Abstract: A thermoplastic pressure vessel consisting of a container having an exterior sidewall, a bottom and an annular top rim. An annular seal surface encircles the top rim. An annular collar extends outwardly and downwardly from the sidewall adjacent the top rim. The downward portion of the collar is spaced from and substantially parallel to the sidewall of the container such that an air cavity is formed between the collar and the sidewall. The collar has a threaded exterior surface. A closure is provided having a top with a depending annular skirt. The skirt has an upper portion of a first diameter with an interior sealing surface and a lower portion of a second diameter larger than the first diameter having a threaded interior surface.