MICROENCAPSULATION TECHNIQUES 01 I Solvent for polymer ] -, Dispersing or J separation J meJ•• • }• Disperse polar solution 11 [ Polar solvent solution as caHmdal droplets m •-- r polymer soluhan Precipitate polymer around •1 [ Liquid m•smble with d•spersed polar solution to polymer soluhon but form suspension of m•crocopsules non-solvent for polymer _ 1• Gradually harden microcapsules by successive washings in liquid [ Non-solvent for polymer ] mixtures decreasing in amounts of solvent for polymer J Separate m,crocapsules • J lndlfferent coating Disperse microcapsules medium in coating medium ? Dry under J J reduced J PRODUCT I Deposit on substrate I PRODUCT Figure 3. Nonaqueous phase separation wall material is caused by the addition of a liquid (nonsolvent) that is miscible with the organic solvent but immiscible with both the core particle and the polymeric wall material. The process may be modified to include a coacervation step and suitable measures for hardening the capsule walls (8). Both liquid and solid core particles may be encapsulated by non- aqueous phase techniques. Capsule sizes may be varied over a wide range from a few microns to millimeters, and a variety of wall materials may be employed. This process has been applied to such diverse core materials as reactive chemicals, glycerine, urea, and pigments. Interfacial Polymerization The formation of a polymer at the interface between two liquid phases is known as interfacial polymerization. This film-forming tech- nique has been used to make capsules with either hydrophilic or hydro- phobic cores. Capsules with polyamide (nylon) walls can be made by a process in which an aqueous core liquid containing an aliphatic diamine (i.e., hexamethylenediamine) is dispersed in droplet form in an organic solvent (i.e., chloroform-cyclohexane, 1:4). When a solution of a dicar- boxylic acid halide (i.e., sebacoyl chloride in the above mixed solvent) is

•2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS added with stirring, the nylon walls of the capsules are formed. The re- action can take place at room temperature and depends on the fact that acid halides of this kind are nearly insoluble in water, while the proper diamines have an appreciable partition coefficient towards the organic phase. The polymer is formed in the organic phase, and since the rate o[ polymer formation exceeds the rate of diffusion of the diamine out of the aqueous phase, the nylon wall is deposited almost entirely at the interface (9). Product applications have included core materials such as dye pre- cursors, oils, proteins, and enzymes. Multiorifice Rotating Cylinder A multiorifice encapsulation device utilizes centrifugal force to form capsules from fluid wall formulations. The wall formulation is hardened by appropriate means after the capsules are formed. Core material •, Wall material Figure 4. Multiorifice rotating cylinder device Orifices around periphery •o hardening bath The apparatus consists of a rotating cylinder having orifices located about its periphery. As shown in Fig. 4, the wall material is fed to two internal grooves from which it flows by centrifugal force to the indi- vidual encapsulating orifices where membranes are formed. The core material is fed onto a concentrically rotating disc and is projected onto the membranes. When the combined mass of core and wall material is such that centrifugal force overcomes the cohesive force of the wall material, individual capsules break loose and are projected outward. The membranes re-form, and the next capsules are formed. Capsule size is inversely proportional to rotational speed. Good size uniformity is claimed, since the capsules do not break loose from the orifices until a given mass is reached (10).