Interaction of divalent cations with carboxylate group in TEMPO-oxidized microfibrillated cellulose systems

Renewable and biodegradable polymers, such as cellulose, starch, chitosan, and hemicelluloses have recently been studied for many applications (Fang et al. 2000; Xu et al. 2005; Durango et al. 2006; Nakagaito and Yano 2008; Escalante et al. 2012). In general, oxidized microfibrillated cellulose (MFC), with high mechanical strength and low density, can be formed into hydrogels, aerogels, films, etc., depending on the target applications, ranging from biomedicine, tissue engineering, pharmaceutics and electronics/optoelectronics to textiles, food, membranes and wood products (Chang et al. 2010; García-González et al. 2011; Ming et al. 2017; Song et al. 2017) MFC suspensions have been converted into films by several methods, like casting, vacuum filtration, solvent exchange, spraying, and spin-coating (Henriksson et al. 2008; Wågberg et al. 2008; Aulin et al. 2010a, b; Spence et al. 2010a, b). Cellulose networks in the films are formed by strong inter- and intramolecular interactions via physical and chemical cross-linking (Sannino et al. 2009; Chang and Zhang 2011). A cross-linked cellulose network can be prepared via light-induced ligation (Hufendiek et al. 2016), glyoxalisation (Quero et al. 2011), through maleic acid and sodium hypophosphite (Kim et al. 2015), or with polymers such as polypyrrole or polyurethane (Yin et al. 2001; Wu et al. 2017) as cross-linking agents. The concentration of MFC in most of the earlier mentioned applications is not higher than 1% wt. The quality of the films depends on the film formation process, drying method and kind of MFC utilised.

The tensile strength of cellulose films has been improved by producing composites with polyvinyl alcohol, chitosan, starch or carboxymethyl cellulose (Hakalahti et al. 2015; Pahimanolis et al. 2013; Prakobna et al. 2015; Toivonen et al. 2015; Wang et al. 2015). Chemical modification of the hydroxyl groups in cellulose, such as esterification and etherification, is another technique to alter the hydrophobicity of cellulose films (Baiardo et al. 2001; Fox et al. 2011; Missoum et al. 2012, 2013; Sehaqui et al. 2014; Khanjani et al. 2018). However, these modifications often decrease the mechanical performance by reducing the crystallinity of the materials. Another approach, introduced by Isogai et al., is to selectively oxidize the accessible hydroxymethyl groups to sodium carboxylate groups by a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) catalyzed hypochlorite treatment (Kitaoka et al. 1999; Saito et al. 2007; John and Thomas 2008; Habibi et al. 2010; Isogai et al. 2011). The oxidation promotes fibrillation without changing the crystallinity of cellulose which is the basis for high quality MFC film formation. The carboxylate groups of oxidized MFC are well-known to strongly bind metal cations. Recently, Dong et al. investigated the hydrogelation of carboxylated cellulose nanofibrils with divalent and trivalent cations (Dong et al. 2013). They also presented a molecular model for the interaction between divalent cations and carboxylate groups from two adjacent chains on one fibril (Dong et al. 2013). Saito et al. reported on an ion-exchange treatment of divalent metals (barium, nickel and calcium) with TEMPO-oxidized cellulose at different pH values (Saito and Isogai 2005). They showed that the cation exchange is less efficient below pH 2.5–2.7 because free carboxylic acids exist at such low pH values.

Additionally, Shimizu et al. fabricated water-resistant films by the formation of interfibril cross-linkages with multivalent metal salts (Shimizu et al. 2016). They first dried oxidized MFC (Na⁺ form) films for 3 days, and then soaked the films in aqueous salt solutions for 2 h. After drying, the films possessed high Young’s moduli (11–20 GPa) and high tensile strength (170–280 MPa).

The tensile strength of paper can be enhanced by refining the pulp and wet pressing the sheets, which improve the inter-fibre bonding properties. However, the wet tensile strength of paper varies depending on the swelling ability of pulps of different origin. For example, wet sheet strength of oxidized MFC depends on its carboxylate content, a higher carboxylate content (0.60 vs. 0.11 mmol/g) leads to a lower wet tensile strength (Saito and Isogai 2005).

