Thermal Analysis and Thermal Hazard Analysis of Ionic Liquids

Ionic liquids (ILs) are organic salts with low melting points (below 100°C). They are attractive for many applications because their properties can be tailored by adjusting both the cation and anion. Thermal stability is an important factor in many potential applications of ILs, such as heat transfer fluids, electrolytes in batteries, and high-temperature lubricants. In addition, energetic ILs have been developed for propulsion applications. The basic thermophysical properties, like decomposition temperature, melting point and glass transition temperature, are important for design and evaluation for these applications. In this work, these properties were measured using thermal analysis tools Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC).

ILs are often described as thermally stable based on their relatively high decomposition onset temperatures from TGA with open sample crucibles. However, thermal runaway reactions have been observed for some ILs with Accelerated Rate Calorimetry (ARC) with a closed sample bomb. This information is crucial from a safety standpoint, because the condition in an ARC system is similar to the real world thermal environment for many potential application of ILs. However, this information is not yet available for most ILs due to the fact that relatively large amounts of sample are required for an ARC test and the cost of ARC itself.

In this work, custom-made DSC high-pressure crucibles have been constructed in-house using a micro-welding system containing a jewelry spot welder and a microscope. They were constructed from different metals and alloys, including 304 stainless steel, 316 stainless steel, titanium, tantalum, gold-plated stainless steel 304, and brass. Testing ILs in different crucibles made from different metal/alloys simulates different real-world storage, transportation, and process operations in the chemical industry. The cost for each pair of crucibles is less than two U.S. dollars. Kinetic parameters can be extracted from only a few DSC runs with few milligrams of samples with these crucibles. Potential thermal hazard and incompatibilities with commonly used metals/alloys were assessed for selected ILs to provide guidance for ILs’ potential high temperature applications.