Abstract:

Fine boron powd. particles with diam. of ≤1 $μ$m were encapsulated in poly(Me methacrylate-Et acrylate) using water/oil/water (W/O/W) double emulsion. The microencapsulation process, factors affecting microcapsule (MIC) formation and dimensions, and flocculation phenomena of the encapsulated powders, were studied to optimize the product. Optical and SEM observations indicate that the double emulsion technique yielded microcapsules with diam. of 30 to 250 $μ$m, depending on the process parameters. Two encapsulation mechanisms are proposed. The first, prevailing at low vol. fraction of the primary emulsion (ϕ2 ≤ 0.01), leads to the formation of MICs derived from discrete secondary emulsion droplets. The second, which is more significant with the increase of ϕ2, at boron-to-polymer ratio (n) of 0.7, is derived from the coalescence of the secondary emulsion drops, leading to sepn. of polymer-boron aggregates. The most problematic step of microencapsulation is the drying process of MICs. At n = 0.7, hypo-osmotic drying leads to partial water withdrawal from the drop interior upon heating and solvent evapn.; the boron particles do not form a solid core. Water diln. of the double emulsion results in addnl. osmotic water withdrawal from the MIC interior. MICs from discrete liq. drops or from sepd. aggregates have similar (30-50 $μ$m) dimensions detd. by the d. matching law. Microgranulated powders prepd. at n = 0.7, may result in flocs 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 ϕ2 = 0.01, when water is completely eliminated from MICs simultaneously with solvent evapn., fast drop solidification leads to the formation of solid core. The diam. of the resulting MICs is 200-250 $μ$m. Powd. boron is a component of propellants, and encapsulation provides for coarsening of particles and surface protection, both of which are required for the application. [on SciFinder(R)]