Effect of morphology on the isothermal decomposition kinetics of nitroguanidine

The thermal stability of nitroguanidine (NQ) with different morphologies was studied using a self-developed isothermal decomposition gas manometric device at both high and low temperatures. At 443.15–463.15 K, the total decomposition of needle-like NQ (NNQ) and hexagonal block-shaped NQ (HNQ) proceeded in two stages: a fast decomposition stage after heating, and a much slower decomposition stage later. It was indicated that all the models for NNQ and HNQ at the first stage followed the Avrami-Erofeev equation but with different values of n, while all the models at the second stage conformed to Parabola order. The decomposition rate of HNQ was faster than NNQ after heating, showing a better stability of NNQ. At 373.15–413.15 K, the initial decomposition kinetics were calculated. The activation energy calculated by mode-fitting method was 146.33 kJ/mol and 182.71 kJ/mol for NNQ and HNQ, respectively, which was consistent with the results obtained through model-free method. The average activation free energy (ΔG≠), enthalpy (ΔH≠) and entropy (ΔS≠) for NNQ were 148.30 kJ/mol, 142.03 kJ/mol, and -15.81 J/(mol•K), respectively, while they were 139.61 kJ/mol, 184.42 kJ/mol, and 107.14 J/(mol•K) for HNQ. The storage life of NNQ and HNQ at 298.15 K were extrapolated to 21.58 and 29.44 years, respectively.