Moreover, the tensile index of the plates increased when the fiber loading after oxygen delignification was sufficiently high. The effects of high alkali impregnation and oxygen delignification of softwood kraft pulp on yield and mechanical properties.
RESULTS AND DISCUSSION 25
Introduction
- Unit processes
- Fiber and pulp properties
- Future of oxygen delignification
- Thesis aim and objectives
Both HexA and MeGlcA contribute to the fiber charge in the pulp when it is deprotonated (Dang et al. 2006). The increase in fiber charge content after delignification by oxygen can be achieved partly by oxidation of residual lignin (Figure 5) retained in the fibers, but also by the formation of aldonic acids in the carbohydrates during the process (Figure 7) (Sjöström 1993) . , Lucia et al. 2002).
Experimental
Materials and chemicals
Screened and hand-picked softwood chips from BillerudKorsnäs Skärblacka Mølle (a mixture of 70% spruce (Picea abies) and 30% pine (Pinus sylvestris)) were used in this study. The white liquor for the boiling experiments was prepared from stock solutions of sodium hydroxide (NaOH) and hydrogen sulfide (Na2S ) to achieve the desired effective alkali charge and sulphidity in the forced boiling For NaOH, lozenges of puriss grade NaOH (VWR International AB, Radnor, PA, USA) were dissolved in deionized water to a concentration of approximately 10 M.
Technical grade Na2S flakes (VWR International AB) were dissolved in deionized water to a concentration of approximately 1.5 M NaSH and 1.3 M NaOH. The raw materials and main topics studied in the different manuscripts are summarized in Table 1.
Methods
The alkali concentration in the cooking stage was adjusted to 0.5 M by dilution with deionized water and the temperature was raised to 160 °C at a rate of 3 °C/min. The bags were heat-sealed, first kneaded by hand, and then placed in a vibrating ink-shaker for uniform mixing of the chemicals. The autoclaves were sealed, pressurized with 7 bar O2 and placed in a steam-heated glycol bath at 100 °C, with rotation and a gentle incline.
The bleaching sequence used was OQ(Op)DED and OQ(Op)PP for both standard and high alkali impregnation boiling experiments. All the bleach trials and chelating phases (Q) were carried out at a consistency of 10 % in strong polyethylene plastic bags, double sealed and kneaded in a vibrating paint shaker for uniform mixing.
Analysis
Physical and mechanical characterization of laboratory hand sheets The laboratory hand sheets were characterized by gram weight (ISO 536), structural thickness (SCAN-P 88:01) and mechanical properties, including tensile index, stiffness index and strain-at-break (ISO 1924-3). The fiber morphology was evaluated in a Lorentzen & Wettre (L&W) Fiber Tester, where the fibers in an aqueous suspension are transported by a strong flow, sufficient to orient them in two dimensions without causing deformations. A digital imaging system collects and analyzes the images taken of the fibers and the physical parameters such as fiber length, fiber width, form factor (defined as the straightness of the fiber) and number of kinks are calculated from the software.
The curl index was calculated based on the shape factor, related to the highest percentage of fibers with lengths between 1.5 and 3 mm, according to Page et al.
Results and discussion
Effect of oxygen delignification on the tensile index The aim of this project was to understand the effect of oxygen delignification on the
The higher the starting kappa number, the higher the amount of charged groups after oxygen delignification. Pulp K57_O30 with 91% more charged groups than power boiled pulp had a significantly better increase in tensile strength than pulp with fewer charged groups. On the other hand, the tensile strength of K46_O30, with almost 50% more charged groups than K31, achieved the same tensile strength as boiled pulp (K31).
It is therefore possible to increase tensile strength by oxygen delignification; but only by extensive lignification, so that a large number of charged groups are obtained. A reduced kraft boiling is therefore required to initiate the extended oxygen delignification with a sufficiently high number of charged groups (high kappa number).
Yield
This difference in xylan content between the REF and HAI pulps remains in the oxygen-delignified pulps. Previous reports are consistent with our results and show that xylan is more soluble under highly alkaline conditions than in REF impregnation (Li Jansson and Brännvall 2011). A high alkali concentration in the impregnation phase has previously been reported to have positive effects on the glucomannan yield (Paananen et al. 2010, Paananen et al. 2013, Brännvall 2018).
It is also worth noting that the pulp yield was slightly higher in HAI pulps, but not significantly. Pulps produced with HAI in kraft cooking achieved a 2% higher yield after sequential bleaching with chlorine dioxide (DED) than identically bleached REF pulps.
