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Grenfell Fire

Dear Reader,

It has come to my attention that a serious and rather horrible fire occurred recently in an apartment block in London. One of the important questions which need to be asked is “why did the fire spread so fast through the building ?”. A key part of the fire safety case for high rise apartment buildings is that the building is divided into a series of small fire cells. The idea being that a fire in one flat should not be able to spread to another flat or even worse another floor.

The idea is that if the fire can be contained for a long time within one flat that fire fighters will have time to bring it under control while the other people in the block stay inside their own flats. Here in the Grenfell Tower this fire cell idea clearly failed.

It has been suggested that the cladding added to the outside of the building did contribute to the spread of fire, Mike Penning (MP and former fire fighter) has been reported as having commented that the “cladding and windows (were) clearly burning” and that “I have never seen a tower block fire move so fast in that sort of way”.

The Sun newspaper reported that the tower was fitted with a cladding which includes polyurethane, if this is true then I have to ask the question of “why was polyurethane used instead of a noncombustible substance such as fibre glass ?”. I live in a wooden house in Sweden, I have had some building work done on my house recently. I also have to ask “how flame retardant was that grade of polyurethane ?”.

The new walls of my house have a layer of fibre glass in them for the thermal insulation. In my lab at Chalmers I have sometimes accidentally exposed glass wool (fiberglass) to a butane torch while heating glassware using the flame. When glass wool is exposed to this flame it melts slightly but as expected (being glass) it never ignites.

I hope to be able to write something soon which will explain the chemistry of what happens when polyurethanes and polystyrene are heated up in a fire. The key thing to keep in mind is that plastic does not normally burn. It might be a rather challenging statement, but almost no plastics will burn. What happens is that the heat of a fire normally converts plastics into flammable gases and other volatiles, this is a process known as pyrolysis. The pyrolysis products from the plastic then react with the oxygen from the air to burn thus making heat.

One of the key methods for preventing or slowing a fire fueled by plastic pyrolysis products is to slow the rate at which the plastic can emit combustable gases, one method I have seen in the plastic for electrical cables is to use cross linked polyethene. If polyethene (polyethylene) is crosslinked with radiation or a chemical treatment then it is much harder for it to generate pyrolysis gases. I have exposed polyethene to radiation and it does change the properties of the material. Sadly my trusty copy of A.J. Swallow’s book on organic radiation chemistry is out of my reach right now (I am typing on the train).

Another option is to include flame retardant chemicals such as brominated additives and antimony oxide, these substances do get a rather bad press. The problem is that there are some brominated flame retardants such as polybromobiphenyls which are clearly very harmful to human and animal health. But there are plenty of brominated additives such as polubromophenyl ethane and brominated polystyrene which are less harmful. One of the problems is that some elements within the green movement want to ban these substances, the fact that some bromine compounds are bad is not a good argument that all bromine compounds are harmful.

The problem is that it is possible to find harmful substances made from any elements, this is not a good excuse for banning all compounds containing an element. Lets consider a human analogy, “Jack the Ripper” was a human, “Jack the Ripper” was very evil and should have been arrested and locked up to prevent further serious crimes. But this is not a justification for locking up all humans to prevent crime.

What happens when brominated plastic is heated is that hydrogen bromide is released into the fire, this alters the flame chemistry. The hydrogen bromide reacts with some of the free radicals in the flame in such a way that it terminates the free radical chain reaction which occurs in the flame. By adding the antimony oxide this effect is greatly improved, the antimony oxide reacts in the hot plastic with the hydrogen bromide to form water and antimony tribromide. The antimony(III) bromide then has an even stronger fire inhibiting effect than the hydrogen bromide.

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