Page 465 - Handbook of Properties of Textile and Technical Fibres

438 Handbook of Properties of Textile and Technical Fibres

stress-strain curve becomes less distinctive. The stress at break exhibits a maximum at
a spinning speed of 6000e7000 m/min and then decreases (Nakajima, 2007). The
elongation decreases monotonously with spinning speed and is less than 25% at a spin-
ning speed of over 8000 m/min (Nakajima, 2007). The birefringence Dn increases
slowly up to a spinning speed of 2000 /min and then quickly above a spinning speed
of 3000 m/min. The maximum value of Dn ¼ 0.12 appears at a spinning speed of
7000 m/min and then decreases to Dn ¼ 0.08 at a spinning speed of 9000 m/min
(Nakajima, 2007). An increase in winding speed up to 3500 m/min results in an
increased orientation without any indication of crystallization (Kolb et al., 2000). At
a winding speed of about 3500 m/min flow-induced crystallization becomes apparent
(Donelly et al., 1997) while at speeds of 5000 m/min and higher, very well-developed
crystals are detected (Heuvel and Huisman, 1978). Another feature of high-speed spin-
ning is nonuniformity of the fiber macro structure, with more orientation and crystal-
linity near the fiber surface as a result of nonuniform solidification. Since fiber stresses
become concentrated in the oriented regions, the taut molecules will break first, trig-
gering rupture of the fiber before the unoriented molecules contribute much resistance.
At higher spinning speeds, the loss of overall fiber strength and tenacity therefore re-
sults (Shimizu and Kikutani, 2002).
Detailed explanation of structural changes during high-speed spinning of PET is
presented in the work by Nakajima (2007). As a result of the special molecular prop-
erties of POY, the temperature at which the heat-induced crystallization occurs is about

30 C lower than for LOY.
Higher spinning speed leads to the higher preorientation of chains. If the tempera-
ture is increased above the glass transition point T g , there is an increased reorientation
of the amorphous (noncrystalline) molecules and shrinkage occurs. The shrinkage
value increases with increasing amorphous orientation. If the spinning speed is further
increased above 2000e3000 m/min tension-induced crystallization occurs. This
blocking mechanism increases the molecular reorientation of the amorphous phase,
even at temperatures above T g , and the result is a reduction of the shrinkage. The ther-
mal shrinkage in boiling water therefore exhibits a maximum (around 60%) at a spin-
ning speed of 2000e3000 m/min, and then decreases to as low as 2%e3% at a
spinning speed of over 6000 m/min (see Fig. 13.8; Nakajima, 2007).
A higher spinning temperature leads to the lowering of tenacity, increase in defor-
mation at break, and lowering of orientation. These tendencies are quite opposite to
that which occurs when a small amount of polymethylmethacrylate is added to PET
(Yoshimura et al., 2002). This additive reduces the structure formation of PET at spin-
ning speeds higher than 3000 m/min.
When PET is melt-spun at high speeds the so-called “necking” deformation in the
threadline region occurs. This phenomenon is in fact the sudden reduction in fiber
diameter detected by a sudden jump in the velocity in the necking zone. At a take-
up velocity of 6000 m/min, the necking zone was found at a distance of
140e150 cm from the spinneret. The temperature profile has a maximum of about

160 C at the end of this zone (Hirahata et al., 1996). From an investigation of changes
of fiber diameter at a take up speed of 4000 m/min, necking was identified at a distance
between 40 and 47 cm from the spinneret (Hirahata et al., 1996).