3. Results and discussion
3.5. Supramolecular structure - Effect of delignification processes
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
Figure 26: Schematic representation of kraft cooking (black arrows) and oxygen delignification (white arrows) for the studied pulps. The length of the arrows shows the degree of delignification. Pulps are denominated KX_OY, where X is the kappa number of the kraft-cooked pulp and Y is the kappa number of the oxygen-delignified pulp.
The trial with double “O” means double oxygen delignification stage without washing in the middle. Single oxygen trials were performed at 100 °C and 7 bar but at varied alkali charge and time as detailed in the figure. For the double oxygen trial, 85 °C and 10 bar were used in the first stage, and 110 °C and 5 bar were used in the second stage.
The FSP along delignification for kraft-cooked and oxygen-delignified pulps, without refining, is shown in Figure 27a). Although the degree of delignification in the kraft cook varied greatly (from kappa number 91 to 28), the FSP of the pulps remained in the range of 1.3-1.6 g/g. Specifically, pulps with the highest (K91) and lowest (K28) kappa number had the same FSP value, 1.7 g/g.
Figure 27: a) Fiber saturation point as a function of kappa number for a) unrefined pulps and b) refined pulps;
the dashed arrows connect the original cooked pulp to the final oxygen-delignified pulps.
Interesting, Andreasson et al. (2003) showed a decrease in FSP from kappa number 110 to 60, and an increase in FSP moving from kappa number 35 to 15. Within the cited study, the fiber wall volume was similar for pulps with kappa number 60 and 35/25 despite the increase in pore radius from 13 to 25 nm. At low delignification degrees the lignin that is removed creates new pores in the fiber wall, but the mechanical strength of the fiber wall structure will restrain the increase in FSP. As lignin is further removed, the rigidity of the fiber wall is reduced and the contracting forces are overcome leading to an increase in FSP, explaining the increase in FSP from 60 to 35/35.
When continuing delignification with oxygen after kraft cooking, the FSP for K91_O53 and K52_O42 remained constant (represented by the black dashed arrows) whereas K91_OO37 increased (represented by the red dashed arrows) - Figure 27a. As discussed in Paper II, oxygen delignification introduces carboxylic acid groups, which potentially reduce the shrinking forces and maintain fiber pore structure due to the removal of chemical components.
Figure 27b shows the effect of PFI-refining on the FSP for kraft-cooked and oxygen- delignified pulps. After 4000 revolutions, all the pulps showed a significant increase in FSP, with values stabilizing near 2 g/g for kraft-cooked pulps and 2.5 g/g for oxygen-delignified pulps. Continuing refining to 6000 and 10 000 PFI-refining for K91 and K52 pulps showed no significant change in FSP suggesting that extensive refining does not affect the internal fiber structure further.
The increase in FSP caused by refining is suggested to be largely due to random breaking of the crosslinks in the interfibrillar matrix, known as internal fibrillation (Stone et al. 1968, Page 1989, Maloney and Paulapuro 1999). The breaking of the interfibrillar matrix increases the fiber pore size leading to greater water retention capability (Stone et al. 1968, Zhao et al. 2016). The increased FSP values observed for oxygen-delignified pulps is potentially due to the increased charge content of the fibers as pulps with high charge content are known to fibrillate more effectively during refining. When the internal fibrillation is completed, additional refining only serves to increase external fibrillation, resulting in FSP stabilization as observed here, and in previous work (Kang and Paulapuro 2006, Gharehkhani et al. 2015).
Figure 28 shows the crystallite size in kraft-cooked and oxygen-delignified pulps as measured by wide-angle X-ray scattering (WAXS). Crystallite size appears to be unaffected by delignification, as all pulps showed a value of 5 nm, regardless of the process (kraft cook vs oxygen delignification).
Figure 28: Crystallite size for unrefined pulps subjected to a kraft cook and oxygen delignification as a function of kappa number.
The crystallite size can be related to the dimensions of the inner crystalline component of the cellulose fibril and correlates with the lateral fibril dimensions (Brännvall et al. 2021). Removal of hemicellulose and lignin has been reported to
increase the average crystallite width from 3.5 nm to 5.0 nm (Driemeier et al. 2011).
In previous studies, crystallite sizes of 2.5 to 3.5 nm were reported in wood chips subjected to either acidic or alkaline pretreatment which removed only a minor part of the lignin (Brännvall et al. 2021). This suggests that the crystallite size rapidly increases at the onset of delignification from which it reaches a plateau of 5 nm.
