Influencing Porosity, Print Gloss and Glueability
The gloss of the printed paper is determined by the smoothness of the paper surface and the ink holdout. In turn, the ink holdout is mainly determined by the porosity and the chemical and physical structure of the coated surface. Coatings need to be porous on a microscopic scale so that the soluble component of printing inks is able to penetrate the paper and dry more quickly. The intensity and bril¬liance of the printed image and the ink consumption depend on the pigments staying on the surface. The hydrophobicity of the coated surface and the surface tension both influence the ink uptake.
Coatings that contain synthetic thickeners and co-binders are more hydrophobic and take up more ink, but their porosity can have a detrimental effect on the print gloss. Synthetic co-binders give rise to an open-pored structure, which can in¬crease the rate of ink absorption and give a lower print gloss. The task here is to find the best compromise between the pore size distribution and the hydropho¬bicity of the coated surface in order to obtain the highest possible print gloss without prolonging the drying time of the ink too much. An open-pored structure is often highly desirable in multiple coats applied to board because this guarantees high ink absorption.
Another important aspect governed by the porosity of the coating is the glue-ability of coated board. Here the affinity of the adhesive also plays an important role. High porosity can be obtained by using binders based on vinyl acetate, and it is for this reason that large quantities of vinyl acetate binders are used in the United States to coat folding boxboard, in spite of disadvantages such as low bind¬ing power, poor printability and build up of stickys in the wet-end of the paper or board machine. The approach taken in Europe is to use styrene-acrylic and sty-rene-butadiene binders, which give a more compact, less porous coating, and to add special co-binders that form a porous film and aid glueability. The porosity of the films formed by acrylic-based products and their compatibility with acrylic and acetate-based adhesives give very good glueability when they are applied to folding boxboard.