Geeking Out About Fly Sheets

April 30, 2015 5 min read

Flysheet coatings: SilNyl vs PU coated PE

There has been much hype about the lightweight SilNyl (silicone coated ripstop nylon). When if comes to shaving a few ounces off your flysheet/rainfly it certainly looks promising. 

Here is an article on backpackinglight.com by member Roger Caffin: 

"I spent 27+ years in Textile Research with the CSIRO, ending up as a Senior Principal Research Scientist with a large team under me.

I have a Suter tester (it measures the pressure rating of a fabric) of my own and I have been testing (on my own behalf) light-weight coated fabrics for many years (6+).

I have tested many of the fabrics from the various suppliers in the American cottage industry - (thanks to various people including Sam for all the samples).

I keep seeing claims of 'our silnylon is superior', but I have yet to find a sample that really is superior. Some show normal variation; others are actually inferior.

I have tested Asian fabrics which are nylon 6.6, have a pressure rating several times that of the stock American silnylon, and are slightly lighter than stock silnylon. (Also polyesters.) Some of those fabrics have pressure rating of up to 80 kPa, compared to stock Westmark silnylon which is around 15 kPa.

The old 'wet-look' Westmark silnylon also got to 80 kPa, but the coating company (Duro) had its coating line closed down by the EPA because it was releasing too much solvent into the atmosphere. They could not be bothered upgrading their plant. As noted by some, operations in Asia are not restricted by the EPA.

The current Skylite silnylon from Westmark is not designed for our outdoors use. It is designed for parachutes and for the huge blow-up advertising puppets you see outside (or on top of) shopping centres. Neither of those markets have a water-pressure requirement. Unfortunately Westmark does not feel enough market pressure to improve the pressure rating - perhaps because those outdoors companies which are concerned have simply gone to Asia for their fabrics.

Some people have claimed that Asian fabrics are inferior. Nice try guys, but no cookie. Some (most?) of the world's major high-tech textile companies are located in Asia. You have only to ask what happened to Pertex: the UK holding company sold the brand and tech knowledge to Mitsui.

Now for some technical details. Please note that every statement made here is one based on actual measurement or observation. No guessing at all.

You might like to note that 10 kPa is roughly equivalent to 1 m (1000 mm) water pressure. I tend to work in kPa.

I will have to take some photos of pressure testing. A fabric with 20 kPa pressure across it is bulging quite significantly. When the fabric gets to 80 kPa (8000 mm) presure the bulge is ... startling. Or alarming.

The idea that a tent fly (tarp, whatever) will experience broad pressures of 15 - 20 kPa from rain is mostly unreal (typhoons excepted!) imho. The tent would explode from the ground due to the forces involved. And you won't get spot pressures on a fly like that either: the existing surface film of water dissipates the impact of every (non-typhoon) rain drop very well. Water flows; bullets don't.

However, you can easily get 60 kPa on a groundsheet when you kneel on it. That's where the problem really arises. That is why so many companies use a heavier fabric for the groundsheet: they don't want customers getting wet knees. Customers don't want wet knees either ...

Misting

I understand that some people like to blame their tent for their getting wet inside. But just wanting to blame someone else is not a substitute for proper testing and decent science. So-called 'misting' is condensation being popped off the inside of the tent due to the impact of raindrops. (There is one exception I will come to later.)

Why should one get condensation on the inside of a fly/tarp/whatever when it is raining? The reasons are a shade complex, but the effect is at least partly driven by the way the temperature of the fabric is lowered by the cooling effect of the rain. You get a wet outer surface and the water on the outside starts to evaporate a bit, cooling the fabric. Ever seen your windscreen fog up inside when it is raining outside? Same effect. So you turn on the heating a bit and warm up the windscreen.

On the inside of the tent you have a person who is warm, possibly with damp clothing, and the ground is wet, all evaporating away. That makes the inside of the tent around 100% RH, but at a slightly warmer temperature than the ambient, and even warmer than the fabric. When that warm saturated air hits the colder fabric, condensation happens. Yes, this can happen even with some ventilation due to a breeze: the breeze may just be cooling the fabric down even more by evaporation.

For the doubters - note that you can get condensation even when it is not raining, like under a clear sky. The fabric is cooling down here because it is losing heat to the night sky by radiation. The night sky can look as low as -70 C (-94 F). You don't need many degrees of temperature drop for condensation to happen.

Yes, there is one exception, and that is EPIC fabric. This is funny stuff. It has air-flow but blocks water - up to an abrupt threshold. That threshold is set by the surface tension of the water on the coating on the fibres, and is around 15 kPa for clean new fabric. BUT, get dirt, skin oils, vegetable oils, or anything else, on the fabric, and the surface tension effect is shot to pieces. Then the fabric starts to leak like an uncoated fabric - at essentially zero pressure. So your nice new EPIC tent is waterproof for a few days, then collapses.

Where do we go from here?

It seems that the silicone coating is porous under pressure. I am not talking about a few leaky spots here and there. I mean the silicone polymer seems to be actually micro-porous. Under pressure the threads inside the fabric start to get wet. I have watched this happen during testing. However, when I test a fabric with a PU coating on one side and a silicone coating on the other, I see thread-wetting from the silicone side but not from the PU side. Yes, that means the PU coating is inherently more pressure-resistant.

The old PU coatings which sat on the surface rather than going into the fabric actually weakened the fabric, while the silicone coating/impregantion does strengthen the fabric. However, we now have PU coatings which go into the fabric the same way as the silicone coatings do. I expect these to strengthen the fabric.

So I am currently betting on the future being fabrics with a silicone/PU coating - either as a copolymer or as a different coating on each side. And they will be coming from Asia, as so far the American suppliers show no sign of competing."


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