Page 469 - Handbook of Properties of Textile and Technical Fibres

442 Handbook of Properties of Textile and Technical Fibres

drawing and shrinkage force of PET. They found that by modeling the deformed PET
as a network, the properties of drawn material could be correlated with deformation
histories. Gordon et al. (1994) also observed that their results from the two-stage
uniaxial stretching of PET could be consistently interpreted using a molecular network
model.
In both cold and hot drawing, the principle of drawing is the longitudinal tensile
deformation of 20%e2000% from a starting length l 0 . The draw point where fiber
drawing takes place is usually done by a stationary heated metal pin around which
the fiber passes (see Fig. 13.10).
The industrial hot drawing process of a fiber involves passing the fiber over a series
of rollers. The rollers rotate at specified constant angular velocities, each faster than the
other. In the two-stage draw process most of the draw is provided in the first stage
(between first and second roller) and a relatively smaller draw in the second stage
(between second and third roller). As the number of stages is increased, it is possible
to keep each roller at a different temperature and induce the maximum possible draw in
each stage in order to obtain the maximum molecular orientation in the fiber (Bechtel
et al., 2001).
In zone drawing and annealing, the molecular chains are oriented by localized heat-
ing, which introduces a steep temperature gradient and a high strain rate (Kunugi et al.,
1981). Localized heating can be carried out using a CO 2 laser, which allows accurate
fixing of the drawing position without contact (Okumura, 2004).
For preparation of high-modulus PET fibers, vibrating hot drawing is very efficient
(Kunugi and Suzuki, 1996). A variety of drawing methods for improving mechanical
properties of PET are summarized in the work by Kunugi and Suzuki (1996).
The main parameter of the drawing process is the drawing ratio, defined as the ratio
of drawn fiber length l to the starting length l 0 .

l v 2
l ¼ ¼ (13.4)
l 0 v 1
In fact, the total drawing ratio l is the sum of partial drawings at individual stages of
the drawing process l 1 , l 2 .i.e.,

v 2 v 3
l ¼ l 1 l 2 . ¼ ::: (13.5)
v 1 v 2
where v 1 is rate of first drawing roller, v 2 is rate of second drawing roller, and v 3 is rate
of third drawing roller (for two-stage drawing).
The maximum attainable draw ratio l m is estimated as the ratio of the extended-
chain length to the root mean square end-to-end distance of an unperturbed random
coil (Kanamoto et al., 1983). To predict the maximum attainable draw ratio as a func-
tion of the molar mass M, the simple relation l m z M 0:5 can be used (Kanamoto et al.,
1983). It was derived for isotropic amorphous PET where l m w 3.45 (Lorentz and
Tassin, 1994). The assumption that the majority of the entanglements in PET will
act as permanent crosslinks leads to the evaluation of l m from rubber elasticity theory