Why does polyethylene melt and how does it affect fire performance?

3 December 2019 Kingspan Insulation UK
In this blog we will explain that polyethylene, a thermoplastic material, melts. First and foremost, you’ll need to watch the video showing polyethylene being subjected to a radiant heat panel. You can watch the video here or below.

In short, the video shows that polyethylene melts and completely thermally decomposes. In this post, we will delve deeper into how melting impacts burning behaviour and why melting can complicate the flame spread.

The video

To recap, the demonstration consists of 3 minutes of exposing polyethylene beads to a radiant heat panel. The polyethylene melts. This melting makes char formation impossible as chars can only be formed from solids. Subsequently, the liquid polyethylene undergoes pyrolysis. This pyrolysis releases combustible gases, which cannot be seen, but the result of their subsequent combustion can, in the flames that occur. These flames are evident from 1 minute and 5 seconds into the demonstration and become increasingly aggressive after 1 minute 25 seconds. These flames continue after the 3 minute test has ended and continue until all of the polyethylene has undergone pyrolysis. This is shown at the end of the video where you can see the polyethylene is completely thermally decomposed.
Now, let’s look at exactly what is happening to the polyethylene to give this outcome.

The science behind the video

All “combustible” materials undergo pyrolysis when they are subjected to a significant heat source. Pyrolysis is the thermal decomposition of a combustible material.
For polyethylene, a thermoplastic material, the immediate reaction to the heat is to soften and melt. Once the material has melted, pyrolysis occurs and causes the liquid to breakdown and generate hot combustible gases (pyrolysis gases). The flames you see during the video are a result of the exothermic reaction (combustion) between the hot combustible gases and oxygen. As pyrolysis of liquid polyethylene does not result in a protective char layer, the rate of emission of combustible pyrolysis gases is maintained and flaming does not die down quickly. The flames continue until the liquid polyethylene is completely thermally decomposed. In a real life fire scenario, this can cause an increased rate of fire spread. Additionally, if positioned vertically within a building system, flaming droplets can fall from the material. These can land on other materials and cause secondary fires, precipitating the spread of fire to other parts of the building.

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