Evaluation of oxygen delignified fibers with high water absorbency, as a greener alternative to fully bleached fibers for tissue paper

2.1 Materials

A mixture of softwood chips from an industrial mill (80 % Spruce and 20 % Pine) was used in this study for kraft cooking and oxygen delignification. A never dried commercial fully bleached pulp (reference pulp) from the same mill was used for comparison. For future comparisons, it is important to highlight that the fully bleached pulp was produced industrially and, therefore subjected to much harsher treatments than the pulps produced in the laboratory.

2.2 Methods

2.2.2 Oxygen delignification

Oxygen delignification was carried out with 60 g of oven-dried (o. d.) pulp, placed in polyethylene bags with the required amount of NaOH, MgSO₄, and water at a consistency of 12 %. The bags were closed by heat-welding, kneaded initially by hand and then in a vibrational shaker. The pulp was then removed from the bags and transferred to pressurized teflon-coated stainless-steel autoclaves. The autoclaves were closed, pressurized with oxygen and placed in an electrically heated glycol bath, with rotation at a slight inclination. The trials were performed with single or double oxygen stages with no washing in between. The single oxygen stages were pressurized with 0.7 MPa of oxygen and the reaction was carried at 100 °C. For the double stages of oxygen delignification the pressure used in the first stage was 1 MPa and the temperature was 85 °C, while for the second stage the pressure was 0.5 MPa and 110 °C. After the oxygen delignification, the pulps were washed with deionized water and filtrated.

The conditions for each trial are illustrated in Figure 1. Pulps were denominated KX_OY, where X is the kappa number of the cooked pulp and Y is the kappa number after the oxygen delignification stage.

Figure 1:

Illustrative representation of the performed kraft cooking and oxygen delignification trials. The black arrows represent the cooking extension and the white arrows the oxygen delignification. The white single arrows are relative to the single oxygen stage, while the two white arrows overlapping are relative to the double oxygen stage. The samples are denominated KX_OY, where X is the kappa number of the cooked pulp and Y the kappa number after oxygen delignification.

2.3 Analysis of pulp and paper properties

Table 1 and 2 present the different pulp and paper characterization standards.

Measuring tissue properties usually depends on several factors that are often poorly controlled such as the sample size, the number of plies, among others. In the present study, the papers were produced and evaluated in the same conditions: lab tissue handsheets of 20 g/m² for the commercial fully bleached pulp and for the oxygen delignified pulps.

3.1 Delignification

The cooking trials were aimed to obtain three different kappa numbers: a high kappa number (109), a medium-high kappa (61) and a low kappa number (29) – Table 3. The kappa numbers aimed for are based on results from previous studies (Esteves 2022; Esteves et al. 2021a; Mai 2021) where it was seen that the higher the starting kappa number is after cooking the higher will be the fiber charge increase. Having kraft-cooked pulps between kappa 109 and 29 will also allow us to evaluate the influence of the lignin content on the final absorption and strength properties in connection with the fiber charge.

The pulp with the highest kappa number (K109) was defibrated in a Sprout Waldron refiner before the oxygen delignification. The oxygen trials were aimed to have extended delignification (more than 60 min) with a high alkali charge (more than 3 % NaOH) and an end pH between 10 and 11.5. The goal was to achieve the highest possible total fiber charge. However, for the lower cooked kappa number (K29) the end pH was higher than it was aimed at since the alkali charge was too high for the delignification efficiency.

3.2 Total fiber charge

It is known that the carboxylic acid groups are the main charged groups found in the chemical pulp fibers (Dang et al. 2006; Zhang et al. 2005). However, the charges suffer modifications during the different pulping processes that can eventually lead to their decline. Figure 2 illustrates the different behaviors that occur during Kraft cooking, oxygen delignification, and bleaching.

Figure 2:

Total fiber charge for non refined pulps at different kappa numbers after kraft cooking (black line) and oxygen delignification (green, orange and blue lines) at different alkali charges, given as % NaOH in the figure. 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 figure (black square).

The linear decrease in the charged groups as the lignin content is reduced during kraft cooking is clear (Figure 2) and it is well documented (Buchert et al. 1997; Chai et al. 2003; Dang et al. 2006; Esteves 2022). This is caused by the lignin removal during the delignification and by some xylan dissolution (Buchert et al. 1997; Chai et al. 2003; Dang et al. 2006; Esteves et al. 2021a) since they are the main components where the charged groups are present.

For oxygen delignification, the total fiber charge development is not straightforward, but the increase in total fiber charge when compared to the kraft cooked pulps for the same lignin content is evident in the higher kappa numbers. The higher values for the charge content were obtained for the pulp with the highest kappa number from the cooking, which was expected. This is mainly due to the oxidation reactions in lignin that lead to a higher content of new charged groups on the pulp due to the higher lignin content. These groups are largely from the muconic acid structures created in lignin (Dang et al. 2006; Snowman et al. 1999; Yang et al. 2003), but 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 fiber charges was almost insignificant after oxygen delignification. This was not a surprise since the lignin content was reduced significantly during the kraft cooking step, and the potential to maximize the charges was reduced.

