Our Polyester (mylar) material is DuPont inert Melinex or suitable equivalent. It has passed the Photo Activity Test (PAT) and is approved by the Library of Congress for indefinite storage. Polyester is a smooth, crystal clear, acid-free, archival material that has a glass-like surface and has no wrinkles or dimples in it of any kind. Our ArchivalArmor™ 4 mil Polyester Sleeves provide an inert, safe environment for papers of all kinds.