Imagine a solider wearing five pounds of chemical protection suit and 25 pounds of anti-ballistic SWAT suit. Although these suits provide the necessary protection to the wearer, weight and lack of comfort are important issues that are critical for their applications and acceptance by warfighters. If the weight of personnel protective gears could be reduced and at the same time they provide enhanced protection, there could be no best option for the next generation chemical protective clothing and anti-ballistic vests. Defense and law enforcement communities hope that nanofiber composites and nano materials such as nano tungsten carbide can sever as panaceas for such complexities. Although nano materials can satisfy such requirements, large scale production and health related concerns of nano materials pose immediate challenges.
Nanoscience is considered to be one of the key technologies of the 21st century, which will play an important role in high-tech sectors such as material science, electronics and even energy. Miniaturization of instruments and products has revolutionized the electronics industry which has provided a platform for the nanotechnology to have practical applications. The textile industry has yet to fully utilize the benefits of nanoscience to that extent as in the case of semiconductor industry. In fact, major developments involving nanotechnology in the textiles industry have been in the finished applications using nano formulations and in the field of filtration. In the case of filtration using nanofilters, a handful of companies have been successful in the past decade in using nanofiber webs in their filtration products. The important barrier to the mass utilization of nanofiber webs in the textile industry has been lack of productivity in the electrospinning technology. More recently, there have been some developments to overcome this barrier, which will speed up the exploitation of nanofibers for applications in filtration, medicine and even for energy storage. Another aspect that will play an important role in the greater utilization of nanoproducts in consumer products is the toxicity issue with submicron and nano-sized materials. There are conflicting theories on the health—effects of nanomaterials. Governments around the world are investing heavily to understand the health issues surrounding nanomaterials. Thorough knowledge of toxicity and health issues related to submicron fibers and nano materials that find applications in the textile industry is much needed for our industry. This article will throw some insight on the scalability issues with regard to the manufacturing of submicron size fibers and toxicity-related aspects of nanomaterials. A greater understanding on these two aspects will be essential to the nonwovens industry to fully exploit the potential of nanomaterials.
Electrospinning Process and Continuous Nanofiber Formation
The electrospinning process is recognized as a simple and easy method to produce submicron size fibers. Original patents on this simple technology date back to the earlier part of 20th century. Since the 1990s, there has been renewed interest and a major upsurge in the use of this electrospinning technology for the production of nanofibers. A nanofiber in the parlance of fiber science generally refers to submicron-sized fibers whose diameter ranges somewhere between 100 nm and 0.5 microns. Electrospinning technique is being used in roughly 100 laboratories around the world to develop fibers using polymers as simple polyethylene oxide to complex protein molecules.
Most recently, researchers from Technion-Israel Institute of Technology are reporting the first successful development of protein nanofibers for wound repairs. This research reports that bovine serum albumin has been successfully electrospun for biomedical applications. Such breakthrough developments are possible using nozzle-based electrospinning technology. However, transferring the laboratory research to marketplace has been the challenge in this field. If the production of nanofibers using simple methods can be translated to commercial space, nanofiber webs can find a myriad of applications, which will have mass utilization. How has the commercialization issue been tackled by the nonwovens industry? Donaldson has been using electrospinning technology to develop value-added products such as filters and synthetic nanofibrillar cell growth surfaces for a number of years. However, widespread commercialization and the use of the electrospinning process by a large number of industries has not occurred. But recently a handful of companies are endeavoring to scale-up the nanofiber production process in order enable commercialization. Ohio-based NanoStatics Corporation utilizes the nozzle-based electrospinning process and has marketed high throughput electrohydrodynamic spinning system (EHS). Czech-based Elmarco has come up with nozzle-free Nanospider continuous technology. Recently, Elmarco has relocated to Raleigh, NC.
NanoStatics Corporation’s electrohydrodynamic spinning process produces continuous nanofibers from polymeric fluids by applying high electric fields. This system has an array of electrospinning nozzles in a matrix of 100 to 400 nozzles per square foot, all producing consistent nanofibers. Each nozzle has a flow rate greater than 0.1 ml per minute. This is relatively lager than the low production rate of 0.02 ml per nozzle in current low productive systems. EHS gives more than five times an increase in productivity per nozzle. NanoStatics claims that with the increase in the throughput and the number of nozzles per square feet, the productivity gain compared to the current electrospinning systems can be as high as 200 times. Typically, a 100-inch-wide EHS unit can contain more than 10,000 nozzles that produce consistent nanofibers at line speeds of 20 to 1000 feet per minute.
