Looking for the superior clarity of polyester in a sheet that will lie flat in a screwpost book? Look no further than our new Flex-Hinge Polyester Sheet Protectors. These top-and-bottom loading 100% archival sheets are much clearer than polypropylene sheets and they lie perfectly flat thanks to a hinged binding edge. Sold in packs of ten. Made of inert, unplasticized, heat resistant and chemically stable archival quality, heavy-duty .0045 polypropylene.