Page 416 - Handbook of Properties of Textile and Technical Fibres

The chemistry, manufacture, and tensile behavior of polyamide fibers 389

It is apparent that the amorphous network structure developed in the spinning thread
is preserved in the fiber structure in spite of post-crystallization (Penning et al., 2003).
It has been found that mechanical properties of high-speed spun PA6 fibers show an
increase in an initial modulus, tenacity and yield strength up to 5000 m/min and a
decrease above 5000 m/min (Vasanthan, 2008).

12.4.2 Drawing

Fully drawn fibers can be obtained by a drawing operation. For undrawn fibers,
drawing is a major operation changing the structure from the partially crystalline
(spherulitic) nonoriented undrawn state to a crystalline (fibrillar) drawn state with
much better end-use properties.
The filaments after solidification have a certain preorientation that is caused by the
spinneret orientation and the spin draw (take-off rate), but they still have a certain
degree of permanent elongation that will only be removed by the drawing process
(Sengupta, 1997a).
For polyamide fibers, cold-drawing (below T g ) is typical where the drawing zone
is at the neck point (the zone where the fiber diameter suddenly drops from a thick
undrawn fiber to a thin drawn fiber). This zone has a length of about 0.3e2 mm for
most fibers. The changes of fiber structure and geometry during cold-drawing are
shown in Fig. 12.24.
An increase in polymeric chains orientation is typical for cold-drawing, but crystal-
linity is not always positively affected (it can be decreased due to partial destruction of
crystallites). Cold-drawing and neck formation are the results of mechanical instability

Drawn

Tie Oriented
molecules fibrillar
structure
Fibrils
Unoriented
structure
Neck
point

Undrawn
Figure 12.24 Changes of fiber geometry and structure during cold-drawing of polyamides.