Introduction

In fiber-based packaging industry, having a cellulosic furnish that provides certain strength properties while maintaining cost and bulk has always been a critical issue. Bulk of a cellulose sheet matters because it can affect end product stiffness. Mechanical refining is a conventional method to enhance bonding within a paper. Refining fibrillates fibers, increases surface area and thus increases inter-fiber bonding. However, refining typically results in slower dewatering rates during the paper-forming process and a denser final product [1,2,3,4]. Wet-end addition of micro-nanofibrillated cellulose (MNFC) is another way that has been studied in the last decade to increase bonding within a paper. Though MNFC addition to the papermaking furnishes increases paper strength, it also tends to cause production problems like slow dewatering rate and high shrinkage due to micro-nanodimension of the fibrils [5,6,7,8,9]. Therefore, it is necessary to find ways to increase the bonding capacity of the fibers while avoiding generating excess fines and fiber fragments, slowing down the dewatering rate and losing bulk of the sheets. This can possibly be achieved by chemical pretreatments of fibers to increase swelling and facilitate mechanical defibrillation [10,11,12,13,14].

Introducing charged groups onto pulp fibers through a chemical modification is one way to enhance delamination of fiber walls. This approach increases osmotic pressure and causes the cell wall to swell. Increased swelling results in lower energy consumption for mechanical fibrillation of fibers [12, 15,16,17]. One of the most well-known chemical pretreatments is the TEMPO-mediated oxidation of cellulose fibers that increases surface charge by introducing carboxylic acid groups to C-6 position [18, 19]. Concentrated sulfuric acid hydrolysis is another technique that partially dissolves the amorphous regions in cellulose and introduces anionic sulfate groups [19,20,21]. Other chemistries have also been exploring the increase of osmotic swelling and the potential for interfiber bonding by introducing anionic or cationic groups onto the cellulose backbone [11, 17, 22,23,24,25,26,27,28,32,33,34,35,36,37]

Figure 5 shows the external fibrillation, fines content and fiber length of the pulps as a function of refining amount. It is most notable that the phosphorylation allows for the enhanced development of external fibrillation (Fig. 5a) and increases number of fibrillar fines while having limited impact on flake-like fines generation (Fig. 5b, d). The flake-like fines are largely fragments of the outer cell wall and ray cells and thus are not greatly impacted by the phosphorylation procedure. However, the phosphorylation helps increase fibril split and deaggregation, thus increasing both external fibrillation and fibrillar fines generation. Fibrillar fines are well known to have a significant impact on fiber bonding and strength properties.

Figure 5
figure 5

The physical properties of reference and phosphorylated fibers as a function of PFI refining revolutions, a fibrillation, b fines A content, c fiber length, d fines B content.

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Figure 6 shows the microscopic images of the UBSW and P-UBSW fibers before and after refining. The P-UBSW fibers are seen to balloon even before refining. In regions where the outer cell wall is weak or entirely missing, the increased osmotic pressure from phosphorylation causes the cell wall to balloon between collars. However, many of the fibers do not show ballooning at all. The enhanced swelling appears to be quite heterogeneous; concentrated on certain fibers and certain parts of the cell wall [37,38,39].

Figure 6
figure 6

Optical microscopy images of fiber, a UBSK, b P-UBSK, c 10 k rev refined UBSK, d 10 k rev refined P-UBSK.

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Sheet properties

The two different groups of sheets were characterized to investigate the potential of highly swollen pulp in paper and board making. The first group (shown by red squares in all figures) of handsheets is made of 100% UBSK pulp refined to different extents. The second group (shown by blue circles in all figures) is a blend of UBSK and 10 k rev refined RP-UBSK. Figure 7 shows the density of both sheet series.

Figure 7
figure 7

The density of sheets from a R-UBSK as a function of PFI refining revolutions, b UBSK as a function of 10 k rev RP-UBSK content.

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The density of UBSK sheets increases from 0.5 to 0.63 g cm−3 after 4 k rev of refining. While necessary to increase bond area, this is negative for producing light weight structures, such as low-density folding boxboard. An alternative approach is to add the highly swollen RP-UBSK to a network of unrefined UBSK. The unrefined UBSK has a stiff enough fiber to prevent the structure from densifying in consolidation. The RP-UBSK has high surface area and swelling and can bond the structure together. Figure 7 shows that composite sheets of UBSK/RP-UBSK have a lower density than the refined UBSK sheets alone. Furthermore, the blend has a more favorable density/modulus relationship as shown in Fig. 8. The elastic forces within a network of relatively stiff reference fibers and flexible modified fibers are sufficient to re-expand and regain some of its bulk after wet pressing. At any modulus, the blend has a lower density than the conventional refining case [2, 3, 40, 41].

Figure 8
figure 8

The mechanical strength properties of sheets from R-UBSK refined at different PFI revolutions and from UBSK containing different contents of 10 k rev RP-UBSK, a specific modulus, b tensile strength index.

