11.08.2015   Innovative Biofibers from Renewable Resources

Limitations of the Lyocell Process

18.1.1 Fibrillation of Lyocell Fibers

NMMO fibers have higher orientation in the amorphous regions and consist of long and thin crystallites that prevent fringing of the fibers and lead to fibrillation when wet and under abrasive conditions such as those found during dyeing. Figure 18.3 shows the SEM images of fabrics fibrillated after dyeing with various dyes. Figure 18.4 shows optical images of an extensively defibrillated lyocell fiber. SEM images in Fig. 18.5 show a comparison of the cross sections of fibers produced using the lyocell and viscose processes. A considerably higher level of fibrillation can be seen in the lyocell fibers compared to the fibers produced using the viscose process. The lateral links between crystallites in fibers produced by the NMMO process are also weaker that contributes to the fibrillation. Cross section of the fracture surface of an NMMO fiber reveals considerable fibrillation compared to the viscose-type fibers. The degree of fibrillation is expressed quantitatively in terms of “fibrillation index” which is the sum of the fibril lengths (£i) divided by the fiber length (l) [02Udo]. It has been reported that the fibrillation index is directly proportional to the % crystallinity. A highly crystalline fiber has high degree of

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Fig. 18.3 SEMs of fibrillated lyocell samples dyed with 5.0 % owf Procion Red H-E3B at (a) 100x and (b) 500x magnification, 5.0 % owf Cibacron Navy W-B at (c) 100x and (d) 500x magnification, and 5.0 % owf Procion Orange H-ER at (e) 100x and (f) 500x magnification. Reproduced with permission from Elsevier [07Gos]

 

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Fig. 18.4 Optical images depicting the extensive fibrillation of lyocell fibers (right) compared to cuprammonium rayon (left) when treated under the same wet conditions (30 min in 1 % NaOH at 90 °C) (Reproduced with permission from Sage) [09Miy]

 

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Fig. 18.5 Comparison of the cross sections of regenerated cellulose fibers produced using the NMMO process (a) and the viscose process (b). A markedly low degree of fibrillation can be seen in the NMMO fibers [02Udo]. Reproduced with permission from Sage publications

orientation and delaminates to a greater extent and therefore has higher extent of delamination.

Several approaches have been proposed to control the extent of fibrillation. Researchers have changed the type of pulp, drying and pretreatment conditions, and solvents used for precipitation to reduce fibrillation mainly by decreasing the % crystallinity and orientation. Fibrillated fibers have been defibrillated using enzymes and other techniques to improve the properties of the fibers [07Gos]. In addition, cross-linking with triazine derivatives, polycarboxylic acid derivative, and aminofunctional polysiloxane polymers and binding and curing with conven­tional resins have also been done to reduce or eliminate fibrillation [10Han]. Partially oxidized lyocell fibers cross-linked with multifunctional amines led to increased amorphous regions, decreased swelling and water sorption, and therefore low degree of defibrillation [10Han].