MICROENCAPSULATION TECHNIQUES 8O products may cause premature rupture of capsules in the bottom layers ot: the stack. Release and Reaction Capsular core release may be instigated by such methods as: pressure and shear to rupture the wall, heat to melt wall material, dissolution ot: the wall by a so]vent, and extraction of the core contents by leaching through the wall. The mechanism selected is governed by the end use ap- plication ot: the product. Release of core contents is usually followed by some chemical reac- tion with a material adjacent to the capsule, or a physical effect such as evaporation or wetting. The elficiency of release and mixing with a co-reactant is important to minimize the quantity of capsules required, hence cost. If speed of reaction is important, release should involve intimate contact with a co-reactant and avoid losses of core material to substrate or surroundings. Product PerJ•ormance Perhaps the most important criterion for judging the over-all per- formance of a capsular product relates to economics. As usual, one must come to grips with the question: "Is the consumer willing to pay for the extra cost of microencapsulation?" Here one must take into con- sideration not only the product novelty aspects of the problem, but also such marketing factors as consumer education, distribution patterns, and price consciousness. Unfortunately, there are no pat answers to this problem especially since there may not be a similar established product to serve as a point of reference. It is evident that such a situation calls for careful product development and market studies. MICROENCAPSULATION PROCESSES There are a large number of microencapsulation processes and modi- fications of processes that have been disclosed in the literature and patents. Processes of potential interest in the cosmetics field were selected for discussion in this paper. Aqueous Phase Separation In aqueous phase separation processes a polymeric or macromolecular wall material is dissolved, or dispersed as a sol, in water. The core ma- terial to be encapsulated, which must be immiscible with water, is dis- persed in particulate form throughout the aqueous phase by stirring.

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Wall formation occurs when the dissolved wall material is caused to "phase out" and "wrap" around the core particles by a suitable system change. Possible system changes include: reduction in temperature, addition of chemical precipitating agent, or pH alteration. A highly successful adaptation of aqueous phase separation is called aqueous phase coacervation, which has been applied to the encapsulation of such materials as carbonless carbon paper reactants, flavor oils, and perfumes (3). In this process the macromolecular wall material (i.e., gelatin) is phased out of aqueous solution as a concentrated liquid phase (coacervate) which forms a uniform liquid coating around the core particle. On further processing a hardened solid (gelatin) wall is formed around the core material. The method is outlined in Fig. 2. Make an emulsion of oll and a gellable colloid aqueous sol 1 [ Coacervate by a!dition of 1 an aqueous salt solution (Steps above are performed at a temperature above melting point of the colloid sol ) Gel the colloid by pouring coacervate mixture in cool salt solution Wash w•th water and filter to remove the salt Harden fdter cake with soluhon of formaldehyde in water Wash with water and filter to remove residual formaldehyde Adjust water concentration to desired amount Figure 2. Aqueous phase separation Aqueous phase separation processes have been applied to both liquid and solid cores. Capsule sizes may vary from a few microns to several millimeters. The capsule wall may be treated to present a good barrier to oily or hydrophobic materials, but it is usually a poor barrier to water or hydrophilic materials. Nonaqueous Phase Separation Nonaqueous phase separation is the inverse of the aqueous phase process in that the continuous wall-containing phase is organic or hy- drophobic in nature, and the core material is usually water-miscible (hydrophilic). In the process shown in Fig. 3, the core liquid (called polar solvent solution) is suspended in droplet form by stirring in poly- mer solution (wall material). Phasing out and core wrapping of the