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| Issue date:01/12/2007 |
| ATA Journal for Asia on Textile & Apparel - Dec 2007 Issue |
| Source:Journal for Asia on Textile & Apparel |
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| Separate new developments claim to be able to turn both polymeric nanofibers and carbon nanotubes into yarns, with far-reaching implications, Adrian Wilson reports. |
Scientists have long known of the remarkable electro-mechanical properties of carbon nanotubes — they are 100 times stronger than steel, one-third the weight of aluminum and extremely good heat and electricity conductors.
This makes them very attractive for broad-based use, with the potential to augment or replace many current materials in end-user products.
Up to now, however, competitive commercial manufacturing processes have generally produced only short carbon nanotubes — usually tens of microns long — making currently available products very much like a powder.
And as with most powders, they can be quite difficult to incorporate into final manufactured goods, and even when this is achieved, exhibit less than optimal strength and conductivity properties.
Today's nanotubes are also quite expensive — usually too expensive for use in volume industrial applications. This is a result of the significant amounts of impurities generated in their manufacture. Extensive and expensive post growth purification is usually needed to remove these impurities.
This situation may be about to change.
New material from long carbon nanotubes
A company called Nanocomp Technologies, based in Concord, New Hampshire, the USA, says it has successfully produced a new textile material from long, carbon nanotubes.
The material, which is being produced both as nonwoven sheets and in yarn form, is said to be extremely lightweight and strong, as well as efficiently conducts both electricity and heat.
As such, it could provide functional performance benefits in defence and aerospace applications ranging from body armour to structural composites.
"We believe we are on the cusp of delivering the promise of carbon nanotube materials," said Peter Antoinette, Nanocomp president and CEO. "Like our predecessors in performance products who developed Gore-Tex and Tyvek, we have a product platform with vast functionality, and together with the system integrators that will ultimately incorporate it into end-use products, we aim to determine just how broad the benefits will extend."
Nanocomp's patent pending process is said to produce long and pure, continuous carbon nanotubes, at high growth rates. Longer nanotubes mean greater strength, higher conductivity, easier handling, and greater product safety. The company's materials require no post-growth purification, and high output rates are believed to hold the promise of achieving excellent process economics and product affordability.
In the near term, Nanocomp expects its materials to be used in conjunction with carbon fibers and aramids to reduce weight and improve performance of body armour. They will also be incorporated into land, air and marine vehicle structures to improve fuel economy.
A team of scientists from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia and the NanoTech Institute of the University of Texas (UTD) won the 2006 NanoVic Prize for Innovation in Nanotechnology for their work on the development of carbon nanotube yarns and transparent sheets.
The award followed the team's success in demonstrating that synthetic carbon nanotubes can be assembled into transparent sheets that are stronger than steel sheets of the same weight.
In 2004, CSIRO Textile and Fiber Technology scientist, Ken Atkinson worked with UTD researchers to co-develop a method for spinning carbon nanotubes by downsizing conventional wool and cotton spinning to the nanoscale. A research partnership combining CSIRO's expertise in spinning technologies with UTD's expertise in "growing" carbon nanotubes, was formed to develop applications and investigate methods to produce commercially viable volumes of the material.
New technology involving electrospinning
Another new technology has recently been patented that may open the door for the commercial production of woven nanofiber applications.
Its inventor is being represented by South Africa-based Licensing Technology Network in seeking a partner willing to license the technology and assist in commercialization.
Technical details on exactly how the method works will only be disclosed under a non-disclosure and non-circumvention agreement, but basically, it involves the well-known electrospinning process.
Electrospinning is a simple but extremely versatile method for obtaining continuous nano and micro fibers of natural and synthetic polymers, as well as inorganic oxide materials.
On average, solution-based electrospinning, using needle spinnerets, has solution throughput rates on the order of one milliliter per hour per needle.
Fibers with diameters in the range of 50 to 100nm are typically spun from solutions with relatively low concentrations, 0.5-10wt% (weight percent) depending on polymer type and molecular weight.
This means that, assuming a polymer density of around 1g/ml, the typical solids throughput rate of a needle-based electrospinning process is 0.005g to 0.01g of fiber per hour per needle. If this calculation is extended, producing a nanofiber web with a planar density of 80gsm at a rate of five square meters per second will require a minimum of 40,000 needles.
In addition to the requirement for such large numbers of needles, electrical field interference between the different needles also limits the minimum separation between them and furthermore, continuous operation of needle-based spinnerets requires frequent cleaning of the needles as polymer deposits block the spinnerets.
Although the electrospinning process is relatively cost effective on a laboratory scale, the low rates of fiber throughput on single-needle setups make production at industrial volumes almost prohibitively expensive for most commodity applications like filtration and absorbent textiles.
The latest yarns from nanofibers spun by the new patented Licensing Technology Network technology |
By increasing fiber production rates, the cost can be dramatically lowered, broadening the scope of application for electrospun nanofibers and opening the door to the development of new technologies.
Licensing Technology Network's inventor claims to have proven that yarns consisting of about 100,000 fibers can be spun consistently with good strength and has filed a provisional patent on the production method for producing these yarns.
Patented methods allow possible commercialization
An industrial scale model is now being built to test the viability of producing nanofiber yarns of sufficient quality and consistency to be used for commercial weaving and knitting applications.
There are currently 13 different patented methods for making continuous yarns from electrospun fibers, according to Licensing Technology Network. Four of these were patented between 1934 and 1944, while the other nine appeared only after 2001. The two most promising patents were filed in 2005 and 2006.
