Mechanical Pulp. Effects of Pretreatment and Wood Source on Fiber Separation and Pulp Properties
The work presented in this thesis concerns the mechanical pulping process and the effects of wood source and pretreatment conditions on the pulp properties. In the first part, the deformation of Norway spruce (Picea abies) wood samples was investigated under shear and torsional loading at different temperatures (20-95°C) and sulfonation conditions. The shear strength and maximum torque decreased with increased temperature. The fiber separation was found to be of the transwall type when the deformation was performed at low temperatures (20-60°C). When high temperatures were combined with sulfonation, the fracture plane was transferred to the compound middle lamella region of the fiber wall, thus exposing a smoother wood surface.
The effect of pretreating of wood chips intended for CTMP production was studied in a pilot plant equipped with a plug screw press. Sulfonation before mechanical deformation of the chips in the screw press gave a more selective initial fiber separation in the deformed chips compared with the case where chips were allowed to pass through the screw press without prior sulfonation. The long fiber content and the energy consumption required to reach comparable freeness values were higher for the pulp produced from chips that were sulfonated before mechanical deformation in the screw press. The tear strength at constant energy consumption was also higher for this pulp, whereas the tensile strength was lower.
Sulfonation of fibers, followed by polyelectrolyte adsorption analysis, was found to be an excellent method for determining surface lignin concentration. The wood raw material was shown to have an influence on the thermomechanical pulp quality: pulps from forest grown loblolly pine (Pinus taeda) (+30 years old) had a slightly lower bonded area at a given total surface area than pulps prepared from plantation grown loblolly pine (15 years old) containing a higher percentage of juvenile wood. This was consistent with the higher surface lignin concentration found for the pulps prepared from the forest grown wood.