Abstract—Two polymeric matrices - silicone and C4

(polyisobutylene plasticized by dioctyl-sebacate and oily material) were prepared and used for preparation of various plastic explosives. The following energetic nitramines were used as explosive fillers: BCHMX (cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole), ε-HNIW (ε-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaiso-wurtzitane), RDX (1,3,5-trinitro-1,3,5-triazinane), and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane). The effect of the polymeric matrices on the thermal stability of the explosives was studied using differential thermal analysis technique (DTA). High performance liquid chromatography (HPLC) was further used to detect the presence of BCHMX dissolved in dioctyl-sebacate. Heat of combustion of the prepared plastic explosives and of the pure explosives was measured using calorimetric bomb. The results show that the polar plasticizer acts as a solvent of the energetic material and decreases the overall thermal stability of prepared plastic explosive.

Keywords—energetic materials, polymers. combustion, stability.

I. INTRODUCTION OLYMERIC matrices are used with the energetic materials to increase the safety during transportation, loading and

handling of these materials in addition to enhancement of their mechanical properties [1], [2]. BCHMX (cis-1,3,4,6-tetranitro-octahydroimidazo-[4,5-d]imidazole) is a new attractive nitramine which has been prepared at the Institute of Energetic Materials (IEM) by 2 stage synthesis (unpublished) [3]. Sensitivity and detonation characteristics of BCHMX in plastic explosive were studied [4]-[6]. The thermal stability was however only briefly evaluated in the past. It is therefore important to look at the decomposition process since practically all energetic materials decay in time. The decomposition of a pure substance is always accelerated by heat (during production, storage or usage) and sometimes by presence of other substances, which are not compatible with the energetic material itself. Decomposition leads to

Ahmed Elbeih is a PhD student at Institute of Energetic Materials, University of Pardubice, Czech Republic (e-mail: elbeih.czech@gmail.com).

Svatopluk Zeman is a Professor at Institute of Energetic Materials, University of Pardubice, Czech Republic.

Jiri Pachman, Institute of Energetic Materials, University of Pardubice, Czech Republic.

Zbynek Akstein, Research Institute of Industrial Chemistry, Explosia Company, Czech Republic

degradation of both chemical and mechanical properties and is generally accompanied with gas and heat generation. In extreme cases self-heating may initiate burning and lead to explosion accident [7]. This paper therefore represents a brief study of the thermal stability of the new energetic material, BCHMX incorporated in a plastic explosive. A two different binder systems were used and for comparison ε-HNIW (ε-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), RDX (1,3,5-trinitro-1,3,5-triazinane), and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) were also evaluated bonded by the same matrices. The effect of the polymeric matrices on the thermal stability of BCHMX and the selected nitramines are presented and the heat of combustion of all the prepared samples as well as the pure explosives fillers was measured.

II. MATERIALS AND METHODS

A. Nitramines RDX was a product of Dyno Nobel (mixture of Classes 2

and 5 according to the standard [8]), β-HMX was imported from Russia and its particle size was close to Class 3 according to the standard [9]), technical-grade ε-HNIW was a product of Explosia pilot plant, and BCHMX was prepared by a two step continuous laboratory synthesis at the IEM.

B. Preparation of Polymeric Matrices Two different polymeric matrices were used in the study.

The first polymeric matrix consisted of 25 wt% of polyisobutylene, 59 wt% of dioctyl-sebacate (DOS) and 16 wt% of oil HM46 and it was designated as “C4 matrix”. The matrix was prepared by placing polyisobutylene pieces into heated mixture of DOS and HM46 oil at 80 °C for one week. The mixture was every day stirred for 15 min.

The second polymeric matrix designated as “silicone matrix” was polydimethyl-siloxane.

C. Preparation of Plastic Explosives All the plastic explosive samples were prepared by research

branch of Explosia Company (Research Institute of Industrial Chemistry), Pardubice, Czech Republic. The preparation method was based on mixing the explosives crystals with the polymeric matrix using a computerized mixer Plastograph BRABENDER. In case of samples based on C4 matrix, 91 wt% of each individual explosive was mixed with 9 wt% of

Influence of Polymeric Matrices on the Thermal Stability and Heat of Combustion of High

Energy Materials

Ahmed Elbeih, Svatopluk Zeman, Jiri Pachman, and Zbynek Akstein

P

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C4 matrix at 70 °C for 70 min. under vacuum. While in case of samples based on silicone matrix, 88 wt% of each individual explosive was mixed with 12 wt% of polydimethyl-siloxane at 30 °C for 90 min. under atmospheric pressure.

