Objectives of Paper Coating
The main purpose of coating is to improve the surface quality of paper or board. The quality improvement can be aimed at optical properties such as brightness, gloss or opacity, at tactile properties such as smoothness, but, most importantly, at printability and print image quality.
The application of (usually white) pigments to the base paper surface enhances the brightness of the paper. In addition, the opacity increases due to the high light scattering of the pigments. This improves the optical appearance, because the shine-through of the back side printing is reduced. Also, the coat layer evens out the surface topography of the sheet, resulting in an improved smoothness, which in turn gives a better gloss.
The coat layer reduces the penetration of ink into the paper sheet. Therefore, the ink does not spread as much and the print image is clear and sharp. The print density and the print gloss are enhanced, and the ink demand is reduced com¬pared to uncoated papers.
For specialty papers, the coat layer can have functional properties. Examples are the thermosensitive layer of thermal papers or the capsule-containing coat layer of carbonless papers.
The benefits of applying a coating layer become very apparent when comparing paper surfaces with different coatings. The SEM (scanning electron microscope) micrograph of an 80 g m–2 woodfree base paper shows multiple layers of inter¬secting fibers (Fig. 7.1). The paper surface is characterized by hills and dales formed by the fiber mesh. The voids between the fibers impair the smoothness and uniformity of the paper surface. The next micrograph shows a precoated paper at the same magnification for comparison.
A precoating of just 10 g m–2 per side suffices to cover up the majority of voids and fiber crossings. This helps to smooth and even out the paper surface. Paper surface quality can be further enhanced by the application of a top coat. Coating the paper with another 12 g m–2 per side distinctly enhances the existing precoated surface quality, with a number of domi¬nant fiber structures and valleys still remaining visible. Subsequent calendering achieves an additional quality gain by enhancing smoothness and gloss. The re¬sulting surface is flat, with a minimum of irregularities. An electron microscope picture of a microtome cut from a coated paper sample is shown in Fig. 7.2. In the center of the sheet, the fibers can be seen. The lighter and more densely packed structure on the outside of the paper sheet is the pigments of the coat layers.
The caliper of the coat layer varies according to the changing thickness of the base sheet. The surface is smooth so, by coating, paper and board can be upgraded to a higher quality level with added value. Furthermore, raw material costs are also an important factor of coated papers and coating colors are in most cases cheaper than chemical or mechanical pulp. So an optimum ratio of coating layer/fiber web has to be found.