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 important at printability and print image quality.
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 compared to uncoated papers.
For specialty papers, the coat layer can have functional properties. Examples are the thermo-sensitive 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 microscop) micrograph of an 80 g/m2 woodfree base paper shows multiple layers of intersecting fibers:
Typical coat weights for graphic papers are 5 to 40 g/m², which means 10 and up to 50% of the final basis weight of the paper. There is a clear trend to increase the pigment portion in comparision to the fiber content:
Wood-containing papers are mainly single-coated (LWC). Coat weights are usually
between 6 and 10 g/m² 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
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, br