Detailed and global models are presented for thermodynamically inhibited nucleation-growth reactions and applied to the β-δ phase transition of HMX (nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine). The detailed model contains separate kinetic parameters for the nucleation process, including an activation energy distribution resulting from a distribution of defect energies, and for movement of the resulting reaction interface within a single particle. A thermodynamic inhibition term is added to both processes so that the rates go to zero at the transition temperature. The global model adds the thermodynamic inhibition term to the extended Prout-Tompkins nucleation-growth formalism for single particles or powders. Model parameters are calibrated from differential scanning calorimetry data. The activation energy for nucleation (333 kJ/mol) is substantially higher than that for growth (29.3 kJ/mol). Use of a small activation energy distribution (∼400 J/mol) for the defects improves the fit to a powered sample for both the early and late stages of the transition. The effective overall activation energy for the global model (208.8 kJ/mol) is between that of nucleation and growth. Comparison of the two models with experiment indicates the thermodynamic inhibition term is more important than the energy distribution feature for this transition. On the basis of the applicability of the Prout-Tompkins kinetics approach to a wide range of organic and inorganic materials, both models should have equally broad applicability for thermodynamically constrained reactions.

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The use of isoconversional, sometimes called model-free, kinetic analysis methods have recently gained favor in the thermal analysis community. Although these methods are very useful and instructive, the conclusion by some that model fitting is a poor approach is largely due to improper use of model fitting, such as fitting a single heating rate or multiple heating rates separately. The current paper shows the ability of model fitting to correlate reaction data over very wide time-temperature regimes for three polymers of interest for formulating high explosives: Estane 5703 P (poly [ester urethane] block copolymer), Viton A (vinylidene-hexafluoropropene copolymer), and Kel-F 800 (vinylidene-chlorotrifluorethene copolymer). The Kel-F required two parallel reactions⎯one describing an early decomposition process accounting for ~1% weight loss and a second autocatalytic reaction describing the remainder of pyrolysis. Essentially no residue was obtained. Viton A and Estane also required two parallel reactions for primary pyrolysis. For Viton A, the first reaction is also a minor, early

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Decomposition kinetics are determined for HMX (nitramine octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine) and CP (2-(5-cyanotetrazalato) pentaammine cobalt (III) perchlorate) separately and together. For high levels of thermal stress, the two materials decompose faster as a mixture than individually. This effect is observed both in hightemperature thermal analysis experiments and in long-term thermal aging experiments. An Arrhenius plot of the 10% level of HMX decomposition by itself from a diverse set of experiments is linear from 120 to 260 o C, with an apparent activation energy of 165 kJ/mol. Similar but less extensive thermal analysis data for the mixture suggests a slightly lower activation energy for the mixture, and an analogous extrapolation is consistent with the amount of gas observed in the long-term detonator aging experiments, which is about 30 times greater than expected from HMX by itself for 50 months at 100 o C. Even with this acceleration, however, it would take ~10,000 years to achieve 10% decomposition at ~30 o C. Correspondingly, negligible decomposition is predicted by this kinetic model for a few decades aging at temperatures slightly above ambient. This prediction is consistent with additional sealed-tube aging experiments at 100-120 o C, which are estimated to have an effective thermal dose greater than that from decades of exposure to temperatures slightly above ambient.