Thursday, May 15, 2008

Making Strong Fibers

Structure of various fibersSchematic structures of various fibers. With decreasing disorder and defect density, the fiber strength increases (left to right). (Left) Typical commodity textile fiber contains amorphous and crystalline regions as well as voids and foreign particles; tensile strength, ~0.5 GPa. (Middle) High-performance polymer fibers contain chain ends, entanglements, voids, and defects; tensile strength, ~5 GPa. The structure of currently produced carbon nanotube fibers resembles this structure. In addition, carbon nanotube fibers often contain foreign particles in the form of catalysts. (Right) On the basis of predicted strain to failure (15), ideal carbon nanotube fibers without defects or entanglements will have a specific tensile strength of 70 N/tex; for a single-wall carbon nanotube fiber a diameter of 2 nm, this would translate to a tensile strength of 70 GPa.

Today’s polymeric and carbon fibers are up to 10 times as strong as that available half a century ago (see the figure, left and middle panels). High-performance polymeric fibers find applications in textiles such as firefighter clothing, bulletproof vests, and cables requiring stiffness and strength in tension; high-performance carbon fibers find applications in structural composites, for example, in airplanes, satellites, and tennis rackets. Even stronger fibers can be made using carbon nanotubes. These fibers will be lighter than existing fibers and may increase the performance of airplanes, space vehicles, and many sports and leisure goods.

In applications such as advanced textiles and structural composites, high-performance polymeric and carbon fibers have two key advantages over high-strength bulk materials such as steel. First, as a result of their low density, the specific strength (strength divided by density) of polymeric and carbon fibers is 5 to 10 times that of high-strength bulk materials. Second, fibers can be more easily processed into complex shapes.

One reason that fibers can have higher strength than bulk materials is that they can be processed with small diameters, which minimizes defects. The diameters of current commercial high-performance polymeric fibers range from 10 to 15 μm, and those of carbon fibers range from 5 to 10 μm; making fibers with even smaller diameters will further increase the strength.

Innovations in polymer synthesis and fiber processing have been critical for making high-performance fibers. The first high performance fiber, Kevlar was developed in the 1960s at DuPont, a company with a long history in fiber synthesis. In the 1980s, polyethylene, first synthesized in the 1930s, was processed into a high-performance commercial fiber (with the trade names Spectra and Dyneema), based on gel-spinning technology invented at DSM in the Netherlands. The high-strength polymeric fiber Zylon, based on rigid-rod polymer research that began in the 1970s at the U.S. Air Force Research Laboratory, was commercialized in 1998 by Toyobo in Japan. High-strength polyacrylonitrile-based carbon fibers were developed by optimizing parameters such as the polyacrylonitrile-copolymer composition, fiber spinning, and polyacrylonitrile stabilization and carbonization. Other high-strength fibers include silicon carbide, alumina, glass, and alumina borosilicate fibers; however, these fibers have relatively high densities.

Kevlar, Zylon, Spectra, Dyneema, and the carbon-fiber precursor polyacrylonitrile are all processed from solutions typically containing 5 to 20 weight percent (wt %) of polymer (1, 5). The rest of the mass is the solvent. Solvent removal disrupts the structure and worsens tensile properties. Entanglements, chain ends, voids, and foreign particles act as stress concentration points, lowering strength (see the figure, middle panel). Conversion of polyacrylonitrile to carbon involves heat treatment under tension in air and then in an inert environment. During this process,
~40% of the mass is lost in the form of gases. Gas diffusion, particularly at high temperature, also disrupts and degrades the structure and lowers strength.

Carbon nanotube fibers can also be processed from liquid media. Alternatively, they may be pulled from nanotube “forests” or drawn as an aerogel fiber from the gas phase in a reactor. The latter process appears to be particularly promising. The resulting fiber consists mostly of flattened double-walled nanotubes with diameters of 5 to 10 nm and a length of about
1 mm. The specific strength of this fiber is up to 2.5 times that of the strongest commercial fiber today.

The method used to make this fiber eliminates two problems encountered in the formation of high-strength polymeric and polyacrylonitrile-based carbon fibers: the solvent and gas removal. However, some key challenges need to be ironed out. The catalyst particles must be eliminated from the fiber; drawing conditions must be optimized to eliminate entanglements between carbon nanotubes; and conditions must be tuned so that the growth of a given carbon nanotube is not terminated and that the fiber is pulled at the rate at which nanotubes are growing.

The structure of carbon nanotube fibers is similar to that of high-strength polymeric fibers (see the figure, middle panel). Both types of fibers are strong and stiff along the fiber axis in tension, but relatively weak in axial compression and transverse to the fiber axis. However, carbon nanotube fibers are electrically and thermally conducting, whereas current high-strength polymeric fibers are insulators of both heat and electricity. Potential applications of carbon nanotube fibers will thus be those requiring high strength and stiffness in tension, high energy absorption, and electrical and thermal conductivity. The low density of these fibers would provide further weight savings.

Carbon nanotubes can also act as a nucleating agent for polymer crystallization and as a template for polymer orientation. No other nucleating agents are as narrow and long as a single-wall carbon nanotube. The tensile strength of a poly (vinyl alcohol) film tripled with the addition of 1 wt % of single-wall carbon nanotubes. Similarly, incorporation of 1 wt % of carbon nanotubes in polyacrylonitrile increased the tensile strength and modulus of the resulting carbon fiber by 64% and 49%, respectively. Polyacrylonitrile/carbon nanotube composites have good tensile and compressive properties. Next-generation carbon fibers used for structural composites will thus likely be processed not from polyacrylonitrile alone but from its composites with carbon nanotubes.

If processing conditions can be developed such that all carbon nanotube ends, catalyst particles, voids, and entanglements are eliminated, this would result in a continuous fiber with perfect structure, low density, and tensile strength close to the theoretical value. Such a carbon nanotube fiber could have 10 times the specific strength of the strongest commercial fiber available today. However, many challenges have to be overcome to achieve this goal.

*.* Source of Information : 15th February 2008 Science

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