Global Industry Analysts, Inc. predicted that the value of smart and interactive textiles will be $1.8 billion by 2015. During the same period, the global textile and apparel trade will be about $800 billion. This is expected to reach $1 trillion by 2020. Although market research does not quantify the amount of nanoproducts and applications in this general estimate, the authors feel that the $1 trillion global market share value of textile and apparel sectors will definitely be enhanced with the use of nanotechnology and functional finishes. More importantly, the contribution of nanomaterials and processes will be primarily in the development of niche products and non-commodity products such as in the energy, biomedical, defense, aerospace sectors, etc. Reports in the nanofield estimate that nanomaterials will play an important role in fuel cells, battery separators and super capacitors. According to the Research and Markets Ltd. of Ireland, the value of nanomaterials in the energy sector was around $203.7 million in 2010 and is estimated to rise to $2.28 billion by 2017. Overall, the nanofield is expected to play an important role in high performance textile materials.
Status of Nanotechnology in Technical Textiles
The textile industry heretofore has focused on two aspects of nanotechnology: 1) fibers and 2) finishes. The following issues surrounding the commercialization and application of nanofibers are well known to the industry such as: 1) productivity; 2) performance-related characteristics; 3) scale-up and 4) cost. There have been a few developments in the recent past to overcome these aforementioned issues. In addition, it is not clear whether the nanotechnology enabled products meet with the consumer expectations with regard to the need for low costs in volume-based commodity textiles sectors.
Nanotechnology-based textile materials can be broadly classified into: a) non-functional textile products b) functionalized/finished textiles. Nano textiles are basically textile structures of nano size such as nanofibers and other fibrous structures. These basic structures can be built-up further to develop higher order structures such as fabrics and composites where nanofibers and other fibrous structures can serve as building blocks. The inherent characteristics of these nanomaterials such as high surface area- and weight-to-volume ratio at the building block level will be of great advantage wherever high performance, life and environmental related end uses are involved.
In our view, the cost of such nano enhanced products will not be an issue in the healthcare, environment, defense, aerospace and advanced technology sectors. Furthermore, because of the dependence of the nano sector on high level research and development and intellectual property protection, developed economies such as the U.S., Europe, Israel, Australia and Canada will have an edge at least until 2030.In addition, because of the nature of the nanotechnology industry in terms of the technology and its size, it gives immense opportunities for small and medium sized enterprises to venture into this field.We endeavor to provide a new, albeit simple, method of classifying nanofibers based on the dispersion process by which they are produced. This article will highlight some of the recent developments with regard to the nanofiber technology and applications.
Origin of Nanoscience
It will be a fruitful exercise to understand the origin of nanoscience and nanotechnology. If we trace the origin of nanoscience and nanotechnology, we can really understand its influence on human life. The DNA molecule, which is the building block of human life, is about 2.5 nm in diameter, proving that life begins at the nano level. Tracing back to the invention related to nanotechnology within the context of textiles, it has come to light that the technology related to the manufacturing of nanofibers using synthetic methods is over a century old. The process of dispersing fluids using electric charges which formed the basis for developing solvent cum electrospunnanofibers dates back to the year 1902 with the granting of two separate and independent patents on dispersing fluids using electricity. Cooley was granted a patent for the apparatus for electrically dispersing fluids (U.S.Patent 692,631) and Morton was granted a patent for the method of dispersing fluids (U.S. Patent 0705691). After couple of decades in 1940, Formhals was granted a patent for using electric charge to develop synthetic threads. These patents basically contributed to the development of nano textiles. After a hiatus of many decades, resurgence in the use of electrostatic charges to develop nanofibers happened due to the research activities at the University of Akron, OH. However, thinking about the possibilities of the opportunities at the nano level began due to the motivation and lecture given by the Nobel Laureate Richard Feynman. In his famous lecture titled “There’s Plenty of Room at the Bottom,” delivered in 1959 at California Institute of Technology Feynman asked,“Why cannot we write the entire 24 volumes of the Encyclopedia Britannica on the head of a pin?”This concept of squeezing the size of materials to fit to a given dimension to accommodate huge information is what nanotechnology is all about. Ever since that time, basic scientists and technologists have been working in their respective fields to harness the potential of materials at nano and micro levels for new and emerging applications. With the invention of the scanning tunneling microscope by IBM scientists Heinrich Rohrer and Gerd Binnig, research and development in the nano field exploded. Since that time, many tools such as atomic force microscopy, nanoindenters became available to the materials science community which kick started the growth of nanotechnology sector.
