Thermal decomposition of HMX

Structurally, HMX exhibits conformational polymorphism, as demonstrated in Fig. 9.1. Brill and Reese (1980) analysed the relative stabilities of the a, p and 8 forms in terms of coulombic forces to interpret the thermophysical behaviour of the three forms. They concluded that the chair conformation found in the p modification is more stable than the chair-chair conformation in the other two forms. The relative stability to pyrolysis could also be accounted for by the analysis of the coulombic attractions and repulsions around the molecule in each of the modifications. More recently, Henson et al. (1999) have followed the change in second harmonic generation response during the jS -> 5 phase change, which has long been implicated in the thermal decomposition of HMX (Karpowicz and Brill 1982). 

Mass spectra of the important explosives RDX, HMX, TNT, TNB and Tetryl were first briefly reported by Meyer (Ref 34) and later investigated in greater detail with high resolution and labeling techniques by Bulusu et al (Ref 45). Mass spectrometric studies of the photodecomposition of labeled dimethyl-nitramine (Ref 56) and the thermal decomposition of HMX and RDX (Refs 27 31) illustrate the application of these techniques to studies of reaction mechanism and bond dissociation processes. Nitroguanidines have only recently been investigated by Beynon (Ref 35)... 

A detailed literature summary and discussions of the thermal decomposition of HMX are presented by Boggs1151, and the general picture of the decomposition processes of HMX may be understood from [15]. When HMX is heated slowly, a single-stage mass loss process is observed the mass loss begins at 550 K and rapid gasification reaction occurs at 553 K. No solid residue remains above 553 K. Two endothermic peaks and one exothermic peak are seen the first endothermic peak at 463 K is the crystal transformation from (3 to 6 and the second endothermic peak at 550 K is the phase change from solid to liquid. The exothermic peak at 553 K is caused by the reaction accompanied by gas phase reaction. 

Kraeutle, K.J. (1981) The Thermal Decomposition of HMX Effect of Experimental Conditions and of Additives Chemical Propulsion Information Agency Publication 3A7, 383-394. 

ABSTRACT. The use of mass spectrometry to collect data on the decomposition chemistry of nitramine compounds and the relevance of the data to the processes occurring in these materials when they are used in actu propellant and explosive applications is discussed. The simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) and time-of-flight (TOF) velocity-spectra techniques and then-application to the study of energetic materials are discussed. The means by which these techniques enhance the amount of information obtained from more conventional mass spectrometric experiments is illustrated with studies on the evaluation of the use of appearance energy measurements to study the thermal decomposition of HMX and on the identiHcation of the HMX pyrolysis products and the determination of their gas formation rates. 

Behrens, Jr., R., (1990), Thermal Decomposition of HMX and RDX Decomposition Processes and Mechanisms Based on STMBMS and TOF Velocity-Spectra Measurements, S. Bulusu (ed.), Proceedings of the NATO Advanced Study Institute, ... 

THERMAL DECOMPOSITION OF HMX AND RDX DECOMPOSITION PROCESSES AND MECHANISMS BASED ON STMBMS AND TOF VELOCrTY-SPECTRA MEASUREMENTS ... 

R.N. Rogers, J.L. Janney, "The Thermal Decomposition of HMX Kinetic—Isotope-Effect Observations", Proceed. 12th No. Am. Thermal Anal. Soc. Conf.. 474-477 (Sep 1983). 

Thermal decomposition of pure explosives such as primary explosives lead azide, lead styphnate, mercury fulminate etc. [35], monomethylamine nitrate [36] and explosive mixtures RDX + HMX mixtures [37]. 

Simultaneous TGA and DTA were used by Maycock et al. to study the kinetics of the isothermal and adiabatic thermal decomposition of 8-HMX (one of the polymorphs of HMX) under helium atmosphere and reported activation energy [44] —63 KcalmoT1. The use of simultaneous DTA/TG has been reported for compatibility testing of TNT with two epoxy-and two alkyd resins paints [21, 45]. 

The endotherm at 192 °C is due to the fi-d crystalline phase change, and the exotherm at 276 °C is due to the violent decomposition of HMX. Thermal pre-ignition and ignition temperatures of explosive substances can be obtained from DTA and TGA thermograms. 

The Thermal Decomposition of /3-HMX , Proc7thSympExpl Pyrots, FIRL, Phila (1971) [DTA analysis of /3-HMX in air at 1 atm using a heating rate of 2°/min revealed the endothermic process of the irreversible cryst phase change from /3 to 6 at 192°, and the violent decompn of <5 -HMX at 276°. Integration of the dx/dt or dT/dt curves for the activation energy exponent yields the data shown in Table 4... 

We examined the thermal decomposition of a number of nitramines in dilute solution and in the melt phase. The nitramines included acyclic dialkyl mononitramines, where the dialkyls were methyl, ethyl, propyl and isopropyl cyclic mononitramines (N-nitro-pipeiidine and N-nitropyrrolidine) and cycle multifunctional nitramines (N-dinitropiperazhe l,3-dinitro-l,3-diazacyclo-pentane l,3-dinitro-l,3-diazacycbhexane RDX and HMX). For all nitramines, the predominant condensed-phase product was the nitrosamine though the amount formed depending on the nitramine and the phase of the thermolysis. The common trigger in the decompositions was N-N02 ho mo lysis, but the fate of the resultant amine radical depended on the phase. In solution the radical was stabilized sufficiently so that it resisted further decomposition and, instead, reacted with NO to form nitrosamine. In vapor or condensed phase, the amine radical underwent further reaction therefore,... 

A key point, with regard to both issues, is the decomposition process of the compound What are its energetics and how readily does it occur Our emphasis in this chapter shall be upon factors that influence the ease with which decomposition can be initiated by unwanted external stimuli, i.e. sensitivity. These stimuli may be of various types, including impact, shock, friction, heat and electrostatic charge [8]. Relative vulnerabilities to these different effects need not be the same for example, the onset temperatures for the thermal decomposition of TNT (1) and HMX (2) are quite similar, but the latter is much more likely to undergo detonation upon impact [8]. It has been shown, however, that there is a general correlation between impact and shock sensitivities [10], which are the ones upon which we will focus. 

Surya Bulusu et al. have determined with isotopes that the thermal degradation of HMX proceeds by breakage of C—bonds. It is important to determine whether charged intermediates are formed during decomposition, and even during detonation because these would change the Fermi level. 

The thermal decomposition process of HMX was studied by time-of-flight mass spectrometry (Model Zhp-6 spectrometer) [71]. It is suggested that the decomposition of HMX involves three stages, i.e. initial decomposition of the solid phase at about 160 C, decomposition with melting and decomposition of the liquid phase. The products which have been confirmed are NOj, NO, HCN and CHO. 

While not necessarily exerting a profound influence of the thermal decomposition of energetic materials, many solid state and condensed phase phenomena occur with energetic materials that play a role in the manufacture, use, and efficacy of energetic materials. HMX and NH11NO3 are notorious for the problems created by their tendency toward polymorphism. Some of the condensed phase phenomena newly observed for energetic materials are described in this section. 

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