Mechanisms of Gloss Development with Matte-coated Paper in Offset Printing (2) -Post-Printing Micro-Dynamics of Ink Film-

Abstract

In a previous report, it was expected that exposure of small-scale roughness with ink vehicle penetration roughened the printed surface of matte-coated paper. The purpose of the present work is to elucidate the post-printing ink micro-dynamics, or microscopic behavior of an ink film immediately after transfer to matte-coated paper. Dynamic print gloss was compared between inks with three different viscosities. For the ink with the highest viscosity, gloss increased initially most slowly of the three inks. The gloss decreased most slowly past the lowest peak. Presumably, for the highest viscosity ink, the ink split pattern formed on the coated paper surface at the printing nip was leveled by fluid flow most slowly, resulting in the initial slowest gloss increase. In order to examine the gloss decrease process, the ink films were solidified with liquid nitrogen at various laps of time after printing, and the ink densities were determined by color-image analysis. Although the ink density distribution immediately after printing was not dependent on the profile of the coated paper, the correlation efficient between the images of paper surface profile and ink density distribution increased with time. This phenomenon supports the mechanism that ink flows along the paper surface geometry with time and builds up at the bottom of valleys resulting in high ink density in there. We confirmed that small-scale roughness due to coating pigments became exposed with time. In short, roughness hidden by ink coverage appeared as the ink film became thinner by vehicle penetration.