Appendix F
High-Performance Fibers
ARAMID FIBERS (KEVLAR AND TECHNORA)
Poly(p-phenylene terephthalamide), Kevlar, was first synthesized by Kwolek at DuPont in the 1960s. Kevlar is processed from sulfuric acid, with the polymer concentration at about 20 wt percent. Surprisingly, there is a decrease in viscosity with increased polymer concentration due to local alignment of polymer molecules in the solution to form a nematic phase. Thus the solution becomes liquid crystalline, a feature that had earlier been predicted by Flory.1 The solution is extruded through an air gap into an acid solvent, such as water, where it coagulates. Removal of the approximately 80 percent acid from solution during fiber drying and tension heat treatment (500°C) leads to the formation of a highly aligned, extended chain fiber. However, the coagulation process also creates undesirable defects. The number of defects can be estimated from the deviation of the actual fiber density from the theoretical crystal density of 100 percent (approximately 1.45 g/cm3 versus 1.50 g/cm3). Kevlar fiber was developed and commercialized at DuPont, originally for completely different applications than for body armor (for example, it was used for reinforcing tires). The potential of Kevlar for use in ballistic protection was realized only when the National Institute of Justice conducted ballistic testing on Kevlar fabric. Other polyaramids followed, including Technora, an aramid copolymer fiber that is produced in the Netherlands and Japan from terephthaloyl chloride and a mixture of p-phenylenediamine and 3,4’-diaminodiphenylether.
POLYETHYLENE (SPECTRA, DYNEEMA)
Unlike the extended rigid-rod molecular structure of Kevlar, polyethylene (PE) is one of the most flexible polymers. Since the 1930s, fibers and films have been manufactured from PE by melt processing. The morphology of these fibers and films is semicrystalline, consisting of 60 to 70 vol percent crystals; the remainder consists of amorphous, entangled polymer chains. Interestingly, melt-processed polyethylene contains chain-folded crystals with a modulus in the 1 GPa range. Trash bags and milk jugs, having typical molecular weights of 50,000 to 200,000 g/mole, are common examples of such polyethylene products. But if PE molecules could be extended into straight chains, the carbon-carbon backbone would give outstanding properties. Indeed, after nearly half a century of process development in the field of polyethylene, a new type of spinning was invented by Smith and Lemstra in the Netherlands in early 1980s.2 Known as gel spinning, this process is able to extend the macromolecules to nearly their full length and results in a highly crystalline extended-chain polyethylene fiber exhibiting high strength and high modulus characteristics that show ballistic protection capability. Because the molecules are processed from a dilute solution, the molecular weight of the polyethylene used in gel spinning can be in excess of 3 million g/mole or higher, much higher than that in any other synthetic polymer. Fiber is processed from a decalin solution that typically contains less than 5 wt percent polymer. The polymer solution is extruded at between 130°C to 150°C or so into a cold coagulant such as water. This resulting gel-like fiber, which contains more than 95 percent solvent, is typically then drawn at between 90°C and 130°C to draw ratios of 50 to 100. The macromolecules become extended and form near-single-crystal fibers.
The theoretical density of polyethylene is 1.00 g/cm3, while the density of Spectra and Dyneema fibers is about 0.97 g/cm3. This underscores the fact that even today’s highly extended-chain polyethylene fibers contain a significant number of defects and suggests an opportunity for even more significant gains in future development of this material.
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1Flory, P. 1956. Phase equilibria in solutions of rod-like particles. Proceedings of the Royal Society of London Series A—Mathematical and Physical Sciences 234(1196):73-89.
2Smith, P., and P. Lemstra. 1980. Ultra-high-strength polyethylene filaments by solution spinning/drawing. Journal of Materials Science 15(2): 505-514.
RIGID-ROD POLYMERS (ZYLON AND M5)
After the successful commercial development of Kevlar in the 1970s, significant research efforts were devoted to the development of other rigid-rod polymers. Rigid-rod polymers programs began in the 1960s at the U.S. Air Force Research Laboratory as well as in Russia. The U.S. program was accelerated in the 1970s, resulting in the development of poly-p-phenylene benzobisthiazole and polybenzoxazole (PBO) fibers.3PBO fiber was further developed initially at SRI International and later at Dow Chemical Company before being commercialized by Toyobo Company (Japan) in 1998 under the trade name Zylon. Among other applications, PBO fiber was also developed for use in fire-protective clothing as well as for ballistic protection. However, in the early 2000s it became clear that there were environmental stability issues with Zylon fiber causing decreased fiber strength over time and negatively affecting its ballistic performance. This is attributed to poor resistance to ultraviolet radiation as well as to poor hydrolytic stability.
