Nalgene 4001 Class B Polymethylpentene Volumetric Flasks with Screw Caps provide the accuracy of standard glass flasks combined with the break resistance of plastic. Polypropylene Screw Closures. Individually calibrated gravimetrically TC/TD at 20°C. Printing is durable; won't wear off. Guaranteed to meet the Class B accuracy requirements of ASTM E288 and ISO Standard 384. No meniscus to confuse readings. Translucent. Autoclavable, Leakproof and Graduated. Before autoclaving, set closure on top of the container without engaging the threads.