Abstract: The present invention relates to a closure giving a tight mechanical seal and preferably made from metal or plastic, with an insert on its inner side as a sealing element for containers, in particular for bottles and glassware, where the insert is composed of at least one polyolefin which can be prepared using catalysts selected from the class consisting of the metallocenes.

Abstract: The subject of the present invention is a process for the production of closures, in which process a sealing composition is applied and baked on the inside of the closures, wherein a sealing composition is applied which containsA) 20 to 95% by weight of at least one compound having on average more than two isocyanate groups per molecule and/or of a diisocyanate prepolymer, the isocyanate groups being blocked,B) 1.5 to 30% by weight of at least one diamine and/or polyamine,C) 0 to 78.5% by weight of at least one plastic and/or elastic and/or reactive organic polymer,D) 0 to 60% by weight of pigments and/or fillers,E) 0 to 50% by weight of one or more reactive diluents andF) 0 to 20% by weight of auxiliaries and additives, the sum of the proportions by weight of the components A to F being 100% in each case.The present invention also relates to the closures produced by the process according to the invention and to packaging containers which contain said closures.