Surface analysis, a way to better understand lithographic printing

In this case study, the effect of water, a cationic polymer, an anionic surfactant and a proprietary surfactant mixture on the surface of non-image area of a lithographic printing plate was studied. Contact angle data was measured and analysed in terms of surface energy components; surface chemistry was monitored with XPS and surface topography was observed with SEM imaging. Surface energy of various treated plate was calculated from contact angle using Fowkes's Geometric Mean, Wu's Harmonic Mean and Good van Oss' Acid-Base approaches. Although the theoretical considerations behind these approaches are different, and they yield different values of surface energy components, they reflect changes of the non-image area of the plate after various treatments. Some discrepancies among these models are discussed. XPS and SEM results showed that cationic polymers absorbed onto the non-image area of the plate, leading to changes in both the surface topographic and energetic characteristics. The influence of the adsorbed cationic polymer to the surface energetics of the plate's non-image area is mainly through a reduction in the polar component of its non-image area. The wettability of the non-image area by water is more sensitive to the change in its polar component than dispersive component. Plate surface analysis showed that a correlation between the adsorption of cationic substances and scumming can be established.