John W. Fronabarger

AN INVESTIGATION OF SOME ALTERNATIVES TO LEAD BASED PRIMARY EXPLOSIVES

Beginning in 1993, a series of Executive Orders were issued that required the review and revision of all federal and military specifications and standards to eliminate or reduce procurement of extremely hazardous substances and chemicals by federal facilities. Accordingly, a program was established at NSWC-Indian Head to study the replacement of lead azides (LA) and normal lead styphnate (NLS) with materials not containing objectionable elements. One branch of this program consisted of the identification of existing compounds that could be exploited. The ten best potential alternates for lead azide and the ten best potential alternates for lead styphnate where identified. During this effort a number of the candidates were eliminated on grounds of synthesis impracticality and/or undesirable characteristics. RD-1333 and/or NLS were used as standards for all subsequent testing. A series of screening tests were performed to determine safety properties. A limited series of studies were conducted regarding solubility and chemical compatibility characteristics. Materials deemed suitable from the screening testing were then subjected to functional testing. One potential LA replacement and two potential NLS replacements have been identified.

Environmentally Acceptable Alternatives to Lead Azide and Lead Styphnate

A new material (copper(I) 5-nitrotetrazolate or DBX-1) has been prepared during the course of our investigations into possible replacements for lead azide. DBX-1 demonstrates a particle morphology that is well suited to handling and loading, has very low water solubility and has thermal stability and output characteristics that are equal to or greater than those of lead azide. Investigations into the suitability of DBX-1 as a lead azide replacement are ongoing. In addition, a lead styphnate replacement, KDNP (7-hydroxy-4,6dinitrobenzofuroxan, potassium salt) has completed initial screening tests at PSEMC and has been selected to undergo performance evaluation and compound qualification testing as outlined in NAVSEAINST 8020.5C. Synthesis, safety characteristics and output performance of these materials is described.

ASTM-E698 Kinetic Study of HNS-IV near the Melting Point

Part of the process for qualifying hexanitrostilbene (HNS), type IV, manufactured by Pacific Scientific, Energetic Materials Co. required the determination of the critical temperature (Tc) for the material. The Tc calculation was performed per MIL-STD-1751A, Method 1074, requiring several thermal and physical properties. Some of the properties can be obtained or estimated from the literature, while others are measured directly. Two of the measured properties are the Arrhenius activation energy (Ea) and frequency factor (Z). ASTM-E698, “Standard Test Method for Arrhenius Kinetic Constants for Thermally Unstable Materials” is a simple, but controversial method of determining these factors. The method uses differential scanning calorimetry (DSC) to measure the shift in the decomposition peak position at different scan rates, ranging from 0.05 °C/min to 50 °C/min, to calculate these parameters. HNS has a melting point at 316-318 °C and decomposition of the material occurs very close to the melt. In the course of the measurements it was observed that at scan rates above 0.5 °C/min, the decomposition peak position was higher than the melt temperature, while it was at a lower temperature than the melt for scan rates of 0.5 °C/min or less. The Kissinger plot displayed reasonable linearity over the data set, but displayed a perturbation at the mid scan rates with non-parallel linear sections at the fastest and slowest scan rates that reflect the liquid and solid state decompositions respectively . The mid scan rate data reflects a combination of the solid state and liquid phase decomposition kinetics. The higher mobility of the molecules in the molten state results in a change in the decomposition kinetics of the material. The activation energy for the solid state HNS-IV can be estimated from the slow heating rate data to be approximately 66 kcal/mole (275 kJ/mole). The high scan rate data can be used to estimate the molten HNS-IV activation energy at approximately 39 kcal/mole (164 kJ/mole). It was also observed that as the scan rate dropped, the melting point also dropped to lower temperature. This is presumably due to the influence of the early decomposition products.

Critical Temperature Determinations for the Lead-Free Primary Explosives DBX-1 and KDNP

A decade long project to develop new, lead-free primary explosives has resulted in two materials that can replace lead azide and lead styphnate. The first compound is a detonating material, known as DBX-1 [copper(I) 5-nitrotetrazolate], with properties that will allow it to function as a drop in replacement for RD-1333 lead azide in the majority of applications. The second compound is a deflagrating material, known as KDNP [4,6-dinitro-7-hydroxybenzofuroxan, potassium salt], which can functionally replace lead styphnate in most applications. During the course of the development and qualifications, the self heating properties in the form of critical temperature determinations were performed for these materials. In order to allow direct comparison of the new materials with lead azide and lead styphnate, a critical temperature determination for these standard materials was also performed under the same conditions. The new materials compare favorably with the legacy materials.