Saflip brand flips are the standard in archival safe flips suitable for long-term protection. They contain no oils or other additives and will not corrode or cloud your coins like other flips can.
Most of the double pocket coin flips today are made of vinyl, the common name of polyvinyl chloride (PVC). These PVC flips are available in both "soft" and "hard" versions and both types are dangerous for storing coins. The chemicals that can bleed out of the vinyl and the hydrogen chloride gas that the vinyl emits are corrosive to coins, causing sticky green slime, cloudy appearance and microscopic pitting of the coin's surfaces. This is why museums don't use vinyl of any kind, because museums know there is no such thing as safe vinyl. Saflips are manufactured under rigorous conditions to keep them uncontaminated by oil or machine dirt. Saflips are packaged in inert poly bags -- they are free of paper and cardboard dust that might cause spotting.