I found this here
and thought it may be of interest
http://www.winwood-outdoor.co.uk/acatalog/Gore-Tex.html
Yonks ago, W.L. Gore created a thin membrane of expanded PTFE (polytetrafluoroethylene). This membrane was waterproof and breathable which amazingly stopped everyone drowning in their own perspiration by letting water in gas form out.
Here's how it works
Yes, everyone knows the spaces between the ePTFE filaments are large enough to let water in gas form through but not water in liquid form. But wait 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 waterproof breathable jacket hydrophobic. It's so scared of water that it can't stand it 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 there you are, nothing to do with hole size, all to do with surface energy.
BUT, if the surface energy of your jacket was lowered it would leak...
Next question: what lowers the surface energy of the surface?
Answer: you, your body oils, your dirt, your detergents.
Clogging the ePTFE pores from the inside creates pore channels with poor water repellency. Which means you get a leaky jacket and your leaky jacket is all your fault.
Don't panic though. Those clever clogs from Gore-Tex have combated this by adding another membrane to protect the first membrane from you.
Polyurethane
The second membrane is polyurethane. So we're okay - the ePTFE is protected from you and everyone's happy.
Nope, we're certainly aren't happy. The pores in polyurethane are not large enough to let water through in either liquid or gas form. So we've disappeared up our own.... I mean we're back to square one. A jacket with two fancy membranes that doesn't really do a thing. It's a waterproof jacket but it's still not breathable. What's the point???
Again stay calm, the egg heads have ruminated over this; they don't use bog standard polyurethane, don't be silly. They have modified the polyurethane and 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.
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. It is always dry because it's underneath the ePTFE and not exposed to the elements. Here it evaporates and is transported through the ePTFE by gas phase diffusion.
Summary
So:
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.
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.
Cheers all dai
and thought it may be of interest
http://www.winwood-outdoor.co.uk/acatalog/Gore-Tex.html
Yonks ago, W.L. Gore created a thin membrane of expanded PTFE (polytetrafluoroethylene). This membrane was waterproof and breathable which amazingly stopped everyone drowning in their own perspiration by letting water in gas form out.
Here's how it works
Yes, everyone knows the spaces between the ePTFE filaments are large enough to let water in gas form through but not water in liquid form. But wait 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 waterproof breathable jacket hydrophobic. It's so scared of water that it can't stand it 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 there you are, nothing to do with hole size, all to do with surface energy.
BUT, if the surface energy of your jacket was lowered it would leak...
Next question: what lowers the surface energy of the surface?
Answer: you, your body oils, your dirt, your detergents.
Clogging the ePTFE pores from the inside creates pore channels with poor water repellency. Which means you get a leaky jacket and your leaky jacket is all your fault.
Don't panic though. Those clever clogs from Gore-Tex have combated this by adding another membrane to protect the first membrane from you.
Polyurethane
The second membrane is polyurethane. So we're okay - the ePTFE is protected from you and everyone's happy.
Nope, we're certainly aren't happy. The pores in polyurethane are not large enough to let water through in either liquid or gas form. So we've disappeared up our own.... I mean we're back to square one. A jacket with two fancy membranes that doesn't really do a thing. It's a waterproof jacket but it's still not breathable. What's the point???
Again stay calm, the egg heads have ruminated over this; they don't use bog standard polyurethane, don't be silly. They have modified the polyurethane and 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.
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. It is always dry because it's underneath the ePTFE and not exposed to the elements. Here it evaporates and is transported through the ePTFE by gas phase diffusion.
Summary
So:
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.
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.
Cheers all dai
