Why do steel structures need their own specific fire-safety precautions?
Extremely high temperatures are generated in a fire. And though the metal itself will not burn, if it is left unprotected, steel columns can soften and collapse in 10 to 20 minutes, depending on the intensity of the fire and the thickness of the columns. To give the fire department enough time to complete evacuation, such buildings need particularly good fire protection. Intumescent coatings are ideal at protecting steel structures against the effect of heat, because in a fire they swell to as much as 100 times their original thickness, repelling the heat. The dense foam formed has a stable carbon backbone, and under heat exposure its components react, compacting even further.
How do you test ingredients for suitability in fire protection coatings?
At our labs in Hürth-Knapsack, we apply intumescent coatings to small steel plates and then expose them to heat. We have special furnaces there that enable us to simulate fire scenarios and replicate various fire profiles in compliance with DIN 4102 Part 8. These furnaces can reach 800 °C in 35 minutes, for example. The foam layers are black at first, but the color gradually fades as the heat rises, until eventually they turn virtually snow-white. Rather like when charred wood turns to ash.
What are the key ingredients of intumescent coatings?
At some 20 to 30 percent, depending on the formulation, the main component is ammonium polyphosphate (APP), which Clariant produces in Germany under the Exolit® AP trademark. Basically, the foam heat shield consists of the decomposition products produced by this substance and another chemical that serves as a carbon donor. The quality of the binder is key to intumescent coatings, though. It determines the success or failure of a good recipe because the binder generates the matrix in which the foam, in turn, will be generated.
How do the the protective foam layers function in the event of a fire?
Their most important job is to keep the surface temperature of the steel as low as possible. By doing that, they extend the time that is available to rescue workers. Depending on the building's fire rating – which varies widely from country to country – a steel column must hold out for a minimum 30, 60, 90 or 120 minutes before it reaches the critical temperature. That's a precious time window for evacuating people. In hospitals, airports, shopping malls and sports stadiums, and wherever people congregate in large numbers, more time to fight the fire translates to more lives saved. It is also important for the fire protection coatings themselves to have a long service life and stay fully functional for many years and even decades. Load-bearing parts are therefore inspected regularly to ensure the coating is intact. If it has been damaged, the affected area is simply sanded down and recoated.