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

Abstract

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.

Coating

“…Fig 2 shows the transmittance factor, T F , as a function of coat weight for a kaolin clay pigment (Astra-Plus) and a ground calcium carbonate pigment (Carbital 90) before and after bleaching. As known from earlier work (Fossum et al 1976;Fjellström et al 2007a;Fjellström et al 2007b) Table1. Data for the kaolin and calcium carbonate (GCC, PCC) pigments used in this work as obtained from Imerys.…”

“…The lower transmission in the UV-region of this coating layer is expected to increase the photo-stability of a coated paper, (calculated from the ISO brightness loss due to irradiation) with about 65% when compared to the yellowing characteristics of a uncoated base paper consisting of 100% mechanical or chemimechanical pulp (cf. Fjellström et al 2007a;Fjellström et al 2007b). The discoloration of a single coated wood-containing paper will thus still be extensive and other photo-stabilising measures are probably necessary for long-life paper products.…”

“…kaolin clay, calcium carbonate, talc) to synthetic inorganic or organic products (e.g., plastic and silica type products). Kaolin clay has been reported to be somewhat more effective in retarding the accelerated light-induced yellowing compared to a coating layer containing calcium carbonate pigments (Fossum et al 1976;Reinhardt, Arneberg 1988;Fjellström et al 2007a;Fjellström et al 2007b). Luo and Göttsching (1991) reported that kaolin was somewhat more effective in retarding the photo-yellowing of a base paper intended for light weight coated paper grades than a brighter calcium carbonate pigment.…”

Review: Light-Induced Yellowing of Lignocellulosic Pulps – Mechanisms and Preventive Methods

“…Coatings also block the damaging ultraviolet radiation by absorbing or scattering radiation or by concealing a part of the discoloration of the base paper (Fjellström et al 2007b). It is most likely necessary to coat a paper if pulps containing lignin are to be used as the main fiber component in the furnish.…”

“…It is most likely necessary to coat a paper if pulps containing lignin are to be used as the main fiber component in the furnish. The coat-weight will affect the ability of the radiation to pass through the coating (Johnson 1989(Johnson , 1991Ghosh et al 2002;Yuan et al 2006;Fjellström et al 2007b). The amount and type of pigment in the coating color will affect the protective capacity; titanium dioxide and zinc oxide pigments being the most effective, followed by kaolin, precipitated, and ground calcium carbonate pigments (Gellerstedt et al 1983;Reinhart and Arneberg 1988;Luo and Göttsching 1991;Beyer et al 2006a;Fjellström et al 2009;Li et al 2011).…”