Coating Color Formulations
The main driving forces to develop new or improved coating color formulations are:
. • customer needs for improved coated paper quality such as higher brightness, print gloss and bulk, better coating coverage
. • requirement of high uniformity of paper quality which calls for accurate process control during coating color formation, color application, and drying
. • trends in printing technology such as higher printing speeds or digital color printing
. • new developments in coating technology such as new coating methods or higher coating speeds requiring good coating color runnability and uniform coating color quality at the same time
. • need for less energy consumption in drying which means higher solids content of coating colors
. • environmental requirements including recirculation and reuse of coating col¬ors.
However, coating color formulations, in any case, must be cost competitive. Coating color application requires its special formulation:
1. 1. The solids content greatly influences the runnability of the coating color in the coating machine. The solids content basically influences the viscosity of the color and its flow behavior at varying shear stresses in the coating machine.
2. 2. The various components of the coating color determine the highest possible solids content attainable for the target viscosity. The highest possible solids content is aimed for to save energy for drying of the coating color.
3. 3. Most coatings are processed in the pH range 7.5–11.5. The alkaline range is preferred because most of the color components are anionic in character and, from a colloidal chemical viewpoint, form considerably more stable systems at higher pH values than at low pH values. In addition, the drying time in offset printing is greatly reduced with increasing pH, because of the oxidative drying system mostly used.
4. 4. For nearly all coating processes, the water retention capacity of the coating color is most important; i. e., the ability of the coating color to retain water in spite of the sucking action of the coating base paper. If the water retention capacity is too low, the coating color may be too strongly dewatered between application and levelling. This leads to an increased solid content of the coating color before it comes into contact with the levelling blade which causes streaking and an uneven coat surface.
Coating color formulations and their solid content and properties differ according to the kind of application and coating process. Typical ranges of coating weight applied to the paper as well as solids content of the coating color for the various principles of pigmenting and coating are shown in Table 7.3.
7.6 Coating Color Formulations
Table 7.3 Coat weights and solid contents of coating colors for different coating methods.
Coating Method Coating Color solid content (%)Coating Speed (m/min) Coat Weig htRemarks
Size Press 20–30 100–1000 2–5 Pigmented
Film-Press 35–68 100–2000 6–12
Blade Coater 45–72 100–3000 7–20
Metering Bar Coater 40–65 100–800 3–12 Pigmented surface
Sizing or coating
Air Knife 30–50 100–500 8–12 Mainly
Spray Coating 45–55 100–2000 5–12
Curtain Coating 45–68 100–2000 8–12
Another crucial factor in coating color formula design is the intended use, i. e., the envisaged paper type and printing process. Formulations differ between matt and glossy grades, as well as for single or double coats with pre- and top coats, and triple coats with precoat, intermediate and topcoat. All coating color formulations aim for maximum solids content for quality and economic reasons. The coating color formulations presented in Tables 7.4 to 7.12. are examples commonly used in Europe at present.
However, they must usually be adapted to suit the particular conditions in the individual paper and board mills. All formulations are in dry parts of the individual substances. In many cases coated web offset paper has to have high stiffness of paper and freedom from blistering and creasing, the latter especially for heat-set web offset. Manufacture of higher-weight papers like MWC and HWC involves formulations similar to those used for woodfree grades.