What are balloons made of?

The balloon bag or "envelope" is made of
reinforced nylon fabric. It is very light
(1.3 ounces per square yard) but
very strong (25 pounds per square inch).
The envelope is treated with a polyurethane
coating to make it more airtight and to help
the fabric withstand the ultraviolet rays of
the sun. The pilot and passengers ride in
a gondola (or basket) that is made of woven
wicker, which is both strong and flexible.
The gondola is connected to the envelope by
stainless steel suspension cables.

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