Extensional Rheology of Coating Colours for Curtain Application, 2004 Coating and Graphic Arts Conference

Online-concepts for the production of coated papers are today widely used within the paper industry. The introduction of the metered size press as an applicator with low mechanical impact on the paper web certainly had an influence on this and the consequential further development of alternative non impact technology, i.e. practically non contact coating systems.

The curtain coater is already a successfully introduced technique for the production of special papers with more than 20 applications world wide. This system because of its good coverage makes it interesting for the field of graphical papers.

The use of conventional offset colours in curtain coating leads to runnability problems and surface defects especially at high speeds. The disturbances are caused primarily as a result of the insufficient stretchability of the coating colour. Therefore, it was essential to optimise the recipe to meet the special demands of the curtain coater.

Rheological measurements were used in a precise investigation which made it possible to evaluate suitable modifications under laboratory conditions. In particular the extensional rheology, a totally new technique in assessing coating colours, showed excellent correlation between the laboratory assessment and behaviour on the curtain coater.

With the correct choice of a suitable co-binder it was possible to control the coating defects associated with conventional coating mixes. Particularly the use of a special water soluble thickener with high molecular weight made it possible to achieve a problem free runnabilty at high speeds and excellent coating quality.