D. Heat of Combustion Measurements Automatic isoperibolic combustion calorimeter MS10A,

Laget, was used for determining the internal energy of combustion of all the prepared samples as well as the pure explosives fillers used. Weighted sample was placed in contact with platinum wire inside the bomb containing 5 ml of water (saturating the internal space with water vapor), and then filled with excess of oxygen (20 atm.) The high pressure of oxygen is necessary to ensure complete combustion. The measurements are based on recording the temperature increase in the calorimetric vessel after the combustion process [10]. The results obtained from the measurements are reported in Table I as heat of combustion.

E. Thermal Stability Studies A DTA 550 Ex apparatus was used for thermal analysis of

explosives [11]. The measurements were carried out at atmospheric pressure, with the tested sample in a direct contact with the air. The tested sample (0.05 g) was placed in a test tube made of Simax glass, 5 mm in diameter and 50 mm long. The reference standard was 0.05 g aluminium oxide. A linear heating rate of 5 °C min-1 was used. The results are typical DTA thermograms with upwards oriented peaks for exothermal and downwards oriented endothermal peaks. The temperatures of the maximum of the decomposition peak at a linear heating rate of 5 °C min-1 are listed for all samples in Table I.

F. Analysis Method High performance liquid chromatography (HPLC) was used

to determine solubility of the tested nitramine explosives in plasticizer used in the matrix. The liquid chromatography system consists of the following components (all from Agilent 1200 series): degasser G1322A, pump G1311A, autosampler G1329, column thermostat G1316A, and diode array detector G1315D operated by software HP Chem Station B03.01. The column material was BDS Hypersil C-18, particle size 5 μm, pore size 12 nm and column dimensions 250 mm x 4.6 mm.

0.1 g of pure BCHMX was placed in a beaker containing 10 ml of dioctyl-sebacate and stirred by magnetic stirrer for 15 min at normal temperature and pressure. The content of the beaker was then filtered to remove the solid BCHMX from the dioctyl-sebacate. The same procedure was repeated with another sample at 40 °C under vacuum. The obtained solutions were diluted by mobile phase (water/methanol 50:50) and then were injected into the HPLC. The flow rate of mobile phase was 1 ml min-1. The peak assignment was based on retention times acquired from pure standards measured under the same conditions.

III. RESULTS AND DISCUSSION From Table I, it is clear that the heat of combustion of all

the pure explosives is lower than that of all the prepared plastic explosives. These results prove that the used polymeric matrices act as a fuel and hence increase the heat of combustion of the pure explosive.

Results of the heat of combustion of all the prepared samples and the pure explosive fillers compared with the percentage of polymeric matrix in each sample are illustrated in Fig. 1, which shows that the heat of combustion of samples based on RDX and HMX have nearly the same value for each individual polymeric matrix. While samples based on BCHMX have lower heat of combustion than samples based on RDX and higher than samples based on HNIW for each individual polymeric matrix.

Fig. 1 Results of heat of combustion compared with the percentage of binder in each studied sample.

It is also clear that addition of C4 matrix increased the heat of combustion of all the studied explosive more than the addition of silicone matrix even though the percentage of silicone matrix (12 wt%) is higher than the percentage of C4

TABLE I HEAT OF COMBUSTION AND PEAKS DECOMPOSITION TEMPERARUE OF ALL THE

STUDIED SAMPLES

No. Sample DTA Peaks at

5 °C min-1 (°C)

Heat of Combustion (J g-1)

1 RDX 213.1 9522 2 HMX 273.2 9485 3 BCHMX 225.3 9124 4 HNIW 222.4 8311 5 RDX-C4 212.9 12356 6 HMX-C4 261.5 12382 7 BCHMX-C4 215.9 12163 8 HNIW-C4 212.3 11452 9 RDX-silcone 213.6 11664

10 HMX-silicone 272.4 11647 11 BCHMX-silicone 226.3 11328 12 HNIW-silicone 222.9 10672

BCHMX-C4

HNIW-sil

BCHMX-silRDX-sil

HNIW

BCHMX

RDX

RDX-C4

HNIW-C4HMX-sil

HMX

HMX-C4

8000

8500

9000

9500

10000

10500

11000

11500

12000

12500

13000

-3 0 3 6 9 12 15

weight percentage of binder in explosive formulations (%)

Hea

t of c

ombu

stio

n (J

g-1

)

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matrix (9 wt%) in each prepared plastic explosive. This might be due to the presence of oily material (fuel) and dioctyl-sebacate in the C4 matrix which leads to increase the heat of combustion of all the samples.

Samples based on HNIW gave the lowest heat of

combustion compared with the other studied nitramines for each individual polymeric matrix.

In Fig. 2, the thermal behaviour of the studied pure explosives compared with the prepared samples based on C4 matrix and silicone matrix are presented. The DTA measurements of samples based on C4 matrix showed that the peak temperatures of decomposition of these mixtures are lower than that of the corresponding explosives in their pure state. These results might be due to the presence of the polar plasticizer (DOS) in acrylate matrix which acts as a solvent for all the studied explosives and leads to the decrease of the mixture’s decomposition temperature.

