Balloon construction depends on how large of payload to what altitude.
50-70kft is not that high and reasonable payloads go up on latex
(elastomer) balloons hundreds of times a day.
Hot air balloons can approach that altitude, but it is getting
a little iffy for the pilot and performance.
Scientific ballooning is normally heat sealed polyethylene bags
(think dry cleaning bag material) in pretty thin gauge. That can
be sporting to source. (ravenaerostar.com)
Bigger/heavier payloads get heavier membrane material that can actually
be easier to find. (many plastic extruders can go reasonably thick)
Beyond that, mylar and other glued materials have been used, and generally
with load tapes for payload support.
One note: The natural balloon shape is a compromise between volumetric
efficiency and payload support. If you haven't found that on-line yet, you
still have more looking to do. ;)
-Gar.
(global-western.com does some)
From: David McMillan <skyefire@xxxxxxxxxxxx>
Subject: [AR] OT: Materials and construction of large high-altitude
balloons
Date: Sun, 3 Dec 2017 00:54:11 -0500
   So, not really on-topic, but this is probably the most likely
place
I can look for leads. For reasons I'm not free to go into, I'm doing a
deep dive into how large high-altitude (50-70k ft) balloons are
constructed, both methods and materials. And I'm having a devil of a
time finding *anything* at all -- it doesn't seem like anyone's being
*secretive,* as such, but more that those "in the know" just don't talk
about it much. Even the NASA technical reports server came up empty --
lots of papers about *using* balloons, but not a single one about how
the ballons are *made*. Maybe I'm just failing to use the right search
keywords, but when I've simply searched for "balloon" and then checked
every single resulting paper....
   So, given the crossover interest in using high-altitude balloons
as
ersatz first stages for launch systems, I'm hoping that some list
members might have some knowledgeable advice on where I should go digging.
