Page 474 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 447

determined by the orientation in the amorphous phase and the mean relative molecular
mass of the polymer. Amorphous shrinkage can occur at low temperatures (less than

100 C) only. Corresponding shrinkage forces lie in the region of 10e20 mN/tex.
Another shrinkage mechanism is the so-called crystalline contraction. It occurs espe-
cially in differentially shrinkable fibers. This type of contraction is provoked by rear-
rangement of the crystalline phase connected with the formation of “perfect”
crystallites with folded chains. The rearrangement of the crystalline phase can occur

at relatively high temperatures only (above 200 C for PET) and its prerequisite is
the presence of a greater number of less than perfect crystallites. This kind of structure
is formed during setting under tension or in modified polyesters.
There are two basic ways of fiber setting:
Isotonic setting: when the shrinkage of the fiber as result of chain retractions and
relaxation of internal stresses occurs. These changes are stabilized by recrystallization.
The orientation and strength of fiber are decreased and elongation to break increases.
Practically, this type of setting is implemented in the free-state without limitation of
dimensional changes (annealing or slack heat setting).
Isometric setting: when there are no dimensional changes of fibers. The main mech-
anism is the relaxation of internal stresses associated with chains sliding and stress-
induced crystallization. The orientation and strengths of fibers are unchanged. This
type of setting practically runs at a constant length, when the fibers cannot be
deformed.
The semicrystalline structure appears mainly during thermal setting in isometric or
isotonic states. The structural differences caused by these types of heat setting are sche-
matically shown in the Fig. 13.12.
Fibers after setting typically have a degree of crystallinity of about 0.4, an orienta-
tion factor of crystalline phase of about f c > 0.95 and an orientation factor of amor-
phous phase of about f a ¼ 0.6. Basic structural units are relatively strong micro
3
fibrils having diameters of 10e15 nm and a length of 10 nm. These units are assem-
bled into fibrils having diameters of 30e45 nm. A very important role is played by taut
tie molecules (TTM). The portion of TTM is around 0.1e0.5. This phase is responsible
for the mechanical properties of PET fibers.

Isotonic Isometric
Figure 13.12 Structural differences due to basic heat setting types.