I have been looking into the construction of Mylar envelopes and have found out a fair few things. The first is that some people refer to Mylar as the stuff that metallic party balloons are made out of. This stuff is actually a layer of polyester coated with nylon on one side and metallised on the other. This allows them to be heat sealed due to the nylon coating but is not the same as proper Mylar, which is BoPET (Biaxially-oriented polyethylene terephthalate). I did, however, come across this forum thread about people making hobby blimps out of Mylar. This got me thinking about ways that I could do a similar thing. I am a bit sceptical as to whether or not the glue they are using will work at high altitude though. Also, I couldn’t find the glue that they refer to at any of my local hardware stores. As a result, I have been doing some research into suitable alternatives that could be found locally. More on this a little later.
Since there is a little bit of confusion when referring to the Mylar trade name, I am not really wanting to go and buy a roll of “Mylar” from ebay or the like just to find out that it is the wrong stuff. I then went on a bit of a hunt to find a suitable alternative that is locally available and cheap. Through further study online, I found out that space blankets, a.k.a. emergency blankets, are actually metallised PE. These are both locally available and cheap, meaning they hit both of my requirements. There are two, still to be answered, problems though. The first is that they may be the proper material, but are they biaxially oriented? This is a requirement because the envelope won’t handle the pressure at high altitude if it is not. The second problem is with the way that they are packaged. Due to them being folded for easy of selling/transporting/etc., the blankets have creases from the folds all over them. I am not sure if this will pose a problem when it comes time to gluing the seals and if they will cause points of weakness that will result in premature eruption of the envelope as the balloon gains altitude.
Time will tell I suppose.