Patent ID: 8217127

Claim:
A (meth)acrylic acid/alkyl (meth)acrylate ester copolymer comprising: 95.42 to 97.48% by mass of (meth)acrylic acid; 2.43 to 4.30% by mass of an alkyl (meth)acrylate ester, which is at least one selected from the group consisting of eicosanyl (meth)acrylate, behenyl (meth)acrylate, and tetracosanyl (meth)acrylate, or a mixture of two to four members selected from the group consisting of stearyl (meth)acrylate, eicosanyl (meth)acrylate, behenyl (meth)acrylate, and tetracosanyl (meth)acrylate; and 0.08 to 0.30% by mass of a compound having two or more ethylenically unsaturated groups, wherein the (meth)acrylic acid/alkyl (meth)acrylate ester copolymer has the following properties: 1) a viscosity of a neutral viscous aqueous solution comprising 1% by mass of the (meth)acrylic acid/alkyl (meth)acrylate ester copolymer and 1% by mass of sodium chloride of 40000 mPa·s or more at 25° C., and a ratio of a viscosity of the neutral viscous aqueous solution at 25° C. to a viscosity of the neutral viscous aqueous solution at 50° C. of from 1:0.8 to 1:1.2; and 2) a ratio of a viscosity of a neutral viscous aqueous solution comprising 1% by mass of the (meth)acrylic acid/alkyl (meth)acrylate ester copolymer and 1% by mass of sodium chloride at 25° C. to a viscosity of a neutral viscous aqueous solution containing 1% by mass of the (meth)acrylic acid/alkyl (meth)acrylate ester copolymer and 0.5% by mass of sodium chloride at 25° C. of from 1:0.75 to 1:1.25.