Colloidal stability of flexographic newsprint deinking dispersions

The clarification of washing deinking filtrate water contaminated with water-based flexographic newsprint inks is a difficult and costly process when a large amount of flexographic ink is present. In the present work, "dual polymer" systems from three different manufacturers were evaluated to assess their clarification effectiveness. It was found that these water clarification chemistries were not always successful in clarifying the residual water to low turbidities. In addition, they have a narrow range of operation that may cause significant problems in water clarification plants. To further understand the possible mechanism(s) of stabilization of flexographic inks, a model system composed of carbon black and an acrylic binder was developed. Particle size characterization of flexographic inks indicated that they are in the colloidal range ({dollar}approx{dollar}200 nm). Experiments with the model system showed that flexographic inks are electrosterically stabilized. Ionization of the carboxyl groups in the acrylic resin and the carbon black resulted in a maximum electrostatic contribution to stabilization at a pH range between 8 and 10. Adsorption isotherms were obtained to verify the adsorption of the acrylic resin onto carbon black. The extent of coverage of the acrylic resin onto carbon black showed the existence of a three-dimensional surface layer that acts as a robust steric stabilizer. An increase in the adsorption of the polyelectrolyte on flexographic ink particles was observed upon charge screening with a monovalent electrolyte. This additional polyelectrolyte adsorption may increase stability of flexographic inks. Due to the importance of zeta potential measurements in the characterization of flexographic ink particles, a theory for the conversion of mobilities to zeta potentials of particles with adsorbed polyelectrolytes was developed. Two important variables were included in the present theory for the conversion of mobilities to zeta potentials of bare particles: (1) the distribution of the double layer as modified by the polyelectrolyte, and (2) the hydrodynamic effect of the polyelectrolyte on the surrounding fluid. In the final phase of this work a strategy to destabilize flexographic inks was developed. It was found that the best strategy to destabilize these dispersions was the elimination of the steric and electrostatic component to stabilization simultaneously. In examining several different potential flocculating materials, cupric chloride was found to be the most successful. The effectiveness of this divalent salt was attributed to the complexation of copper(II) with the carboxyl groups of the adsorbed polyelectrolyte. This complexation reduces particle charge and polyelectrolyte solubility simultaneously. At the same molar concentration, cupric chloride was more effective than electrolytes of cations with higher valence. To investigate the effect of washing deinking chemicals on water clarification with cupric chloride, a complete factorial experimental design was performed. It revealed that calcium and calcium oleate soaps had a positive effect on the clarification of water. In contrast, silicate and oleate ions were found to be detrimental to water clarification.