In an attempt to improve intermolecular interactions in rigid-rod polymers with the intent of increasing the fiber compressive strength and torsional modulus, the Akzo Nobel firm in the Netherlands synthesized and processed polypyridobisimidazole (under the name M5) fiber during the 1990s.4 The fiber was further developed by Magellan Systems International, and the technology now resides with DuPont, although the fiber has not yet been commercialized.
Similar to Kevlar, both the Zylon and M5 fibers are processed from a liquid crystalline polymer solution, except in this case the solution is one of polyphosphoric acid. Depending on the polymer molecular weight, for fiber spinning, polymer concentration in solution is again typically between 5 weight percent 15 weight percent. Like the process used to make Kevlar, the nematic solution is extruded through an air gap into an acid solvent such as water. The coagulated fiber is then heat-treated under tension up to about 500°C. Structure formation mechanism in the rigid-rod chains of Zylon and M5 fibers is very similar to the structure formation mechanism in Kevlar and is quite different from that of the flexible-chain gel-spun polyethylene (Dyneema and Spectra).
Intermolecular interactions in polyethylene are only van der Waals interactions, whereas in Kevlar there is hydrogen bonding in one dimension transverse to the fiber axis, and in M5 fibers there is hydrogen bonding in two transverse directions. Ranking fibers in from greatest to least, in terms of compressive and torsional properties, shows that M5 has highest compressive and torsional properties, followed by Kevlar, then Zylon, then Spectra and Dyneema, which are approximately equal.
THERMOTROPIC LIQUID CRYSTALLINE POLYMERIC FIBERS
Thermotropic liquid crystalline polymeric fibers, developed in the 1970s, are melt processed (no solvent). These polymers exhibit liquid crystalline behavior in the melt state. Vectran, a copolyester and an example of a commercial fiber in this class, is spun at temperatures of 275°C or more. To further enhance mechanical properties, as-spun fiber may be further drawn and annealed below the polymer melting temperature. During this process, fiber may also undergo further solid state polymerization, resulting in a polymer of higher molecular weight. Unlike the liquid-crystalline-solution processing of rigid-rod polymers and the gel spinning of flexible-chain polyethylene—both of which are processed from polymer solutions containing 85 percent to 95 percent solvent (which must be removed during fiber processing)—there is no solvent to be removed in the processing of thermotropic liquid crystalline polymers. Compared to polyethylene, however, the molecular weights (and hence the chain length) of aramids, rigid-rod polymers, and thermotropic liquid crystalline polymers are much more limited. Vectran has more applications in injection-molded products than in fiber form.
CARBON FIBERS
The development of modern carbon fibers dates back to the 1960s with research by Shindo in Japan, Watt in England, and Bacon at Union Carbide in the United States. Early carbon fibers were made by pyrolyzing cellulose; today, carbon fibers are made starting from petroleum pitch or from polyacrylonitrile (PAN) copolymers. Pitch-based carbon fibers can have a very high tensile modulus and high electrical and thermal conductivities but exhibit relatively low tensile and compressive strength. By contrast, PAN-based carbon fibers have high tensile strength, good compressive strength, and intermediate modulus and electrical and thermal conductivities. High-purity mesophase pitch (a liquid crystalline pitch) is melted, extruded typically at about 400°C, and then carbonized in stages (Stage 1 at 600°C to 1000°C, Stage 2 at 1100°C to 1600°C, and Stage 3 at 2200°C to 2700°C) in an inert environment. Fibers carbonized at about 2700°C can exhibit up to 90 percent of the theoretical modulus. The theoretical modulus of graphite along graphene planes is 1,060 GPa, giving it a specific theoretical modulus of 469 N/tex,5 which is equivalent to 469 GPa/(g/cm3).
PAN fibers are either wet spun or dry-jet wet spun from solutions in sodium thiocyanate and water, dimethyl acetate,
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3Chae, H., and S. Kumar. 2006. Rigid-rod polymeric fibers. Journal of Applied Polymer Science 100(1): 791-802.
4Sikkema, D. 1998. Design, synthesis and properties of a novel rigid rod polymer, PIPD or ‘M5’: High modulus and tenacity fibres with substantial compressive strength. Polymer 39(24): 5981-5986.
