|WHAT'S THAT STUFF?
Volume 79, Number 1
CENEAR 79 1 p.
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The pilot has just announced that the weather is going to hold and that your airplane is ready to push back from the gate. It looks like your flight won't be canceled after all, despite the sleet and freezing rain that has fallen. You relax. But the plane isn't moving. Now what? A technician appears in a lift over the wing and starts spraying something. A shroud of steam rises from the wing and you remember--deicer. Then you wonder, what's that stuff?
Most people are familiar with solid chemical deicers and anti-icers that are used on sidewalks and roadways. Deicers work to break up snow or ice so that it can be removed more easily. The chemicals work by dissolving slowly on contact to create a brine, with the heat of solvation for some chemicals helping to melt the ice or snow. Anti-icers work to keep water from freezing or refreezing.
Chemical deicers/anti-icers typically are urea or various chloride salts: calcium, magnesium, potassium, or sodium. These five chemicals and combinations of them account for essentially all roadway deicers, with CaCl2 being the most effective and most popular.
However, chloride salts are prohibited for use on aircraft because they can be corrosive. So like automobile antifreeze, aircraft deicing fluids are aqueous solutions of a glycol, or mixture of glycols, along with proprietary additives. Depending on the formulation required, the additives might include a surfactant, polymer thickening agent, pH buffer, corrosion inhibitor, flame retardant, or dye.
Glycols and other deicing chemicals are efficient freezing-point depressants--that is, they act as a solute to lower the freezing point of the solvent, in this case water. Propylene glycol (CH3CHOHCH2OH) or ethylene glycol (HOCH2CH2OH) generally is the major component of aircraft deicers, making up 30 to 70% of the solution. Either alone or in combination, the glycols, calcium magnesium acetate, sodium acetate, sodium formate, and urea are used on runways and airport roadways.
Ethylene glycol has been the standard for antifreezes and deicers for years because of its low cost. It is a colorless, slightly viscous liquid with a freezing point of -13 C, and it can lower the freezing point of water to about -50 C, depending on dilution. The sweet taste of ethylene glycol belies its acute toxicity to mammals, and occasionally it has led to the death of animals that have licked up antifreeze that has leaked onto the ground. When ingested, it depresses the central nervous system and can be fatal to humans even in small quantities.
Propylene glycol also is a colorless, viscous liquid at room temperature. It doesn't have a true freezing point, but becomes glasslike at -51 C, and it can lower the freezing point of water to about -60 C. Because propylene glycol is essentially nontoxic, its share of the U.S. aviation deicer market has grown from 10% (124 million lb) to more than 70% (about 140 million lb) during the past five years at the expense of ethylene glycol, according to R&D consulting firm SRI International .
Aviation deicing fluids, at about $5 to $7 per gal, generate more than $200 million per year in sales worldwide, according to industry sources. Union Carbide , Lyondell , and Clariant are among the major players in the market, which has been slowly moving along at 2 to 3% annual growth for several years.
A single application of Type I deicing fluid to the critical surfaces of an aircraft, such as the wings, flaps, and fuselage, is the most common treatment method. Type I fluid is about 90% glycol and 8% water, but it is diluted as needed depending on ambient temperatures. It is sprayed on hot (150 to 180 F) at high pressure to melt or remove ice, snow, or sometimes just frost. Deicing may take place at the departure gate or at a central facility near the runway mainly by using handheld nozzles, although automated machines called gantries have been developed.
Deicer performance is measured by holdover time, which is the length of time an aircraft can wait after being treated prior to takeoff. For Type I fluids, the holdover time is only about five to 15 minutes, so the aircraft has to take off right away or else wait to be deiced again.
When a longer holdover time is needed, a two-step process is used in which deicing is followed by treatment with an anti-icing fluid. Anti-icing fluids can be either Type II or Type IV, although Type II is slowly being discontinued in favor of Type IV. Type III anti-icing fluid, designed for small commuter aircraft, is no longer used. The anti-icers, which are sprayed on unheated, are 65% glycol and usually are not diluted further. They contain a sufficient amount of a polymer thickener to enable a heavier layer to be applied. For Type II and Type IV fluids, the holdover times are about 30 minutes and 80 minutes, respectively.
Apart from safety, environmental protection is an important aspect of deicing. Besides the mammalian toxicity of ethylene glycol, there is concern about aquatic toxicity of the glycols because they can deplete dissolved oxygen in streams or lakes as they biodegrade. There also is concern over the toxicity of urea and the additives, particularly tolyltriazoles used as corrosion inhibitors and flame retardants. The Environmental Protection Agency requires airports to monitor storm water runoff, which is usually controlled by local discharge permits. Depending on permits and economics, airports may contain and treat storm water on-site, send it to a municipal wastewater treatment facility, have it hauled away by a contractor to be treated or recycled, or discharge it untreated.
Stricter regulations probably will force a reduction in the amount of chemicals used for deicing/anti-icing in the coming years. A few airports are beginning to use deicing fluid recovery systems to recycle the glycols and capture the additives. Others are looking to chemical-free alternatives for deicing, such as using infrared light or hot air, installing electrically heated panels in aircraft wings, or using cameras to detect which sections of an airplane need to be treated.
So the next time your flight takes a few extra minutes to get off the ground during freezing weather, look out your window and see chemistry at work.
Chemical & Engineering News