Chemistry Reference
In-Depth Information
was obtained. The high molecular weight PBO showed very good spinnabil-
ity resulting in as-spun fibers of very high tenacity (3 GPa) and very high
modulus (300 GPa). After heat-treatment the properties were even better,
3.4 GPa (
∼
25 gpd) and 400 GPa (
∼
3
,
000 gpd) respectively.
PBT and PBO fibers were spun from their liquid crystalline solution
using the dry-jet wet spinning process. For instance, PBO fibers may be
prepared by spinning of 13 wt-%-17 wt-% PBO/PPA solution at 60
◦
Cto
90
◦
C. The as-spun fibers were then heat-treated under tension at
500
◦
C
for 60 seconds. As for the preparation of Kevlar fibers, the heat-treatment
under tension is the key to achieving the highest-possible property of PBT
and PBO fibers. Allen
et al
. (1985) studied the effects of heat-treatment
in a nitrogen atmosphere on PBT fibers. The variables include the tem-
perature, the tension, and the time taken for the as-spun fiber samples for
the study to be made by dry-jet wet spinning of a PBT with an inherent
viscosity of 14 dL/g. The tensile modulus of the as spun fiber was 150 GPa.
Its tensile strength was 1.6 GPa. The tensile mechanical properties were
doubled to 300 GPa and 3 GPa by the most suitable heat-treatment. To
achieve these values, temperatures of 630
◦
C-680
◦
C with a duration time
of under one minute were required. The tensions applied were in the range
of 150 MPa-200 MPa. To interpret the significant improvement in property
by heat-treatment, Allen and coworkers studied the structure change using
wide-angle X-ray diffraction. They found the crystallite size perpendicu-
lar to the fiber axis increased from approximately 2 nm in as-spun fibers
to 10 nm-12 nm in fibers after heat-treatment. Fiber tensile strength was
found to increase with this increase in the extent of the lateral molecular
order. With measurement of the azimuthal spread of the major equatorial
reflections of the fibers, they also found that a higher applied tension dur-
ing heat treatment had led to a higher overall molecular orientation with
respect to the fiber axis. For example, the azimuthal spreads of the two
major equatorial reflections were 12
.
5
◦
,
7
.
3
◦
and 5
.
5
◦
respectively for three
samples heat-treated at 640
◦
C in nitrogen for 8 seconds under different
tensions (0, 32 and 190 MPa). The Herman's orientation factor
f
was 0.93,
0.98 and 0.99 while the value for the as-spun fiber was 0.87. This increase
in axial orientation is associated with significant increases in both tensile
modulus and tensile strength. Thus the primary factors affecting the tensile
properties by heat-treatment are the increase of overall axial molecular ori-
entation combined with increases in the extent and perfection of the lateral
molecular order in the fiber. In Table 5.3 are typical tensile properties of
PBT and PBO fibers.
∼
