Lyocell dopes

Fig. 11 Cellulose (beech sulfite pulp) dissolved in NMMO (Lyocell dope). Left DSco and differential MWD of the starting pulp, molecular weights of DP = 50 and DP = 200 are indicated by vertical dashed lines. Right Time course of the overall carbonyl content for three selected molecular weight ranges. Reprinted with permission from Biomacromolecules (2002) 4 743. Copyright (2002) American Chemical Society...

Typical values given in the literature for Lyocell dopes without stabilizers added are DP losses from 472 to 177 over 6 hours at 105 °C [22], or from 570 to 185 over 2 hours at 90 °C [23]. The activation energy for the cellulose chain cleavage in NMMO at temperatures above 115 °C was determined to be 69 kj/mol. This observation strongly supported the hypothesis that the actual rate-determining step of the cellulose chain cleavage was a /1-alkoxy elimination, with an activation energy of 67-72 kj/mol [24,25]. 

Scheme9 Trapping of formaldehyde with dimedone (18) under Lyocell conditions in the two-phase system Lyocell dope/o-dichlorobenzene...

Scheme 10 Trapping of N-(methylene)morpholinium cations (17) in Lyocell dopes by 2-acetonaphthone (20)...

From the observation that N-(mcLhylcnc)morpholinium cations (17) induced the autocatalytic decomposition of tertiary amine W-oxides in combination with the proof that these intermediates were actually present in Lyocell dopes, the question arose why Lyocell solutions were stable at all. The answer is found in the fact that carbenium-iminium ions generated are immediately consumed by reaction with water and carbohydrate structures as the most abundant and most reactive nucleophiles present. Only when the local concentration of 17 increases to such a high level that the consumption by reaction with water or cellulose cannot compensate for its production, does the reaction with NMMO become uncontrollable and lead to an exothermic event. The pulp used in the Lyocell process acts as a quasi-stabilizer for the solvent NMMO, albeit with the drawback of increased chromophore generation. 

In summary, trapping methodology helped to confirm the presence of the three main NMMO-derived radical species, the nitrogen-centered radical cation 3 and the carbon-centered radicals 4 and 5 in Lyocell dopes. Trapping... 

As Rosenau et al. [49] indicated that paper grade pulp, unbleached chemical pulp, cotton and rayon fiber wastes, or even paper wastes can be used as raw materials for Lyocell fiber production, even though problems with spinnability may be encountered in some cases. In preparation for spinning dope, a 50-60% aqueous NMMO is used with the addition of 0.01 0.10% antioxidant to prevent cellulose degradation. A typical antioxidant is PG [50]. In a typical Lyocell industrial process, the slurry is produced from cellulose pulp and an aqueous NMMO solution. Typical compositions are 50-60% NMMO, 20 30% water, and 10-15% pulp [48]. Subsequently excess water is efficiently evaporated at temperatures lower than 150°C and... 

Most recent, extensive rheological studies [59] on the stabilized NMMO-water solution proved that the apparent viscosity of the anisotropic solutions is relatively high (see Figure 10.26). Therefore, most commercial Lyocell spinning dopes usually have concentrations less than 17% and are isotropic. 

Fibers can also be spun from isotropic solutions of cellulose in NMMO, as a variation of the Lyocell process. Theoretically, fibers from isotropic solutions would exhibit different structures and inferior mechanical properties vis-a-vis those spun from mesophases. But high tensile properties, strength and modulus can be obtained by conditioning at elevated temperatures. Chae et al. [82] spim cellulose fibers from isotropic cellulose spinning dope in NMMO monohydrate. They foimd little difference in crystal structure and tensile strength between their fibers and conventional Lyocell libers. However, the eellulose fibers from isotropic solution had a lower tensile modulus, which may be helpful for cellulose fibers used in textiles. 

Chae, D. W., et al. (2002). Physical Properties of Lyocell Fibers Spun from Isotropic Cellulose Dope in NMMO Monohydrate. Textile Research Journal, 72(4), 335-340. McCormick, C. L. (1981). Novel Cellulose Solutions, U.S. Patent 4,278,790, Jul. 14. Turbak, A. (1993)., Newer cellulose solvent systems, in Wood and Agricultural Residues, E.J. Soltes (Editor), Academic Press. 87-99. 

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