Thermoplastic cap with internal oxygen impervious sheet and plastisol ring seal

Leakproof screw stopper device for receptacles such as bottles and jars containing products which must be kept protected from oxygen, comprising a cup-shaped cap with a bottom and a skirt with an internal thread, the bottom of the cap having, in the vicinity of its connection to the skirt, a ring seal intended to function together with the edge of the mouth or neck of the receptacle and consisting of a plastisol cast in the liquid state and gelled, characterized in that the cap (1) is made of an oxygen-permeable, injection-moulded thermoplastic, that the bottom (2) of the cap is covered internally with an oxygen-impervious composite sheet (5) comprising a layer of paper (6) and a layer (8) of PVC and adhering to the bottom because of the injection of the cap onto the said sheet, on the paper side, and that the plastisol seal (9) is cast on the said composite sheet (5) on the PVC side so as to adhere to the PVC layer (8) after gelling.