In a reactor equipped with a stirring rod and a thermometer were placed 754 parts of water, 13 parts of a sodium salt of sulfuric acid ester of ethylene oxide adduct of methacrylic acid ELEMINOL RS-30 (trade name, available from Sanyo Chemical Industries, Ltd.,), 83 parts of styrene, 83 parts of methacrylic acid, 110 parts of butyl acrylate and 1 part of ammonium persulfate, and the mixture was stirred at 400 rpm for 15 minutes to yield a white emulsion. The emulsion was heated to an inner temperature of 75° C., followed by reaction for 5 hours. The reaction mixture was further treated with 30 parts of a 1% aqueous solution of ammonium persulfate, was aged at 75° C. for 5 hours and thereby yielded an aqueous dispersion [Polymer Fine Particle Dispersion 1] of a vinyl resin (a copolymer of styrene-methacrylic acid-butyl acrylate-sodium salt of sulfuric acid ester of ethylene oxide adduct of methacrylic acid). Fine Particle Dispersion 1 had a volume-average particle diameter of 0.10 μm as determined with a laser diffraction-scattering size distribution analyzer LA-920 (trade name, available from Horiba, Ltd.). Part of Fine Particle Dispersion 1 was dried to isolate a resin component. The resin component had a Tg of 57° C.
