are weaker and the intensity of characteristic diffraction peaks of TNT are stronger,which may be attributed to the difference in the structure of the samples.

3.4. Thermal performance

The thermal behavior of refined CL-20,raw TNT and CL-20/TNT samples were tested by differential scanning calorimetry (DSC).Because of the difference of DSC curves of TNT under unpressurized and pressurized conditions,the thermal decomposition of samples under two conditions was studied to further facilitate study of the differences between samples.DSC curves of CL-20,raw TNT and CL-20/TNT samples are shown in Fig. 7 and the experimental results are showed in Table 4 under the unpressurized and pressurized condition.

As shown in Fig. 7 and Table 4, under the unpressurized condition (0.1 MPa), the DSC curves show an endothermic peak at 163.99°C,and an exothermic peak at 253.69°C for the refined CL-20,which can be attributed to the phase transition from ?to γ form at 163.99°C and the characteristic peak for the thermal decomposition of CL-20 at 253.69°C, respectively. The thermal decomposition peak of CL-20 is controlled by the fracture of N-NO2bond in the range of 250-400°C.It can be seen from Fig.7(a,b)that the DSC peak of refined CL-20 is strongly influenced by pressure.With the increase of pressure, the exothermic reaction is intensified under the catalysis of NO2and other gases, and the peak deformation becomes sharper, leading to the peak shape backward.

Compared with the CL-20,The DSC curves of TNT are obviously different with or without pressure. Under the unpressurized condition (0.1 MPa), raw TNT has two endothermic peaks, one is the endothermic melting peak at 80.06°C,and the other is ascribed to volatilization or vaporization of molten TNT.However,the thermal decomposition curve of TNT in molten state can be obtained under high pressure (2 MPa). Such phenomenon primarily could be accounted for the fact that pressure inhibits the volatilization orvaporization of molten TNT, which makes the amount of experiments involved in thermal decomposition increasing. In the meantime, the oxidation-reduction reaction between gaseous products and parent aromatic rings may be more sufficient as a result of the increase of pressure.

Table 4 Summarized data from DSC curves of samples under the unpressurized and pressurized conditions.

After introducing of TNT,both core-shell CL-20/TNT composites and CL-20/TNT mixtures have two endothermic peaks and two exothermic peaks under the unpressurized or pressurized condition. It indicates that CL-20/TNT composites and CL-20/TNT mixtures have similar components. In particular, there are two exothermic peaks (around 200-310°C) of CL-20/TNT samples.Because of the fact that the fracture energy of N-NO2bond initially decomposed by CL-20 (188.3-230.1 kJ/mol) is lower than that of C-NO2bond initially decomposed by TNT (272-301 kJ/mol) [40],there are two exothermic peaks in CL-20/TNT samples (the first peak is the exothermic peak of CL-20 and the second peak is the exothermic peak of TNT).

Whether the CL-20/TNT samples are in pressurized condition or not, the first endothermic peak temperature of CL-20/TNT composites is slightly lower than that of CL-20/TNT mixture at the same ratio. The main reason is that TNT of core-shell CL-20/TNT composites is precipitated from the CL-20/TNT suspension, which not only leads to the diminish of TNT particle size but also results in TNT particles unstable and easy to melt under heat. As shown in Fig.7(c,d)and Table 4,with the increase of TNT content,it is found that second endothermic peak of CL-20/TNT composites is delayed,which is due to the endothermic reaction of TNT, more energy is needed to make transformation of the crystal by CL-20 particles.

Compared with the endothermic peaks of CL-20/TNT composites,the exothermic peaks of samples are significantly different.The exothermic peak temperature of CL-20 in CL-20/TNT samples is lower than that of pure CL-20.The above-mentioned phenomenon can be explained that the exothermic decomposition process of CL-20 is synchronous with that of TNT,which makes TNT promote the exothermic decomposition of CL-20 [41]. With the exothermic decomposition of CL-20, the accumulation of more energy accelerates the decomposition of TNT, and the exothermic peak temperature is lower than the melting temperature of TNT. The first exothermic peak of CL-20/TNTcomposites,with the increase of TNT content,is accelerated and the second exothermic peak is delayed.These phenomena are mainly attributed to the fact that increasing amount of molten TNT participates in the decomposition with the increase of TNT content,which makes the second exothermic peak in the sample closer to the peak of TNT.Under the condition of no pressure,CL-20 particles in CL-20/TNT composites are decomposed and gases are generated at the high temperature, which makes molten TNT in a micro container decomposed with the effect of pressure and heat. Compared with CL-20/TNT mixture, the exothermic peak of core-shell CL-20/TNT composite is slightly delayed,which is ascribed to the fact that the CL-20 particles of the composites coated by TNT,which makes the heat source absorbed and conducted by the outer shell (TNT).