Previous studies have reported that the fiber charge increase from oxygen delignification will be dependent on several factors, such as initial lignin content, alkali charge applied, delignification degree and the extension of the reaction (Esteves 2022; Esteves et al. 2021a; Zhang et al. 2006). From Figure 2 the variation in those parameters can be observed. Pulps with different starting kappa number (109, 61 and 29) will have different final charge content after extended oxygen delignification depending on the conditions in the oxygen stage (alkali charge and reaction time).

Table 4 presents the final total fiber charge obtained after oxygen delignification and the increased amount of total fiber charge if the pulp would be only cooked to the same kappa numbers, calculated by equation (1) and (2).

Previous studies (Esteves 2022; Esteves et al. 2020) have shown that a minimum increase (>60 %) in the fiber charge is vital for the change in pulp properties. From Table 4 the samples that presented the highest fiber charge increase were selected and considered for further studies (refining and paper testing). The samples from the cooked pulp with kappa number 29 didn’t have a significant increase in the fiber charge, achieving only 23 % increase, nevertheless, the sample was also considered for further trials to evaluate the influence of the lignin content on the final properties.

3.3 Water retention value

The water retention value (WRV) was significantly higher for the oxygen delignified pulps compared to the commercial fully bleached pulp (reference) – Figure 3A. This behavior was already expected since the oxygen delignified fibers presented a higher total fiber charge (higher content of carboxylic acid groups) which leads to an increase in fiber 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 3:

Water retention value for unbleached oxygen delignified pulps and fully bleached pulp as a function of A) PFI-refining, B) kappa number and C) total fiber charge.

Figure 3B shows the water retention value correlation along the kappa number when pulps are unrefined or refined with 4000 revolutions. As expected, the WRV increases with refining and decreases with kappa number. A similar relation can be seen in Figure 3C where the WRV is plotted against the total fiber charge. As previously suggested, WRV is highly dependent on the total fiber charge (Esteves et al. 2021b).

The significant increase in the water retention value is very beneficial to the absorption and strength properties of the pulps, however, it might impact the paper drying section, leading to higher energy requirements. However, the increase in the water retention value also leads to a decrease in the refining required to a certain tensile strength, which in turn will lead to lower energy consumption.

3.4 Absorption and capillarity properties

To evaluate the absorption and capillarity properties of the studied pulps, the basket method and Klemm test were used, respectively. The absorption capacity can be defined as a measurement of the fiber ability to absorb water (g water/g fiber) until it gets saturated (Schuchard and Berg 1991). The absorption quantification also considers the fiber capacity of holding water, since there is a dewatering period before measuring the wet weight. The absorption capacity for the analyzed samples is presented in Figure 4. As expected, the absorption capacity is directly related to the paper bulk despite the large span in kappa number, ranging from 0 (REF) to 47.

Figure 4:

Absorption capacity for unbleached oxygen delignified pulps and fully bleached pulp as a function of bulk (20 g/m² laboratory tissue handsheets were used). Decrease in bulk for each pulp sample is obtained by increased refining.

The commercial fully bleached pulp and the pulps K61_O and K61_OO presented very similar results for the absorption capacity (Figure 4) while the oxygen delignified pulps with the highest kappa number (K109_O and K109_OO) and the lowest (K29_O10) after the cooking step presented lower absorption capacity.

Absorbency can be significantly increased through the increase in the product bulk (Sedin and Vomhoff 2017). Bulk is therefore an important property of tissue paper, as it regulates the paper absorption, paper network porosity and softness sensation. The oxygen delignified pulps (K61_O25 and K61_OO18) presented similar absorption capacity as the reference pulp with slightly higher bulk. The oxygen delignified pulps K109_O47, K109_OO35 and K29_O10 presented a much lower bulk when compared to the other pulps and, consequently lower absorption capacity. For the unrefined samples, K109_O47 and K29_O10, the bulk and absorption capacity are very similar, even though the lignin content is significantly different.

Contrary to what happens with the water retention value (Figure 3A), when refining is applied the water absorption capacity of the pulps decreases – Figure 4. While the water retention value is mainly related to the water absorption in the fiber wall, the absorption capacity is related to the water absorption within the fiber network. The external and internal fibrillation created by the refining leads to a greater swelling potential by increasing the water penetration between the fibrils seen as water retention value (Hartman 1985; Kang 2007), but simultaneously, the increase in fibrillation also leads to better fiber bonding which promotes a denser paper structure, leading to a lower absorption values (Lumiainen 2000).

