Abstract: Sheets of thermoplastic material for lining structural containers and conduits, and having portions of their surfaces at the edges to be joined to each other treated with urethane, are joined and sealed by a strip of polyvinyl chloride, the interacting surface of which is prepared by hydroxyl modification for sealing with the thermoplastic sheets. The joinder is accomplished by a method that dispenses with any necessity of specially treating the surfaces at any prescribed time prior to accomplishing the joinder, and dispenses with any requirement for heat elevation during joinder. A method of joinder with the thermoplastic sheets alone is also shown.