Nanotechnology In Textiles
Ongoing development of nanofiber fabrication, fiber and fabric surface modification, and nanoparticle composite fiber technologies promises to spur development of new commercial products.
Dr. Russell E. Gorga
In 2000, the Nanoscale Science, Engineering, and Technology Subcommittee (NSET) of the National Science and Technology Council's Committee on Technology defined nanotechnology as a "research and technology development at the atomic, molecular or macromolecular levels, in the length scale of approximately 1-100 nanometers [nm], to provide a fundamental understanding of phenomena and materials at the nanoscale and to create and use structures, devices and systems that have novel properties and functions because of their small and/or intermediate size. ... Nanotechnology research and development includes manipulation under control of the nanoscale structures and their integration into larger material components, systems and architectures. Within these larger scale assemblies, the control and construction of their structures and components remains at the nanometer scale." NSET also notes exceptions at both ends of the scale, for example, nanoparticle-reinforced polymers that exhibit novel properties and phenomena at around 200 to 300 nm owing to bonds between the nanoparticles and the polymer.
In light of this definition, one can break nanotechnology in textiles into three broad categories: fabrication of nanofibers; surface modification of fibers and fabrics; and nanoparticle composite fibers. This article will touch briefly on these three topics with respect to technology, applications, and future developments.
Figure 1: Electron micrograph of nanofibers fabricated from needleless electrospinning; average diameter is 250 nm; courtesy of Laura Clarke and Russell Gorga, NCSU
Fabrication Of Nanofibers
Fabrication of nanofibers — fibers with diameters of several hundred nanometers or less — is a growing area of interest both academically and commercially. Commercially, the use of nanofibers has exploded over the last five years or so. In August 2010, a conference was held in Raleigh, N.C., called "Nanofibers for the 3rd Millennium 2010," and organized by nanofiber production equipment supplier Elmarco Inc., Morrisville, N.C., and the Nonwovens Institute at North Carolina State University (NCSU). The field has had tremendous growth over the last 10 to 15 years.
The best-established process for nanofibers fabrication is electrospinning, which has patents extending back to the early and mid-1900s. In the process, electrostatic forces are used to draw a solution or melt polymer fluid into a fibrous form. Depending on the materials system and processing conditions, resulting fibers can range from several microns to less than 100 nanometers (See Figure 1). There are several companies that manufacture equipment to produce nanofibers — including Elmarco for electrospinning and West Melbourne, Fla.-based Hills Inc. for islands-in-the-sea extrusion. Recently, FibeRio Technology Corp., Edinburg, Texas, has developed a process called Forcespinning™ Technology to produce nanofibers through centrifugal forces instead of electrostatic forces. The company has just introduced production equipment for research and development purposes, and is in the process of developing commercial-scale equipment.
In addition to equipment manufacturers, there are several companies that commercially manufacture nanofibers. Donaldson Co., Minneapolis, manufactures a wide variety of Ultra-Web® filter systems that incorporate electrospun nanofibers into some of its filtration media. Companies such as Hollingsworth and Vose, East Walpole, Mass.; BASF SE, Germany; Freudenberg Group, Germany; SNS Nano Fiber Technology LLC, Hudson, Ohio; and Nanofiber Filters LLC, Wilmington, Del., are all either commercially producing or developing nanofibrous platforms for product lines ranging from filtration to absorbent, breathable and/or water-repellent fabrics. Fiber Innovation Technology Inc., Johnson City, Tenn., uses the Hills technology to produce unique fibrous structures to diameters of about 2 microns. This list of companies is not intended to be complete, but rather is a cross-section from larger corporations to small start-ups.
Surface modification is used to impart unique properties to fibers and fabrics. Of the three categories, it is the best-established — although not necessarily from a strictly "nano" perspective — and has a large variety of technologies and applications. It is also an area that raises the most scrutiny as to whether or not the technology can actually be considered nanotechnology based on the modification process and the coating thickness.
