Before Waterproof Breathable Fabrics were introduced by W.L. Gore in the late seventies, waterproof garments were either rubberized (like sailor’s foul weather gear) or nylon coated with Polyurethane (PU). Both were waterproof, but suffered from the problem that they were total vapor barriers. Any sweat that came off an athlete wearing these fabrics stayed inside the garment, soaking the wearer within a short amount of time. So you had the option of foregoing raingear and getting soaked, or wearing it and stewing in your own juices. Gore-Tex introduced the concept that garments could be both waterproof and breathable at the same time. That idea inspired many competing technologies and revolutionized the outerwear industry.
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Waterproof Breathable fabrics all have some type of waterproof laminate or coating that is attached to a face fabric, usually nylon. The difference between them and the old PU coatings is that they are porous on a microscopic level. As we all remember from chemistry class, water molecules are small, just H20. The pores in waterproof breathable membranes are plenty large enough for floating vapor molecules to pass right through but way too small for droplets of liquid moisture to pass through. To put this in perspective, Gore-Tex membranes contain over 9 billion pores per square inch, and these pores are 700 times larger than a water vapor molecule but 20,000 times smaller than a water droplet. So it keeps weather out but allows vapor from the inside to escape, doing a way better job of keeping active people truly dry.
It is worth it to keep in mind that this technology presents a continuum of possible pore size and that the word “waterproof” is a surprisingly flexible term. Larger pores equal better breathability, but they run the risk of allowing water to be forced through. Under high enough pressure (kneeling on melting snow, for instance) water could be pushed through pores that are too large, causing the fabric to leak and the wearer to lament that his pants aren’t waterproof. Many companies address this issue directly, designating the level of waterproofness in their fabrics by listing the height of water in a column (and hence the pressure) that the fabrics will withhold. The typical range is 10,000mm on the low end and 25,000mm on the upper end for the most waterproof fabrics.
There are two basic ways to make fabric waterproof and breathable. The first is to use membranes, materials manufactured to specifications and then laminated (basically glued) onto face fabrics. These boast reliable waterproofness and breathability, because they can be uniformly manufactured, and good durability. The downside is that they are relatively expensive to produce and incorporate into garments.
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