Fibre surface properties of kraft pulp : The influence of wood raw material, bleaching and storage

Sammanfattning: The aim of this thesis was to study the fibre surface properties of Birch (Betula ssp.), Eucalyptus (E. urograndis and E. globulus) and Spruce (Picea Abies) pulps bleached using two different methods: ECF (Elementary Chlorine Free) and TCF (Totally Chlorine Free). The hardwood pulps were subjected to a hot acidic stage prior to ECF bleaching in order to remove hexenuronic acid. The effect of prolonged storage (ageing) on the fibre surface properties was studied for all of the pulps. The characteristics of the fibre surfaces were determined by measuring the dynamic contact angle of single fibres (DCA) and by using Electron Spectroscopy for Chemical Analysis (ESCA) and Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). The correlation between the fibre surface properties and the sizing efficiency of the hardwood pulps were evaluated by measuring the amount of alkyl ketene dimer (AKD) needed to reach a certain water absorption level(measured as Cobb60). The birch pulp was more hydrophilic than the eucalyptus pulps, thus requiring more AKD to reach a certain water absorption value, and also had a lower DCA. The introduction of a hot acidic stage (A stage) in the ECF bleaching sequence applied to the hardwood pulps (i.e. A/D(EP)DD versus D(EP)DD) lowered, as expected, the total fibre charge due to the removal of hexenuronic acids groups (HexA). According to the DCA measurements all of the pulps became more hydrophilic after the A/D(EP)DD bleaching than after the D(EP)DD bleaching sequence. The sizing ability of the pulps, measured as the AKD demand to a certain Cobb60 value, showed that the AKD demand was more or less the same for the eucalyptus pulps regardless of the bleaching sequence used. The birch pulp, on the other hand, required somewhat less AKD in the A/D(EP)DD sequence than in the D(EP)DD. The AKD demand to a certain Cobb60 value decreased further for the bleached birch and E. globulus pulps after ageing due to an increase in the fibre surface hydrophobicity. ESCA analysis revealed an increase of surface extractives on the pulps after ageing, which was also supported by ToF-SIMS analysis. It was concluded that there was an increase in fatty acids, fatty acid salts and possibly even sterols and glycerides on the fibre surfaces due to ageing. A migration of these components from the interior of the fibre wall to the fibre surface during ageing was verified in this study. The TCF and ECF bleached softwood pulps also showed differences in their fibre surface properties that were tangible and thereby measurable. The dynamic contact angle was highest for the TCF bleached softwood fibres, i.e. these were more hydrophobic. Although changes caused by ageing were much more pronounced for the ECF fibres, all pulps became more hydrophobic upon ageing. The hydrophobicity of pulp fibres can be determined using DCA, which is a rather rapid and economically viable analysis. Using this information, a paper/board mill can base its sizing strategy on scientific findings. The ageing effects seen on the fibre surfaces in the form of the migration of extractives might be an important parameter from an industrial point of view, especially when pulps are stored and/or transported for a long time period. Nowadays it is quite common for pulps to be shipped around the world and subjected to warm and humid conditions. It is therefore probable that the properties of the pulp change in such a way that the papermaking process requires adjustment.