Лако-красочные материалы — производство

Microencapsulation is a revolutionary technology enabling liquids to be treated as solids. The technology was first used to produce carbonless copying paper, but today it is widely used in a number of industrial fields such as medicine, agricultural chemicals, thermochromic materials, cos­metics, and toiletries.

The original microcapsule was produced by a complex coacervation between gelatin and gum arabic. Gelatin changes its charge depending on pH value; it has negative and positive charges at pH values above and below its isoelectric point, respectively. On the other hand, gum arabic has a negative charge regardless of pH value. The colloidal mixture of gelatin and gum arabic, therefore, brings about a neutralizing reaction at pH values below the isoelectric point of gelatin, resulting in polymerization.

The typical microencapsulation process via complex coacervation is illustrated in Figure 6.7.

aqueous gum arabic

Figure 6.7. Microencapsulation by complex coacervation.

A leuco dye solution (3-5%, 100 g) is emulsified in an aqueous solution of gelatin (5%, 100 g) at 50 °C until the droplet is 5 to 10 pm in diameter. The droplet size is governed by the rate of agitation. Once the appropriate droplet size is achieved, an aqueous solution of gum arabic (5%, 100g) is added. The mixture is then diluted with water (50°C, 200ml), and the pH is adjusted to 4-5 with acetic acid to permit coacervation resulting in deposition of a thin film of liquid polymer on the core of the droplet of leuco dye solution. The liquid polymer is gelated by cooling below 10 °C, and then hardened with formaldehyde. The mixture is finally adjusted to pH 9 to give microcapsules containing leuco dye solution.

Recently, many synthetic polymers such as urea/formalin resin, melamine/formalin resin, polyester, and polyurethane have been widely used as the wall material for the microcapsule, though the gelatin microcapsule is still used. Microcapsules using a synthetic polymer wall have several advantages over those using a gelatin wall: (1) the preparation process is simple, (2) the size of the microcapsules is well balanced, (3) the microcap­sule concentration can be increased twofold or more and (4) the microcap­sules have a high resistance to water and many chemicals. Synthetic microcapsules are prepared by interfacial polymerization or in situ polymer­ization.