Thermal decomposition of NTO: An explanation of the high activation energy

Burning rate characteristics of the low-sensitivity explosive 5nitro-1,2,4-triazol-3-one (NTO) have been investigated in the pressure interval of 0.1-40 MPa. ne temperature distribution in the combustion wave of NTO has been measured at pressures of 0.4-2.1 MPa. Based on burning rate and thermocouple measurements, rate constants of NTO decomposition in the molten layer at 370-425 degrees C have been derived from a condensed-phase combustion model (k = 8.08. 10(13) center dot exp(- 19420/T) s(-1). NTO vapor pressure above the liquid (In P=-9914.4/T+14.82) and solid phases (In P = - 12984.4/T+ 20.48) has been calculated. Decomposition rates of NTO at low temperatures have been defined more exactly and it has been shown that in the interval of 180-230 degrees C the decomposition of solid NTO is described by the following expression: k 2.9- 10(12) -exp(-20680/T). Taking into account the vapor pressure data obtained, the decomposition of NTO in the gas phase at 240-250 degrees C has been studied. Decomposition rate constants in the gaseous phase have been found to be comparable with rate constants in the solid state. Therefore, a partial decomposition in the gas cannot substantially increase the total rate. High values of the activation energy for solid-state decomposition of NTO are not likely to be connected with a sub-melting effect, because decomposition occurs at temperatures well below the melting point. It has been suggested that the abnormally high activation energy in the interval of 230-270 degrees C is a consequence of peculiarities of the NTO transitional process rather than strong bonds in the molecule. In this area, the NTO molecule undergoes isomerization into the aci-form, followed by C3-N2 heterocyclic bond rupture. Both processes depend on temperature, resulting in an abnormally high value of the observed activation energy.