Improved understanding regarding how to optimise the degree of swelling in the wood fibre wall may be a way to reduce the energy demand in the refining process. The forces necessary to achieve fibrillation (internal as well as external) are greatly influenced by the swelling properties of the fibre wall. The degree and position of swelling is also the main cause to where in the wood matrix the fibre separation will take place. Refining of wood chips in different state of swelling will result in mechanical pulp fibres with different optical and mechanical properties.

The typical parameters that influence the wood fibres’ ability to swell are temperature, pH, ionic form, amount of charged groups and ionic strength. Water retention value is one way to measure the pulp´s capacity to retain water after centrifugation and is strongly correlated to the swelling ability of wood fibres.

A blowline-sample of Norway spruce thermomechanical pulp (TMP) taken out directly after the chip-refining step was used as reference material for this study. The sample was treated with sodium sulphite under different conditions to introduce a range of very low to very high degrees of sulphonic acid groups in the cell walls. In a similar way alkaline hydrogen peroxide was used to introduce a range of very low to very high degrees of carboxylic acid groups. Each sample was then ion-exchanged into proton, sodium, calcium and aluminium form.

The effect of the amount of sulphonic and carboxylic acid groups in combination with the effect of counter ion, on the swelling capacity of mechanical pulp fibres was investigated. In addition, all samples were measured in a temperature interval between 25°C and 95°C to monitor changes in the softening temperature due to lignin structure modifications.

The influence of sulphonic and carboxylic acid groups in combination with the effect of counter ion form, on the swelling capacity of mechanical pulp fibres was studied by means of water retention value (WRV) measured in the temperature range from 25 to 95^oC. Mechanical pulp fibres (TMP) were treated with hydrogen peroxide and/or sodium sulphite during conditions resembling those used in chemimechanical and bleaching processes commonly used in the industry. In conventional chemimechanical processes sulphite treatment is used before refining while peroxide treatment can be utilized both before and after refining. In this study we did however also use sulphite after peroxide treatment. When subjecting sodium sulphite treated pulps to a subsequent hydrogen peroxide step, all pulps show a decrease in sulphonic acid groups, which could be owed to dissolution of highly charged lignin. Pulps treated with a high hydrogen peroxide charge (4%), showed a loss in carboxylic acid groups during subsequent treatment with sodium sulphite. This loss is probably due to dissolution of highly charged fibre material such as demethylated pectins. Both increased degree of sulphonation and carboxylation of the lignin reduces the softening temperature by means of reducing the degree of cross-linking in the lignin matrix. This softening probably improves the compressibility of the fibre pads in the sample holders of the WRV centrifuge, which would counteract an otherwise expected increasing WRV-value due to increased swelling potential. This makes it difficult to see clear trends in WRV as a function of increase in degree of sulphonation and carboxylation. When changing counter ion form from proton or calcium form to sodium form there is however always a clear increase in WRV in the range from 20 to 30%.

The influence of sulphonic and carboxylic acid groups in combination with the effect of counter-ion form on the swelling capacity of mechanical pulp fibres was studied by means of water retention value (WRV) measured in the temperature range from 25oC to 95oC. Mechanical pulp fibres (TMP) were treated with hydrogen peroxide, sodium sulphite, or both under conditions resembling those used in chemi-thermomechanical and bleaching processes commonly used in the industry. In conventional chemi-thermo-mechanical processes, sulphite treatment is used before refining, whereas peroxide treatment can be used both before and after refining. However, in this study, sulphite was also used after peroxide treatment. When sodium sulphite-treated pulps are subjected to a subsequent hydrogen peroxide step, all pulps show a decrease in sulphonic acid groups, which can be attributed to dissolution of highly charged lignin. Pulps treated with a high hydrogen peroxide charge (4%) show a loss in carboxylic acid groups during subsequent treatment with sodium sulphite. This loss is probably due to dissolution of highly charged fibre material such as demethylated pectins. Both the increased degree of sulphonation and carboxylation of lignin reduce the softening temperature reducing the degree of cross-linking in the lignin matrix. This softening probably improves the compressibility of the fibre pads in the sample holders of the WRV centrifuge, which would counteract the otherwise expected increase in WRV due to increased swelling potential. This phenomenon makes it difficult to see clear trends in WRV as a function of increasing degrees of sulphonation and carboxylation. When changing the counter-ion form from proton or calcium form to sodium form, there is, however, always a clear increase in WRV in the range from 20% to 30%.

