MICROENCAPSULATION TECHNIQUES 95 •-Heater i•i..•• rixte•b e oil :.'i':.' I •-Fluidized coating :.://• material i•:•il •Distributorplate •-- Plenum --Air inlet Figure 6. Melt prilling in fluidized bed energy for fusing the wall material particles together. Applications of this process have yielded slow-release fertilizers, glycerine capsules, and biologically active encapsulated products. Miscellaneous Processes There are a number of additional microencapsulation processes that might be included in this paper but, in the interest of brevity, can only be mentioned here. "Spray drying" encapsulation is an old process that has received new interest in recent years (14). It involves the atomization and spray drying ot: an emulsion in which the core is the discontinuous phase and the wall material is a constituent of the continuous liquid phase. In a process known as "meltable-dispersion," the wall material in a molten state and the core material are dispersed in a medium (in which both are insoluble) at a temperature high enough to maintain the wall material in liquid form. By means of agitation and use of wetting agents, the wall material is caused to envelop the core particles, and solidi- fies on-cooling to complete capsule formation (15).

96 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In the "diffusional exchange" processes, a previously formed capsule with a porous coating is immersed in a preferred liquid so that the origi- nal core contents are diffused out of the capsule and the liquid diffused in. The resulting encapsulated liquid is then overcoated or subjected to a treatment that imparts the desired degree of wall impermeability (16). Another aqueous phase separation process employs a process fluid such as mineral oil. An emulsion of a flavor oil, for example, in an aqueous gelatin solution is dispersed in the process fluid and the mixture is cooled to phase out the gelatin. The capsules may be hardened by dehydration of the water with anhydrous alcohol (17). PROBLEMS As in many areas of science and technology, practical or applied de- velopments have outrun an understanding of the fundamental factors of microencapsulation. This situation has probably been accentuated by the proprietary aspects of many of the applications of interest. It is not surprising, therefore, that the literature in this field is primarily con- cerned with methodology and products rather than with underlying principles. It is a truism in the field of microencapsulation that ex- perience is the best teacher, and that even the most proficient practi- tioner learns something almost every time he goes into the laboratory. Perhaps one of the most difficult technical problems is to control the properties of the capsule wall. Here one tries to extrapolate the avail- able knowledge on formation and properties of polymer films and or- ganic and inorganic coatings to the encapsulation process of interest. The results of such attempts often leave much to be desired, since there is usually only a slight resemblance between the conventional film form- ing operations and those in microencapsulation. In general, some of the factors that should be considered include: formulation of wall material, solvents and nonsolvents employed, temperature, and rate of deposition. A problem that is common to many microencapsulation processes is the agglomeration of capsules during wall formation. As the wall ma- terials change from liquid to solid form they often go through a sticky stage which makes agglomeration difficult to avoid. The problem be- comes more pronounced as particle size is reduced. Various methods which may be employed to minimize this difficulty include the use of chemical hardening agents, careful selection of wall materials and of sol- vents, addition of parting agents, and application of mechanical devices to physically separate capsules.