3. Results and discussion
3.4. Fiber swelling
In these equations, TFCOxygen is the total fiber charge of pulp after oxygen delignification to kappa number “𝑥”, and TTFCCook is the theoretical total fiber charge of pulp after kraft cooking until the kappa number (𝑥) was the same as the oxygen- delignified pulps.
Increases in the alkali charge (Figure 21a), reaction time and reduction of the kappa number (Figure 21b), were all found to increase the total fiber charge. This suggests that in order to significantly increase carboxylic acid groups content, the reaction time and alkali concentration should be increased and as great a reduction in kappa number as possible should be obtained during delignification.
Figure 21: Effect of a) alkali charge, b) time and delignification degree during oxygen delignification, on the increase in total fiber charge.
Figure 22: Schopper-Riegler degree as a function of PFI-refining for kraft-cooked (black squares) and oxygen- delignified (circles) pulps with kappa numbers between 25 and 30. The total charge (meq/kg) is provided on the right of the figure.
Schopper-Riegler degree is connected with the dewatering of the pulps and is influenced by the fiber morphology, fiber fibrillation and fines contents. The “fines content” of the different pulps can therefore be estimated from these Schopper- Riegler values. Fines tend to swell more than fibers and can also fill the open spaces in the fiber network, leading to slower dewatering (Motamedian et al. 2019). The oxygen-delignified pulps with the highest SR° (K57_O30 and K50_O25) are thought to have had more fibrillation, therefore a higher fine content composition and slower dewatering.
3.4.2. Water retention value
The correlation between the WRV and total fiber charge for unrefined pulps is shown in Figure 23a, and the relationship between the WRV and kappa number is plotted in Figure 23b. It is clear that the water retention value is more dependent on the total fiber charge rather than the lignin content, regardless of the pulping process.
Figure 23: Water retention values for unrefined pulps as a function of a) total fiber charge and b) kappa number.
In Figure 23b it can be seen that almost all the pulps exhibited a linear relationship between the WRV and kappa number. The outlying dataset, highlighted in Figure 23b, which has higher WRV values was obtained from reduced kraft cooking (kappa
number higher than 50) followed by an extended oxygen delignification. These pulps, had a greater difference in total fiber charge between the kraft-cooked pulp and the oxygen-delignified pulp, compared to similar kappa number - Figure 23b.
Even though the pulps had similar kappa numbers, those with higher fiber charge contents exhibited WRVs of approximately 1.8 g/g, while pulps with lower charge contents presented WRVs of 1.6 g/g. Lindström and Carlsson (1982b) asserted that when the total fiber charges are substantially increased, the swelling forces can overcome the restraining forces of the fiber cell wall, allowing the fibers to swell to a greater extend, which could explain this difference.
For kraft-cooked pulps, the WRV decreased from 1.7 g/g to 1.6 g/g when the pulps were delignified from kappa number 57 to 25. However, this finding contradicts that of Andreasson et al. (2003) who reported an increase in the WRV from 1.8 g/g to 2.2 g/g for pulps with similar kappa numbers (6o to 25). In these two studies the raw material used was the same (spruce and pines wood chips) but the cooking conditions were slightly different.
Unlike unrefined pulps, the WRV did not correlate with either the total fiber charge or the kappa number for refined pulps - Figure 24a and b. However, a significant increase in the WRV of oxygen-delignified pulps with high fiber charges was observed for PFI refined pulps - Figure 24.
Figure 24: Water retention values for refined pulps (4000 PFI-revolutions) as a function of a) total fiber charge and b) kappa number.
The data in Figure 24b shows that the kappa number is not a determinant factor in fiber swelling, in agreement with the data in Figure 23b. Pulps with similar kappa numbers can have significantly different water absorption values, and pulps with similar WRVs can have totally different lignin contents, regardless of the unit processes used. The kraft-cooked pulp with the highest kappa number (K57), had a WRV of 1.8 g/g, which was not significantly different to the WRVs of an oxygen- delignified pulp with kappa number of 23 and a bleached pulp with kappa number 7.
3.4.3. Fiber saturation point
Fiber saturation point (FSP) is another method of evaluating the fiber swelling. It differs from the WRV by measuring only the water inside the fiber wall (W in Figure 14). The retention of water within the fiber wall is largely influenced by the fiber wall structure, which is changed by the fiber processing methods. Figure 25a shows the FSP of cooked and oxygen-delignified pulps with kappa numbers ranging from 17 to 57. In contrast to the findings of Paper II, Stone and Scallan (1967) and Andreasson et al. (2003), in this work the FSP was not found to decrease as the degree of delignification increased. In fact, pulps with kappa values of 57 and 28 were found to possess similar FSP values. For oxygen-delignified pulps, the values highlighted in Figure 25a have similar kappa numbers and the FSP can range from 1.3 to 1.6 g/g, showing no correlation with the delignification degree nor the delignification process. However, a linear relationship between the FSP and total fiber charge was observed for oxygen-delignified pulps - Figure 25b.
Figure 25: FSP as a function of a) kappa number and b) total fiber charge, for unrefined kraft-cooked pulps (black squares) and oxygen-delignified pulps (blue and white circles).
In the following subsection of this chapter, the FSP will be evaluated in more detail, together with X-ray scattering to investigate the fiber supramolecular structure of the fibers.
3.5. Supramolecular structure - Effect of delignification