Print Mottle Reduction Through Clay Engineering and Pore Structuring in Paper Coating, 2005 Coating Conference

In general, fine particle size clays have been implicated in poor print performance (print gloss etc.) by creating pore structures that are not optimal for excellent printing. This paper clearly demonstrates that fine particle clays can indeed be designed that will provide excellent printing characteristics. In this study, coated solid bleached sulfate board bulk coating structure was investigated by mercury porosimetry. The surface structure and its effect on ink and substrate interactions were studied. The print mottle from sheetfed offset printing was evaluated by digital imaging. Based on Laplace equation, it was calculated that in the pore range of 0.1 - 0.02 µm of a coating layer, typical for a top coating of gloss grade, the capillary force increases from 20 to 100 factors. In the range of 0.2 - 0.1 µm, the capillary force increases from 10 to 20 only. It was found that for a given coating structure, together with peak pore size and volume, pore distribution can be used to gauge the affects of ink adsorption. In the same coating layer, the smaller pores affect the initial ink vehicle adsorption trend more than do the larger pores. The initial ink tack slope, which is measured and fitted by a mathematical model, can be used to measure the initial adsorption. The differential adsorption caused by pore size distribution within the same coating layer affects the initial tack slope and time to reach the maximum tack. We propose that the differential adsorption in clay coatings features two phenomena: 1) the differential adsorption of coating layer surfaces at the initial contact of ink and paper and 2) is the continued differential adsorption by the coating under the coating surface. Print back trap mottle (BTM) level can be linked to the initial tack slope differences. Through an understanding of coating pore size and distribution, initial ink tack slope, BTM and the influence of each, clay properties can be engineered and selected to create optimum structure for balanced coated board properties, BTM reduction in particular.