Figure 5 shows the capillary rise behavior of the different pulps obtained by the Klemm method. Similar to the results in Figure 4, the pulps K61_O25 and K61_OO18 presented the most similar results when compared to the reference fully bleached pulp.

Figure 5:

Water capillarity rise distance for unbleached oxygen delignified pulps and fully bleached pulp as a function of bulk (20 g/m² laboratory tissue handsheets were used). Decrease in bulk for each pulp sample is obtained by increased refining.

Capillary rise depends on the cohesion and adhesion forces of the water molecules that will rise along the fiber walls or within the pores (Morais and Curto 2022). The size of those pores will define the capillary pressure inside the fibers and the fiber network, determining how fast the water will rise. Micropores are present in native fibers, but during pulping the lignin–hemicellulose matrix is gradually removed creating some pores in the cell wall, called macropores (Brännvall et al. 2021). The micro and macropores will influence the water absorption capacity of the fibers, once they influence the fiber capillary pressure (Beuther et al. 2010). Bigger pores tend to decrease the capillary forces and the holding capacity (Joutsimo and Asikainen 2013), while smaller pores are capable of retaining more liquid at higher pressures than larger pores due to greater capillary force (Brodin and Theliander 2012).

When fibers undergo refining, the capillary pressure tends to decrease, probably due to internal and external fibrillation. As fiber interactions get stronger, the sheet thickness is reduced by the increase in sheet densification, and consequently, the open voids available in the sheet in which water can be held will be reduced (Kullander 2012). The decrease in the capillary pressure with the increase in fibrillation was suggested before (Morais et al. 2021). The higher the bulk, the higher the volume of pores inside the paper network.

Figure 6 shows the different speeds in the capillary rise for the studied pulps. For unrefined pulps, the oxygen delignified K29_O10 and K61_O25 had the fastest water rise (Figure 6A). The pulps with slower water rising were the oxygen delignified pulps with higher kappa numbers, K109_O47 and K109_OO35. The faster capillarity rise seen for the K61_O and K61_OO is probably due to the more open structure of the paper network since both pulp presented the highest bulk.

Figure 6:

Rate of water capillarity rise for unbleached oxygen delignified pulps and fully bleached pulp for A) unrefined and B) 4000 PFI-refined pulps. (20 g/m² laboratory tissue handsheets were used).

When refining is applied, the fastest water rise is for the commercial fully bleached pulp, while the oxygen delignified pulps land in lower water rising, especially for the K29_O10.

3.5 Mechanical properties

3.5.1 Dry tensile properties

The reference fully bleached pulp presented a much lower tensile index than the unbleached oxygen delignified pulps – Figure 7A. The difference in the tensile index is even more significant for unrefined pulps. This is caused by the different chemistry of the fully bleached and unbleached fibers and the harsher industrial treatment that the fully bleached pulp was subjected to. The oxygen delignified pulps obtained a higher tensile index. All the unrefined oxygen delignified pulps, obtained similar tensile index (between 74 and 83 Nm/g) and higher bulk (1.55–1.45 cm³/g) when compared to the reference fully bleached pulp refined with 2000 PFI-revolutions (72 Nm/g and 80 g/cm³). For higher refining levels, the unbleached pulps continued to present higher tensile index values than the reference pulp. The results showed that oxygen delignification pulps obtain similar absorption properties with much higher tensile strength than fully bleached pulps, with lower refining energy required.

Figure 7:

Tensile index for the reference and oxygen delignified pulps as a function of A) bulk (60 g/m² laboratory tissue handsheets were used) and B) absorption capacity (20 g/m² laboratory tissue handsheets were used). The increase in tensile strength for each pulp sample is obtained by increased refining.

The increase in paper strength was surely achieved by the increase in internal and external fibrillation in the refining step. Refining breaks some inner bonds in the fibers wall, making them more flexible and swellable, leading to better fiber bonding (Gharehkhani et al. 2015; Kullander 2012; Maloney and Paulapuro 1999). However, for the fully bleached pulps, the increase in the tensile index seems to be more influenced by the increase of sheet densification due to refining, rather than the increase in the fiber bonding strength. Oxygen delignified pulps presented higher strength for similar bulk compared to the reference pulp. This suggests that a less bonded area is needed to accomplish a certain strength. Nordström (2014) and Esteves et al. (2021b) have previously reported similar results, where pulps with higher lignin content obtained a higher tensile index to a given low density.

From Figure 7B, it is interesting to highlight that the oxygen delignified pulps have a significantly higher strength for similar absorption capacity. As seen for the other properties, the samples K61_O25 and K61_OO18 presented the best strength and absorption performance, as shown in Figure 4.

