Miniemulsion copolymerization of styrene/ n-butyl acrylate was investigated as a means of encapsulating hydrophilic titanium dioxide (TiO 2 ) in a film-forming polymer. Dispersion studies of the TiO 2 were first carried out to determine the choice of stabilizer, its concentration, and the dispersion process conditions for obtaining stable TiO 2 particles with minimum particle size. Through screening studies of various functional stabilizers and shelf-life stability studies at both room and polymerization temperatures, Solsperse 32,000 was selected to give relatively small and stable TiO 2 particles at 1 wt % stabilizer and with 20 -25 min sonification. The subsequent encapsulation of the dispersed TiO 2 particles in styrene/n-butyl acrylate copolymer (St/BA) via miniemulsion polymerization was carried out and compared with a control study using styrene monomer alone. The lattices resulting from the miniemulsion encapsulation polymerizations were characterized in terms of the encapsulation efficiencies (via density gradient column separations; DGC) and particle size (via dynamic light scattering). Encapsulation efficiencies revealed that complete encapsulation of all of the TiO 2 by all of the polymer was not achieved. The maximum encapsulation efficiencies were 79.1% TiO 2 inside 61.7% polystyrene and 63.6% TiO 2 inside 38.5% St/BA copolymer. As the density of the particles collected from the DGC increased from one layer to another, both the average particle size and the number of the TiO 2 particles contained in each latex particle increased.

ABSTRACT:The optical properties of films cast from latexes comprising TiO 2 particles encapsulated in poly(styrene-co-n-butyl acrylate) prepared by miniemulsification and having high loading (11-70% pigment volume concentration (PVC)) were investigated. Contrast ratio (CR) measurements for all batches showed that a 3 mil film (obtained at 43% PVC) was the minimum thickness at which 98% CR could be achieved. Also, the greatest hiding power (HP) (535 ft 2 /gal) was obtained at 43% PVC. In addition, gloss measurements showed that as the TiO 2 loading increased, the gloss decreased, indicating that the increasing amount of TiO 2 contributed to a reduction in the surface smoothness. The effects of surfactant concentration (sodium lauryl sulfate) on the encapsulated particle size and the optical properties of the encapsulated films for the 11% PVC system are described. The effect of varying the sonifier operating conditions on the TiO 2 particle size, encapsulated particle size, and the optical properties of the encapsulated films for the 43% PVC system was also investigated. A comparative study of the optical properties of films cast from encapsulated latex particles with films cast from physical blends of polymer particles and pigment at the same TiO 2 loading and film thickness was made. The optical properties (i.e., CR and gloss) showed a clear advantage of the miniemulsification process over the blends.

Miniemulsification technology was used to encapsulate TiO 2 particles inside a styrene/n-butyl acrylate copolymer with high loading levels (11 to 70% PVC (pigment volume concentration)). In this approach, a St/BA copolymer dissolved in toluene in the presence of a costabilizer (hexadecane) was mixed with a dispersion of TiO 2 particles in toluene and sonified, and then emulsified in an aqueous surfactant solution by sonification. The effect of sonification time on both the dispersibility of the TiO 2 particles in the presence of the copolymer and hexadecane and on the encapsulated particle size was investigated. Particle size analysis by dynamic light scattering showed that these composite latexes are quite stable. It was also found that as the TiO 2 loading increased from 11 to 43% PVC, the particle size of the TiO 2 dispersion decreased while the polymerencapsulated TiO 2 particle size increased. The effect of surfactant concentration (sodium lauryl sulfate, SLS) on the encapsulated particle size was investigated using four different SLS concentrations in the 11% PVC system. The results showed that as the SLS concentration increased the particle size decreased, as expected. Also it was found that the minimum surfactant concentration that gives stable encapsulated TiO 2 particles is above 10 mM SLS. The role of HD in the recipe was studied for an artificial latex containing no TiO 2 and one prepared at 11% PVC, in terms of particle size before and after solvent stripping, and its effect on the T g .

ABSTRACT:The application of a density gradient column (DGC) method using sodium polytungstate (SPT) solutions as the medium was investigated for determining the encapsulation efficiency of 11-30% pigment volume concentration (PVC)) latex particles prepared by the miniemulsification process. The encapsulation efficiencies for 11, 20, and 30% PVCs were found to be 100% of the TiO 2 encapsulated inside 86.3, 98, and 98.9% of the styrene/n-butyl acrylate copolymer, respectively. The copolymer not participating in the encapsulation (free copolymer) was found in the 1.04 g/mL density layer of the DGC. Particle size analysis by DLS (dynamic light scattering) showed that the encapsulated particle size increased with increasing density. Thus, the number of TiO 2 particles (primary particles) inside each encapsulated particle increased to accommodate both the increased size and density. The results obtained by DLS for each DGC layer of the 30% PVC system were confirmed qualitatively by TEM in terms of the increasing encapsulated particle size and broadening of the size distribution as the density increased in the DGC.