Optimization of Warm-Cloud Seeding Agents by Microencapsulation Techniques¹

Abstract

Microencapsulation technology, whereby single crystals or solution droplets are chemically "packaged" inside thin coating shells, provides a method of optimizing the chemical and physical properties of warm-cloud seeding agents. Ethylcellulose-encapsulated urea and sodium chloride particles have been produced in the lsboratory and their hygroscopic properties investigated gravimetrically, microscopically in a diffusion chamber, and optically in a large fog chamber. These investigations indicate that microencapsulation provides for a relatively narrow particle spectrum with a sharp lower size limit, gives the particles more structural integrity, prevents degeneration of the particle spectrum and formation of "fines" during handling and dispersal, and greatly reduces clumping and caking during storage, handling, and dissemination. Hygroscopic properties of the seeding particles are not affected by the presence of the encapsulating coating. As an encapsulated particle grows by vapor diffusion in a humid environment, its core material is released by diffusion through its permeable but insoluble ethylcellulose shell. Individual seeding particles are thus unmistakably "tagged," providing an objective method for evaluating the seeding effects. The process lends itself to large-scale production and promises to be more ecosomical than mechanical milling and sizing to an equivalent size spectrum. 1 This research was supported by the Laboratory Directors Fund of the Air Force Cambridge Research Laboratorles. on the microphysical and chemical properties and, thus, on the warm-cloud seeding potential of hygroscopic materials, are discussed in this paper. The optimization of urea and sodium chloride, in particular, as warm cloud seeding agents is described. 2. MICROENCAPSULATION TECHNOLOGY Microencapsulation may be defined as a special technique of chemical packaging whereby very small crystals, particulates, or solution droplets are encased in thin coating shells. Possible encapsulation methods include organic phase separation, aqueous coacervation, meltable dispersion, and interfacial polymerization. Potential wall materials include nearly all waxes, and polymers possessing sharp melting points. By proper choice of capsular wall material, wall thickness, and morphology, microencapsulation can be used to alter significantly the chemical and physical properties of the encapsulated substance. Microencapsulation techniques are widely used in foodstuffs, pharmacology, and chemical engineering. A well-known example is the "tiny time pills" advertised by several commerical cold-remedies that employ microencapsulation to effect a controlled, sustained release of the pharmaceutical contents. A detailed review of microencapsulation techniques, effects, and usages is given by Herbig (1967). All microencapsulations discussed in this paper use ethylcellulose 'as the wall material, deposited on urea and sodium chloride particles by the process of organic phase separation.2 They are intended to serve as examples only. Other wall materials,...