There is a potential to increase the share of recovered fiber stock in higher-quality graphic paper grades (uncoated and coated wood-containing papers as SC and LWC papers). The optical properties are of decisive importance for the use of recycled recovered fiber material (deinked pulp) in higher-quality graphic paper grades. However, any such intentions are opposed by the occasionally experienced red discolouration of deinked stock. Red discolouration is considered to have occurred if the measured value a* of the CIELAB colour space exceeds the value of +1. Various chemical analysis processes were developed to enable investigation of the causes of red discolouration and explore countermeasures. The use of thin-layer and liquid chromatography permitted identification of the causal substances. The colorimetric determination of the concentration of the substances was done by means of a molecular absorption spectroscopy. The evaluation of the red discolouration took equal account of the concentration and the optical effect ¡V expressed by the measured values of the CIELAB colour space ¡V of a colorant. The cause of the red discolouration are laked BONA and Ò-naphthol pigments as well as rhodamine-based colorants. The aforementioned colorants enter the recovered paper mixture via rotogravure and offset inks. As the result of a modelling of the pulping process based on a print sample, the addition of sodium hydroxide proves to be the most important process parameter that negatively influences the deinkability of the laked azo pigments. Colorants based on rhodamine B are often used to nuance rotogravure inks. Within the framework of this modelling, no significant dependence of the deinkability of rhodamine colorants on the process parameters was apparent. The deinkability of the colorant in relation to a laked azo pigment proved less effective due to a poorer detachment from the fibers. The difference to other coloured inks (cyan, yellow) is not so much the result of a poorer level of effective ink removal, but rather the general influence of red ink particles on the reflectivity. Any remaining concentrations in the deinked recovered fiber pulp have no positive influence on the brightness or luminance, but have a visually disturbing effect. Besides by ink flotation, red discolouration during recovered paper processing, due to laked azo pigments, can be effectively reduced by means of reductive dithionite bleaching, switching to a pseudoneutral operation, and dissolved air flotation of the corresponding process water. As opposed to this, a modification of the pulping conditions and colour stripping by conventional bleaching steps do not achieve any reduction of rhodamine colorants. As a preventive measure, in the long term every effort should be made to improve the deinkability of printed products that cause a red discolouration. Besides the substrate, the ink composition is also of major importance. The deinkability of prints cannot be decisively improved by the production process of a laked azo pigment, the use of alkali-stable pigments, nor a higher share of binding agent. Bearing in mind that laked azo pigments are the most important red pigments and that the aforementioned technical process options exist, there is no urgent need for optimisation. Flotation of rhodamine colorants cannot be carried out as efficiently as laked azo pigments, nor can colour stripping be done by conventional bleaching steps. For this reason, consideration should be given to reducing, substituting or eliminating rhodamine colorants in inks. From the point of view of the deinking process, a substitute should allow efficient flotation and irreversible colour stripping. | English |