Characterization of Ink–Paper Coating Adhesion Failure: Effect of Pre-dampening of Carbonate Containing Coatings

To attain an improved knowledge about ink–paper coating adhesion during offset printing, we have developed a laboratory procedure to measure adhesion failure between ink and coated paper during ink setting. This paper describes the technique and a special case where two ground calcium carbonate (GCC)-based coatings have been printed, one normal coating containing the delivered amount of dispersing agent already present in a typical GCC slurry and another coating made from the same GCC slurry but with excess additional free dispersant in the form of sodium polyacrylate. The areas where the ink–paper coating adhesion had failed at the pull-off points after tack testing were characterized by determining white spots in the retained print using image analysis. The white spots represent areas where ink has detached from the coating after contact with a rubber coated cylinder. This area is referred to as ink–paper coating adhesion failure area (IPCAFA). A minimum in IPCAFA at the point of maximum in tack force was found for both coatings where the print was applied onto dry coating. When the coatings, however, were pre-damped, IPCAFA increased on the normal coating over a short time period, independent of the dampening level. For the coating with excess dispersant, on the other hand, the higher dampening level strongly increased the IPCAFA. The strong impact of excess dispersing agent in this work may be explained by a number of related effects, including increased polarity of the coating surface, an associated moisture layer and an effective reduction of fine pores due to water-swelling polyacrylate that blocks these pores.