| 摘要 |
PURPOSE: To improve contact rusting resistance by specifying the composition, coating weight and film thickness of a coating film in a steel sheet made by forming a chromate coating film on the surface of a galvanized steel sheet and next a water-based coating material film composed of a copolymer resin, containing a rust preventive agent and a polyfunctional polyisocyanate. CONSTITUTION: A polyfunctional polyisocyanate compound B (hereafter called merely compound) in an amount of 5 to 80 pts.wt. (solid portion hereafter the same) is incorporated in 100 water-based dispresion resin A of one or more kinds of an ethylene-acrylic acid copolymer, a styrene-acrylic acid copolymer, a polyacrylic acid and its ester and their respective copolymers, a polymethacrylic acid and its ester and their respective copolymers. The compound B has at least 3 isocylanate groups in a molecule. Further the rust preventive agent C is added in the ratio (A+B)/C of 90/10 to 40/60 by weight. The water-based resin is applied on the galvanized steel sheet, on which the chromate coating film has been formed in 5-200mg/m<2> coating weight expressed in terms of metal chromium, to form the resin coating film of 0.2-3.0μm. dry film thickness.
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