Cap Only with Lip Seal, Natural, Non-Sterile, 1000/cs (T501N)Caps with 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.