NOVEL FIBER FORMS

MICROFIBERS
Microfiber is arbitrarily defined as a filament of less than 1.0 dpf. Normal filament yarn
polyester is around 3. 0–5.0 dpf. Microfibers are many times finer than a human hair and finer
than the finest silk: diameters are generally less than 10 mm. A typical polyester microfiber has
a titer of about 0.5 dpf. Such fine fibers in the form of yarns have excellent textile properties.
They are very flexible, giving a soft ‘‘hand’’ and excellent drape to fabrics. The high density of
fibers in a typical microfiber fabric makes it inherently windproof and waterproof. There are
only tiny gaps for air to blow through, yet the fabrics are largely unwettable, because surface
tension effects prevent water from penetrating the interstices in the fabric. These fabrics are
comfortable to wear as water vapor from perspiration evaporates easily. Their fabric properties
make them ideal for women’s wear, sportswear, active, and outdoor wear. They have
(radiant) heat-insulating properties because the filaments are of the same dimensional order
as the wavelengths of infrared radiation. A 0.5 dpf polyester filament (density1.4 g=cm3)
has a diameter of about 7 mm, right in the middle of the IR wavelength range (2–20 mm).
Hence, radiation is efficiently scattered by the microfibers and radiation loss of body heat is
reduced.

Microfibers lend themselves very well to fabric esthetics. Dyed fabrics appear with solid,
bright colors due to the fine size of individual filaments. They are semimatte in appearance,
without the need for treatments such as sand washing. The vulnerability to damage from
careless ironing is one disadvantage. The thermal capacity of the tiny filaments is so low that
it is easy to overheat them. They also snag easily and, as with all fine fabrics, they need to be
handled with a degree of care.
MELT-SPINNING MICROFIBERS
The first commercial microfibers were produced in Japan [76] in the 1970s and were made by
spinning a bicomponent fiber with polyester fibrils dispersed in a matrix polymer in the
‘‘islands in a sea’’ configuration [77]. This was drawn into fibers and processed into fabric
and finally the matrix polymer was dissolved, leaving tiny polyester fibrils. These were
processed into a synthetic suede material marketed as Ultrasuede. The polyester fibrils
were extremely fine, less than 1 mm in diameter. The process was expensive, but the product
was successful. At the same time, numerous variations on this theme later followed. One
ingenious idea by Sato and coworkers used was a blend of PET in a PET–sulfoisophthalate
copolymer rich in SO3Na groups, which dissolved readily in aqueous base leaving the
unaffected PET.
There are many patents in the literature, mostly on static devices for melt-spinning
multiple fibers with the ‘‘islands in a sea’’ configuration. This is usually done by a multiple
series of flow-divider plates that take the initial side–side polymer flow (as in a heterofil
spinner) and subdivide it and cross over the flow many times before the spinneret plate, so
that each spun filament emerges with the desired structure. Some examples are further ideas
of Okamoto [78] and Dugan [79].More recently, the dissolvable matrix has been made of such
materials as polylactic acid (already mentioned), thermoplastic starches, or water-soluble
copolyesters. Good review articles on microfibers have been written by Robeson [80], Murata
[81], and Isaacs [82].
Another method of making microfibers is the ‘‘splittable pie’’ technique, where a bicomponent
fiber of special configurations is spun from two incompatible polymers that adhere
poorly. On subjecting these fibers to mechanical stress, as during carding, they split apart
to form bundles of microfibers with a wedge-shaped cross section. PET–polypropylene or
Polyester Fibers 25
PET–nylon-6 are examples of suitable polymer combinations. Such microfibers are frequently
used in nonwoven fabrics such as filter materials and specialty fabrics like cleaning cloths for
microelectronic components or polishing cloths for lenses and delicate optical instruments.