Why does phenolic insulation char and how does it benefit fire performance?

28 March 2019 Kingspan Insulation UK


If you’re reading this blog, you’ve already watched our demonstration of a sample of Kingspan Kooltherm insulation, Kingspan’s range of phenolic insulation, being subjected to a blow torch flame. If you haven’t, you can watch it here or below.

In a nutshell, the video shows that Kooltherm insulation chars when it is burnt. In this post, we will delve deeper into how charring impacts burning behaviour, why this charring causes Kooltherm insulation to self-extinguish when it is removed from a heat source, and that, therefore, just because it is by definition combustible, doesn’t mean that Kooltherm insulation is flammable.

The video

To recap, the demonstration consists of around eight minutes of exposing the insulation board to a blow torch flame. The heat from the flame causes the insulation to undergo pyrolysis. This pyrolysis results in the black char that is formed on the surface of the board. This pyrolysis also releases combustible gases, which cannot be seen, but the result of their subsequent combustion can, in the additional flames the occur. These additional flames are most obvious in the first 30 seconds after the blow torch flame is introduced, whilst the initial char is formed. After this, the flames die down a little, and the amount of char gradually increases.

After 8 minutes the blow torch is removed and you can see that the board itself does not propagate flaming across its surface without the blow torch flame, and that the board self-extinguishes.

So, what exactly is happening to the Kooltherm insulation to give this outcome. Let’s explore.

The science behind the video 

Firstly, pyrolysis is the process of thermal decomposition of a combustible material. It happens when the material, which in this case is a sample of Kooltherm insulation, is subjected to heat energy. In this instance, the pyrolysis results in two products: hot combustible gases (pyrolysis gases) and char. Pyrolysis is an endothermic reaction, meaning it absorbs heat.

The additional flames you see during the video (over and above the blow torch flame) are a result of the exothermic reaction (combustion) between the hot combustible gases and oxygen. The additional flaming dies down after the first 30 seconds of the video because of the char formation. This char protects the uncharred insulation beneath it from the blow torch flame (and the heat from the combustion of pyrolysis gases) and thus retards its pyrolysis. Whilst the surface of the char keeps on pyrolising, the rate of emission of pyrolysis gases lessens, and the nature of the pyrolysis gases changes, the more pyrolised the char becomes. The net effect is that the rate of production of pyrolysis gases, and the intensity of the flaming produced by their combustion, reduces after the formation of the initial char layer until equilibrium is attained.

The energy released from combustion of the pyrolysis gases provides some of the energy needed to cause further pyrolysis. However, when the blow torch flame is taken away, the energy from combustion of pyrolysis gases alone is insufficient to support further pyrolysis, and the insulation self-extinguishes.

This shows that materials classed as “combustible” will not necessarily burn or combust in all eventualities. So, “combustible” does not automatically mean “flammable”. “Flammability” is scenario specific whereas “combustibility” is an intrinsic material property based solely on the calorific content of a material.        

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