Deicing/anti-icing is a two-step process.
Heated Type I fluid is used remove snow, ice, and frost that is already on the airplane. Type I can be diluted, as it is very expensive, but diluting it increases its freezing temperature so undiluted Type I can be used in colder temperatures than when it is diluted. A minimum temperature is established based on the level of dilution. i.e. 50/50, 75/25, etc.
While heated Type I fluid is very good at cleaning off contamination, it does not provide much protection against the accumulation of additional contamination if the snow, sleet, frost, etc. is still falling. In fact, if the icing conditions are continuing, it is likely impossible to deice and takeoff before the Type I has lost its ability to protect the airplane.
If the icing conditions are ongoing, the heated Type I application is followed by Type IV fluid, applied cold and undiluted, as an anti-icing step. The Type IV goes on thick, almost like a gel, which sits on the aircraft and prevents additional accumulation of contaminants on the airplane. During the takeoff roll, the coating of Type IV sheers off the airplane taking the built up snow/ice with it leaving the airplane clean by liftoff speed.
In conditions where Type I can protect the airplane for ten or fifteen minutes, Type IV can protect it for two to three hours. We deiced with Type I and IV last night in Chicago. Our Type I holdover time was 11 minutes. Our Type IV holdover time was 154 minutes. Timing starts at the BEGINNING of the last anti-icing application process so, with Type I alone, there's no way we would have been able to takeoff.
The purpose of these fluid applications is to provide a clean airplane at liftoff. They offer no protection once airborne where the aircraft's own anti-icing systems take over.
