Anti-icers and deicers prevent ice formation, remove existing surface ice, and remove or prevent frost. Deicers break up existing snow and ice while anti-icers prevent ice formation and adhesion.

Deicing and anti-icing involves the removal and prevention of snow, ice, or frost buildup. The deicing process may involve manual scraping, heat application, or the use of chemicals to lower the freezing point of water; the latter type is discussed herein.

Chemical deicers and anti-icers are prominently used within the aerospace, commercial aircraft, and transportation industries. Aircraft grounded during freezing conditions and precipitation typically develop ice or frost on control surfaces; the resulting uneven surfaces disrupt lift, increase drag, and may ultimately impair aerodynamic performance. Additionally, large chunks of ice may dislodge in flight and damage engines or propellers, and supercooled water droplets within clouds can form ice on wing edges during flight. For this reason, chemical deicing is mandatory at airports when temperatures are expected to be near freezing (0° C or 32° F).

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Types of Deicers

All deicers, both granular and liquid, melt ice by lowering the freezing point of water. For example, the common deicer sodium chloride immediately begins to break hydrogen bonds formed during freezing when applied to ice. The resulting brine (NaCl and water) dissolves downward through the ice and accumulates under it, breaking its surface bond. The loose ice then falls away from the surface or is removed mechanically.

Deicing chemicals may take a granular or liquid form, while anti-icers are produced as viscous fluids, as described below.  

Granular 

Granular deicers are extensively used in the transportation industry to clear roads of snow and ice. It is commonly called rock salt. Most deicers in this category are provided with two temperature ratings. The eutectic temperature represents the lowest possible temperature at which the granules can perform any melting action. The practical temperature describes the temperature at which the deicer accomplishes substantial melting within a reasonable time span (often 20 minutes).

The table below describes the most common types of granular deicers.

In addition to the chlorides listed above, acetates such as calcium magnesium acetate (CMgAc) and potassium acetate (KAc) are occasionally used as granular deicers. While acetates offer superior performance when compared to chlorides (potassium acetate has a eutectic temperature of -76° F), they are also twenty to thirty times the price of sodium chloride. Potassium acetate is sometimes employed by airports located in extremely cold climates.

Environmental Concerns

Because granular deicers are often spread over wide areas, their impact on the surrounding environment is a concern. While non-toxic to humans and animals, all chlorides are moderately corrosive to unprotected metals and can harm bordering vegetation if over-applied to roads and other paved surfaces. An advantage of acetates is that they are less toxic to plant life, but contain a large amount of organic content. The introduction of this material to rivers and lakes can elevate oxygen demand and elevated CO₂ levels.

Liquid

Both deicers and anti-icers can take a liquid form. Deicers are typically based on ethylene glycol or propylene glycol and contain additives such as thickeners, surfactants, corrosion inhibitors, and dyes.

Society for Automotive Engineers (SAE) standards AMS 1424 and AMS 1428 specify four types of aviation deicing fluid, as described below.

• Type I fluids are unthickened and therefore have low viscosity, flowing off of surfaces shortly after application. They are sprayed onto surfaces at high temperatures (up to 180° F) and high pressures. Type I fluids are usually dyed orange and make up approximately 3/4 of total fluids used at airports in the United States (2008 data).
• Type II fluids are thickened with a polymeric agent and are more viscous than Type I deicers. They are designed to flow off of aircraft surfaces at speeds above 100 knots, making them most suitable for larger, faster aircraft. Type II fluids are dyed light yellow and are gradually being supplanted by Type IV.
• Type III are dyed a darker yellow and may be considered a compromise between Type I and II fluids. They are more viscous than Type I fluids but are designed to flow off of surfaces at speeds less than 100 knots, in contrast to Type II.
• Type IV deicers are similar to Type II but are designed for longer holdover times. They are typically dyed green. 

Liquid deicers and anti-icers may be applied as part of a one- or two-step process. One-step processes combine both fluid types in a single application of heated, thickened fluid mixed with water. The two-step process involves deicing with heated Type I fluid or a mixture of a thickened fluid (Type II, III, or IV) and water, followed by a separate anti-icer application. While the two-step operation is more cumbersome, it tends to clear previous applications of anti-icer, safely removing residue.

Anti-icers

Anti-icers are applied to a surface to help prevent ice and frost formation, absorbing moisture before it has the opportunity to freeze. Chemical anti-icers offer limited protection and are prone to failure after absorbing a certain quantity of contaminant.

Anti-icers are typically comprised of unheated propylene glycol which has been thickened to a consistency similar to that of unset gelatin. This is in contrast to deicing fluid, which has a viscosity closer to that of water. For these reasons, deicers are sometimes classified as Newtonian fluids, while anti-icers can be considered non-Newtonian fluids. The image of an anti-icer being poured at right illustrates this increased viscosity.

In the aircraft and aerospace industries, anti-icers are typically specified using the product's holdover time, or the length of time during which the fluid can prevent the formation of ice before takeoff.

Standards

The manufacture, classification, and use of deicers and anti-icers is regulated through the use of standards and industry recommendations. The U.S. Federal Aviation Administration (FAA) maintains comprehensive departure planning information related to deicing and promotes its Standardized International Aircraft Ground Deice Program (SIAGDP). From an environmental discharge standpoint, the U.S. Environmental Protection Agency (EPA) issues performance standards related to the management of deicing waste at certain cold-weather airports.

Both of these agencies also issue individual standards related to deicing, some of which are listed below. 

• FAA N 8900.167 FAA-approved deicing program updates (winter 2011-12 revision)
• FAA AC 120-58 Ground deicing of large aircraft

References

Pollard Highway Products—Comparison of CaCl₂ and MgCl₂ 

Boeing—Safe winter operations

Images credits:

Oregon Dept. of Transportation / CC BY 2.0 | HomeSpot HQ / CC BY 2.0