To a reaction vessel equipped with a stirrer and a thermometer, 683 parts of water, 11 parts of sodium salt of the sulfuric acid ester of methacrylic acid ethylene oxide adduct (ELEMINOL RS-30, available from Sanyo Chemical Industries, Ltd.), 139 parts of styrene, 138 parts of methacrylic acid, 110 parts of butyl acrylate, 12 parts of butyl thioglycolate and 1 part of ammonium persuiphate were poured, and the components were stirred at 400 rpm for 15 minutes to obtain a white emulsion. The white emulsion was heated, the system temperature was raised to 75° C. and the reaction was performed for 5 hours. Next, 30 parts of an aqueous solution of 1% ammonium persulphate was added thereto, and the reaction mixture was aged at 75° C. for 5 hours to obtain an aqueous dispersion of a Vinyl resin (copolymer of styrene-methacrylic acid-butyl acrylate-sodium salt of the sulfuric acid ester of methacrylic acid ethylene oxide adduct). This aqueous dispersion was taken as particulate dispersion 1. The volume average particle diameter of the particulate dispersion 1 measured by a laser diffraction particle size distribution analyzer (LA-9201 available from Shimadzu Corporation) was 120 nm. A part of the particulate dispersion 1 was dried, and the resin therein was isolated. The glass transition temperature (Tg) of the resin was 42° C. and the mass average molecular mass of the resin was 30,000.
