# Fabricating Sustainable All-Cellulose Composites
## 3 Dissolution of Cellulose
### Non-Derivatising Solvents  
#### 3.4.1 Non-Aqueous Solvents  
The Lyocell process, in commercial use since 1991, allows for the dissolution of cellulose without derivatisation, complexation or special activation [40]. In the Lyocell process, cellulose pulp is dissolved in an aqueous solution of N-Methyl-morpholine N-oxide (NMMO). Using spinnerets, the formed solution is extruded across an air gap into a coagulation bath containing water, which works as a non-solvent. While NMMO is separated by extraction, the cellulose fibres are regenerated in the water by precipitation [95]. The Lyocell process is considered to be a more environmentally friendly option than the viscose process because NMMO hydrates are non-toxic to the environment [96]. However, the  
Figure 4: Classification of cellulose solvents. The overview includes the most commonly used and newly researched solvents.  
NMMO also has limited thermal stability, due to which a variety of additional by-products may appear [97, 98].  
The dissolution of cellulose using organic solvents in combination with salts, such as in the case of the DMAc/LiCl solvent system, has also captured both industrial and academic interest [99, 100, 101, 102]. The DMAc/LiCl solvent system has been widely applied, and it can also dissolve cellulose with high molecular weight without noticeable degradation [103]. The drawbacks of this process are the high cost and difficulties with recycling the solvent due to its low volatility, which limits its further development for commercial applications [68, 104].  
Ionic liquids have risen as alternative, environmentally friendly and powerful solvent systems for shaping cellulose [68, 105]. What makes ionic liquids interesting is that they are organic salts that exist as liquids at relatively low temperatures (<100 \({}^{\circ}\)C) and can obtain good thermal stability, low melting point and low vapor pressure [106, 107]. Ionic liquids such as 1-butyl-3-methylimidazolium chloride can dissolve refined or natural cellulose without causing derivatisation [108, 109]. One emerging process, which is currently in its pilot scale and utilizes ionic liquids to dissolve cellulose, is the Ioncell process [110, 111]. The Ioncell pulp (Ioncell-P) method can be used to produce high-purity cellulose from bleached paper-grade birch kraft [112], and Ioncell fibres (Ioncell-F) can be formed using the same dry-jet wet spinning technique as the Lyocell-NMMO system [113]. One of the current challenges with the commercialisation of the Ioncell process is recovering the solvent from the coagulation bath without impairing its solvation power. Therefore, more research is still required to improve the recyclability and long-term chemical stability of the solvent system without compromising energy efficiency [80, 112, 113].  
Deep eutectic solvents (DESs) share many characteristics and physiochemical properties with ionic liquids, but they are generally considered easier to prepare with high purity at low cost [81, 114]. DESs are composed of two or three often cheap and safe chemicals that consist of a hydrogen bond donor and a hydrogen bond acceptor [115, 116]. These components are capable of self-association and form a eutectic mixture with a melting point lower than that of each individual component. In most cases, DESs are liquids between room temperature and 70 \({}^{\circ}\)C [115]. One promising DES system can be formed by combining choline chloride (ChCl) and urea [116]. The benefits of this DES system are the good availability of chemicals and their low melting point. However, based on current knowledge, the solubility of cellulose in DESs is far below that of ionic liquids or in the Lyocell process, but it can be improved in the future [81, 115].