Particle milling and encapsulation in supercritical CO₂ mixtures

Over the past several decades ink jet printing has grown to become a multibillion-dollar industry, dominating the home computing market. While sales of printer hardware have recently stagnated, sales of printing supplies - most notably inks - continue to grow at a rate of roughly $1 billion per year.

Ink jet inks are essentially aqueous suspensions of pigment nanoparticles, with particle diameters in the range of 70-100 nm. The suspended pigment particles are coated with a layer of hydrophobic dispersant resin in order to prevent agglomeration and to promote adhesion to the paper after printing. Ink dispersions are typically produced by mechanical comminution of millimeter- or micron-scale pigment particles to the desired size in a wet grinding media mill, with simultaneous coating of the particles with dispersant resin molecules. Currently, this process is carried out in aqueous media; the goal of our project is to produce resin-encapsulated pigment nanoparticles in supercritical carbon dioxide (scCO₂) in order to exploit potential processing advantages and improve product quality.

Our project is a collaboration with the DuPont Ink Jet group, and the proposed process to produce ink jet inks is based upon a patented high-pressure media mill (HPPM) developed at DuPont. Our research objective is confirm the feasibility of employing supercritical fluid processing techniques to produce ink jet inks, and to establish quantitative understanding of the factors that influence and control particle milling and encapsulation in supercritical fluid media. In parallel with lab-scale feasibility studies, we will develop methods to characterize the micro- and nanoparticles produced by size reduction and coating operations in supercritical fluid media, as well as the resulting aqueous dispersions. To complement experimental studies, we plan to construct a model to predict size reduction rates and particle encapsulation a priori based on fundamental transport, comminution, and other kinetic processes. Such a model will not only be valuable in optimizing important product parameters but will also lead to a more fundamental understanding of interactions among the desired product particles, the abrasive grinding media, the dispersant resin, and the supercritical fluid media.

• DuPont Ink Jet