Understanding TATB (1,3,5-triamino-2,4,6-trinitrobenzene) thermal decomposition

Abstract

First synthesized in 1888 as a dye, TATB (1,3,5-triamino-2,4,6-trinitrobenzene) was not recognized as a high explosive until 1956 when it was reported that TATB possesses a combination of performance properties, high thermal stability, and low impact sensitivity 1. Now, decades later, TATB is widely regarded as the most established insensitive high explosive. However, despite having been widely used, the thermal decomposition kinetics of TATB remain elusive. Thermal decomposition literature widely disagrees on the importance and identity of intermediate species, in both the gas and solid phases, and the interpretation of results is often clouded by mass-transport and self-heating effects. Understanding the molecular reactions contributing to TATB decomposition is essential for determining reactivity and safety of high explosives subjected to abnormal environmental conditions, such as fires.

In this issue of Propellants, Explosives, Pyrotechnics a special collection of current research on TATB is presented. For the past several years, researchers at the Lawrence Livermore National Laboratory's Energetic Materials Center, have been working to deconvolute the multi-physics nature of TATB thermal decomposition, with the goal of producing models capable of predicting response over a wide range of temperature and pressure, as well as possible changes to material composition, structure, and sensitivity.

Many-parameter models require calibration to achieve this objective, and our general approach is to use a variety of small-scale experiments, isolating individual components of the mechanisms (chemical and physical), then using larger-scale experiments to test and validate predictive capabilities. Thus, these efforts have progressed on scales across several orders of magnitude with an array of analytical techniques, many of which have not previously been applied to TATB. These techniques were developed specifically to accomplish this experimental and modelling progress on TATB thermal decomposition. These include analysis by solid-state nuclear magnetic resonance, mass, and infrared spectrometry, for identifying molecular species by isolation and detection of light gases, extracted soluble fractions, and insoluble residues.

TATB sublimation and thermal degradation are intrinsically competitive processes during heating. Simultaneous differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) under varying degrees of confinement, initial mass, and heating profiles can enable teasing apart the conflicting effects between sublimation and degradation. This approach more fully characterizes the nature of sublimation in TATB and the importance of gas-phase residence time on the degree of autocatalysis. Additionally, minimizing self-heating, by limiting sample mass to control the maximum heat flow, decouples reaction from heat transfer limitations permitting study of the intrinsic kinetics.

Chemical kinetic models are at their best when they honor mass and energy balances. Our recent work was the first to quantitatively relate the amount of heat generated from TATB as a function of time and the amounts of soluble, insoluble, and gaseous products. In addition, soluble and insoluble products were characterized by Fourier-transform infrared (FTIR) and liquid chromatography mass spectrometry (LC–MS). Several new intermediate degradation compounds were identified both in the gas phase and solid residues, through analytical methods developed for this effort. This includes identification of a family of compounds resulting from the combination of two TATB degradation products into larger molecules, one step closer to amorphous carbon char. We have also added to the identification of the nature of this transition to weakly organized graphitic and amorphous carbon nitride, with inclusion of condensed C−C (sp²) and C−N (ring) structures losing functionality as thermal severity increases. A wide variety of thermal histories were explored, including interrupted heating schedules, to constrain a global chemical kinetic model. We found no published TATB kinetic model accurate in fundamental structure or capable of matching the reaction extent for thermal analysis conditions.

While furazan producing condensation reactions and their H₂O release have been previously reported, early CO₂ evolution in decomposition experiments has thus far been unexplained. Through isotopic labeling to produce ¹³C₆-TATB with the ¹³C incorporated into the benzene backbone, the resulting shift from mass 44 to 45 confirmed the source of the CO₂ was indeed the TATB ring, not adsorbed or adventitious gas. Additionally, ¹⁵N labeled TATB revealed the specific composition of multiple degradation intermediates.

Prior to our work, the furazan and furoxan mechanisms were generally thought to be dominant. However, based on the results of several small-scale tests, we have introduced NO₂ causing a cascade of oxidation steps at least in some conditions, which have been incorporated into the new model. Using cryo-focused pyrolysis gas chromatography mass spectrometry (pyGC-MS), our team has identified a new molecular profile emerging as TATB is heated to decomposition temperatures. We identified known furazan decomposition products and discovered additional compounds associated with loss of nitro functional groups and opening of the furazan ring structure. The release of nitrogen dioxide gas from loss of nitro functional groups is likely driving the auto-oxidation of TATB, while furazan ring opening facilitates formation of lower molecular weight gas-phase molecules. Work to date suggests the dominant mechanism changes between 100 and 300 °C. These findings are vital for understanding the heat flow, enthalpy, and kinetic decomposition models over a wide range of temperatures and pressures.

Taken together, the techniques and data were used to create a multi-step thermal decomposition model. The methods herein should influence future studies involving decomposition measurements and modeling.

Acknowledgments

This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. LLNS, LLC. LLNL-JRNL-857233.