Deicing fluids come in a variety of types, and are typically composed of ethylene glycol (EG) or propylene glycol (PG), along with other ingredients such as thickening agents, surfactants (wetting agents), corrosion inhibitors, colors, and UV-sensitive dye. Propylene glycol-based fluid is more common due to the fact that it is less toxic than ethylene glycol.
There are four standard aircraft de-icing and anti-icing fluid types: Type I, II, III, and IV.
Type I fluids are the thinnest of fluids. As such, they can be used on any aircraft, as they shear/blow off even at low speeds. They also have the shortest hold-over times (HOT) or estimated times of protection in active frost or freezing precipitation.
Type II and IV fluids add thickening agents to increase viscosity. The thickeners allow fluid to remain on the aircraft longer to absorb and melt the frost or freezing precipitation. This translates to longer HOT, but it also means a higher speed is required to shear off the fluid.
Type III fluids are relatively new and have properties in between Type I and Type II/IV fluids. Type III fluids also contain thickening agents and offer longer HOTs than Type I, but are formulated to shear off at lower speeds. They are designed specifically for small commuter-type aircraft, but work as well for larger aircraft.
*Note: Holdover Times (HOT) are published in a range to account for variations in precipitation intensity: shorter time for heavier intensity, longer time for lighter intensit
Type I fluids are always applied heated and diluted. For de-icing, it is the heat and hydraulic force that accomplish the task. For anti-icing, it is primarily the heat imparted to the airframe that accomplishes the task. Caution: Type I fluids have the shortest HOT. When a Type I fluid fails, it fails suddenly.
Type II and IV fluids may be applied heated or cold, and diluted or full strength. In North America, typically Type IV fluids are applied cold, and only for anti-icing. In the UK, typically Type II or IV fluids are applied heated to accomplish de-icing as well as anti-icing.