Rigid electric surface heating element

Claim 1

A rigid surface heating element heated by electricity and consisting of a thin, rigid, heat-resistant, electrically insulating, air impervious layer for emitting heat, connected by means of an electrically conducting film provided with corresponding electrodes and electrical connectors to a thicker, rigid, air impervious electrically insulating polyurethane foam layer, said heating element being prepared by mounting said electrodes on said thin, rigid, heat-resistant, electrically insulating, air impervious, heat emitting layer, depositing an aqueous plastic dispersion consisting essentially of a synthetic resin, electrically conducting particles, and an alkai silicate selected from the group consisting of potassium silicate and sodium silicate in an amount sufficient to provide homogeneous voltage potential properties to form said electrically conducting film over said electrodes and heat emitting layer, drying said film, and depositing said thicker, air impervious, heat insulating, polyurethane resin foam layer over the entire surface of said film by bringing a foam-forming reaction mixture into direct contact with the previously deposited electrically conducting film, said silicates being present in said heating film in sufficient quantity to protect said heating film from attack by the starting reaction components of the polyurethane resin.