Optimization of Warm-Cloud Seeding Agents by Microencapsulation Techniques

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.