The relevance of acidic groups and metal ions in the production and end use of super-calendered (SC) magazine paper

Abstract

In this thesis, clay-induced discolouration of mechanical pulp and the electrostatic properties of supercalendered (SC) magazine paper have been investigated. New knowledge has been established in both areas. It was shown that acidic groups and metal ions are important factors affecting these issues. Phenolic groups and ferric/ferrous ions have a significant influence on the clay-induced discolouration. Accessible hydroxyl and carboxyl groups, as well as sodium ions, are of high importance for the electrostatic properties. The effect goal of the study was to increase cost efficiency in the production process of SC paper, i.e. to reduce discolouration of mechanical pulp, and to improve and stabilize the electrical properties of SC paper in order to obtain a paper with superior print quality in rotogravure printing. Mill experiences and laboratory model studies have shown that clay-induced discolouration of bleached mechanical pulp inside an integrated paper mill can result in a brightness reduction of several ISO-units of the mechanical pulp. Moreover, it was demonstrated that this can have a considerable effect on the consumption of bleaching chemicals in an SC magazine paper mill. The clay-induced discolouration of mechanical pulp was ascribed to the accessible iron in kaolin clay. It was shown that the degree of clay-induced discolouration varied significantly between different commercial clays. Thus, it is essential to ensure that the content of accessible iron in filler kaolin clay is as low as possible. Clay producers have to work with their mining and production process in this respect. The clay-induced discolouration test method developed in this work can be used as an acceptance check of clay in the clay quality control strategy. The kinetics and mechanisms involved in the clay-induced discolouration of bleached thermomechanical pulp (TMP) were identified. There was a considerable increase in the clay-induced discolouration rate with increasing temperatures in the range from 50°C to 65°C. Most of the chromophores formed at temperatures above 50°C were irreversible in terms of a subsequent chelating stage. This was attributed to oxidation reactions, which were catalyzed by iron ions released from kaolin. The results indicated that a few of the chromophores formed were due to iron-chelate complexes of phenolic structures. It was demonstrated that the addition of chelating agents before costorage of mechanical pulp and clay may eliminate or to a large extent reduce the clay-induced discolouration. Because of the relatively high cost of chelating agents and the environmental concerns related to the use of certain chelating agents, other ways of reducing the clay-induced discolouration should be considered. Furthermore, the ability of some iron-chelate complexes (for instance iron- DTPA complexes) to accelerate the decomposition of hydrogen peroxide must be taken into consideration in counter-current water circulation arrangements. There were significant differences in the clay-induced discolouration between different mechanical pulps. Efforts have been made to explain these differences. Moreover, the clay-induced discolouration of mechanical pulp was significantly lower with sodium dithionite than without sodium dithionite present during co-storage of TMP and clay. It was suggested that two of the factors contributing to the colour stabilizing effect are the ability of sodium dithionite and/or associated compounds to suppress the formation of coloured quinones and the ability of sodium dithionite to reduce coloured ferric compounds to less coloured ferrous compounds. The increased release of iron from kaolin, promoted by reductive conditions, appears to be of secondary importance. It seemed that the positive effect of using sodium dithionite remains as long as there are no “wet” oxidative conditions before sheet forming. In order to achieve a good print quality with SC magazine papers in rotogravure printing, a high surface smoothness is required. However, the electrical properties of magazine paper may also affect the print quality. Therefore, it is important that manufacturers of papers for rotogravure printing make an effort to ensure that the papers are provided with stable and appropriate electrical properties. This work revealed that the electrostatic properties of commercial SC papers vary widely. It was shown byIn this thesis, clay-induced discolouration of mechanical pulp and the electrostatic properties of supercalendered (SC) magazine paper have been investigated. New knowledge has been established in both areas. It was shown that acidic groups and metal ions are important factors affecting these issues. Phenolic groups and ferric/ferrous ions have a significant influence on the clay-induced discolouration. Accessible hydroxyl and carboxyl groups, as well as sodium ions, are of high importance for the electrostatic properties. The effect goal of the study was to increase cost efficiency in the production process of SC paper, i.e. to reduce discolouration of mechanical pulp, and to improve and stabilize the electrical properties of SC paper in order to obtain a paper with superior print quality in rotogravure printing. Mill experiences and laboratory model studies have shown that clay-induced discolouration of bleached mechanical pulp inside an integrated paper mill can result in a brightness reduction of several ISO-units of the mechanical pulp. Moreover, it was demonstrated that this can have a considerable effect on the consumption of bleaching chemicals in an SC magazine paper mill. The clay-induced discolouration of mechanical pulp was ascribed to the accessible iron in kaolin clay. It was shown that the degree of clay-induced discolouration varied significantly between different commercial clays. Thus, it is essential to ensure that the content of accessible iron in filler kaolin clay is as low as possible. Clay producers have to work with their mining and production process in this respect. The clay-induced discolouration test method developed in this work can be used as an acceptance check of clay in the clay quality control strategy. The kinetics and mechanisms involved in the clay-induced discolouration of bleached thermomechanical pulp (TMP) were identified. There was a considerable increase in the clay-induced discolouration rate with increasing temperatures in the range from 50°C to 65°C. Most of the chromophores formed at temperatures above 50°C were irreversible in terms of a