3. Results and discussion
3.6. Fiber morphology
Between each delignification process, it appears that the weight intensities do not significantly change following oxygen delignification (highlighted with dashed lines).
For example, both pulps K91 and K91_O53, had weighted intensities of 2 %, 27 % and 71 % for I1, I2 and I3, respectively, despite the latter undergoing an additional oxygen delignification to lower kappa number. This suggests that changes to the supramolecular structure largely occur during kraft delignification and not during the oxygen delignification step.
The weighted intensities of the components of kraft-cooked pulps that have undergone different degrees of refining are presented in Figure 30. Refining increases the weight intensity of both the largest (I1) and smallest (I3) structures, while the intermediate (I2) structures decrease. This suggests that refining can break apart the aggregated particles into smaller structures.
Figure 30: Relative weight intensities for the three modelled intensity components (I1, I2 and I3) used to model the recorded SAXS data for PFI-refining a) K52 and b) K91. The components represent the abundance of larger (I1), intermediate (I2) and smaller (I3) structural features in the pulps.
(from 9 % to 14 %) and number of kinks (from 0.27 to 0.8 kinks per fiber) when the delignification goes from kappa number 50 to 15 - Figure 31b.
The highest curl index and number of kinks per fibers were observed for the bleached pulps, which was to be expected since they had the lowest kappa numbers. These results are in a good agreement with previous studies (Page et al. 1985, Mohlin and Alfredsson 1990, Yang et al. 2003).
Figure 31: a) Curl index and b) number of kinks, for unrefined pulp fibers after kraft cooking (black squares), oxygen delignification (blue circles) and bleaching (asterix) at different kappa numbers. The curl values are for fibers with lengths between 1.5 and 3 mm.
During delignification, a significant amount of material is removed from the fiber wall, making the fibers more susceptible to deformation. This susceptibility of the fibers to curling and kinking, therefore, increases with the degree of delignification, since more material is removed. However, a significant difference is seen between kraft-cooked pulps and the pulps subjected to oxygen delignification and bleaching.
Multiple pulps delignified to the same degree by oxygen delignification will have vastly different degrees of deformation, whereas kraft pulps have similar deformation values. This suggests that, during kraft delignification, the wood matrix helps to prevent fiber deformation, due to the support of the stiff chip structure and neighboring fibers. In oxygen delignification, the fibers are already separated and therefore more susceptible to deformation during delignification. This is discussed in Paper II in more detail.
In Figure 32, the width of unrefined pulps after different unit processes are shown along with the kappa number. The fiber width decreased with increasing degrees of delignification for all the unit processes evaluated, suggesting that the width of the fibers is affected by the delignification degree and independent of the unit process used. Bleached pulps exhibited the lowest width, but also the lowest kappa number.
This can be correlated with the amount of material removed during the delignification. Scallan and Green (2007) also reported a decrease in fiber width with degree of delignification for kraft-cooked pulps. They explained the reduction in width by the inward contraction of the fiber wall as lignin and hemicellulose removal leads to the shrinking of the fiber walls.
Figure 32: Mean fiber width for unrefined pulps fibers with a length of 1.5 and 3mm after kraft cooking (black squares), oxygen delignification (white circles) and bleaching (asterixis) at different kappa numbers.
3.6.2. Effect of Refining
The mechanical forces of the refining process lead to the modification of the fiber wall structure. Although PFI-refining does not completely reproduce the mechanical action that the fibers are subjected industrially, it is a homogeneous treatment that gives important information about the behavior and the morphology of the fibers.
In Figure 33, the effect of refining on fiber curl index is shown for the three different unit processes evaluated. The curl index of unrefined kraft cooked pulps was seen to increase from 8 % to 11 % after refining. These findings are in agreement with those of Nordström (2015) who observed the same trend when refining unbleached pulps.
However, PFI-refining is known to reduce the number of fiber deformations, like curl and kinks (Page 1985, Mohlin and Alfredsson 1990, Mohlin et al. 1996). Unrefined oxygen-delignified pulps had a higher curl index than the corresponding kraft cooked pulps prior to refining. During refining however, the number of fiber deformations decreased until they reached the same level as the unrefined kraft- cooked pulps - Figure 33a and b. Finally, bleached pulps had the highest initial curl index, 16-18 %, which the refining decrease to 13 – 14 % - Figure 33c, the greatest decrease of any of the unit processes evaluated.
Figure 33: Curl index at different levels of PFI-refining, for pulps after a) kraft cooking, b) oxygen delignification and c) bleaching.
The differences in the curl index between the kraft-cooked and oxygen-delignified pulps disappeared after 4000-PFI revolutions. However, for bleached pulps the curl index was significantly higher than the other two even after 4000-PFI revolutions.
The refining effect on the number of kinks per fibers is very similar to the curl index - Figure 34. Kraft-cooked pulps had the fewest kinks per fiber and refining had very little impact. For unrefined oxygen-delignified pulps and bleached pulps the kinks are much higher when compared to the kraft-cooked pulps. For these pulps the refining had a massive impact, decreasing the number of kinks from 0.7 to 0.2 and 0.8 to 0.5 for oxygen and bleached pulps, respectively.
Figure 34: Number of kinks per fiber at different levels of PFI-refining, for pulps after a) kraft cooking, b) oxygen delignification and c) bleaching.