Original Research Article

Application and properties of nanometer RDX in PBX

Mr.Jie Liu,

Jie Liu*, Fengsheng Li ,Wei Jiang, Gazi Hao, Lei Xiao, Han Gao, Teng Chen and Xiang Ke

National Special Superfine Powder Engineering Research Center of China, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, P. R. China

 

In the present work, nanometer RDX-based polymer bonded explosive (PBX) was prepared using a solution-water slurry technique. The obtained PBX is composed of 94.5 % of RDX, 3% of dinitrotoluene (DNT), 0.5% of stearic acid (SA) and 2% of polyvinyl acetate (PVAc).

*Corresponding author:

E-mail:lfs_njust@126.com, jie_liu_njust@126.com
 

Keywords:

RDX, PBX, Nanotechnology, Sensitivity, Mechanical property

The properties of the as-prepared PBX, such as friction sensitivity, impact sensitivity, shock sensitivity, and compressive performance, were systematically investigated. The results were encouraging. Compared with the micron-sized RDX-based PBX, the friction sensitivity, impact sensitivity and shock sensitivity of the nanometer RDX-based PBX were lower by 21.1%, 55.4% and 13.6%, respectively. Furthermore,the compressive strength and strain of the nanometer RDX-based PBX were higher by 91.8% and 39.7%, respectively. That is, both the safety and the mechanical resistibility are significantly improved, when nanometer RDX is applied in PBX.
The brisant nitramine explosives, such as Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 2,4,6,8,10,12- hexanitro- 2,4,6,8,10,12-hexaazaisowurtzitane (CL-20),are characterized with high detonation heat, detonation velocity and detonation pressure. They have been extensively used in plastic bonded explosives (PBX) [1-6] and propellants [7-17]. However, the micron-sized explosives are very sensitive, which seriously threatens the safety of the ammunitions. The reduction in sensitivity has become a research focus. The studies have shown that the sensitivities of nitramine explosives are affected obviously by the sizes and size distributions of the explosive particles [18-20]. The sensitivities of explosives can be cut down effectively by reducing the particle sizes. If the nanoparticles were obtained, the sensitivities would be greatly decreased [21-23].
 
As an inspiring fact, in our previous studies, the nano-RDX, which are showing spherical shapes and narrow size distributions, have been produced in certain batches (1-10kg) using a wet ball mill, and the prepared nano-sized particles can be effectively extracted by freezing drying [24].However, to the best of our knowledge, there is still no report on the practical application of nano-RDX in PBX.
 
For these reasons, the nanometer RDX-based PBX was successfully prepared using a solution-water slurry technique in this study, and its sensitivity and mechanical property were investigated.
 
2.1 Materials
 
All chemicals used in the experiments were of analytical purity and used without further purification. Dinitrotoluene (DNT), stearic acid (SA), polyvinyl acetate (PVAc) and ethyl acetate were bought from Sinopharm Chemical Reagent Co., Ltd of China. The micron-sized RDX (d50=88μm, d10=25μm, d90=140μm) was obtained from Gansu Yinguang Chemical Industry Group Co., Ltd of China. The nanometer RDX (d50=60nm, d10=20nm, d90=150nm) was prepared by milling [25].
 
2.2 Preparation of the nanometer RDX-based PBX mold powder
 
The RDX-based PBX mold powder is prepared by slurry technique. RDX is coated with dinitrotoluene (DNT), polyvinyl acetate (PVAc) and stearic acid (SA). The mass percentage of RDX: DNT: PVAc: SA is 94.5: 3: 2: 0.5.
 
Firstly, RDX powders are stirred in water. The stirring speed is 600rpm, the temperature of the water bath is 80℃, and the ratios of water to material are 1:2.5 for micron-sized samples and 1:4 for nano-sized samples.
 
Secondly, the binders (DNT and PVAc) are dissolved by ethyl acetate, and then dropping into the water at the rate range from 1 mL/min to 3mL/min.
Lastly, when the binder solution is dropped, the SA is added in the water. Stir for 30 min, and then naturally cool down until the temperature of water is 35 ℃. The theoretical maximum density of this formulation is 1.78g/cm3.
 
For simplicity, the micron-sized RDX-based PBX and the nanometer RDX-based PBX are briefly named micron-sized RDX (JH) and nano-sized RDX (JH), respectively.
The size of the micron-sized RDX(JH) mold powder is mainly ranging in 500-1500μm, and the size of the nano-sized RDX(JH) mold powder is ranging mainly in 250-500μm.
 
2.3 Sensitivity test of the RDX-based PBX
 
2.3.1 Friction sensitivity
The friction sensitivities of the mold powder samples are measured by sliding friction test at 3.92 MPa. Fifty tests are carried out to obtain the mean explosion probability (P, %) [26]. the test conditions were as follows: each specimen mass is 20 mg, test temperature is 20±2 ℃ and a relative humidity is 60±5%. The mold powders are stacked without pressing.
 
