Page 422 - Handbook of Properties of Textile and Technical Fibres

The chemistry, manufacture, and tensile behavior of polyamide fibers 395

Figure 12.31 Effect of heat-setting
conditions on stress at break of PA 6
fibers (Park et al., 1978).
1000
11.7 % elongation
Stress at break (MPa) 900 Control No elongation

Free-state

800

120 140 160 180 °C
Heat setting temperature T (°C)

lead to accelerated flaw growth, i.e., to a loss of strength. Heat setting at fixed fiber
ends leads to a certain loss in homogeneity but maintains the average relative lengths
of tie molecules. The loss in strength as compared to the control sample is small. Heat
setting under a 11.7% prestrain increases the relative length of tie molecules, their
homogeneity, and leads to a marked increase of fiber strength.
Heat setting also acts as a thermal healing operation. It helps in sealing fissures,
surface cracks, etc. caused by drawing or stressing operations (Sengupta, 1997a).
The effectiveness of heat setting in polyamides is considerably enhanced by using
steam, as observed from changes in density (Warwicker, 1971). In the presence of
moisture, there is considerably greater mobility of the molecular chains, which leads
to a higher degree of set.
Polyamides gradually lose set over the time span of the experiment depending on
recovery temperature. For PA 6, which is set at 170 C, and having a degree of set

of 85%, (Prevorsek et al., 1975) when recovered at 23 C, this value reduces to about

80% at 50 C and 35% at 100 C. For a similar heat set of polyester, the degree of set

remains unchanged over this range of temperatures (Sengupta, 1997a).
12.4.4 Influence of processing on fiber structure
An extensive investigation by Lim et al. (1989) on PA 6 quantified the influence of the
drawing process on molecular orientation preferentially, either parallel, or approxi-
mately parallel to the fiber axis. Ito et al. (2001) have shown that the molecular weight
of polyamides affects these factors significantly. From a comparison of a tensile
modulus versus draw ratio, the researchers revealed that the draw efficiency of PA 6