Nano: the next wave in nonwoven textiles: the latest development in nanotechnology applied to textiles
Nanotechnology has enormous buzz these days. It is a bottom-up science, which enables us to understand the process that makes macromolecules from their building blocks and their properties at atomic levels. Such a technology helps with the manipulation of the processes and properties to suit targeted end use applications. Nonwoven textiles have played an invisible role in the development of nanoscience. The original patent on the electrospinning of cellulose acetate fibers was issued in 1934, way ahead of the Nobel laureate Richard Feynman’s speech in 1959, “There is Plenty of Room at the Bottom,” on nanotechnology, which is widely regarded as the basis of nanotechnology.
Nanotechnology was adapted early on by the electronics industry. The textile industry has been slow in adapting this technology and there are a few commercial products in the market today. Nanofiber filters by Donaldson and new Nano-Tex’s spill repellant fabric are some of the few commercial products that have penetrated the market. According to Young Chung of Donaldson, one third of all Donaldson’s products contain some form of nanomaterials. Today, there are more than 100 academic and industrial research groups around the world working in nano-related research in fibers, textiles and polymers. Governments around the world have invested heavily in nano research and development. According to National Science Foundation, research investment in nanotechnology in 2005 exceeded $4 billion. The U.S., EU and Japan are leaders in the field. A quick search at the Institute of Scientific Information’s Web of Knowledge database using the keyword nanofibers turns out 2015 papers published since 1992. Nanotechnology is growing exponentially with a huge surge in the number of papers and patents published around the world. There have been some interesting developments recently on the use of nanotechnology in fibers and textiles, which are discussed in this article.
What are Nanofibers?
Fibers with nanoscale diameters-including high surface area, flexibility, breathability, porous structure, light weight, desired level of modulus of elasticity, and functionality-have been developed in laboratories that have had commercial success to some extent recently. These nanofibers find applications as filters, liners for toxic chemical protective fabrics, tissue scaffolds and many other advanced engineering applications. In general, the diameters of nanofibers range between 100-500 nm.
Anton Formhals invented the electrospinning method to produce artificial threads in 1934, which is a forerunner for today’s electrospinning of nonwoven nanofibers. Electrospinning processes use high voltage electric field to produce electrically charged jets from polymer solution, which dry by means of evaporation of the solvent produces nanofiber webs. In a strict sense, nanofibers are nonwoven webs made of submicron sized fibers. Depending on the end use applications, different polymers such as natural, synthetic and biodegradable can be easily processed into nanowebs by electrospinning. Research on the electrospinning of nanofibers got a boost in the 1990s due to the work of Professor Darrell Reneker at the University of Akron. Jayesh Doshi and Reneker reported their findings in a landmark paper in 1995. Later on, Dr. Doshi started a nanotech company, eSpin Technologies, Inc. based in Chattanooga TN for commercial production of electropsun nanofibers from a variety of polymers.
Gregory Rutledge’s group at MIT is carrying out fundamental work on electrospinning. His group’s research has led to the discovery of a terminal jet diameter, which determines the fiber diameter that can be spun for a given polymer.
Nanofibers And Military Applications
Apart from filtration applications, functionalized nanofibers are generating a lot of interest in the military research and development communities due to their potential chemical and biological warfare countermeasure capabilities. In order to protect the warfighters from harmful toxins and provide necessary comfort, nanofiber liner materials are highly promising candidates. Nanofibers lined chem-bio suits result in lightweight, breathable, multifunctional clothing with the addition of chemical functionalities to offer protection against harmful liquids, vapors and aerosolized toxins.
Heidi Schreuder-Gibson and Phil Gibson at the U.S. Army Natick Soldier Center have been carrying out collaborative research with government, industrial, and academic partners to find practical applications of nanofibers and nanoparticles for protective clothing. Some of their exciting research projects include the electrospinning of thermoplastic elastomeric polyurethanes, which exhibited good fabric properties like high elasticity and good strength without further processing and treatments. Their current test and development efforts focus on functionalizing melt blown and electrospun webs by commingling reactive nano alumina and titania in addition to other approaches aimed at incorporating reactive compounds into fabrics while maintaining self-decontaminating properties.
