A plastic foil, whose surface is bombarded in a vacuum by ions with 40 to 100 keV energy and 10<16> to 10<17> cm<-2> flux density, is made of a polymer whose monomers are either pure hydrocarbons, or may also contain oxygen and/or nitrogen, sulphur or chlorine. This plastic foil is useful as steam barrier and/or for insulating purposes in electric machines.