(429b) Nanoparticle Milling and Encapsulation in Supercritical Carbon Dioxide Mixtures

In the conventional process used to produce pigment dispersions, agglomerates of primary pigment particles in an aqueous slurry are broken down to aggregates with a diameter of approximately 100 nm in a media mill. As the milling process proceeds, polymeric dispersant molecules dissolved in the aqueous phase adsorb onto newly exposed particle surfaces, forming a continuous coating which prevents reagglomeration of the particles and enhances the stability of the resulting dispersion. In this work we present a new process to produce polymer-encapsulated carbon black nanoparticles, in which milling and encapsulation with benzyl methacrylate/methacrylic acid random copolymers are carried out simultaneously in high-pressure CO₂-expanded liquids. In the final process step, the solvent is flushed from the system with CO₂ and the encapsulated particles are recovered as a dry powder which can be redispersed in aqueous or solvent-based carriers to yield pigment dispersions. The use of CO₂-expanded liquids enables precise control of the polymer solubility and adsorption behavior, and also allows the use of a wide variety of water-insoluble polymers that are inaccessible with the conventional process. Experimental investigations of the high-pressure phase behavior of polymer-CO₂-solvent systems as well as the adsorption of polymers onto pigment particles from CO₂-expanded solvents provided a basis for analysis and optimization of the new process. Encapsulated particles obtained using the optimized CO₂-based process had a number-average diameter of 65 nm, and were easily redispersed in water to form pigment dispersions. Characterization of the encapsulated particles was performed using TEM and dynamic light scattering techniques.