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Kinetics and enthalpy of nitroglycerine evaporation from double base propellants by isothermal thermogravimetry

Double base propellants are composed of two basic components: nitrocellulose as a matrix and nitroglycerine as a plasticiser and blasting oil at the same time, and of additives such as stabilisers, burning catalysts, modifiers of ballistic properties, etc. In the course of time, a number of chemical and physical processes (e.g. stabiliser consumption, migration and evaporation of nitroglycerine, decomposition of nitroglycerine and nitrocellulose, etc.) take place in a propellant grain. The result is a change of propellants’ physical, chemical, thermal, ballistic and mechanical properties. The final consequence is reduction of performances and decrease of safe service lifetime of propellants. Reduction of mechanical properties of a propellant grain during ageing can be the factor that limits rockets safe service time. On the other hand, change in the amount of nitroglycerine significantly affects mechanical properties. This is the reason why the evaporation of nitroglycerine from double base propellants is a subject of great importance. In this work we have studied evaporation of nitroglycerine from a double base rocket propellant applying isothermal thermogravimetry experiments. The experiments were done with a propellant containing 27% nitroglycerine, using thin plate samples having a thickness of 0.2-0.4 mm. It was found that only at a very early stage evaporation can be approximated by the zero-order reaction model, while the entire process is characterised by decrease of the evaporation rate with conversion and can be described by the n-th order kinetic model. The activation energy of nitroglycerine evaporation was calculated to be 81.9 kJ/mol and the pre-exponential factor 5.6107 1/s. The enthalpy of nitroglycerine evaporation, calculated in two different ways based on the method which combines the Langmuir and the Clausius-Clapeyron equations, was calculated to be 82.6 kJ/mol. Closed values of the evaporation enthalpy and the activation energy indicate a non-activated mechanism of evaporation.

kinetics of evaporation; evaporation; propellant; double base; nitroglycerine; enthalpy of evaporation; thermogravimetry

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