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F. Hermanutz et al.
polymer with both highly structured crystalline and amorphous regions. One
of the most remarkable properties of cellulose is the insolubility in water as
well as in most organic solvents [3]. The challenge in dissolving cellulose is
to destroy the strong hydrogen bonds. Nevertheless, there are some solvent
systems for cellulose described in the literature, such as N-methylmorpholineN-oxide (NMMO), N,N-dimethylacetamide/lithium chloride (DMAc/LiCl), N,Ndimethylsulfoxide/tetrabutylammonium fluoride (DMSO/TBAF), and several ionic
liquids (ILs) [4–8]. For industrial applications, NMMO is now the most common
direct solvent system for cellulose and the corresponding NMMO/cellulose solutions
are used for spinning cellulosic fibers. In the so-called Lyocell process, NMMO is
used at concentrations between 10 and 14 wt%. The spinning dope has to be stabilized by additives, like isopropyl gallate, to prevent side reactions [9, 10]. By spinning
through an air gap prior to coagulation in an aqueous bath, high stretch ratios can
be realised, leading to high orientation of the cellulose chains. After drying, long
and thin crystallites are formed, which align along the fiber axis in a highly oriented manner. Consequently, there is little lateral interaction between the individual
macrofibrils, which, in the wet state, leads to a high tendency to fibrillation [9]. However, due to the economic and ecological drawbacks of both processes with regard
to the dissolution and processing of cellulose, more efficient and environmentally
friendly solvents are required [9].
Since Swatloski et al. discovered the ability of some ILs to dissolve cellulose,
a new research field opened up and new possibilities to process cellulose are now
imaginable [7]. This so-called IL-technology is an economically and environmentally
friendly alternative process due to thermal and chemical stability, non-flammable
nature, and miscibility with many other solvent systems of ILs. Besides the spinning
of cellulosic fibers, this technology also offers new ways for the preparation of
cellulose-based composites, coatings, and the chemical modification of cellulose
[11–16].
10.2 Selection of Ionic Liquids
10.2.1 Ionic Liquids and Green Chemistry
ILs are salts in the liquid state and were first described by Walden [17]. Ionic liquids
consist of a cation and an anion like “normal” salts. However, in contrast to these,
ILs have melting points below 100 °C, and ILs with melting points below 25 °C, socalled room-temperature ionic liquids (RTILs), are known as well [18–22]. The low
melting points of such ILs are a result of the selection of cation and anion [23–27].
A key feature of ILs is that their physical properties can be tailored by the selection of ions and substituents on the cations (R-group) [18]. Thus, their solubility in
organic solvents and water can be controlled by the nature of the R-group [28, 29].
However, the miscibility of ILs with polar and non-polar solvents also depends on the
F. Hermanutz et al.
polymer with both highly structured crystalline and amorphous regions. One
of the most remarkable properties of cellulose is the insolubility in water as
well as in most organic solvents [3]. The challenge in dissolving cellulose is
to destroy the strong hydrogen bonds. Nevertheless, there are some solvent
systems for cellulose described in the literature, such as N-methylmorpholineN-oxide (NMMO), N,N-dimethylacetamide/lithium chloride (DMAc/LiCl), N,Ndimethylsulfoxide/tetrabutylammonium fluoride (DMSO/TBAF), and several ionic
liquids (ILs) [4–8]. For industrial applications, NMMO is now the most common
direct solvent system for cellulose and the corresponding NMMO/cellulose solutions
are used for spinning cellulosic fibers. In the so-called Lyocell process, NMMO is
used at concentrations between 10 and 14 wt%. The spinning dope has to be stabilized by additives, like isopropyl gallate, to prevent side reactions [9, 10]. By spinning
through an air gap prior to coagulation in an aqueous bath, high stretch ratios can
be realised, leading to high orientation of the cellulose chains. After drying, long
and thin crystallites are formed, which align along the fiber axis in a highly oriented manner. Consequently, there is little lateral interaction between the individual
macrofibrils, which, in the wet state, leads to a high tendency to fibrillation [9]. However, due to the economic and ecological drawbacks of both processes with regard
to the dissolution and processing of cellulose, more efficient and environmentally
friendly solvents are required [9].
Since Swatloski et al. discovered the ability of some ILs to dissolve cellulose,
a new research field opened up and new possibilities to process cellulose are now
imaginable [7]. This so-called IL-technology is an economically and environmentally
friendly alternative process due to thermal and chemical stability, non-flammable
nature, and miscibility with many other solvent systems of ILs. Besides the spinning
of cellulosic fibers, this technology also offers new ways for the preparation of
cellulose-based composites, coatings, and the chemical modification of cellulose
[11–16].
10.2 Selection of Ionic Liquids
10.2.1 Ionic Liquids and Green Chemistry
ILs are salts in the liquid state and were first described by Walden [17]. Ionic liquids
consist of a cation and an anion like “normal” salts. However, in contrast to these,
ILs have melting points below 100 °C, and ILs with melting points below 25 °C, socalled room-temperature ionic liquids (RTILs), are known as well [18–22]. The low
melting points of such ILs are a result of the selection of cation and anion [23–27].
A key feature of ILs is that their physical properties can be tailored by the selection of ions and substituents on the cations (R-group) [18]. Thus, their solubility in
organic solvents and water can be controlled by the nature of the R-group [28, 29].
However, the miscibility of ILs with polar and non-polar solvents also depends on the