This blog is for entertainment purposes only, and is not meant to teach you how to build anything. The author is not responsible for any accident, injury, or loss that occurs as a result of reading this blog. Read this blog at your own risk.

Friday, March 07, 2014

Epoxy 104

Thermal runaway

I suppose it’s a bit late in the project to be talking about such things, but while this is not the first time it has happened to me, it is the first time I had the presence of mind to ask myself “Where is the nearest camera?”, before asking “Where’s the nearest fire extinguisher?”.

“What’s a thermal runaway?” you might ask, and I’m glad you did.

I hope my Chemical Engineer daughter will forgive me for butchering and oversimplifying this explanation, but here it goes...

Mixing pure epoxy from resin and hardener is an exothermic reaction, meaning it produces heat as the molecules bond to each other. 

One interesting fact about this reaction is that the warmer the components become, the faster the reaction proceeds, and the more heat is produced. 

Like in a nuclear power-plant accident, if the reaction is not stabilized by proper cooling (enough to prevent a runaway, but not too much to prevent the reaction from happening), a situation where an uncontrolled and destructive positive feedback loop takes over.

If the heat from the reaction is unable to escape, the medium warms up, and that makes the reaction happens faster, which produces more heat, which in turn is unable to escape, and heats up the mixture even more, which speeds up the reaction, and... you get the picture, things can go wrong very quickly indeed.

Exotherm positive feedback loop

The key to this process is to keep the batches of epoxy small, in order to reduce the amount of heating, and use containers that maximize external surface such as wide containers, where the epoxy can lay out thinner, and have more avenue to transfer excessive heat to the surrounding air.

Different epoxies have slightly different behaviors in regards to thermal runaway, and although all of them have the potential, I have only had a couple of issues with the West System. 

This is another situation where “supersize” is bad. In this case, I must have gotten a little greedy with my batch size, and set off the subject of the video below.

Exotherm runaway scramble!

This doesn’t mean the West System is bad, as a matter of facts it is a very useful epoxy for a number of things, and I still plan on using it for a long time to come, it just means you have to be careful, and use it as it was intended.

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