Tuesday, March 19, 2013

Modern Waterproofing Technology for Dummies

One more area of technology that is not so simple any more is waterproofing.

The first thing to understand is that "waterproof" does not mean what it used to mean.  It used to mean "water will not get through this material, ever". Unless you tore the material, or unless the water came in through the open neck or cuff, or front zipper.  It was not completely idiot-proof, but the idea had remained the same for hundreds if not thousands of years, so the human mind could grasp it.

Today, we still have that type of old-fashioned waterproofing, in plastic material.  You can also get it in fiber material (cotton, or man made fibers) if the material is treated with oils, waxes, or tar.  Rock and steel can be  waterproof too, but we rarely make waterproof clothing out of them.

In the good old days, if you wanted to go for a walk or ride in the rain, you put "waterproof" clothing over your other clothing, and went out in the rain. If you still got wet, you at least had a chance to understand why.

So what technology do we have today to make our lives more complicated?  Now we have a new type of material called "waterproof breathable".  The problem with the old fashioned "waterproof" was that it did not breathe, meaning that if you didn't take it off when the rain let up, you would quickly be soaked in water condensing on the inside of the garment, and almost as uncomfortable as if you had simply let the rain hit your normal clothes (which were breathable).

Unfortunately "waterproof breathable" is often called "waterproof", which obviously is the first step in the road to confusion.  Actually, something that is breathable is never truly "waterproof" in the old sense of the word.  Because if you add enough pressure to the water, it will get through a "waterproof breathable" material.  Also, the level of waterproofing decreases with time, so as you get the material dirty, or wash it, the amount of pressure required is reduced.  In a relatively short time (say one season), the waterproof pressure may be so low that rain hitting your jacket at 50 kph can be enough pressure to get through.  Or even just falling rain.

Understanding how this can happen may help you stay dry, but it's complicated.  So I'm going to skip all the mathematical formulas that I don't understand anyway, and say it's due to some kind of magnetic repellency.  In other words, some materials repel water (or vice versa), and other materials attract water.  Some repel really strongly, other weakly.  So your breathable garment is either made of woven fibers that naturally repel water (rare), or the fibers of cotton, polyester, leather etc. are coated with a material that repels water.  These water-repelling substances are called "hydrophobic" which means they are scared of water, or water is scared of them, I'm not sure which.  But you will see water ball up and roll away on a very hydrophobic material, just as though it had seen a ghost.

About ten years ago, there was a spray called Scotchguard that would make ordinary fabrics waterproof-breathable.  It apparently has been withdrawn from the market.  The maker decided that it did not biodegrade in a safe manner, and so stopped promoting and selling it.  The leading sellers of waterproof breathable today are Gore-tex and Nikwax. (and others like them)  I guess their formulas are secret or something, so I don't know any more about them.  Goretex is a specially woven material, which apparently is treated or embedded with hydrophobic material, and the Gore-tex layer is sewed into the garment.  So you probably cannot add Gore-tex to a given garment after it is manufactured.

Nikwax is a spray on, or wash-in material that will work on ordinary non-waterproof fabrics.  For example, Nikwax can work on a polyester shell, ordinary leather boots and gloves, or a pair of cotton jeans, down feathers, or even a paper map.  Now some of these fabrics are not very waterproof to start with, like cotton.  So when you treat a pair of jeans with Nik-wax (even using the proper procedure), they will not be truly waterproof.  But they will not absorb water anywhere near as quickly, and they will dry out faster than natural cotton.  Nikwax is more waterproof on materials that have some degree of natural waterproofing already (like tight weave, or small spaces)

One neat feature of Nikwax is that it is water-based, not oil based.  So it smells better, and feels better when dry.  It's easier to clean and less toxic, much like water based paint vs. oil based paint.

Nikwax has developed a whole range of products that can be quite bewildering, and make you suspect this is all a scam to get more money.  It's not a complete scam as you can actually see that the water will bead up and run off your jeans or boots, for example.  But in the real world, if you are riding in a rainstorm on the 401 with only Nikwaxed jeans, you will get wet.  Also, this treatment wears off quite fast, so you may treat your jeans, then never encounter a rain storm before it wears off, which can happen if you need to wash them again.

Picture: An insect walking on water using hydrophobic feet.  This is not a trick or a miracle, I'm sure you have seen this on many ponds in the summer.  http://www.asknature.org/product/4feddb09a84cb65ac0ed01d2109fa731


  1. My understanding of those 'breathable' fabrics was that the material was designed so as to have holes or 'pores' too small to pass water in its liquid form (from the outside of the garment), but large enough to permit autonomous water vapour molecules to pass through (from the inside of the garment).

    'Gore-Tex' was one of the first and charged a massive premium for their materials when they were first introduced back in the 1970s. Since then, of course, all kinds of imitations have been introduced and 'breathable' fabric is now available at nominal price differentials over regular material.

    As far as your insect and its 'hydrophobic feet' ... would not agree that surface tension would account for a considerable portion of its ability to remain on the surface? In fact, some researchers have discovered that those insects also have tiny hairs on their feet, which trap air bubbles, enabling the insect to in effect 'float' - and the researchers are referring to this as 'superhydrophobic' - somewhat according with your contention ;-)


    1. I think Gore-tex and insect flotation both depend on surface tension, which is in turn explained by hydrophobic properties.

      The Gore-tex material is hydrophobic, which repels the liquid water but I guess not water vapour, which is free to pass through the holes. There has to be some kind of theory to explain the different behaviour of water molecules when in a liquid vs a gas, as the molecules of both are the same size. I think it has something to do with hydrophobic properties or surface tension, that I think only works with liquid water. When water as a liquid is near to a hydrophobic substance, the water molecules arrange themselves in a certain pattern, and one way we observe that is as surface tension.

      Most of the flotation of the insect is due to water displaced by the feet, which is basically the water displaced by the big dimples in the surface of the pond because surface tension (or super-hydrophobia). I don't understand the importance of the bubbles (of air?) trapped by the hairs on the legs. By themselves the (almost invisible) bubbles would be much too small to float the insect. For an object to float, it has to displace it's own weight in water, and that would be roughly the amount of water equal in size to the whole insect.