Ink Gloss Development Mechanisms After Printing Part 2 - The Influence of Substrate, 2003 Advanced Coating Fundamentals Symposium Proceedings

It has been well documented that the roughness of a coated paper has a significant influence on the final printed gloss of the offset printed paper. The porosity of the coating has also been shown to have an influence on the print gloss by controlling the rate that the ink film sets and dries. It has been shown that coatings with a fast ink setting rate usually exhibit poor print gloss.

Two mechanisms have been discussed for the influence of pore structure on ink setting rate. The first concerns the amount of time available for the ink filaments to level before the ink film is immobilized. A fast ink setting substrate causes the ink to be immobilized quickly after passing through the printing nip. This prevents complete levelling of the ink filament remains, and results in a rougher printed surface and lower gloss. A second mechanism involves the separation of the resin and ink pigment at the surface of the print. Fast setting coatings withdraw more of the ink vehicle and resin into the coating, leading to the protrusion of ink pigment particles and a more micro-rough surface. This work seeks to establish the relative importance of each mechanism in determining the print gloss of different coated paper surfaces as a function of ink film thickness.

A range of coated papers was selected for evaluation, which were prepared with different pigment coatings. These were calendered to the same paper gloss prior to being printed with a range of ink film thickness (~0.3 - 3 µm ink) using a sheet-fed offset ink. Gloss goniophotometry, laser profilometry and dynamic gloss measurements were used to follow the changes in roughness, gloss and refractive index of the ink film with time after printing.

It is concluded that both ink film leveling and resin depletion at the print surface have an important influence on the final print gloss. The relative importance of each mechanism is however related to the ink film thickness. The timescales at which these mechanisms occur after printing are also discussed.