Flexographic ink behaviour during newspaper repulping
Creators: name: Nesbit, Susan Elizabeth
Publisher: name: University of British Columbia
Year: 2009

description: Newspapers printed with water-based flexographic ink create great difficulties in deinking plants by causing unacceptably low levels of product pulp brightness. Therefore, notwithstanding this ink's relatively benign environmental impact and low safety hazard during printing, deinking mills shun flexographically printed newspaper stock. The paper is avoided because mitigating strategies based on a fundamental understanding of flexographic ink behaviour during deinking mill unit operations are scarce. One reason that little scientific work in this area has been published relates to difficulties in quantifying the response of the ink to different deinking conditions. Flexographic ink behaviour has been studied here by performing experiments on a model repulping system. A novel technique has been developed to measure the degree of residual ink in the repulped paper fibre from experimental trials. This technique, along with pulp brightness, has been used to investigate the effect of different operating conditions during repulping on the net detachment of ink. It is hypothesised that the mechanisms of flexographic ink detachment during repulping are pH dependent and relate primarily to the solubility of the ink's resin component. Once detached, the interaction of ink particles with pulp fibres follows the competing capture and escape phenomena predicted by Langmuir. Thus, if the ink particles are sufficiently small and the ratio of colloidal to hydrodynamic forces is appropriate, then deposition onto both the outside and inside surfaces of fibres can occur. Measuring the effects of pH, repulping time, and repulping power during the model experiments has tested the hypothesis. The effect of pH on both ink particle size and ζ-potential has been measured. Also, the solubility of the ink's acrylic polymer binder has been measured indirectly as a function of pH. Results from these trials, as well as analysis of published data, support the hypothesis. It has been found that the residual ink on fibres repulped in model experiments is low when the repulping pH is high and, conversely, high when fibres are repulped under acidic conditions. Ink binder solubility appears to correlate closely with these results. The relatively high degree of ink detachment achieved during basic repulping likely results from binder dissolution and subsequent ink particle dispersion. An estimate of colloidal interaction energies between ink particles and fibres under high pH conditions indicates that the counterion concentration in these repulping experiments is not sufficiently large to extinguish the repulsive electrostatic interaction energy between ink particles and pulp fibres. However, calculations have shown that the dampening of this repulsive energy is significant during conditions typically found during commercial repulping. Thus, although ink particles may be stabilised by both electrostatic and steric effects during the model experiments, during commercial repulping they are likely stabilised only by a steric barrier that results from ink binder adsorption onto the surface of ink pigment particles. Interestingly, the high levels of ink detachment achieved during high pH repulping may not be attained by binder dissolution alone. Experimental results suggest that some mechanical action (i.e., repulping energy) improves ink detachment even under alkaline conditions. It is suggested here that the repulping energy whose purpose is to defibre the paper, also liberates at least some ink particles that would otherwise remain trapped within the tortuous confines of the newsprint. Too much repulping energy decreases the final pulp brightness of repulped fibres. Thus mechanical energy should be optimised in the repulping unit if the pulp brightness is to be maximised. Because ink binder does not dissolve in acidic solutions, it is proposed that ink detachment during low pH repulping occurs as a result of shear induced by mechanical energy at the fibre-ink interface. Experimental results support this explanation of ink detachment. However, it has also been found that high levels of repulping power (i.e. approximately 800 W/kg pulp) cause an increase in the residual ink in pulp. This increase may be due to comminution and subsequent deposition of detached ink particles under high power conditions. It has been shown here that such deposition likely follows the Langmuir model mentioned previously. As in the case of alkaline repulping, the mechanical energy imparted to the acidic repulping system should be optimised if the maximum net ink detachment is to be achieved. Photomicrographs of repulped fibres indicate that, while ink particle deposition onto the inside surfaces of pulp fibres can occur under basic repulping conditions, lumen loading of ink does not occur during acidic repulping. A Langmuir model, that neglects lumen loading has successfully been fitted to data from acidic repulping experiments during which previously dried and ground ink was added to defibred newsprint that had not been printed. This supports the qualitative experimental results and suggests that lumen loading of flexographic ink during neutral or acidic industrial repulping would be insignificant. Data collected under acidic conditions during the model experiments during which printed paper was repulped do not follow the trend predicted by the Langmuir equation. This implies that phenomena other than deposition and detachment, such as particle comminution, occur during low pH repulping.