Thermal decomposition mechanism of particulate core-shell KClO 3 -HMX composite energetic material

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1 Indian Journal of Engineering & Materials Sciences Vol. 18, October 2011, pp Thermal decomposition mechanism of particulate core-shell KClO 3 -HMX composite energetic material Lin-Quan Liao a,b, Qi-Long Yan c *, Ya Zheng a, Zhen-Wei Song b, Jun-Qiang Li b & Peng Liu b a College of Astronautics, Northwestern Polytechnical University, Xi'an, China, b Xi an Modern Chemistry Research Institute, Xi'an, China, c Institute of Energetic Materials, University of Pardubice, CZ Pardubice, Czech Republic Received 25 April 20011; accepted 11 November 2011 The thermal decomposition mechanism of a newly designed composite material KClO 3 -HMX (KC-HMX) is investigated by combined TG-DSC-FTIR technique and T/Jump in-situ thermolysis cell/ftir (T/Jump FTIR) technique.it is shown that KC-HMX began to decompose at about 266 C without melting, and the fast stage of mass loss at the temperature range of 268.4~290.1 C is considered to be the result of the thermolysis and complex reactions of KClO 3 and HMX with energy release of 1859 J.g -1, which exceeded that of pure HMX about 40%. It is also shown that CO, CO 2, NO 2 and H 2 O were the main gaseous products. The T/Jump FTIR analysis showed that the competing reactions of N-N and C-N bonds cleavage occurred in initial stage of HMX decomposition are greatly affected by KClO 3. In contrast of pure HMX, there is no CH 2 O and HCN detected in its thermolysis products. In presence of electronegative oxygen radical produced by thermolysis of KClO 3 oxidized CH 2 O and HCN through gas-phase reaction (NO 2 +4O 2 ) + (2N 2 O+5CH 2 O) 5NO+3CO+2CO 2 +5H 2 O, which is probably the dominating reaction, being immediately followed by the decomposition reaction of HMX. Keywords: TG-DSC-FTIR, HMX, Potassium chlorate, Decomposition mechanism HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane) is one of the highly energetic material which can be used as a major ingredient of solid propellants in such applications as guns and rocket motors. Therefore, over the past several decades, many studies have been devoted to its ignition, decomposition and combustion behavior 1,2. As its known to all, most of the high explosives including HMX are oxygen-lean compounds and it always burn without any oxygen support from the air due to the design of the rocket motor. The heat releases of these energetic materials are usually restricted due to incomplete oxidation reactions. In order to make the oxidation reactions more complete, the oxygen must be incorporated with such oxygen-lean energetic materials. One feasible way is combining an oxygen rich compound with them such as HMX to form new composite energetic materials. For instance, ammonium perchlorate (AP) and potassium chlorate (KClO 3 ) are oxygen rich compounds. In fact, AP has been widely used as an ingredient of propellants and high explosives. Compared with AP, it is more appropriate for KClO 3 to be used in pyrotechnic mixtures due to its lower energy content, and the combustion behavior of the *Correcpoding author ( terry.well@163.com) mixtures could be greatly improved by using this compound. Undoubtedly, by compatibility and processability, the potassium chlorate could also be considered as the oxygen rich compound which could be used in modified double base propellants containing HMX. In order to make the oxygen element more effective in the propellant, KClO 3 should be combined with HMX at molecular level. In this study, a new core-shell KC-HMX composite energetic material was prepared by recrystalization method and a combined use of different thermoanalytical methods was taken to characterize its extensive thermal properties. In fact, a thermogravimetric analyzer coupled with Fourier transform infrared analysis of evolving products (TG-FTIR) was also used to analyze the volatile products relevant to KC-HMX pyrolysis. Significant efforts have been devoted to identifying an initial stage of the thermal decomposition pathways in the condensed phase and its thermolysis mechanism. Experimental Procedure Materials and apparatus KClO 3 -HMX composite material (KC-HMX; Mass ratio of HMX/KC was about 2/1) was prepared and

