5
C.V.G. Esteves: Oxygen deligni fi ed fi bers as a greener alternative for tissue paper
Figure 2: Total fi ber charge for non re fi ned pulps at di ff erent kappa numbers after kraft cooking (black line) and oxygen deligni fi cation (green, orange and blue lines) at di ff erent alkali charges, given as % NaOH in the fi gure. The oxygen trials presented in green were performed on kraft cooked pulp with an initial kappa (iKa) number of 29, while the oxygen trials presented in orange were performed on kraft cooked pulp with an initial kappa number (iKa) of 61 and the oxygen trials presented in blue were performed on kraft cooked pulp with an initial kappa number (iKa) of 109. The commercial fully bleached pulp is denominated as reference in the fi gure (black square).
Previous studies (Esteves 2022; Esteves et al. 2020) have shown that a minimum increase (>60 %) in the fi ber charge is vital for the change in pulp properties. From Table 4 the samples that presented the highest fi ber charge increase were selected and considered for further studies (re fi ning and paper testing). The samples from the cooked pulp with kappa number 29 didn ’ t have a signi fi cant increase in the fi ber charge, achieving only 23 % increase, nevertheless, the sample was also considered for further trials to evaluate the in fl uence of the lignin content on the fi nal properties.
also from oxidized carbohydrates (Tao et al. 2011; Zhang et al. 2006; Zhao et al. 2016). For the lower kappa number (K29) the increase in fi ber charges was almost insigni fi cant after oxy- gen deligni fi cation. This was not a surprise since the lignin content was reduced signi fi cantly during the kraft cooking step, and the potential to maximize the charges was reduced. Previous studies have reported that the fi ber charge increase from oxygen deligni fi cation will be dependent on several factors, such as initial lignin content, alkali charge applied, deligni fi cation degree and the extension of the re- action (Esteves 2022; Esteves et al. 2021a; Zhang et al. 2006). From Figure 2 the variation in those parameters can be observed. Pulps with di ff erent starting kappa number (109, 61 and 29) will have di ff erent fi nal charge content after extended oxygen deligni fi cation depending on the condi- tions in the oxygen stage (alkali charge and reaction time). Table 4 presents the fi nal total fi ber charge obtained after oxygen deligni fi cation and the increased amount of total fi ber charge if the pulp would be only cooked to the same kappa numbers, calculated by equation (1) and (2).
3.3 Water retention value
The water retention value (WRV) was signi fi cantly higher for the oxygen deligni fi ed pulps compared to the commercial fully bleached pulp (reference) – Figure 3A. This behavior was already expected since the oxygen deligni fi ed fi bers presented a higher total fi ber charge (higher content of carboxylic acid groups) which leads to an increase in fi ber swelling (Esteves et al. 2020; Sjöstedt et al. 2015; Zhao et al. 2016). The pulps with the highest kappa number were the ones with the highest water retention values, while the reference fully bleached pulp presented the lowest charge and the lowest WRV – Figure 3B and C. Figure 3B shows the water retention value correlation along the kappa number when pulps are unre fi ned or re fi ned with 4000 revolutions. As expected, the WRV in- creases with re fi ning and decreases with kappa number. A similar relation can be seen in Figure 3C where the WRV is plotted against the total fi ber charge. As previously sug- gested, WRV is highly dependent on the total fi ber charge (Esteves et al. 2021b). The signi fi cant increase in the water retention value is verybene fi cial to the absorption and strength properties of
Table : Fiber charge increase for the studied pulps. The samples that obtained the highest increase of fi ber charges within each kraft cooking series are highlighted in bold.
Samples
Kappa number
Total fi ber charge, meq/kg
Fiber charge in- crease,%
K _O K _O K _OO K _O K _O K _OO K _O K _O K _OO
No increase
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