Fiber charge
Oxygen delignification increases the total fiber loading of pulp compared to pulp obtained by kraft delignification only (when pulps with similar kappa numbers are compared). This increase in fiber loading achieved by oxygen delignification is well reported in the literature (Laine 1997, Zhang et al. 2005, Zhang et al. 2006). In bleached pulps, the amount of residual lignin is almost negligible, and therefore the residual charge can be assumed to originate exclusively from carboxylic acid groups (Zhang et al. 2006).
In the work published by Zhang et al. 2006) the highest fiber loading from oxygen delignification was achieved with an alkali loading of 2.5 %, but the pulp used had a much lower initial kappa number of 32. Increases in the alkali load (Figure 21a), reaction time and reduction of the kappa number (Figure 21b) were all found to increase the total fiber load.
Fiber swelling
The relationship between WRV and total fiber load for unrefined bricks is shown in Figure 23a, and the relationship between WRV and kappa number is shown in Figure 23b. In Figure 23b it can be seen that almost all pulps exhibit a linear relationship between WRV and kappa number. These pulps had a greater difference in total fiber loading between kraft cooked pulp and oxygen delignified pulp compared to similar kappa number - Figure 23b.
For kraft cooked pulp, WRV decreased from 1.7 g/g to 1.6 g/g when pulps were delignified from a kappa number of 57 to 25. In contrast to unrefined pulp, WRV did not correlate with either total fiber charge or kappa number for refined pulp - figures 24a and b.
Supramolecular structure - Effect of delignification processes
In the next 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. Although the degree of delignification in the kraft coke varied greatly (from kappa number 91 to 28), the FSP of the pulp remained in the range of 1.3-1.6 g/g. The breaking of the interfibrillar matrix increases the fiber pore size leading to greater water retention capacity (Stone et al. 1968, Zhao et al. 2016).
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). The apparent average particle size (APS) describes the size of the various solid structures within the fiber wall, as determined by X-ray scattering.
Fiber morphology
The weighted intensities of the components of kraft boiled pulp that have undergone varying degrees of refining are shown in Figure 30. This susceptibility of the fibers to curling and kinking therefore increases with the degree of deflection as more material is removed. Fiber width decreased with increasing degree of offset for all unit processes evaluated, suggesting that fiber width is influenced by degree of offset and is independent of the unit process used.
The mechanical forces of the refining process lead to the change of the fiber wall structure. However, during refining, 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 decreased with refining to Figure 33c, the largest decrease of all unit processes evaluated.
Pulp strength
Without refining, oxygen-delignified pulps with higher loadings obtained higher sheet density and higher mechanical properties than kraft-cooked pulps and low-loading oxygen-delignified pulps. Similar values for pulp strength and WRV were obtained for pulps with significantly different kappa numbers, such as K50 and K31_O17. As with the pull index, TEA increased for pulps with higher fiber loadings and required less refining.
Oxygen-delignified pulps had similar tensile indices to power-boiled pulps with similar curl indices. Oxygen-delignified pulps can be stronger than power-boiled pulps with similar degrees of deformation - Figure 41a.
Concluding remarks
The hypothesis that fibrillation and fiber flexibility are improved by the increase in fiber swelling volume is also supported by previous studies (Scallan 1983, Page 1985, Laine and Stenius 1997, Mohlin 2002, Bäckström et al. 2013). If the delignification degree, time and alkali load are not sufficiently high in the oxygen-delignification process, the charges are aggregated and are not distributed uniformly in the fiber wall, Figure 44c, which negatively affects the fiber swelling and the bonding potential by having lower fibrillation - Figure 44d. In summary, to significantly increase the mechanical strength of the pulp, the increase in the total fiber loading should be at least 80 % higher than the kraft-boiled pulp.
This provides a homogeneous distribution of the carboxylic acid groups in the fibers, resulting in more pronounced swelling, more fibrillation, greater fiber flexibility, increased fiber adaptability, a larger bonding area and ultimately improved strength. The increase in each of these characteristics is illustrated by: ●●● corresponds to the highest, ●● to an intermediate and ● to the lowest increase.
Conclusions
Acknowledgments
1995) Effect of cooking and bleaching on xylan structure in conventional pine kraft pulp. Effect of surface chemical composition and loading on fiber and paper properties of unbleached and bleached kraft pulps. 2011) Effect of kraft cooking conditions on surface and bulk chemical composition of spruce fibers.
1982a) The effect of carboxyl groups and their ionic form during drying on the hornification of cellulose fibers. 1996) Fiber deformation and plate strength. 2019) Mechanisms of paper strength and stiffness improvement after PFI refining with focus on the effect of fines.
