Ice that forms on the wings of military aircraft affects flight performance and therefore must be completely removed before aircraft takeoff. The typical deicing method involves spraying an aircraft’s wings with a hot 80/20 mixture of propylene glycol/water just before takeoff. Although propylene glycol is ‘generally regarded as safe’ by the US Food & Drug Administration, there is a negative impact on the environment from the large biochemical oxygen demand (BOD) created during the metabolism of the propylene glycol by aquatic microorganisms. In addition, the propylene glycol based mixtures contain toxic additives. The U.S. Environmental Protection Agency will soon enact stricter regulations regarding the runoff from deicing operation at airports. An environmentally benign deicer would thus bring tremendous benefits to military and civilian airfields.
This objective of this project focused on the mitigation of technical risks associated with the development of new deicing and anti-icing material formulations based on ionic liquids (ILs) derived from natural products. The first steps involved identifying compositions of ILs made from naturally-occurring organic salts that are both liquid at room temperature as well as highly water soluble. Testing of the ionic liquid compositions was be directed at demonstrating the possibilities for compliance with key aspects of Society of Automotive Engineers (SAE) American National Standards Institute (ANSI) 1424 and 1428 along with corrosion prevention, compatibility with key aircraft parts, viscosity, residue, biochemical oxygen demand, and aquatic toxicity.
Ionic liquids (ILs) are, in all known instances, bimolecular complexes of an anion and a cation. Over the last ten years, many new ILs have been prepared and studied as replacements for common organic solvents used in the chemical industry. It appears, however, that the use of ILs has not been applied substantially to the problem of aircraft wing deicing. The physical properties of ILs are wide ranging, though; some have vitrification temperatures below -60°C. The physical properties of ILs may be tuned easily by altering the chemical composition of the constituent anions and cations.
The ILs evaluated in this project appear to have excellent freezing-point depression of water. Coupled to presumably high heat capacities, these ILs might be much more efficient at deicing than glycol-based products. The ILs described here are much less toxic and therefore ‘greener’ than the typical commercial imidazolium-type ILs. However, from a toxicity and oxygen demand stand-point, these choline-based ILs will not make a direct replacement of current deicing formulations used at airfields. However, they may be useful as a kind of surfactant that could be added to these deicing products to decrease the toxicity of the ‘additive package’ contained in these products.
The results of this project indicate that while ionic liquids have the ability to reduce the freezing point of water, they are not likely to replace current deicers because they would not provide a reduced BOD compared to glycol-based deicers. This study does provide useful information on the use of ILs for this application and identified other potential applications for these materials.