The intermediate strip 2 may have a thickness of the order of 15 to 200 microns (1.5 to 20.10-5 m) and be made from metal, more especially from aluminium or an aluminium alloy, from a cellulosic material such as paper, or a synthetic material such as polyvinylchloride, a polyester, a polypropylene, a polyethylene, a polyamide, a polybutylene etc.