Silpoly / PU Coatings

Posted: February 3rd, 2019 by: h2

Here is an interesting discussion on on Silpoly durability and PU coating types: Geoff Caplan on backpackinglite. Some things tend to get lost in backpackinglight from the endless churn, so I want to preserve the main points here.

I thought I’d give a heads up to the views of Mike Cecot-Scherer. He’s a freelance designer of backpacking tents, with over 250 designs to his name for many of the major brands. He has access to their labs and repair departments, so is in an usually strong position to cast light on this issue.

He is a strong proponent of 20d poly as a fly fabric, particularly with a sil/PU coating.

Some take-home points from his site:

  • Sag when wet: silnylon can stretch as much as 4″ across the longest dimension of a small tent, leading to serious flapping and increased wind load. Silpoly retains its pitch in the wet. He sees this as a big deal.
  • Tear strength: while nylon is a few percent stronger in theory, in practice strength will vary a lot between manufacturers and batches. After a few days of UV exposure, the silpoly may well overtake the nylon because of its much superior UV resistance. And in any case, repair departments report that fabric tears aren’t a significant cause of failure.
  • Puncture & abrasion resistance: he argues that these are terrible on all lightweight tent fabrics, so there isn’t much to choose between them. Provided you reinforce the stress points, he feels that all you can do is carry a good repair tape in case of issues.
  • Waterproofing: he feels that a 1500-2000 coating is fine for both flies and floors. If you want a bit more robustness, you could take the floor up to 3000. Anything more is overkill. In his experience, users often mistake condensation or vapour transmission (lightweight fabrics breathe) for leakage. He prefers a sil/PU coating as you can seal the seams reliably. He doesn’t discuss the longevity of the PU coating, so I’ve dropped him a line and will post if he replies.

He’s recently developed his own comfort-oriented range of freestanding backpacking tents with some bleeding-edge features. He subjected them to multi-directional wind tests – possible the most comprehensive yet conducted. They were stable and quiet up to Force 8. When tested to failure, it was the DAC poles that went, not the fabric. There are videos of the tests on his site, which is well worth exploring:

A bit further down that thread George posts another comment by Mike:

… But here’s the thing: no one actually bases their tear strength requirements or even their desires on what’s NEEDED because no one knows what that too-low number is. So we’ve gotten into a sort of tear strength arms race even though it’s obvious that it’s the tensile strength that a tent primarily needs (and all fabrics available are WAY overkill).

So I’ve taken it upon myself to ask repair departments of my clients (and Kelty, of course, when I was there) what, if any repairs they saw were caused by the tear strength being too low. Tents age, UV damages fabric, surely there should be some indication of a lower limit to tear strength in use. But the answer was always no. No repairs ever because tear strength was too low. Actually is was quite striking. There were even tents sent in for repair that you could literally push your finger through which weren’t being sent back to be fixed because of that; the customer hadn’t damaged it and it was sent back for something else entirely!

As to abrasion resistance or puncture resistance, ALL the lightweight fabrics we use are just terrible – it takes the barest swoosh against a sharp rock or a gnarled tree to put a bunch of holes or a tear into a rainfly. About the only thing one can say about higher tear strength is that there’s the theoretical possibility (and hope) that a tear won’t propagate as much in a higher tear strength fabric. But prevent a tear? No way. This is when having some of the truly excellent repair tapes available can really save the day. I love modern repair tapes.

Then an important bit of information on PU coatings from Mark:

I asked him about the longevity of PU coatings on lightweight fabrics, and also to look over this thread and give his responses. As you read this, it’s worth bearing in mind that he’s one of the most experienced people in the industry.

The durability of PU coatings can be all over the map and they can’t be distinguished between without some very fancy equipment. The most common PU coatings are called polyester-based polyurethanes. These will degrade with water (!) over time through a process called hydrolysis which basically means that water causes the polymer chains to break apart. It’s what’s responsible for sticky and stinky coatings and short tent life on all kinds of fronts.

A far better type of PU is poly-ether based PU (sometimes called PEU) which I’ve found to be very durable and very well behaved in general: soft, not sticky or stinky, reliably adheres to fabric, folds well, and so on. Mountain Hardwear once made a big deal about this kind of coating but it’s been a while since I’ve seen it on their website (though I could just be missing it). I want to say that all good brands use it but it’s so easy for them to forget to spec it in and with staff turnover – who knows. Also, Asian fabric finishers worry more about UV and yellowing resistance than water stability. They just don’t ‘get’ our concerns; tent makers are oddballs in the market. So they default to polyester-based PUs.

As to basic coating quality, like questions of tear strength, it seems mostly to be a matter of process control – expertise to make the coating without pinholes or too thick or not to wreck the tear strength of a fabric. It’s not an exaggeration to say that the tear strength of a particular fabric depends more on the finisher than on the plastic the fabric is made from.
Too much coating is a HUGE problem for makers concerned with weight. So as with tear strength, an established source with a good track record is important. But, unless someone goes out of their way to tell you who they use for fabrics, it all seems like a big black box at this end (leading to all kinds of false generalities).

I took a quick look at the thread you mention. Nice to see people care. Some notes:
• Ripstops can easily be found with very flat weaves. So coating and abrasion on those materials are just like non-ripstops. There’s no bumps to make wear points or have thin spots in the coating. Feel a fabric; if it feels ribbed, then it fits the old stereotypes. If it feels essentially flat, it’s good to go. I would say that a flat ripstop weave is one of the selection criteria for tent fabrics that experienced tent designers use when evaluating fabrics.

• Plenty of overlap between inner tent and rainfly has proven itself to handle very harsh rain conditions. What they offer is the ability to take the fly off when it’s hot or to sky watch or even just to dry the tent faster (where polyester IS faster than nylon). Also less condensation when those conditions occur.

• 20D polyester can easily have enough tear strength. Again the finisher can’t be throwing it away. Obviously it’s more prone to little holes and tears from unknown causes than heavier materials – “wear and tear.” Some repair tape is a great idea to prevent little things from getting bigger.

• The impact resistance argument is complete nonsense. No winds can hit with the speed and force that is required to get into the realm of “impact” in the engineering, materials science sense. Also note that the actual impact strength of an oriented plastic fiber is very good. Finally, note that the concept of impact is often conflated with notch sensitivity and here again, woven fabrics have no notches in the engineering sense.

• Related to the notion of impact is the idea that a stretchy guyline is helpful. Again this is simply not true. A stretching line makes a scooping fabric shape which leads to heavier loads on the tent (and fiercer flapping). I highly recommend completely non-stretch dyneema lines or near-completely non-stretch polyester lines. Neither of these lines will loosen when wet either.

• I can’t track down a reference but think “wind hammer” is an idea from sailing that crucially depends on a boat’s rocking motion to compound the “impact”. In other words it occurs when a mast and sail swings against the wind. If so, please note that the whole idea here depends on a rigid connection of the boat to the mast to make “wind hammer” a destructive thing. Tent poles (the mast of a tent) flex relative to the ground (the boat) so the concept really doesn’t apply.

• I know of no difference between re-coating nylon versus re-coating polyester fabrics. Both plastics have plenty of binding sites for adhesives and presumedly you’re bonding on the PU anyway. But trying anything but a silicone on the siliconed side will fail.

• All conventional woven fabrics have mechanical stretch in the bias direction. A “triaxial” fabric is available in the composite’s industry but it is not available for anything we would call lightweight or affordable.

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