Primer coating composition and method for forming coating film

A primer coating composition comprising:

• an aqueous polyolefin-based resin (A);
• an aqueous epoxy resin (B);
• a hydrophilicized-modified carbodiimide compound (C);
• and
• an aqueous polyurethane resin (D),
• wherein
• the aqueous polyolefin-based resin (A) comprises an aqueous polypropylene-based resin having a weight-average molecular weight of 50,000 to 200,000,
• a content of the hydrophilicized-modified carbodiimide compound (C) is 3 to 10 parts by mass based on 100 parts by mass of the resin solid content of the primer coating composition, and
• the hydrophilicized-modified carbodiimide compound (C) is a compound represented by a formula (I), (II) or (III) below:
• YOCONH—X—NHCOO—Z—OCONH—X—NHCOOY (I)
• wherein Xs are each a bifunctional organic group having at least one carbodiimide group, Ys are identical or different structures each resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Z is a structure resulting from elimination of a hydroxyl group from a bifunctional polyol having a number-average molecular weight of 200 to 5,000,
• wherein Xs are each a bifunctional organic group having at least one carbodiimide group, Ys are identical or different structures each resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether, R0 is hydrogen, a methyl group or an ethyl group, R1 is an alkylene group having 4 or less carbon atoms, n is 0 or 1, and m is 0 to 60,
• YOCONH—X—NHCOOY (III)
• wherein X is a bifunctional organic group having at least one carbodiimide group, and Ys are identical or different structures each resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether.