by Chris Woodford. Last updated: July 2, 2017.
The great American poet Henry Wadsworth Longfellow (1807–1882) once said: "The best thing one can do when it's raining is to let it rain." If he'd lived a few decades longer, he might have come to a different conclusion. Generally, the best thing you can do nowadays when it's raining is to reach for the GORE-TEX® to keep yourself dry.
GORE-TEX is an amazing breathable, waterproof textile found in high-performance clothes such as walking/hiking boots and mountain coats. Unlike ordinary synthetic textiles like nylon, GORE-TEX stops rain from getting in but lets perspiration out. So it keeps you dry on the outside and dry on the inside at the same time. Sounds remarkable, doesn't it? But how exactly does it work?
Photo: My fantastic GORE-TEX walking boots. They're leather on the outside, but the lining is made of completely waterproof GORE-TEX. You can jump in puddles all day long in a pair of these and your feet won't get wet.
The perspiration problem
Photo: Look inside the boots and you can clearly see the GORE-TEX lining. The GORE-TEX fabric is inside the leather "uppers". The leather keeps out some of the water; the GORE-TEX keeps out the rest.
Suppose you're in the kitchen on a cold winter's day and you've got pans boiling away on the stove. Pretty soon, the windows are steaming up with condensation and the whole place feels like a sauna. But there's a storm outside and the rain is practically blowing sideways. What do you do? Well if you have sash windows (ones that open vertically at the top and bottom), you could open the top window just a fraction. Then the steam will drift out without the rain getting in. You'll let water out without letting rain in. Roughly speaking, GORE-TEX works the same way. It allows perspiration to escape one way through your clothes without letting rain come in the other way.
Is that some kind of magic trick? How can water flow through your clothes in only one direction? GORE-TEX isn't one simple material: it's actually a sandwich of three layers. There are two layers of nylon making up the "bread" and then a layer of microporous Teflon® (a brand name for polytetrafluoroethylene or PTFE) in between. You might know Teflon as the slippery coating on non-stick cookware. (Many people think it's a hi-tech remnant from the Apollo moon-landing program, but it was accidentally invented back in 1938 by a DuPont™ chemist called Roy Plunkett (1910–1994), who was trying to make a better refrigerator.) Teflon's slippery nature makes it great for waterproofing things. Some buildings, including the infamous Millennium Dome in London, are even made with gigantic Teflon roofs. Now no-one's interested in boiling an egg on top of a tent in east London; the Teflon's there to keep out the rain.
Photo: Keep it clean! One drawback of GORE-TEX is that you need to keep it clean to keep it waterproof—and clean it properly. Over time, as perspiration from your body escapes out through the material, it can cause a buildup of surfactants (detergent-like chemicals) that reduces the effectiveness of the waterproofing. In a similar way, the surfactants in ordinary detergents will clean GORE-TEX clothes but leave them considerably less waterproof. Always follow the manufacturer's cleaning instructions and be sure to use proper cleaning and waterproofing products designed specially for GORE-TEX and similar materials.
Liquid and gas
Now the Teflon in GORE-TEX isn't quite waterproof because it has tiny holes (or pores) in it. That's why it's called microporous Teflon. The pores are less than one micrometer (one millionth of a meter) in diameter—less than one fiftieth the size of a human hair. And this helps to explain why water in one form can't pass through but water in a different form can.
When you sweat, your body produces steam, which is water in the form of a gas. As you probably know, the molecules in a gas are not really joined together. They can whizz freely all over the place, which is why a gas fills whatever it's contained in. Now a water molecule is about 700 times smaller than the pores in microporous GORE-TEX, so when you sweat, the steam can easily flow from your skin, drift through the GORE-TEX (by a process called diffusion), and out of your clothes. But water in rain is totally different from sweat. It's a liquid made up of droplets, each of which contains trillions of water molecules. A single water drop is about 20,000 times bigger than the holes in microporous GORE-TEX, so there's no way it's coming through.
That, then, is the clever little secret of GORE-TEX—one of the most amazing materials in the modern world. But you could say it's actually the clever little secret of water—perhaps the most amazing "material" of all time!
Artwork: How GORE-TEX can be both waterproof and breathable at the same time. Steam caused by perspiration passes out easily because the water molecules are smaller than the pores in the GORE-TEX liner; water droplets can't get in the other way because they're much bigger.
Two layers are better than one
That's a very basic explanation—and if it's all you want to know, you can stop reading now. In practice, things are a little bit more sophisticated!
