Gore-Tex is the registered trade name often applied to a particular design of (usually outdoor) clothing fabric that is designed to keep you dry in the rain without getting sweaty.

• Strictly speaking it is the patented name a porous form of the polymer PTFE made by stretching the polymer fibres in a controlled way to create fine pores.
• So the GoreTex itself is actually one layer in the fabric design for a particular clothing application e.g. anorak or wetsuit.

GoreTex fabric is made up of a layer of a plastic based on expanded PTFE and this is laminated onto a layer of another fabric.

• The layer contains very many tiny holes called pores.
• There are apparently around 14 million pores per square millimetre!
• It is the tiny holes in the PTFE layer that let the water vapour molecules through, i.e. it is breathable.
• BUT layer is waterproof because liquid water droplets cannot pass through in the opposite direction, in fact the fabric surface repels water hydrophobic.
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Each pore is too small for water droplets to pass through, but big enough to let water vapour molecules from sweat to go through (transpiration or 'breathability').

• This illustrated in the diagram above, the fabric material is multi-layered (a sort of 'sandwich' composite).
• So, if you sweat in this 'breathable' material, the water vapour can escape keeping you cooler, and you don't get the discomfort from sweat condensate,
  • AND, because water droplets can't pass through the outer tough protective layer, you should keep dry in wet weather.
• Also note, that without the outer tough protective layer e.g. nylon or polyester, the GoreTex membrane would be easily damaged and it wouldn't be waterproof and windproof.
• Gore-Tex products have all the useful properties of nylon-polyurethane fabrics but breathable materials!