Moreover, the counter of the carboxylate groups affect the sheet properties of pulps through their swelling (Scallan and Grignon 1979; Maloney 2015). Thus, the tensile index of dry pulp sheets decreased with increasing valency the cationic counter ions (Scallan and Grignon 1979; Scallan 1983).

In this paper we report on fixing of wet films of hardwood kraft pulps and oxidized MFC (Na⁺ form) with calcium and magnesium salts as a means to improve the strength of the sheets.

The principle of fixing wet films of oxidized MFC and pulp with divalent cations is illustrated in Fig. 1. A homogeneous suspension of MFC (Na⁺ form) and pulp is first spread on a support and then immersed in an aqueous solution of a divalent metal salt, such as CaCl₂ or MgCl₂. The sodium counter ions in MFC/pulp are substituted for Ca²⁺ or Mg²⁺ by the actions of diffusion, ion exchange and the Donnan effect that favors bonding of divalent cations on the anionic MFC/pulp. The divalent cations bridge the carboxylate groups between the fibrils and fibers which leads to solidification of the film within a time required for completing the diffusion over the film thickness. The excess of salt is then removed from the wet film by diffusion in pure water.

When a wet MFC/pulp film on a support was immersed in water, it dispersed the MFC/pulp mixture. Direct mixing of MFC/pulp with the multivalent cation salts led to immediate flocculation, which prevented formation of even films. On the contrary, soaking wet MFC/pulp film (Na⁺ form) in aqueous CaCl₂ or MgCl₂ started to solidify the film immediately. Typically 1–3 h were required to complete the solidification depending on several factors, such as the film thickness and the concentration of the salt solution. The second treatment in water for 15 min or 1 h did not change the appearance of the wet, solidified film. Drying of the films in air left them moist, elastic and skin-like material. Typically softeners, like glycerol or other polyalcohols, may provide a similar elasticity of MFC/pulp films. Obviously, some chloride was still present in the structure since rewetting and redrying of the films made them drier and more paper-like. After a short rewetting (15 min) the calcium/magnesium content of the CaCl₂/MgCl₂ treated films was 210–330 mmol/kg in comparison with their carboxylate content of ca. 450 mmol/kg, approaching the expected 1:2 stoichiometric ratio between Ca²⁺/Mg²⁺ and the carboxylate groups (Table 2). Photoacoustic FTIR spectroscopy of the films confirmed that they were mostly in the carboxylate form, although the content of free carboxylic acids was also significant (< 100 mmol/kg) (Fig. 2). Figure 2 shows that HCl treated and untreated cellulose film with 410 mmol/kg and 80 mmol/kg of carboxylic acid content, respectively. On the contrary, a more extended immersion in water for 60 min removed most of Mg²⁺ (MgCl₂ treated films) and Na⁺ (untreated films) from the films whereas Ca²⁺ mostly remained in the CaCl₂ treated films.

The binding of metal cations of the treated MFC/pulp depends on several factors, such as the original carboxylate content, the ion-exchange and washing procedures, and the strength of the metal carboxylate interaction (Saito and Isogai 2005; Fukuzumi et al. 2010). The pK_a value of the C6-carboxyl group of oxidized cellulose has been reported to be 2.8–3.7 (Fujisawa et al. 2011), and therefore, at the pH 5.5 applied, most of the carboxyl groups were dissociated and associated with the metal cations (Na⁺, Ca²⁺ or Mg²⁺). On the other hand, as the data of Table 2 shows, prolonged soaking the films in a large excess of water, shifted the equilibrium to favour the undissociated carboxylic acid form, depending on the metal salt used. Earlier, Isogai et al. demonstrated that the sorption of Ca²⁺ on oxidized nanofibrillated cellulose (NFC) is stronger than that of Mg²⁺ after soaking in aqueous nitric acid (HNO₃) or aqueous sodium nitrate (NaNO₃) (Saito and Isogai 2005).

Freeze-drying of wet MFC/pulp films in Na⁺ form and their subsequent SEM imaging visualized the porous, fibrillar and membrane-type structures that are typical of MFC (Fig. 3a). These structures dominated the images over the pulp fibers that were hardly visible. In contrast, when the films were freeze dried right after their solidification in aqueous CaCl₂ or MgCl₂, the pulp fibers dominated the images (Fig. 3b, c). Obviously, MFC was attached on the surfaces of the pulp fibers that formed a porous network in this case.

