A multilayer seal, plug or container neck comprising such a seal, and methods of making

The invention concerns a cap liner (9) comprising: a middle layer made of a plastic material with gas barrier effect; first and second outer layers comprising each a polyolefin resin and arranged on either side of the middle layer; an intermediate polyethylene layer, placed between the middle layer and each of the outer layers, the intermediate layers being designed to protect the middle layer against moisture and to prevent degradation of said middle layer gas barrier properties. The cap liner can be placed in a cap near its transverse wall or towards the free end of its skirt or on a container neck. It can be planar or thermoformed. The invention also concerns methods for producing a neck-joint cap assembly.