2.3.2 Impact sensitivity
 
The impact sensitivities of the mold powder samples are measured by drop-hammer test and characterized by the characteristic heights (50% probability of initiation (H50)), which are statistically calculated by 25 effective test values obtained by using a 5kg drop-hammer[27]. The test conditions were as follows: each specimen mass is 35 mg, test temperature is 20±2 ℃ and a relative humidity is 60±5%. The mold powders are stacked without pressing.
 
2.3.3 Shock sensitivity
 
The scale gap test (SGT) is selected to measure the shock sensitivities [28]. In this test, the booster charge is made from RDX refined by acetone, with a density of 1.48g/cm3, the gap material is polymethyl methacrylate polymer, and the acceptor charges have the densities of 1.64g/cm3(micron-sized RDX(JH) and nano-sized RDX(JH)). The inside diameter and length of charges are 25.0 mm and 76.0 mm, respectively. The gap thicknesses (δ) are calculated by 25 effective values. This is a standard test that is used to obtain 50% detonation probability. The residual void percentage of the acceptor charge is 7.9%.
 
2.4Compressive strength and strain
 
The compression method was used to determine compression performance of the RDX (JH). The main principle can be stated as follows: The RDX (JH) sample was placed between the upper and lower pressure plate of material testing machine, and then apply to a specified speed along the axis of the sample under a certain temperature. The pressure plate would continuously compress the RDX (JH) specimen until the RDX (JH) sample was crushed. In this case, the compressive strength (Sc) is defined as the force (maximum load, QC) divided by the cross sectional area (A).The strain (ε) is defined as the effective compression distance (ΔLc) divided by the original specimen length (L0)
 
In this part, a CTM8050 computer controlled universal testing machine with digital display is used to test the compressive strength of JH specimen. All tests were carried out at the loading rate of 10.00 mm min-1 under an ambient temperature of 20℃.The length, the diameter and the weight of each compression specimen are 19.30 mm, 19.82 mm and 10.00 g, respectively, and then the density was1.68 g cm-3.The residual void percentage of the compression specimen is 5.6%. All results are calculated by 5 effective tests. As the RDX (JH) sample is compressed, a stress–strain curve is plotted by the instrument. The maximum load of the material would be corresponded with the maximum stress on the stress–strain curve. The effective compression distance (ΔLc) was corresponded with the strain between 10%Qc and Qc.
 
By its basic definition the compressive strength (Sc) and the strain (ε) are given by:
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The Sc and ε values of RDX (JH) sample is obtained from the average of 5 effective tests.
 
3.1 The Morphology of RDX (JH)
 
The particle size and the morphology of RDX particles, and the microstructure of outside surface and inner section of the RDX (JH) are characterized using the SEM.
Figure1 shows the SEM images of the micron-sized RDX and nano-sized RDX. From Figure 1a, it can be seen that the micron-sized RDX is composed of mostly a polyhedral structure with sharp edges and possess an average particle size of about 80-100 μm. The size of the micron-sized RDX ranges from 1 to 200 μm. As shown in Figure 1b, the prepared nano-sized RDX is semi-spherical and homogeneous, with a fairly uniform size of 60 nm.
 
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Figure 1. SEM images of (a) micron-sized RDX and (b) nano-sized RDX
 
The microstructure of outside surface and inner section of micron-sized RDX (JH) and nano-sized RDX (JH) were carried out and are shown as Figure 2 and Figure 3. As shown in Figure 2a and Figure 3a, micron-sized RDX (JH) presents large defects, such as cracks and holes, and therefore produces lots of pits on its cross section. In comparison, the nano-sized RDX (JH) has a smooth surface and flat cross section, as shown in Figure 2b and Figure 3b.
 
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Figure 2. SEM images of the surface of (a) micron-sized RDX (JH) and (b) nano-sized RDX (JH)
 
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Figure 3. SEM images of cross section of (a) micron-sized RDX (JH) and (b) nano-sized RDX (JH)
 
The nano-sized RDX is with much larger specific surface area than that of micron-sized RDX. When RDX particles are coated by the binders, the nanometer particles are more easily adhered to each other and the binder. Then the micron-sized RDX (JH) samples are much more porous. Especially when the mold powders are pressed, the nano-sized RDX (JH) will be characterized with densification. That is, the microstructures of RDX (JH) will change with particle size and morphology of RDX, and the properties (such as sensitivity and mechanical performance) will change with the morphology and particle size of RDX particles.
 
3.2 Sensitivities
 
3.2.1 Friction sensitivity
The friction sensitivities of micron-sized RDX (JH) and nano-sized RDX (JH) samples are listed in Table 1.
 
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As listed in Table 1, compared with the micron-sized RDX (JH), the average explosion percentage of the nano-sized RDX (JH), which characterizes the fiction sensitivities, is relatively decreased by 21.1% from 38% to 30%, suggesting that the friction sensitivity of RDX (JH) is reduced as the decrease of size of RDX.
 
3.2.2 Impact sensitivity
Table 2 shows the impact sensitivities of micron-sized RDX (JH) and nano-sized RDX (JH) samples. 
 
