A comprehensive mechanism for liquid-phase decomposition of 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX): Thermolysis experiments and detailed kinetic modeling

Abstract

The nitramines 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), and 1,3,5-trinitro-1,3,5-triazinane (RDX) are energetic materials commonly used in solid propellants and explosives. In order to predict ignition and deflagration of propellants containing these ingredients, their thermal decomposition behaviors must be thoroughly understood. In this study, the thermal decomposition of HMX was investigated using synergetic application of experimental and computational methods. Mole fraction profiles of the gaseous decomposition products evolving from the liquid-phase HMX were obtained using Fourier transform infrared (FTIR) spectroscopy for two types of thermolysis experiments – thermogravimetric analysis (TGA) and confined rapid thermolysis (CRT). Four heating rates (5, 10, 15, and 20 K/min) in TGA experiments and four set temperatures (290, 300, 310 and 320°C) in CRT experiments were considered. In the TGA and differential scanning calorimetry (DSC) results, steep mass loss and rapid decomposition were observed after the melting of the HMX at 280°C. CH₂O and N₂O were identified as the major decomposition products. Smaller quantities of H₂O, HCN, NO and NO₂, CO and CO₂ were also formed. In the complementary computational study, liquid-phase elementary reactions were investigated using quantum mechanics calculations at B3LYP/6-311++G(d,p) level of theory with the conductor-like polarizable continuum model (CPCM). A zero-dimensional model was developed to simulate the TGA and CRT experiments based on conservation of mass and species in the condensed-phase and the gas-phase control volumes. The predicted mass loss and gas-phase mole fraction profiles of the decomposition products are in good agreements with the corresponding experimental results, indicating that the comprehensive mechanism proposed here captures the important reactions occurring during liquid-phase decomposition of HMX.

Introduction

The cyclic nitramines HMX and RDX are important energetic ingredients commonly used in many applications, including, among others, explosives and rocket propellants. A quantitative understanding of chemical kinetics during thermal decomposition is necessary for predictive modeling of ignition, combustion and detonation behavior of explosives and solid propellants containing these ingredients. Thus, RDX and HMX have been extensively investigated with a focus on structural characterization [1], [2], [3], [4], [5], [6], phase transformation [7], [8], [9], [10], [11], [12], thermo-physical properties [13], [14], [15], thermal decomposition behavior [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], detonation characteristics [26], [27], [28], [29], [30], [31], [32], as well as burn-rate and gas-phase flame structure [33], [34], [35], [36], [37], [38], [39]. During the past few decades, theoretical studies have also become increasingly important for predictive modeling of the ignition, combustion and detonation behaviors of these energetic materials [40], [41], [42], [43], [44], [45], [46].

Brill and Karpowicz [8] used FTIR spectroscopy to obtain Arrhenius parameters for the β-δ phase transition in HMX and proposed that the breakdown of intermolecular forces controls the rate of thermal decomposition. A detailed quantum molecular dynamics study was performed by Ye et al. [12] to explain the structural changes associated with β-HMX to δ-HMX phase transition and the initial unimolecular reaction pathways of these two conformers. Second-harmonic imaging microscopy was used to investigate the β-δ phase transition mechanism [9] and develop a kinetic model [10,11]. Products formed during thermal decomposition of HMX have been identified by rapid-scan/FTIR spectroscopy [16] and triple quadruple mass spectrometry [19]. Temporal evolution profiles of gaseous decomposition products of various energetic materials have been probed using simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) [20,21] and confined rapid thermolysis (CRT)/FTIR spectroscopy [22]. The kinetics of phase change, thermal ignition, and decomposition have been investigated using cook-off studies [26,27]. Using the measured gas formation rates from STMBMS experiments of Behrens, kinetic parameters for global decomposition models were obtained and applied to slow cookoff in accident scenarios [23,24].

The times-to-explosion for HMX-based plastic-bonded explosives calculated by Tarver and Tran [29] using a global kinetic model are in reasonable agreement with the experimental results [30], [31], [32]. Nonetheless, the times-to-explosion are over-predicted using the global model above the liquefaction temperature of HMX, i.e., 553 K, which may be attributed to the inability of the model to account for faster autocatalytic decomposition in the liquid phase. Recently, Hobbs et al. [25] have successfully modeled the cookoff of HMX-based plastic bonded explosive (PBX 9501) using a melt rate accelerator factor at temperatures above melting point. This approach is reasonable from an engineering perspective, however, for a fundamental understanding at the molecular level, the detailed liquid-phase mechanism is required to explain faster rates at higher temperatures.

