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| Issue date:01/04/2008 |
| ATA Journal for Asia on Textile & Apparel - Apr 2008 Issue |
| Source:Journal for Asia on Textile & Apparel |
| by Adrian Wilson |
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| Nanofibers are traditionally defined as cylindrical structures with an outer diameter below 1,000 nm and an aspect ratio (the ratio between length and width) greater than 50. Over the years, several types have been developed, polymeric, carbon, ceramic, glass, metallic and composite, and they are still the subject of intensive research and development worldwide.They have current and potential use in a broad variety of applications including electronics, mechanical, chemical, sensors and instrumentation, energy, medical, bioengineering, automotive, aerospace, thermal and acoustic insulation, consumer applications, and defence and security. |
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Neumag and Elmarco jointly commercialize nanofibers
The recent announcement of a joint venture between Elmarco of the Czech Republic and Oerlikon Neumag under Oerlikon Textile, the world's largest manufacturer of textile machinery, may mark a significant step in the commercialization of the technology for production of nanofibers, for instance, Nanospider AcousticWeb, which is a sound absorbent material.
Elmarco has already sold a number of its Nanospider lines since their introduction at the INDEX show in Geneva in 2005.
The Nanospider system employs centrifugal spinning, which is similar to electrospinning, using a high-speed rotating cylinder with nozzles to create fibers. It has a high productivity but the fibers are not as fine as electrospun types.
In the electrospinning process, high voltage is used to create and electrically charge a stream of polymer solution that is electrospun by capillary action using a spinneret.
Nanospider technology's productivity is much higher since it does not use nozzles or capillaries to form fibers.
The fibers are formed by an electrostatic field from a thin film of an aqueous or solvent solution and are collected in the form of a nonwoven textile on a collector. Fiber diameter is 100-300 nm and the weight of the nonwoven web is 0.1-5gsm.
Innovative meltblown nanofibers fend off counterfeiting
Another company, Hills Inc, based in the US state of Florida has developed technology to produce meltblown nanofibers with an average size of 250nm and a range between 25nm and 400nm.
The company has used its patented printed-circuit-style extrusion dies to produce the fibers from high-melt-flow-index polypropylene.
Hills Inc's custom-designed nanofibers for authentication |
According to Hills, a hole count of 100 holes per inch and up, and extremely high length-to-diameter ratios enable production of these nanofibers at reasonable rates, and puts meltblown production in the same size range that was previously the exclusive domain of multicomponent spinning or electrospinning technology.
In 2007, nanofibers were introduced in the US as logo security fibers by a new company, ARmark Authentication Technologies, using Hills technology.
ARmark is custom-designing the nanofibers to help brand owners combat the growing problem of counterfeiting. They can be woven into textiles for use in garments, luggage, upholstery, handbags and other luxury items, as well as being employed in currency and security documents.
Based in Glen Rock, Philadelphia, the US, the company also manufactures detection systems, which authenticate products by a simple process of examining fiber cross-sections.
"ARmark has a Hills system with four extruders so it is possible to produce the cross-section design in four colors," said Ben Shuler, vice-president of business development and project management for Hills. "It can produce such fibers from common textile polymers including nylon, polypropylene, polyester and aramids. The fibers and yarns can also be UV and IR responsive."
Global market for nanofibers
The global market for nanofibers rose from US$43.2 million in 2006 with sales at approximately US$48 million by the end of 2007 and is set to be worth around US$176 million by 2012.
Nanofiber revenue growth is being driven primarily by the use of these materials in the mechanical/chemical sector, in particular for manufacturing filtration media, which is expected to grow in value from US$35.3 million in 2007 to about US$127.6 million in 2012.
The fastest growing sector, however, will be electronics, increasing from US$2.2 million in 2007 to US$7.2 million in 2012 and US$137.9 million by 2017. |
DuPont produces advanced fibers in Korea
Unsurprisingly, as a pioneer in fiber sciences, DuPont has made considerable strides in the area of nanofibers with its Hybrid Membrane Technology (HMT), which is said to bridge the gap between nonwoven substrates and membranes.
Sandra S. VanWormer |
The development of HMT involved two DuPont businesses, Nonwovens and Advanced Fiber Systems, responsible for Nomex and Kevlar performance fibers. Ground was broken at a new manufacturing plant in Korea in 2005 and production started up in May 2006.
"The role of Advanced Fiber Systems was very important in this development," said president of DuPont's Hybrid Membrane Venture, Sandra S. VanWormer.
Consistency is important, she added. At the moment, HMT products are made of nylon but the technology is being developed for a range of polymers and fibers.
The key advantages of the submicron fibers in filtration applications are a high surface area, high flow and low pressure drop.
"This leads to energy savings and a particular breakthrough is the high throughput," said Ms VanWormer. "One other advantage is that HMT products do not rely on a static charge for efficiency, unlike electrostatically treated media."
Market size of up to US$15 million
While producing a fully functional web of nanofibers -- as opposed to simply adding a coating or layer to a medium produced by other cheaper methods -- remains a future development, the potential rewards are huge.
Nonwovens manufacturers such as Ahlstrom are now employing nanofiber layers in filter media |
"If I were to make a conservative estimate of the global value of nanofiber filter media, it would be currently in the region of an annual US$8-10 million worldwide, and probably as much as US$12-15 million," said Lutz Bergmann of Filter Media Consulting.
