Flexographic ink-coating interactions: effects of latex variations in coating layers, TAPPI JOURNAL April 2016

Please Note: This document will be available in PDF format in the "My Electronic Documents" link on the home page once your order has been completed. Please make sure you have the latest version of Acrobat Reader. Click on the Acrobat Reader icon to check for the latest version, it’s FREE. To print a hardcopy of a PDF file correctly you must have a postscript printer. If you are not sure if your printer is a postscript printer please refer to your owner’s manual.

Purchase of electronic (downloadable) documents made at www.tappi.org by credit cards, followed by instant download CANNOT be cancelled. We do not offer refunds on electronic download documents.

ABSTRACT: The effect of coating structure variations on flexographic print quality was studied using pilot-coated paperboard samples with different latex content and latex particle sizes. Two latexes, with particle sizes of 120 nm and 160 nm, were added at either 12 parts per hundred (pph) or 18 pph to the coating formulation. The samples were printed with full tone areas at print forces of 25 N and 50 N in a laboratory flexographic printing press using a water-based ink. A high ratio of uncovered areas (UCAs) could be detected for the samples that contained 18 pph latex printed at a print force of 25 N. UCAs decreased with increased print force and with decreased amounts of latex in the coating formulation. The fraction of latex covered area on the coating surface was estimated to be 0.35–0.40 for the 12 pph, and 0.70–0.75 for the 18 pph samples. The ink penetration depth into the coating layer could be linked to the fraction of latex-free areas on the coating surface. Optical cross section microscopy indicated that a higher print-ing force did not increase the depth of penetrated ink to any greater extent. Higher printing force did increase con-tact between plate and substrate, leading to an improved distribution of the ink. This, in turn, increased print density and decreased UCAs. On closer inspection, the UCAs could be categorized as being induced by steep topographic changes. When appearing at other locations, they were more likely to be caused by poor wetting of the surface. To understand the wetting behavior of the coating surface, observed contact angles were compared with calculated contact angles on surfaces of mixed composition.

Application: The detrimental effects of surface heterogeneity can be overcome by increasing printing force. The results can be used to improve print quality by understanding the balance between surface properties and printing parameters.