Sealing agent (1) comprises a barrier layer (2) and an adhesive layer (3) comprising: a polymer, which is polyethylene, polypropylene or a copolymer of ethylene and propylene; a polyolefin-based polymer having at least one functional group of carboxylic acids, hydroxy-groups, anhydrides, acetates and glycidyl methacrylates; and azo-dicarbonamide. Independent claims are also included for: (1) sealing a substrate, comprising applying a sealing agent on a substrate, where the adhesive layer faces the substrate, and heating the adhesive layer to a temperature of 80-600[deg] C; and (2) molding of a sealing agent, comprising connecting the molding with adhesive layer, which is previously heated and fixed with the sealing agent.