New Classification of Nanofibers
A new albeit simple classification based on nanofiber production methods is given at left. Technically, this classification is based on the method of dispersing raw materials such as molten polymer solution and solid particles into its nano state using three main mechanisms such as: 1) mechanical dispersion; 2) chemical Dispersion and 3) electrical Dispersion. To our best knowledge, such a classification is first of its kind and will be helpful to the nonwovens industry for furthering research and development in the field.
The three basic mechanisms given above can be utilized in different forms such as meltblowing, centrifugal spinning, sol-gel process, syringe/needle electrostatic methods, non-needle electrostatic methods and electroblowing.
Of all these processes, electrospining became the first method to be used commercially in a broad manner. The earlier version of electrostatic spinning was predominantly based on needles/syringes which are used as spinnerets for electro-charging the polymers dissolved in the solvent. One of the perquisites in the electrostatic spinning is that the solvent has to be volatile in nature, which restricts the type of materials that can be used in the solvent cum electrospining. Apart from this perquisite, other disadvantages associated with needle based methods lie in its low productivity, solvent dependency, electrical compatibility of the solvent and the polymer, risks associated with use of electricity, solvent reuse, actual utilization of the solvent in the development of nanofiber, etc. These led to incremental developments such as multi syringe spinning heads and needless methods which involve different shapes and sizes of the polymer solution carrier such as solid cylinder, spring, single and multiple discs. Even with enhanced productivity, the method of electrospining is still based on solvent which curtails its versatility with regard to the type of polymer and solvents that can be used. More importantly, in the case of nonwovens and technical textiles sector, for the development of single-use products the availability of cheap raw materials is a perquisite. For example, the spunmelt sector which has 47% of the total share of the nonwoven market is predominantly based on polypropylene, which is a cheapest raw material. In the case of the solvent-based electrospining, in order to be competitive with other systems such as meltblowing, the raw material and process cost should be minimized. As polypropylene is not a viable polymer for solvent spinning, it cannot be used as a starting raw material for electrospining. Furthermore, the amount of solvent required compared to its effective usage in the solvent spun electrospining process is huge which also adds up to the cost of the process. Of late, in order to counter some of the disadvantages with solvent spun electrospining process, efforts are being made to look for possibilities in meltblowing and other hybrid processes such as electroblowing. Interest has also emerged in mechanical methods such as centrifugal force based forcespinning. Meltblowing has been revisited with the design of finer die holes to develop nanosized fiber. Mechanical system based nanofiber processes can melt-extrude a wide variety of thermoplastic fibers which can bring down the cost of raw material. This is not possible in the conventional solvent spun electrospining process. In addition to pure mechanical methods, hybrid methods which involve electricity and air to produce nano-sized fibers are also coming into vogue these days. Based on this principle, meltblowing which involves melt extrusion and hot air dispersion, and electroblowing which involves solvent spinning followed by hot air blowing can be categorized as hybrid methods.
Electroblowing: The patent on electroblowing issued in November 2009 assigned to DuPont involves the combinatorial approach of electrospining and hot air blowing. DuPont’s commercial product developed by this method is known as DuPontEnergain. DuPont is targeting its primary application in energy sectors such as battery separators. According to a news release from DuPont, DuPontEnergainbattery separators enhance the power by 15-30%, increase the life of the battery by 20% and gives better stability to the battery at high temperatures. DuPontEnergainseparators will be initially used in hybrid and electrical vehicle batteries. Additional applications will be in filtration involving broad range of industries such as pharmaceutical, food and beverage and micro electronics. DuPontEnergain battery separators produced using the patented electroblowing method (U.S. Patent 7,618,579) have continuous fibers with diameters between 200 nm to 1 micron. According to DuPont, electroblown webs can provide thinner webs with lower ionic resistance and high temperature stability which are not achievable in conventional nonwovens and microporous films. Electroblown polyimide separators have high temperature melt integrity greater than 250°C, low shrinkage, good chemical resistance, etc. Data from DuPont show that polyimide electroblown webs have been shown to have zero shrinkage at 150°C even after exposure for three hours compared to ultra high molecular weight polyethylene which performs poorly at this condition. DuPont has started constructing a manufacturing facility in Chesterfield County, VA for commercial production.