3.5.2 Wet tensile properties

Cellulose-based materials such as tissue products are highly sensitive to humidity since cellulose molecules hold a large number of hydroxyl groups. Paper is, therefore, a hydrophilic material; when in contact with water or moisture, it starts to swell, breaking down the hydrogen bonds and losing its structure. Therefore, the wet strength becomes critical for tissue papers exposed to water and moisture.

Wet tensile strength is one of the important properties of tissue products since they should be resistant and maintain their structure when wetted. Figure 8 presents the wet tensile index for the oxygen delignified pulps and the reference fully bleached pulp. The wet tensile index for the reference unrefined pulp was not possible to determine due to the lack of wet strength since no wet strength additives were used in this study.

Figure 8:

Wet tensile index for the reference and oxygen delignified pulps as a function of A) bulk, B) water retention value and C) absorption capacity. (20 g/m² laboratory tissue handsheets were used).

The reference fully bleached pulp after 2000 and 4000 PFI revolutions did not present an increase in the wet tensile strength (Figure 8A). The unbleached pulps, on the other hand, had an almost linear increase of the wet tensile index with the refining, obtaining between 200 and 400 % higher wet tensile than the reference fully bleached pulp for similar bulk values.

When the wet tensile index is seen as a function of the water retention value, the relation is almost linear for the unbleached pulps (Figure 8B). The higher the pulp swelling the higher the wet tensile index.

Figure 8C shows similar behaviour as seen in Figure 8A, since bulk and absorption capacity have a linear relationship. For similar absorption capacity (around 7 g/g), the pulps with the higher kappa number (K109_O47 and K109_OO35) present the highest wet strength (2.5 Nm/g).

The increase in fiber fibrillation and fiber bonding, that is expected from refining, seems to vanish when the paper is wetted for the commercial fully bleached pulp. The unrefined unbleached pulps presented wet tensile strength in the same range as the commercial fully bleached pulp when it was refined with 2,000 and 4,000 revolutions, with much higher bulk. The higher wet strength seen for the unbleached pulps shows a great potential for towel papers, for example, where they can still have a high absorption capacity and higher strength.

When the strength values are plotted as a function of the kappa number the relation is clear: higher lignin content in the pulps leads to higher wet tensile strength (Figure 9). Some old laboratory studies have also shown a connection between high lignin content and increased wet strength (Gunnarsson 2012). Figure 9B shows the fairly linear relation of the pulps’ relative strength and lignin content.

Figure 9:

Strength properties for the reference and oxygen delignified pulps at different levels of refining as a function of kappa number. A) Wet tensile index and B) relative strength. (20 g/m² laboratory tissue handsheets were used). The numbers on the top of the symbols in graph A) are the total fiber charge of the samples. The values are in meq/kg.

These results show the potential of using pulps with high lignin content to increase the wet strength of the tissue products. The equilibrium between wet strength and absorption capacity of unbleached tissue products can be altered by the oxygen delignification conditions and by the starting kappa number of the pulps.

3.6 Softness

Softness can be described as the human perception of something that is bulky and delicate with no sharp edges. However, it is also a quite complex phenomenon to evaluate and quantify. Softness depends on several factors, such as individual sensibility, bulk, flexibility, thickness, among others. Softness is considered one of the most essential properties of tissue products (de Assis et al. 2018).

The studied samples were evaluated using the Tissue Softness Analyzer (TSA), which is claimed to give a perception of the sample’s softness (Figure 10). The softness can be estimated by the TS7 parameter, where lower TS7 values are attributed to higher softness (de Assis et al. 2020).

Figure 10:

Softness for unbleached oxygen delignified pulps and fully bleached pulp as a function of, a) bulk, b) wet and c) dry tensile index (20 g/m² laboratory tissue handsheets were used).

The unbleached pulps (K61_O25 and K61_OO18) presented similar TS7 values for slightly higher bulk as the reference pulp when no refining was applied. The highest and lowest kappa samples also obtained a similar TS7 value as the reference but for lower bulk values. The increase in TS7 with refining was expected since the higher strength is usually associated with lower softness (Debnath et al. 2021; Scott et al. 1995). Thus, an increased strength results in impaired softness and vice versa (Figure 10B and C).

An increase in the paper strength is generally followed by a decrease in softness (Salem et al. 2023). However, it can be seen that unbleached pulps presented similar softness levels with much higher wet and dry strength (Figure 10B and C). Figure 10B shows that oxygen delignified pulps, when not refined, present higher wet strength for lower TS7 values (higher softness). When the pulps are refined, the wet strength significantly increases without varying the TS7 factor significantly (K109_O47 and K109_OO35). Quite similar behavior was seen for the dry strength – Figure 10C. The oxygen delignified pulps even showed slightly better softness at specific tensile strength than commercially bleached pulp. Unfortunately, the softness measurements through the TSA equipment come with a significantly large standard deviation for some samples, which gives some uncertainty about the softness comparison between the samples.