Page 421 - Handbook of Properties of Textile and Technical Fibres

394 Handbook of Properties of Textile and Technical Fibres

conditions, and microstructural changes of these fibers is very important so as to be
able to control mechanical properties and dimensional stability (shrinkage). Boiling
water shrinkage of the standard polyamide fiber is about 12%. By heat setting it is
possible to reduce this value significantly during fiber production or during textile
processing. Heat treatment (setting) during fiber production is often called annealing.
Annealing is usually applied to modify the microstructure and stabilize the geometry
of fibers. Heat setting of PA fibers increases the crystallinity, crystallite size, and crys-
tal perfection. Based on the conditions, the orientation of individual structural phases
can be changed as well. Heat setting of PA fibers is complicated by crystalline poly-
morphic transitions and moisture sorption.
The effects of heat setting on the structural characteristics of nylon 6 fibers was
investigated by Murthy et al. (1995). From their experimental results, they deduced
that heat setting increases the crystallinity and crystalline perfection, and in addition
increases the density of the fiber to a large extent. This is in contrast to the cold-
drawing process.
The changes of a PA 6 fiber structure after free-state and under high-tension heat
setting is shown in Fig. 12.30.
The corresponding changes of a PA 6 fiber tensile stress at break is shown in
Fig. 12.31 (Park et al., 1978).
The free-state heat setting is accompanied by a relative lengthening of tie molecules
and by a widening of their length distribution (Kausch, 1985). These inhomogeneities

Free-state heat setting Highest tension heat
at 199°C Control setting at 199°C

Figure 12.30 String model of heat setting (Park et al., 1978).