2 394 INDIAN J. ENG. MATER. SCI., OCTOBER 2011 studied with regard to the thermal behavior and thermolysis mechanism by using thermogravimetry (TG), infrared spectroscopy (IR) and differential scanning calorimetry (DSC). The IR series spectra of gaseous thermolysis products of KC-HMX were also recorded and the bands assigned. Experimental conditions The particulate KC-HMX(about 35.5 mg)was analyzed in the PDSC (TA910s, made in America) instrument was introduced in the static nitrogen atmosphere, and the pressure was 0.1 MPa. The sample quantum was about 1.5 mg, and the heating rate is about 5 C min -1. It was also analyzed in TG 209 coupled to a TENSOR 27 FTIR spectrometer. The sample was heated at a rate of 5 C min -1 in a nitrogen flow of 45 ml min -1 up to 400 C. The in-situ T/Jump FTIR instrument (Nexus 870, Nicolet America) was introduced to detect its condensed thermolysis products with the data collection rate of 1.89 cm s -1, the differentiating rate of 4 cm -1 and the heating rate of about 10 C min -1. Results and Discussion Thermal properties of pure compounds DSC and TG analysis of KClO 3 particle As for the pure potassium chlorate, no thermal event is observed prior to its melting point near 356 C, at which potassium chlorate undergoes a sharp endothermic phenomenon including melting. In fact, there is a long interval of temperature change from when potassium chlorate undergoes fusion at 356 C and first rapidly decomposes exothermically at 472 C. Thus, 472 C corresponds to an ignition temperature. Such behavior suggests that pure potassium chlorate is kinetically stable at its melting point. After complete decomposition of the sample, oxygen and potassium chloride are produced through 2KClO 3 2KCl + 3O 2 (R1) This process is globally recognized and the reaction mechanism is easy to obtain. As a oxygen rich compound, KClO 3 could provide a large amount of free oxygen radicals in its thermolysis process which could be used to oxidize the energetic intermediates produced by thermal decomposition of explosives such as HMX. Thermal decomposition mechanism of HMX HMX as a nitroamine is characterized by -N-NO 2 chemical bonds that are attached to hydrocarbon structures. For its thermolysis process, the bond breakage of N-N produces NO 2, which acts as an oxidizer. The remaining hydrocarbon fragments act as fuel components. Previous theoretical studies have included electronic structure calculations of various decomposition channels of the gas-phase HMX molecule. Lewis et al. 3 calculated four possible decomposition pathways of the HMX polymorph: N-NO 2 bond dissociation, HONO elimination, C-N bond scission, and the concerted ring fission. Based on the energetics, it was determined that N-NO 2 dissociation was the initial mechanism of decomposition in the gas phase, while they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The FTIR/DSC combination technology was introduced by Kimura and Kubota 4 to judge the thermolysis mechanism of HMX, and it was indicated that the initial step was the cleavage of N-NO 2 and N 2 O, CH 2 O, CO, CO 2 and H 2 O was its main gaseous products. Recent experiments using thermogravimetric modulated beam mass spectrometry and isotope scrambling identified gaseous pyrolysis products such as H 2 O, HCN, CO, CH 2 O, NO, and N 2 O between 210 and 235 C 5-7. Brill suggested two competing global mechanisms for thermal decomposition 8, the first leading to 4HONO and 4HCN while the second leading to the formation of 4CH 2 O and 4N 2 O. The above noted experimental work on thermal decomposition of condensed phase HMX is largely restricted to relatively low temperature (277 C) and pressure (0.1 MPa) regimes. In the condensed phase, however, Farber 9 made the observation that alternative decomposition mechanisms can occur for thermolysis of pure HMX. The deposed NO 2 fragment can recombine as a nitride, which is then decompose by breaking the O-N bond to form NO, or attract weakly hydrogen atoms and form HONO. The HONO molecules can then rapidly equilibrate to form water via reaction (R2) as follows. 2HONO H 2 O + NO 2 + NO (R2) The results also showed that the formation of CH 2 O and N 2 O could occur preferably from secondary decomposition of methylenenitramine. The final thermolysis reaction equation could be established as (R3): HMX CH 2 O + N 2 O + CO + CO 2 + N 2 + H 2 O+ NO (R3)

