Conical 15ml and 50ml centrifuge tubes are made of premium polypropylene for clarity and centrifugal strength. Tubes and caps may be frozen at temperatures as low as -86ÀöC. The maximum centrifugal force is 12,000xg for the 15ml tubes and 15,000xg for the 50 ml tubes*. Caps have a flat top surface for easy marking and labeling. Catalog No. C2605 features a ‚Äòskirted' bottom that allows the tube to stand upright without a separate test tube rack.