Scott M. Paap1, Michael S. Wolfe2, and Jefferson W. Tester1. (1) Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 66-053, Cambridge, MA 02139, (2) Imaging Technologies, DuPont, P.O. Box 80402, Wilmington, DE 19880-0402
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.