No abstract

The ionic liquids 1-ethyl-3-methylimidazolium acetate [emim]OAc, N,N,N,N-tetramethylguanidium propionate [TMGH]EtCO(2), and N,N,N,N-tetramethylguanidium acetate [TMGH]OAc, and the traditional cellulose solvent N-methylmorpholine N-oxide NMMO were characterized for their Kamlet-Taft (KT) values at several water contents and temperatures. For the ionic liquids and NMMO, thresholds of regeneration of cellulose solutions by water were determined using nephelometry and rheometry. Regeneration from wet IL was found to be asymmetric compared to dissolution into wet IL. KT parameters were found to remain almost constant at temperatures, between 20-100 °C, even at different water contents. Among the KT parameters, the β value was found to change most drastically, with an almost linear decrease upon addition of water. The ability of the mixtures to dissolve cellulose was best explained by the difference β-α (net basicity), rather than β alone. Regeneration of cellulose starts at thresholds values of approximately β < 0.8 (β-α < 0.35) and displayed four phases.

Different acid-base conjugates were made by combining a range of bases and superbases with acetic and propionic acid. Only the combinations that contained superbases were capable of dissolving cellulose. Proton affinities were calculated for the bases. A range, within which cellulose dissolution occurred, when combined with acetic or propionic acid, was defined for further use. This was above a proton affinity value of about 240 kcal mol(-1) at the MP2/6-311+G(d,p)//MP2/ 6-311+G(d,p) ab initio level. Understanding dissolution allowed us to determine that cation acidity contributed considerably to the ability of ionic liquids to dissolve cellulose and not just the basicity of the anion. By XRD analyses of suitable crystals, hydrogen bonding interactions between anion and cation were found to be the dominant interactions in the crystalline state. From determination of viscosities of these conjugates over a temperature range, certain structures were found to have as low a viscosity as 1-ethyl-3-methylimidazolium acetate, which was reflected in their high rate of cellulose dissolution but not necessarily the quantitative solubility of cellulose in those ionic liquids. 1,5-Diazabicyclo[4.3.0]non-5-enium propionate, which is one of the best structures for cellulose dissolution, was then distilled using laboratory equipment to demonstrate its recyclability.

Ioncell-F, a recently developed process for the production of man-made cellulosic fibers from ionic liquid solutions by dry-jet wet spinning, is presented as an alternative to the viscose and N-methylmorpholine N-oxide (NMMO)-based Lyocell processes. The ionic liquid 1,5-diazabicyclo[4.3.0]non-5-ene acetate was identified as excellent cellulose solvent allowing for a rapid dissolution at moderate temperatures and subsequent shaping into continuous filaments. The highly oriented cellulose fibers obtained upon coagulation in cold water exhibited superior tenacity, exceeding that of commercial viscose and NMMO-based Lyocell (Tencel®) fibers. The respective staple fibers, which have been converted into two-ply yarn by ring spinning technology, presented very high tenacity. Furthermore, the Ioncell yarn showed very good behavior during the knitting and weaving processes, reflecting the quality of the produced yarn. The successfully knitted and woven garments from the Ioncell yarn demonstrate the suitability of this particular ionic liquid for the production of man-made cellulosic fibers and thus give a promising outlook for the future of the Ioncell-F process.

The constant worldwide increase in consumption of goods will also affect the textile market. The demand for cellulosic textile fibers is predicted to increase at such a rate that by 2030 there will be a considerable shortage, estimated at~15 million tons annually. Currently, man-made cellulosic fibers are produced commercially via the viscose and Lyocell™ processes. Ionic liquids (ILs) have been proposed as alternative solvents to circumvent certain problems associated with these existing processes. We first provide a comprehensive review of the progress in fiber spinning based on ILs over the last decade. A summary of the reports on the preparation of pure cellulosic and composite fibers is complemented by an overview of the rheological characteristics and thermal degradation of cellulose-IL solutions. In the second part, we present a non-imidazolium-based ionic liquid, 1,5-diazabicyclo[4.3.0]non-5-enium acetate, as an excellent solvent for cellulose fiber spinning. The use of moderate process temperatures in this process avoids the otherwise extensive cellulose degradation. The structural and morphological properties of the spun fibers are described, as determined by WAXS, birefringence, and SEM measurements. Mechanical properties are also reported. Further, the suitability of the spun fibers to produce yarns for various textile applications is discussed.