Page 472 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 445

Figure 13.11 SEM micrograph of a longitudinal section through the neck region of an aged
polyethylene terephthalate fiber (http://www.irishscientist.ie/p186b.htm).
Sarna E, Wlochowicz A: Damage to cotton surface during spinning process, Microsc Microanal
9(Suppl. 3):366e367, 2003.

readily transforms into a crystalline phase (Keum, 2008). The result is an amorphous
contraction and simultaneous crystallization into the normal crystalline (triclinic)
form. It was also observed that crystallite size (ranging from 2.5 to 4.0 nm) is increased
with draw ratio (Salem, 1992). Oriented PET fibers have a half-time of crystallization
shorter than 0.01 s (Smith and Steward, 1974) and this process is in competition with
shrinkage. Complete shrinkage is therefore usually obtained by shock heating only.
When sub T g physically aged, unoriented, amorphous PET fibers are cold drawn,
high draw ratios can be achieved, and a sheath/fibrillar core microstructure results.
In the fibrillar core slitlike voids of typical width 0.4 mm and length 5 mm can be
seen (see Fig. 13.11). The slit voids represent 1% to 2% of the fiber volume and appear
to be formed at the instant when the fibrils are formed and simultaneously pulled apart
by the extreme stress of cold drawing (Hughes et al., 1999).
It has been shown that PET fibers can be treated with acetone or dimethylforma-
mide/water solutions to plasticize the fiber, along with creating an imperfect crystal
structure that enhances drawability at cold drawing (Ito et al., 1992b). The resultant
fibers can be drawn to higher draw ratios with the use of a two-stage draw technique
resulting in 20% higher strength values. Use of subcritical (at pressures below critical
value 72.8 atm) and supercritical CO 2 as a drawing media has similar effects as use of
the above-mentioned solvents (Hobbs and Lesser, 1999).

Rietsch et al. (1979) studied tensile drawing of PET from 20 to 80 C. Cold drawn
PET was observed to neck at a natural draw ratio of 4.3, roughly independent of rate
and temperature (rates ranged from 0.05 to 5 cm/min). Hot drawn PET, however,
deformed uniformly. Sweeney et al. (1999) found that necking would occur in PET

below 60 C and would not occur above 80 C.
The necking phenomenon starts at the yield point where the engineering or nominal
stress reaches a maximum before it drops as a result of localization of the plastic