Coating Pigments
Coating is a means of enhancing the surface quality of paper and paperboard. The coating process largely levels out the underlying fibrous structures, leaving a more uniform surface that ensures superior printing results.
The benefits of applying a coating layer become apparent from a comparison of equally magnified views of paper surfaces with different coatings.
The SEM micrograph (1) of an 80 g/m2 woodfree base paper clearly shows multiple layers of intersecting fibers. 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 (2) shows a precoated paper at the same magnification for comparison. A precoating of just 10 g/m2 per side suffices to cover up the majority of voids and fiber crossings. This process helps to smooth and even out the paper surface.
It is possible to enhance paper surface quality through application of a top coat (micrograph 3). Coating the substrate with another 12 g/m2 per side distinctly enhances the existing precoated surface quality even before the paper is calendered. At this stage of treatment, a number of dominant fiber structures and valleys still remain visible.
Subsequent calendering further improves quality by enhancing smoothness and gloss. The resulting surface is flat, with a minimum of irregularities (micrograph 4).
That, overall, is what the coating process is all about: achieving a near-perfect paper surface.
1.........................................................................2
3.........................................................................4
The above pictures illustrate the options available to papermakers for upgrading paper surfaces. Current trends are toward high brightness, high opacity, high smoothness and adequate bulk and stiffness. Although calendering adds smoothness, it does to some extent impair all the other desirable properties.
Additional coated qualities include LWC (light weight coated) papers, which are characterized by very low grammages or basis weights.
Applying coatings up to 10 g/m2 per side to mechanical LWC base papers with basis weights up to 50 g/m2 results in coated weights on the order of 51g/m2 to 72 g/m2.
Subsequent calendering further enhances the surface quality in terms of paper gloss and smoothness.
A series of products consisting mainly of natural ground calcium carbonate is also available for coating. These products differ in particle size, particle size distribution, specific surface and brightness. They are usually shipped as dispersion in water with solid content of up to 78%.
Typical Omya products include such well known products as
- Hydrocarb®,
- Setacarb® and
- Covercarb®.
High Quality
State-of-the-art technology is used to manufacture calcium carbonate products that are tailor-made for a variety of applications.
