SUMMARY:The change in colour of different types of unbleached and bleached high-yield pulps under both accelerated and long-term ambient light-induced ageing conditions was studied using the CIELAB colour system. It was found that accelerated ageing conditions that mimic indoor daylight exposure tend to increase mainly the b*-value and decrease the L*-value (i.e., yellow/darken the pulp). Long-term ambient ageing without indirect sunlight also increases the a*-value, which makes the pulp more reddish. It was also found that accelerated light-induced ageing is characterised by rapid initial discoloration that is followed by a slower, secondary phase. The discoloration caused by long-term ambient light-induced ageing with no indirect sunlight present was more even over time. ADDRESSES OF THE AUTHORS: Helena Fjellström(helena.fjellstrom@miun.se) and Hans Höglund (hans. hoglund@miun.se): Mid Sweden University,

SUMMARY:This paper presents a method of studying the reflectance/transmittance properties of thin coating layers in relation to their inhibition of light-induced yellowing. The ability of a coating layer to retard the photo-yellowing of a hydrogen-peroxide-bleached birch chemithermomechanical pulp (CTMP) is exemplified using the developed model system. The repeatability of the method is shown to be excellent, even for coat weights below 10 g/m 2 . Furthermore, transmittance in the UV-region is shown to be strongly correlated with a coating layer's ability to protect birch CTMP against photo-yellowing.

SUMMARY:The ability of coating colours to obstruct ultraviolet (UV) radiation in the 300-385 nm region was examined with the aim of finding the best photo-stabilising formulation to inhibit discoloration of high-yield pulps. The influence of pigment type, bleaching of the pigments, pigment size, pigment size distribution, type of binder and addition of UV-absorbing compounds were examined using a newly developed method for studying the reflectance and transmittance properties of thin coating layers.The pigment type and coat weight was found to be the most important factors for reducing the transmittance of UV-radiation. Kaolin clays were more effective than calcium carbonate pigments and are therefore a better coating pigment for photostability reasons. Bleaching of the pigments, resulted in an overall minor decrease in transmittance for both kaolin and ground calcium carbonate (GCC) pigments, especially at low coat weights. Bleaching of the pigments changed the particle size distribution somewhat, which probably alters the structure in the coating layer. Precipitated calcium carbonate (PCC) pigments have a higher UV-screening potential compared to GCC pigments and should therefore be a better choice among the calcium carbonates. It was further concluded that a narrow pigment size distribution was beneficial for reducing the amount of transmitted UV-radiation that reaches the base paper. Styrene butadiene latex and polyvinylpyrrolidone were better in reducing the transmittance in the UV-region than polyvinyl alcohol.Adding a fluorescent whitening agent to a coating colour decreased the transmittance in the UV-region when the pigment was of the GCC type, and increased the transmittance when kaolin pigment was used. Addition of titanium dioxide (3 parts) to a coating colour containing kaolin pigment blocked about 90% of the UV-radiation at a coat weight of 10 g/m 2 , which is a common coat weight for a single coated paper. Another possibility is to double coat to increase the coat weight, which turns the transmittance factor closed to zero.

SUMMARY:A newly developed method making it possible to study the reflectance/transmittance factor 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-peroxidebleached birch chemithermomechanical pulp (CTMP) was used throughout the study.The transmittance factor of the coating layers in the UVregion (300-385 nm) was found to be very low (below 1.0% at coat weights >5g/m 2 ) for all of the tested pigments, it strongly depends on the coat weight especially for coat weights below 10 g/m 2 . The examined coating colours containing kaolin pigments were found to be better in inhibiting the brightness reversion compared to the coating colours containing calcium carbonate pigments, even though the differences between the pigment types in the coating colours 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 containing kaolin pigments (coat weight of 15.6 g/m 2 ) could conceal about 30% of the discoloration of the CTMP base paper, when measuring the change in ISO brightness.

SUMMARY:The light-induced brightness reversion of different types of unbleached and bleached high-yield pulps was studied under both accelerated and long-term ambient light-induced ageing conditions. It was found that accelerated ageing conditions that mimic indoor daylight exposures overestimate the yellowing tendencies of hardwood high-yield pulps (especially aspen pulps) compared to long-term ambient aging with no indirect sunlight present. It was, however, possible to predict the yellowing characteristics of softwood high-yield pulps. The lignin content of the pulps was found to be strongly correlated with the degree of photo-yellowing in the case of long-term ambient light-induced ageing. The presence of small amounts of birch chemithermomechanical pulp (CTMP) in a fine paper furnish resulted in extensive light-induced discoloration. The extent of photo-yellowing was found to be nearly linearly related to the amount of birch CTMP in the paper.