Functionalizing nanofiber webs with additional materials enhance the use-value of nanowebs. Metal oxide embedded nanofibers can catalyze organophosphorous chemical warfare agents. Recently, research at Texas Tech University has been successful in embedding nano magnesium oxide (MgO) to polymer fibers. By carefully manipulating the process, it has been possible to deposit the nanoparticles on the surface of the fiber to have maximum chemical reactivity for offering greater protection from toxins. The electrospinning technique has been found effective in developing honeycomb filter-in-filter polyurethane nanowebs. These filters will have enhanced filtration capabilities due to nano meshes for entrapping particles.
Professor Seeram Ramakrishna’s group at National University of Singapore is collaborating with the Defense Science and Technology Agency (DSTA) of Singapore to develop nanofiber facemasks for chem-bio protection. According to the scientists from NUS, activated carbon in the facemasks can be replaced by nanofiber webs to trap toxins from air. They are embedding nano metals and cyclodextrins to nanofibers to decompose chemical toxins. Preliminary results with chemical warfare simulants such as Paraoxon have been successful. Their ultimate goal is to develop washable and durable military garments that have nanofibers.
Meanwhile, at MIT Professor Rutledge and his associates have shed light on the strategies for developing super-hydrophobicity in electrospun nanofiber fabrics, which are influenced by the fiber surface chemistry and topology. These water repellant nanowebs will have end-uses in protective clothing and biomedical applications.
And, Dr. Gajanan Bhat at University of Tennessee’s TANDEC in Knoxville and Dr. Raj Vempati of ChK Group, Inc. of Dallas are incorporating nanophase Mn (VII) oxide (Agent M-7-O) in nonwoven fabrics for defense applications. Agent M-7-O is environmentally benign and is a strong Lewis acid oxidant. According to Dr. Bhat, some of the key benefits of these nonwoven fabrics are that they can be safely transported, are flexible enough to be manufactured in different forms depending on end-use requirements and are reactive materials for the decontamination of chemical warfare agents and toxic industrial chemicals.
Nanofibers and Biomedical Applications
Professor Margaret Frey and her colleagues at Cornell University are exploiting the high surface area and water affinity of biodegradable polymers for drug delivery applications. Other exciting projects involve pesticide delivery and biosensor applications. According to Dr. Frey, the high surface area of nanofibers enables more sensors active sites in a small volume of fiber.
Donaldson Company is playing a leading role in the biomedical applications of nanofiber webs. Donaldson has been in the nanofiber business for more than two decades. Donaldson commercialized Ultra-Web nanofiber filters in 1981 and has branched into new applications such as nanofiber based cell culture materials and aerosol barrier garments. In 2002, Donaldson created a new group that focuses on commercializing nanofibers with new applications and on fostering research partnerships and joint ventures for broadening commercial applications. Recent activities at Donaldson involve the development of three-dimensional cell culture medium, which mimics the extra-cellular matrix in the body. Biodegradable nanofibers due to their resemblance with the extracellular matrix (ECM) can be used as tissue scaffolds. These scaffolds allow the cells to be in close proximity to each other and enable the development of three dimensional tissue structures. Mechanical stability, biocompatibility, cell proliferation and cell-matrix interactions are the critical factors that determine the use of nanofibers for biomedical applications.
Scale-up and Commercialization
One of the probable reasons for not having a great commercial success and widespread use of electrospinning technique is the lack of availability of industrial scale machinery that can be purchased. However, Ohio based NanoStatics has developed an electrospinning manufacturing technology that has the commercial scale throughput capacity for high volume production of nanofibers and nanofiber containing materials.
