ANTISTATIC AND ANTISOILING FIBERS

These topics are related because the origins of the problems are interrelated. Synthetic fibers
in general, and PET in particular, are hydrophobic materials—PET has a moisture regain of
0.4% at 60% RH. PET fibers are difficult to wet and rapidly build up static electrical charges
by friction because as water effectively leaks away, voltage is produced. It is possible to build
up potentials as high as 50 kV by rubbing a polyester fabric, e.g., by walking on a polyester
carpet when the relative humidity is low (5%). Such a potential, discharged by grasping a
grounded door handle, would give a very unpleasant electric shock. Static charges also lead to
attraction of dust and dirt.
To avoid these problems, the moisture uptake of the polyester should be increased by
combining it with hydrophilic materials that are wash-fast. One additive that has been used
20 Handbook of Fiber Chemistry
repeatedly is polyethylene oxide (PEG), a stable, functional, highly hydrophilic, watersoluble,
and humectant polymer (see below):

NONCIRCULAR CROSS-SECTION FIBERS

Synthetic fibers like PET and nylon are normally round in cross section, however no natural
fiber has a circular cross section. Wool is irregular, cotton is ‘‘dogbone’’ shaped, and silk is
triangular. In the early 1970s, people began to study the effect of noncircular cross-section
(NCCS) fibers on yarn and fabric esthetics, which is a subjective topic involving such arcane
terms as ‘‘feel,’’ ‘‘drape,’’ and ‘‘handle.’’ Fortunately, a melt-spun fiber lends itself NCCS
well to the production of (NCCS) fibers by varying the shape of spinneret orifice, provided
the melt viscosity is high enough so that surface tension does not cause the filament to resume
a circular shape. Since the holes had to be very small (about 0.015 in. overall), machining a
multiplicity of holes at a uniform size and shape was a major engineering problem, particularly
in the hard metal alloys used for spinneret plates. Laser etching is one technique used.
A hole shaped like a T gave trilobal filaments. In the pioneering days, much of this work was
entirely heuristic, but gradually emerged some rules of thumb. Multilobed yarn cross sections
(trilobal and octalobal) can give quite different appearances. Trilobal is glittery as the incident
light reflects off the fiber surface, while octalobal gives an opaque matte effect, as the light is
effectively absorbed by multiple reflections from the many acute angles. Sharp-edged filaments
have the prized rustle and high frictional characteristics of pure silk, where it is called
‘‘scroop.’’ Flat rectangular filaments give fabrics an unpleasant ‘‘slimy’’ handle. Gradually,
these principles were applied to commercial yarns, and many filament yarns for the apparel
and BCF carpet markets now use NCCS fibers.

LOW-PILL STAPLE POLYESTER

PET staple blends with wool and cotton were highly successful from the very first introduction
of PET in the 1950s. However, consumers soon noticed an annoying problem. It was the
formation of small fuzzy balls (called ‘‘pills’’) on the surface of fabrics. This phenomenon is
Polyester Fibers 19
known as ‘‘pilling’’ and it is common to all staple fibers, particularly if the level of yarn twist is
low, so that the fiber has many loose ends. The pills rub off harmlessly with wool because
wool is a weak fiber. However, PET is a strong fiber (tenacity ca. 5 g=decitex) and therefore
pills do not rub off; instead, they cling and have a negative impact on fabric esthetics. To
reduce pilling, the IV of the polyester is reduced to make weaker fibers. These do not pill so
obviously because the pills break away. A polymer of IV¼0.42 was selected as the best
compromise for a low-pill PET staple fiber, but it caused many problems. The melt viscosity
was so low and the molten polymer so fluid that the process became unstable. A method had
to be found to raise the effective melt viscosity of the polymer while maintaining the low-pill
properties to give an acceptable melt-spinning process. The method adopted was to introduce
branching points into the polymer chain by adding a multifunctional component
(either a polyacid or a polyhydric alcohol) so as to produce a star-branched polymer.
Such polymers are known to have higher melt viscosities for the same (nominal) polymer
IV. The branching agent added (ca. 1 mol%) was usually pentaerythritol. Too much
additive would lead to gel formation by forming cross-linked networks, but this is not a
problem at low levels [2].

MODIFICATION OF POLYESTER FIBERS—SPECIFIC SOLUTIONS FOR SPECIFIC

This wide topic covers both chemical and physical modifications to both the polymer and the
fiber. We shall deal with only a few of the more important variations possible on this theme,
but all are based on an understanding of polyester chemistry and processing described earlier
in this chapter.
SPIN FINISHES
Fibers need to be treated with surface finishes or lubricants to allow high-speed processing.
The various processing steps such as drawing, bulking, and textile processing would be
impossible without these spin finishes because so many of them rely on specific frictional
properties of the fiber (for example, friction twisting). Spin finishes are often water emulsions
of various surface-active agents and lubricant oils; their formulation is a complex process and
sometimes more of an art as well as a science. Finish application is made early in the process,
before the cooling threadline from the spinner hits the first godet. Earlier, finish was applied
from a lick roll rotating slowly in a bath. As spinning speeds increased, the finish was
applied directly via a special hollow ceramic yarn guide as a neat oil formulation and metered
at precise levels via a metering pump. Staple fiber is sprayed with emulsified finish or the
whole tow may be immersed in large baths of finish. Some staple processes use a draw stage in
a hot bath of finish.

BIODEGRADABLE FIBERS

Biodegradable polyesters comprise a diverse field, but the most well-developed fiber (monofil)
market is resorbable surgical sutures, which slowly disappear in vivo and do not need
subsequent surgical removal. The first commercial samples were introduced in the early
1970s by Ethicon Corporation [51]. These sutures were monofil fibers spun from a copolymer
of glycolic acid and D-lactic acid. Such aliphatic hydroxy acids are completely biocompatible
and harmless: in U.S. Food and Drug Administration (FDA) terms, these materials are
‘‘generally recognized as safe (GRAS).’’ The properties of polyglycolide and stereochemically
pure D- or L-polylactide polymers are quite good, and they form strong, highly crystalline
fibers by melt spinning. Other biodegradable polyester fibers have been explored. Synthetic
lactones such as e-caprolactone and 2-dioxanone have been copolymerized with glycolide and
lactide [52,53]. ICI began working on poly (3-hydroxybutyric acid) in the 1970s and later
developed a copolymer with 3-hydroxyvaleric acid. Both polyhydroxyacids are stereochemically
pure and give crystalline polymers, which can be processed into fibers and films. The
interesting feature of these polymers is that they are made in very high molecular weight form
by bacteria. Certain microorganisms, when cultivated and starved of nitrogen sources,
synthesize aliphatic polyesters instead of proteins. The number average molecular weight of
the as-harvested polymer can be several million daltons and it must be reduced to allow the
polymer to be processed and fabricated. ICI (now Astra-Zeneca) first developed ‘‘Biopol’’ as
one product and although others have been introduced by different companies, little has been
targeted towards fiber end-use [54]. All the polyhydroxyacids are unstable and degrade on
exposure or composting, but the degradation rate is very much governed by the ratio of
hydrophobic=hydrophilic properties. While hydrolysis is important, catalyzed degradation by
various lipases is also a factor.