Ignition and Pyrolisis of Explosive Components

Pyrolisis reactions, existing in explosive components and mixtures, are very difficult to follow by experiments, because these processes are very fast and proceed with increasing pressure and temperature. The thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) tests and conventional cook-off methods do not give precise and correct answer to the problem of detecting the life time of chemical explosive or propellant. The method, here presented, is based in experimental analysis correlated with a simple ignition and transition model. The original experimental set-up, presented in an a previous paper, is based in classical thermogravimetry equipment (TGA), recording the evolution of sample weight as a function of temperature, for samples larger enough to solve heterogeneity problems. The sample is enclosed in an open cylindrical container, inside a glass column, where the products of combustion of propane/air flow. Different heating levels can be selected as a function of the distance from the propane/ air burner. The temperature and the mass of the sample are continuously measured and recorded during heating process. The warming transient regime and the ignition zones are clearly visible in obtained thermal results, in the range of 800 1000 K. The selected energetic materials were very well known energetic components of industrial and military plastic bonded explosives: Ammonium Nitrate (AN), pentaerythritol tetranitrate (PETN) and cyclo1,3,5 trimethylene-2,4,6-trinitramine (RDX). A reaction path, final composition and thermodynamic properties of products are predicted as a function of temperature and pressure and for isobar and isochor adiabatic combustion conditions, using a thermochemical computer code, named THOR [Campos 96]. The results are discussed with theoretical prediction of ignition temperature and time delay, based in Semenov criteria for one single particle [Campos 85]. The ignition criterium is based on the changes of reaction intensity during the heating regime, showing the start of combustion or explosion regime. Experimental results prove the validity of this ignition criterion. Kinetic parameters are based on Coats and Redfern approach. From the selected materials, PETN and RDX present an interesting temperature jumping phenomena, after ignition, predicted in theoretical model and observed in experimental results.