Bicomponent 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 Melbourne, Fla.-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 Johnson City, Tenn., 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 – including polycyclohexanedimethanol terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, PET glycol and a huge range of copolyesters – 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 including nylon 6, 6,6, 11 and 12; copolyamides; high-density polyethylene (PE); linear low-density PE; syndiotactic PP; and polymethylpentene. 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. The list of these polymers is long, and includes polyphenylene sulfide, acetal, ionomers, polyvinyl alcohol, polyetherimide, and thermoplastic polyurethanes, to name just a few.
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 polymer.
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. Each application can now seek a fiber that is precisely tailored to fit the specific needs of that application.
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. The bonding temperature can be set from a low of about 110°C to a high of about 180°C. 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. Binder fibers for high-loft nonwovens used as seat cushions in place of polyurethane foam use a sheath polymer with elastic recovery, so that repeated stressing of the bond points does not fracture the bond.
The fundamental sheath/core cross section is also 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 – typically, a hydroentangling process – 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 hollow and partial-wrap versions of this cross section are refinements that allow adjustment of the fiber’s relative splittability.
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. This approach incurs a cost penalty because some of the fiber is washed down the drain. But the smallest microfibers from sea/islands technology are much smaller than those achievable using mechanical splitting technology.
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. The tagged product need not be a fibrous product, but can include electronics, pharmaceuticals, gemstones, explosives, or virtually anything used in an application in which forensic identification could be of value.
Of course, this is not the end of the story. 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. When FIT was first formed, the state of the art was 37 islands in a sea/islands fiber, which could produce microfibers as fine as 0.02 dpf. 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.
It will be necessary to wait for some of these advances to become widely available, but with the state of bicomponent technology available today for commercial production, there may no longer be any need to wait for a staple fiber or filament yarn that offers the exact performance a particular application requires.