Summary of Thermal Stability and our Approach

Following preliminary DSC studies, isothermal decompositions of small quantities (1-3 mg) of compound are performed at temperatures generally below the observed DSC exothermic maximum. Samples are usually thermolyzed in sealed Pyrex tubes. Use of Pyrex usually precludes reaction with the container that often occurs with metal reaction vessels. Sealed vessels also prevent corrosive decomposition products, e.g. NO2 or HF, from damaging laboratory instrumentation. Sealed reaction vessels confine the decomposition products where they can easily be identified and quantified. It is obvious that highly reactive decomposition products such as formaldehyde are not observed by this technique, but they would not be expected to survive over the time of these decomposition experiments (seconds to hours, depending on the temperature). Seal vessel thermoylses mimic real storage scenarios where the sample is self-confined. However, autocatalysis may occur in sealed vessels that would not be observed in open ones. On the other hand, in unsealed tubes sublimation of the sample may become competitive with decomposition. 

Decomposition rate constants are measured over as wide a temperature range as possible. Only the first one third to one half of the decomposition can be analyzed before it becomes severely autocatalytic. With the rate constants, an Arrhenius plot can be constructed and activation parameters calculated. Activation energies and pre-exponential factors correlate the decomposition rates with temperature. In addition, the magnitude of the activation energy may shed light on the key step in the decomposition process, and Arrhenius parameters are necessary in many explosive code calculations. Our procedure is to input the activation parameters into the Frank-Kamentskii equation [145] and use it to predict critical temperature of a reasonable size (e.g. 1 kilogram) of the energetic material  

Probing for proton transfer in the rate-determining step can be accomplished using deuterated compounds or solvents [63,86,91,138,140], If the deuterated explosive is available, it can be examined neat and in solution. If no deuterium kinetic isotope effect (DKIE) is exhibited by the neat deuterated explosive, then hydrogen transfer can be ruled out in the 

The value of studying a family of compounds should not be underestimated. For example, studying the thermal decomposition of NTO was difficult because the range of temperatures over which it could be examined was limited (activation energy was 79 kcal/mol), it reacted with 

Different research groups have different approaches to elucidating decomposition pathways. Thus, a wide variety of activation parameters have been published (Table 1). This is a result of examining the decomposition on different timescales or temperatures. Often if the rate constants are calculated for a common temperature, they will be similar in magnitude [97], It is rare that the first paper published about a compound tells the entire story. For example, the papers on nitramine decomposition, cited herein, span thirty-five years. Each reveals a different piece of the story or supports previous postulates. Consider the mechanisms illustrated by Figs. 8-10. These are the results of several researchers approaching the problem from different perspectives yet, they make similar conclusions. As new ways of probing reaction chemistry become available, they should be implemented. At the same time the reaction chemistry should be examined by the more conventional techniques, and an effort should be made to correlate the results. 

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