Thermosets vs. Thermoplastics

17 December 2019 Kingspan Insulation UK
Thermosets vs Thermoplastics

 

Kooltherm phenolic insulation and polyethylene are both “combustible” materials. Previously, we explained that Kooltherm, a thermoset material, chars when it is subjected to a significant heat source. In this blog we will explain how that differs to polyethylene, a thermoplastic material, which melts.

To test the facts and fully demonstrate how differently thermosets and thermoplastics behave when exposed to heat, we carried out a series of indicative ISO 5660-1 tests. If you haven't already watched the video of these indicative tests, you can find it here or below.

In short, the video shows that phenolic chars when it is burnt whereas polyethylene melts and completely thermally decomposes. In this post, we will delve deeper into how the behaviour of different “combustible” materials can vary.

The test

ISO 5660-1 specifies a method for assessing the heat release rate and dynamic smoke production rate of a material. It is otherwise referred to as the cone calorimeter test.

In preparation for the test, a 100 x 100 mm test specimen is wrapped in a single layer of foil with the upper surface of the specimen left exposed. Once placed in the specimen holder, any excess foil above the top surface of the specimen is removed. To ensure the test specimen is not exposed to irradiant heat from the cone-shaped radiant heat panel before the test commences, a removable radiation shield is positioned above the test specimen. After 15 seconds the shield is removed and the ignitor is powered. The test duration should be carried out in accordance with section 11.3.5 of the test standard.

The videos

To recap, the first demonstration shown in the video consists of 3 minutes of exposing a specimen of phenolic insulation to a radiant heat panel. Upon commencement of the test, the heat from the radiant heat panel causes the specimen 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 that occur 23 seconds into the test. These additional flames are most obvious in the first 30 seconds of the test, after the radiant heat panel is introduced, whilst the initial char is formed. After this, the flames die down a little and the amount of char gradually increases. The energy released from combustion of the pyrolysis gases provides some of the energy needed to cause further pyrolysis. However at 3 minutes and 5 seconds, when the radiation shield is returned above the specimen to mark the end of the test, the energy from combustion of pyrolysis gases alone is insufficient to support further pyrolysis and the specimen immediately self-extinguishes.   

The second demonstration exposes a specimen of 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 these materials to give these different outcomes.

The science behind the video

As we’ve previously mentioned, combustible materials undergo pyrolysis when they are subjected to heat – this is as true for polyethylene as it is for phenolic insulation. Pyrolysis is the thermal decomposition of a combustible material. However, the outcome of the pyrolysis is extremely different for the two materials.

When Kooltherm insulation, a phenolic thermoset material is subjected to heat energy, the pyrolysis results in two products: hot combustible gases (pyrolysis gases) and char. To recap, the additional flames you see during the video 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 radiant heat source (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 radiant heat source is taken away, the energy from combustion of pyrolysis gases alone is insufficient to support further pyrolysis, and the insulation self-extinguishes.

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.

This shows that there is a huge range in the burning behaviour of “combustible” materials.

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