Micro-denier nonwovens hold the key to the success of future technical applications.
By Dr. Behnam Pourdeyhimi
The meaning of the word “durable” is not always clear in this context; nonwovens can be long-lasting or have a short lifecycle. Most nonwovens currently are engineered to be single-use products, and function adequately for the applications for which they’re designed. Automotive nonwovens, geosynthetic nonwovens and the like are intended to last for a long time, and are often called “durable.” At the Nonwovens Institute, we prefer to refer to these as “long-life” nonwovens, rather than “durable.”
We also have the classification known as “multi-use” nonwovens. For example, many commercial wipes used in Europe today are multi-use; that is, they can be used to wipe a surface, be washed/rinsed/cleaned and re-used multiple times. From the perspective of functional clothing, the materials need to withstand multiple launderings without loss of functionality (or appearance). We must make a distinction here: Long-life nonwovens are not necessarily launderable, although they can function for a very long time. Durable, launderable nonwovens are a different class altogether. There are not too many such products on the market—yet.
Durable, launderable nonwovens
There is a traditional belief that nonwovens cannot replace traditional fabrics. In a recent article in the Specialty Fabrics Review, it was stated that: “Although nonwovens may be giving woven fabrics a run for their money in some industry segments, they are not strictly substitutes for traditional woven or knitted fabrics, particularly in clothing applications.”
This is not strictly correct. There are nonwovens that can function perfectly well as clothing today. Start with the basics: Durable, launderable nonwovens by necessity must be hydroentangled. Hydroentangling produces fabrics that are flexible, soft and conformable, similar to traditional woven fabrics.
These fabrics made from staple fibers can withstand only a limited number of launderings, because the laundering process reverses the hydroentangling process. Additional chemical or thermal binding agents are often necessary to increase launderability, with the concomitant consequences of reducing breathability and increasing stiffness leading to loss of “hand.” The advantage of this class, however, is that staple fibers of many types are already available. With clever design they can be made into useful items, as demonstrated by the earlier work of the Polymer Group Inc. (PGI) with Miratec® fabrics, and more recently, Norafin Industries GmbH and Komanda® fabrics, as well as the work underway by NanoSynTex Inc. and the Natick Soldier Systems Center. Many of these structures are inherently weak, however, and many staple-based products use a reinforcement (a scrim, for example) to enforce the fabric to increase tear and tensile resistance.
The same structures made from spunbonded fabrics show much greater promise. For example, Freudenberg’s Evolon® is the first and most revolutionary departure from single-use nonwovens. A segmented pie spunbonded filament is split and entangled in one step by hydroentangling to form a micro-denier high-performance durable, launderable fabric. Evolon is produced today as a segmented pie spunbond; the jets split the segments of filaments, resulting in two different (polyester and nylon) filaments. This process is environmentally responsible and relatively cost effective, and results in a micro-denier structure composed of fibers of about 2 microns in diameter (equivalent fiber diameter if the fibers were round). A recent fashion show by NCSU undergraduate fashion students demonstrated that unique clothing can be produced today by using structures similar to those used in producing Evolon.
Conventional splittable bicomponent fibers for microfiber nonwovens. Splittables are fibers in which the two polymers share only one common interface and both polymer phases are also exposed.
Microfibers are becoming increasingly popular. They’re not new; a simple search on Google resulted in more than 11 million hits. Micro-denier fibers are well established in cleaning products, towels, and suede products.
The value addition due to an increased surface area is not limited to wipes, but can be extended to include filtration and military and medical applications as well. The larger surface area fabrics enhance properties related to insulation, fluid retention, drapeability and durability, and when entangled, to better durability.
The classic splittable fiber cross-sections are shown in Figure 2. The side-by-side method can often also lead to curled (crimped) fibers, depending upon the choice of polymers employed. The segmented pie is the most popular. The tipped trilobal is not typically used as a splittable but is used to provide the functional polymer on the tips—to provide softness, for example. The segmented ribbon is very similar to segmented pie, except that the end segments have to be made from the same polymer to “balance” the structure and prevent it from wrapping, leading to an odd number of segments.
With the exception of the tipped trilobal, the others result in structures that fall under the Evolon umbrella. These structures, as demonstrated by Evolon, are durable, launderable, breathable and flexible, and can be dyed or printed the same way as woven fabrics.
One problem with the segmented pie structure is that the fibers form wedges and pack fairly tightly. Here the challenge lies in balancing the level of consolidation with the mechanical properties (tear and tensile) of the fabric. Higher consolidation results in improved tensile and pilling character, but lowers tear properties because there is little or no mobility in the structure, and fibers are broken individually when the tear propagates. Lower consolidation improves tear properties but results in lower tensile, pilling and abrasion resistance. For example, a 100 gram fabric may only have a tear strength of 6 to 10 newtons and a tensile strength of about 250 N/5 cm3. These structures contain fibers that are about 80 to 90 percent split after hydroentangling.
A new generation
Bicomponent fibers provide significant flexibility in product design and function. Unlike splittables, in which the two polymers share only one common interface and both polymer phases are also exposed, in the “Islands-in-the-Sea” (I/S) process, the islands share a common interface with the sea polymer, but the islands are not exposed. Traditionally, the structure is formed and then the sea is removed by washing it away in an appropriate solvent. Islands-in-the-sea fibers (typically 37 to 49 islands) are used in the production of suede and leather fabrics. The process is often not environmentally responsible due to the solvents used in production.
It has been demonstrated, however, that the feasibility of I/S cross-sections in the filaments can produce high-strength fabrics after bonding the spunbond webs using mechanical (hydroentangling) or thermal (calendering) techniques. Contrary to common wisdom, I/S fibers can be fractured or fibrillated, resulting in Evolon-like fabrics with much improved tear strength. When the sea phase is fibrillated and fractured, unlike other Islands-in-the-Sea examples, the sea remains in the structure—potentially, a much more cost-effective and environmentally responsible process than chemically washing away the sea. The process for fibrillating the fibers is relatively simple: The fibers are passed through a calender—to cause shear—and are then subjected to hydroentangling.
Figure 3 shows the cross-section of the fibers in I/S fabric before fracturing. These fibers were consolidated by passing them through the calender, cold. The mechanical pressure in the nip has already caused the fracture of some fibers.
Figure 4 shows a partially fractured and Figure 5 a fully fractured topical view of the same fabric. As the fibers are fully fractured, at least on the surface, the fibrils would tend to easily dislodge on the surface, giving a suede-like effect to the fabric.
Figure 5 shows the fully fibrillated fabric after being calendered as well. Calendering tends to tie down the fibers on the surface, and often improves the abrasion and mechanical properties of the fabric.
Using a different strategy, the tipped trilobal structure can also yield splittable/fracturable structures, which can form fabrics with stretch and recovery when an elastomer is used strategically in the fiber cross section.
There is a wide range of applications that can effectively utilize this new generation of durable, launderable nonwovens. Micro-denier nonwovens hold the key to the success of future technical applications requiring multi-functional fabrics. There is a portfolio of technologies available, and still others are under development currently at the NCSU Nonwovens Institute. (For more information, please e-mail email@example.com.) There will be significant growth in areas and markets in which nonwovens have traditionally not been used.