Paper Coating Machine Application Concepts
Typical coat weights for graphic papers are given in Fig. 7.28, plotted against the basis weight of the base paper. The coat weight is typically 25–40 % of the final basis weight of the paper.
Wood-containing papers are mainly single-coated (LWC). Coat weights are usu¬ally between 6 and 10 g m–2 per side. Traditionally, the coat is applied with a blade coater. More modern installations use a film press for the single coat.
At higher basis weights (MWC), two coat layers may be applied per paper side. Here, the top coat is usually a blade coat. The pre-coat may be either a film or a blade coat.
Woodfree papers cover a very wide basis weight range. Depending on the basis weight, the application can be single coat, double coat or triple coat. The coat weight per layer can vary in a wide range between approximately 8 and 20 g m–2.
For board grades, the coat weight depends on the requirements of the final product and on the quality of the base board. Coverage, brightness and smooth¬ness are the main properties that are improved by the coating. Depending on the required coat weight, two or three coat layers are applied.
Due to their functions, different formulations are used for the individual coat layers. For instance, pigments with high opacity are used in the first layers, whereas very fine pigments with good smoothness potential are used for the top coat.