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Under mechanical pulping conditions, elevated temperature initiates reactions in the wood constituents resulting in the formation of coloured structures. In an attempt to reduce the formation of colour, a number of additives have been examined to assess their ability to inhibit heat-induced darkening of a Norway spruce (Picea abies) stone groundwood (SGW) pulp. SGW pulp was used as a model pulp with the purpose of imitating a thermomechanical pulp (IMP). The heat-treated SGW pulp, both with and without additives present, was subsequently bleached with 4.5% hydrogen peroxide at high pulp consistency and with varying alkali charges in order to examine the hydrogen peroxide bleachability of such pulps. Heat treatment at 170 °C resulted in a loss of brightness of about 4% ISO after heating for 60 seconds. The reduction in brightness could only partly be regained through high consistency hydrogen peroxide bleaching; the SGW pulp displayed a brightness of 83.7% ISO whereas the heat-treated SGW pulp reached a brightness of 80.4% ISO. Heat treatment with a low addition rate (1 %) of alkaline hydrogen peroxide, sodium bisulfite or sodium sulfite, with a subsequent hydrogen peroxide bleaching stage, resulted in an improved bleached brightness of about 81.6% ISO, i.e. the loss of brightness due to heat was still approximately 2% ISO. Sodium borohydride (1 %) did not suppress the thermal darkening but improved the bleached brightness by approximately 2% ISO compared to the heat-treated SGW pulp and resulted in a brightness of 82.6% ISO. Addition of a chelating agent (DTPA, 0.11 and 0.45%) and the radical scavengers phthalic hydrazide (1 %) and 1 -hexanol (1 %) suppressed the heat-induced darkening somewhat but did not improve the hydrogen peroxide bleachability to any noticeable extent. Addition of citric acid (0.6%) did not suppress the thermal darkening and resulted in impaired bleachability.

The effect of spray application of xyloglucan on strength and recyclability of bleached chemical pulp was studied. Xyloglucan alone and with a cross-linking chemical was applied on sheets made of unbeaten never-dried and once-dried virgin pulps. Recyclability of these pulps was evaluated based on the tensile strength of wet and dry handsheets. Dry strength measurements showed that the spray application reduces the loss of strength and bonding ability in recycling. The use of cross-linking chemical was found to be essential in enhancing both the wet and dry strength of the virgin and recycled pulps. The chemical application was efficient with both never-dried and once-dried pulps. Wet web strength results indicate that the amount of web breaks could be decreased in paper and board machines using recovered fibers by having cross-linked xyloglucan layer on fiber surface. Spray application was found to be an effective way to introduce non-charged chemicals into fiber network. As a consequence of improved fiber bonding the need of refining decreases which further improves the recyclability of fibers. In addition to xyloglucan also other polysaccharides, for example those provided by chemical or mechanical pulp mills, could be used for improving the recyclability and extending the lifetime of fiber material., QC 20140903

The objective of this investigation was to evaluate how physical properties of laboratory sheets, made from different thermomechanical and chemithermomechanical pulps, depend on the pressure and temperature levels during pressing and drying. Sheet were formed from long fibre fractions, defined as the pulp retained on a 50 mesh wire in a Britt Dynamic Drainage Jar, and pressed and dried according to a number of different pressure and temperature settings. The results showed that there were large differences in the sheet properties depending on what type of method that were used for pressing and drying. High freeness pulps were more sensitive to the type of method used for pressing and drying than low freeness pulps. The results indicate that the difference in sheet properties between ISO and Rapid Köthen sheets might be explained by the fact that ISO sheets are only wet pressed, the fibres are then allowed to spring back when pressure is released whereas the fibres in Rapid Köthen sheets are locked in their positions as the sheets are pressed and dried till over dry.

By measuring adsorption using a stagnation point reflectometer, it was shown that multilayers consisting of cationic polymers and dissolved and colloidal material released from wood fibres into process waters are formed on an anionic silica surface. The similarity of this experiment to the papermaking process was established. It was established using the JKR-method that this multilayer adsorption had a profound effect on the adhesion, in air, of an elastic polydimethyl siloxane (PDMS) probe to these surfaces. The adhesion of PDMS to a bare silica surface showed a large hysteresis, with a much stronger adhesion measured upon the separation of the two surfaces compared with the value obtained from gradually pressing them together. It was suggested that this hysteresis was due to specific interactions (hydrogen bonding) that develop over time. The first adsorbed layer, a cationic poly(dimethyldiallylammoniumchloride) (polyDMDAAC), decreased the magnitude of the hysteresis and gave a relatively low adhesion, which may be due to the fact that these specific interactions were blocked. The subsequent build up of layers of lipophilic wood extractives in the form of colloidal particles and cationic polymer increased the adhesion. This was interpreted as being due to the build up of a soft layer on the stiff mineral surface causing additional energy dissipation upon separation, even though the molecular adhesion was still decreased compared with the bare silica. This was supported by the fact that none of the individual components of the multilayer increased the adhesion when applied in a relatively thin layer on the silica, and by the fact that a thick deposition of cast coated wood extractives gave a very high adhesion. Stearic acid was chosen as a model substance for extractives and deposited onto silica from vapour phase. This resulted in a partial coverage of the surface where the stearic acid was present as patches of different size. This gave a significant reduction of, QC 20100525 QC 20111103

