Page 524 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 497

assumption has been proved experimentally (Wu and Cucullo, 1999). The lowering of
T m is then described by the Nishi Wang equation:

f
T m ¼ T 0 1 þ BV PET 2 (13.79)
m
H PET
3
0
where T is the equilibrium melting point, V PET ¼ 0.7477 cm /g is the molar volume,
m
H PET ¼ 121.24 J/g is the heat of fusion for PET, B is the interaction energy density,
3
and f is the volume fraction of PEN in the fibers. The value of B ¼ 17.6 J/cm has
0

been found (Shy and Jabarin, 2001). The values of T are about 25e30 C higher than
m
T m for the same f. The value of B is directly connected with the Flory Huggins
interaction parameter:
c ¼ BV PET =ðRTÞ (13.80)
where R is the universal gas constant. The small negative value of c ( 0.407 for

300 C) shows that the blend system can form a thermodynamically stable, compatible
mixture in the melt (Shy and Jabarin, 2001). The dependence of the melting point
temperature on the concentration of PEN is nearly linear and corresponds to a least
squares regression model with the form.
T m ¼ 256:101 202:256f (13.81)

This linearity is probably valid in the restricted range of PEN concentration only. It
is well known that the T g of random copolymers is lowered due to the presence of
modifying units. Because the PET/PEN forms a random copolymer the glass transition
temperature is well described by the linear rule of mixtures:

T g ð f Þ¼ T gPEN f þ T gPET ð1 f Þ (13.82)

where T g ( f ) is in units ( C). From published values of T gPET ¼ 81 C, T gPEN ¼ 123 C
an ideal T g ( f ) can be predicted. In reality T g is dependent on an f molecular charac-
teristics (molar mass and corresponding distribution) and structural factors and
therefore these parameters should be computed for real polymers. The experimental
dependence of T g on f is shown in Fig. 13.37.
The linearity in dependence of T g on f is clearly visible. The corresponding least
squares regression model has the form.

T g ¼ 77:402 þ 50:86f (13.83)

From these values the T g of both polyesters has been computed as T gPET ¼ 77.43,
T gPEN ¼ 128.29.
For the characterization of the tensile mechanical behavior of PET/PEN samples
(see Table 13.2) the following characteristics were evaluated (Militký et al., 2011):

P Tenacity (cN/dtex) as load at break
E Elongation (%) as deformation at break