In addition to cook-off models, various theoretical models, as reviewed by Beckstead et al. [41] have been formulated to study ignition [46] and deflagration [42], [43], [44] of HMX. These models analyze the combustion wave structure in three regions (solid phase, melt layer and gas phase) by solving the equations for the conservation for mass, momentum, energy, and species. In the solid phase, β-HMX to δ-HMX phase transition is modeled. Other solid-phase decomposition reactions are assumed not to occur in these combustion models. The melt layer is either ignored or modeled by a three-step global mechanism [47]. A detailed reaction mechanism, originally developed by Melius [48,49], refined by Yetter et al. [50], and further updated by Chakraborty et al. [51] is used in the gas phase.

During thermal decomposition of HMX, various gaseous species, including CH₂O, N₂O, HCN, NO, NO₂, CO, CO₂, and H₂O, evolve from the condensed phase. Based on the analysis of these decomposition products, various reaction pathways were proposed. However, due to fast reactions and short-lived intermediates, experimental identification of the reaction products and the elementary reactions is quite challenging. Recently, quantum mechanics calculations have been performed in many theoretical studies to probe solid-phase and gas-phase decomposition of HMX. Truong and co-workers [52], [53], [54] performed direct dynamics calculations for the gas-phase decomposition of α-HMX at B3LYP/cc-pVDZ level of theory. Based on the analysis of rate constants calculated using transition state theory, they found that the NNO₂ bond fission is the preferred decomposition pathway. Molt et al. [5,6] used high-level ab initio methods to compute the geometries and energetic ordering of various conformers of RDX and HMX in the gas phase. In a subsequent study [55], they were the first to locate a well-defined transition state for NNO₂ bond homolysis during gas-phase decomposition of RDX. With a barrier of 41.9 kcal/mol, HONO elimination was found to be the dominant initial reaction. The equivalent barrier for NNO₂ homolysis was 53.9 kcal/mol. However, a recent study [56] on rate constants using variable reaction coordinate transition state theory have concluded NNO₂ bond dissociation as the dominant pathway in the gas phase.

Decomposition of RDX and HMX from their excited electronic states and related anionic species was investigated by Bernstein and co-workers [57], [58], [59]. NO₂ and HONO were ruled out as sources for NO formation, which indicates that the decomposition mechanism is different in the excited states as compared to the ground states. Zhang et al. [60] found that H⁺and OH⁻ accelerates the decomposition of HMX in the gas phase, however, in the aqueous solution, this is true for OH⁻ only. Kuklja and co-workers [61], [62], [63], [64] investigated gas-phase and solid-phase decomposition of β- and δ-HMX, and observed that NNO₂ bond fission is the dominant pathway in the gas phase. In the condensed phase, NNO₂ bond fission is also the dominant pathway for β-HMX, however, the exothermic HONO elimination channel is found to be more favorable in δ-HMX.

Most of these theoretical studies have investigated the gas-phase or solid-phase decomposition of HMX and RDX. A large number of reactive molecular dynamics studies [12,[65], [66], [67], [68], [69], [70] have also been performed with a focus on detonation characteristics. However, very little attention has been given to liquid-phase decomposition of HMX, even though the melt layer has been observed at the surface of burning propellants [71]. The liquid-phase decomposition in various combustion models is still represented by a global mechanism [42], [43], [44], which is inadequate for rigorous combustion modeling. The global mechanisms do not accurately capture the evolution of various decomposition products from the condensed phase into the gas phase above the surface of the burning propellants. This may also lead to inaccuracies in the gas-phase combustion mechanisms. Hence, liquid-phase chemical kinetics needs further investigation.

In this study, we demonstrate synergetic application of quantum mechanics calculations, chemical kinetic modeling and thermolysis experiments to develop and validate a comprehensive mechanism for liquid-phase decomposition of energetic materials consisting of elementary reactions. HMX is chosen as the representative energetic material but the methodology is applicable to other energetic materials as well. The first objective is to quantify species evolution rates during thermal decomposition of HMX using FTIR spectroscopy for low heating rates using TGA setup and for high heating rates using CRT setup. In our previous work [72,73], quantum mechanics calculations were performed to develop initiation mechanism for the liquid-phase decomposition of RDX and HMX. Thus, the second objective is to further expand and validate the HMX initiation mechanism consisting of elementary reactions. The detailed chemical kinetic mechanism in the liquid phase developed and validated here can then be used in three-phase deflagration models along with appropriate solid-phase and gas-phase mechanisms to predict the ignition and combustion behavior of HMX.