POLYPROPYLENE SHEET PROTECTORS (for use in Screwpost Book Mechanisms) are welded and have become an industry standard for quality. Packed in a metallic, reusable bag. Archival quality heavy-duty .0045 polypropylene. Inert, unplasticized, heat resistant and chemically stable. 10 pt. black acid and lignin-free paper. Both plastic and paper pass the Photographic Activity Test (PAT) and meet all ANSI requirements for archival storage.