When producing mechanical pulps the fibre separation will take place in the weakest part of the wood matrix. A prerequisite to swelling in wood and mechanical pulps is that the wood matrix is softened. The position of where the weakest part of the wood matrix is situated can to a large extent be controlled by adjustment of the swelling and softening properties of each of the wood polymers (lignin, hemicelluloses and cellulose). Most probably the efficiency of the external and internal fibrillation of the fibre walls is also influenced by how the swelling and softening properties are controlled. The combination of position of fibre-fibre separation together with the efficiency of the external and internal fibrillation will to a large extent determine the energy demand to produce mechanical pulps. Refining of wood chips with different state of softening and swelling will give rise to fibres with different optical and physical properties. The most important parameters that influence the wood matrix and wood fibres’ ability to swell are temperature, pH, ionic form of and the amount of charged groups in the hemicelluloses and lignin of the cell walls. In order to improve the level of knowledge on how to influence the degree of wood matrix and fibre wall swelling of mechanical pulps we have undertaken to study the swelling properties of wood and fibres produced by means of different mechanical pulping processes.

It was found that pulps not containing sulphonic acid groups need to be heated above the softening temperature of lignin in order to be able to swell to their full capacity. Introduction of sulphonic acid groups also opens up the rigid structure of lignin which lowers the softening temperature and increases the swelling potential even at lower temperatures. The effect of valence of the counter ion was also shown to be more pronounced after adding more carboxylic acid groups to a pulp. Depending on the number and type of anionic acid groups in the fibres, high-yield pulps will have different combinations of properties in different ionic conditions. The preliminary conclusions from this study are that the ability to control swelling properties of mechanical pulps is an important feature to take into account when producing pulp and paper.

It has earlier been shown that the counter-ion to the charged groups in the fibre has a significant effect on the beatability of fibres, but large-scale investigations of this effect are scarce in the literature. The objectives of the present study were therefore to develop a technique to ion-exchange large quantities of industrial pulp into the Na+-form using complexing agents and to study the effect of industrial-scale refining on pulp fibres in the Na+-form and how the fibres respond to industrial-like papermaking. The results show that ion-exchange can indeed be conducted on a pilot-scale using complexing agents such as DTPA. The study further indicates that an energy reduction of 50% at a given WRV or tensile index may be achieved if the fibres are converted to Na+-form prior to pilot-scale refining. By applying these techniques in full-scale production, it should thus be possible to save significant amount of energy, especially in the case of papers made from unbleached pulp that usually demands a higher degree of beating to achieve sufficient strength.

For the first time, well-defined and stable lignin model surfaces have been utilised as substrates in polyelectrolyte adsorption studies. The adsorption of polyallylamine (PAH), polyacrylic acid (PAA), and polyelectrolyte complexes (PECs) was monitored using quartz crystal microgravimetry with dissipation (QCM-D). The PECs were prepared by mixing PAH and PAA at different ratios and sequences, creating both cationic and anionic PECs with various charge levels. The adsorption experiments were performed in 1 and 10 mM sodium chloride solutions at pH 5 and 7.5. The highest adsorption of PAH and cationic PECs was found at pH 7.5, where the slightly negative charged nature of the lignin substrate is more pronounced, governing electrostatic attraction of oppositely charged polymeric substances. An increase in the adsorption was further found when the electrolyte concentration was increased. In comparison, both PAA and the anionic PEC showed remarkably high adsorption to the lignin model film. The adsorption of PAA was further studied on silica and was found to be relatively low even at high electrolyte concentrations. This indicated that the high PAA adsorption on the lignin films was not induced by a decreased solubility of the anionic polyelectrolyte. The high levels of adsorption on lignin model surfaces found both for PAA and the anionic PAA-PAH polyelectrolyte complex, points to the presence of strong non-ionic interactions in these systems.

Fines from white water were compared with fresh TMP fines from the same mill. The white-water fines gave a low er tensile strength of handsheets and a stronger light absorption. The surface composition (ESCA) and contact angle indicated more extractives on the surface of these fines. The total extractives content was much higher; no other large differences in chemical composition or morphology were found. Addition of unbleached white water to fresh fines had no effect on the tensile strength, whereas addition of white water subjected to hydrogen peroxide bleaching gave an immediate decrease.

Changes in properties of TMP fines brought about by the method of fractionation were investigated. Fines fractionated from white water by Baller McNett showed a higher tensile index and a lower light absorption compared with fines obtained using a Britt dynamic drainage jar (BDDJ). This was attributed to the large volume of tap water and thus more extensive washing of the fines in the Bauer McNett. This was strongly supported by the fact that acetone extraction improved the strength properties of these fines drastically. Fines fractionated from a fresh TMP by the Bauer McNett had a stronger light absorption and lower light scattering than those obtained by the BDDJ. This was attributed to the use of tap water and the loss of small particles in the Bauer McNett. Since the washing of mechanical pulp influences the properties significantly, the BDDJ method is recommended because it uses less water and of a more defined quality than does the Bauer McNett.