nanocellulose, cationic starch and paper strength · 2021. 2. 2. · within nfc are sufficient to...

Peer Review Appita Journal

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NANOCELLULOSE, CATIONIC STARCH AND PAPER STRENGTH

MARTIN. A. HUBBE

Professor and corresponding author ([email protected]) North Carolina State University, Campus Box 8005, Raleigh, NC 27695-8005, USA

SUMMARY

The purpose of this work is to help understand the effects of adding nanocellulose – in combination with a dry-strength agent such as cationic starch – on the strength properties of paper. Recently it has been found that combinations of nanofibrillated cellulose (NFC) with high levels of cationic starch can achieve much greater increases in paper’s tensile strength and stiffness compared to either additive on its own. This paper considers how the observed effects can be accounted for based on capillary forces, the forming of hydrogen bonds during paper drying, and forming of polyelectrolyte complexes. Recent research showed that adding pre-mixed cationic starch and NFC to papermaking furnish made it possible to maintain tensile strength and stiffness while decreasing the refining level and achieving much lower levels of apparent density. The resulting paper was bulkier but still strong. It is proposed here that the observed effects involve formation of dense attachments at the nano-scale, but that a “spring-back” effect of the NFC-starch complex avoids the overall densification that occurs when papermakers rely on refining of the main furnish to achieve paper strength objectives. The initial, nano-scale densification is predicted by the capillary force theories of Campbell and Page. Elastic forces appear to facilitate re-expansion of clusters of nanofibrillated cellulose, allowing the paper to regain some of its bulk after wet-pressing. It appears that NFC can play a synergistic role as a kind of “extender” for cationic starch, allowing the starch to behave as if it were a long, fibrillar material capable of filling the pore spaces within typical paper. Pronounced decreases in air-permeability of the resulting paper, even when the apparent density has been decreased relative to a default paper sheet, provide supporting evidence for such a concept. Because the cationic starch and NFC surfaces have opposite charge, the concepts of polyelectrolyte complexes may apply.

Keywords: Nanofibrillated cellulose, dry strength, bonding agent, synergistic


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INTRODUCTION

What can a papermaker do when the mechanical refining of fibres is not sufficient, by itself, to achieve the desired strength requirements of a paper product? Part of the answer has been well known for over fifty years – add a dry-strength agent such as cationic starch (1-3). If the need for additional strength is severe enough, papermakers may even consider emerging strategies, such as the use of highly fibrillated cellulose (4-8). Both of these approaches have their own limitations and challenges, and these need to be kept in mind. The drawing in Figure 1 represents typical nanofibrillated cellulose (NFC) material, which has a complex, branched morphology in its wet, swollen state.

Challenges when using nanocellulose include the difficulty of retaining the very fine material as the sheet is being formed (10-11), monitoring the retention efficiency, and maintaining acceptable rates of dewatering (11-12). By analogy to what happens when papermaking furnish contains a relatively high level of cellulosic fines (13-15), it seems likely that the highly fibrillated cellulose slows dewatering by choking off drainage passages within the wet web. In addition, one can be concerned that nanocellulose might be unevenly distributed with the thickness of a sheet, possibly leading to curl problems and loss of dimensional stability, especially when humidity levels vary (16).

Fig. 1. Representation of typical NFC. Adapted from (9) by copyright owner

Though cationic starch is well known as a dry-strength agent, its effects are limited in two

key ways. First, tests have shown that there is a limit to the amount of cationic starch that will adsorb efficiently on to typical papermaking fibres; after the first 1% to 1.5% of cationic starch, on a mass basis, any additional cationic starch tends to build up in the white water (17-19). Such unretained starch can contribute to stabilisation of foam, make slime problems worse, and add to the biological oxygen demand of the water going to wastewater treatment. Secondly, and probably related, the strength benefits of cationic starch added to the wet end often reach a point of diminishing returns.


