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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).

More on Grenfell cladding and fridge design

Dear Reader,

It appears that a polyurethane type material was present in the cladding at Grenfell, one newspaper is claiming that a product named “Celotex RS5000”. When I looked up this material it turned out to be a polyisocyanate type polyurethane which suggests that it is going to be a bit harder to burn than a simple polyurethane.

The daily mail did print the rather dire line “Insulation burns at sufficient temperatures and gives off hydrogen cyanide”, I would comment that we need to be careful about some words.

Flammable (inflammable) liquid means that the fumes of a substance will ignite if a flame is presented to above the liquid which is below a particular temperture. Flammable solids are a little harder to define, one good definition is a solid which is one of the following

  1. A desensitized explosive which has sufficent water, plasicizer or some other additive to prevent detonation.
  2. A self reactive material which can burn without added oxygen or air.
  3. Solids which can ignite through friction (such as matches)
  4. Pyrophoric solids and solids which can selfheat to the point of ignition.

I very much doubt if the cladding will be able to fall into one of these subclasses, what the cladding is more likely to be is “combustible”. Combustible means “possible to burn the substance if it is subject to sufficient heating”. For example a pool of JET-A fuel will not burn if it is exposed to flame, but if you were to soak a rug in the jet fuel or add a wick then it would be quite easy to start the fuel burning.

The emission of toxic fumes during burning is not a rare thing, many fuels will when burning under the “right” conditions form toxic gases such as carbon monoxide and sometimes hydrogen cyanide. I hold the view that all carbon containing fuels can form carbon monoxide when burnt. Also the generation of hydrogen cyanide during burning under air poor conditions is very common, cigarette smoke tends to contain hydrogen cyanide.

The problem is that all smoke is harmful but smoke from some materials is worse than others, for example overheating PVC and CSPE cables emit fumes which include hydrogen chloride (hydrochloric acid gas). This will make a fire which involves these materials worse than a fire which involves cables such as XLPE cables when we consider how corrosive the smoke is to objects and how harmful it is to people. On the other hand it is important to note that the very chlorine rich plastics such as PVC and CSPE are very hard to ignite. The great problem I see is that if we purge the world of things with chlorine in them like PVC then while a fire might emit less hydrogen chloride once it gets underway we might end up having more fires unless we find a decent replacement.

We need to consider both the frequency (how often) and the consequence of fires and other misadventures. If we look at the Grenfell fire, then if the cladding was the absent from the building then the consequence (how bad) of the fire which started in one flat would have been lower. Removal of the cladding will not reduce the frequency of the fridge fires but it will alter the likely consequence. If on the other hand we were to improve the design / construction of household electric equipment and thus reduce number which burst into flames each year then we could also reduce the number of deaths, injuries and monetary cost per year due to fires.

Back in 2015 the London Fire Brigade published a statement on fridges, what they want is the design and construction of fridges to be altered to reduce the consequence of a fire in a defective fridge or freezer. They want the casing of the device to be metal (to slow the development and spread of fire) or something else which will resist fire better. Sadly again I think that they are misusing the term “inflammable”.

I think that they are right for calling for the fire safety of fridges and freezers to be improved, one of the problems was that in common with PCB transformer oil the freon used in fridges was introduced as a safe non-combustable material to reduce fires. Freon was used originally in fridges as as a non toxic and non combustable alternative to the toxic and flammable gases (such as sulfur dioxide, ammonia and other horrors) which were used in the first ever fridges.

Now instead of things like freon-12 (dichlorodifluoromethane) fridges are using things like cyclopentane, propane or other similar flammable hydrocarbons. I have to ask the question of why do we have to use these compounds. I would like to know if some fluoro-iodohydrocarbon could be devised which would be non toxic, non flammable, too unstable in the lower atmosphere to pose a threat to the ozone layer and unable to cause global warming. But right now we are stuck with fridges which are using flammable gases for their working fluid.

I would like to suggest that we should consider the question of could we improve a fridge in terms of fire safety. I would like to accept the idea that we go for a metal layer covering the plastic foam insulation and we would change to a flame retardant plastic. Such as a layer of XLPE over the polystyrene foam. This would reduce the rate at which a fridge burns. But are there things we could do to stop fridges igniting in the first place.

I would like to suggest that we could change the law to reduce the chances of a fridge creating the spark which ignites leaking gas, for example I would suggest changing the thermostat for one which has all spark generating bits either sealed in a stainless steel capsule, potted in plastic or designed out. This would reduce the danger that a fridge poses if the pipes inside the fridge start to leak. I would also suggest changing to a brushless motor on the compressor to stop the fridge motor making sparks. I would also target the lamp in the fridge, I would opt for a LED lamp which would last the life of the fridge and is in a sealed module designed to prevent a spark encountering flammable gas.

Another improvement would be to add two semiconductor flammable gas alarms. One inside the fridge and one outside the fridge. The idea is that if the one inside the fridge is triggered that an alarm should start ringing and the power should be cut to the fridge. While the second should be outside the fridge at floor level as the working fluids in a fridge form gases which are more heavy than air. If this one goes off it is a more important matter. It should make the fridge scream for help and also shut down. It could also offer a warning in the event of a gas leak in the kitchen (particularly if the people use LPG to run the cooker).