Meltblown Nanofiber Webs: Since 2008, Dr. Gajanan Bhat with his research team at the Nonwovens Research Laboratory, University of Tennessee-Knoxville (UTNRL) have been working with modular die manufacturers such as West Melbourne, FL based Hills Inc. and Bristol, CT based Arthur G. Russell to produce nano sized meltblown webs. UTNRL currently has the capability to produce meltblown nanofibers on its six-inch line using both the Hills technology and the Arthur G. Russell die from the Oyster Bay, NY-based Nonwoven Technologies, Inc. In both the cases, the existing meltblown dies have been retrofitted with nanofiber dies without any modifications. Trials with several thermoplastic polymers, such as PP, PET, PBT and PLA have been performed. In all these cases, fiber diameters observed have been less than a micron with the average diameter being about 400 nm. As the research is continuing to improve fiber diameter distribution, web structure, according to Bhat, UTNRL has retrofitted the 20-inch-wide line with the Arthur G. Russell modular meltblown die to allow production of nanofiber webs at semi-commercial scale. According to Dr. Bhat, a main advantage of nano meltblown technology is that the non-usage of volatile solvents, as in the case of electrospinning process. In commenting Dr. Bhat said, the productivity in the thermoplastic meltblown nano process is at least one order magnitude higher than that of needleless electrospinning and a few orders magnitude higher than that of needled electrospinning.
Some Emerging Developments
Recent research at the Nonwovens & Advanced Materials Laboratory at Texas Tech University is focused on developing cotton nanofiber composites with enhanced filtration capabilities. Recent results show that aerosol filtration efficiency of nanofiber embedded cotton fabric is almost doubled when compared with untreated cotton fabric. Of course, the durability of such nanofibers layers is still an issue as it is the weak van Der Wall force that is holding the layers onto the base substrates. To overcome this, it is convenient to use one or several layers of nanofibers between base substrates as sandwich. Donaldson Company has developed such products for filtration and it is common practice these days in the industry to use nanofibers in sandwiched structure.
Centrifugal force based forcespinning which is now commercialized by Edinburg, TX based FibeRio Technology Corporation has an advantage over solvent based electrospining.In this system, nanofibers can be developed from molten polymers and solid metal particles. The State of Texas through Texas Emerging Technology Fund has awarded $1.5 million in September 2010. El Paso based Cottonwood Technology-a venture capital fund has also partnered with FibeRio Technology Corporation towards the commercialization of the centrifugal force spinning technology.
Xungai Wang, professor and director of the Centre for Material and Fiber Innovation, Deakin University, Australia has been working to develop novel materials using nanotechnology. According to Professor Wang, a new form of solvent spinning has been invented and patented by his group (WO 2010/043002). In their patented technology, a metallic disc is used to produce electrospun nanofibers instead of syringes. Uniform fiber distribution, higher production capability, simple operating procedure and simplified machinery give this technology an edge over other solvent based electrospining techniques, said Professor Wang.
Nanowebs as Hydrogen Storage Matrix
A fascinating development in nanofiber application is from the U.K.-based Cella Energy Limited which utilizes electrospinning to develop a hydrogen storage matrix. Co-axial electrospinning set-up has been used to develop a hydride core nanoweb. Electrospining concept has been used to incorporate ammonia borane core and polystyrene sheath in a nano matrix which is permeable to hydrogen. The core and sheath nanostructure developed using co-axial electrospining encapsulates the hydride efficiently. It is hoped that such nanoweb structures will be suitable candidates for hydrogen storage which can release and absorb hydrogen quickly.
Summing-up, nanofiber development has reached Phase-3 with non-solvent based methods of production coming into the commercial market. Earlier phases focused on two primary aspects: 1) developing commercially acceptable nano products and 2) enhancing the productivity of the processes. The current phase which focuses on highly productive and non solvent electrostatic methods seem to have overcome certain bottle necks, which the industry has been facing with regard to raw material perquisites and process constraints. Another important challenge for the nano sector is to thoroughly understand the environmental and health risks associated with nanotechnology based products.