Another set of SEM images were collected from the solidified MFC/pulp films after soaking in water, drying, rewetting and redrying (Fig. 4). The films were freeze-fractured and the images were taken on the fracture surfaces representing cross sections of the films. The cross sections of the films in Ca²⁺ and Mg²⁺ form revealed a consolidated structure with repeating < 0.5 µm thick horizontal lamella (Fig. 4b, c). No fiber separation was observed at the fracture surfaces. In contrast, the film in Na⁺ form obviously delaminated during the freeze-fracture demonstrating a lower internal strength (Fig. 4a). In this case, individual fibers became visible in the fracture surface although the intact films in Na⁺ form were, in average, denser than the films in Ca²⁺ and Mg²⁺ form (Tables 3 and 4). The consolidation of the sheets occurred only in the vertical direction as their lateral dimensions remained practically constant during the drying.

In addition to their lower density, the films in Ca²⁺ and Mg²⁺ form had higher tensile indexes than the films in Na⁺ form had demonstrating again the positive effect of the divalent cations on internal strength of the material (Fig. 5). Although the experimental error of the elastic moduli was large, in average, they seemed to increase when the films were converted into the Ca²⁺ or Mg²⁺ form (Table 3). After longer treatment in water, the elastic moduli of the films in the divalent cation form decreased (Table 4), possibly due to removal of the salt excess (CaCl₂ or MgCl₂). For the films in Na⁺ form the effect of prolonged water treatment was opposite. In this case, the increased average elastic modulus could be explained by the larger fraction of free carboxylic acid groups that increase hydrogen bonding in the films (Fujisawa et al. 2011). The counter ion did not significantly affect the moisture content of the sheets under the constant conditions, 23 °C and 50% RH, applied in testing of the film properties. Therefore, the differences in the strength were not caused by differences in the equilibrium moisture content. Somewhat surprisingly, the length of the water treatment (15 and 60 min) and the simultaneous removal of the counter ions from the wet, solidified films did not really affect the tensile strength of the dried films. Obviously, the bond formation of the MFC/pulp gel was initiated and fixed already during the 3 h solidification process in the presence of the divalent salts. Although drying of the wet films naturally increased their strength, the network structure was fixed already before the drying. Therefore, releasing more Mg²⁺ cations from cellulose sheets by 60 min water treatment did not change the tensile index values of the dried cellulose films.

The treatment of the films with CaCl₂ and MgCl₂ also increased their strain at break (Fig. 5). The increase was largest with HT BHKP the fibers of which were curlier than those of the untreated kraft pulps (Tables 3 and 4). Washing time of the solidified film did not clearly affect the strain at the break.

According to previous studies, introducing some additional carboxylate groups into pulp by oxidation (from 0.06 to 0.12 mmol/g) can improve the wet tensile index, whereas increasing the carboxylate group content further is negative for the wet strength (Kitaoka et al. 1999). In contrast, the dry tensile strength may increase with increasing carboxylate content to the level of 0.4 mmol/g (Kitaoka et al. 1999). The high ionic charge of the pulp increases its hydration (water retention value) and ability to form bonds and consolidate the fiber network during drying (Scallan 1983; Barzyk et al. 1996).

In this study, substitution of Na⁺ ions with Ca²⁺ or Mg²⁺ ions simultaneously decreased the density and increased the tensile strength and breaking strain (Fig. 6). This unusual behaviour could originate from the layered structure where the fibers are surrounded by the MFC ‘matrix’ rather than directly bonded with each other.

Exchange of Na⁺ to Ca²⁺ or Mg²⁺ as the counter ion of wet films of oxidized MFC and pulp increases the internal strength of the films in dry state. In the presence of an excess of the divalent metal salt, such as CaCl₂ or MgCl₂, drying of the film forms an elastic, skin-like material. Removal of the excess of the salt before drying leads to a paper-like film the properties of which depend also on the type of the pulp used. Curly pulp fibers provide films with high elongation at break. Although soaking of the films in aqueous solutions, and water, may be impractical to accomplish in large scale, the approach could be applied in tailoring the material properties of cellulosic films for specific uses.