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Results of recent work show the possibility of strong synergistic effects when using nanofibrillated cellulose (NFC) and cationic starch in combination (20). The additives, when combined in advance before addition to the main fibre suspension, yielded strength improvements that were much greater than could be achieved with either additive by itself. Furthermore, by addition of colloidal silica, it was possible to maintain drainage rates even when adding the cationic starch-treated NFC to the furnish and achieving large gains in strength. By applying lower levels of refining of the main pulp, it was possible to achieve high levels of stiffness and bulk (lower apparent density) (20). Related increases in paper strength due to addition of highly fibrillated cellulose have been reported by others (10-12, 21-32). In light of these findings it is reasonable to consider whether such effects are consistent with known theories of paper strength.

MECHANISTIC CONSIDERATIONS

Capillary Forces Before hydrogen bonds can form between adjacent fibres, giving rise to the strength characteristics of dry paper, the fibres first need to be brought into molecular contact. Forman and Campbell were among the first to propose that capillary forces are responsible for such “drawing together” of adjacent fibres during the drying of paper (33-34). The mathematics and mechanism were spelled out more clearly by Page (35), based on the well-established Young-Laplace equation that predicts the strength of capillary forces within a meniscus between flat surfaces. The situation envisioned by Page is illustrated in Figure 2.

Fig. 2. Sketch to define the radii R1 and R2 in an idealised circular meniscus between

hypothetical smooth facing cellulose surfaces.

Cellulose surface

R1

R2

Meniscus

Cellulose surface

Air


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Based on Page’s analysis (35), the negative pressure (or vacuum) within the meniscus can be expressed as:

ΔP = 4 γ cosθ (1/R1 = 1/R2) [1] Where γ is the surface tension of the water, θ is the contact angle of water on the solid

(which is treated as being absolutely flat), and R1 and R2 are the two radii associated with a circular meniscus of water between adjacent flat areas of fibre. After making a few simplifying assumptions, such as perfect wetting of the surface (cosθ = 1), and that the meniscus becomes very thin (R1 << R2), the equation can be approximated by:

ΔP ≈ 2 γ /x [2] where x is the distance between the surfaces. As more and more water evapourates from

the paper, the value of x approaches zero, leading to prediction of extremely strong forces of consolidation. Evidence to support the action of very strong forces includes the mutual distortion of the shapes of cellulosic fibres that have been dried while in contact with each other (36-39).

Figure 3 provides a conceptual sketch of one way in which the presence of NFC between a pair of fibres might affect the mechanism of action of capillary forces when such a sheet of paper is dried. Here it is envisioned that one of the functions of the nanocellulose might be to inhibit the retraction of wetted area of the meniscus as water becomes evaporated. If this is what happens, the consequence would be a larger bonded area between the fibres, in comparison to the same sheet dried in the absence of the NFC. While the concept (Figure 3) still needs to be confirmed by suitable experiments, it is one way to account for the observed ability of relatively small proportions of NFC to greatly increase paper’s dry strength (10-12, 20-32).

Fig. 3. Retraction of a water meniscus during evaporation of water during paper drying may decrease relative bonded area; NFC might inhibit such retraction, leading to greater bonded area. A: Initial state of water meniscus

between two fibres; B: After some evaporation; C: When NFC is present in mensicus during evaporation.

Fiber

A Watermeniscus B Retracted

meniscusC Not-retracted

meniscus


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While there seems no reason to doubt the approach just described, which is mainly based on Page’s analysis (35), the underlying assumptions might not cover all situations, especially in cases where the surfaces of the adjacent fibres in the wet web are relatively rough and non-conformable. For example high-yield fibres, with a lot of lignin, can be expected to be rough and resistant to development of high values of relative bonded area (40). Likewise, recycled kraft fibres can be comparatively stiff when wet (41). In such cases it might be more appropriate to envision a model more like that shown in Figure 4, where the NFC is imagined as acting like a hydrogel. A hydrogel can be described as a water-loving, three-dimensional hydrophilic polymer having sufficient structure to remain insoluble, but which can contain large amounts of water relative to its mass (42). Such a description is consistent with the network-like shapes shown in many studies of NFC materials (9). Due to the bulky, swollen, but flexible nature of gel particles, it is reasonable to expect that the NFC can be effective in filling spaces between adjacent fibres after wet-pressing of the sheet and maintaining intimate contact until drying is complete. Such a chain of events would lead to a higher relative bonded area, which again could be a way to account for the ability of NFC to greatly increase paper strength under certain circumstances.

