From Wikipedia:
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.
Type I fluids have a low viscosity, and are considered "unthickened". They provide only short term protection because they quickly flow off surfaces after use. They are typically sprayed on hot (130–180 °F, 55–80 °C) at high pressure to remove snow, ice, and frost. Usually they are dyed orange to aid in identification and application. Type II fluids are pseudoplastic, which means they contain a polymeric thickening agent to prevent their immediate flow off aircraft surfaces. Typically the fluid film will remain in place until the aircraft attains 100 knots (190 km/h) or so, at which point the viscosity breaks down due to shear stress. The high speeds required for viscosity breakdown means that this type of fluid is useful only for larger aircraft. The use of Type II fluids is diminishing in favor of Type IV. Type II fluids are generally clear in color. Type III fluids can be thought of as a compromise between Type I and Type II fluids. They are intended for use on slower aircraft, with a rotation speed of less than 100 knots. Type III fluids are generally bright yellow in color. Type IV fluids meet the same AMS standards as Type II fluids, but they provide a longer holdover time. They are typically dyed green to aid in the application of a consistent layer of fluid.
From NASA:
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.
