Page 464 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 437

is slow at temperatures above the melting point. The melt-quenching of the molten
PET then leads to a locally regular structure. Typical is an existence of the “nodular”
structure (Militký et al., 1991).
Due to the take up speed preorientation of amorphous PET occurs. The speed of the
molten polymer emerging from the spinneret is much less than the speed at the godet
wheel and this stretching in the semimolten state induces molecular order and orien-
tation in the fiber. Stretching during spinning leads to a huge increase of the surface
area per unit volume, which tends to reduce the chance of crystallization. On the other
hand, stretching by spinning results in molecular orientation and therefore accelerates
crystallization. It depends on the spinning conditions which effect will dominate
(Cao et al., 1989).
Depending upon the spinning speed the various kind of fibers are obtained:
• 1000e1800 m/min LOY (low-oriented yarn). The fibers are amorphous and have to be drawn
almost immediately. A semicrystalline structure is produced in drawn and annealed fibers.
• 1800e2800 m/min MOY (medium-oriented yarn). The fibers are amorphous and more
oriented. They are used in combination with immediate drawing/annealing or texturing.
• 2800e4200 m/min POY (preoriented yarn). These fibers are clearly not crystalline but they
are oriented enough to support storing for several months without becoming brittle. They are
generally used in technologies based on simultaneous drawing and texturing (Warwicker and
Vevers, 1980). POY fibers are currently produced from PET at spinning speeds between
3000 and 3500 m/min.
• 4200e6000 m/min HOY (highly oriented yarn). These fibers are partially crystalline and
exhibit high orientation. They are not fully drawn. It was found that crystallization begins
at a critical amorphous orientation of 0.18 (Gowd et al., 2004). Larger increase in amorphous
phase orientation is due to the strong interaction between crystallites and surrounding amor-
phous matrix (Gowd et al., 2004).
• 6000e10 000 m/min FOY (fully oriented yarn). The fibers are sufficiently oriented and crys-
talline, of sufficiently low extension to break and of sufficiently high tenacity to be used for
many purposes without further drawing. These products therefore eliminated the need for
drawing, although not for all uses. A problem is the growth of diameter variation (uneven-
ness) and gradual loss of strength due to the increasing spinning speed.
It is interesting that the highest tenacity and modulus are best approached by an
LOY plus high draw ratio route (Geller, 1997). It was observed for LOY (spinning
speed 500e2500 m/min) that after the simulated drawing, the stress at break is a
strongly decreasing function of spinning speed (Kim, 1986). This is in accordance
with Brody’s (1983) findings that high-speed spinning resulted in a broader or more
nonuniform distribution of the chain contour length of tie molecules in the amorphous
phase. The molecular orientation in the amorphous regions of the drawn LOY PET
fibers is much higher than in high-speed spun PET fibers (Kawahara et al., 2000).
In a typical LOY fiber spinning process the polymer viscosity increases exponentially
along the spinning thread line as the polymer is transformed from a molten to a solid
body.
Due to the increase of the spinning speed an increase of the fiber stress at break and
decrease of the elongation at break occur. When the spinning speed exceeds
5000 m/min, the elongation at break is below 70% and the yield point in the