Introduction Mini-emulsions are relatively stable and small (20– 500 nm) oil droplets dispersed in an aqueous medium [1, 2, 3, 4, 5]. Such droplets are prepared by using high force dispersion devices such as an ultrasonicator and high-pressure homogenizers. The droplet size can be adjusted by the volume fraction and solubility of dis- perse phase, and the type and amount of surfactant. The stability of small droplets in mini-emulsions arises from the use of highly water-insoluble molecule (hydrophobe) and surfactant. The hydrophobes depress molecular diffusion degradation, so called ‘‘Ostwald ripening’’, and surfactants depress coalescence by collisions. Mini-emulsion polymerization is carried out by polymerizing the monomer droplets in mini-emulsions. The polymerization of mini-emulsions provides advan- tages with respect to incorporation of various hydro- phobic compounds, because molecules do not have to diffuse into the polymerizing loci through the aqueous phase during particle growth. These days, in the field of colorants such as ink and paint, organic volatiles are undesirable because of safety, health, and environmental reasons, and use of an aqueous system is generally required. While colorants consisting of water-soluble dyes have a variety of colors and provide brilliant colors, the light fastness and the waterproofness of them are not suffi- cient. On the other hand, colorants consisting of pig- ments provide excellent light fastness but the color brilliance is not satisfactory. We aimed to prepare ‘‘colored latex’’ with high dye content by mini-emulsion polymerization for the purpose of the development of a third colorant next to dye and pigment. ‘‘Colored latex’’ is the oil-soluble dye-loaded latex with particle size of less than 500 nm. Up to now, many other groups have tried to prepare colored latex Mayuko Takasu Toshifumi Shiroya Kimiya Takeshita Munehiro Sakamoto Haruma Kawaguchi Preparation of colored latex containing oil-soluble dyes with high dye content by mini-emulsion polymerization Received: 16 January 2003 Accepted: 5 March 2003 Published online: 9 May 2003 Ó Springer-Verlag 2003 Abstract Polymer particles con- taining oil-soluble dyes (colored latex particles) were prepared by mini-emulsion polymerization. Copper phthalocyanine dyes and styryl dyes were used as oil-soluble dyes. Highly hydrophobic dyes played the role of hydrophobe by themselves and enabled the full incorporation of dyes in the latex without additional hydrophobes. Two phthalocyanine dyes having similar color were blended in a mini- emulsion polymerization so that the resulting colored latex showed enough strong color depth as a colorant. Colored latexes with high dye content (more than 30 wt% for phthalocyanine dye system and 40 wt% for styryl one) and with particle size less than 100 nm were obtained. Keywords Mini-emulsion polymeri- zation Hydrophobe Dye polymer composite Colored latex Oil-soluble dye Colloid Polym Sci (2003) 282: 119–126 DOI 10.1007/s00396-003-0902-5 ORIGINAL CONTRIBUTION M. Takasu H. Kawaguchi (&) Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3–14–1 Hiyoshi, 223–8522 Yokohama, Japan E-mail: haruma@applc.keio.ac.jp Fax: +81-45-564-5095 M. Takasu T. Shiroya K. Takeshita M. Sakamoto Mitsubishi Chemical Co., 1000 Kamoshida, 227–8502 Yokohama, Japan