YER EXPERIENCE WITH DYNEEMA TENT FLOORS?
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“Regular nylon is about 1/3 stronger than poly for the same weight. So 1.1 Nylon would be roughly equivalent to around 1.5oz poly. Now there are a couple different nylon;s. Nylon [6,6] is about 40-60% stronger than poly. This is around 1.54 to 1.76oz poly”
Fabric tear strengths seem to be all over the place, but I disagree with the general conclusion that regular nylon (nylon 6) is 1/3 stronger and nylon 6,6 is 40-60% stronger. I’m sure there are examples like this and I think we’ve discussed this a few times and thus probably won’t agree, but for the actual fibers themselves poly is about 99% as strong as nylon 6,6, per DuPont who actually make these fibers:
Of course, going from fibers to fabrics involves a lot of other factors (threadcount, calendaring etc) and nylon does seem to gain an edge there, but according to Mike Cecot-Scherer (arguably the most prolific tent designer of all time) nylon 6,6 typically has a 15% advantage (source) – not 40-60%. That’s the same as what I’m seeing. The tear strength of the 20D polyester that we use tests 85-90% as strong as putting identical coatings on a nylon 6,6. So with nylon losing about 10% strength when wet, they’re very similar in the field even before factoring in UV degradation.
I suspect a lot of the results showing silpoly being substantially weaker are because a lot of silpoly isn’t true silpoly. A lot of it is using a sil/PU blend coating (like what RSBTR sells) that is substantially weaker than pure sil, and often this is the case even when the person using the fabric isn’t aware of that. Coatings tend to have a much larger effect on tear strength than nylon vs poly.
In this thread, I shared a fabric performance chart prepared by an employee of Ripstop By the Roll, a large seller of fabrics for use in MYOG as well manufacture of gear. The chart rates several of the fabrics and runs all the way from low to high ratings, so is critical of some of the seller’s products in many respects.  The ratings are not just of raw fabric yarns, but rate specific fully manufactured fabrics offered for sale.  I think that makes the chart more helpful, because many of the qualities of a fabric depend not just on the yarns; but also on the quality and process of manufacture, including the weaving, coatings, etc.
The chart’s evaluation of PU coats added to membrane silpoly is consistent with the comment on this page by Roger Caffin about the effect of PU on tear strength. He has long experience with the fabric business. The chart rates the Membrane silpoly PU 4000, “low” in that regard, and low also with respect to puncture resistance.
The chart also is consistent with my experience with some of the fabrics. The one in question earlier in this thread was the quality of one such fabric, the RBTR 15 denier non-ripstop Membrane Silpoly, weighing .93 oz/sq/yd and coated on both sides with a “mix” of silicone and polyurethane.
I find it very useful to often make small scale models of tents before attempting to build them. Until recently, I used silnylon to build a number of models, but more recently tried using the above 15 denier polyester for the outer skin of the models, stretched over frames made of music wire, and pinned to the the peaks of the frame above, and to a plastic or cork board below. Here is a very early model that dates back to the years when Epic Malibu, a heavier polyester fabric, was used by Black Diamond on their single wall tents:
Some may recognize this model from a project I’ve posted about based on a double cross pole configuration, XX for short. The Epic Malibu polyester fabric worked fine, and demonstrated that polyester fabric does have significant bias stretch, diagonal to the fabric grain. But later, when I tried using the RBTR Membrane 15 denier, it unraveled from the pins at the peaks, causing the model’s canopy to come apart.  Repeated attempts led to the same result. So went back to silnylon for the models.
So my experience was also consistent with the above RBTR chart that rated their fabric medium-low or vice-versa for tear strength, puncture resistance and abrasion resistance. For the above reasons, I think the RBTR chart is worthy of some reliance, especially considering that it is highly unusual for sellers of MYOG materials to specifically rate their own products so critically. When we ask questions that we already know the answer to, the responses provide guidance about how much we can rely on the answers to questions we don’t know the answers to. We have a few lab workers on BPL, but in large part we have to rely on our own knowledge and experiences in making judgments about materials as well as a lot of other things.
Please note that tear resistance, as distinguished from puncture and abrasion resistance, can be very different. An example is provided by DCF, a material often discussed on these forums. I’ve often found it easy to stick a straight pin through tent weight DCF, or even cut it with a sharp utility knife. Whereas tearing the same material can be very difficult.
Please don’t get me wrong. Silpoly has a place, and RBTR sells a lot of it I’m sure. Especially the heavier 20 denier ripstop silpoly weighing 1.24 oz/sq/yd. And I’ve decided to use that silpoly for the vestibules on a tent because it appears much more resilient than the non-ripstop Membrane 15 denier, and also because the leading edge of each vestibule is key in the design to tensioning and supporting the tent, and keeping it taut in an application where silnylon would sag and loosen the walls of the tent.
But I’ve decided to use silnylon for the main body of the tent, the portion shown with fabric stretched over the frame in the above photo, because as you can see, the frame stretches the fabric on the bias. I’ve found from experiments that a permanent bias stretch on a silnylon canopy will minimize sag from dew and wet weather.