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As listed in Table 2, the special heights of micron-sized RDX (JH) and nano-sized RDX (JH) samples are 29.8 cm and 46.3 cm, respectively. In other words, the impact sensitivity of the nano-sized RDX (JH) sample is 55.4% lower than that of the micron-sized RDX (JH) sample, indicating that the impact sensitivity of RDX (JH) would be significantly decreased as the usage of nanometer RDX. Additionally, the standard deviations (Sdev.) of the nano-sized RDX(JH) is smaller than that of the micron-sized RDX(JH), which means the impact initiation probability of the samples are close to each other, resulting from that the size and morphology of the nano-sized RDX(JH) particles are close to each other.
 
3.2.3 Shock sensitivity
The shock sensitivities of micron-sized RDX (JH) and nano-sized RDX (JH) samples are presented in Table 3.
 
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As shown in Table 3, the gap thicknesses are 32.10 mm and 27.75 mm for micron-sized RDX (JH) and nano-sized RDX (JH), respectively, which reflects the shock sensitivities of the nano-sized RDX (JH) is 13.6% lower than those of the micron-sized RDX (JH). What’s more, the standard deviations (Sdev.) of the nano-sized RDX (JH) is smaller than that of the micron-sized RDX (JH), which implies that the size and morphology of the particles are close to each other. As a result, the shock initiation probabilities of PBX samples are close to each other.
 
According to the above results of sensitivity tests, we found that the differences in microstructure between micron-sized RDX (JH) and nano-sized RDX (JH) lead to different results. For micron-sized RDX (JH), because there are lots of defects in the interface of coarse RDX particles, it suffers a greater amount of high temperature “hot-spot” groups and is gradually detonated with much more probability. Conversely, for nano-sized RDX (JH), it has a low probability of explosion and a relatively lower sensitivity, due to the flat and dense structure with fewer defects generating a relatively small amount of high temperature “hot-spot” groups. Therefore, nano-sized RDX (JH) has lower defects and thereby possesses lower sensitivity.
 
3.3 Compressive properties
The compressive strength is obtained experimentally by means of a compressive test. As the RDX (JH) samples are compressed, a stress–strain curve are plotted by the instrument and are shown in Figure 4. It can be seen form Figure 4, the stress–strain curve of nano-sized RDX (JH) is similar to that of micro-sized RDX (JH). When the load is less than 10% the maximum load (Qc), the strains of RDX (JH) samples change a little. Above 10%Qc, the strains of the specimen will have a huge change until they are crushed. However, compared with the micro-sized RDX (JH), nano-sized RDX (JH) has an amazing Qc, confirming that it has an excellent resistibility to compression.
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Figure 4. The stress–strain curves of (a) micron-sized RDX (JH) and (b) nano-sized RDX (JH)
 
In this compression test, the effective compression distance (ΔLc) correspond to the strain between 10%Qc and Qc. According to the formula 1 and 2, the compressive strength (Sc) and strain (ε) of RDX (JH) samples can be calculated and listed in Table 4 and Table 5.
 
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Table 4 and Table 5 show that the Qc, Sc and ε are 4498.6N, 14.59MPa and 4.04% for micron-sized RDX(JH), and 8632.3N, 27.99MPa and 5.65% for nano-sized RDX(JH), respectively. Briefly, the compressive strength and strain of the nano-sized RDX (JH) increased relatively by 91.8% and 39.7%, respectively, which indicates that nano-sized RDX (JH) has superior mechanical properties.
 
When nano-sized RDX is applied in PBX, the small size effect and surface effect of nanometer materials will obviously benefit the densification of the pressed RDX (JH). The cracks and holes will be dramatically reduced, and the bonding strength will be significantly enhanced, because of the nanometer materials. Then the compression performance will be improved on a large scale after nano-sized RDX was applied in PBX. Furthermore, the micron-sized RDX particles, as shown in figure1, will be easily to be crushed into pieces when compression force was loaded on, however, the nanometer RDX particles won’t be crushed. Then the mechanical performance is further strengthened after nanometer RDX applied in PBX.
 
4 Conclusions
 
In summary, the nanometer RDX-based PBX was successfully prepared using a solution-water slurry technique and its microstructure, sensitivity and mechanical property were investigated. The experimental results of the analysis indicate:
 
(1) SEM observations show that micron-sized RDX (JH) presents large defects, such as cracks and holes, and therefore produces lots of pits on its cross section. In contrast, the nano-sized RDX (JH) has a smooth surface and flat cross section.
 
(2) The particle size of RDX has a significantly effect on the sensitivity and mechanical performance of RDX based PBX. Compared with the micron-sized RDX-based PBX, the friction sensitivity, impact sensitivity and shock sensitivity of the nanometer RDX-based PBX were lower by 21.1%, 55.4% and 13.6% respectively, and the compressive strength and strain of the nanometer RDX-based PBX were higher by 91.8% and 39.7%, respectively.
 
These beneficial results show that nanometer RDX endows the nano-sized RDX-based PBX with high security and excellent mechanical properties.
 

This work was financially sponsored by the National Natural Science Foundation of China (NSFC, 51606102) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Published: 14 December 2016

Reviewed By : Dr. Qingyin Wu.Dr. Ezra Elias.

Copyright:

Copyright: Copyright: © 2016 Jie Liu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.