The encapsulation of fine aluminum hydroxide powder was carried out by emulsion polymerization of methyl methacrylate(MMA). Aqueous soluble radical initiators such as 2,2-azobis(2-amidinopropane)dihydrochloride(AIBA) and potassium persulfate (KPS) were used.
To explore the effect of electrostatic interactions between the inorganic powder surface and the ionic end group of the surfactant, anionic sodium dodecyl sulfate (SDS) and cationic dodecyl trimethyl ammonium bromide (DTAB) were added prior to the polymerization, and their adsorption behaviors were investigated under various conditions. The lower limit is the concentration required to permit initial formation of bilayer on the inorganic powder surface is $1.46\times10^{-4}g$. The addition of 2mmol SDS/$1H_2O$ or 9mmol DTAB/$1H_2O$ was found to be necessary to cover aluminum hydroxide powder particles uniformly with polymer film. The higher limit will be considered the CMC; Above this CMC, the surfactant molecules form micelles which do not take part in adsorption on the powder surface. These two surfactants have the same carbon chain length, but opposite electric charges in ionic end group.
Aluminum hydroxide, with hydroxyl groups on the surface, was controlled by the pH of the aqueous solution; the isoelectric point of $Al(OH)_3$ is 9.02. When the powder surface and the ionic end groups of the surfactants were oppositely charged, adsorption was high; the surfactants hardly adsorbed at all when the same electric charges were combined. This aspect shows a different yield of capsulating polymer in accordance with the amount of adsorption of the surfactants on the powder. For this reason, a small amount adsorption of surfactant was added to the reaction system prior to polymerization in order to form preferential polymerization loci on the powder surface. The encapsulation of aluminum hydroxide powder was carried out with the combination of electric charges between the ionic end groups of surfactant and initiator. The encapsulation state of the polymer on the surface of inorganic powder was closely related to the amount of surfactant adsorbed on the powder, and an amount of adsorption above a certain value was necessary for uniform encapsulation. The adsorption of ionic surfactant on inorganic powders increases with an increase in the surfactant concentration at a concentration lower than the CMC. As the adsorption amount increases, the charge of the powder surface is changed by the end group of the surfactant adsorbed. With the DTAB pretreatment system, anionic radicals($SO_4$) induced from the $K_2S_2O_8$ initiator will enter easily into the adsorbed surfactant layer due to electrostatic interactions. The combination of electric charges between the ionic end groups of the surfactant and the initiator influence the molecular weight of the capsulating polymer. The combination of SDS-AIBA system resulted a high molecular weight than that case of DTAB-KPS system. The result was examined by scanning electron microscopy (SEM) of $Al(OH)_3$-PMMA powder obtained by emulsion polymerization process and indicated excellant uniform encapsulation state which applied a certain surfactant concentration. In the combination of SDS-AIBA and DTAB-KPS system, the surfactant concentrations were 4.0mmol/l and 16.0mmol/l respectively.
In the present paper, the uniform encapsulation of a aluminum hydroxide powder by emulsion polymerization was attained by the adsorption of sufficient surfactant molecules for the formation of preferential polymerization loci prior to the polymerization.
