Kinetic evaluation of propellants decomposition via Kissinger and Flynn-Wall-Ozawa method (Poster)

Nitrocellulose (NC) based propellants are intrinsically unstable due to degradation of NC as a function of time and temperature. A propellant that decomposes, dissipates heat to the surrounding. Self-heating of the propellants starts when this heat production becomes larger than its dissipation to the surrounding. Since this is a safety issue, propellants are monitored for their life-time. An important factor in determining this life-time is the activation energy of decomposition. This is used in the calculation of the life-time, based on accelerated ageing at elevated temperatures. One of the procedures to study the stability of these NC based propellants is described in a standard STANAG 4582 [1]. Here, Heat Flow Calorimetry (HFC) is used to examine the remaining life-time of NC based propellants based on a dual activation energy (80kJ/mol<60°C, 120kJ/mol>60°C)[l]. Literature however shows that the reported activation energies vary over a large range and have a tendency to be higher than used in STANAG 4582[2]. The purpose of this study was to determine the apparent activation energy of three NC based propellant formulations by means of dynamic measurements with Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) applying the Kissinger [3,4] and Flynn-Wall-Ozawa [5] method. Although it is known that there is a large difference in measuring conditions between HFC and DSC (sample size, atmosphere, open/closed), dynamic DSC/TGA measurements were used because they are fast and require only minimum sample size (mg range). Data treatment was performed by the two methods. The first is the Kissinger method, or alternatively called peak-maximum evolution method [3,4]. This method uses the point where the rate of reaction during heating is at its maximum (Tp). Another approach is the Flynn-Wall-Ozawa method [5]. In contrast to Kissinger, which is designed for the peak temperature, Flynn-Wall-Ozawa worked out a method to determine the activation energy based on the temperature that corresponds to a certain conversion degree (Ta), a so called iso-conversional method. This allows to calculate the apparent activation energy as a function of the conversion degree. This is useful to check the invariance of the activation energy during the course of the reaction. Activation energies for all three propellants via the two techniques and the two models were determined. Comparison and critical interpretation of the results and methods were made.