Page 470 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 443

in the form l m z N 0:5 , where N e is the number of statistical chain segments between
e
entanglements (Huang et al., 1994).
Drawing is mainly influenced by temperature, rate of deformation, and presence of
plastifying agents. Polymeric chains in LOY undrawn fibers are only slightly oriented.
Polymeric chains in a drawn fiber are mainly oriented parallel to the fiber axis (around
80%e90% of chains). Drawing is therefore responsible for increasing fiber strength,
decreasing the deformation at break, and originating the fibrous (fibrillar) structure.
This new fibrillar oriented structure is deformed (plastically) as well. The crystalliza-
tion rate is increased with the draw ratio due to the strain-induced crystallization effect.
Ziabicki (1974) has shown for the case of uniaxial tensile deformation that the half
time of crystallization t 0.5 (f a ) is related to the mean orientation factor f a :

t 0:5 ð0Þ 2 2
¼ exp A f þ B f þ . (13.6)
a
a
t 0:5 ð f a Þ
where t 0:5 ð0Þ is the half time of crystallization of unoriented PET and A, B are
empirical constants. For small f a the higher terms are often neglected (B ¼ 0).
There exist two basic drawing processes.
Hot Drawing (homogeneous) takes place above the drawing temperature T D :

T D > T g . For polyester the minimum T D is equal to 80 C. The rate of drawing is
increased by increasing the drawing temperature or by using water as a heating me-
dium. At hot drawing temperatures, the polymer is in a rubbery state, the chains are
free to move at a molecular level, and can reorganize and reorient themselves under
the mechanical stress of the drawing process. Orientation is here mainly due to sliding
of chains. High temperatures require higher tensile stresses to ensure an orientation.
Increase of tensile stress leads to the increase of melting temperature (e.g., polypro-
pylene can be drawn at 180 C, in spite of its melting point being 173 C). Duration

of drawing is between 1 and 20 s. In course of hot drawing uniform fiber thinning oc-
curs. Since the drawing process gives additional orientation to products the draw ratio
l (3e6) varies according to the final end-use. For higher tenacities higher draw ratios
are required. Shrinkage s s force induced by orientation during hot drawing can be
expressed by a relation derived from theory of rubber elasticity (Ward et al., 1999):

2
s s ¼ N 0 kT l 1=l (13.7)

where T is temperature, k is the Boltzman constant, and N 0 is the number of chains in a
unit volume of idealized network of chains.
The drawing process generates molecular orientation, which can lead to strain-
induced crystallization. Crystallinity induced in this way is strongly dependent on
the strain rate, the temperature, and the drawing conditions (Salem, 1992, 1994). Crys-

tallinity may be developed during hot drawing in the temperature range of 130e220 C

(Cook, 1968). The so-called cold crystallization temperature T c is 128 C for PET
(Langevin et al., 1994). LeBourvellec et al. (1987) found that crystallinity and crystal-
lization kinetics depended on the degree of molecular orientation, i.e., PET deformed