Investigation on the thermal stability of hexanitrostilbene by thermal analysis and multivariate regression

Abstract

The thermal decomposition of hexanitrostilbene (HNS), a well-known heat resistant explosive, has been investigated by simultaneous TGA/DTA and DSC at heating rates between 0.05 and 40°C/min. Depending on the temperature time history, the reaction takes place either in the solid-phase or in the liquid-phase after the melting of the sample. To be able to observe the solid-phase reaction, experiments with constant heating rates well below 2.5°C/min have to be performed. Therefore, it is impossible to judge the thermal stability of solid HNS using kinetic models derived from DSC experiments at heating rates of 10–20°C/min, as it is the standard procedure. In this work, separate global kinetic models have been developed for the thermal decomposition of high bulk density HNS in the solid, respectively, the liquid-phase by applying the non-linear multivariate regression technique. The multivariate regression technique is an indispensable method for deriving reliable kinetic models. The solid-phase model consists of three consecutive reaction steps: (1) a three-dimensional phase boundary reaction, dominantly a sublimation, (2) an autocatalytic decomposition reaction, and (3) a slow reaction of fractal order, supposedly a high-temperature pyrolysis of primary solid products. The liquid-phase model contains four consecutive reaction steps: (1) a reaction with high activation energy and a small order, simulating the melting, (2) a reaction with low activation energy and an order smaller one, dominantly an evaporation, (3) an autocatalytic reaction, which is the same as the second reaction in the solid-phase model, and (4) a slow reaction resembling the third reaction step of the solid-phase model. The solid-phase model was used to simulate the stability of HNS under isothermal conditions at temperatures below 300°C.