| Abstract
| - Through the use of simultaneous thermogravimetry modulated beam mass spectrometry, optical microscopy,hot-stage time-lapsed microscopy, and scanning electron microscopy measurements, the physical and chemicalprocesses that control the thermal decomposition of 1,3,5-trinitrohexahydro-s-triazine (RDX) below its meltingpoint (160−189 °C) have been identified. Two gas-phase reactions of RDX are predominant during the earlystages of an experiment. One involves the loss of HONO and HNO and leads to the formation of H2O, NO,NO2, and oxy-s-triazine (OST) or s-triazine. The other involves the reaction of NO with RDX to form NO2and 1-nitroso-3,5-dinitrohexahydro-s-triazine (ONDNTA), which subsequently decomposes to form a set ofproducts of which CH2O and N2O are the most abundant. Products from the gas-phase RDX decompositionreactions, such as ONDNTA, deposit on the surface of the RDX particles and lead to the development of anew set of reaction pathways that occur on the surface of the RDX particles. The initial surface reactionsoccur on surfaces of those RDX particles in the sample that can accumulate the greatest amount of productsfrom the gas-phase reactions. Initial surface reactions are characterized by the formation of islands of reactivityon the RDX surface and lead to the development of an orange-colored nonvolatile residue (NVR) film on thesurface of the RDX particles. The NVR film is most likely formed via the decomposition of ONDNTA onthe surface of the RDX particles. The NVR film is a nonstoichiometric and dynamic material, which reactsdirectly with RDX and ONDNTA, and is composed of remnants from RDX and ONDNTA molecules thathave reacted with the NVR. Reactions involving the NVR become dominant during the later stage of thedecomposition process. The NVR reacts with RDX to form ONDNTA via abstraction of an oxygen atomfrom an NO2 group. ONDNTA may undergo rapid loss of N2 and NO2 with the remaining portion of themolecule being incorporated into the dynamic NVR. The dynamic NVR also decomposes and leads to theformation of H2O, CH2O, N2O, NH2CHO, (CH3)2NCHO, (CH3)2NNO, C2H2N2O, and (CH3)3N or CH3NCH2CH3. The competition between reaction of the dynamic NVR with RDX and its own thermal decompositionmanifests itself in a rapid increase in the rate of evolution of the NVR decomposition products as the amountof RDX remaining in the sample nears depletion. The reactions between the NVR film and RDX on thesurface of the RDX particles leads to a localized environment that creates a layer of molten RDX on thesurface of the particles where reactions associated with the liquid-phase decomposition of RDX may occur.The combination of these reaction processes leads to an acceleration of the reaction rate in the later stage ofthe decomposition process and creates an apparent reaction rate behavior that has been referred to asautocatalytic in many previous studies of RDX decomposition. A reaction scheme summarizing the reactionpathways that contribute to the decomposition of RDX below its melting point is presented.
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