Mylar is not transparent in IR but reflective.
Metallic party balloons don't need to be made of Mylar (BoPET).
If anything, use of Mylar per se for party balloons seems quite rare, which surprised me
I assumed all metallic balloons were some sort of BoPET material like Mylar.
Advert on Amazon retail website,
https://www.amazon.co.uk/Balloons-Balloon-Birthday-Wedding-Decorations/dp/B0C7KS9HZP?th=1
From section "Color Issues and Packaging" on the
BalloonHQ website, part of Balloon Suite group, here
https://balloonhq.com/faq/making/:
External Quote:
Pearl Tones.
Pearl tone latex is created by adding crushed mica to the latex. This process can make the latex more brittle, and less
twistable. If you want to see proof of this, you have to look no further than at Tilly Pearl 130's.
Gold/Silver/Metallic 260's
Metallic latex is made in the same way as pearl latex.
That webpage also tells us,
External Quote:
How Foil Balloons Are Made
The concept and technology for the "metalization" of plastic sheeting that has given us foil balloons comes directly out of the NASA Space Mission. By the way, all of us should stop referring to foil balloons as Mylar (a trademarked name for a certain type of polyester film mad by DuPont) balloons. The balloon industry refers to them as "foil" balloons, because they are made of nylon sheet, coated on one side with polyethylene and metallized on the other. It's evidently so much harder to make balloons out of aluminized Mylar (and probably so much more expensive) that nobody does it.
(My emphasis, except the heading).
From the context and likely audience of the
BalloonHQ website, I think "...nobody does it" might be better understood as "...very few suppliers of party balloons do it". Mylar balloons obviously exist.
The
BalloonHQ webpage continues (citing The
Incredible Balloon webpage),
External Quote:
In the late 1970's, silver metalized balloons were developed for the New York City Ballet. These balloons are commonly called Mylar, but they are actually made from a metalized nylon and are more expensive than latex balloons. Someone wrote me that it is 48 gauge (.48 mil) aluminized biaxial nylon w/ a special coating (capron emblem) for heat sealing.
So there are metallic-appearance latex party balloons and nylon/ polyethylene metallized party balloons.
I have absolutely no idea if their respective IR signatures / opacity/ reflectivity etc. differ significantly from "traditional" coloured latex or true BoPET products.