Surface control agent for water-based coating, water-based coating composition, coating film, and multilayer coating film

A water-based coating composition comprising:

• a surface control agent for a water-based coating in an amount of 0.1% by mass or more and 10% by mass or less when a total resin solid content is 100% by mass, wherein
• the surface control agent for a water-based coating contains a copolymer containing, as constituent monomers,
• a dibasic acid ester (A) having a polymerizable unsaturated double bond in an amount of 10% by mass or more and 99% by mass or less, and
• a polymerizable unsaturated monomer (B) containing an ether group represented by a general formula (1):
• R1—(CmH2mO)n—R2 (1)
• (where R1 represents a (meth)acryloyl group, a vinyl ether group, or an allyl group, R2 represents a hydrogen atom, a (meth)acryloyl group, a vinyl ether group, an allyl group, or an alkyl group with 1 to 22 carbon atoms, m is a number from 2 to 4, and n is a number from 2 to 100) in an amount of 1% by mass or more and 90% by mass or less,
• the copolymer has a weight-average molecular weight of 1,000 or more and 100,000 or less, and
• the dibasic acid ester (A) is at least one monomer selected from the group consisting of maleic acid diester, maleic acid monoester, fumaric acid diester, fumaric acid monoester, itaconic acid diester.

A coating film obtained by curing a water-based coating composition, wherein

• the water-based coating composition contains a surface control agent for a water-based coating in an amount of 0.1% by mass or more and 10% by mass or less when a total resin solid content is 100% by mass,
• the surface control agent for a water-based coating contains a copolymer containing, as constituent monomers,
• a dibasic acid ester (A) having a polymerizable unsaturated double bond in an amount of 10% by mass or more and 99% by mass or less, and
• a polymerizable unsaturated monomer (B) containing an ether group represented by a general formula (1):
• R1—(CmH2mO)n—R2 (1)
• (where R1 represents a (meth)acryloyl group, a vinyl ether group, or an allyl group, R2 represents a hydrogen atom, a (meth)acryloyl group, a vinyl ether group, an allyl group, or an alkyl group with 1 to 22 carbon atoms, m is a number from 2 to 4, and n is a number from 2 to 100) in an amount of 1% by mass or more and 90% by mass or less,
• the copolymer has a weight-average molecular weight of 1,000 or more and 100,000 or less, and
• the dibasic acid ester (A) is at least one monomer selected from the group consisting of maleic acid diester, maleic acid monoester, fumaric acid diester, fumaric acid monoester, itaconic acid diester.