Thermal Behaviour of Energetic Materials in Adiabatic Selfheating Determined by ARC~(TM)

摘要

The thermal behaviour of energetic materials is an important property of them. Several useful methods exist to determine it: DSC (dynamic scanning calorimetry), TGA (thermal gravimetric analysis), heat flow microcalorimetry (HFMC), mass loss by weighing, EGA (evolved gas analysis) coupled with other methods, temperature resolved X-ray scattering, measurement of thermal transport properties by transient source methods as HotDisk~(TM), and other specialized methods. Here the adiabatic selfheating of a variety of energetic materials is discussed, determined by ARC~(TM) (Accelerating Rate Calorimeter~(TM)). In comparison to DSC and TGA, which uses mostly a forced linear heating up of the sample and to HFMC, which is mostly used iso-thermally, ARC~(TM) determines the self heating of the sample in a pseudo adiabatic environment. So the decomposition process is mainly sample controlled and not forced from outside. The state of adiabaticity is obtained by a precisely controlled heating up of an oven in which the sample is centrally positioned in a closed metallic container. The heating up of the oven follows the self heating of the sample. A typical result is the selfheating rate of the sample as function of the adiabatically reached temperature, which is caused by the exothermal decomposition of the sample, not by the instrument, which serves only as device to maintain adiabaticity also with small sample amounts. Typical sample amounts with energetic materials are in ARC~(TM) between 200 mg and 600 mg, which is much more than normal DSC and TGA instruments can handle. This pseudo adiabatic state of a decomposing sample is also found in real situations: samples with bigger size and low heat conductivity (materials in barrels or shipping drums), samples heated up by forced heating and showing a temperature increase in their center by heat accumulation (as in slow cook-off), energetic materials in reaction vessels, NC-based gun propellants in unstabilized state and starting with the thermal runaway, NC-based gun propellants still stabilized but piled up to heaps or collected in big drums, and more situations. From this point of view the determination of selfheating has the intention to assess the safety against thermal explosions. Another aspect is the determination of thermally induced decomposition behaviour. The samples are in a defined environment (adiabatic situation) and therefore their decomposition behaviour is well comparable. Because of the closed measuring device, decomposition gases are contained and may act back on the sample. This means autocatalyti-cally caused decomposition is well recognizable by a steep increase of the seifheatrate. Selfheating determind by ARC~(TM) is well suited to compare materials with respect to their slow cook-off behaviour. With well conducted measurements one can obtain controlled deflagration of the sample inside the measurement cell and the associated temperatures are quite near to the cook-off temperatures determined with typical slow cook-off devices. Because of the closed measurement conditions also the pressure increase and the decomposition gas generation can be determined. Moreover, other atmospheric conditions can be provided as vacuum, argon or helium or nitrogen atmospheres, or reactive atmospheres as synthetic air, NO2 or what is intended to investigate. After an introduction of the principles of operation and of data evaluation, the paper presents results on several types of energetic materials: Formulations as gun propellants (single, double, triple base), rocket propellants and high explosive charges; ingredients alone as ADN, HNF, FOX 7, FOX 12, nitroguanidine, TAGN, AN, RDX, HMX, TNT, NTO, CL-20, GAP, NC and others. Also substances in toluene solution have been measured, which allows a further control on the decomposition behaviour and the determination of decomposition parameters.