In addition to the scaling up of the process, NanoStatics is also using environmentally friendly solvents to produce nanofibers with functional properties. These are trademarked as “Green Spinning” and “Engineered Nanofibers.” According to Jack Shirmer, vice president of market and business development of NanoStatics Corporation, “In 2008, NanoStatics has introduced filter media suitable for the HVAC market with a brand name NS Enhance.”
Elmarco’s Nanospider is a unique technology that has been designed to overcome the problems caused by the close packing of spinning nozzles. The drum-based system takes care of the problems associated with the Taylor cone, which is very crucial for the development of submicron-sized webs. This technology is capable of producing nanofiber webs from 50 nanometers to one micron for various industrial applications. It also enables scaling up of the nanofiber production in energy efficient way to achieve industrial requirements.
The modular equipment can be stacked in serial up to four units to scale up the throughput and reach a maximum production speed of 15 meters per minute producing 0.03 gsm fabric with 200 nanometers. According to Fred Lybrand, vice president of North America Elmarco Inc., “Nanospider technology has clearly demonstrated with Elmarco’s customers in their production environment that nanofibers are ready for industrial applications.” Elmarco makes high volume production equipment up to 1.6 meters wide.
The two aforementioned systems basically overcome the production barrier that is existing in the conventional nozzle-based electrospinning technology. With the advent of high productive systems such as Nanostatics and Nanospider, industry now has the option to go into high gear in the development of nanofiber webs for a number of applications.
Toxicity of Nanomaterials
Nanomaterials that are used to functionalize nanowebs are prone to have contact with human skin and enter into the human body. Therefore, investigation into the toxicity of nanomaterials such as nanometal oxides, which are catalytic in nature, is extremely important. For example, nanometal oxides such as iron oxide and zinc oxide, which have catalytic activities on toxic gases, are used to coat nanofiber webs. How toxic will these particles be? The health-related toxicity of these materials, which is a determining factor in the acceptance and application of nanofiber composite materials used in air and toxic gas filtration. Our industry has not scratched the surface of this field yet. The evaluation of toxicity of nanomaterials is a multidisciplinary activity involving material scientists and toxicologists. Even for basic toxicologists, the investigation of toxicity offers enormous challenges and requires interdisciplinary approaches. It involves careful selection of animal models and experimental protocols and costly experiments. Scientists at The Institute of Environmental and Human Health at Texas Tech University (TIEHH-TTU) have recently begun dedicated research efforts to understand the toxicity of nanomaterials such as metal oxides and carbonbased nanomaterials. Shawna Nations and Dr. George Cobb at TIEHH-TTU are investigating the acute and chronic toxicity of metal oxide nanoparticle such as Fe2O3, ZnO, CuO and TiO2 to frogs. They have found that acute exposure to nanoparticles were not embryo lethal, but did have a dose-dependent effect on the growth of frogs. CuO and ZnO nanoparticles induced malformations resulting in EC15¬ of 39 mg/L and 2 mg/L respectively. Both these nanoparticles were found to inhibit metamorphosis upon chronic exposure. As is evident, long-term exposure of nanoparticles may lead to negative effects. However, the effects in humans have to be determined to have regulations on the use of such nanomaterials for applications in filters, etc. Such a study is warranted and will be of enormous help to our industry. The toxicity of carbon nanotube remains an unresolved issue. As carbon nanotubes are used in a number of fiber-based composites, the health issues surrounding the use of carbon nanotubes are of extreme importance. A recent study by Jonathan Maul at TIEHH-TTU on the interaction between functionalized fullerenes and agriculture chemicals has shown that fullerenes functionalized with carboxylic acid reduced the reproductive toxicity of bifenthrin to aquatic organisms such as Daphnia magna. This result is of particular relevance to our industry as nanofiber-based materials provide certain properties such as distructive adsorption, filtration, etc. So the toxicity of functionalized nanofibrous materials will be a determining factor in the development and use of chemically modified nanofibrous materials. A collective effort is needed between the nonwoven industry and toxicologists to gain an overall acceptance of nanofiber materials for those applications when nanowebs will be in contact with human beings.
