Binder composition, rigid body, and method for manufacturing rigid body

A binder composition comprising

• a rust inhibitor comprising at least one selected from the group consisting of 2-Mercaptothiazoline, 2-Amino-5-mercapto-1,3,4-thiadiazole, 2,2′-Dibenzothiazole disulfide, 3-Mercaptopropionic acid, 3,3′-Dithiodipropionic acid, 1,3-Diethyl-2-thiourea, L-cysteine, L-cystine, 2-Mercaptobenzothiazole, 2-Mercaptobenzothiazole sodium, 2,5-dimercapto-1,3,4-thiadiazole, 2-thioacetic acid-5-mercapto-1,3,4-thiadiazole, and 2,5-dithioacetic acid-1,3,4-thiadiazole, and
• a carboxyl group-containing polymer, in which a percentage of structural units containing a carboxyl group to the total structural units constituting the carboxyl group-containing polymer is 50 mol % or more,
• wherein the binder composition is an aqueous solution, and a pH of the aqueous solution is 1 or more and less than 5,
• wherein the content of the rust inhibitor in the binder composition is 10 to 3000 ppm, and
• wherein the composition causes a weight reduction of less than 0.5% to a rolled steel sheet having an opening (grate:
• SPCC-SD, size:
• a thickness of 1 mm, a length of 30 mm, and a width of 30 mm) hung with a string-like Teflon® and immersed into the binder composition.