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

The thermal decomposition of hexanitrostilbene (HNS), a well-known heal res istant explosive, has been investigated by simultaneous TGA/DTA and DSC at heating rates between 0.05 and 40 degreesC/min. Depending on the temperatur e time history, the reaction takes place either in the solid-phase or in th e liquid-phase after the melting of the sample. To be able to observe the s olid-phase reaction, experiments with constant heating rates well below 2.5 degreesC/min have to be performed. Therefore, it is impossible to judge th e thermal stability of solid I-INS using kinetic models derived from DSC ex periments at heating rates of 10-20 degreesC/min, as it is the standard pro cedure. In this work, separate global kinetic models have been developed fo r the thermal decomposition of high bulk density I-INS in the solid, respec tively, the liquid-phase by applying the non-linear multivariate regression technique. The multivariate regression technique is an indispensable metho d for deriving reliable kinetic models. The solid-phase model consists of t hree consecutive reaction steps: (1) a three-dimensional phase boundary rea ction, dominantly a sublimation, (2) an autocatalytic decomposition reactio n, and (3) a slow reaction of fractal order, supposedly a high-temperature pyrolysis of primary solid products. The liquid-phase model contains four c onsecutive reaction steps: (1) a reaction with high activation energy and a small order, simulating the melting, (2) a reaction with low activation en ergy and an order smaller one; dominantly an evaporation, (3) an autocataly lic reaction, which is the same as the second reaction in the solid-phase m odel, and (4) a slow reaction resembling the third reaction step of the sol id-phase model. The solid-phase model was used to simulate the stability of I-INS under isothermal conditions at temperatures below 300 degreesC. (C) 2001 Elsevier Science Ltd. All rights reserved.