I always assumed that it was clear mylar (polyester) ...
That said, I doubt that the material is particularly important since it is in a sheltered environment and probably won't see much abuse.
Thanks for the reply! The Grafix sheets are made of Dura-Lar - I figured they`d be ok and it looks like that should be the case. From a product description:
"Dura-Lar is a polyester film that’s a mix between Mylar and Acetate. This film comes in a variety of different types (wet media, matte, clear, etc.) which all have specific features but overall, Dura-Lar is heat-resistant, durable (will not tear, absorb color, or discolor), and is acid free and archival."
I`m going to pick some up as see how it works.