3. Results and discussion
3.7. Pulp strength
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.
lower refining energy input, suggesting that a higher charge content improves the refining process, this is in agreement with literature (Laine et al. 1997).
Figure 35: Tensile index as a function of structural density for kraft-cooked and oxygen-delignified pulps with kappa number (a) 25 and (b) 30.
Without refining, the oxygen-delignified pulps with higher amount of charges obtained higher sheet density and higher mechanical properties than kraft-cooked pulps and oxygen-delignified pulps with low charge. It is noteworthy that the oxygen- delignified pulps had much higher curl index (11 to 14 %) than the kraft-cooked pulps (around 8 %), and yet were slightly stronger - Figure 33 and Figure 35a. This finding contradicts previous claims that oxygen delignification decreases the pulp strength (Allison et al. 1998, Rauvanto and Henricson 2009, Joutsimo and Giacomozzi 2015).
Figure 36 compares the WRV of kraft-cooked and oxygen-delignified pulps with different fiber charges during refining. The data shows that the WRV increases proportionally with the fiber charge and increased refining. There is also a noticeable trend in that those pulps with the greatest charge showed the greatest increase in WRV with increased refining; The WRV of the pulps with the highest fiber charge increased from 1.8 g/g to 2 g/g, while the pulp with the lowest charge increased from 1.6 g/g to 1.7 g/g after 4000-PFI revolutions.
Figure 36: Water retention value as a function of the PFI-refining, for pulps with kappa number (a) 25 and (b) 30.
The total fiber charge of the pulps is given on the right-hand side of the graphs.
Figure 15 and Figure 36 correlate very well, it can therefore be concluded that for pulps with similar kappa numbers, the pulp strength is related to the fiber charge and degree of swelling. This is in agreement with other findings in the literature (Zhang et al. 1994, Laine and Stenius 1997).
However, despite previous reports of the strong relationship between fiber swelling and paper strength (Ingmanson and Thode 1959), for different kappa numbers the correlation is more complex. Figure 37a shows the development of the tensile index as a function of WRV for pulps with significantly different kappa numbers (from 50 to 17). In this data, no strong correlation between swelling and pulp strength was observed. By means of an example, pulps with tensile indices between 90 to 100 N.m/g could have WRVs ranging from 1.5 g/g to 1.9 g/g. Similar values for pulp strength and WRV were obtained for pulps with significantly different kappa numbers, like K50 and K31_O17.
Figure 37b shows the relationship between the tensile index and structural density for the same pulps as presented in Figure 37a. Pulps with higher lignin contents (K50 and K57_O45) had higher WRVs than those pulps with lower lignin contents (e.g.
K31 and K31_O17) yet they obtained much lower sheet densities after PFI-refining.
Pulps with high kappa numbers do not conform to each other as easily as pulps with lower kappa numbers, even if with higher swelling ability. Fibers with high lignin contents may require more refining to achieve good fiber conformability. According to Paavilainen (1993), Scallan and Grignon (1979) and Lindström and Carlsson (1982a) the increase in swelling affects fiber flexibility and conformability by loosening the cell wall structure and consequently facilitating the bonding between fibers, increasing their density, this was verified by the data provided in Figure 35.
Figure 37: Tensile index as a function of a) WRV and b) structural density for kraft-cooked and oxygen-delignified pulps with different kappa numbers of: 50, 45, 31, 30, 23 and 17 (see inset legends).
The increase in the tensile stiffness index with refining was very similar for kraft- cooked and oxygen-delignified pulps - Figure 38.
Figure 38: Tensile stiffness after different levels of PFI-refining as a function of structural density, for pulps with kappa number (a) 25 and (b) 30.
Refining leads to sheet densification by increasing the bonded area between fibers, which results in an increased number of activated load-bearing segments, which results in a higher tensile stiffness index of the sheet. However, there is a significant
difference in the stiffness development between the pulps K50_O25 and K46_O30_O23. The second of these pulps (K46_O30_O23) had a lower tensile stiffness value despite having a similar density. This pulp (K46_O30_O23) was the only one obtained by two oxygen delignification steps (with washing in between) and had the highest curl index of any of the oxygen-delignified pulps evaluated.
