COATING COMPOSITION FOR PRECOATED STEEL SHEET, PRECOATED STEEL SHEET, AND PROCESS FOR PREPARING THE SAME

摘要

The invention relates to a thermosetting coating composition for precoated steel sheets which are required to have high hardness, good workability and stain resistance, to a precoated steel sheet having a film of the coating composition, and to a method for producing such precoated steel sheets. The coating composition for precoated steel sheets of the invention consists essentially of a polyol and contains a specific compound and a curing agent composed of polyisocyanate compounds added thereto, in which the curing agent is a mixture composed of a blocked polyisocyanate compound as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (a) and their derivatives, and a blocked polyisocyanate compound as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (b) and their derivatives: (a) 4,4'-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphthalene-1,5-diisocyanate; (b) hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate. The coating composition forms a coating film having good workability, high hardness and good stain resistance.