Fig. 4. Concept that NFC material, shown here as rounded gel-like material, can bridge the spaces within damp paper and contribute to bonded area of the paper after drying.

Spring-Back Concept Though the concepts depicted in Figures 3 and 4 may help to account for the relatively large strength increases observed in certain cases when NFC has been added to papermaking furnish, there is a need also to account for the surprisingly low apparent density achieved in a recent study employing relatively high treatment of NFC with cationic starch (20). Especially when the treated NFC was added to lightly-refined kraft fibres, the apparent density of the resulting sheets was relatively low.

Damp Paper After Drying

Fiber

NFC/CS


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One way to begin to account for such findings is to suppose that the elastic forces present within NFC are sufficient to allow at least some “spring-back”. As illlustrated in Figure 5, one envisions that detachment occurs at some of the contact points within a the NFC material occupying the space between two fibres in the damp paper web that is about to be dried, thus allowing the bent strands of NFC to relax to a more bulky overall shape. It might be hypothesised that such contact points might sometimes be too small or improperly oriented for the capillary forces envisioned by Campbell, Page, and others to be effective. Though the model in Figure 5 again would benefit from direct confirmatory experiments, it points to the potential to use NFC to prepare relatively low-density but strong paper products, as was demonstrated in the cited work (20).

Fig. 5. Proposed tendency of NFC, due to its elasticity, to regain some of its bulk after being subjected to wet-pressing and capillary forces.

Polyelectrolyte Effects A recent study by Brockman and Hubbe showed that, under specific circumstances, nanocellulose can amplify the effects of a cationic polyelectrolyte (43). In the cited work, cellulose nanocrystals (CNC) were partly covered with a high-charge cationic polymer, and the resulting combination exhibited dewatering effects that were much greater than could be achieved by the cationic polymer by itself. Though the situation being considered here (20) involved a different pair of additives – NFC and cationic starch – it is worth exploring possible explanations for the observed synergistic effects. For instance, as shown in Table 1, the stiffness achieved with the combination of NFC and cationic starch greatly exceeded what could be achieved with either additive alone at the about the same level. Separate tests showed that the cationic starch treatment alone yielded tensile strength results similar to those of untreated fibre furnish (20).

It is proposed here that the synergistic strengthening effect achieved with the combination of NFC and cationic starch can be understood based on the four points listed in Table 2.

Hypothetical max densification

Hypothetical actual structure

Fiber surfaceFiber surface

NFC


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Table 1: Stiffness and tensile strength achieved with cationic starch, NFC, and their combination (20)

Additive Tabor stiffness

[g⋅cm] Breaking force

[kN/m]

Cationic starch 26 10

NFC 39 11

Both additives 50 17

Table 2: Key factors to account for the synergistic effect of pre-mixed NFC and cationic starch on the strength properties of the resulting paper.

1 Larger amounts of cationic starch retained due to the high specific surface

area of NFC

2 Increased retention of NFC in the paper due to the adsorbed cationic starch

3 Increased bonded area within the paper because NFC can span spaces in the

wet web

4 Less loss of cationic starch into fibre pores due to its pre-adsorption onto

the NFC

Past studies have shown that only about 1% or 1.5% of cationic starch solids can be

effectively retained on ordinary papermaking fibres (18). But since NFC has a much higher surface area per unit mass, it is reasonable to expect a much higher ratio of cationic starch adsorption onto NFC. Meanwhile, the retention efficiency of the NFC during formation of paper can be expected to benefit greatly from the adsorbed cationic starch, which leads to attraction of the positively charged coated NFC to the negatively charged surfaces of the fibres. Thus, various cationic polymers have been shown to positively affect the performance of NFC in related studies (11, 16, 20, 23, 27, 32).

Figures 6 and 7 present a conceptual view of how NFC could function as an “extender” for added cationic starch. The idea is that the combination of NFC and cationic starch might overcome some key limitations of cationic starch related to its much smaller size (about 10-50 nm in solution) and limited amount that can be taken up by ordinary papermaking fibres.


