Effect of fiber wall chemistry on pulping processes of novel Eucalyptus hybrids

Abstract

This thesis investigates relevant issues regarding the use of Eucalyptus wood: the factors affecting energy consumption during wood defibration, with focus on fiber wall chemistry and the assessment of wood quality from a wide range of novel Brazilian Eucalyptus hybrids. The obtained results are reflected to mechanical and traditional chemical processes. Wood refining experiments were carried out in laboratory scale, with and without chemical pretreatment: alkaline peroxide mechanical pulping (APMP) and thermomechanical pulping (TMP) processes, respectively. The most important finding was the direct influence of the lignin structure on the defibration energy. Especially, the relative contents of the guaiacyl moieties in the wood lignin seem to play a crucial role. The results showed that the higher the amount of guaiacyl structures in the lignin, especially in the middle lamella between the fibers, the higher the specific energy consumption (SEC) in the APMP process. However, for the TMP process the correlation was not as clear. This is most probably due to the fact that in the APMP process the defibration takes place in the middle lamella while in the TMP process other mechanism prevails. However, in the samples with very low amount of guaiacyl structures, SEC decreased substantially also in the TMP process. A different approach was promoting fiber wall deconstruction via autohydrolysis of wood. The autohydrolysis process changed notably the mechanical properties of the Eucalyptus chips, with impressive SEC decrease. Although autohydrolysis was performed under mild temperature (120 oC), lignin structure was changed. This assumption was based on the characterization of the fiber surfaces via x-ray photoelectron spectroscopy (XPS). Results showed high coverage of the fiber surfaces with lignin, indicating that the defibration of autohydrolyzed Eucalyptus wood chips takes place along the middle lamella. This behavior differs from the traditional thermomechanical pulping, where the rupture occurs mostly in the fiber wall (especially in the S1 layer), being similar to chemimechanical processes. As a final step, traditional kraft pulping was performed, alongside with soda-anthraquinone (NaOH-AQ) pulping, with focus on the surface properties of the produced fibers, analyzed by SEM and XPS. The surface lignin content of NaOH-AQ pulp fibers was lower than that of the kraft counterpart. However, kraft pulp handsheets showed better physical and mechanical properties. XPS data (C2/C3 ratio) strongly suggested, together with the pulp bulk chemical composition that xylan is more abundant on the surface of kraft fibers, probably enhancing their mechanical properties.