Made with Mylar, one of the strongest plastics available—offering strength comparable to one-third that of steel—these sleeves provide exceptional protection against humidity, poly bag breakdown, acid migration, oxygen exposure, ultraviolet fading, mold and mildew, as well as oils and greases. The half-back comic backing boards feature a 3% alkaline buffer throughout and maintain a pH of 8.0+ to help ensure the long-term preservation of your collectibles. Meets strict U.S. Government standards for archival storage.