Page 476 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 449

rule-of-thumb, which defines the necessity to go to higher temperatures to achieve a
new state, with the distribution of crystal size and perfection being shifted to higher
values.
To secure the results obtained by setting it is therefore essential that the maximum

temperature used in subsequent thermal treatments is 15e20 C lower than the setting
temperature. The temperature used in subsequent aqueous treatments under pressure

should be roughly 30 C lower than the temperature of heat setting. This means that

before high-temperature dyeing at 130 C, the material must be processed by heat

setting at a temperature higher than 170 C, for instance at 180 C.

The heat setting effect in PET is highly time-dependent. The rate of setting in-
creases rapidly with increase of temperature. The structural and morphological
changes due to heat setting are presented in the review (Gupta, 2002).
13.3.3.1 Isotonic setting

Peszkin and Schultz (1986) annealed PET fibers at temperatures ranging from 100 to
200 C under a small tensile force. They observed that competition existed between

chain-recoiling (shrinkage) and crystallization. The crystallization kinetics increases
with higher temperatures and higher tension. Chain orientation also increases with
tension (Peszkin and Schultz, 1986). Heat setting of PET fibers under a higher tensile
force provokes crystallization in two stages (Lee and Schultz, 1993). In the first stage,
nucleation starts and fibrillar crystallites grow as the fibers begin to elongate. Chain
mobility in the amorphous phase is restricted gradually and reaches its asymptotic
value. Concurrently, tensile modulus and tenacity increase markedly. In the second
stage, a microstructure is created and the perfection of crystallites continues to
increase. Fiber elongation during heat treatment reaches its asymptotic value. Simul-
taneously, the increasing trend in tensile modulus is stopped and reaches its asymptotic
value (Lee and Schultz, 1993).
High-temperature (above 200 C), high-tension heat setting maintains a high level

of orientation in the amorphous regions, hence a high fiber modulus and a relatively
lower dyeing rate. The latter can be improved by reducing the heat setting temperature
so that less crystallization occurs. If the fibers are heated with low tension, disorienta-
tion of the oriented amorphous regions occurs and the fibers are left with low shrinkage
forces (and modulus) but high dyeability.
The three stage model of oriented PET structural changes during its isothermal crys-
tallization was proposed in the work by Radhakrishnan and Kaito (2001). The first
stage is thermodynamic relaxation, which occurs when the material is heated above
its T g . The second stage is the structural change of the oriented amorphous structure,
where the degree of orientation increases from the nearly isotropic state and the gauche
conformation is transformed into the trans-conformation. A crystalline structure
appears in the third stage, only after the orientation process is completed. The bundles
of highly oriented tie chains in the amorphous phase (mesophase) are probably respon-

sible for the first stage of crystallization (80e100 C); the less ordered or nonoriented
chains are responsible for the second stage of crystallization at temperatures above

140 C(Keum, 2003).