B
icomponent melt-spun fibers were first commercialized in the middle of the 20th century,
in the form of fibers with sheath/core and side-by-side cross sections. Very quickly, a primary
application for the sheath/core bicomponent cross section evolved: By employing a
lower-melting-temperature (Tm) polymer in the sheath and a higher-Tm polymer in the core, these
fibers could be used in nonwoven webs to thermally bond the webs together without losing the fiber
shape of the binder fiber. This allowed more bond points, which improved fabric strength and
allowed for increased line speeds.
Since that time, sheath/core binder fibers have become widely accepted and have set the
stage for the introduction of bicomponent staple fibers, tows and filament yarns with a wide range
of enhanced performance features offered by more advanced bicomponent technologies. An important
step forward in the commercialization of some of the more advanced possibilities was the invention
by United States-based Hills Inc. of a process for producing spin pack parts using photochemical
etching. This advance increased the fineness and precision of control over polymer flow paths and
did so while simultaneously reducing the cost of the parts. Subsequently, Fiber Innovation
Technology, Inc. (FIT) was established in 1996 in the United States as a specialty fiber producer
not controlled by any polymer producer having a single-polymer, commodity focus. With access to all
available thermoplastic materials, and using the Hills technology, FIT has been able to pioneer a
large number of different bicomponent fiber types in a wide variety of applications in a relatively
short time. As a result, fiber consumers now have access to commercial supply of an almost endless
variety of bicomponent fibers, with an exponentially larger range of performance features than when
the simplest bicomponent fibers were first introduced.
Highly Tailored Fiber Properties
Today, the choice of polymers used in a bicomponent fiber is not restricted to a handful of
commodity polymers such as polyethylene terephthalate (PET), nylon, and polypropylene (PP).
Instead, the entire range of polyesters is being augmented by aliphatic polyesters such as
polylactic acid and polyhydroxyalkanoates, which introduce the new environmental benefit of being
derived from renewable resources. Similar range extension is now available with polyamides and
polyolefins. But perhaps the most intriguing new possibility is the incorporation of engineering
polymers, whose properties are typically exceptional but whose cost has traditionally prevented any
investigation of use in commodity fiber applications.
Added to the newly-expanded polymer choices is a much greater variety of bicomponent cross
sections made possible by Hills technology and some pack-part innovations by FIT. Now it is
possible to put the polymers pretty much wherever desired in the fiber’s cross section.
And it’s no longer necessary to limit the choice to round fibers. Shaped-cross-section
fibers can also be coextruded using two polymers.
Finally, the entire range of polymer additives that can be used in single-polymer fibers can
also be used in one or both of the polymers in a bicomponent fiber to achieve targeted performance
characteristics. These additives include such things as colorants, flame retardants,
antimicrobials, conductive materials and carbon nanotubes, among other additives.
With this very large matrix of material properties and ways of combining them into each
fiber, it will be apparent that bicomponent fibers are no longer a one-trick pony. Whereas in the
past, fabric design meant trying to optimize the fixed attributes of a commodity fiber into each
different application, bicomponent fibers now offer a way to engineer finely-tuned performance into
the fiber.
Exemplary Uses Of Bicomponent Fibers
There are far too many different end-uses for bicomponent fibers to cover in a brief
article, but a few illustrative examples are discussed below.
Even the basic sheath/core binder fiber has been updated since the early days. Today, there
is access to a range of copolymers of polyesters, polyamides and polyolefins that allow precise
targeting of the desired thermal bonding behavior. It is even possible to select bonding polymers
outside this range, but these options can impose significant caveats. Beyond the bonding
temperature, the adhesive character of the bonding polymer can be adjusted to adhere better to
polar surfaces or nonpolar ones. And the crystalline nature of the polymer can be adjusted to give
a broader or narrower melt-temperature range.
The fundamental sheath/core cross section also is useful in many applications demanding
engineering polymers. Typically, such an application depends entirely on the surface properties of
the more exotic, and more expensive, polymer. In these cases, the fiber’s core can be made with a
suitable lower-cost polymer to deliver all of the benefit of the more expensive polymer at a
materials cost well below that of a fiber made from the surface polymer alone.
Side-by-side bicomponent fibers typically rely on the difference in shrinkage between the
two polymers. At any point in the fabric formation process, if the fibers are not physically
constrained, shrinkage can be induced by the application of heat. Since the two polymers shrink at
different rates, the fiber resolves the resulting tension by curling into a helix. This behavior
allows a fabric to be made flat and then bulked when and where it suits the application.
The pie-wedge cross sections typically are used to make microfibers. Direct spinning of
microfibers is difficult — and practically impossible below about 0.3 to 0.5 denier per
filament (dpf) — and expensive, as throughputs are low. But a 2- to 3-dpf pie-wedge fiber does not
suffer throughput limitations, and is robust through fiber and fabric production processes. Once a
nonwoven web is formed from these fibers, it can be subjected to mechanical agitation, which will
split the segments into microfibers typically about 16 segments per bicomponent fiber. The result
is a microfiber fabric at significantly reduced cost compared to one made using direct-spun
microfibers.
The sea/islands cross section also generates microfibers. In this case, the sea polymer can
be easily removed by dissolution in a suitable solvent — typically, a light, hot caustic bath or
warm water. A fabric made of sea/islands fibers is passed through the solvent, and the result is a
microfiber fabric.
The taggant cross section is one that FIT initially developed just to show off its
capabilities. But since then, the company has discovered that the inclusion of a logo or some other
complex shape in the fiber’s cross section can be of value in taggant fibers for applications in
which liability protection is desired. The logo can even be a two-dimensional barcode that can be
read by a machine vision system, thereby stealthily incorporating large amounts of information into
a product.
Future Directions
Innovation will continue and build upon the advances that have brought the technology to
this stage. Already, tricomponent spinning systems are being developed to coextrude three different
polymers into each fiber rather than just two. And some of the simpler bicomponent cross sections
are appearing in spunbond fabrics, in which filaments are extruded directly into a nonwoven web
without forming fibers as an intermediate product. The precision of polymer control to form the
cross section also continues to advance.
In recent years, Hills has produced spin packs capable of stuffing hundreds of islands into
each fiber cross section, which enables the production of submicron microfibers. There is even one
sea/islands cross section with close to 10,000 islands. And before electrospinning technology even
makes it out of the cradle, researchers are beginning to experiment with bicomponent electrospun
filaments, using polymer solutions rather than polymer melts.
Editor’s Note: Jeffrey S. Dugan is vice president, research, of Fiber Innovation Technology
Inc.
July/August/September 2010