No Sweat - How Gore-Tex jackets really work
28 January 2010
Yonks ago, W.L. Gore developed a thin membrane of expanded PTFE, polytetrafluoroethylene for the scientists out there, and we got to use a cool new word 'breathability'. This membrane stopped us sloshing around in our own armpit juice by letting sweat out of our jackets while the jacket still remained waterproof, how could this possibly be?
|Here's how it works:|
Everyone knows the spaces between the ePTFE filaments are large enough to let water in gas form (sweat) through but not water in liquid form. But this isn't quite the truth.
In reality breathability is all about surface energy. The high surface energy of the ePTFE filament makes your Gore-Tex jacket hydrophobic. It's so scared of water that it can't stand to be near it, so the water is repelled. It takes a great deal of pressure to force water through, more pressure than your average downpour, so it beads and drips off.
| So there you are, nothing to do with hole size, all to do with surface energy. This surface energy is maintained by a durable water repellent (DWR) finish on the surface of the jacket.|
BUT, if the surface energy of your jacket was lowered the breathability would go down and you'd be sloshing around in your own juices again...
|Next question: what lowers the surface energy?|
Answer: you, you stinker, your body oils, your dirt, your detergents.
If the DWR coating wears off or gets too dirty the outer surface stops beading water, the Gore-tex membrane can't get the sweat out and the breathability goes down. Also, clogging the ePTFE pores from the inside with your body oils will do the same thing. Your jacket is still waterproof but no longer breathable.
What's the point then? You might as well have gone down to Lidl and bought one of their jackets for £3.99.
|Don't panic, let's talk about polyurethane.|
Those clever clogs from Gore-Tex have combated your oiliness by adding another layer to protect the ePTFE from you. The extra layer is polyurethane, laminated to the inside of your jacket. This isn't your normal polyurethane because that doesn't let water through in either liquid or gas form, this is special. They've made it water absorbent
Water coming from your body is absorbed by the polyurethane, then each molecule is transported through the polyurethane by SOLID STATE DIFFUSION.
Nobody said this was going to be easy, but keep going we're nearly there.
|Solid State Diffusion|
I shook the Sodium Hydroxide from my old science books and checked out diffusion, here's what it said:
'a substance at a high concentration is transported to a region of lower concentration'.
High concentrations of water (sweat) on the inside are transported to the lower concentrations on the dry outer surface of the polyurethane. The outer surface is always dry because it's underneath the ePTFE and not exposed to the elements. From here it evaporates and is transported through the ePTFE by gas phase diffusion.
|I think a summary is required.|
I've made it sound complicated but there are only two basic steps to breathability:
1) Solid state diffusion at a molecular level takes the liquid water away from the body through the polyurethane (hydrophilic monolithic membrane).
2) Water evaporates on the polyurethane and diffuses through the ePTFE (hydrophobic microporous membrane) as a gas.
|One final but...|
Unfortunately, the polyurethane membrane moves water at a lesser rate than the ePTFE, so the polyurethane membrane has to be very thin to compete. Because it is so thin it is delicate and needs protection.
That is why second generation Gore-Tex has a free hanging nylon liner on the inside of the garment - to protect the polyurethane .
|3 layer Gore-Tex and Gore-Tex Paclite|
Instead of the free hanging nylon liner, it is now common to get a laminated protective layer of polyester tricot on the inside. This is 3 layer Gore-Tex.
3 layer Gore-Tex is a bit bulky. 2.5 layer Paclite gets rid of the bulk problems by substituting a textured surface pattern on the inner surface.
If you're still awake, you may ask: What does the ePTFE membrane do anyway? The polyurethane membrane is now the hold up on breathability, so the super breathable ePTFE need not be there at all???
As luck would have it the ePTFE provides enough surface roughness to bond a very thin layer of polyurethane to without imperfections. If you try and laminate polyurethane to another surface eg nylon it would need a thicker polyurethane layer and hence impede breathability. So ePTFE is useful in the end, if not for the same reasons as when we first set off.
However, it is the very thin polyurethane layer which is the key to Gore-Tex's breathability success.
Thanks for reading.