The increase in the tensile stiffness index was much lower than that of the tensile index. The stiffness index of the K50_O25 pulp increased by 6 % and only 1 % for K57_O31, versus the kraft-cooked pulps with 4000 PFI-revolutions.
Figure 39 shows the tensile energy absorption (TEA) as a function of structural density. As with the tensile index, the TEA increased for pulps with higher fiber charges and required less refining.
Figure 39: TEA index after different levels of PFI-refining as a function of structural density, for pulps with kappa number (a) 25 and (b) 30;
The relationship between strain-at-break and sheet density was similar for kraft- cooked and oxygen-delignified pulp sheets - Figure 40. The increase in density, by refining, increased the fiber-fiber contact area and therefore the number of load- bearing segments. This is even more pronounced for pulps with higher curl index, such as K46_O30_O23. Oxygen-delignified pulp sheets with the highest fiber charge achieved the same elongation at lower refining energy than kraft-cooked pulps.
Figure 40: Strain-at-break after different levels of PFI-refining as a function of structural density, for pulps with kappa number (a) 25 and (b) 30.
3.7.2. Effect of morphology
Strength is greatly influenced by fiber morphology, especially by curl index and kinks per fiber (Perez and Kallmes 1965, Page and Seth 1988). Figure 41 shows the tensile index values for pulps with different unit processes versus their curl index. The higher the curl index, the lower the tensile index - Figure 41a. The pulps shown in the graphs have different lignin contents as the pulp strength was seen to decrease with increased delignification. The pulps with the highest fiber charges and tensile indices are stronger than other pulps with similar curl indices, most probably due to their increased swelling and fiber conformability, once again standing out from the tendency line seen in Figure 41a.
The linear relation between the tensile index and curl index is even more pronounced when the results are compared between samples with similar sheet densities - Figure 41b. The density of the paper reflects the number of fiber-fiber contacts, it is therefore prudent to compare sheets with the same nomimal density. When doing so, the pulps K50_O25 and K57_O30 follow the same linear dependance on curl index as the other pulps. The decrease in tensile index with the curl increase is well reported in the literature (Page 1985, Mohlin et al. 1996, Seth 2004).
Figure 41: Tensile index as a function of the curl index for a) unrefined pulps and b) pulps with a structural density of 0.69 g/cm3. The curl indices are for fibers between 1.5 mm and 3 mm long.
Oxygen-delignified pulps had similar tensile indices as kraft-cooked pulps with similar curl indices. It has previously been reported that such reduction in pulp strength can be caused by fiber deformation and chemical modification of the fiber wall (Rauvanto and Henricson 2009). However, is important to highlight that this is not what is happening in this work. Oxygen-delignified pulps can be stronger than kraft-cooked pulps with similar degrees of deformation - Figure 41a.
In Figure 42, the tensile stiffness index in relation to curl index is analyzed. No clear relation is observable, especially for the kraft-cooked pulps - Figure 42a. As the curl index increases, the fiber length decreases, this reduces the number of load bearing segments and therefore paper stiffness. The reduction in fiber curl leads to an increase in sheet strength by increasing the load transferring efficiency of the paper
(Page and Seth 1988, Niskanen 2000, Vainio and Paulapuro 2005). This happens due to segment activation; curly fibers cannot carry load, while straight fibers can.
Figure 42: Tensile stiffness index as a function of the curl index for a) unrefined pulps and b) pulps with a structural density of 0.69 g/cm3. The curl indices are for fibers between 1.5 mm and 3 mm long.
Lastly, the strain-at-break was analyzed as a function of the curl index - Figure 43.
Unlike the tensile index and tensile stiffness index, increased curling increases the strain-at-break, as curled fibers have the ability to straighten when stretched rather than breaking (Seth 2005) - Figure 43a. An almost linear relationship is seen for unrefined pulps, except for kraft-cooked pulp where no significant difference in strain-at-break is seen for different curl index.
Figure 43: Strain-at-break as a function of the curl index for a) unrefined pulps and b) pulps with a structural density of 0.69 g/cm3. The curl indices are for fibers between 1.5 mm and 3 mm long.