The Role of Surface Properties in Digital Printing on Extrusion Coated Paperboard, 2006 TAPPI Advanced Coating Fundatmentals Symposium

Traditionally package printing has been conducted with offset, flexo or gravure printing. Dry toner-based electrophotography is a digital printing method that has intruded into package printing and is increasingly competing with conventional printing techniques. This technology differs quite extensively from the others mentioned above, and sets its own specific demands for the print media. The aim of this work is to study the printability of extrusion coated paperboards, and with a focus on the surface properties, to deepen the knowledge of the factors influencing print quality.

This paper evaluates the printability of three different extrusion coatings used for packaging boards: low density polyethylene (PE-LD), ethylene methyl acrylate (E/MA) and polyethylene terephthalate (PET). The surface chemistry of the extrusion coatings has been modified with corona treatment. The effects of corona treatment have been evaluated with common paper tests and microscopic measurements. Based on these measurements, the correlation between surface chemistry, toner adhesion and print quality is discussed. The effects of surface properties, particularly surface energy, on both toner adhesion and visual appearance are evaluated. In addition, the electret properties of polymer surfaces are defined. One central research method in this work is electrostatic force microscopy (EFM) that is principally used in the characterization of conducting, piezoelectric, ferroelectric, etc. materials. In this study, the use of this method is extended to also cover extrusion coated fiber-based materials.

Printability properties of an extrusion coated paperboard vary quite a lot depending on the polymer type, when digital printing is concerned. By using suitable surface modification a product can be achieved that succeeds in the electrophotographic printing process and in converting operations. This study shows that when the surface energy of a polymer coating is above that of toner (e.g. Xeikon’s magenta toner 38 mN/m /18/), the toner adhesion approaches 100%. Surface charge uniformity is also necessary for uniform print quality and toner adhesion. Some polymer coatings, e.g. E/MA and PET, have the required surface energy level without corona treatment, but some coatings, e.g. PE-LD, need surface modification in order to succeed in the digital printing process.