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More about polyurethane and the Grenfell Tower

Dear Reader,

Regarding the horrible fire in London at the Grenfell Tower, some questions are being quite rightly asked about the safety of using polyurethane foam in the gap between the building and the weather resistant cladding. My first thought was “why not use fibre glass in the gap” as it is a good thermal insulator and it is impossible to ignite (unless one uses chlorine trifluoride).

I saw an interesting comment that polyisocyanate foam would have been safer. This made me consider what is a “polyisocyanate foam”. Before we start it is important to understand what a polyurethane is, it is normally a great big long chain molecule formed by the reaction of a di-isocyanate with a long molecule which has an alcohol at both ends. It is impossible to draw a single structure for polyurethane as many different combinations of diisocyanates and di alcohols exist.

Here is one example of a polyurethane synthesis, I have used polyethylene glycol and bis(4-isocyanatophenyl)methane.

polyurethane synthesis

It has been known for some time that aryl isocyanates oligomerise when exposed to phosphines. A common method of making a polyisocyanurate resin is to take a typical diisocyanate and react it with its self using a catalyst to form a triisocyanate which has a central isocyanurate core. Sorry is this seems like a series of tongue twisters.

resin synthesis

What happens next is that the 1,3,5-tris(4-(4-isocyanatobenzyl)phenyl)-1,3,5-triazinane-2,4,6-trione is reacted with a diol to form a special type of polyurethane. While it is not perfectly noncombustable the hybrid polyurethane isocyanurates are harder to burn than the polyurethanes.

 

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