Uniform titanium dioxide (TiO 2) microcapsules prepared by glass membrane emulsification with subsequent solvent evaporation

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Abstract
Anatase-type titanium dioxide (TiO 2) was encapsulated using an Shirasu porous glass (SPG) membrane emulsification technique and followed by solvent evaporation. The oil phase, consisting of fine powder of anatase TiO 2, Disperbyk-180, the hydrophobic oil phase additive, and polymer wall solution, was pushed through the membrane pores into the aqueous phase of poly(vinyl alcohol) and sodium dodecyl sulfate to form the solid-in-oil-in water, (S/O)/W, emulsion droplets. Three types of styrene-based copolymer poly(styrene- co -acrylic acid) (PS-AA), poly(styrene- co -2-ethyl hexyl acrylate) (PS-2EHA) and poly(styrene- co -dimethyl aminoethylmethacrylate) (PS-DMAEMA) were used as an encapsulating shell. Uniform droplets were successfully obtained by modifying the oil phase using methyl laurate or hexadecanol as the oil phase additive, together with carefully monitoring the emulsification flow rate during the emulsification. The (S/O)/W emulsion was gently stirred in a sealed reactor, and evacuation of solvent started under moderate heating with increasing a vacuum intensity. Those uniform-sized TiO 2 microcapsules revealed fine porous morphologies on their surfaces as a result of a mild phase separation induced from the addition of the oil phase additive. The encapsulation efficiency was influenced by the stability of TiO 2 in the oil phase, the polymer wall employed, and the operational control of the glass membrane emulsification process. The membrane emulsification process could prepare the TiO 2 microcapsules with about ~6-8.5 wt% of encapsulation loadings. PS-AA and PS-2EHA copolymers provided better encapsulation efficiency compared to PS-DMAEMA. SPG membranes with 1.42, 2.8, 5.25, 7.0, or 9.5 #181;m were employed and 2-20 #181;m microcapsules were subsequently obtained.

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