subsequent chelating stage. This was attributed to oxidation reactions, which were catalyzed by iron ions released from kaolin. The results indicated that a few of the chromophores formed were due to iron-chelate complexes of phenolic structures. It was demonstrated that the addition of chelating agents before costorage of mechanical pulp and clay may eliminate or to a large extent reduce the clay-induced discolouration. Because of the relatively high cost of chelating agents and the environmental concerns related to the use of certain chelating agents, other ways of reducing the clay-induced discolouration should be considered. Furthermore, the ability of some iron-chelate complexes (for instance iron- DTPA complexes) to accelerate the decomposition of hydrogen peroxide must be taken into consideration in counter-current water circulation arrangements. There were significant differences in the clay-induced discolouration between different mechanical pulps. Efforts have been made to explain these differences. Moreover, the clay-induced discolouration of mechanical pulp was significantly lower with sodium dithionite than without sodium dithionite present during co-storage of TMP and clay. It was suggested that two of the factors contributing to the colour stabilizing effect are the ability of sodium dithionite and/or associated compounds to suppress the formation of coloured quinones and the ability of sodium dithionite to reduce coloured ferric compounds to less coloured ferrous compounds. The increased release of iron from kaolin, promoted by reductive conditions, appears to be of secondary importance. It seemed that the positive effect of using sodium dithionite remains as long as there are no “wet” oxidative conditions before sheet forming. In order to achieve a good print quality with SC magazine papers in rotogravure printing, a high surface smoothness is required. However, the electrical properties of magazine paper may also affect the print quality. Therefore, it is important that manufacturers of papers for rotogravure printing make an effort to ensure that the papers are provided with stable and appropriate electrical properties. This work revealed that the electrostatic properties of commercial SC papers vary widely. It was shown by a full scale printing trial that this variation may be of importance for the print quality in rotogravure with an electrostatic assisted ink transfer system (ESA). The electrostatic properties were analyzed by measuring the surface potential after corona charging with a fast response electrostatic fieldmeter as a function of time at varying water content of paper (relative humidity). The influence of different factors (equilibrium content of water, grammage, calendering, filler kaolin clay, pH, accessible acidic groups and ion exchange with H+ and Na+) on the electrostatic properties of paper made of TMP has been investigated in laboratory model studies. Moreover, the results were compared with mill experiences. It was shown that it is the total water content in the sheet in relation to the weight of fibres plus water, expressed as % (% on fibre basis), that primarily determines the potential decay rate of papers with kaolin as filler. Furthermore, the amount of kaolin in the range of 17% to 41% on oven dry basis did not appear to systematically affect the surface potential decay rate or the maximum surface potential. The number of conducting links in the paper appeared to have a considerable effect on the potential decay rate at low water content (<7-8% on fibre basis) in the paper. At this low content of water, when the number of charge carriers and their mobility are limited, the surface area in the bulk of paper (grammage) appears to be an important factor for the potential decay rate (conductivity). The potential decay rate increases with increasing grammage (in the range of 40 g/m2 to 70 g/m2). A full scale printing trial showed that SC papers with high potential decay rates have less tendency of whiskering in rotogravure printing compared to papers with lower potential decay rates. From a practical point of view, this suggests that low grammage SC papers could be more prone to whiskering than high grammage papers. Findings obtained in laboratory model studies are in line with the following hypotheses: - Changes in the nanostructure of the paper (fibres) (e.g. through calendering) may reduce the number of conducting links and thus decrease the potential decay rate at a given water content. - The addition of filler kaolin clay to TMP reduces the inter-fibre contacts and hence also the number of links available for electrical conduction. - An increase in the amount of accessible acidic groups (e.g. carboxyl groups) may strengthen the network of conducting links. - Protons support the charge transport process in lignocellulosic materials. - The presence of sodium ions (introduced by ion exchange) contributes to an increased concentration and/or mobility of charge carriers relative to a fully protonated TMP. The role of monovalent metal ions on the electrical conduction mechanism in lignocellulosic materials in the hygroscopic range is unclear. Further work is needed to clarify if the metal ions stimulate "proton hopping" and/or if they contribute to the charge transport themselves. Further research is also needed on how bivalent metal ions affect the conduction process. Lowering of pH in furnish before sheet forming enhanced the electrical conduction in paper. This result is consistent with the literature that points out the significance of protons and hydronium ions (H3O+) to support the charge transport process in lignocellulosic and cellulosic materials. It was shown by mill experiences and a mill trial that the surface potential decay characteristics of paper to some degree may be controlled by varying the dosage of sodium dithionite (and/or sodium bisulfite). The effect was attributed to both a change in pH and the concentration of sodium in the paper. Hence, it follows that a stable pH and a relatively constant content of sodium ions in paper are beneficial in order to stabilize the electrical properties. The water content of the paper may to a certain degree be used to adjust electrical properties. Thus, it is advantageous to have a stable and appropriate water content of paper before wrapping of rolls. It is also important to control the water content of the paper in the printing process. This work has deliberately focused on how different parameters in the production process of SC paper may influence the electrostatic properties of paper and not on the absolute quantitative requirements