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Physical Characterization, Accelerating Rate Calorimetry and Thermal Conductivity of Ammonium Nitrate Emulsion
Abstract
In the recent years, there have been numerous incidents involving runaway reactions in ammonium nitrate emulsion (ANE) explosives. In most cases, thermal explosions have occurred during abnormal (dry-running or deadheading) pumping operations Current work at the Canadian Explosives Research Laboratory involves assessing the thermal hazards of ANE explosives in order to better understand the effect of their formulation and physical properties on their thermal stability. Since manufacturers’ (proprietary) ANE formulations vary considerably to suit the end-user, multiple small (1-2 kg) batches of ANE were prepared in-house using a standard formula of diesel oil/surfactant/water/ammonium nitrate. We report on the basic physical properties of these ANEs: viscosity, density, water content, emulsion droplet size and size distribution. Additionally, using a recently established technique for measuring the thermal conductivity () of small volumes of energetic materials, the base emulsion and base emulsion blended with sensitizing agents such as aluminum particles or glass microballoons, is reported.
The thermal decomposition of these ANEs was characterized using accelerating rate calorimetry (ARC). Preliminary data suggests an interesting correlation between the measured refinement (emulsion droplet size) and the ARC detected onset temperatures for thermal runaway. Correlation of the thermal conductivity measurements with the ARC detected thermal decomposition behavior is also discussed.
The paper will review the feasibility of adapting the Modified Transient Plane Source (MTPS) method as a screening tool for early-detection of explosives and hazardous materials. Materials can be distinguished from others based on their inherent thermal properties (e.g. thermal effusivity) in testing through different types of barrier materials. A complimentary advantage to this technique relative to other traditional detection technologies is that it can penetrate reflective barrier materials, such as aluminum, easily. A strong proof-of-principle is presented on application of the MTPS transient thermal property measuring in the early-screening of liquid explosives. The work demonstrates a significant sensitivity to distinguishing a wide range of fluids based on their thermal properties through a barrier material. The work covers various complicating factors to the longer-term adoption of such a method including the impact of carbonization and viscosity. While some technical challenges remain, the technique offers significant advantages in complimenting existing detection methods in being able to penetrate reflective metal containers (e.g. aluminum soft drinkscans) with ease.
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