The 20D silnylon from Estrem Textil is .16 oz lighter per sq/yd than the 20 D silpoly from RBTR, but it is not just that. It is that I’m persuaded that the silnylon will be more reliable “when thing go wrong,” as in Roger’s excellent article for BPL. And after looking at all the info linked in this and other threads on BPL, I think there is a consensus about that.
Stumphges,
Re: ” I think your hypothesis – that pulling the bias stretch out while tensioning the fly reduces moisture/temperature-related sag of silnylon – is probably right.”
I said: “I’ve found from experiments that a permanent bias stretch on a silnylon canopy will minimize sag from dew and wet weather.”
A bit more on that: Various samples of superlight nylon tent fabrics (0.8 – 1.08 oz/sq/yd) were stretched tautly on 9″ diameter plastic embroidery loops. Here are two examples:
Note the the one on the left is shown with the fabric underneath the loop, and the one on the right has the fabric on top. Then a number of loops were propped up vertically between sliding casement windows and sliding outside window screens. There is an overhang on the roof to keep rain from hitting the windows and screens; but when the wind blows, it happens. Initially, this was just to see how much expansion, meaning wrinkles and sagging, developed on the taut surfaces of the fabrics.Each of the fabrics stayed in the windows through sun and rain for as long as was needed to gauge the amount of wrinkling and sagging, which was substantial, except that the nylons coated with both sil and PU, developed slightly less wrinkling; but it was more wrinkling than desired for the canopy of a tent. Note FWIW that all the fabrics were tested for HH by either Richard Nisley or Stephen Seeber, and ranged in HH from mid-2000’s up to mid-3000’s after simulated aging by runs though washing machines.
Much more recently, plastic knitting needles slightly over 9″ long with music wire cores were inserted in some of the loops with the fabrics taut and facing down, and then rotated so that the fabrics were stretched diagonally on the bias across the grain of the weaves. Even on 9″ diameter circles, the bias stretch of nylon allowed the fabric to dome outward substantially. Then the exposure tests were repeated. While there was slight wrinkling just inside the perimeters of the loops, the fabrics otherwise remained taut during fluctuations in the weather. Hence my confidence that permanent bias stretching in a tent design will address issues with nylon sagging without the need for shock cord attachments at front stakes and other such measures. But soon the full scale erected prototype canopy being used for design and measuring will also be subjected to weather changes. Fall is a good time for that.
Of course, the above may all be moot as suggested by Xavier’s experiments contrasting the puncture strength of nylons and polyesters with different coatings. One problem with that is the low rankings RBTR has published for its own polyesters vs nylons, linked in my post on the previous forum page: https://ripstopbytheroll.zendesk.com/hc/en-us/articles/236101568-Coated-Fabric-Comparison-Chart
Were it not for that, plus the opinions of many, I would be first in line for a sub one oz/sq/yd polyester light tent fabric with a mini-ripstop weave. But I’m pretty sure from my own experiences that RBTR “membrane” silpoly 15 denier is not it. Even if it were, I’d want something beefier for a floor, like the Silnylon 6.6 20 denier from RBTR, especially because I don’t use ground cloths. It’s just that I’ve used high quality silnyon floors for many years with no punctures, abrasion, tearing, or leakage; and for floors, haven’t found drum tightness to be an issue. These were all 20 to 30 denier fabrics, and all were nylon 6.6, ever since Thru-Hiker began offering it for sale.
To demonstrate how wildly tear strength can vary here is a list of specs for 30-40D nylon and poly fabrics from a fabric mill. Not shown here are the type of nylon (e.g 6 vs 6,6), type of coatings, and other factors.
My main point is that you can’t conclude much in the way of general truths from a small sample of fabrics. We see three 30D nylons ranging widely from 500 – 3000g, and then 30D poly at 1400g, and results for the 40D fabrics that are quite similar to the 30D. Even if you accounted for coatings etc, there would be enough variation that you wouldn’t want to base any general conclusions on only 1 or a few samples.It would be easy to cherry pick data to support any conclusion. One could say “Wow 30D poly (sample 6) is 3x stronger than 30D nylon (sample 1) and even 40% stronger than 40D nylon (sample 4)“. Or the opposite: “Wow 30D nylon (sample 3) is twice as strong as 30D poly (sample 6)” etc. Actually those differences are most likely the result of different coatings, much moreso than fiber. I don’t have coating specs for these fabrics  to investigate further. If I did, I’m sure they would explain some but not all of the differences. Fiber differences are relatively minor compared to the other factors at play.
I wish I knew. I’m not aware of any advantages to blending in the PU. Maybe you can seam tape it, but no one buying from RSBTR is seam taping.
I got some more info on these three 30D nylon’s that vary in tear strength by 6x.
The coatings are Sil/PU 1500, PE5000, and Sil/PE 2000, respectively. Note:
1) The numbers are marketing terms moreso than actual HH tests, but still give a rough idea of how much total coating is on the fabric.
2) For the dual coated fabrics, I have no information on the relative weight of these coatings but they are one coating per side, not a blend.
3) There are other factors like threadcount (listed), calendaring, type of nylon, etc.I still think it is remarkable that #3 is 6x stronger. It is 20% higher threadcount and 20% heavier, and PE is better than PU, but even if #3 is nylon 6,6 vs 6 it is still a remarkable difference. I have samples of these, so I made do some hasty tear strength tests to see if the large advantage for #3 seems real.
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