| 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 then a resin coating film composed of a modified epoxy resin, containing a silane coupling agent and a rust preventive pigment, and a polyfunctional polyisocyanate. CONSTITUTION: A modified epoxy resin A is formed by adding 0.8-1.0mol dialkanolamine per one equivalent of epoxy group of an epichlorohydrin- bisphenol A type epoxy resin having the epoxy equivalent of 500-5000. A silane coupling agent B in an amount of 0.5 to 10 pts.wt. polyfunctional polyisocyanate C (containing >=3 isocyanate groups) and the rust preventive additive D in the weight ratio (A+B+C)/D of 90/10 to 40/60 aree added to 100 pts.wt. resin A, and the resultant resin composition 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, and its dry film thickness is controlled to 0.2-3.0μm.
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