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

Abstract

Double base propellants are composed of two basic components: nitrocellulose as a matrix and nitroglycerin as a plasticizer and blasting oil at the same time, and of additives such as stabilizers, burning catalysts, modifiers of ballistic properties, etc. In the course of time, a number of chemical and physical processes (e.g. stabilizer consumption, migration and evaporation of nitroglycerin, decomposition of nitroglycerin 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, the change in amount of nitroglycerin significantly affects mechanical properties. This is the reason why the evaporation of nitroglycerin from double base propellants is a subject of great importance. In this work we have studied a very early stage of evaporation of nitroglycerin from a double base rocket propellant applying isothermal thermogravimetry experiments. The experiments were done with a propellant containing 27% nitroglycerin, using thin plate samples having a thickness of 0.2–0.4 mm and weighing ∼4 mg. It was found that at a very early stage the evaporation can be described by the zero-order reaction model, while the entire process is characterized by power law decrees of the evaporation rate with time. The Langmuir vaporization equation, and equations proposed by Pieterse and Focke, and by Beverley et al. are used to relate nitroglycerin mass loss data and the vapor pressure. The activation energy of nitroglycerin evaporation was calculated to be 81.9 kJ mol −1 and the pre-exponential factor 5.6 × 10 7 s −1, while calculated enthalpy of nitroglycerin evaporation at 298.15 K lies between 80.0 and 90.5 kJ mol −1, depending on the calculation method applied.