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Few researchers have evaluated minimized refining and compensation with surface modification of pulp fibers. Rice et al. utilized nanofibrillated cellulose pretreated with cationic starch as a bonding system in handsheets. Pretreated NFC was particularly effective in increasing the tensile strength and stiffness of low-refined sheets. Their strategy allowed improved tensile strength at a lower apparent density (high bulk) of the handsheets which authors suggest using it as a substitute for mechanical refining of pulp fibers for specific paper grades [2]. Pettersson et al. used starch and CMC for CTMP fibers surface treatment. They reduced refining energy and produced strong and bulky paperboards [42]. Jan Eric berg et al. optimized process conditions in low-consistency (LC) refining, i.e., LC refiner filling patterns, in order to produce fibrillar fines and improve the separation of fibers from each other while preserving the natural fiber morphology as much as possible. A fine filling pattern resulted in sheets with higher tensile index at maintained bulk compared to a standard filling pattern [43].

The structure of interfiber bonds is influenced by refining or the addition of phosphorylated pulp. Light scattering coefficient can be used as an indicator of interfiber bonding area, with lower light scattering, indicating higher relative bond area. In this experiment, the light scattering decreased either by increased refining or addition of phosphorylated pulp (Fig. 9a). This means that both approaches increase fiber contact. Besides, the sheets containing 10 k rev RP-UBSK have a higher light scattering coefficient in a certain tensile strength which suggest that the phosphorylated pulps give a higher specific bond strength than the conventional refining (Fig. 9a) [41, 44]. Furthermore, the sheets containing 10 k RP-UBSK are less permeable than R-UBSK sheets, while they are bulkier and more porous (Fig. 9b). Highly fibrillated fibers and fibrillar fines fill voids between unrefined fibers, creating a low permeability closed sheet. This is consistent with fibrillation and Fines B content in Fig. 5d.

Figure 9
figure 9

The properties of sheets from R-UBSK refined at different PFI revolutions and from UBSK containing different contents of 10 k rev RP-UBSK a light scattering, b air permeability.

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In Fig. 10a, the SEM images of the UBSK sheets show intact, unfibrillated fibers. In the sheet made of R-UBSK, only partial surface fibrillation and small interfiber films are visible. For the sheet containing phosphorylated pulp, the surface is highly closed by a film. Film forming is due to presence of highly fibrillated fibers and fibrillar fines. There is a covered area by a film (red arrows) that we believe it is one relatively long microfibrillated fiber rather than individual bonded fibrils. Typically, a sealed wire-side surface is the result of fines enrichment. This is well known to have a serious negative impact on sheet dewatering. In this case, it is actually the microfibrillated fibers which contribute to sheet sealing. At the 25% concentration, used in this experiment, there is a high enough concentration of these highly fibrillated fibers to seal the wire side surface effectively.

Figure 10
figure 10

SEM images of the sheets (wire-side) made from a) reference UBSK, b 4 k rev R-UBSK, c blend of UBSK and 25% 10 k rev RP-UBSK.

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It is important that new furnish solutions not only lead to better functionality but can be manufactured efficiently. Thus, dewatering is a relevant property. In this case, the two furnish solutions are compared at equal tensile strength. The is shown in Fig. 11 for dewatering rate and couch solids. Both the water removal rate and the solids content following vacuum dewatering (couch solids) are relevant for overall water removal efficiency and paper machine speed.

Figure 11
figure 11

Dewatering properties of the sheets from R-UBSK refined at different PFI revolution and from UBSK containing different contents of 10 k rev RP-UBSK a dewatering rate, b couch solids.

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The dewatering rate is higher for the conventional refining over most of the range when compared on a tensile strength basis. We believe this is due to the effect of the phosphorylated pulp on permeability and sheet sealing discussed above. It appears that the plugging of interfiber pores manifests itself already in the water-saturated state where dewatering rate is relevant.

The couch solids relate to the swelling and capillarity of the web after a specified vacuum period. Couch solids are a function of both the compression of the web under vacuum and the displacement of water from interfiber capillaries. Unrefined pulp would normally provide a permeable capillary system that is easy to dewater by vacuum. However, when the unrefined fibers are mixed with refined phosphorylated pulp, with a WRV around 12 mL g−1, the interstitial pores are effectively filled with high swelling microfibrils that effectively retain water. So, the couch solids of the blended pulp are very low compared to conventional refining. These results indicate that further optimization of variables will be required to utilize composite pulp systems. Although the strength/density relationship of the phosphorylated/unrefined pulp blends is very promising, the low dewatering rate is a serious drawback.

Conclusion

In this work, a deep eutectic solvent (DES) functional drying method was utilized to enhance hierarchical fiber swelling. As a result, we could facilitate cell wall delamination and deaggregation of cellulose fibrils during mechanical fibrillation. It was shown that mesoscale swelling and surface area increased greatly through a combination of phosphorylation and mechanical fibrillation. This resulted from increased osmotic pressure within the cell wall in combination with shear and cyclic compression in refining. The phosphorylated pulp was found to swell and fibrillate in refining, but not degrade to flake-like fines any faster than unmodified pulp. Adding this highly fibrillated swollen pulp to an unrefined fiber furnish made it possible to produce a family of sheets with higher strength/density ratio compared to conventional refining. The pretreated pulp was found to have lower sheet permeability and film form on the exit layer. Dewatering analysis showed that the phosphorylated/unrefined pulp blend had a lower dewatering rate and couch solids than the conventionally refined reference. Though the composite pulp strategy leads to less refining need of the main fiber furnish and promising sheet properties, further optimizations are needed to decrease the negative impact on dewatering behavior for commercial applications.