The first, the twisted nonwoven web yarn method, involves electrospinning nanofibers through multiple nozzles to obtain a nonwoven nanofiber web, either in a larger form, or directly in a ribbon form. Subsequent to this, these nanofiber web ribbons are passed through an air twister to obtain a twisted nanofiber yarn.
The second recent patent describes a ribbon-shaped nanofiber web that is prepared by electrospinning onto a collector, which consists of a continual belt type plate with longitudinal grooves at regular intervals. The nonwoven nanofiber webs are deposited into the grooves. These are later separated from the collector, concentrated, strained and spun into a yarn.
The fact that the fibers collected selectively on the metal in a thin nonwoven web fiber strip provides a measure of confinement on the strip that leads to a certain degree of alignment.
These patents, however, have inherent limitations said to be overcome by this latest invention.
Microscopic picture of nanofiber structure
Nanofibers compared to a human hair |
None of the existing patented methods result in the spinning of long enough yarns, or are commercially viable enough to be adopted by the mainstream industry. The new method results in a quality twisted yarn, but also produces such yarns in quantities or speeds that would make production of these yarns commercially viable.
In contrast to the electrospun twisted nonwoven web yarn method patented in 2005, this new method is said to provide excellent parallel alignment of fibers.
It also provides much better control over and consistency of the staple-loop lengths of the fibers compared with the grooved belt collector yarn.
These two properties, combined in one process, result in a superior yarn with high strength due to the aligned fibers, and high reproducibility due to control over staple-loop lengths. This makes it ideal for weaving and knitting.
The replacement of only a small percentage of the fibers or yarns in a traditional textile fabric with yarns of similar diameter, but now made up of several thousands of nanofibers, can significantly increase the toughness and specific surface area of the fabric without increasing its overall mass. This could be of particular importance for applications like lighter airline blankets, increased strength of military clothes, overalls, outdoor sport clothing like hiking or ski wear.
Incorporating nanofibers into producing traditional textiles creates several opportunities. The complete fabric can even be made from nanofiber yarns. This has important implications in protective clothing applications, where lightweight, breathable fabrics with protection against extreme temperatures, moisture, ballistics, and chemical or biological agents are often required. This would also mean the mass production of existing developments like:
Stain free and wrinkle free clothes
Liquid resistant fabrics
Clothing that can absorb body odors
Clothing that emits slow release of deodorants
Clothing that changes color in light
Clothing that changes color with external or body heat
Nanospider and other nanofiber technologies
Oerlikon Neumag — part of Oerlikon Textile, the world's largest manufacturer of textile machinery — has announced a co-operation agreement with Elmarco of the Czech Republic — the first company worldwide to supply industrial production lines for nanofibers.
Nanospider technology is a modified electrospinning system developed by Elmarco in co-operation with the University of Liberec. It has the ability to process a wide range of polymers in diameters of 50-300nm into nonwoven webs of 0.1-5gsm.
The fibers are formed by an electrostatic field from a thin film of an aqueous or solvent solution, and a wide range of polymers, including PA6, PVA and PUR, as well as more exotic types such as chitosan and gelatine, have been successfully spun.
A substrate with a nanofiber layer (Photo from Elmarco) |
Centrifugal spinning is similar to electrospinning, using a high-speed rotating cylinder instead of nozzles or capillaries to create the fibers. 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.
Nanofibers currently account for 10% of Elmarco's annual US$23.3 million sales but in the future are expected to become its key product. Last year, the company sold 12 of its Nanospider machines to companies in Japan and the US.
With the full weight of Oerlikon Textile behind it, Nanospider technology is likely to rapidly become big commercial news.
DuPont is already achieving success in various fields including filtration and bedding with its Hybrid Membrane Technology (HMT).
HMT is comprised of continuous sub-micron polymeric filaments with a diameter of between 100 nanometers and 1 micron and is said to bridge the performance gap between microporous membranes and traditional nonwovens. The technology uses a proprietary spinning process that creates a membrane-like nonwoven sheet structure to deliver functionality across an extensive range of microfiltration applications.
In addition to improving the flux/barrier performance for air and liquid filtration applications, DuPont HMT adds breathability and protection to bedding and apparel products and can significantly improve safety, durability, and energy performance in a variety of battery and capacitor applications.
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, with commercialization of products commencing immediately.
"The role of Advanvced Fiber Systems was very important in this development," said president of DuPont's Hybrid Membrane Venture, Sandra S. VanWormer. "The electrospinning method of producing nanofibers has been around since the 1930s but the unique breakthrough made by DuPont was in developing production on a commercial basis, which we have done in Korea."
Consistency is very 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 global market for nanofibers was already worth US$43.2 million in 2006, having increased from US$40.2 million in 2005, according to a report by BCC Research.
The market will be worth an estimated US$48 million by the end of 2007, growing to US$176 million in 2012 and to US$825 million by 2017.
Nanofiber revenue growth is being driven primarily by the use of these materials in the mechanical/chemical sector, in particular for manufacturing filtration media.
Nanofibers are traditionally defined as cylindrical with an outer diameter below 1,000nm and an aspect ratio (the ratio between length and width) greater than 50. Over the years, several types of nanofibers have been developed — polymeric, carbon, ceramic, glass, metallic, and composite.
Nanofibers have current and potential use in a large variety of applications including electronics, mechanical, chemical, sensors and instrumentation, energy, medical, bioengineering, automotive, aerospace, thermal and acoustic insulation, consumer apparel, and defence and security.
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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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