Process for manufacturing a surface-treated metal sheet of high durability

A process for manufacturing a surface treated metal sheet of high durability, the process consisting of the steps of:

• a. subjecting a metal sheet to a chemical treatment to form a chemically treated metal sheet;
• b. applying by roll coating an organic resin paint containing a rust-inhibitive pigment onto the chemically treated metal sheet and drying the organic resin paint to form an undercoat layer on the chemically treated metal sheet;
• c. extruding a molten modified polyolefin resin and a molten polyolefin resin simultaneously in two layers from an extruder through a T-die to form an intermediate coating layer having a thickness of 50 to 500 microns on the undercoat layer, the intermediate coating layer consisting of an adhesive layer of 5 to 30 microns of the modified polyolefin resin on the undercoat layer and a layer of the polyolefin resin of 45 to 495 microns on the adhesive layer;
• and
• d. applying, as a topcoat paint, an acryl-modified vinylidene fluoride resin paint onto the intermediate coating layer and drying the topcoat paint to form a topcoat layer on the intermediate coating layer and
• optionally subjecting said intermediate layer to flame or corona discharge treatment before forming said topcoat layer thereon.