The preferred films for use in the layer 3 are films of synthetic organic polymers such as polyethylene, polypropylene or other polyolefin; polyamide, polyester, e.g. polyethylene terephthalate, polyurethane, polyvinyl chloride, a copolymer of vinyl chloride and vinyldiene chloride, synthetic rubber such as polychloroprene or butyl rubber, etc.