Influencing Printability in Paper Coating
The evenness of the printed image is a very important criterion for quality. The acceptance of coated paper by printers depends on its runnability in the printing process and its printability in terms of an even image that is free of defects.
The printability of paper is principally determined by the surface of the coating, and its structure and topology, pore size and pore size distribution, chemical na¬ture, and homogeneity all play an important part in influencing the interaction between paper and ink. The binder has the greatest influence on printability, but co-binders and thickeners also play a part and they have to be adapted to the printing process.
In offset litho, mottling is mainly caused by the uneven distribution of coating ingredients at the surface of the coating. There are different types of mottling depending on the causes and the way in which the mottling manifests itself. In gravure printing, missing dots can be avoided if the paper has a very smooth surface and a defined microscopic roughness.
In both printing processes, it can be assumed that co-binders and thickeners with very high water retention will have a detrimental effect on print quality. Very high water retention prolongs the drying time of the wet coating and the various different ingredients in the coating migrate at different rates, which causes the binder, co-binder, and thickener to become unevenly distributed. This patchiness gives rise to mottling in offset litho printing processes. Low web speeds and low drying capacity tend to exacerbate this problem. Excessively high water retention can also give rise to a rough, uneven coating which can cause missing dots in gravure printing.
There are no hard and fast rules with co-binders and thickeners when it comes to avoiding mottling. All natural and synthetic products can cause mottling under adverse conditions. Coatings that contain starch are known to have a high ten¬dency to mottle, even if starch is only contained in the precoat. Soy protein behaves similarly. Synthetic products are not usually prone to mottling, although CMC can give rise to missing dots in gravure printing processes.