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As in the model proposed earlier (43), the nanocellulose makes the cationic polymer act as if it had much larger dimensions. Though the true length of NFC is difficult to determine and is seldom reported (9), it is reasonable to expect length-related dimensions of NFC in the range of about 1 to 200 µm, which is big enough to bridge spaces within a paper sheet as it is being dried. Finally, past work has shown that some polyelectrolyte material can permeate into the cell walls of kraft fibres (44-46). Such polyelectrolytes, including cationic starch, then would not be available to participate in bonding between the fibres. In principle, the situation envisioned in Figure 6 minimises such losses in efficiency by first adsorbing the cationic starch onto the exterior surfaces of NFC.

Fig. 6. Representation of typical nanofibrillated cellulose with adsorbed cationic starch (shown as amylose). Modified from (9) by copyright owner.

Fig. 7. Concept of how a combination of NFC with adsorbed cationic starch (CS) may effectively enhance retention of the NFC.

NFC/CS

1 µm

Fiber

NFC/CS

Fiber


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Polyelectrolyte Complexation Earlier research has shown that very strong inter-fibre bonding can be achieved by means of in-situ formation of polyelectrolyte complexes in the fibre suspension just before forming the paper sheet (47). Possibly such principles may also govern the effects of cationic starch-NFC mixtures. One can envision NFC as one of the polyelectrolytes, based on the narrowness of the fibrils and their negative surface charges. Cationic starch can play the role of the second polyelectrolyte. If this model is valid, then one would expect maximum effects near to (but maybe not equal to) the ratio of additives that will give a neutral charged complex (47). Further research would be needed to establish such a relationship.

An intriguing finding of the recent work with cationic starch and NFC (20) was that superior strength, together with more rapid dewatering of the paper sheet, could be achieved by addition of colloidal silica to the mixture of NFC and cationic starch. The effects on dewatering are consistent with what has been observed for microparticle-type retention and drainage programs for papermaking (48). In such systems the greatest dewatering generally is achieved near to a charge-neutralised condition.

A mechanism to help explain the dewatering benefit when colloidal silica is added to a suspension in which cationic starch is adsorbed onto the surface of NFC is illustrated in Figure 8. Because of the extremely small size of the colloidal silica (assumed to be about 5 to 20 nm as sol particles or fused groups of primary particles), it is reasonable to expect them to diffuse within the coils of cationic starch adsorbed on the NFC surfaces (48). The formation of a complex between the cationic starch and the NFC surfaces involves neutralisation of ionic charges. Such neutralisation implies removal of the osmotic pressure that tends to maintain the cationic starch in a water-swollen, expanded molecular conformation. The expected result, as illustrated in Part B of the figure, is a less water-swollen structure of the adsorbed layer of cationic on the NFC surfaces in the suspension. This less swollen macromolecular structure is consistent with the observed higher rates of dewatering (20). Though such a mechanism needs more research, it offers a potential way to account for what has been observed in the lab.

Fig. 8. Proposed mechanism whereby addition of colloidal silica leads to a less water-swollen and expanded layer of adsorbed cationic starch, thus promoting the release of water during paper forming. Part A: Before addition of the colloidal silica. Part B: After.

A. “Tails” of adsorbed cationic starch molecules

Fibril of highly fibrillated cellulose

Aqueous phase

B. Gel-type colloidal silica particle

Less extended cationic starch, complexed with colloidal silica


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CONCLUSIONS

In this paper, a series of concepts have been proposed to help explain an observed synergistic action of combinations of highly fibrillated cellulose (NFC) and cationic starch (CS) to enable large gains in paper strength. The approach has potential to maintain paper strength while achieving lower apparent density of the paper by backing off on refining. It is proposed here that the effects can be attributed to (a) the ability of NFC to serve as an “extender” for cationic starch as a dry-strength agent, (b) the ability of cationic starch to enhance the retention of NFC during formation of the paper, (c) possibly a tendency of the NFC to resist retraction of the edges of menisci between fibres as water is being evaporated, and (d) possibly a tendency of the NFC, due to its elasticity, to regain some of its expanded character during the drying of the paper.


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Original manuscript received 26 January 2019, accepted 2 March 2019

nanocellulose, cationic starch and paper strength · 2021. 2. 2. · within nfc are sufficient to...

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