Inhibition of light-induced brightness reversion of high-yield pulps: The UV-screening properties of coating layers containing kaolin or calcium carbonate pigments

A newly developed method making it possible to study the reflectance/transmittance of thin coating layers and to link this to the inhibition of the light-induced yellowing, was used to describe the photo-stabilising properties of different types of commercial kaolin and calcium carbonate pigments. A base paper produced from a hydrogen-peroxide-bleached birch chemithermomechanical pulp (CTMP) was used throughout the study.The transmittance of the coating layers in the UV-region (300-385 nm) was found to be very low (below 1.0% at coat weights < 5g/m2) for all of the tested pigments, it strongly depends on the coat weight especially for coat weights below 10 g/m2. The kaolin pigments examined were found to be better in inhibiting the brightness reversion compared to the calcium carbonate pigments, even though the differences between the pigment types were only a few brightness units. The coating layer was, however, able to conceal some of the colour formed through the light-induce brightness reversion. For example, a coating layer consisting of kaolin pigments (coat weight of 15.6 g/m2) could conceal about 30% of the discoloration of the CTMP base paper.