What actually makes a waterproof coat waterproof?

Chris put this question to physicist Jess Wade from Imperial College London...

“What actually makes a waterproof coat waterproof?”

Jess - I think this a really nice question because it’s another one of those accidental discovery ones. It’s made out of a polymer, so a polymer is a long chain of carbon atoms all connected together and it’s actually similar to the stuff that I research. But I research conjugated polymers, so there you have single double, single double bonds between these carbons.

Back to the waterproofs - this is a polymer called PTFE (polytetrafluoroethylene) and a guy called Robert Gore, as in GORE-TEX, was playing around with PTFE trying to pull apart rods of this to create a really, really, long thread of it. What they were trying to do was pull apart these rods at really, really, high temperatures, really slowly to eek it out into this really long string to make things that will be really, really sturdy resistant fibres and stuff like that. What he realised, after a long time of playing with this, was that if he pulled it apart incredibly quickly rather than doing it slowly at all, he got this really porous structure. So he generated this structure that had loads and loads of tiny, tiny little holes inside it and that only happened if he did it really, really quickly. So it increased it by 800 times it’s own size and created something that was about 70% air.

This structure’s actually really neat for making clothes out of because it doesn't let water get through, so water drops that you get on the surface - like if you’re outside in the rain on a mountain - don’t get through, but you can let sweat out. So that’s the difference between a waterproof jacket, a jacket that you can wear out and wear it and sweat in, or something that’s just water resistant like PVC that you get really sweaty in and you can’t wear it for very long. It’s because of this porous structure which I think is really neat and it was a complete accident that he did that at all!

Chris - The reason the raindrops won’t come through, but the sweat does, is the sweat’s water vapour, so that’s individual groups of molecules of water which can get through these tiny holes. But by the time they’ve formed a big droplet, that clings to itself very hard and won’t break up into small enough particles to get back through the hole.

Jess - Exactly right. It’s like a sieve. When we have a sieve the holes are exactly the right size to let this tiny water vapour through but they’re too small to let this massive water drop through.