GORE-TEX clothes are generally made from three layers: a conventional outer shell fabric, plus two separate GORE-TEX layers with similar but quite different waterproof-breathable properties. On the inside, there's a relatively thin, relatively delicate inner membrane or film that's hydrophilic ("water-loving"), which readily allows water vapor to escape but doesn't allow liquid water in. The hydrophilic layer absorbs perspiration produced by your body and transports it, by diffusion, to the outside: unlike with many other textiles, the water doesn't move by wicking and capillary action (where the water soaks into the fabric and then channels through it, like water moving up a plant from the roots to the leaves), but because of the difference in water concentration between the inside and the outside. Next to the inner layer, there's a thicker outer layer made from a microporous plastic polymer, such as Teflon. This layer is hydrophobic ("water-hating"), which means it resists water from outside, even when it's impacting at high speed or pressure (as in heavy rain) or when the fabric is flexing back and forth (as you walk or run along). The hydrophobic layer doesn't soak up water coming in from the outside (droplets simply "bead" on its surface and stay there, without soaking in), but like the inner hydrophilic layer, it does allow perspiration to escape from the inside by diffusion.
Why have two layers to GORE-TEX instead of one? The outer layer provides protection and support for the inner layer, but it also provides thermal insulation: it reduces heat loss and helps to keep you warm. If it's not properly hydrophobic, the outer layer soaks up the rain, which cools the inner layer and makes your body cold. The two layers work together as a team, keeping water out, keeping heat in, and allowing perspiration to escape.
Alternatives to GORE-TEX
Photo: Dependable waterproof and breathable textiles are a big selling point for high-performance outdoor clothes. Here's a selection of jackets and waterproof pants I've just pulled from my own home. Like rival materials, Schoeller's quick-drying 3XDRY has a hydrophobic outside and a hydrophilic inside, but they're both incorporated into a single layer, woven fabric. According to Schoeller, the fabric moves moisture so quickly that sweat doesn't have time to build up inside it. That's certainly been my own experience when I've worn the 3XDRY jacket shown here, but as a single layer of fabric it's not so good on colder days. Rohan uses its own waterproof and breathable textile called Barricade.
GORE-TEX is certainly the best known brand of high-performance, waterproof and breathable textile—but it's by no means the only one. If you're shopping around for outdoor clothing, bear in mind that there are plenty of rival textiles to consider; just because it's not branded "GORE-TEX," doesn't mean it's not going to be good. Two very popular alternatives are eVent and SympaTex. eVent clothing works in a similar way to GORE-TEX: it has three layers of fabric, with a backing fabric on the inside, a wicking Teflon-type (PTFE) membrane in the middle to draw moisture away, and a water-repellent outside. SympaTex is quite different. Instead of having micropores, like GORE-TEX and eVent, it uses what's called a copolymer, made of hydrophobic polyester and hydrophilic polyether. The polyester repels water from the outside; the polyether absorbs moisture from the inside and transports it out, away from your body. But there are no actual pores through which the water travels.
Artwork: A) How SympaTex works: Unlike in GORE-TEX, there are no micropores in SympaTex to carry moisture away from your body. Instead, hydrophilic polyether molecules in the SympaTex membrane effectively act like little "channels," carrying moisture from the inside to the outside.
Artwork: B) Inside a multi-layer aerobic sports garment (such as a waterproof and breathable cycle sweater). 1) On the outside, there's a durable shell made of something like 85 percent polypropylene and 15 percent spandex (to give it body-hugging elasticity). 2) In the middle, there's a layer of waterproof and breathable SympaTex (or GORE-TEX). 3) On the inside, there's a tricot, warp-mesh lining to help moisture wick away from the body and increase comfort.
Who invented GORE-TEX?
In October 1969, Dr Robert ("Bob") W. Gore (1937–) discovered how to turn PTFE into a microporous material called expanded polytetrafluoroethylene (ePTFE), which makes up the waterproof and breathable membrane used in GORE-TEX. Gore was the son of Wilbert Lee ("Bill") Gore (1912–1986), a chemical engineer who worked at DuPont before starting his own firm, W.L. Gore & Associates, from his basement in 1957. Initially, Bill Gore's company used DuPont's Teflon to make electrically insulated ribbon cable (parallel connecting cables that link printed circuit boards together inside electronic devices). But it was Bob Gore's discovery of ePTFE that led to real success, spawning all kinds of profitable uses, from waterproof clothing and boots to guitar strings and medical implants, and turning W.L. Gore & Associates into a world-class chemical company with sales of over $2 billion a year.