Page 473 - Handbook of Properties of Textile and Technical Fibres

446 Handbook of Properties of Textile and Technical Fibres

deformation. Plastic flow localization proceeds gradually up to a strain value that is
called the natural draw ratio l p . The natural draw ratio is dependent on temperature
and rate of deformation (Seguela, 2007). This parameter is dependent on the nature
of the polymer:
• Stiff polymers (polystyrene, aramids) have l p ¼ 1.5e2.5 (as for viscose).
• Semicrystalline polymers with lower stiffness such as polyamides and PET have l p ¼ 3e6.
• High crystalline flexible polymers such as polypropylene and polyethylene have l p ¼ 5e10.
The draw ratio has a major effect on fiber elongation and tenacity. High draw ratios
give high-tenacity fibers with higher moduli and lower extensions to break; low draw
ratios give lower tenacities with much more extension. A semiempirical rule connect-
ing elongation to break ε b and tenacity s b (cN/dtex) has been proposed:
a
s b ¼ Kε b (13.8)

where K and a are constants. Experimentally, a z 0:3 and K is a measure of the
inherent fiber strength and is related to the molecular weight. This parameter will also
increase if, after drawing, heat setting is used to crystallize the oriented structure.

13.3.3 Heat treatment
The heat setting step usually accompanies the drawing process. The purpose of this
step is:
• stabilization of the dimensions of the fibers (reduction of shrinkage caused by chains
retractions),
• relaxation of internal stresses in the fiber,
• creation or stabilization of the crystalline structure (the melting of small imperfect crystallites
and the formation of more perfect larger ones).
The level of fiber tension and the heat setting temperature can both have significant
effects on the final properties of fibers. Basically, there is a competition between two
separate processes, i.e.,
• Crystallization, starting in the oriented amorphous mesophase (extended chain crystalliza-
tion) and then extending into low oriented amorphous phase (folded chain crystallization)
(Keum, 2008).
• Shrinkage due to chain disorientation via bond rotation in amorphous regions, i.e., changes
of amorphous chains conformation from the trans to gauche form.
The relative rates of these two processes are mainly affected by tension and temper-
ature, thus a huge range of different fiber properties can be achieved.
Shrinkage varies strongly with the mode of treatment. If relaxation of stress and
strain in the oriented fiber is allowed to occur through shrinkage during heat setting,
then shrinkage at the textile processing stage is reduced and initial modulus is lowered.
Heat setting with a fixed fiber length, i.e., under tension during heat treatment, is less
affected by a change in modulus and reduced shrinkage values can still be obtained. In
semicrystalline PET fibers “amorphous” shrinkage takes place partially. The amount is