Influence of ink chemistry and uncoated paper surface energy on setting and transfer of water-based flexographic inks

The setting and transfer of water-based flexographic inks is of critical importance for the print quality of uncoated papers and linerboards. While paper surface free energy is often of importance for print properties, ink chemistry such as binder type and its interaction with the paper is also known to influence ink transfer. Some of the same fundamental mechanisms that control coating operations should be related to these inks, but the important paper and fluid parameters are not clear in the literature. Four different base papers, a woodfree, newsprint, supercalendered (SC) and linerboard, were treated with a solution of alkyl ketene dimer (AKD) in hexane to change the surface energy without changing fiber pore structure. The samples were printed with a laboratory flexographic proof press at a range of anilox volumes using two water-based inks. For one ink formulated with an emulsion binder, the sized paper produced a significant decrease in ink transfer and print density compared to the unsized paper. However, for the solution binder ink, ink transfer and print density were not influenced by the substrate contact angle. The two inks had similar viscosity and surface tensions. The difference in behavior seems to be related to the filtercake resistance of the inks. The ink that was not sensitive to the paper surface energy had a high filtercake resistance: this resistance causes the ink transfer to depend on the ink film split, not the paper surface energy. A model is proposed that predicts the correct trends in ink transfer for parameters that match the laboratory conditions.