3 Liao et al.: KClO 3 -HMX COMPOSITE ENERGETIC MATERIAL 395 (f(α) =1/2α; one dimension diffusing chemical reaction; E a = kj.mol -1 [10] The reaction models above (conditions: multiple heating rate non-isothermal kinetics; Coats-Redfern method; 20% < α (conversion) < 80%) could accommodate the thermal decomposition of HMX, as well as subsequent reactions in the foam layer. Thermal behavior of KC-HMX composite material TG and DSC analysis TG and DSC curves for HMX and KC-HMX (Figs 1 and 2) show a residue-free decomposition. By applying the TG and DSC method, only one decomposition steps can be clearly observed and quantitatively described for both HMX and KC-HMX. As shown in Fig. 1 and Table 1, the TG curve of HMX in dynamic nitrogen atmosphere at a heating rate of 5 C min -1 showed one stage of very fast mass loss of 96.6% which is attributed to the thermolysis of HMX. The mass change occurred only in the temperature range of C with energy release of 1239 J.g -1. The thermal decomposition of KC-HMX composite material is similar to HMX, and there is one stage of more fast mass loss of 72.1% in its thermolysis process. The fast stage of mass loss is considered to be the result of the thermolysis and complex reactions of KClO 3 and HMX at the temperature range of 268.4~290.1 C with energy release of 1858 J.g -1. As shown in Fig. 2 and Table 1, the thermolysis process of KC-NC/NG could be divided into as two stages, the thermolysis of NG + KClO 3 with a heat release of 1755 J.g -1 and thermolysis of NC with J.g -1. The heat release from the thermolysis of NG made the thermolysis of KClO 3 take place at a lower temperature range. It was also indicated that the residue of pure HMX decomposition is less than 3.4%, while KC-HMX shows a residue of 27.9% which was probably due to the formation of KCl. However, when the initial mass ratio of HMX/KC is 2/1, the calculated residue (KCl and remain of HMX thermolysis) should be 22.7%. Hence the actual percentage of KC in the composite might be less than 1/2 due to the slight mass loss of HMX during the preparation process. Besides, as shown in Fig. 2, KClO 3 has a significant influence on the thermolysis behavior of HMX. In addition to lowering of the thermolysispoint of HMX, a more intensive exothermic peak could be observed at lower temperature for KC-HMX Table 1 TG and DSC results of HMX, KC-HMX and KC-NC/NG under pressure of 0.1 MPa Parameters of TG Parameters of DSC samples T fw, C L a, (%) T Vmax C Mass remain T 0, C T p, (%) H d J g -1 HMX % KC-HMX % KC-NC/NG P P *Note: T fw temperature range; L a percentage of mass loss; T Vmax temperature of maximum mass loss rate; heating rate: 5 C.min -1 Fig. 1 Characteristic TG curve of KClO 3 -HMX and pure HMX (heating rate 5 C/min; sample mass about 2 mg; nitrogen flow of 45 ml min -1 ) Fig. 2 Characteristic DSC curve of KClO 3 -HMX, HMX and KC-NC/NG (heating rate 5 C /min; sample mass 2.15 mg)

