Cellulose dissolution

Another activation treatment, suitable for most celluloses (although with great variation of the time required, 1 to 48 h) is polar solvent displacement at room temperature. The polymer is treated with a series of solvents, ending with the one that will be employed in the derivatization step. Thus, cellulose is treated with the following sequence of solvents, before it is dissolved in LiCl/DMAc water, methanol, and DMAc [37,45-48]. This method, however, is both laborious, needs ca. one day for micro crystalline cellulose, and expensive, since 25 mL of water 64 mb of methanol, and 80 mb of DMAc are required to activate one gram of cellulose. Its use may be reserved for special cases, e.g., where cellulose dissolution with almost no degradation is relatively important [49]. 

The basic requirement for cellulose dissolution is that the solvent is capable of interacting with the hydroxyl groups of the AGU, so as to eliminate, at least partially, the strong inter-molecular hydrogen-bonding between the polymer chains. There are two basic schemes for cellulose dissolution (i) Where it results from physical interactions between cellulose and the solvent (ii) where it is achieved via a chemical reaction, leading to covalent bond formation derivatizing solvents . Both routes are addressed in details below. 

A comment on these structures is relevant to understanding the mechanism of cellulose dissolution by these solvent systems. Support for association... 

Recently, use of LiCl/DMAc and LiCl/l,3-dimethyl-2-imidazolidinone as solvent systems for acetylation of cellulose by acetic anhydride/pyridine has been compared. A DS of 1.4 was obtained the substituent distribution in the products synthesized in both solvents was found to be the same, with reactivity order Ce > C2 > C3. Therefore, the latter solvent system does not appear to be better than the much less expensive LiCl/DMAc, at least for this reaction. It appears, however, to be especially efficient for etherification reactions [178]. It is possible, however, that the effect of cellulose aggregation is more important for its reaction with the (less reactive) halides than with acid anhydrides this being the reason for the better performance of the latter solvent system in ether formation, since it is more efficient in cellulose dissolution. 

Remsing, R.C., Swatloski, R.P., Rogers, R.D., and Moyna, G., Mechanism of cellulose dissolution in the ionic liquid l-n-butyl-3-methylimidazolium chloride a and 35/37(21 NMR relaxation study on model systems, Chem. Commun., 1271-1273, 2006. 

As NMMO is a solid at room temperature, dissolution and processing of the spinning dope require elevated temperatures of about 100 °C. The dope is spun into an air gap and water, where cellulose is regenerated and NMMO is washed out. After purification and evaporation of the water, the amine N-oxide is reintroduced into the system and used again for cellulose dissolution. 

Among the best-known nonderivatizing solvent systems is a combination between copper, alkali, and ammonia termed Schweizer s reagent. Solutions of cuprammonium hydroxide have been used for both analytical and industrial cellulose dissolution. Regenerated fibers with silk-like appearance and dialysis membrane have been (and partially continue to be) industrial products on the basis of cellulose dissolution in cuprammonium hydroxide. The success of this solvent is based on the ability of copper and ammonia to complex with the glycol functionality of cellulose as shown inO Fig. 11. Because of the potential side reactions (oxidation and crosslinking, Norman compound formation), alternatives to both ammonia as well as copper have been developed. Cuen and cadoxen are related formulations based on the use of ethylene diamine and cadmium, respectively. The various combinations of alkali, ammonia. 

Dissolution of wood using [C mimHCl] has also been reported [127-129], Based on the same mechanism of cellulose dissolution, the dissolution of wood also requires the virtual absence of water, which necessitates extensive drying of the wood, small particle size, drying of the ionic liquid, and reaction under an inert atmosphere. Long dissolution times are usually required unless microwave heating is used. 

Ionic liquids are able to dissolve carbohydrates to high concentrations as mentioned in the previous section. The use of ionic liquids in cellulose dissolution and functionalisation is particularly significant considering the problems associated with conventional processes such as the cupramonium and xanthate processes [117, 118, 120, 134-136], Lignin is soluble in ionic liquids, as discussed in Sect. 4.2. 

Microwave-assisted polycondensation reactions in ILs have also allowed the enhanced synthesis of polyamides and polyurethanes the comparison between microwave synthesis conditions in ILs with conventional heating methods and conventional organic solvents has also been addressed [92, 93]. Pretreatment methods combining microwave irradiation and ILs for cellulose dissolution and modification have been also proposed [94, 95]. Microwave irradiation can enhance the solubility of cellulose in ILs and decrease the degree of polymerization of regenerated cellulose after IL dissolution, which can be beneficial for improving cellulose hydrolysis [95]. 

In recent years cellulose dissolution has been researched quite extensively and new solvents have been discovered that are more environmentally friendly. Several new processes that rely on these solvents have been developed for manufacturing fibers. Furthermore, research has also been focused on cellulose derivatization processes that pollute less and are more economical. 

The discussion of cellulose dissolution must recognize that cellulose can exist in four polymorphic forms native cellulose known as cellulose I polymorph cellulose II obtained by regeneration of cellulose I cellulose III, which is derived from the liquid ammonia treatment of cellulose I or cellulose II and cellulose IV, which refers to the thermal treatment of cellulose I or cellulose 111 [2]. It is important to recognize these distinctions because the respective cellulose polymorphs can have different solubility characteristics in particular solvents, as will become evident further in this chapter. 

As suggested by Laszkiewicz [3], cellulose dissolution may be divided into four groups as specified in Table 10.1. 

FIGURE 10.16 Effects of various factors on cellulose dissolution in NMMO-water. (From Rosenau, T., Potthast, A., Sixta, H., and Kosma, P., Prog. Polym. Sci., 26,1763, 2001. Reprinted with permission of Elsevier B. V.)... 

Minimization of NMMO use for cellulose dissolution, thus lowering the cost of its... 

Rapid cellulose dissolution in anhydrous phosphoric acid is the basis for the Akzo phosphoric acid process [36,74]. A similar concept has been practiced for making Kevlar fibers, in which anhydrous sulfuric acid is used to dissolve poly(p-phenylene terephthalamide). However, anhydrous sulfuric acid is highly corrosive and is produced by mixing concentrated sulfuric acid with small amounts of fuming sulfuric acid. 

FIGURE 10.43 Cellulose dissolution time in and IKA-Duplex kneader as a function of temperature. Gceiiuiose= 17.1% w/w DPcenuiose = 800 Cp o, = 74.4% w/w. (From Boerstoel, H., Liquid Crystalline Solutions of Cellulose in Phosphoric Acid for Preparing. Cellulose Yams, Ph.D. dissertation, University of Groningen, 1998.)... 

Yang YJ, Shin JM, Kang TH, Kimura S, Wada M, Kim UJ (2014) Cellulose dissolution in aqueous lithium bromide solutions. Cellulose 21 1175-1181... 

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