Page 423 - Handbook of Properties of Textile and Technical Fibres

396 Handbook of Properties of Textile and Technical Fibres

was greatly affected by the drawing technique, predrawn morphology, and molecular
weight. They concluded that the draw efficiency increases with molecular weight, with
a more prominent effect in the noncrystalline networks than in the crystalline
networks.
There are a plethora of studies that have been conducted on single polyamide fibers
with the aim of improving their structural and mechanical properties. Methods to
enhance the microstructure and physical properties of polyamide fibers include
high-temperature zone drawing (HTZD), in which the drawing process was performed
in different temperature phases (Suzuki and Endo, 1997). This method led to an
increased initial modulus and fracture strength for the fibers. A later study applied
high-tension annealing (HTA) along with HTZD to obtain further increased crystal-
linity and mechanical properties (Suzuki et al., 1999). Further increases in the initial
modulus and tensile strength were obtained, due to increases in the crystallinity and
orientation factors. A summary of the improvements in tensile properties from
HTZD and HTA treatments for PA 66 fibers is shown in Table 12.4. Suzuki et al.
(1998) also developed a continuous zone-drawing technique for the enhancement of
microstructural and mechanical properties of PA 66 fibers. For this method, the crys-
tallinity increased from 25% to 37% and the orientation factor increased dramatically
for the fibers tested. As with the other methods, increases in the initial modulus and
fracture strength were also obtained.
Penning et al. (2003) applied a network model to describe the deformation mech-
anisms in PA 6 fibers under numerous spinning and drawing conditions. They found
that the network draw ratio can be revealed by superposition of the true stress-strain
curves and can be associated with fiber orientation using relationships for network
deformation. These enrichment techniques can be used in conjunction with the fracture
mechanics findings to fabricate a robust, high-performance polyamide fiber.
Ward and Hadley (1993) have summarized that the mechanical anisotropy of solid
polymers is determined by factors such as the structure of the molecular chain and the
crystal structure, the molecular orientation and morphology, and thermally activated

Table 12.4 Increase in tensile properties of PA 66 fibers by HTZD and
HTA treatments (Suzuki et al., 1999)
Elongation at
Fiber Initial modulus (GPa) Tensile strength (GPa) break (%)
Original 1.1 0.18 385.9
HTZD 1 4.7 0.54 34.5
HTZD 2 6.4 0.75 12.9
HTA 1 8.2 1.1 13.4
HTA 2 9.5 1.18 12.6
HTA 3 12.3 1.42 13.1