These disposable sterile tubes are made of high quality polypropylene (PP) plastic that will withstand temperatures from -190˚ to +120˚C. They are also available in crystal clear polystyrene (PS) with a range of 0˚ to 70˚C. Cuo function caps are made of polyethylene (PE) and have both a screwcap design dand the popular two position design (up/loose for aerobic work and down/sealed for anaerobic cultures). When using screwcap function, rotate the cap ¼-turn to partially seal tube, or rotate ½-turn to completely seal tube. Polystyrene tubes may be centrifuged at up to 2000xg. Polypropylene tubes withstand higher forces, up to 4000xg.