Made of polypropylene. The O-Ring secured in the top of the cap ensures a positive leakproof seal, time after time, keeping the integrity of small samples under even the most adverse conditions. The caps do not have the attachment loops for users who prefer to remove the caps completely from the tubes when filling or sampling. Perfect for cryogenic work. Can be centrifuged up to 17,000g. Will withstand temperatures from -196°C to +121°C.