Ideal for the storage of biological materials, our polypropylene tube with high-density polyethylene (HDPE) cap has an incredibly secure 2-turn closure that includes an external thread. Specifically designed to remove the risk of trapping sample on the inner wall. The difference in hardness between the tube and cap coupled with the capâ€™s molded in sealing ring forms a leak resistant seal. These round-bottomed tubes have molded in locking bases that work best with the Stockwell, Simport, Corning, and Nunc Storage Racks. When used in appropriate storage racks, tubes are easily opened or closed with one-handed operation. Tubes are graduated in 0.5mL increments and have a frosted writing area. Non-sterile tubes are packaged separately from caps. Sterile tubes are packaged with caps on. Storage Vials Features: High Density Polyethylene (used for the cap) provides a softer surface that conforms to a harder material like polypropylene-copolymer (used for the tube) to increase the integrity of the seal. Internal sealing ring envelops the top of the tube to prevent liquid by-pass. External threads eliminate the problem of sample retention common to internally threaded tubes. Universal locks hold tubes securely allowing for one-handed operation. Round bottom design assists to maximize sample recovery.