4 396 INDIAN J. ENG. MATER. SCI., OCTOBER 2011 composite material than that of pure HMX. On one hand, the presence of KClO 3 lowered the peak temperature of HMX, and the oxygen produced by KClO 3 accelerated the thermolysis velocity of HMX meanwhile made the exothermic reaction more complete, the heat release of HMX was thereby enhanced. On the other hand, there are phase changes for pure HMX at C and C, while HMX decomposed rapidly before change of phase in presence of KClO 3. In order to clarify how the heat release of HMX was enhanced by KClO 3 and which are the main chemical reactions for the extra heat release, the detailed chemical reaction pathways should be brought forward. Therefore, the evolved gases and condensed products of KC-HMX decomposition were further analyzed. Evolved gas analysis (EGA) With FTIR spectroscopic EGA, the gaseous decomposition products of KC-HMX can be identified. Figure 3 shows the IR gas-phase spectra which were obtained online during linear heating of KC-HMX. As shown in Fig. 3, the main gaseous products for the decomposition of KC-HMX should be CO 2, CO, NO 2, and H 2 O, and about 72.1% mass loss occurred during the thermolysis process. The possible decomposition pathway of KC-HMX pyrolysis could be described as reactions (R4)-(R6). However, mass spectrum (Fig. 5) show that N 2 O, CH 2 O, CO 2 and H 2 O were detected as the main thermal decomposition gases of HMX. HMX + KClO 3 KCl + CH 2 O + N 2 O + NO + HCN + O 2 CH 2 O + N 2 O + HCN + O 2 NO + CO + CO 2 + H 2 O NO + CO + O 2 NO 2 + CO 2 (R4) (R5) (R6) The change of concentration with the temperature for the main gaseous products of KC-HMX and pure HMX mentioned in (R3) and (R4) are also shown in Figs 4 and 5. As shown in Fig. 5, the maximum concentrations for all the gaseous products appeared at about for the pure HMX, which are a little lag compared with the peak temperature (285.6 C) of the HMX thermolysis. But for decomposition of KC-HMX, the maximum concentration for all the gaseous products appeared at about 290 C which is higher than that of pure HMX, even though the peak temperature of its decomposition is lower than that of pure HMX. This might be caused by the difference of the sense organ for different test facility. However, there is no CH 2 O and N 2 O for decomposition of KC-HMX, and the Fig. 4 EGA profiles of the KClO 3 -HMX decomposition gases H 2 O ( cm -1 ), CO ( cm -1 ), CO 2 ( cm -1 ) and NO 2 ( cm -1 ) Fig. 3 FTIR of the main gaseous products of KClO 3 -HMX at temperature of 280 C Fig. 5 Fragments concentration curves of gaseous products for pure HMX thermolysis

5 Liao et al.: KClO 3 -HMX COMPOSITE ENERGETIC MATERIAL 397 curve of CO 2 concentration rebounded after its first tiptop at the points of and C. This should be caused by continued oxidation reactions in presence of oxygen, such as: HCN + O 2 CO + NO (or NO 2 ) + H 2 O CH 2 O + O 2 NO (or NO 2 ) + CO + H 2 O CO + O 2 CO 2 (R7) (R8) (R9) The above mentioned chemical reactions are exothermic which could enhance the whole heat release of KC-HMX decomposition which is confirmed in its DSC traces (Fig. 2). The EGA/MS profiles of gaseous products confirm its degradation due to consecutive gas-phase reactions for HMX and a large amount of data with regard to its thermal decomposition mechanism are available. Thermal decomposition mechanism of KC-HMX composite material The thermal decomposition mechanism of KC-HMX could not be clarified just based on its gaseous thermolysis products. The autocatalytic acceleration of the thermal decomposition process of KC-HMX is further confirmed by in-situ T/Jump FTIR measurements (Figs 6 and 7). It was shown that there is a quantitative change for the main peak intensity at about 266 C, following which some new peaks ( cm -1 ) appeared, which are characteristic peaks of N=O. In thermolysis of potassium chlorate, the Cl - is adequate to exhaust K + producing KCl, while the strong electronegative O 2- ion was produced to react with other fuel free radicals 11. In order to make sure what had happened in this thermolysis process, the initial IR spectra and intermediate condensed products were collected and identified which is shown in Fig. 7. It was observed that there is a broad peak at about cm -1 which is the characteristic peak of crystal H 2 O, and it disappeared at about 110 C at which the water would sublimate. Generally, there are two competing pathways for HMX decomposition. One is aroused by a cleavage of C-N bond while the other is started with a uniform cleavage of N-N bonds. However, in presence of potassium