Gore-Tex is probably the best known breathable waterproof material in the outdoor industry, and like Hoover it has become a bit of a catch-all name for the style of waterproof breathable garment which it usually comprises.
The ‘basic’ version of the Gore-Tex membrane is used on a very wide variety of garments, and is suitable for multi-activities like hiking, biking and general outdoor wear.
The Gore-Tex membrane itself is a very fragile, very complex mesh structure which is made from a material called PTFE. This is closely related to the non-stick coating on your frying pan, and that stretchy white tape which plumbers use to seal joints. The process for creating the super-fine web was discovered when a massive stretching force was applied to a bit of solid PTFE very quickly. It stretched to around 8 times its original length and created a structure with tiny little blobs of material connected by strands. This mesh is then bonded to an extremely thin layer of polyurethane (PU) which protects it and stops your body’s oils and dirt from clogging up and damaging the membrane itself.
The PTFE membrane works quite simply because the size of water droplets is ENORMOUS compared to the size of water vapour. And the way that your body works is by sweating water droplets then heating these droplets up so they vapourise and take the heat with them to cool you down. So, the sweat vapour you give off can pass through the Gore-Tex membrane, but rain droplets can’t.
In reality, it’s a little more complicated than that because the PU protective layer requires the sweat to re-form in to liquid, pass through it and then re-evaporate through the PTFE mesh using your body heat again.
What that does mean though is that if you sweat quicker than your body can evaporate the sweat (i.e. you get damp), you will still be damp inside the Gore-Tex garment. It doesn’t work like magic to suck the sweat off you.
HOWEVER, the Gore-Tex membrane is really fragile, and must be protected from your clumsy oafishness by encapsulating it between layers of protective fabric which also have nice big holes in them for vapour to pass through. On the inside, against your skin, that will be a soft or smooth face-fabric. And on the outside it will be a tougher, abrasion proof fabric. This outer face will usually be chemically treated with something called Durable Waterproof Repellent (DWR) to make rain bead-up and run off it. After all, if the garment is covered in rain, it can’t breathe to let your sweat out. This is why Gore-Tex garments need to be kept clean and re-treated with DWR after a while.
Gore-Tex is a Trademark of W.L.Gore and Associates.