One of the problems is that some words which have very precise meanings within science are used sometimes in the media in places where they should not be. For example “volatile” has two meanings within the Cambridge Dictionary. The second one is the one which is the “scientific” meaning of volatile.

  1. likely to change suddenly and unexpectedly or suddenly become violent or angry.
  2. A volatile liquid or solid substance will change easily into a gas.

But it was used when discussing fireworks, in Malta there has been a string of rather horrible firework accidents. One of the problems is that the firework industry there use pyrotechnic mixtures which are not permitted in many parts of Europe. In one newspaper report it was commented that “local firework factories use highly volatile chemical mixtures banned in many other countries“. Unless the firework makers are using some rather odd mixture such as ammonium nitrate / nitromethane or a low molecular weight organic explosive I would very much doubt if the energetic materials in the fireworks emit a large amount of vapor. Instead what they were meaning is that the firework makers in Malta are willing to use mixtures of things like chlorates and fuels which are very sensitive to static electricity, friction and impact.

It would have been better to have written “the local firework factories use excessively sensitive chemical mixtures banned in many other countries” or “the local firework factories use chemical mixtures which are easily triggered under accident conditions, some of these are banned in many other countries”. I will not go into a deep discussion of firework chemistry here but I will comment that many chlorate / fuel mixtures are unsafe. Some of them are quite rightly banned by UK law and by the laws on fireworks in many other parts of the world.

I have to ask the question of how should we choose the materials for a high rise tower block, we have two issues. The first is how easy (or how hard) is it to burn the material. While the second is how toxic can the smoke be before we decide it is too toxic. Sadly I am unable to give easy answers to these questions.

Calcium carbide and Tianjin

Dear Reader,

It has come to my attention that an almighty explosion has occurred in China. It has been reported that the site was used for the storage of calcium carbide and other chemicals. Some of you might want to know what calcium carbide is, it is a chemical formed in electric arc furnaces from graphite and limestone (calcium carbonate).

In contrast to aluminium carbide when it is reacted with water it forms acetylene rather than methane. If it was a simple carbide with carbon four minus ions (C4-) then it would be expected to form methane when it is treated with a source of protons. Instead it forms ethyne (acetylene) when it is treated with water.

This suggests that it contains C22- ions rather than simpler C4- ions, this formation of non-methane hydrocarbons from metal carbides is not unique. It has been reported that uranium monocarbide (UC) forms a host of different hydrocarbons when it is treated with water.

I hope to be able to write more about calcium carbide soon

 

 

A kitchen is not the place for petrol

Dear Reader,

It has come to my attention that a crisis has developed over petrol, so far there has been no strike by fuel delivery drivers but Francis Maude has stirred up some panic by suggesting that the public should fill up their cars and put some petrol to one side in their homes.

I would like to remind my readers not to handle petrol near any of the following.

1. Gas cookers or other devices which use flames

2. Electrical equipment, this includes light switches. Never turn a light switch on or off when there is a gas leak

3. Anything else which makes sparks

I have seen an interesting quote from a fire fighter (Matt Wrack ), he said. “Petrol is highly flammable, highly  explosive, easily ignited and toxic

Now lets go through this statement one part at a time.

Petrol has a very low flash point, the flash point is the lowest temperature at which the fumes above a combustible liquid will ignite when a flame is presented to them. A highly flammable liquid has a flash point of 32 degrees C or lower, as typical petrol (motor car fuel) has a flash point below zero degrees C it counts as a highly flammable liquid.

The firefighter is quotes as saying that petrol is explosive, I would disagree. Liquid petrol will not detonate in the same way as TNT can so it is not a true explosive. But mixtures of air and petrol can ignite and deflagrate. A deflagration is an event where a flame front travels through a material, a deflagration can emit a lot of heat and even create an overpreassure. Without high speed photography it can be hard to distinguish between deflagration and detonation.

So while I think that the fire fighter is wrong to label petrol as an explosive, I would say that a clear violent reaction hazard exists with petrol.

In some ways a deflagration (or fuel air explosion) with petrol can give a higher energy yield than many explosives. For example TNT is very oxygen poor and tends to form a lot of soot when it detonates. But the same mass of petrol when mixed in a suitable way with air can release more energy but I would say that I would expect that the “explosion” will be a slower and slightly more gentle event.

The autoignition temperature of petrol is an important thing, the autoignition temperature is the lowest temperature of a surface which is able to ignite a mixture of air and fuel. This is a measure of how easy it is to ignite a fuel.

I have found a table of autoignition temperatures, petrol ignites at only 280 oC, while acetone (nail varnish remover) needs 465 oC and toluene (a paint thinner) needs 535 oC to ignite it. So the fireman is right to say that petrol is easy to ignite.

The fireman stated that petrol was toxic, I would say that “toxic” can be hard to judge. While petrol is no where near the worst toxic substance I can think of (just trust me I can think of some real nasties) I would say that exposure to petrol can lead to some horrible health effects. So petrol has to be at least “harmful” and it can have some toxic effects (like giving petrol sniffers brain damage) so I think we will agree with the fireman.

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