INK-COATING ADHESION: the importance of pore size and pigment surface chemistry

The characteristics of ink vehicle absorption into porous structures has formed the basis of a number of recent studies. Kalela, Ridgway and Gane (11th International Printing and Graphic Arts Conference, Bordeaux, 2002) considered an example of common impression cylinder deposits as a result of poor coldset ink adhesion especially in the presence of excess fountain solution. These studies centre on the absorption mechanisms of pigmented coatings and how these can be adjusted to enhance ink coating adhesion. By developing techniques of tack cycle analysis and correlation with adhesion, i.e. observing print density on the pull-off areas using the InkSurface Interaction Tester (ISIT), it has been possible to assess ink-coating adhesion. To enhance the adhesion properties of inks, two strategies for coating designs are discussed. The first is to increase the number of ultrafine pores and to increase simultaneously the pigment surface area to enable a capillary-driven separation of oils from solved resins and to provide higher adsorptive power for resins. This was tested by the inclusion of a novel surface-structured calcium carbonate. The second is to introduce an oleophilic species into the coating structure, achieved by the inclusion of hydrophobic talc via a costructure between talc and hydrophilic dispersed calcium carbonate (Gane, P.A.C., Buri, M. and Blum, R., International Symposium on Paper Coating Coverage, Finland 1999). Mercury porosimetry measurements of model coatings using these pigment combinations are used to illustrate their effect on coating structure. These structures are modelled using a computer network simulator, an absorption algorithm is applied to generate the absorption dynamic for each structure. The differences in wetting front are illustrated. It is seen that only at the shortest timescales is there any difference in the absorption dynamic (ignoring any surface chemistry effects). It can therefore be concluded that the observed improvements in this study of ink adhesion are related to adsorptive mechanisms, and absorption rate governs the dwell time over which this adsorption can occur.