PET YARN AFTER PROCESSING—HEAT-SETTING AND BULKING

Drawn filament yarn can be treated in a number of ways. It may simply be wound onto a yarn
package, twisted on a ring frame, or sent for a yarn bulking process such as false-twist
bulking. One of the major breakthroughs in the 1970s was the introduction of high-speed
yarn winders, which gave large cylindrical yarn packages (up to 15 lb of yarn) and ran at
3000 m=min (113 mph). The yarn traverse was a major technological enabler, as without a
reliable high-speed traverse to keep pace with the windup speeds, the process was not
runnable (i.e., conversion efficiency of polymer to salable yarn was <~90%). The problem
was that the traverse guide had to reverse instantaneously and reliably at the end of each
traverse stroke. Any ‘‘dwell’’ would cause a buildup yarn at the bobbin edges and the yarn
would simply slough off. Engineering solutions were eventually found and nowadays windup
speeds can be 6000 m=min or even higher.
Many apparel yarns need to be textured or ‘‘bulked’’ to give desirable esthetic properties,
particularly for cotton blends and women’s wear markets. This may be done during drawing
(draw-bulking) or in a separate process. The number of bulking processes is numerous and for
those wanting more detailed descriptions, a reference to a specialist publication is provided [37].
The principle of the so-called ‘‘false-twist’’ bulking is to create minor side-to-side variations in
molecular orientation across a given yarn, causing the yarn to bend during controlled thermal
shrinkage to create a 3D structure with a bulky feel. The process entails running a continuous
yarn through a device that twists it in the middle. Since no net twist is applied, it is called a ‘‘false’’
twist; the yarn ahead of the machine is wound up and the false twist escapes, but the yarn behind
the twister passes through a long tube heated above fiber Tg, so that, as it exits, the false twist is
‘‘set’’ into the yarn. When this twist tries to spring back and unwind, it causes the treated yarn to
bulk up into a spiral crimp. The degree of twist is quite high, several hundred twists per meter, so
that, if the yarn is running at productive speeds, the rotation of the twister device has to be
extremely high, of the order of 1 million rpm. This produces formidable mechanical problems.
One ingenious solution is the friction-bulking process, in which the yarn itself is twisted either by
running against the internal surface of a rotating friction bush or by contact with the edges of a
series of friction disks. Since the yarn diameter is very small compared to that of the bush or the
disk, a very high ‘‘gear-up’’ ratio is achieved and the friction device can rotate at far more
reasonable speeds. A typical texturing process is shown in Figure 1.8.
Bulked continuous filament (BCF) carpet yarns are heavy decitex bundles of fiber that are
bulked by passage through a turbulent blast of steam or hot air well above Tg. The turbulence
blows the yarn about and entangles the filaments, and then heat sets them into place, giving
them a permanent crimp. Polymers like PET do not have very good resilience as carpet fibers,
but PTT (Tg¼458C) lends itself very well to the BCF process and has excellent resilience [38].