According to NanoStatics, their commercial technology can produce nanofibers whose diameters range from 50 to 1000 nm. The thickness of the nanowebs can range from 100 nm to more than 200 microns. With the availability of a commercial electrospinning technology, it will be possible for the industry to undertake capital investments in the nanofiber arena.
Nanometer diameter spunmelt fibers have been of tremendous interest recently. Uniform nano sized melt spun fibers down to 250 nm diameters have been developed via island-in-sea method by Hills, Inc. According to Hills, these fibers can achieve high strength of 3 grams/denier and can be wound for further processing. Spunbond island-in-sea fabrics with fiber sizes of 2 to 0.3 microns have been developed by Hills. Nanotubes developed by island-in-sea method down to 300 nm diameters with wall thickness of 50 to 100 nm have been successfully developed for which a patent application has been filed. Hills’ nanotube fibers can find application in chemical warfare defense, drug release, micro filtration and micro hydraulics.
Carbon Nanotubes and Composites
In 1991, Sumio Ijima of NEC Laboratories of Japan discovered the multiwall carbon nanotubes whose diameters were of nanoscale. Some of the unique characteristics of nanotubes are lightweight, high strength, electrical properties and thermal resistance. Scientists at the University of Texas at Dallas (UTD) NanoTech Institute, along with CSIRO, Australia have achieved a major technological breakthrough by spinning multi-walled carbon nanotube yarns that are strong, tough and extremely flexible and are both electrically and thermally conducting. According to the researchers, these carbon nanotube yarns will result in “smart” clothing that stores electricity, provides ballistic protection and adjusts temperature and porosity to provide greater comfort. Professor Ray Baughman and Dr. Mei Zhang of UTD collaborated with Dr. Ken Atkinson of CSIRO to develop multiwalled nanotube yarns, which are cost effective, compared to single walled nanotube structures. These researchers have produced transparent carbon nanotubes, which are stronger than similar weight steel sheets. These sheets are said to find applications such as of light-emitting displays, low-noise electronic sensors, artificial muscles, conducting appliques and broad-band polarized light sources that can be switched in one ten-thousandths of a second.
Professor Satish Kumar at Georgia Institute of Technology uses single wall, double wall and multiwall carbon nanotubes (CNT), as well as vapor grown carbon nanofibers for dispersing in various polymer matrices by in situ polymerization, in melt, and in solution. According to Professor Kumar, matrix systems studied to date include poly(p-phenylene benzo bisoxazole) (PBO), polypropylene (PP), poly (vinyl alcohol) (PVA), poly(methyl methacrylate) (PMMA), and polyacrylonitrile (PAN). These composite systems have been extruded into continuous fibers by conventional melt and solution spinning technologies. These have enhanced tensile strength, modulus, chemical resistance, increased glass transition temperature and reduced thermal shrinkage. Polymer/carbon nanotubes are processed into porous nanofibers, nanowires and electrospun microscopic cups.
Gajanan Bhat at UTK’s TANDEC has incorporated nanoclay in polypropylene spunbond fabrics. His result showed that addition of one percent of nanoclay increases tenacity and modulus with no decrease in breaking elongation.
Atomic Future in the Nonwoven Industry
The development of nano fuel cells, which could use nonwovens, is not in distant future. According to ACON, AG a science and business consultancy in Zurich, Switzerland, worldwide nanotechnology market is estimated to be around 900 billion dollars by 2015.’ Given the myriad of applications, which nanofiber nonwovens can find, it will be beneficial for the nonwoven and textile industries to explore different and value-added applications using nanoscience to enhance their market share. It is exciting that this year’s Annual Technical Conference of INDA will have a session on nanofibers for the second time in its history. Douglas Mulhall in his book, “Our Molecular Future”, says manipulating things at atomic level will influence our future and all things as big as our planet. Will nanotechnology influence the nonwoven industry? A coordinated effort between research infrastructure and industry will be able to force molecular future in the nonwoven industry for a win-win situation!
Seshadri Ramkumar and Mohammad Munin Hussain
Texas Tech University
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