Qualitätssteuerung von SC-papier durch kolloidchemische Veränderungen

In recent years, the printability of highly filled mechanical uncoated gravure papers has reached an outstanding level owing to successful optimization of machinery and equipment as well as improved raw material treatment techniques. Since several years now, DIPs are being used as an additional furnish component. An extensive study was devoted to find out whether printability levels could be further enhanced via controlling the electrokinetics in PM circuits. The following measurements were made for charge characterization: zeta potential (streaming potential) in the laboratory ; SCD values in the laboratory and online in a PM circuit. The trials were conducted in the Augsburg mill of Haindl Papier on a modern-design twin wire former (Roll Blade former). The investigations revealed a correlation between zeta potential SCD value and printability. The solid printability as compared to continuous tone and quarter tone printabilities developed in opposite directions. Charge conditions just below the isoelectric point were found to favourably influence the solid printability, whereas satisfactory continuous tone and quarter tone printabilities are typically achieved at higher zeta potential values. The impact of different colloid chemistries on printability are explained by a modified retention and dewatering behaviour and the resulting sheet forming variation in the paper machine. According to a priority list of influences that was compiled for this purpose the effects of colloid chemistry on printability rank fifth after the variables furnish, pulp quality, PM setting or supercalendering. No correlation was found to exist between zeta potential and SCD values. On the other hand, very good correlations were established between SCD values and retention as well as the dewatering behaviour in the PM. These findings were obtained after the tests and trials had been optimized.