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. As for tubes, these closures are also made of polypropylene, providing the same coefficient of expansion for both, which further enhances the leakproof qualities of the vials at changing temperatures. The cap features a long skirt and a super-fast thread design that allows it to be removed or sealed with a single turn. Made of polypropylene.