Admicellar polymerization of methyl methacrylate on aluminum pigments

Micrometer-sized aluminum particles used as pigments in silver inks and coatings are reactive in water-based formulations. This leads to hydrogen gas evolution in the paint containers and loss of the silvery appearance of the coating. The protection of aluminum pigments from water is called inhibition and it was shown in earlier work that anionic surfactants of the phosphate ester type are effective as inhibitors, forming a protective bilayer on the aluminum pigment surface. In this work, the protection of aluminum pigments has been extended by means of an encapsulating polymer layer. A poly(methyl methacrylate) (PMMA) coating was applied on aluminum pigment particles by admicellar polymerization of methyl methacrylate. A surfactant is first adsorbed on the aluminum pigment surface and a hydrophobic monomer and initiator is subsequently solubilized inside the hydrophobic domain of the surfactant aggregates that cover the pigment. Diffuse Reflective Infrared Fourier Transform (DRIFT) spectrometry showed that PMMA was formed on the pigment particles and the amount of organic material was up to 24% of the particle weight, as measured by Thermo Gravimetric Analysis (TGA). A hydrophobic initiator, such as benzoyl peroxide, gave good results but the hydrophilic sodium persulfate resulted in poor yield of encapsulating polymer. Sodium dodecyl sulfate, which by itself is not an efficient inhibitor, was used as surfactant. Good results were obtained in terms of protection from an alkaline solution, indicating that the polymer coating per se is an efficient inhibitor. The highest amount of polymer formed on the pigment surface was obtained when the surfactant concentration was around the CMC.