Page 364 - Fiber Fracture

346 J.W.S. Hearle

SOLUTION-SPUN FIBRES

Structure and Stress-Strain Curves

Regenerated cellulose, cellulose acetate, acrylic and some other fibres are spun from
solution, either by dry spinning, with evaporation of solvent, or by wet spinning into a
coagulation bath. In viscose rayon, the solute is sodium cellulose xanthate, which is a
chemical derivative of cellulose soluble in caustic soda, so that a chemical reaction is
involved in fibre formation.
There is structural uncertainty similar to that for the melt-spun synthetics. The
fibres are partially crystalline and partially oriented. The density and other analyses of
regenerated cellulose fibres indicate an effective crystallinity of about 33%. The method
of regeneration affects the structure. In ordinary viscose rayon, there is a micellar
structure, which could be represented somewhat as in Fig. 9b. Due to the differential
mobility of sodium and hydrogen ions, the regeneration produces a skin, which has a
fine texture and is stronger than the core, which has a coarser texture. Modification of
the chemistry gives higher-strength rayons, which are 'all-skin'. Other variations give
high-wet-modulus rayons, which have a fibrillar texture. The newer iyocell fibres, which
are spun from a solution of cellulose in an organic solvent, also have a fibrillar texture.
Secondary cellulose acetate, with one -OH group to five acetate groups, is poorly
crystalline due to the irregularity of the molecules. More information on regenerated
cellulose fibres is given in Woodings (2000). Acrylic fibres are atactic copolymers, with
a small percentage of a monomer other than acrylonitrile, and the structure is assumed
to be quasicrystalline, with regions of locally aligned molecules that are not in 3D
crystallographic register.
In both cellulose and acrylic fibres, there are strong intermolecular forces in the
disordered as well as the more ordered regions, though these are weaker than the
covalent bonds in the main chains. In cellulose, the cross-links are hydrogen bonds, and
in acrylic fibres they are polar interactions of the -CN groups. The stress-strain curves
of an acrylic fibre, shown in Fig. 1 1, are typical of these materials. The curves, S, ST and
W20, at 20°C all show a marked yielding at about 2% extension, when the intermolecular
bonds in the disordered regions start to break. The upper graph shows that the elastic
recovery falls off sharply at the same strain. There is a second-order transition at around
80"C, when the polar interactions become mobile, and this leads to the low modulus
and high break extension at 95°C in curve W95. As shown below, viscose rayon shows a
similar behaviour, except that it is absorption of water, not increase of temperature, that
results in the low modulus due to the mobility of hydrogen bonds.
There has been little analytical modelling of the mechanical properties of this group
of fibres. Hearle (1967) treated the wet and dry properties of rayon fibres in terms of
the composite models shown in Fig. 12 by following the well-known mixture laws.
The series structure, Fig. 12a, which is dominated by the soft component, averages the
strains at the same stress, and the parallel structure, Fig. 12c, which is dominated by
the stiff component, averages stresses at the same strain. The stress for the micellar,
Fig. 12b, form is somewhat arbitrarily placed in a mid-way position. In the wet state,
Fig. 12d, the component stress-strain curves are assumed to be linear, with a high