Page 471 - Handbook of Properties of Textile and Technical Fibres

444 Handbook of Properties of Textile and Technical Fibres

at higher temperatures and crystallized more slowly due to the fact that more molecular
relaxation occurs. It is interesting that strain-induced crystallization did probably not
occur during drawing but was postponed until the moment when deformation stopped
(Blundell et al., 1996). Crystallization can therefore occur during deformation at lower
strain rates, with more crystallization occurring during drawing at low strain rates than
at high strain rates. At very high strain rates no crystallization would occur during
drawing.
When the temperature of drawing is relatively high and the strain rates low, PET
fibers will stretch without resultant orientation occurring. This is often denoted as
flow drawing (Radhakrishnan and Gupta, 1993). It was suggested that under these con-
ditions molecular relaxation processes predominate over the orientation process.
Consequently the drawing tension drops to low levels.
In the work by Dargent et al. (2000) the influence of water presence on the hot
drawing process was investigated. It was found that water does not modify the degree
of crystallinity of drawn PET but blocks the growth of a part of the crystallites and
modifies their crystalline size. At a large draw ratio, the effect of water molecules
on the orientation of the amorphous phase decreases and vanishes for approximately
a draw ratio of 6. Water molecules have also an influence on strain-induced crystalli-
zation by shifting the crystallites size distribution to lower values.
The blowing of hot air during drawing at a temperature of 220 C and using a

1
maximum strain rate of 18.7 s produced fibers with improved mechanical properties
(Suzuki et al., 1999). The resulting fiber had a degree of crystallinity of 44%. Despite
drawing at high temperatures close to the melting point, the momentary heating at high
temperatures promotes the strain-induced crystallization and alignment of chains
rather than chain slippages (Suzuki et al., 1999).
Cold drawing (heterogeneous adiabatic) at temperatures lower than the glass tran-
sition temperature, T g is characterized by huge plastic deformations at constant stress
and the appearance of necking. The cold drawing process is quite exothermic. In the
neck region the energy is dissipated into heat, viscosity is locally decreased, and chain
orientation occurs. In the case of semicrystalline polymers the melting of lamellar
structures occurs as well. With increased drawing speed the necking zone becomes
more distinct because the draw heat cannot dissipate as quickly. The duration time
of drawing is here very short, around 0.005 s only. The amorphous phase in drawn
PET fibers that have not yet been thermally treated comprises 90% or more of the
structure. Their structure is in the form of a “frozen” physical network with chain
entanglements as knots. Cold drawing of PET amorphous samples induces also the for-
mation of a highly ordered metastable mesomorphic form (Bonart, 1966), which leads
to a drastic increase of the packing density (Keum and Song, 2005). This mesomorphic
form of PET corresponds to a solid mesophase characterized by a parallel arrangement
of chain axes and long-range positional order of a structural feature only in one dimen-
sion, i.e., along the chain axis and the absence of any long-range order in the lateral
packing of the chains (Cole et al., 2002). This phase is stable at temperatures lower
than T g but highly unstable above T g . When cold drawn fibers are heated above the

glass transition temperature (for amorphous PET it is about 70 C in the dry conditions

and 50 C in wet) the mobility of the physical network is released and the mesophase