Characterization of Explosives & Precursors
A. Overview and Significance
All new materials require characterization; but in the case of explosives, complete characterization is especially important in terms of safety concerns–safety for those who handle the materials and safety for those with expectations that the materials perform. In the case of homemade explosives (HMEs), the materials may not be exactly new (many were reported in the late 1800’s), but their “routine” handling by those involved in
counterterrorism has resulted in accidents and raises questions about detectability.
To detect, destroy, handle safely or prevent the synthesis of HMEs, complete understanding of the following information is essential:
• How the HME is formed and what accelerates or retards its formation;
• How the HME decomposes and what accelerates or retards that decomposition;
• How the HME crystallizes;
• What is its vapor pressure and what is its headspace signature;
• What is its density;
• What is its sensitivity to accidental ignition as well as purposeful ignition;
• What is its performance under shock and fire conditions?
Admittedly this mission is too large and R1-A.1 has approached it material by material. In previous years, we have examined triacetone triperoxide (TATP) and erythritol tetranitrate (ETN). The detailed examination of TATP resulted in 10 publications and led to a method of preparing safe, long-lasting canine and instrument training aids. ETN, a compound chemically similar to the often used explosive PETN (pentaerythritol tetranitrate), is still being examined. Our present focus has been on hexamethylene triperoxide diamine (HMTD) and fuel/oxidizer (FOX) mixtures, in general.
Many laboratories which work directly or indirectly on homeland security issues are not able to purchase or store explosives, especially HMEs. Our database provides a valuable service to those laboratories. Standard chemical properties are measured and uploaded to a database for assessment by registered users. In addition, advice is available in terms of how to perform analyses in their own laboratory; and, in a few cases, personnel have been sent to train in the URI laboratory. Disposal of small quantities of HMEs can also be a concern. URI is a leader in the field of chemical digestion of unwanted HMEs. Research on FOX mixtures is a field where little definitive information is available but there is much speculation in terms of what “works” and what “ought to work”. Our research in this area has two goals: (1) To allow the homeland security enterprise (HSE) to narrow or widen the list of threat oxidizers; and (2) To collect and match sufficient small-scale data to large-scale performance so that small-scale data has greater predictive value.
R1-A.1 is currently focused on HMTD formation and decomposition and on bounding the range of FOX mixtures which can be used as explosives. Publications regarding our findings can be found in section V.A. One of our first approaches to the study of HMTD was examining analysis methods. HMTD exhibits an unusual gas phase phenomenon in the presence of alcohols, and we used positive ion atmospheric pressure chemical ionization (APCI) mass spectrometry to examine this behavior. HMTD was infused with various solvents, including 18O and 2H labeled methanol, and based on the labeled experiments, it was determined that under APCI conditions, the alcohol oxygen attacks a methylene carbon of HMTD and releases H2O2.
Our work continued to study synthesis and decomposition of HMTD in condensed phase. Mechanisms are proposed based on isotopic labeling and mass spectral interpretation of both condensed phase products and headspace products. Formation of HMTD from hexamine appeared to proceed from dissociated hexamine, as evident from the scrambling of the N label when synthesis was carried out with equal molar labeled/unlabeled hexamine. The decomposition of HMTD was considered with additives and in the presence and absence of moisture. In addition to mass spectral interpretation, density functional theory (DFT) was used to calculate energy differences of transition states and the entropies of intermediates along the decomposition pathway. HMTD is destabilized by water and citric acid, making purification following initial synthesis essential in order to avoid an unanticipated violent reaction.
A survey of the stability and performance of FOX mixtures examined 11 solid oxidizers with 13 fuels by differential scanning calorimetry (DSC), simultaneous differential thermolysis (SDT) and hot-wire ignition. Sugars, alcohols, hydrocarbons, benzoic acid, sulfur, charcoal and aluminum were used as fuels; all fuels except charcoal and aluminum melted at or below 200oC. It was found that the reaction between the oxidizer and the fuel was usually triggered by a thermal event, i.e. melt, phase change or decomposition. Although the fuel usually underwent such a transition at a lower temperature than the oxidizer, the phase change of the fuel was not always the triggering event. When sugars or sulfur were the fuels, their phase change usually triggered their oxidation. However, 3 oxidizers, KNO3, KClO4, NH4ClO4, tended to react only after they underwent a phase change or began to decompose, which meant that their oxidization reactions, regardless of the fuel, was usually above 400oC. KClO4fuel mixtures decomposed at the highest temperatures, often over 500oC, with the ammonium salt decomposing almost 100oC lower. Mixtures with ammonium nitrate (AN) also decomposed at much lower temperatures than those with the corresponding potassium salt. With the exception of the oxidizers triggered to react by the phase changes of the polyols and sulfur, FOX mixtures generally decomposed between 230oC and 300oC, with AN formulations generally decomposing at the lowest temperature. In terms of heat release, potassium dichromate/fuel mixtures were the least energetic, generally releasing less than 200 Jg-1. Most of the mixtures released 1000 to 1500 Jg-1, with potassium chlorate, ammonium perchlorate and AN releasing significantly more heat, around 2000 Jg-1. When the fuel was aluminum, most of the oxidizers decomposed below 500oC, leaving the aluminum to oxidize at over 800oC.
Only two oxidizers reduced the temperature of the aluminium exotherm: chlorate and potassium nitrite. To go to temperatures above 500oC, unsealed crucibles were necessary, but with these containers, the endothermic volatilization of reactants and products effectively competed against the exothermic decomposition so that heat release values were artificially low.
Studies suggest that thwarting the synthesis of HMTD will be challenging. Further mechanistic studies are underway in order to devise the best approach to this problem.Phase 2 Year 2 Annual Report
Faculty and Staff Currently Involved in Project
Students Currently Involved in Project
- Maria Donnelly
- Matt Porter
- Austin Brown
- Devon Swanson
- Lindsay McLennan
- Jon Canino
- Kevin Colizza
- Stephanie Rayome