Cellulose (2018) 25:3595–3607
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it possible to produce new, biodegradable composites of exclusive characteristics, e.g. barrier properties (Hubbe et al. 2017) or even substrates for printed electronic products (Lee et al. 2009; Peresin and Rojas 2014). Moreover, using the same polymer for the surface modification as that present in the natural fibers may result in strong adhesion between the coating layer and the base paper. Despite being a hydrophilic and hydrogen-bonded substance, cellulose neither dissolves in water nor in many popular volatile polar and non-polar organic solvents (e.g. ethanol, methanol). The lack of simple dissolving systems is a direct outcome of the complex chemical structure of cellulose. The cellulose macromolecule contains a high number of hydroxyl groups prone to the forma- tion of a strong and highly structured intra- and intermolecular hydrogen-bonding network, which resists water and most organic solvents. Therefore, cellulose solubility depends on the ability of the solvent to break these interactions (Yamane et al. 2006; Lindman et al. 2010; Medronho and Lindman 2014, 2015). Cellulose contains multiple hydroxyl groups on the glucopyranose ring and, therefore, its insolubility in water is rather unexpected. It might be explained—according to recent investigations—by its amphiphilic nature. This means that the glucopyranose ring exhibits both a hydrophobic and hydrophilic character. The axial direction of the ring is hydropho- bic as a result of the location of C–H bonds along the axial position. Hydrogen atoms connected directly to the carbons do not contribute to the hydrogen bonding. According to simulations carried out by Mazeau (2011), the surface energy of the layer of C–H moieties is the lowest and, as a consequence, the attachment energy, which is mainly of van der Waals type, is less favourable than that of other surfaces of the cellulose. Hydrophobic interactions obviously limit cellulose solubility in polar solvents. The equa- torial direction of the ring is hydrophilic since the hydroxyl groups—responsible for hydrogen bond- ing—are located along this direction. Hence, cellulose macromolecules exhibit differences in polarity (Ya- mane et al. 2006; Medronho et al. 2012). The situation is more complex due to the presence of various crystalline phases in native cellulose. There are three principal phases: type I a (triclinic), I b (monoclinic) and type II (Biermann et al. 2001). Affinity of all these phases to water and to many organic solvents has not yet been fully investigated. Some authors also link
cellulose insolubility with its crystallinity (Cao et al. 1994; Medronho et al. 2012). The chemical structure and properties of cellulose may suggest that amphi- philic solvents would be the most suitable for cellulose dissolution. Ionic liquids (Kosan et al. 2008) and some organic solvents exhibit such properties (Kalash- nikova et al. 2012; Lindman et al. 2010; Medronho and Lindman 2014). It should be emphasised that, even though many different systems that dissolve cellulose are described in the scientific papers, not all of them are amphiphilic (Alves et al. 2016). N - Methylmorpholine N -oxide (NMMO) is an example of such a system. The mechanism of cellulose dissolution in NMMO systems has been described in the literature (Medronho and Lindman 2014). The proposed mech- anism, however, does not fully explain the occurring changes. For instance, it does not include the impact of water molecules, which are crucial for the NMMO cellulose dissolving system. Experimental practice indicates (Lindman et al. 2010) that lack of water results in cellulose insolubility in NMMO. Con- versely, at too high water content, cellulose would not dissolve either. This emphasises the water-sensi- tivity of the process. Nonetheless, among different cellulose solvents, NMMO seems to be the most interesting and promising from the viewpoint of paper material modifications. This solvent has been the subject matter of research for many years (Kulpinski 2007; Kulpinski et al. 2011; Erdman et al. 2016). Nowadays, it is applied on an industrial scale for the production of cellulose fibers, known under the brand name of Lyocell or Tencel. Low toxicity is one of the most important advantages of this solvent. The possibility to obtain cellulose solutions in a wide range of concentrations, from less than 1% to approx- imately 28% by weight, is another advantage. No less important is the fact that—in the case of NMMO—the cellulose solidification process is carried out in water baths. So far, methods for modifying paper with the application of NMMO have been scarcely studied. Johnson (1969) patented a method in which only NMMO (without cellulose) was used for paper mechanical property improvement. In another patent, Melville et al. (2014) applied a mixture of NMMO, water and fluorinated polymer particle suspension to modify paper surface so that its abrasion resistance would be increased and, simultaneously, its friction coefficient decreased. Paper with an applied wet layer was heated at a temperature of 100 C to ensure good
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