MICROENCAPSULATION TECHNIQUES 93 The walls ot• the t•ormed capsules may be hardened by various means including chemical reaction, evaporation of a solvent, and cooling. The process would appear to be suitable for making a variety of products in- volving primarily liquid cores. It may have limitations when capsules below about 100 t• are required. It is expected that scale-up of the process might have some mechanical limitations. However, high pro- duction rates are claimed for some products. Applications include cap- sules containing water, solvents, wax solutions, and pesticides. Fluidized-Bed Spray-Coating Microencapsulation ot: core particles that can be fluidized by a gas may be accomplished by spraying a coating agent (wall) onto the sur- face ot: the particles. The wall may be formed by congealing ot: a mol- ten material, by chemical reaction on the surface, or by evaporation ot• a solvent from a coating solution. The solvent is removed with the gas leaving the bed. Coating thickness may be easily controlled by the amount ot• wall material applied. A conventional fluidized-bed spray- coating unit is shown on the left in Fig. 5 (11). COATING SOLUTION DENSE-PHASE FLUID BED VERTED CONE J' FLUIDIZING AIR CONVENTIONAI- MODI FI ED Figure 5. Fluidized-bed spray-coating units

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Conventional fluidized-bed spray-coating methods are generally em- ployed in the encapsulation ot5 solid particles, and a minimum capsule size ot5 about 100 g is the usual rule. Liquids may be encapsulated if they can be frozen in particulate form and coated at temperatures below their freezing point (12). Fluidized-bed encapsulation has yielded such products as: slow release fertilizer, coated iron particles, seeds, salts, and clays. Deagglomerating-Jet Spray-Coating Numerous modifications of the conventional fluidized-bed micro- encapsulation concept have been developed to satisfy the needs of par- ticular problems. A deagglomerating jet unit was created to coat core particles ot5 small size (less than about 100 g) which tend to agglomerate in a conventional fluidized bed. Here the key feature is the use ot5 a high velocity gas jet and a conical conduit in a fluidized bed to deagglom- erate the partially coated particles before additional coating material is applied from the coating spray nozzle. This method is shown on the right in Fig. 5 and may be employed to encapsulate solid particles down in the 10-g size range. It does not lend itself to liquid cores nor to solid core particles larger than about 300 g (5). Products include pharma- ceuticals, resin catalysts, inorganic salts, and pigmented plastics. Melt Prilling in a Fluidized Bed This process is illustrated in Fig. 6. Here the wall material must be in solid particle form so that it can be fluidized by a gas. The core material is heated and is in liquid form for atomization from a nozzle to yield droplets ot5 the desired size. The droplets of core material fall into the fluidized bed and are simultaneously cooled and coated with the wall material particles. The heat liberated from the core droplets is transferred to the wall material particles causing them to melt, adhere to the core surface, and flow together to form a coherent capsule wail structure. A mixture ot5 capsules and bed material is removed from the fluidizing column and the capsules are separated by screening. The ex- cess bed material is returned to the system. The process has been made continuous by providing for continuous capsule removal and bed ma- terial make-up (13). Capsule sizes possible by this process range from about 300 to 3000 •. Both liquid and solid core capsules can be made however, the core ma- terial must be able to withstand the temperature required to provide the