Page 367 - Handbook of Properties of Textile and Technical Fibres

340 Handbook of Properties of Textile and Technical Fibres

axis (Mortimer et al., 1996; Mortimer and Peguy, 1996a). The filament diameter de-
creases, and chain orientation increases, with increasing distance from the spinneret
(there is an initial swelling of filaments as they exit the spinneret caused by the visco-
elastic nature of the solution, which is termed “die-swell”). The temperature of the fil-
aments also decreases with increasing distance from the spinneret, and there is a
corresponding rise of the solution viscosity. At a distance from the spinneret, there
are no further changes of orientation or of filament diameter as the filament tempera-
ture reduces to a point that allows no further movement of the molecular chains. The
ultimate degree of orientation at this point is inversely correlated both with the spin-
neret diameter and extrusion velocity, and directly correlated with the filament take-
up velocity. No further changes of orientation occur when the extruded filaments enter
the spin bath at this time, but there is evidence of the chains organizing into crystalline
structures, and this is consolidated when the fibers are dried. Thus, a large part of the
focus in improving the tensile properties of the regenerated fibers is on controlling the
temperature, humidity, and length of the air gap (Fink et al., 2001).
As the extruded filaments enter the coagulation bath, the rapid exchange of the sol-
vent with nonsolvent causes a fast coagulation of the cellulose and a skin is formed
(Biganska and Navard, 2008; Moss et al., 2002; Gindl-Altmutter et al., 2014). The
skin exhibits thinner fibrils, greater orientation, and higher crystallinity than the
core, and its thickness increases with cellulose concentration in the dope and decreases
with rising temperature of the bath. However, the average thickness of the skin is about
50e100 nm (Abu-Rous et al., 2006), and therefore it does not exert any significant in-
fluence on the tensile properties of fibers.

10.5 Summary

The field of regenerated cellulosics continues to develop both in terms of the technol-
ogy of fiber production as well as in their applications. New solvent systems continue
to be investigated with a view to improve the mechanical performance of the resulting
fibers, to reduce the environmental impact of the manufacturing process, and to
improve process safety. One interesting development is to create fibers by assembling
and aligning nanofibrils obtained from wood pulp, i.e., without dissolution and regen-
eration of the polymer (Håkansson et al., 2014). These would definitely classify as arti-
ficial but perhaps not necessarily as “regenerated” cellulosic fibers.

References

Abu-Rous M, Ingolic E, Schuster KC: Visualization of the nano-structure of Tencel (R)
(Lyocell) and other cellulosics as an approach to explaining functional and wellness
properties in textiles, Lenzing Berichte 85:31e37, 2006.
Adusumalli RB, Reifferscheid M, Weber H, Roeder T, Sixta H, Gindl W: Mechanical properties
of regenerated cellulose fibres for composites, Macromol Symp 244:119e125, 2006.