Page 480 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 453

and conformational defects leading to the perfection of the crystalline structure and the
formation of larger, more perfect crystals. Second, there is formation of a new crystal-
line morphology in the form of folded chain lamellae, which grow perpendicular to the
chain axis. These secondary crystallites formed during heat setting are qualitatively
different and apparently much less stable than those formed during stretching (Greener
et al., 1999). Substantial disorientation due to relaxation of the tie chains connecting
neighboring crystals was noted in fibers subjected to prolonged isometric heat setting
(Gupte et al., 1983). However, no changes were observed in crystal orientation and a
marked increase in tensile modulus occurred for oriented PET held under stress upon
prolonged heat treatment (Itoyama, 1987).
Isometric heat setting does not reduce the fraction of tie molecules but relaxes them
by increasing their contour length, without changing their end-to-end distance. If their
number is sufficiently high, i.e., at a high draw ratio, they can slowly crystallize at
room temperature and form axial crystal bridges. They very efficiently transmit the
axial forces and prevent shrinkage during a new heat treatment (Peterlin, 1978a).

13.3.4 Structure of polyester fibers

The fiber structure depends greatly on the process parameters of fiber formation such
as spinning speed, drawing, and heat setting. Final fiber structure depends consider-
ably on the temperature, rate of stretching; draw ratio, relaxation, and heat setting con-
dition. The crystalline and amorphous orientation and the percentage of crystallinity
can be significantly adjusted in response to these process parameters. The polymer
structure is generally described by two hierarchical levels:
• the molecular level (molecular chains and their construction)
• the supramolecular level (crystalline and amorphous regions)
These two levels are determined by the chemical composition of the polymers.
Theoretically, PET should contain hydroxyl (eOH) end groups only. But owing to
the effect of various degradation reactions, such as hydrolysis, thermal oxidation,
etc., taking place during polycondensation or melting of PET, carboxyl (eCOOH)
end groups are also produced. In different PET fibers the acidity caused by these
end groups ranges from 2 10 2 to 4$10 2 mol/kg (D’Allo, 1977). Furthermore,
carboxyl groups are responsible for additional degradation because they catalyze the
hydrolysis of ester bonds. Therefore, when high-strength industrial fibers (tire cords)
are prepared, inhibitors are used to react with the carboxyl groups restricting the degra-
dation process.
Due to effect of side reactions about 1.5%e3% of DEG is always produced. DEG is
introduced into the chains as a statistical copolymer. Finally, the PET fiber contains
1.4%e3.8% of oligomers (cyclic trimer mainly) on average.

13.3.4.1 Molecular structure

The PET fibers have a rigid benzene ring in its backbone. Individual chains contain
sequences of six aliphatic groups (dCOdOdCH 2 dCH 2 dOdCOd). Practically,