(37c) Thermal Hazard Evaluation of 1-Hydroxybenzotriazole Hydrate By DSC, ARC and Calvet Calorimeter (C80)

1-Hydroxybenzotriazole (HOBT) is an important fine chemical that is widely used in various chemical and pharmaceutical processes. HOBT is classified by the UN/DOT as a hazard class 1.3C explosive when in anhydrous form, while the commercial form of HOBT contains 10-20 wt% water and is classified as a hazard class 4.1 desensitized explosive, or “self-reactive” solid. In recent years, there were multiple explosion accidents reported in the production of HOBT, causing serious injuries and property damage. The references of hazard assessment for HOBT are scarce. Hence, a comprehensive thermal hazard evaluation for HOBT production, storage, transport, and use deserves further attention.

The results of dynamic Differential Scanning Calorimetry (DSC) for HOBT hydrate indicated an endothermic melting event occurred at a peak temperature of 94^oC and a large exotherm of 1.6 KJ/g peaked at 215^oC. Accelerating Rate Calorimetry (ARC) data showed that a runaway thermal decomposition started at 151^oC, that released a large amount of heat and gases. The exotherm was extremely rapid with a maximum temperature rise rate of ~3400 K/min and a maximum pressure rise rate of ~13K bar/min. Time to maximum rate (TMR_ad) for the thermal decomposition of HOBT was conservatively estimated under the assumption of a zero-order reaction using ARC data. The severity and probability of thermal runaway was also assessed.

In many cases, the self-accelerating decomposition temperature (SADT), which defines the safe temperature of storage and transport, can be estimated from ARC data. This estimation is provided by extrapolation to lower temperatures, and the results agree well with the United States SADT test recommended by the UN manual. However, in the case of HOBT hydrate, the decomposition reaction takes place in the liquid phase after it is melted, whereas the assessment for storage and transportation must be done for the solid phase. Using the exothermic peak to determine the SADT at lower temperature is equivalent to using the kinetic data of the liquid to assess the stability of solid. This could result in a very unreliable evaluation. To address this concern, we employed a highly sensitive Calvet calorimeter (Setaram C80) to study the thermal behavior of the HOBT hydrate in the solid phase and determine its SADT. Exothermic activity was indeed detected before HOBT hydrate melted. The thermal kinetic model and the results of SADT determination will be presented.

Water has been used as a desensitizer in the commercial form of HOBT to suppress explosive potential and to reduce the sensitivity to friction and shock. However, our study revealed that the addition of more water into HOBT hydrate resulted in more heat release from the thermal decomposition, suggesting that water is potentially reactive and may cause secondary reactions with the decomposition products. The primary thermal parameters (e.g., exothermic onset temperature, self-heat rate, heat of reaction, apparent activation energy) for the decomposition of HOBT with various water contents (11.7wt% - 21.5wt%) will be compared and discussed.