Moreover, Figure 28 indicates that the internal structure of the cellulose elementary fibrils remains unchanged after each unit process.
Table 4 shows the values of the cavities and particles obtained by the modeling method developed by Larsson et al. (2022). The intensities obtained from SAXS were divided into three different components, I1, I2 and I3. These components are based on the electron density path difference, where: I1 represents the abundance of larger structures, I2 intermediate and I3 the smaller structures. The structures represent both cavities (pores in the fiber wall), and particles (solid structures). The different component's intensities were simulated using FSP values as the volumetric fill factor.
Table 4: Cavities and particles sizes for the three simulated superposition components (I1, I2 and I3) used to model the recorded SAXS data for unrefined a) kraft-cooked and b) oxygen-delignified pulps. The components represent the abundance of larger (I1), intermediate (I2) and smaller (I3) structural features in the pulps.
AACS (nm) Stda. Dev. AAPS (nm) Stda. Dev.
K91
I1 31 0.4 12 0.2
I2 10 0.1 4 0.1
I3 5 0.1 2 0.0
K52
I1 30 0.4 15 0.2
I2 8 0.1 4 0.1
I3 4 0.1 2 0.0
K31
I1 35 0.4 18 0.2
I2 8 0.1 4 0.1
I3 4 0.1 2 0.0
K28
I1 39 0.5 15 0.2
I2 9 0.1 4 0.1
I3 5 0.1 2 0.0
K28repetition
I1 32 0.4 14 0.2
I2 8 0.1 4 0.1
I3 4 0.1 2 0.0
K91_O53
I1 32 0.4 13 0.2
I2 10 0.2 4 0.1
I3 5 0.1 2 0.0
K52_O42
I1 29 0.4 14 0.2
I2 8 0.1 4 0.1
I3 4 0.1 2 0.1
K91_OO37
I1 40 0.5 13 0.2
I2 11 0.2 4 0.1
I3 6 0.1 2 0.0
The apparent average cavity size (AACS) is used to describe the water-filled interstitial spaces between solid particles. The intensity component I1 for cavities was in the size range of 29 to 40 nm, which correlates well with the size of pores in the fiber wall. Forsström et al. (2005a) reported pore diameter of 30-35 nm in pulps with kappa numbers ranging from 110 to 16.
The intensity components I2 and I3 corresponding to cavity size, ranged from 8-11 nm and 4-6 nm, respectively. The pulps with the highest I1 size also had the highest I2 and I3 size. Correspondingly, pulps with the smallest I1 values also had the smallest I2 and I3 component sizes. Of the studied pulps, the I2 and I3 AACS values remained relatively constant, and only the I1 values varied (30 - 40nm). Kimura et al. (2014) used BET analysis on softwood fibers and reported a range of pore sizes near 10–20 nm in diameter and also mesopores with diameters of 4–5 nm. These two size ranges fit well with the size ranges of I2 and I3 components, respectively.
The apparent average particle size (AAPS) describes the size of the different solid structures within the fiber wall, as determined by X-ray scattering. The intensity component I1 ranged from 12 - 18 nm, which is approximately one half of the fibril aggregate sizes obtained by CP/MAS 13C-NMR measurements (Larsson et al. 2022).
The size of AAPS related to intensity component I2 (4 nm) is similar to the size of elemental fibrils measured by CP/MAS 13C-NMR and the lateral dimensions measured by WAXS (Brännvall et al. 2021). The intensity component I3 was 2.0 nm.
Unlike the AACS (cavity size), no significant difference in the AAPS (particle size) was seen the pulps investigated.
Figure 29 shows the relative weight intensities of each component, for kraft and oxygen-delignified pulps. Since the AAPS values are more or less constant between the pulps, the difference seen in the relative weight intensities is related to the changes in cavity size. The weight intensity of smaller structures (I3) seems to dominate both in kraft-cooked and oxygen-delignified pulps with no clear correlation with the degree of delignification.
Figure 29: Relative weights intensities for the three modelled intensity components (I1, I2 and I3) used to model the recorded SAXS data for unrefined a) kraft-cooked pulps and b) oxygen-delignified pulps. The components represent the abundance of larger (I1), intermediate (I2) and smaller (I3) structural features in the pulps.
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.