Visualisation of the distribution of ink components in printed coated paper using focused ion beam techniques

Abstract

Rotogravure printing is the preferred method when large print runs are required, such as for catalogues. Penetration of ink into the paper is not desirable as this increases the amount of ink needed to obtain a certain print density. Thus the differences in ink demand between different papers have a significant impact on the economics of the process. In order to control the ink penetration, paper is often coated with a thin layer of kaolin clay. The extent of penetration depends on the structure of this coating and the characteristics of the ink used. High spatial resolution gallium ion SIMS has been used to study ink penetration by embedding the paper in resin and imaging in cross-section, as well as depth profiling the ink film on the paper. For inks that react with the embedding resin however, the former technique fails. More recently, focused ion beam (FIB) cross-sections were used to visualise the penetration of ink into the paper. The more conductive nature of the ink used here compared with the coating gave high contrast between them, allowing easy assessment of the penetration. Results obtained from this method on paper with two different coatings were consistent with print gloss, print density and size of the printed dots. Ink is seen to have penetrated further into a paper coating with a ‘blocky’ morphology than one with a ‘platey’ morphology. This is consistent with the smaller dot area, lower print gloss and lower print density achieved on the blocky paper coating. TEM sections were obtained using the FIB with ex situ lift-out, providing a more direct view of the ink-paper interface.