Schritte zur Optimierung des Weissgrades von gestrichenen Papieren

It is common practice to make use of the carrier properties of polyvinyl alcohol (PVA) in order to enhance the efficiency of optical brighteners (OBA) in paper coatings. This report describes - with emphasis on PVA - how grade, amount and addition mode of both additives contribute to paper whiteness. Proposals are made as to how these products may be economically used to achieve high brightness yields. For an efficient brightness increase PVA should be used at somewhat higher OBA amounts where the brightness curve of sole OBA normally flattens out ; a fully hydrolyzed grade should be preferred. A standard OBA grade with 4 sulpho groups such as ®Leukophor UHF plus PVA added to the coating color performs better than the same OBA with built-in carrier. However, it is possible to further enhance brightness of the latter by adding PVA. Extensive rheological studies of coating colors containing OBA and different grades of PVA showed that shear thickening does not occur at crucial shear rates of around 10 6 s -1 when using very low molecular weight PVA such as ®Mowiol 4-98 and ®Mowiol 3-98. PVA contributes to OBA being more firmly fixed to the pigment (80/20 calcium carbonate/clay) which is believed to reduce its tendency to migrate.