5“Tex” is the mass of a 1,000-meter length of fiber in grams.
dimethylsulfoxide, or zinc chloride and water.6 Depending on the molecular weight, solvent, and the copolymer composition, the polymer concentration in solution is typically 5 to 25 wt percent. After spinning, fibers are successively drawn at several different temperatures (typically between room temperature and 175°C). Drawn fibers are oxidized under tension typically between 200°C and 350°C for approximately 2 hours. Oxidized fibers are then carbonized under tension in stages, similar to the carbonization of pitch-based fiber. Fibers with the highest tensile strength are typically obtained at about 1300°C to 1500°C.
CARBON NANOTUBE FIBERS
Carbon nanotube (CNT) fibers to date have been processed primarily by one of the following two techniques: (1) CNT smoke drawn directly from the chemical vapor deposition reactor in the form of aerogel fibers7 and (2) fiber processed from aqueous8 or acidic9 dispersions of CNTs. In both cases, it is important that the CNTs be as long as possible and as perfect as possible, and they should be free of catalyst and other foreign impurities, including amorphous carbon. The tube-to-tube diameter variation should be minimized and the diameter should be relatively small. Nanotube orientation also plays a critical role with respect to mechanical properties.10 Multiwall CNTs tend to undergo telescoping, with the individual tubular shells slipping past one another, whereas single-wall CNTs are essentially the ultimate for a high-strength polymer molecule, having a theoretical strength as high as 150 GPa and modulus values as high as 1,050 GPa, respectively. The theoretical modulus of carbon nanotubes is dependent on their diameter since their central portion is empty; however, their specific theoretical modulus is 469 N/tex irrespective of the diameter.
ALUMINA, BORON, SILICON CARBIDE, GLASS, AND ALUMINA BOROSILICATE CERAMIC FIBERS
Boron fiber is processed using chemical vapor deposition on substrates such as tungsten or carbon, whereas silicon carbide fibers can be processed either by chemical vapor deposition or by a precursor method similar to the processing of carbon fibers. Alumina and alumina borosilicate fibers are typically processed using a sol-gel precursor followed by sintering. Nextel fibers (from 3M Company) are ceramic oxide fibers that belong to the category of alumina-boro-silicate. Compared to polymeric and carbon fibers, these fibers retain their mechanical properties to much higher temperatures. Although the tensile strength of these fibers is not quite as high as that of some of the polymeric fibers, their compressive strength can be comparable to or higher than that of carbon fiber having the best compressive strength. Owing to ionic-covalent bonds in all directions, these fibers are much more isotropic than are carbon and polymer fibers, which exhibit a very high degree of anisotropy.11,12,13
Glass is melt-extruded and drawn into fibers typically at 1000°C to 1200°C. Fiber tensile strength is limited by defects, residual stresses, and structural inhomogeneities in the fibers.
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6Gupta, V., and V. Kothari. 1997. Manufactured Fibre Technology. New York, N.Y.:Chapman and Hall.
7Koziol, K., J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett, and A. Windle. 2007. High-performance carbon nanotube fiber. Science 318(5858): 1892-1895.
8Vigolo, B., A. Penicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, P. Bernier, and P. Poulin. 2000. Macroscopic fibers and ribbons of oriented carbon nanotubes. Science 290(5495): 1331-1334.
9Ericson, L., H. Fan, H. Peng, V. Davis, W. Zhou, J. Sulpizio, Y. Wang, R. Booker, J. Vavro, C. Guthy, A. Parra-Vasquez, M. Kim, S. Ramesh, R. Saini, C. Kittrell, G. Lavin, H. Schmidt, W. Adams, W. Billups, M. Pasquali, W-F. Hwang, R. Hauge, J. Fisher, and R. Smalley. 2004. Macroscopic, neat, single-walled carbon nanotube fibers. Science 305(5689): 1447-1450.
10Liu, T., and S. Kumar. 2003. Effect of orientation on the modulus of SWNT films and fibers. Nano Letters 3 (5): 647-650.
11Chawla, K. 1998. Fibrous Materials. Cambridge, U.K.: Cambridge University Press.
12Elices, M., and J. Llorca. 2002. Fiber Fracture. Oxford, U.K.: Elsevier Science.
13Watt, W., and B. Perov, eds. 1986. Strong Fibers. North-Holland: Elsevier Science.