Acrylic Acid → Urea

Thoroughly mixed were 50 parts of an anionic water-soluble polymer (a 20 wt. % aqueous solution; viscosity: 300 cps) having a monomer composition of 55 mole % acrylic acid, 40 mole % acrylonitrile and 5 mole % of 2-acrylamido-2-methylpropanesulfonic acid and obtained by radical-polymerizing the monomers in an aqueous system, 10 parts of urea, 1 part of resorcin and 250 parts of water. The pH of the resultant mixture was adjusted to 3.3 with a 20% aqueous NaOH solution. Then, as core materials, 200 parts of phenylxylylethane ("Hisol SAS-296", product of Nippon Petrochemical Co., Ltd.) containing 3 wt. % of Crystal Violet Lactone and 0.8 wt. % of Benzoyl Leuco Methylene Blue, both dissolved in the phenylxylylethane, were added. The resultant mixture was emulsified using a Homo-Mixerat 9,000 rpm. Three minutes later, there was obtained a stable o/w-type emulsion having an average droplet size of 4.0 micrometers. Twenty-five parts of formalin (a 37% aqueous solution of formaldehyde) were then added to the above system. The system was heated to 60° C. with stirring. While continuing the stirring, the system was held at this temperature for 4 hours to provide microcapsules having dense wall films, which were made of a urea-formaldehyde resin, around the core material. The system was then cooled, and a 28% aqueous ammonia solution was added little by little with stirring to raise the pH of the system to 8.5. Thus, the smell of formaldehyde was completely wiped off from the system. The thus-obtained microcapsule slurry had a solid content of 43 wt. % and a low viscosity of 85 cps. No substantial viscosity changes were observed on the slurry over a pH range of from pH 3.3 (at the time of condensation) to pH 8.5 (after removal of formaldehyde).