Many printing papers are pigment coated in order to obtain a paper surface with superior printing properties. To achieve this it may be assumed that a coating layer of uniform thickness or mass is extremely important.
A soft X-ray technique has been developed in order to study the coating mass distribution on coated papers. In order to demonstrate the significance of this technique a wood-free and a wood-containing base paper were coated in a pilot coater. On the wood-containing base paper, blade coating with a flooded nip unit was compared with coating in a short dwell time (SDTA) unit. In the former unit, two blades of the same thickness but with different tip areas were used. The result indicated that the most uniform coating layer was obtained when the coating was performed in the flooded nip coater and with a blade of low tip area. This is interpreted as being due to a high specific load on the base paper when it passes under the blade tip, which smoothes out the irregularities in the base paper surface so that the coating film flowing out under the blade tip and the corresponding coating layer are of uniform thickness or mass.
On the fine paper the effect of the strategy of drying the coating layer has been investigated. It was found that the coating layer is redistributed during its consolidation process, which is interpreted as being due to movements in the base paper, as a result of the water pick up from the wet coating layer which swells its fibers and break bonds between them.
The coated fine paper was printed in a sheet-fed offset press and the mottle in the print was evaluated. The correlation between the print mottle and the coating mass distribution was very good.