Nano-knitting now possible in conventional fabrics.
Although nanofibres have been incorporated into conventional fabrics--such as with a nano version of silicon woven into socks to absorb body odour--the task of knitting nanofibres together in isolation has proved more challenging. As a result, the commercial end of the science has concentrated on bonding nano coatings and nano particles to existing fibres. But now researchers in the United States and Australia have created a "nano yarn" that works on the same principle as spinning conventional yarn from cotton.
Scientists created the yarn by growing a mat of fibres on a substrate, called a nanotube forest. A sharp instrument then pulled at the fibres along the plane of the substrate, twisting and wrapping the fibres around one another. Typically, the yarn spun is one-third of a millimetre long, but while to the lay person this may sound too small to be spun, the key is the length to diameter ratio in which fibres with a 10 nanometre diameter were used. The process works by embedding a nanofibre in a carbon nano-tube, which in turn is in a polymer-made textile. This creates a new textile fibre that is made of multi-walled carbon nano-tubes (MWCNTs) included in an organic copolymer matrix by extrusion processes. Such fibres are consequently assembled in a knit.
One of the challenges confronting scientists has been that there can be weaknesses in these fabrics. This is not because of the fibres, as the polymers offer strengthening properties, but because carbon nanotubes join together in what is described as a "non-harmonious way"--this means they do not always join in a supple and flexible way. The resulting knit is extremely strong but at the same time quite rigid.
Consequently, some areas are strong, others are breakable.
Scientists at the Nanotech Institute, University of Texas, Dallas, working with Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO), have now achieved what they believe is a significant breakthrough. They have spun multi-walled carbon nanotube yarns that are strong, tough and extremely flexible, and are both electrically and thermally conducting. Unlike conventional yarns, they do not degrade in strength as a result of overhand knotting (abrasion and knotting, especially with an overhand knot, seriously degrades the strength of most polymer fibres).
"If you take one yarn which forms a loop and connect it with a similar loop in the opposite direction you develop an intense loop strength," said Professor Ray Baughman, director of the Nanotech Institute at the University of Texas, Dallas. "Just like any knitted fabric it is interpenetrated with another loop. With carbon nanotubes, the fibre diameter is so small compared to any knots that the individual fibres don't even notice they are knotted," he said. Prof Baughman likens the scenario to a bamboo forest, with the carbon nanotubes standing erect on a substrate (usually silicon) like bamboo stalks. Using this analogy, if the nanotubes were bamboo, the bamboo would be an inch in diameter but half a mile high.
"These very tall, very small 'trees' tend to bend a little to make intermittent bundles," he explained. "The consequence is that when you pull on one side of the forest, one nanotube pulls a neighbouring one and the process propagates. We can make strong yarn by twisting them, just like you twist spun wool.
Prof Baughman and his colleagues have filed patents for the spinning process, addressing the issue of rigidity, covering the carbon nanotube spinning technology and other semiconducting, metallic and superconducting nanofibres and nanoribbons. The patent document, 420 pages long, lists a range of potential ramifications and applications and has been filed to comply with local patent laws in the United States, China, Europe, South Korea, India and Australia. "Individual carbon nanotubes are like minute bits of string, and many trillions of these invisible strings must be assembled to make useful macroscopic articles," said Dr Baughman. "Every pound of spun yawn would consist of two million miles of nanotubes."
In practical terms, it may transpire that production of nano yarns will resemble traditional weaving and production techniques. To establish a prototype yarn, Prof Baughman invited a research student to weave what is thought to be the first ever carbon nanotube-based textile on a conventional wooden loom. The loom was strung with polyester yarn and wove in 10-ply yarn made from multi-walled carbon nanotubes synthesized in the laboratory. The result was a fingernail-sized swatch of fabric.
"I can imagine growing carbon nano tubes as a forest at one end of a 12-metre conveyor belt and stripping them at the other end," he said. "If you are using twist-based spinning it is basically the same technique that has been used for cotton and wool. The end result is just like holding cotton in your hands, except it looks black."
The potential implications are huge, according to Prof Baughman. Among other things, the futuristic yarns could result in "smart" clothing that stores electricity, provides ballistic protection and adjusts temperature and porosity to provide greater comfort. Other potential applications could include bandages that help injured limbs move again, tighten to stem bleeding or send a signal to say someone was hurt.
Professor Sandy Black, reader in fashion and textiles design and technology at the London College of Fashion, believes such developments could transform the way the textile industry goes about its business. This is because nanotechnology is likely to offer the opportunity for the colourisation of fabrics--that is, that a blouse would be able to be switched from green to blue either by the manufacturer or the customer.
"At the moment, manufacturers in fashion have to guess at how many items in a colour or size they need to order," said Prof Black. "But if there were a shorter pipeline then that would mean less waste and cost for manufacturers. Customers would get a bespoke service but not at the high cost of labour that is traditionally the case."
"Although the overheads in R&D are large at the moment, that will change and as a marketing tool, nanotechnology offers manufacturers an extra function. There have been fantasies about small capsule wardrobes--or nano-wardrobes--that enable clothes to change colour or texture. That would offer novelty and variety to the market."
Prof Baughman also envisages some extraordinary applications and potential uses. "The chameleon effect this material offers is particularly important for the military and would enable soldiers to blend into their environment," he said. Other potential products would involve overcoats made from carbon nanotube yarns that change colour when they heat up." However, he cautioned that such commercial products would take time to reach the market. "Commercial viable is some way off," he said. "It's a question of scaling up."
As for cost, Prof Baughman believes nano yarns are already viable and costs will drop dramatically, as multi-walled nanotubes will be cheaper than single-walled ones. "Everyone thinks carbon nanotubes must be expensive but the lowest cost for MWCNTs is around US$100 per pound," he said. "That makes it inexpensive enough to be used for fairly routine applications."
www.tfrg.org.uk Textile Futures Research Group
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|Publication:||International News Services.com|
|Date:||Jul 1, 2007|
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