Ionic Liquids for Cleaning Operations at Air Force Logistics Centers
By Janelle Yerty, Melissa Klingenberg, Elizabeth Berman and Natasha Voevodin
A number of industrial solvents used in cleaning operations have been designated by the U.S. Environmental Protection Agency as producing greenhouse gases (GHGs) or containing volatile organic compounds (VOCs), hazardous air pollutants (HAPs), and/or ozone-depleting substances (ODSs). Although many earlier ODS issues were addressed previously, some issues involving HAPs, VOCs, or GHGs still remain. These include concerns related to worker health and safety with cleaning operations. Ionic Liquids (ILs) have been investigated previously for coatings deposition, energy harvesting, deicing, and as green solvents for munitions applications. Recently, they have also been considered for cleaning applications (largely textile related). Because of their negligible vapor pressure, high thermal and electrochemical stability, and low melting points (less than 100 degrees Celsius (°C)), ILs offer potential as alternative cleaning agents
. The toxicology of ILs depends on the specific anions and cations. For example, 1-ethyl-3-methylimidazolium chloride (EMIM [Cl]) is non-toxic, while a close derivative, 1-butyl-3-methylimidazolium chloride (BMIM [Cl]) is toxic,
Typically, the cation determines the viscosity and conductivity, which ultimately affects the, mass transport properties of the IL
. Anion chemistry can determine the reactivity with water, coordinating ability, and hydrophobicity, though not all ILs are hydrophobic
. Due to the paired nature, ILs can be designed to suit a specific need. However, the myriad of combinations also results in reduced availability, potentially high synthesis costs, and limited life cycle knowledge, which may prove to be an obstacle for using ILs as “green” solvents
A literature search and vendor survey was conducted to identify suitable IL chemistries for cleaning applications. It was found that 2-ethylhexyl lactate (2ehl) was tested as a cleaner. The published work showed that 2ehl displayed similar performance to hydrofluorinated ether (HFE) 7100, which is currently used in vapor degreasing operations at Air Logistics Centers (ALCs)
. It was also found that 1-ethyl-3-methylimidazolium (EMIM) acetate, EMIM methane sulfonate, EMIM ethylsulfate, and triethylsulfonium bis(trifluoromethylsulfonyl)imide were good candidates as cleaning agents
Performance requirements for cleaning operations were derived from technical orders and military specifications supplied by ALCs paint/depaint and plating facilities-17
. Cleaning efficiency, toxicity, health hazards, and materials compatibility were prioritized concerns. The ILs were largely evaluated based on environmental health and safety (EH&S) risks in terms of Health Materials Information System (HMIS) ratings and any issues indicated on the associated material safety data sheet (MSDS) and/or technical data sheet (TDS) because little to no performance data was available related to cleaning operations.
EMIM acetate, EMIM ethylsulfate, and 2ehl were selected for investigations. EMIM acetate was procured from Government Scientific Source and EMIM ethylsulfate was procured from Expotech USA, Inc. while 2ehl was procured from Purac America, Inc. Testing included an evaluation of cleaning ability and effects on metal surfaces. All testing followed industry and federal standards, including ASTM test methods, U.S. EPA standard test methods, and ALC military specification test methods, listed in Table 1.
Cleaning Efficiency Evaluation
Cleaning efficiency was conducted using pre-weighed 4130 steel and 2024-T3 aluminum (Al) panels coated with 200 milligrams (mg) of molybdenum disulfide grease (containing at least 50 mg of free soil) that were baked in an oven at 100 °C for 60 minutes (min). Five wear cycles, using each IL or dilutions thereof, were performed on each of three panels using an in-house fabricated apparatus equivalent to Gardner wear tester.
Each tested IL was diluted with nine parts water per the test specification, but also dilution ratios of 1:1, 1:2, 1:3, and the undiluted form were conducted to determine optimum performance. Percent (%) cleaning efficiency was calculated as a comparative value to that of the control solution [% by weight (wt.) d-limonene, 5% by wt. diethanolamine, 5% by wt. Triton X-100, and 60% by wt. distilled water per ASTM D 1193, Type IV]. For 2024-T3 Al, the control removed 97% of the grease, while 89% of the grease was removed from 4130 steel. Table 1 summarizes cleaning efficiency values in comparison to the control on 4130 steel and 2024-T3 substrates and associated cleanliness inspections.
Table 1. Cleaning Efficiency Tests on 4130 Steel and 2024-T3 Al