Fig. 2 A typical DTA measured peaks for all the studied samples

At the same time the peak temperatures for all the prepared samples based on silicone matrix are higher than those of pure explosives except in the case of HMX based sample. The slight increase in the decomposition temperature of samples based on silicone matrix compared with the corresponding explosive in the pure state proof that silicone matrix increases the stability of the studied explosives.

In case of HMX-silcone, the lower decomposition temperature of this sample compared with the pure HMX might be due to the high decomposition temperature of the pure HMX at which the silicone matrix was completely melted and so decreased the decomposition temperature of HMX.

Regarding the new plastic explosive named BCHMX-silicone, it is obvious that the decomposition temperature of this sample is higher than all the other samples based on RDX and HNIW which means that the thermal stability of this new material is better than RDX-C4 which is well known Composition C4.

On the other hand, the decomposition temperature of BCHMX-C4 is lower than that of pure BCHMX. To

understand the reason of this result, new sample was prepared by mixing the pure BCHMX with 5.3 wt% of DOS plasticizer (the same wt% used in the C4 matrix) without any binderand was designated as BCHMX-DOS.

The thermal stability of this new sample was studied by DTA technique using the same conditions as the previous studied samples. The DTA peaks of the new sample BCHMX-DOS, pure BCHMX and BCHMX-C4 are presented in Fig. 3.

Fig. 3 A typical DTA measured peaks for BCHMX-DOS compared with BCHMX-C4 and pure BCHMX at 5 °C min-1.

Fig. 3 clearly shows that BCHMX-DOS has lower decomposition temperature than both pure BCHMX and BCHMX-C4. This fact confirms the idea that DOS decreases the thermal stability of the pure explosive. Also the higher decomposition temperature of BCHMX-C4 compared with BCHMX-DOS proofs that the binder polyisobutylene acts as inhibitor and decreases the negative effect of DOS on the thermal stability of the explosives.

In order to check the solubility of the explosives by the polar plasticizers pure BCHMX was placed in DOS and stirred for certain period as discussed in the analysis method. The results of the liquid chromatography measurements are presented in Fig. 4.

Fig. 4 Chromatogram of standard BCHMX and BCHMX on DOS

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In the chromatogram standard BCHMX appears as a peak

at retention time 2.88 min. It further shows relatively high purity of the sample. In case of the DOS sample, a peak appeared at 2.89 retention time, this peak represents the BCHMX, which means that the tested sample of DOS contains BCHMX. It was therefore proven that the plasticizer dissolves part of the energetic filler in the plastic explosive.

IV. CONCLUSION Addition of the selected polymeric matrices to the studied

nitramines increased their heat of combustion. Samples based on the C4 matrix have higher heat of combustion than those based on silicone matrix for each individual explosive. The silicone matrix showed a positive effect on the thermal stability of the explosives while the C4 matrix decreased the thermal stability of these explosives. The proposed idea that DOS (polar plasticizer) acts as a solvent for BCHMX (explosives) was confirmed by HPLC. Also the idea that DOS decreased the thermal stability of the explosives was confirmed by the DTA measurements. BCHMX-silicone has higher decomposition temperature than all the studied samples based on RDX and HMX.

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[3] D. Klasovity, S. Zeman, A. Ruzicka, M. Jungova, M. Rohac, “cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole (BCHMX), its properties and initiation reactivity”, Journal of Hazardous Materials, vol. 164, pp 954–961, 2009.

[4] A. Elbeih, J. Pachman, W. Trzcinski, S. Zeman, Z. Akstein, J. Selesovsky, "Study of plastic explosives based on attractive cyclic nitramines, Part I. Detonation characteristics of explosives with PIB binder" Propellants, Explosives, Pyrotechnics, vol. 36, online.

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[7] W. Perry, J. Zucker, P. Dickson, G. Parker, B. Asay, “Interplay of explosive thermal reaction dynamics and structural confinement”, Journal of Applied Physics, vol. 101, no. 7, 2007.

[8] Detail specification RDX, Military Standard MIL-DTL-398D, US Dept. of Defence, April 17, 1999.

[9] Detail specification HMX, Military Standard MIL-DTL-45444C, US Dept. of Defence, Nov. 26, 1996.

[10] Solid fuels – Determination of heat of combustion by calorimetric method in the pressure vessel and calculation of calorific value, Czech Tech. Standard ČSN ISO 1928, Czech Inst. of Standards, June 1999.

[11] Krupka M., “Devices and equipments for testing of energetic materials”, in: Proc. 4th Seminar New Trends in Research of Energetic Materials, Pardubice, pp. 222, 2001.

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