Page 420 - Handbook of Properties of Textile and Technical Fibres

The chemistry, manufacture, and tensile behavior of polyamide fibers 393

The basic conditions for good setting are fiber plastification achieved by hot air

(heat setting optimum temperature for PA 6 is 190 C and 225 C for PA 66 (Fourné,

1999)), water steam (heat setting optimum temperature for PA 6 is 128 C and 130 C
for PA 66 (Fourné, 1999)), and plastification compounds (optimum temperature
depends on swelling agents). The properties and shape of fibers set at temperature
T S are not influenced by thermal treatment till T S e 30 C(Hearle and Miles, 1966).

The upper limit of T S is around T m e 30 C. For PA 6 and PA 66 the density increases
with the heat setting temperature (Fourné, 1999). For obtaining fibers with a residual
shrinkage of <0.5% the free-state heat setting should be used.
It was found that free-state heat setting leads to change in some of the molecules in
oriented PA 66 fibers from the elongated to the folded conformation, with no change in
crystal orientation (Dismore and Statton, 1966). These changes of structure cause
shrinkage of the fibers during the heat setting (see Fig. 12.29(a)) probably governed
by the number of folds that are introduced. Tensile strength is also reduced (see
Fig. 12.29(b)).
During heat setting in the free-state, the intermolecular bonds are dissociated allow-
ing short lengths of molecules to rearrange and form more stable bonds. The small,
nonperfect, crystallites are remelted into bigger and more stable ones (Sengupta,
1997a).
During heat setting under tension, the T g and breaking load have been seen to reach
a maximum depending on the heat-setting conditions (Valk et al., 1980). This can be
explained by the assumption that the average size of crystallites reaches a minimum,
and therefore the number of crystallites per unit volume shows maximum values at a

heat-setting temperature of about 190 C. By increasing the heat-setting temperature
above 190 C, the crystallites become larger and their segregation in the noncrystalline

matrix occurs (Valk et al., 1980). This kind of behavior, which has been found for
polyester, will be similar for polyamides as well.
Polyamide fibers undergo various heat-setting processes during processing in
textile production. Knowledge of the relation between thermal history, processing

(a) (b)
50
10
40
8
Shrinkage (%) 30 Breaking load (lb) 6 4

20
10 2

0 0
160 180 200 220 240 260 140 180 220 260
Heat setting temperature T (°C) Heat setting temperature T (°C)
Figure 12.29 PA 66 fiber (a) shrinkage and (b) breaking load during free-state heat setting
(Dismore and Statton, 1966).