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Isocyanates

Dear Reader,

We have had a lot to think about recent regarding polyurethane, so I think it is a good idea if we discuss the chemistry which is the basis of polyurethanes. The key chemicals for making polyurethanes are the isocyanates.

Now for those of you who have never encountered the isocyanates, I can tell you that they are rather reactive electrophiles which are often strong irritants. It is normal to make then from phosgene (carbonyl chloride) and a primary amine. They are similar to both carbon dioxide and the protein synthesis reagent (DCC, N,N’-Dicyclohexylcarbodiimide). All three have a sp carbon in an allene like system.

Using extended Huckel theory we can predict that carbon dioxide will have a charge of + 0.6 on the carbon, while diphenylcarbodiimide will have a charge of + 0.37 on the central sp carbon while the phenyl isocyanate will have a charge of + 0.46 on the central sp carbon. Here is a picture of phenyl isocyanate.

phenyl isocyanate space

Nucleophiles such as water, alcohol and amines will attack this carbon to form addition products. Here is a picture of phenyl isocyanate in which I have calculated the charges on the atoms, these are projected as colours onto the solvent accessable surface of the molecule. The more red they are then the more positive they are and the more blue then the more negative they are.

phenyl isocyanate charges

What you should be able to see is that the carbon in the isocyanate group is the most positive part of the molecule. I have a line drawing for you which will explain what happens when the molecule reacts with an alcohol, which is below.

urethane formation

In this way the isocyanate group can react with an alcohol group to form a bond between the alcohol molecule and the isocyanate molecule. The synthesis of polyurethane normally uses a diisocyanate and a long molecule which has two alcohol groups at the different ends of it. This will allow the creation of bigger and bigger molecules (polymerization) which transforms the small molecules into a very large molecule (a macromolecule).

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Colour and ignition

Dear Reader,

I saw an interesting demonstration done on the news, channel four news took a lump of thermal insulation of the same type as was used at the Grenfell Tower to a fire testing lab. A sample was sawn off the block and exposed to heat from a heated cone. It looked to me like a Cone Calorimeter. The test used for the TV news appeared to me to be a radiant ignitability test.

The story was in short that the organic plastic had a shiny aluminium layer. When this aluminium layer was exposed to the infra-red light nothing happened. While when the block was turned through 90 degrees to allow the plastic to be exposed to the infra red light without the aluminium layer in the way, the plastic ignited within seconds.

This made me think of something which is well known, there is a <sarcasm>rather cheerful</sarcasm> book on the subject of nuclear warfare with the title “The Effects of Nuclear Weapons” which was edited by Samuel Glasstone and Philip J. Dolan. This is a classic book which was written during the cold war in the USA. It has a chapter (chapter 7) devoted to Thermal Radiation and Its Effects. I think that thermal radiation is one of the most deadly effects of a bomb detonation, it is something which caused a lot of the deaths in Japan so while it might not be as scary sounding as fallout or ionizing radiation it is still a very important issue.

If you look at table 7.35 you will see that the color of cloth alters the amount of energy needed to ignite it. If we consider cotton clothing then white cotton needs a dose of 48 calories per square centimeter to ignite it if the heat pulse comes from a 1.4 megaton air burst. But if we look at the data for dark blue cloth then the energy required to ignite is only 19 calories per square centimeter. The reason I knew about this data was that some years ago I had to assess some work on pyrotechnics and one of the threats I had to consider was an infra-red light induced clothing fire.

I think that the aluminium layer is going to be even more reflective of infra-red light than white cotton. So I think that the cone ignition experiment will have far more difficulty igniting the plastic when the heat strikes the aluminium layer. I saw the plastic and it looked like a pale yellow solid which is the type of color I would expect for a polyurethane foam.

The colour of the foam suggests that large amounts of graphite was absent, I have seen an interesting paper in which it is shown that 15 % of expandable graphite is able to reduce the ability of polyurethane foam to burn according to Sophie Duquesne, Michel Le Bras, Serge Bourbigot, René Delobel, Hervé Vezin, Giovanni Camino, Berend Eling, Chris Lindsay and Toon Roels, Fire and Materials, 2003, 27(3), 103-117. I do not know how much expandable graphite might have helped prevent the fire from spreading but the colour of the plastic suggests it was not present.

Another option would be to use polyammonium phosphate as an additive to the polyurethane, it has been reported that this substance will make it harder to burn polyurethane. I hope to get a chance to write some more about the chemistry of polyurethane.

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