| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, February 2010. === Cataloged from PDF version of thesis. === Includes bibliographical references. === Ink jet printing is a demanding application that requires carefully formulated inks in order to quickly and reliably produce high-quality printed images. Although ink jet inks are currently produced via an aqueous process, supercritical fluids (SCF) and gas-expanded liquids (GXL) present alternative processing media for particle coating operations that may offer significant benefits with respect to the production of polymer-encapsulated pigment particles for these inks. The main thesis objective is the demonstration and analysis of a particle size reduction and encapsulation process which takes place in CO₂-expanded acetone and produces colloidal carbon black particles. These particles should be uniformly coated with functionalized hydrophobic resins such that they are easily redispersed in water or solvent to form stable nanoparticle dispersions suitable for use in ink jet inks. A prototype size reduction and encapsulation system has been constructed based on a high-pressure stirred reaction vessel designed to operate at pressures up to 200 bar (3000 psi). The prototype vessel has a fluid volume of 1 liter with a multidisc agitator capable of rotating at more than 3400 RPM. Pigment particles are initially milled in a solution of non-aqueous solvent and dissolved dispersing resin. Size reduction is achieved within the apparatus via the grinding action of 1.2 mm spherical ceramic media contacting the micron-size pigment particles. As milling progresses, high-pressure CO₂ is slowly introduced to the vessel; the CO₂ acts as an anti-solvent, lowering polymer solubility and driving adsorption of the dispersing resin onto the pigment particles as new surface area is exposed. === (cont.) After encapsulation is complete, the system is flushed with CO₂ and the product particles are retained as a dry powder in a high-pressure filter. The solvent-free particles are then recovered by venting the system to atmospheric pressure, and subsequently re-dispersed in water for analysis in inks. The apparatus under investigation provides a new process approach to particle size reduction and coating that affords greater freedom in ink formulation, while offering a path to improved ink quality and possible cost savings in a highly competitive market. Specifically, the use of CO₂-expanded liquids enables the deposition of hydrophobic polymers on the surface of particles for use in aqueous inks, thus significantly increasing the variety of polymers that are available for use in these systems. A representative model system of carbon black pigment and benzyl methacrylate/methacrylic acid (BzMA/MAA) copolymer dispersing resins of varying monomer compositions (BzMA/MAA mass ratio = 85/15, 80/20, and 75/25) has been studied in order to assess the feasibility of the high-pressure milling and encapsulation process for ink jet applications. These components have been successfully employed in high-pressure coating operations to produce encapsulated carbon black particles which were recovered as a dry, flowable powder. Dry product particles were redispersed in water to obtain stable aqueous dispersions with a number average particle size of 135-190 nm. === (cont.) In order to guide the selection of appropriate process conditions for the encapsulation system, the high-pressure solid-liquid-vapor phase equilibrium of ternary CO₂-solvent-polymer systems has been probed experimentally and modeled with the PC-SAFT equation of state. Precipitation of BzMA/MAA copolymers generally required a larger overall CO 2 mole fraction - and thus a higher system pressure - for more dilute polymer solutions; however, a minimum in the precipitation pressure was observed for all polymer compositions and temperatures near a CO₂-free polymer mass fraction of 0.03. The ternary systems were characterized by a rapid reduction in polymer solubility over a relatively narrow range of pressure (between 200 psig and 400 psig, depending on the polymer and system temperature); the precipitation pressure increased with increasing temperature and BzMA mass fraction (per polymer mass unit). The PC-SAFT EOS was successfully employed to correlate the phase behavior data by adjusting only two binary interaction parameters; the average relative error associated with the predictions of precipitation pressure for each polymer was 3.7%. Characterization of the encapsulation process also requires knowledge of the thermodynamics and kinetics of polymer adsorption onto particle surfaces from CO₂- expanded solvents. To this end, interactions with the particle surface have been investigated through the collection and correlation of experimental adsorption isotherm data. === (cont.) Adsorption of 85/15 and 75/25 BzMA/MAA polymers onto carbon black from CO₂-expanded acetone was measured at 35°C and pressures between 0 psig and 300 psig over a range of mixture compositions relevant to particle coating operations. Pressurization with CO₂ to pressures up to 200 psig caused a decrease in the amount of polymer adsorbed on particle surfaces, but further increases in pressure resulted in higher polymer loadings. In the case of 75/25 BzMA/MAA polymer, the polymer loading increased significantly between 200 psig and 300 psig as the solubility limit was approached or exceeded. Our results are valuable not only in providing quantitative data to facilitate process optimization, but also in offering a more fundamental understanding of interactions among the pigment particles, the dispersant resin, and the gas-expanded liquid media. Such information is important to both process and product design. === by Scott M. Paap. === Ph.D.
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