3.5. Sensitivity study

The impact sensitivity is one important indicator to measure the safety of explosives.Impact sensitivity investigation was carried out and the characteristic drop height is presented in Table 5. It is observed that refined CL-20 has a characteristic drop height of 25.9 cm and raw TNT is much more insensitive, whose characteristic drop height is 105 cm.

As shown in Table 5, compared with refined CL-20, the characteristic drop height of core-shell CL-20/TNT composites increase by 42%, 68%, 79% and 176%, respectively. It is proved that to a certain extent,the impact sensitivity of CL-20/TNT samples can be reducedby increasing TNT content from 10%to 55%.This is attributed to the fact that the shell(TNT)in the core-shell structure has high impact stability, which makes hot spots difficult to form. In addition, for the CL-20/TNT-3 (25% TNT) prepared by spray-drying method, the H50increases by 47.6% compared with the CL-20/TNT mixture prepared by physical-mixing method,which implies that there are differences in self-assembled structure of components in samples prepared by the two preparation methods. CL-20/TNT mixture is simply a mixture of components, and the formation of hot spots depends mainly on the relatively sensitive CL-20.Conversely,when CL-20 undergoes external impact action, TNT particles coated on the surface are firstly attacked and play a key role for cushion from the impact action.Thus,hot spots are hardly generated in the coreshell CL-20/TNT composites.In addition,TNT particles of core-shell CL-20/TNT composites are refined by spray-drying method, which reduces the internal cavities inside TNT crystals and makes it hard for the formation of hot spots to come into being under the same impact stimulus.

Table 5 Impact sensitivity of samples.

3.6. Detonation performance

Detonation velocity and detonation pressure have been regarded as the index parameters of explosive output energy [42], and the theoretical maximum detonation performance of CL-20, TNT,and different formulations of CL-20/TNT samples were calculated by EXPLO5 program in Table 6. It was found that when CL-20/TNT = 75/25, detonation velocity and detonation pressure of sample can reach 9038.14 m/s and 37.388 GPa, which are almost equivalent to that of the current military standard explosive HMX[43]. More importantly, compared with the other samples, CL-20/TNT-3 (75/25) have unique structure, which can reduce the generation of hot spot and greatly improve the safety of CL-20. In conclusion, the CL-20/TNT-3 composites prepared spray-drying technique not only can have high detonation performance, but also was insensitive to impact.

4. Conclusion

Core-shell CL-20/TNT composites were successfully prepared by spray-drying method. The morphology and formation process of core-shell CL-20/TNT composites were analyzed by SEM,TEM,XPS and XRD.The results show that with the increase of TNTcontent,an increasing amount of TNT is coated onto the surface of ?-CL-20 particles,which indicates that the core-shell CL-20/TNT compositeis gradually formed. When the formula of CL-20/TNT is 75/25, CL-20 particles are completely coated by the recrystallized TNT, and thickness of the shell is uniform(about 20-30 nm).and there is no crystal transformation of CL-20 in the process of preparation.When it comes to thermal decomposition,the first exothermic peak in the composite is advanced and the second exothermic peak is delayed compared with that of CL-20/TNT mixture(75/25)at the same ratio,whether under pressurized condition (2 MPa) or not (0.1 MPa).With the increase of TNT content in the composite,especially under pressure, the distance between the two exothermic peaks of the composite is increasing. The results show that the structure and core-shell ratio of the composites have enormous influence on their thermal decomposition process.In addition,CL-20/TNT composites with core-shell structure are superior to the CL-20/TNT mixture(75/25)and its sensitivity decreases by 47.6%,which indicates that the core-shell structure can effectively improve the safety performance and retain the good characteristics of high energy. To sum up, the preparation of core-shell structure provides an effective way to improve the performance of CL-20 crystal.

Table 6 Detonation performance of samples.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.