How is Cuben mylar not-equal space blanket mylar?

"As I read it, each is a BoPET, or Mylar (biaxially oriented PE) laminated to fibers–High Molecular Weight PE, in the case of Cuben. Aluminization involved for SBs. If you didn't care much about the stretch/strength benefit of the HMWPE fibers, would you say that my experience with the puncture resistance of my space blanket would be a good gauge of what I'd experience with Cuben? Since I could get Cuben thicker, could I expect vastly superior puncture resistance?"

Charley, I would recommend doing more reading on this forums and elsewhere about Cuben and space blankets before you make a decision about what material to use for your project. No condescension intended; I just think this site has a lot of good archived forum material about cuben and space blankets, and it will help you to avoid wasting any money on the wrong material.

For starters, mylar is not biaxially oriented PE (polyethylene). It's biaxially oriented PET (polyester). The properties of those two materials are very different. There are three basic classes of "space blankets": conventional aluminized mylar ones, aluminized PE ("Heatsheets" type), and heavy-duty ones that are an aluminized film laminated to a layer of nonwoven PP gauze. As far as I know, there are no fiber-reinforced space blankets that are at all similar in their construction to Cuben. A typical crinkly mylar conventional space blanket is made with film about 0.5 mil in thickness. The newer "Heatsheets" type space blankets are made with thicker PE film (1-2 mil), and their softer film tends to stretch and deform like a trash bag rather than tear into ribbons like mylar when they fail.

The usual kind of Cuben that backpacking gear is made from is a laminate in which a grid of straight Dyneema (UHMWPE) fibers is sandwiched between two layers of mylar film (each 0.08 or 0.18 mil in thickness). The UHMWPE fibers in the laminate are tremendously strong and have near-zero stretch. The specific (weight-relative) tensile strength of Cuben far surpasses space blankets of any kind and woven fabrics. Cuben has poor puncture resistance because pointy things can penetrate the thin mylar and push aside (pass between) the reinforcing fibers. The abrasion resistance of Cuben is poor because the mylar on the surface is 1/6 to 1/3 the thickness of space blanket mylar, and tears easily. Once abrasion has torn the mylar face of the material, the reinforcing fibers can fuzz and pull away from the opposite layer of mylar, which is fragile and easily tears, leaving a shaggy little wound in the material. The more abrasion-resistant styles of Cuben have thicker mylar.

Most styles of Cuben are substantially more puncture resistant than a mylar space blanket. People who use space blankets for groundsheets typically just toss and replace them when they get shredded, but cuben is expensive, so people using cuben groundsheets usually choose a heavier (0.18 mil mylar) type that will last a few seasons. Consequently, the typical cuben groundsheet is much heavier (and tougher) than a space blanket. For a poncho, many people use cuben materials in the 0.5 – 0.75 oz/yd range.

Hope this helps.

A few corrections – Mylar is an extruded sheet, no biaxial there. So far, so pedantic… but in a roundabout way I'm going on with this to (I think) answer your question!

Cuben Fiber has at its core a dense biaxial (or on some qualities quadaxial) Dyneema* fibre structure. This offers the puncture resistance in much the same way as tightly woven fabrics made with other high-tensile fibres (e.g. of Kevlar or Vectran) do in ballistic resistant garments.

Cuben Fiber isn't trumped as anti ballistic because they're using finer fibres in what is a much thinner and lighter package. But it'll perform far better against punctures than a space blanket.

Other aspects of performance aren't comparable either.

The flex resistance of the lamination of two or more Dyneema layers will lessen the kind of creasing damage that freely occurs with one or two sheets of mylar with an aluminium layer/coating – i.e. a space blanket. Cubic offer different thicknesses of mylar sheets and also offer qualities with a woven fabric laminated face.

Needless to say tensile strength is where Cuben Fiber really performs.

So a few similarities, but on many fronts, very different things.

*They make up qualities with other substrates too… but only for special applications and no-doubt only for special customers!

Was going to let this sink from its weightlessness, but here's some light clarification, probably in reverse order

I got my info on mylar from Wikipedia. I realize, and thought I'd stated or implied, the fiber reinforcement difference between cuben and spaceblanket was vastly different, and I was curious only about the "Biaxially-oriented Polyethylene Tetrapthalate" portion of Cuben. Unless WPedia is wrong on this, the "biaxial" in mylar refers to the process of "drawing" the film in two directions during the extrusion.

Now I'll skip to the punchline. It turns out my "spaceblanket" is probably not Mylar at all, probably utilizing polyethelyne film. It is this "all weather blanket." Maybe it is a "Grabber." I thank you Colin for the search term "HeetSheet" (regi. tm)leading me to info. on what is likely in a "spaceblanket."

The fabric mesh in this is "Astrolar"; don't know or care what it is. Didn't know Cuben fabric was that tightly woven.

Skipping further ahead, I've abandoned my idea of a cuben poncho/groundcloth. I like the idea of Cuben staying put on the ground way better than silnylon or my polycryo. I don't care intensely about pinholes in a groundcloth under my sleeping pads, or poncho in Sierra showers. But in reading all the subsequent discussion and noting the $$ of Cuben, find it easy to stay my course.

As to Colin's courteous & warranted search-chide, I will backup all the way to my omitted disclaimer–I did search and read a lot, but due to my failing acuity or nature of the internet, find searching within the profusion always seems to disperse rather than hone-in when I'm looking for something very specific.

EDIT to list kernels gleaned:
–Biaxially-oriented polyethylene terephthalate is biaxially-oriented polyethylene terephthalate.
–Thickness matters a lot and you can get Cuben with differing thicknesses of Biaxially-oriented polyethylene terephthalate. Probably not so with "space blankets."

I'm glad you didn't let it sink/float away! I'm better feeling a bit red faced but better educated than remaining in ignorance… thanks for putting me right!

I'd wrongly assumed there'd been a misunderstanding by both you and Colin – with what I thought was a mistaken attribution of the term 'biaxial' to the Mylar (which, as I explained, is the correct term to describe the non-woven Dyneema arrangement in most Cuben Fibre qualities). An erroneous assumption made due my shameful ignorance of sheet PET manufacturing process, which I see DOES involve biaxial stretching AND biaxial polymer orientation too! (…I was also unfamiliar with the term BoPET, and it's not a term I've seen used by Cubic Tech, but I think it's fair to assume their Mylar sheets will indeed be BoPET, if the two terms aren't actually synonymous!)

In my defence, looking at PET film, it doesn't look biaxial! :-)

I stand corrected and better educated!

But I didn't make clear what Cuben Fiber is, which involves a bit of description. It isn't a dense weave. Cuben Fiber Dyneema "non-woven" structure is really interesting and fairly unique due to the uniqueness of Dyneema (by which I mean UHMWPE). It's best visualising as two or more layers of spaghetti – each layer individual strands of spaghetti lined tightly up together on one axis, and the next layer the same thing but aligned up on an opposing axis.

While I've let myself down with my poor knowledge of films, I do feel I've some insight into Dyneema, Cuben Fiber and many other performance fabrics, but sometimes my enthusiasm for the stuff probably makes me come across as a damn know-it-all, which really isn't my intention. I'm mad about the stuff, and I'm lucky that I get to play with it in my job.

I'm totally with you on the internet profusion thing, the easiest policy to take is fire question now… realise you look lazy (or in my case silly) later!