ARCHIVAL POLYESTER (PET) (Mylar) is an inert material of the highest clarity. We use DuPont inert Melinex® or a suitable equivalent. Polyester (Mylar) will not discolor, damage, or adhere to items placed inside it. It is a sturdy film that beautifully displays, timelessly protects, and gives extra support to fragile documents. Used by the Library of Congress. Indefinite Storage.