chlorate, the mechanism of HMX decomposition is influenced by high concentration of electronegative O 2- ion. Based on the investigation made by Palopoli 12, for pure HMX, when the pressure was elevated, more CH 2 O was produced by cleavage of C-N, and the decomposition mechanism is dominated by cleavage of C-N. In this way, more fuel was produced and the heat release is thereby enhanced at higher pressure. It would be in favor of C-N cleavage when potassium chlorate was used as oxidizer. If the cleavage of C-N were considered as the initial way of its decomposition, an intermediate product hydroxide methyl methacylamine (HMFA, HOCH 2 NHCHO) would be formed according to Palopoli et al. 13. The dominant reaction in this oxidation stage should be described as (R10). HMX HOCH 2 NHCHO 4CH 2 O + 4N 2 O (R10) Without potassium chlorate, NO 2 and N 2 O would act as oxidizers and CH 2 O as fuel component. When electronegative O 2- ion was produced by potassium chlorate, both N 2 O and CH 2 O would act as fuel, minor NO 2 and the entire O 2- ion would act as oxidizers. Some other gaseous products of HMX Fig. 6 In-situ T/Jump FTIR spectra of KC-HMX pyrolysis (heating rate 5 C/min) Fig. 7 FTIR of the condensed intermediate products of KClO 3 -HMX

6 398 INDIAN J. ENG. MATER. SCI., OCTOBER 2011 decomposition would also be oxidized by oxygen through the reactions of (R6)-(R9). Since nitrogen dioxide and oxygen reacts quite rapidly with formaldehyde, the overall gas phases reaction (NO 2 + 4O 2 ) + (2N 2 O + 5CH 2 O) 5NO + 3CO + 2CO 2 + 5H 2 O (R11) Is probably the dominating reaction, being immediately followed by the decomposition reaction of HMX. Therefore, the predicted thermolysis pathways of KC-HMX could be established as shown in Scheme 1. The reaction pathway was based on a cleavage of N-N bond releasing NO 2 as an initial decomposition product which is conventionally observed for organic nitroamines. Conclusions The thermal decomposition mechanism of a newly designed composite material KC-HMX was investigated by combined TG-DSC-FTIR techniques and T/Jump FTIR analysis.the following conclusions could be made: (i) KC-HMX began to decompose at C without melting, and the fast stage of mass loss at the temperature range of 268.4~290.1 C is considered to be the result of the thermolysis and complex reactions of KClO 3 and HMX with energy release of 1859 J.g -1, which exceeded that of pure HMX about 40%; (ii) The CO, CO 2, NO 2 and H 2 O were detected as the main gaseous products of KC-HMX decomposition, and the competing reactions of N-N and C-N bonds cleavage occurred in initial stage of HMX decomposition are greatly affected by KClO 3 ; (iii) In contrast of pure HMX, there is no CH 2 O and HCN was detected in its thermolysis products. In presence of electronegative oxygen radical produced by thermolysis of KClO 3 oxidized CH 2 O and HCN through gas-phase reaction (NO 2 +4O 2 ) + (2N 2 O+5CH 2 O) 5NO + 3CO + 2CO 2 + 5H 2 O, which was probably the dominating reaction, being immediately followed by the decomposition reaction of HMX. Acknowledgement The authors gratefully acknowledge helpful discussion with Dr Liu Zi-Ru and Dr Li Ji-zhen of Xi an Modern Chemistry Research Institute. Special thanks to Wang Xiao-Hong, who performed the DSC and TG experiments. References 1 Kuldeep Parsad, Richard A Yetter & Michelld D Smooke, Combust Flame, 115 (1998) Craig M Tarver & Tri D Tran, Combust Flame, 137 (2004) Lewis J P, Glaesemann K R, Van Opdorp K & Voth G A, J Phys Chem A, 104 (2000) Kimura J & Kubota N, Propellants Explos, 5 (1980) Behrens R, Int J Chem Kinet, 22 (1990) Behrens R, J Phys Chem, 94 (1990) Behrens R & Bulusu S, J Phys Chem, 95 (1991) Brill T B, J Propellants Power, 11 (1995) Farber M & Srivastava R D, 16 th JANNAF Combustion Meeting, CPIA Pub. 308, Dec 1979, Yan Qi-Long, Song Zhen-Wei, Shi Xiao-Bing, et al., Acta Astron, 64 (2009) Shiro Shimada, Thermochim Acta, 255 (1995) Palopoli S F & Brill T B, Combust Flame, 87 (1991) Miller J A & Bowman C T, Prog Energy Combust Sci, 15 (1989)