Comparative analysis of vaporization and thermal decomposition from simultaneous thermal analysis studies of cyclotrimethylenetrinitramine (RDX)

Unavailability of proper vaporization rates is one of the constraints for the development of advanced predictive liquid or solid propellant combustion models. In this work, we present the estimated sublimation and vaporization rates of cyclotrimethylenetrinitramine (RDX) over a wide range of temperatures. Simultaneous thermal analysis was carried for RDX at various slow heating rates – 5, 10, and 15 ºC/min using a coupled TGA/DSC-FTIR system. In the absence of significant solid-phase decomposition reactions, the experimental mass loss below the melting point temperature of RDX occurs mainly due to the sublimation of RDX from the free surface. Whereas, in the liquid phase, a substantial amount of RDX decomposes into various smaller molecular weight species. The net experimental mass loss occurs because of the intricate interplay of simultaneous decomposition and vaporization of various species, including RDX. The thermal decomposition process was modeled using a recently developed detailed liquid-phase decomposition mechanism for RDX with 500 elementary reactions. A computational model based on conservation equations was developed to compute the temporal evolution of various species in the gas phase. Also, FTIR spectral transmission data were processed to obtain experimental species evolution profiles evolved into the gas phase. For each of the heating rates, a suitable match between computational and experimental mass loss and species evolution profiles was achieved using estimated sublimation and vaporization rates. As evident from the FTIR data, a major part of the mass loss occurs because of the evolution of decomposition products, such as N₂O, CH₂ O, NO₂, NO, HCN, H₂O, CO, and CO₂. Results show that vaporization accounts for 29.6, 34, and 35.9 % of the total mass loss for the 5, 10, and 15 ºC/min heating rates, respectively. Relatively more RDX vaporizes at higher heating rates because the sample temperature quickly exceeds the normal boiling point of RDX after which vaporization rate increases due to the initiation of the boiling phenomenon.