Influence of ink chemistry and surface energy on flexographic print quality

The transfer and setting of water-based flexographic inks on uncoated woodfree fine paper and linerboard is reported as a function of ink properties and paper surface energy. Inks with different co-solvents were characterized in terms of viscosity and filtration resistance. Papers were treated with alkyl ketene dimer (AKD) to reduce their surface energy. The absorption rate and ink density after printing was characterized. For some inks, the surface energy of the paper had no influence on ink transfer, while for other inks, a significant difference was seen. These inks had similar viscosities and surface tensions. The filtration behavior of the ink was found to have a significant impact on ink transfer. Results show that for inks with low filtercake resistance, the surface energy of the material has an effect on ink transfer and print density, decreasing ink transfer after sizing. However, for inks with high filtercake resistances, surface energy had a minimal effect on ink transfer and the dewatering of the ink in a printing nip must be controlled by the ink filtercake and not by the surface energy of the paper. The absorption of ink co-solvents into AKD may also explain differences in print quality. Sorption isotherms of methyl isobutyl ketone (MIBK) and isopropanol on uncoated, AKD sized linerboard were measured by inverse gas chromatography. At the same vapor pressure, the absorption of MIBK is greater than that of isopropanol. The strong interaction between MIBK in the ink and the AKD may explain the increased ink penetration, leading to lower print densities compared to inks formulated with the isopropanol co-solvent.