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First lie wins

Dear Reader,

I want to write to you and tell you about a problem, this is a cognitive problem (thinking problem) which many people have. It is that they have a bias in favor of things which they already believe. As a scientist I battle to be the best I can and part of this battle is against the worst aspects of myself. I have to admit that scientists can and do suffer from cognitive biases, I try to work and think in such a way that these biases are suppressed.

I have seen reports which suggest that a polyethene product may have had a key role in spreading the fire so I think it is a good idea I write something about polyethene and fires.  I accept that the important fuel might have been polyethene rather than polyurethane. Maybe for all I know both were present in the cladding.

In the aftermarth of the Grenfell fire polyurethane was mentioned in news reports,  but it now appears that polyethene played an important part in the event. What I have written about polyurethane will stay on the blog, I do not think it is worthless. It might explain to someone something about how polyurethanes degrade and burn.

Now as we suspect that polyethene was a key fuel I want to discuss some polyethene issues, if we confine ourselves to high density polyethene (HDPE) then it is quite a simple molecule. It is simple strings of CH2 units. Here are five polymer chains.

polyethene chains

if we randomly break the chains by heating the polymer in the intense heat of a fire then we can degrade the chains. Now we will have

degraded polyethene

We have a C4, two C6, a C7, a C8, a C10, a C11, three C13, a C16, a C17, a C19, two C21 and a a C22 chain. All of these chains will be much more able to burn than the polyethene. However if we were to use irradated polyethene which is crosslinked then the situation is more rosy. I added some random crosslinks and now we would get the following

crosslinked degraded polyethene

Now we would get a C4, a C6, a C7, a C11, a C13 chain and a lot of crosslinked materials which is much harder to burn. Thus the fire would not burn with the same ease. The cross linking would tend to reduce the supply of fuel to the fire. The plastics fire is fueled by volatile gases which are formed by the thermal breakdown of the plastic, as the fire creates heat this would make it harder to get a self supporting chemical reaction (fire).

There are several methods of cross linking polythene, one method is to use an electron beam irradiation machine. I hope to get a chance to write something about such machines another day.

The problem with the “first lie wins” bias is that it stops people considering new evidence correctly, it also can make people want to defend their pet idea with great vigor. I hope to also have the chance to write more about this as well.


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