My comment on the durability of mylar in comparison to nylon is based on real world experience in using it for: molding, vacuum bagging, RC airplane covering. IE running it over corners, wadding it up, pretty much not paying much attention to it like I do to nylon bags.
Its not bad like acetate for tearing/brittleness. In fact, its pretty decent for bending and folding. Just not up there with nylon. It also depends on the thickness of said mylar(duh). Probably also depends on how highly polymerized said batch of mylar is or cross link polymerized it is. I forget how much cross linking is going on. Could be none and I don't feel like looking it up. If you really want to know open your trusty plastics materials handbooks. The good ones are larger than the normal materials(steel, bizimuth, etc) handbooks as there are so many varieties of every conceivable plastic out there.
For this discussion, I think we can assume that Cuben Tech folks aren't idiots, and got their mylar properly polymerized to their desired maleability and tear strength criteria. Though in this case their polymerization criteria has far more to do with bonding the dyneema to the mylar I would presume, than say maleability(waddability)
After all space suits use mylar to maintain air pressure inside. Though I think this is layered aluminized mylar as coating mylar with an aluminum coating is for that needed extra real air permeability of 0 is fairly easy due to its high electrostatic properties compared to other plastics.
Also mylar has that nice property of shrinking when heated, creating a bunching effect of the dyneema between the two layers. At least this is what I am assuming they are doing. Once again said shrinking property of mylar is dependent IIRC on how much polymerization is done on said mylar to begin with.
I have had RC(Radio Control) mylars that literally fall apart if you run them over even a slightly sharp edge or crack and split. Others you can munch/bunch them a bit, but it will leave crinkle lines. For this reason when molding you you have to have no sharp corners on your molds. On certain types of nylon, you could literally have a knife pointing straight up and the nylon bagging material would form around it without a hitch. Try doing that with your aluminized mylar balloon. Not a chance.
Long way of saying, that for Cuben Techs products, they probably don't give a hoot and a hollar about wadding up their products. Sails are NOT wadded up in a bag, well some folks do... Not their racing sails! They are generally ROLLED, or at worst folded loosely creating no sharp points/lines. In other words their specs for polymerization of their sails could probably be far looser than that wanted for stuffable tents/ground sheets you name it. It could simply be a bad batch on the edge of their specs perfectly OK for sails on a boat, but not all that hot when it comes down to waterproofness.
Sorry Roger, though the name Mylar is trademarked, it can actually have a wide range of material properties and lets not get started on what happens when additives are introduced into the discussion! Its kinda like talking about nylon in that regard from what I have read about it and from my user end experiences with the stuff.
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