The white polypropylene ribbed caps has a teflon face over a silicone liner. The white plastic caps are available in 4 sizes, 53/400, 58/400, 63/400, and 89/400, and can be purchased by the bag. Be sure to test all caps and containers options with your products for proper compatibility. Typical uses of Bonded 60 mil PTFE/Silicon lined plastic caps include: analytical lab samples, high purity chemicals & strong acid solvents. Uses include environmental samples, pharmaceuticals, and diagnostic reagents.