Lyocell process

Coupling sugar, 23 480 Courtaulds lyocell process, 11 266-267, 267-269. See also Samuel Courtauld Co., Ltd. 

Lyocell process, environmental issues related to, 22 279-280. See also Courtaulds lyocell process Lyondell s process, 23 345 Lyophilic colloids, 7 283-284 Lyophilization, 28 716 Lyotropic liquid crystalline polyesters,... 

N-McLhylmorpholine-N-oxidc monohydrate, a tertiary, aliphatic amine N-oxide, is able to dissolve cellulose directly, i.e. without chemical derivatization, which is used on an industrial scale as the basis of the Lyocell process [ 1, 2], This technology only requires a comparatively low number of process steps compared for instance to traditional viscose production. Cellulose material - mainly fibers - are directly obtained from the cellulose solution in NMMO no chemical derivatization, such as alkalization and xanthation for rayon fibers, is required [3]. The main advantage of the Lyocell process lies in its environmental compatibility very few process chemicals are applied, and in the idealized case NMMO and water are completely recycled, which is also an important economic factor. Even in industrial production systems NMMO recovery is greater than 99%. Thus, compared with cotton and viscose the Lyocell process pertains a significantly lower specific environmental challenge [4]. Today, Lyocell fibers are produced on an industrial scale, and other cellulosic products, such as films, beads, membranes and filaments, are also currently being developed or are already produced commercially. 

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. 

Origin and formation pathways of the different vinyl ether and enamine structures in mixtures from the uncontrolled NMMO degradation were difficult to assess, since the high temperatures during such processes also allowed disfavored decomposition processes to proceed, which are thermodynamically forbidden or disfavored under the usual Lyocell process conditions. 

NMMO process has a short production cycle. The dissolution and spinning process is usually done within 5 h. Currently, full continuous dissolution of cellulose is implemented in the Lyocell process. 

FIGURE 10.12 The Lyocell process in general. (From Rosenau, T., Potthast, A., Sixta, H., and Kosma, P., Prog. Polym. Sci., 26, 1763, 2001. Reprinted with permission of Elsevier B.V.)... 

The degradation of cellulose in NMMO-water solution was one of the major obstacles in the early stages of Lyocell development process. Laszkiewicz studied the degradation of 5% cellulose solution in an NMMO-water solution at 80°C. He clearly showed that if there is no antioxidant present, the DP rapidly decreases [3]. As shown in Figure 10.23, DP decreased from 700 to around 140 after 120 min at 80°C. The use of antioxidants, particularly PG (see Figure 10.24) that is commonly used in the Lyocell process, prevents cellulose degradation. 

The Lyocell process— potential negative effects of side reactions... 

Th Courtaulds Lyocell Process. The increasing costs of reducing the environmental impact of the viscose process coupled with the increasing likelihood that the newer cellulose solvents would be capable of yielding a commercially viable fiber process led Courtaulds Research to embark on a systematic search for a new cellulosic fiber process in the late 1970s. 

However, since 1998 the Lyocell process has been used in Austria and the special feature of this process is the solution of the pulp which is in an organic solvent (NMMO = N-methyl-morpholine-N-oxide), instead of CS2/NaOH, which has the effect of eliminating the odorous and noxious emission of sulphurous gases. The properties of the produets from this Lyocell process are different to the standard viscose fibre, and therefore this process should not be regarded as an environmentally compatible alternative to the viscose process. 

The core part of the Lyocell process is the direct dissolution of cellulose through NMMO (N-methyl-morpholine-n-oxide). The solvent, which is 100 % biodegradable, is able to dissolve cellulose physically without any chemical pretreatment. 

Figure 11.2 shows a simplified diagram of the Lyocell process. 

The cupro and lyocell processes are based on the application of direct dissolution systems. The copper ammonia complex is prepared from copper sulphate and sodium carbonate with sodium and ammonium hydroxides, as presented in the equation in Fig. 4.3. The dissolution process is carried out for cuprammonium via weakening the intra/intermolecular hydrogen bonds and complex formation. Cupro and especially lyocell processes consume lower amounts of water, but a similar magnitude of energy. 

Wood pulp is the principal raw material of the lyocell process in terms of cost and volume. The grade used is similar to the dissolving pulp used for viscose rayon but has a slightly lower degree of polymerisation (DP) Tencel fibres have a DP of 500 to 550. The pulp is pulled from the reels into a shredder, which cuts the pulp into small pieces for mixing with the amine oxide solvent. The amount of pulp fed to the mixer has to be accurately measured so that the cellulose content in solution is closely controlled. 

The conversion of cellulosic wood pulp into a textile fiber dates to the earliest periods of synthetic fiber manufacture viscose rayon and lyocell processes are well known. However, it is stiU conceivable to improve upon... 

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