Paper and Board Coating General Aspects
The paper coating process was first developed in the USA late in the 19th century, but did not find broader application until the middle of the 20th century. Since that time the European paper industry has become a leader in coating technology. Coated papers suit the highest requirements as regards printability. In paper and board coating an aqueous suspension, called coating color, is applied to one side of the sheet (mainly in the case of board) or to both sides (mainly for printing papers). After application of the required amount, the coating is dried and finished. In finishing, the coated paper and board achieve their smoothness and gloss poten¬tial. Coating is done either in the paper machine (on-machine coating) or in a separate step after the base paper production (off-machine coating). It is desirable that, besides filling the cavities, the coating also covers the highest lying fibers on the base paper surface (Fig. 3.11). There are methods that tend to favor filling of the cavities while higher spots remain covered by only thin or practically no coat¬ing (equalizing or leveling coat, e. g. with blade equipment). Some other methods give a coating of more or less uniform thickness, thus also covering the highest spots on the base paper surface, yet the cavities remain only partially filled (con¬tour coat, e. g. roll applicators).
Coating colors consist of several components, white pigments (e. g. clay, calcium carbonate, talc, titanium dioxide) and so-called binders (e. g. starch, latexes) being the most important as regards volume and cost. Further specific additives influ¬ence and control the applicable solid content, the rheology, water retention and immobilization of the coating color during the coating process (e. g. dispersant, co-binders, thickeners), and others influence the physical and optical surface struc¬ture and properties of the coating layer (e. g. associative thickeners, lubricants, hardening agents, fluorescent whitening agents, defoamers, degassing agents). These components are described in more detail in Section 188.8.131.52.
Water is an essential component of a coating color making it possible to mix the components of a coating color, e. g., so that all the pigment particles are separated from each other, which is impossible in the dry state. Water also makes it possible to transport the color elsewhere and apply it onto the base paper so that the coating color remains uniformly dispersed. As water evaporates from the coating layer, the coating layer consolidates when the binder forms bridges between pigment parti¬cles and base paper. Coating colors should contain only as much water as the flow properties need, in order to save energy and costs for drying. The solid contents of the coating colors can be as high as about 70 wt. %.
The composition of coating colors resembles that of paints, containing similar components. Of course, there are differences in detail: the additives may be totally different, there are different kinds of binders, and paper coatings are white while most paints are colored so the pigments are different, etc. One major difference between paints and coating colors is the amount of binder: paints contain much more binder than do coating colors. This is partly due to their different objectives: The purpose of painting is to improve the looks of the surface to be painted, and also to create a protective layer.
This latter function demands that the paint layer should be nonporous in most cases. This is achieved by using sufficient binder to completely fill the spaces between the pigment particles.
As for the coating colors, other than to improve the looks of the paper, the purpose of their application is to achieve the desired properties on the paper sur¬face, the most important being the printing properties. The coating layer should be strong enough to resist the stresses of the printing process; the surface strength of the coating dictates the allowable minimum amount of binder. For example, offset printing inks are tacky, which requires a certain z-strength, or pick-strength, of the coating. On the other hand, increasing the amount of binder has a negative effect on several coating properties, and excess binder may cause quality concerns, e. g., low opacity and gloss, or glueability problems. Therefore adding more binder to the coating than the surface strength requires should be avoided; there is also a cost consideration. However, the coating process and base paper absorbency can make high binder levels necessary.