The effect of molecular structure on thermal stability, decomposition kinetics and reaction models of nitric esters

In this paper, the thermal stability and decomposition mechanism functions of 10 nitric esters including nitroglycerine (NG), pentaerythritol tetranitrate (PETN), trimethylolethane trinitrate (TMETN), dipentaerythritol hexanitrate (DiPEHN), trimethylolpropane trinitrate (TMPTN), erythritol tetranitrate (ETN), xylitol pentanitrate (XPN), sorbitol hexanitrate (SHN), mannitol hexanitrate (MHN) and nitroisobutylglycerol trinitrate (NIBGT) are determined by means of non-isothermal TG and DSC techniques. It has been found that the mean activation energies for most nitric esters are comparable at constant heating rate (around 145 kJ mol^-1), indicating that their main decomposition pathways might be the same. The mass loss activation energies of NG, TMETN and TMPTN are less than 100 kJ mol^-1 due to partial evaporation. Based on the critical temperature of thermal decomposition, the order of molecular stability for involved nitric esters is found to be MHN < XPN < TMPTN < SHN < NIBGT < ETN < PETN < DiPEHN. The introduction of function groups to the tertiary carbon is in favor of increasing thermal stability due to increase of symmetry and rigidity of the molecule. The decomposition kinetics was described in terms of the Johnson-Mehl-Avrami and Šesták-Berggren models. Two types of kinetic behavior were observed and most nitrate esters followed typical decomposition kinetics close to the first order reaction. However, certain materials showed complex behavior caused by overlapping of more mechanisms/processes, which were represented either by simultaneous evaporation and decomposition or by different decomposition mechanisms originating from varying morphology and structure of the samples.