The effects of enzymatic treatments of dissolved and colloidal substances (DCS) released from thermomechanical pulp (TMP) have been investigated. A model dispersion of DCS was made by leaching several portions of TMP in distilled water and separating the fibrous material. Some samples were enriched in colloidal particles by removing dissolved substances using ultra-filtration. The DCS, which had been subjected to different enzymatic treatments, were added in a fixed quantity to TMP fines that had been made cationic, and were subsequently used to form handsheets. All DCS additions increased the content of lipophilic extractives in the sheets. Lipase gave a complete hydrolysation of triglycerides into free fatty acids. The untreated DCS gave no significant decrease in tensile strength, because of the relatively small addition. A treatment of the DCS with Lipase gave a higher extractives content and a tensile strength on the same level as the reference. A Mannanase treatment gave a decrease in strength compared with the reference at the same amount of extractives in the sheet. A combined treatment with Mannanase and Lipase gave a more pronounced decrease in tensile strength. Two possible reasons for the differences in strength at a given amount of extractives were suggested: (i) the destabilisation of the colloidal wood extractives due to the Mannanase could affect the distribution of the colloid in the sheet, making it more detrimental to sheet strength compared with the stable colloid. This would account for the observation that Lipase did not affect sheet strength as such, but the combination with Mannanase gave the lowest tensile strength; (ii) the decomposition of galactoglucomannans in aqueous solution would diminish their positive effect on tensile strength and/or affect the adsorption of the colloid. A reflectometry technique was used to quantify the adsorption of the differently treated DCS onto a model surface of the cationic fines. Colloidal wood extractives, QC 20100525

An experimental method to manufacture a model white water from mechanical pulp by leaching of the pulp in distilled water is presented. This water, which contains dissolved and colloidal substances DCS, is fractionated into two fractions: a fraction which contains solely dissolved substances, DS, and a fraction enriched in colloidal particles, CS. The major part of the lipophilic extractives were found in the CS fraction and had a similar composition as the extractives in the non-fractionted water sample, the DS fraction contained no detectable amounts of neutral components and small amounts of fatty and resin acids. Galactoglucomannan was the dominating dissolved polysaccharide, Arabinose or Xylose were not detected in the water. The effect of these types of white water on the characteristics of TMP was studied by addition of different amounts to cationised TMP fines prior to sheet making. Fines were chosen as a model system because they are probably more susceptible to adsorption of DCS than coarser fractions, due to higher specific surface area. It was shown that water containing colloidal particles decreased the tensile strength of handsheets and that this decrease becomes more pronounced if the addition of colloial particles is increased. It is suggested that this decrease is linked to the amount of lipophilic extractives present on the surface of the fines and it is shown that the decrease begins to occur at a surface coverage of about 45%, as calculated from ESCA data. The tensile index was not affected by addtion of DS under the experimental conditions used. The light absorption coefficent of the sheets increased upon addition of all water fractions, which indicates that the dissolved substances are an important reason for the observed darker colour., QC 20100525

An experimental method to manufacture a model "white water" from mechanical pulp by leaching of the pulp in distilled water is presented. This water, which contains dissolved and colloidal substances "DCS", is fractionated into two fractions: a fraction which contains solely dissolved substances, "DS", and a fraction enriched in colloidal particles, "CS". The major part of the lipophilic extractives were found in the "CS" fraction and had a similar composition as the extractives in the non-fractionted water sample, the "DS" fraction contained no detectable amounts of neutral components and small amounts of fatty and resin acids. Galactoglucomannan was the dominating dissolved polysaccharide, Arabinose or Xylose were not detected in the water. The effect of these types of "white water" on the characteristics of TMP was studied by addition of different amounts to cationised TMP fines prior to sheet making. Fines were chosen as a model system because they are probably more susceptible to adsorption of "DCS" than coarser fractions, due to higher specific surface area. It was shown that water containing colloidal particles decreased the tensile strength of handsheets and that this decrease becomes more pronounced if the addition of colloial particles is increased. It is suggested that this decrease is linked to the amount of lipophilic extractives present on the surface of the fines and it is shown that the decrease begins to occur at a surface coverage of about 45%, as calculated from ESCA data. The tensile index was not affected by addtion of "DS" under the experimental conditions used. The light absorption coefficent of the sheets increased upon addition of all water fractions, which indicates that the dissolved substances are an important reason for the observed darker colour.