Practical and mechanistic considerations in the use of W/O/W double emulsions for microencapsulation of fine boron particles

Encapsulations of fine powders within polymer matrices have many different technical applications in the areas of powder practice. Fine boron powdered particles with diameter of less than or equal to 1 mum were encapsulated in poly(methylmethacrylate-ethylacrylate) copolymer using water/oil/water (W/O/W) double emulsions as preparative media. Concepts related to the microencapsulation process, and factors influencing microcapsule (MIC) formation and their dimension along with flocculation phenomena of the encapsulated powders, have been explored in order to obtain good microencapsulation. Optical and scanning electron microscopy (SEM) observations have shown that using the double emulsion technique yielded microcapsules with diameters of 30 to 250 mum depending on the process parameters. Two encapsulation mechanisms were disclosed and illustrated. The first, prevailing at low volume fraction of the primary emulsion (phi(2) less than or equal to 0.01), leads to the formation of MICs derived from discrete secondary emulsion droplets. The second, which is more significant with the increase of phi(2), at boron-to-polymer ratio (n) of 0.7, is derived from the coalescence of the secondary emulsion drops, leading to separation of polymer-boron aggregates. The most problematic step of microencapsulation is the drying process of the MICs. At n = 0.7, hypoosmotic drying leads to partial water withdrawal from the drops' interior during emulsion heating and solvent evaporation; the boron particles do not form a solid core. Water dilution of the double emulsion results in additional osmotic water withdrawal from the MIC's interior. MICs from discrete liquid drops or from separated aggregates have similar (30-50 mum) dimensions determined by the density matching law. Microgranulated powders prepared at n = 0.7, may result in floes due to the agglomeration of polymer-boron aggregates during the drying process. The dimensions of the flocs depend on the drying regime that can be controlled to form particles of desired dimensions. At n = 1 and phi(2) = 0.01, when water is completely eliminated from MICs simultaneously with solvent evaporation, fast drop solidification leads to the formation of solid core. The diameter of the resulting MICs is 200-250 mum.