Many leading industrial nonwovens manufacturers including Ahlstrom, Hollingsworth & Vose, and Johns Manville are now employing the nanofiber layers in filter media.
The technology is widely acknowledged to be pioneered by Donaldson Company with its UltraWeb range, and Hollingsworth & Vose has a global licensing agreement with Donaldson, covering the patented technology used to produce nanofiber filter media and nanofiber containing filter elements for pleated air filters used in on-road vehicle applications.
The use of nanofibers as actual complete filter media, however, is limited due to a number of factors, chiefly the lack of full-scale production equipment, high costs and limitations of the electrospinning manufacturing route.
Electrospun fibers are said to result in webs of great homogeneity and the process is very flexible, with the ability to handle additives.
At the laboratory level, it is easy to develop proprietary chemicals and it is a good method of producing individualized fibers. The main stumbling block to electrospinning at the moment, however, is the slow production rate, in addition to the need to handle solvent gas and the possible risk of explosion in the process. The resulting webs also have low mechanical strength and poor surface adhesion.
As a result, alternatives to electrospinning are currently the subject of much research and development, while there is also a drive to achieve finer fibers via progress with established technologies such as meltblown.
Korean producer supplies polyamide 6.6
Finetex, a company with its origin in a Korean research university project, now has three commercial nanofiber nonwoven lines running in the Philippines, each with ability to produce some eight million square meters of nanofiber webs per year.
To put the company's annual output in context, with an average weight of just 0.05gsm, eight million square meters of these extremely fine webs only amount to around 20 tons of material.
The solvent electrospinning process on which the company's technology is based involves introducing a polymer solution into an electrostatic field via a nozzle or other device.
The electrostatic field attracts the solution, breaking the surface tension and creating an unstable jet. A whipping action of the jet causes the solvent to evaporate off and leaves a long fiber that is collected on the opposite electrode.
The polymers employed can be single or multi-component, reactive or non-reactive, synthetic or inorganic and biodegradable. The commercial products supplied by Finetex are mainly polyamide 6.6.
Additives that can be incorporated, both soluble and non-soluble, can provide a range of properties including hydrophobic or hydrophilic and anti-bacterial. It is also possible to add low volatile liquids, odors or scents, and colors.
Finetex manufactures its webs either in roll goods in widths up to 1.6 meters and 500 meters long, or can add the layers to other substrates by direct deposition, particularly for interlinings.
The webs can be laminated with conventional equipment, but since their strength is not high, the machine has to be slowed down and also different. Often, more adhesives have to be applied.
The markets Finetex serves fall into four major groups, namely filtration, technical textile products such as protective workwear, medical wound dressings and pads, and in energy generation as fuel cell components, such as battery separators and super capacitors.
In technical textile clothing membranes, the company sees its webs bridging the performance and price gaps between hydrophilic monolithic types based on PU and hydrophobic microporous membranes employing expanded PTFE.
"We have very flexible technology and can accommodate, for example, multiple layers with differing aspects and fiber diameters," Mr Luukkonen said. "We are filling the gap that microporous membranes cannot reach and PTFE is too expensive for."
Finetex recently announced a joint venture with Net69Sports of Korea, marking its formal entry into the global textile business last November. The new joint venture company is called Finetex Textile Company Limited. It would sell fabrics incorporating laminated nanomembranes and launch these products at outdoor wear exhibitions in the US and Europe early this year.
Laser spinning for nanofiber production under research
Among the many projects currently underway on nanofiber production, laser spinning is being proposed by researchers at the University of Vigo in Spain and Rutgers University in New Jersey, the US.
They have produced very long amorphous nanofibers in a simple physical process that does not involve catalysts, templates or any other reagents except the precursor material with the desired fiber composition. Not only does the method produce nanosized fibers, it can also produce nanofibers directly from materials that melt at high temperatures, something that is not possible with other similar techniques such as electrospinning.
Researchers have used igneous rock from Mount Etna to produce carbon nanofibers |
Laser spinning essentially involves using a high-power laser to make a cut in a plate of the precursor ceramic material, such as silica or alumina. This approach ensures that only a small volume of ceramic is in the fluid state at the melt front (the leading edge of the cut) at any given time. While this is being done, a supersonic nozzle injects a high-velocity gas jet in the area of the cut. The viscous molten material produced is then quickly stretched and cooled by the gas jet in a simple elongation process to yield a disordered net of intertwined amorphous micro and nanofibers.
Researchers at the Fritz Haber Institute in Berlin, Germany, meanwhile, have used igneous rock from Mount Etna volcano to produce carbon nanotubes and fibers directly by deposition from the gas phase. The naturally occurring iron oxide particles in lava make it an effective natural catalyst, possibly smoothing the way to a more efficient production method.
Other natural materials such as spider silk, collagen and cotton could be a cheap and abundant source of nanofibers, said scientists in China and the US.
Feng Xi-qiao of Tsinghua University in Beijing and colleagues at Brown University in the US showed that nanofibers measuring between 25nm and 100nm across can be extracted from natural sources using ultrasound. The simple technique could be scaled up to industrial scales.
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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