Page 366 - Handbook of Properties of Textile and Technical Fibres

Regenerated cellulosic fibers 339

of the interior gel-like phase as the solvent diffuses out due to osmosis. The gel-like
phase shrinks causing the outer skin to collapse and forms folds that lead to skin-core
structures with a serrated cross section (Sisson, 1960; Hearle, 2001). At low acid con-
centrations in the spin bath, the osmosis rate is low and “all-skin” fibers are produced,
whereas with increase in the acid concentration, the rate of osmosis is high and the ratio
of skin-to-core phases reduces. Thus, the cross section of viscose fibers may be changed
by altering H 2 SO 4 concentrations in the spin bath as shown in Fig. 10.8.
In the lyocell process, unlike in the viscose process, the extruded filaments pass
through an air gap before they enter the regeneration bath, in a method termed “dry-

jet wet spinning.” The polymer dope is maintained at temperatures of 90e120 Cto
avoid crystallization of the NMMO (the melting point of NMMO monohydrate is

74 C), and that makes it difficult to directly extrude filaments into aqueous regenera-
tion baths (Coulsey and Smith, 1996; Fink et al., 2001). Instead, the extruded filaments
emerge into an air gap and are cooled with a gas flow before they enter the regeneration
bath (White, 2001). The viscosity of the celluloseeNMMO solution is high, and there
occurs shear-induced orientation in the spinnerets followed by flow-induced orienta-
tion in the air gap.
In the viscose process, the cellulose precipitation in the regeneration bath proceeds
through chemical reactions, and their rates may be reduced with process modifications
as described above. Thus, it is possible to maintain the extruded filaments in an inter-
mediate gel stage and to apply stretching or elongational strains to improve chain
orientation. In contrast, in the lyocell process, cellulose precipitation proceeds via spi-
nodal decomposition through desolvation, as the nonsolvent (water) diffuses into the
extruded filaments and the solvent (NMMO) diffuses out. These diffusion rates are in
2
the range of 10 11 to 10 9 m /s, which is sufficiently high for the precipitation of the
cellulose in extruded filaments to be completed in a time frame of milliseconds to sec-
onds (Biganska and Navard, 2005). The result is that most of the chain orientation
improvement needs to be achieved before the extruded filaments enter the regeneration
bath (Mortimer and Peguy, 1996b; Mortimer and Péguy, 1996).
As the cellulose solution is extruded through the spinneret into the air gap, a shear
force is exerted on the polymer chains and increases their orientation along the filament

Figure 10.8 Cross section of viscose fibers regenerated with different levels of H 2 SO 4
concentration in the spin bath: 60 g/L (left), 100 g/L (middle), and 160 g/L (right).
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Reproduced from Roder T, Moosbauer J, WOSs K, Schlader S, Kraft G: Man-made cellulose
fibres - a comparison based on morphology and mechanical properties, Lenzing Berichte 91:
7e12, 2013 with kind permission of the publisher.