Caps are available either with or without a silicone washer between the cap and the tube to ensure a positive leakproof seal at all temperatures. Closures and tubes are both manufactured of polypropylene, providing the same coefficient of expansion. The cap features a long skirt and a super-fast thread design allowing it to be removed or screwed on with a single turn. Autoclavable. These silicone washer caps will guarantee a positive leak proof sealat all temperatures. Made of polypropylene.