Applications range from water and stain repellency, wrinkle resistance and flame retardation to higher-tech applications such as microbe resistance, electro-textiles — such as printed circuit boards — and chemical/biological detection and other protective applications. Technologies employed for such applications range from plasma treatment to atomic layer deposition (ALD) to grafting of polymer chains to self-assembly of monolayers.
Commercial applications range from the Speedo® LZR swimsuit — using a cold plasma technology to repel water — to the Nano-Tex technique of pad-applying to a fabric or dip/spray-applying to a garment with solution containing particles to create so-called nano-whiskers on the surface of a cotton fiber. The biggest issue with surface modification, especially in older technologies, is longevity. Many coatings become depleted as a function of wear, or abrasion, and washing. Therefore, nanotechnologies employing strong bonding to the surface of the fiber are eagerly sought and under development.
Many interesting technologies, such as ALD and polyelectrolyte multilayers, while potentially commercially viable, remain primarily developmental stages for fibers and fabric surfaces. For example, recent work done at NCSU has shown ALD can be used to coat electrospun nanofibers with an aluminum oxide layer (See Figure 2).
Figure 2: Electron micrograph of electrospun polymer nanofibers coated with aluminum oxide using the ALD process; courtesy of Dr. Greg Parsons, NCSU
A growing area of interest is antimicrobial applications. Companies like AEgis Technologies Group Inc., Huntsville, Ala., have developed silane layers that are grafted from the fiber surfaces. LAAMScience, Morrisville, N.C., has developed antimicrobial technology in which the molecule of interest is grafted to the fiber surface and emits singlet oxygen when exposed to visible light.
Nanoparticle Composite Fibers
Of the three categories, nanoparticle composite fibers currently have the smallest commercial presence and remain largely of academic interest. However, there is no shortage of basic and developmental research occurring in this area. Incorporation of nanoparticles, such as silver particles or carbon nanotubes, can be used to create fibers that are antimicrobial or have increased strength and/or thermal/electrical conductivity. The largest challenge remains obtaining adequate dispersions of nanoparticles in polymer matrices. Research in this field includes incorporation of nanoparticles to improve strength, conductivity, gas permeation, cellular response, static charge dissipation, sensors, actuators and other factors. There is no shortage of potential applications for nanocomposite fiber, and this field remains wide-open for significant commercial breakthroughs.
For example, work done at NCSU has demonstrated that a small fraction of carbon nanotubes can improve the conductivity of non-conductive polymer nanofibers by more than 10 orders of magnitude. Further, depending upon the fiber geometry — single component versus core-sheath bicomponent, the percolation threshold — the transition from non-conductive to conductive — can be pushed to lower nanotube concentrations when the nanotubes are isolated in the sheath of the bicomponent fibers (See Figure 3). This, in turn, could have implications for applications such as signaling and sensing.
Figure 3: Conductance values for single component and bicomponent fibers doped with multiwall carbon nanotubes; graph courtesy of Laura Clarke and Russell Gorga
To summarize, the field of nanotechnology can be broadly or narrowly applied to any industry, and the textiles market is no exception. In essence, there should be less emphasis on the catch phrase "nanotechnology" and more emphasis on the diverse technologies to create nanofibers, modify the surface of fibers and fabrics, and incorporate small particles into or onto fibers and nanofibers, regardless of whether or not they fall strictly into NSET's definition. With the commercial success of nanofibers production and many surface modification technologies, and the growing academic interest in all three areas discussed above, there will, no doubt, be not only many new technologies that can be utilized, but also many new commercial products to enhance people's lives and keep them safe.
Editor’s Note: Dr. Russell E. Gorga is an associate professor and program director, Textile Engineering and Fiber and Polymer Science, at NCSU College of Textiles. The author wishes to thank Drs. Stephen Michielsen and Peter Hauser, NCSU College of Textiles, for discussions regarding surface modification in the textile industry. A list of references for further reading on the topics discussed is included with the online version of this article at www. TextileWorld.com. Visit www.nano3millennium.com for more detail on the Nanofibers For The 3rd Millennium 2010 conference.