MICROENCAPSULATION TECHNIQUES 87 Wall The selection of the proper wall material or materials for a given product application is often determined by the requirements of the system under consideration. For example, if the capsular product must resist the leaching action of an aqueous medium, the wall material selected should be hydrophobic in nature and provide a good water barrier. Since the wall deposited tends to follow the contour of the core (as indicated in Fig. 1), the final form of the capsular product is alTected by the core configuration. Depending upon the encapsulation process employed, it may be more or less difficult to coat uniformly core particles whose surfaces possess sharp peaks and depressions much like the problems encountered in conventional painting or plating. Microencapsulation Process Selection The selection of the preferred microencapsulation process for a given application is not always a simple task for there are a large number of processes from which to choose. One must be concerned with such [actors as: whether the core is solid or liquid the solubility charac- teristics of the core the reactivity of the core with candidate wall ma- terials and solvents the size of the desired capsule the method of attach- ing the capsule to the desired substrate the method of core release and process and product economics. A detailed discussion of the above factors is beyond the scope of this paper. Some of the more important aspects are discussed in a later section in which microencapsulation processes are described. Capsule Post-Treatment In many instances the capsules made by a given process require ad- ditional treatment to impact the desired properties. Post-treatment may involve chemical and/or physical methods. Hardening of gelatin capsule walls by treatment with formaldehyde is employed in capsules used for carbonless carbon paper. Polymer wall films deposited from organic solvent solutions may be hardened by treatment with a selected nonsolvent for the wall material. Heat-fusible wall materials deposited on core particles may be improved in barrier properties by a suitable heat treatment. Thin coatings of liquids and finely divided solids may be applied to the capsule wall surface to reduce the tendency of capsules to adhere to each other and to improve barrier properties in selected environments.

88 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Caps•t le-Substrate Interaction Although there are instances where capsules, as produced, are used directly in a product, in numerous cases the capsules must be fastened to a substrate or suspended in a •nedium for proper functioning in the end use. For example, encapsulated perfume is attached to paper tissues so that pressure release may be instigated when the tissue is crumpled in use. Food flavor capsules may be suspended in a matrix of jelly in a tart and must remain stable until release is effected by heating in the toaster or oven. Here two proble•ns are encountered: the need for a method of applying capsules to substrate and the need to avoid adverse effects of substrate on capsule stability in storage. The method selected for apply- ing the capsules to the substrate must avoid the use of solvents that tend to leach core •naterial through the capsule wall during the application period. In addition, the presence of the binder employed to fasten cap- sule to substrate should not significantly interfere with subsequent cap- sular release. Products which require suspension of capsules in a •natrix must be designed so that the capsule contents and wall remain intact during the time the product is stored prior to consumption. If the cap- sule is surrounded by an aqueous medium (for example, in a food product) the wall material (which •nay be a fatty material) must limit the rate of leaching of core so that sufficient active core material remains at the time release is desired. Storage Conditions under which capsules and products containing capsules are stored vary widely. Factors of concern include temperature, hu- midity, pressure, light, or other forms of radiation and air pollutants. Capsules containing volatile materials or certain reactive chemicals must be protected from excess temperature to avoid premature evaporation or decomposition of the core contents. If the core is hygroscopic, cap- stiles may absorb water from a high humidity atmosphere to the point of wall rupture, in some cases. Conversely, capsules containing some water in the core may lose water by evaporation in a low humidity environment, thereby reducing the reactivity of the capsular system. Excessive presstire on capsules in storage, such as certain slow release fertilizers, may cause blocking or welding together of the capsules so that the product is no longer free-flowing. Stacking of capsule-containing