Coating composition for precoated steel sheet, precoated steel sheet, and process for preparing the same

A coating composition for precoated steel sheets, which consists essentially of a resin component comprising the following i), ii) and iii):

• i) from 1 to 18% by weight, relative to the solid resin content of the composition, of a polyester compound comprising main repeating units of a chemical formula:
• ##STR16## wherein n represents an integer of 1 or more;
• and R represents an alkylene group having 10 or less carbon atoms, or a 1,4-cyclohexylene-dimethylene group, or a neopentylene group, or a polyoxyalkylene group;
• ii) a polyol except i);
• and
• iii) a curing agent of a mixture composed of a blocked polyisocyanate compound (A) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanate of the following group (a), and a blocked polyisocyanate compound (B) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (b), in a ratio by weight in terms of the solid contents, (A)/(B). of from 70/30 to 30/70:
• (a) 4,4'-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphtdalene-1,5-diisocyanate;
• (b) hexamethylene diiisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate,
• with the amount of the curing agent of iii) being so defined that the ratio by mol of the isocyanato groups, (NCO), in the curing agent to the total of the hydroxyl groups, (OH), in the polyester compound of i) and the polyol of (ii), (NCO)/(OH), is from 0.8 to 1.2.

A coating composition for precoated steel sheets, which consists essentially of a resin component comprising the following i), ii) and iii):

• i) from 1 to 15% by weight, relative to the solid resin content of the composition, of a compound of a chemical formula:
• HO--(CH.sub.2).sub.n --O--R--O--(CH.sub.2).sub.n --OH
• wherein n represents an integer of from 2 to 10;
• and R represents ##STR17## ii) a polyol except i);
• and iii) a curing agent of a mixture composed of a blocked polyisocyanate compound (A) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanate of the following group (a), and a blocked polyisocyanate compound (B) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (b), in a ratio by weight in terms of the solid contents, (A)/(B), of from 70/30 to 30/70:
• (a) 4,4 '-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphthalene-1,5-diisocyanate;
• (b) hexamethylene diiisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate,
• with the amount of the curing agent of iii) being so defined that the ratio by mol of the isocyanato groups, (NCO), in the curing agent to the total of the hydroxyl groups, (OH), in the polyester compound of i) and the polyol of (ii), (NCO)/(OH), is from 0.8 to 1.2.