Recent Developments in Nanotextiles
Nanotechnology with regard to textiles has evolved in the last decade and now predominantly focuses on functionalization of basic nanowebs. Functionalization refers to value addition to regular nanofiber webs by incorporating some chemicals or by modifying the physical structure. These functionalized nanofibers provide enhanced applications such as catalysts, sensors, liners for chemical and biological protective clothing, tissue scaffolds, bioengineered materials, etc. A recent study in Ramkumar’s group at TIEHH-TTU has manipulated the electrospinning collector substrate to develop filter-within-filter honeycomb nanowebs. These nanowebs in addition to high surface area, due to mesh-in-mesh structure can act as good filters and can trap fine particles. Such a process of assembling nanofibers to obtain unique patterns for superior performance is commonly referred to as self-assembly. A similar self-assembling project is being carried out by Professor Juan Hinestroza at Cornell University, which focuses on understanding the self-assembling mechanisms of nanoparticles on the surface of fibers. Dr. Hinestroza has found that high surface coverage of fibers with nanoparticles may induce interesting and unique phenomena such as the creation of color without dyes. This may open up new avenues for the use of nanofibers in camouflage and military applications. Also his work focuses on measuring the mechanical properties of bicomponent and tricomponent nanofibers using acoustic force atomic microscopy.
Dr. Gajanan Bhat and his coworkers at the University of Tennessee Nonwovens Research Laboratory in collaboration with eSpin Technologies have investigated the structure and properties of electrospun nanofiber composites with spunbond and meltblown fabrics in order to produce sustainable biomedical webs. In another collaborative work with ChK Group, Inc. in Dallas, TX, Dr. Bhat and his group were able to develop nanophase Mn (VII) oxide (NM740)- incorporated nonwovens to neutralize chemical warfare agents. This nanoparticle-incorporated nonwoven can also be used as a smart fabric in military and civilian applications.
The cotton research unit at the Southern Regional Research Center, Agricultural Research Services, USDA in New Orleans, LA has been utilizing nanomaterials to develop value-added cotton products. According to Dr. Paul Sawhney of the above unit, layer-by-layer deposition of nanoparticles of appropriate chemicals and their size may be a more efficient and cost-effective route to develop value-added textiles. He is of the opinion that the layer-by-layer deposition will not change the intrinsic properties of textiles as in the case with the traditional method of deposition of nanopartilces. As is evident, functionalized nanomaterials find a number of applications heretofore unexplored. More importantly, functionalized nanofiber webs have a number of biomedical applications. The following table gives a brief overview of the types of polymers that are finding enormous biomaterial applications. Elaborate information and the corresponding bibliography are available in the article published in the March 2006 issue of Indian Journal of Fibre and Textile Research.
Nanotechnology appears to come in handy in solving one of the important issues facing the nonwoven industry. Enhancing the comfort aspect of polypropylene is one of the immediate challenges for our industry. There seems to be a solution to this by effectively utilizing environmentally-benign technologies. Early indication from a recent research activity at Texas Tech University has shown that plasma treatment may create submicron size indentations on polypropylene nonwovens that may serve as air pockets similar to those in wool. If such results are proven successful, polyolefin-based nonwovens can very well penetrate into apparel sectors. It is a long road to travel. Our industry is a late bloomer to effectively utilize nanotechnology tools such as Atomic Force Microscopy (AFM) and nanoindenters to understand the physical characteristics of nonwoven substrates. Nano-level characterization using sophisticated tools may lead to some new understanding on the physical attributes of nonwovens, which may lead to some new applications. Atomic force microscopy reveals the smooth surface and the diameter of individual fibers in a spunbond nonwoven lies within the range of 10-15 microns. The utilization of AFM will be useful in understanding the surface features of functionalized nanowebs.
Where Do We Go Next?
Nanoscience as related to textiles is nearly a century old. Since the invention of the electrospraying technology using cellulose acetate fibers in the early 1900s, the science has slowly evolved to reach a production level of 0.1 ml per minute per nozzle. Currently the barriers with regard to higher productivity have been overcome with the availability of multinozzle electrospinning and nozzle-free continuous methods. These technologies should create interest in our industry to seriously exploit the commercialization of nanofibrous materials for a myriad of industrial applications. In addition to concentrating on the productivity aspect, efficient methods to functionalize nanofibrous materials using environmentally benign approaches and the evaluation of health-related issues will be the immediate tasks for our industry. Nontoxic commercially producible nanofibrous materials will have a number of applications in consumer and healthcare-related products and can give competitive advantage to our industry.